theoretical and experimental challenges

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QED in laser-plasma interactions: theoretical and experimental challenges Tom Blackburn Chalmers University of Technology 4th July 2017 Workshop on Plasma Astrophysics: From the Laboratory to the Thermal Universe

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QED in laser-plasma interactionstheoretical and experimental challenges

Tom BlackburnChalmers University of Technology

4th July 2017Workshop on Plasma Astrophysics From the Laboratory to the Thermal Universe

Introduction

Goals

Understanding QED in laser-plasmas is necessary

Understanding QED in laser-plasmas is interesting

(Understanding QED in laser-plasmas is difficult)

Introduction

Goals

Understanding QED in laser-plasmas is necessary

Understanding QED in laser-plasmas is interesting

(Understanding QED in laser-plasmas is difficult)

Introduction

Goals

Understanding QED in laser-plasmas is necessary

Understanding QED in laser-plasmas is interesting

(Understanding QED in laser-plasmas is difficult)

Introduction

Quantum electrodynamics

Fundamental theory of the interaction between light and matter

Unifies quantum mechanics with special relativity

Charged particles interact via the emission and absorption of photons

Introduction

Quantum electrodynamics

Fundamental theory of the interaction between light and matter

Unifies quantum mechanics with special relativity

Charged particles interact via the emission and absorption of photons

Introduction

Quantum electrodynamics

Each vertex carries a factor of e the electron charge and a δ-function encoding the conservation of momentum

Simplest two photon emission (radiation reaction) and pair creation

Introduction

Quantum electrodynamics

Two-photon pair creation (Breit-Wheeler process) not observed experimentally (heavy ion collisions hohlraums)

Multiphoton Breit-Wheeler seen at SLAC (50 GeV electron beam into 1018 Wcm-2 laser 140 positrons) Non-linear index n = 5 6

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Zeeman effect Coulomb potential perturbed by static magnetic field

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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  • Slide 82

Introduction

Goals

Understanding QED in laser-plasmas is necessary

Understanding QED in laser-plasmas is interesting

(Understanding QED in laser-plasmas is difficult)

Introduction

Goals

Understanding QED in laser-plasmas is necessary

Understanding QED in laser-plasmas is interesting

(Understanding QED in laser-plasmas is difficult)

Introduction

Goals

Understanding QED in laser-plasmas is necessary

Understanding QED in laser-plasmas is interesting

(Understanding QED in laser-plasmas is difficult)

Introduction

Quantum electrodynamics

Fundamental theory of the interaction between light and matter

Unifies quantum mechanics with special relativity

Charged particles interact via the emission and absorption of photons

Introduction

Quantum electrodynamics

Fundamental theory of the interaction between light and matter

Unifies quantum mechanics with special relativity

Charged particles interact via the emission and absorption of photons

Introduction

Quantum electrodynamics

Each vertex carries a factor of e the electron charge and a δ-function encoding the conservation of momentum

Simplest two photon emission (radiation reaction) and pair creation

Introduction

Quantum electrodynamics

Two-photon pair creation (Breit-Wheeler process) not observed experimentally (heavy ion collisions hohlraums)

Multiphoton Breit-Wheeler seen at SLAC (50 GeV electron beam into 1018 Wcm-2 laser 140 positrons) Non-linear index n = 5 6

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Zeeman effect Coulomb potential perturbed by static magnetic field

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
  • Slide 2
  • Slide 3
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Introduction

Goals

Understanding QED in laser-plasmas is necessary

Understanding QED in laser-plasmas is interesting

(Understanding QED in laser-plasmas is difficult)

Introduction

Goals

Understanding QED in laser-plasmas is necessary

Understanding QED in laser-plasmas is interesting

(Understanding QED in laser-plasmas is difficult)

Introduction

Quantum electrodynamics

Fundamental theory of the interaction between light and matter

Unifies quantum mechanics with special relativity

Charged particles interact via the emission and absorption of photons

Introduction

Quantum electrodynamics

Fundamental theory of the interaction between light and matter

Unifies quantum mechanics with special relativity

Charged particles interact via the emission and absorption of photons

Introduction

Quantum electrodynamics

Each vertex carries a factor of e the electron charge and a δ-function encoding the conservation of momentum

Simplest two photon emission (radiation reaction) and pair creation

Introduction

Quantum electrodynamics

Two-photon pair creation (Breit-Wheeler process) not observed experimentally (heavy ion collisions hohlraums)

Multiphoton Breit-Wheeler seen at SLAC (50 GeV electron beam into 1018 Wcm-2 laser 140 positrons) Non-linear index n = 5 6

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Zeeman effect Coulomb potential perturbed by static magnetic field

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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  • Slide 82

Introduction

Goals

Understanding QED in laser-plasmas is necessary

Understanding QED in laser-plasmas is interesting

(Understanding QED in laser-plasmas is difficult)

Introduction

Quantum electrodynamics

Fundamental theory of the interaction between light and matter

Unifies quantum mechanics with special relativity

Charged particles interact via the emission and absorption of photons

Introduction

Quantum electrodynamics

Fundamental theory of the interaction between light and matter

Unifies quantum mechanics with special relativity

Charged particles interact via the emission and absorption of photons

Introduction

Quantum electrodynamics

Each vertex carries a factor of e the electron charge and a δ-function encoding the conservation of momentum

Simplest two photon emission (radiation reaction) and pair creation

Introduction

Quantum electrodynamics

Two-photon pair creation (Breit-Wheeler process) not observed experimentally (heavy ion collisions hohlraums)

Multiphoton Breit-Wheeler seen at SLAC (50 GeV electron beam into 1018 Wcm-2 laser 140 positrons) Non-linear index n = 5 6

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Zeeman effect Coulomb potential perturbed by static magnetic field

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
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Introduction

Quantum electrodynamics

Fundamental theory of the interaction between light and matter

Unifies quantum mechanics with special relativity

Charged particles interact via the emission and absorption of photons

Introduction

Quantum electrodynamics

Fundamental theory of the interaction between light and matter

Unifies quantum mechanics with special relativity

Charged particles interact via the emission and absorption of photons

Introduction

Quantum electrodynamics

Each vertex carries a factor of e the electron charge and a δ-function encoding the conservation of momentum

Simplest two photon emission (radiation reaction) and pair creation

Introduction

Quantum electrodynamics

Two-photon pair creation (Breit-Wheeler process) not observed experimentally (heavy ion collisions hohlraums)

Multiphoton Breit-Wheeler seen at SLAC (50 GeV electron beam into 1018 Wcm-2 laser 140 positrons) Non-linear index n = 5 6

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Zeeman effect Coulomb potential perturbed by static magnetic field

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
  • Slide 2
  • Slide 3
  • Slide 4
  • Slide 5
  • Slide 6
  • Slide 7
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  • Slide 82

Introduction

Quantum electrodynamics

Fundamental theory of the interaction between light and matter

Unifies quantum mechanics with special relativity

Charged particles interact via the emission and absorption of photons

Introduction

Quantum electrodynamics

Each vertex carries a factor of e the electron charge and a δ-function encoding the conservation of momentum

Simplest two photon emission (radiation reaction) and pair creation

Introduction

Quantum electrodynamics

Two-photon pair creation (Breit-Wheeler process) not observed experimentally (heavy ion collisions hohlraums)

Multiphoton Breit-Wheeler seen at SLAC (50 GeV electron beam into 1018 Wcm-2 laser 140 positrons) Non-linear index n = 5 6

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Zeeman effect Coulomb potential perturbed by static magnetic field

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
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Introduction

Quantum electrodynamics

Each vertex carries a factor of e the electron charge and a δ-function encoding the conservation of momentum

Simplest two photon emission (radiation reaction) and pair creation

Introduction

Quantum electrodynamics

Two-photon pair creation (Breit-Wheeler process) not observed experimentally (heavy ion collisions hohlraums)

Multiphoton Breit-Wheeler seen at SLAC (50 GeV electron beam into 1018 Wcm-2 laser 140 positrons) Non-linear index n = 5 6

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Zeeman effect Coulomb potential perturbed by static magnetic field

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
  • Slide 2
  • Slide 3
  • Slide 4
  • Slide 5
  • Slide 6
  • Slide 7
  • Slide 8
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Introduction

Quantum electrodynamics

Two-photon pair creation (Breit-Wheeler process) not observed experimentally (heavy ion collisions hohlraums)

Multiphoton Breit-Wheeler seen at SLAC (50 GeV electron beam into 1018 Wcm-2 laser 140 positrons) Non-linear index n = 5 6

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Zeeman effect Coulomb potential perturbed by static magnetic field

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
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Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Zeeman effect Coulomb potential perturbed by static magnetic field

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
  • Slide 2
  • Slide 3
  • Slide 4
  • Slide 5
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Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Zeeman effect Coulomb potential perturbed by static magnetic field

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
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Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Zeeman effect Coulomb potential perturbed by static magnetic field

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
  • Slide 2
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  • Slide 78
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82

Reduce a hard problemby splitting it into a ldquoeasyrdquo part and a ldquosmallrdquo correction

Results take the form of a series expansion around the easy (ie unperturbed) solution

eg ground state energies in quantum mechanics

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Zeeman effect Coulomb potential perturbed by static magnetic field

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
  • Slide 2
  • Slide 3
  • Slide 4
  • Slide 5
  • Slide 6
  • Slide 7
  • Slide 8
  • Slide 9
  • Slide 10
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Stark effect Coulomb potential perturbed by static electric field

Zeeman effect Coulomb potential perturbed by static magnetic field

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
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Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
  • Slide 2
  • Slide 3
  • Slide 4
  • Slide 5
  • Slide 6
  • Slide 7
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Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

More = better

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

More = better

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
  • Slide 2
  • Slide 3
  • Slide 4
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Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

But the series diverges

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

But the series diverges

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Perturbation theory generically leads to divergent series

This has been known for a long time ndash see Bender and Wu Phys Rev 184 1231 (1969)

Besides infinities are interesting

Beyond perturbative QED

Perturbation theory in quantum mechanics

ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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ldquoThe series is divergent therefore we may be able to do something with itrdquo - O Heaviside

ldquoThis is infinity here It could be infinity We donrsquot really donrsquot know But it could be It has to be something ndash but it could be infinity rightrdquo- D J Trump

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
  • Slide 2
  • Slide 3
  • Slide 4
  • Slide 5
  • Slide 6
  • Slide 7
  • Slide 8
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We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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We can handle divergent series using Borel summation

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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We can handle divergent series using Borel summation

Use the integral definition of a factorial

Beyond perturbative QED

Perturbation theory in quantum mechanics

We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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We can handle divergent series using Borel summation

Use the integral definition of a factorial

Swap the summation and integration

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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What does this mean for factorially-divergent series

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

Beyond perturbative QED

Perturbation theory in quantum mechanics

What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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What does this mean for factorially-divergent series

If the terms alternate in sign the result is real and finite

If they dont the integrand has a pole on the real axis

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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  • Slide 82

Recall the dispersion relation for an EM wave in a warm plasma

The permittivity has a pole where the phase velocity of the wave and the particle velocity are equal

Gives rise to Landau damping

This has physicalsignificance

Beyond perturbative QED

Perturbation theory in quantum mechanics

If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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If the terms do not alternate in sign Borel summation gives you an imaginary term that is non-perturbative in the coupling strength

Non-perturbative in the sense that exp(-1x) has Taylor expansion about 0 that is 0 to all orders

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Stark effect Coulomb potential perturbed by static electric field

Divergent perturbative series with coefficients that do not alternate in sign

Zeeman effect Coulomb potential perturbed by static magnetic field

Divergent perturbative series with coefficients alternating in sign

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Stark effect Coulomb potential perturbed by static electric field

Energy shift of the ground state of H

Ionization rate

Zeeman effect Coulomb potential perturbed by static magnetic field

Energy shift of the ground state of H

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Perturbation theory is generically divergent

Controlling those divergences naturally introduces physics from the non-perturbative sector

Why non-perturbative Why perturbative

Beyond perturbative QED

Perturbation theory in quantum mechanics

Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Does the same occur in quantum field theory

After all perturbation theory has proved extraordinarily successful

Beyond perturbative QED

Perturbation theory in quantum mechanics

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Static magnetic field Euler-Heisenberg effective action (exact result)

Beyond perturbative QED

Perturbation theory in QED

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Static electric field The field invariants are E2 ndash B2 and EB

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Static electric field The field invariants are E2 ndash B2 and EB

Transform B2 rarr ndashE2 and the terms no longer alternate in sign

S has a non-perturbative imaginary part

Static magnetic field Euler-Heisenberg effective action (exact result)

Perturbative expansion is divergent but the signs alternate

Beyond perturbative QED

Perturbation theory in QED

The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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  • Slide 3
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The action for a static electric field acquires an imaginary part that is non-perturbative in the electric field strength

Stark effect = ionisation rate Here we get the pair-creation rate from the vacuum

Ecrit = 13times1018 Vm

Beyond perturbative QED

Perturbation theory in QED

How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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How do we access fields of this magnitude in the lab

Important parameter is χ (ratio of electric field in electron rest frame to the critical field of QED)

Beyond perturbative QED

QED in intense fields

χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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χ = 1 means the electron sees in its rest frame a field strong enough for non-perturbative effects to dominate

Low order terms are suppressed only high order terms (ie involving large numbers of background photons) contribute

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Treat the intense background as a classical field calculate transition rates to all orders

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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  • Slide 3
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  • Slide 78
  • Slide 79
  • Slide 80
  • Slide 81
  • Slide 82

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

Beyond perturbative QED

QED in intense fields

Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Treat the intense background as a classical field calculate transition rates to all orders

Rates no longer diverge but now we need entire space-time structure of the background field

What about arbitrary field structures

Beyond perturbative QED

QED in intense fields

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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If the formation length is short enough we can assume the fields are quasi-static over the emission process

The formation length is smaller than the laser wavelength by a factor of the strength parameter a0

Coupling sfQED and classical processes

Locally constant field approximation

What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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What do we lose by doing this

Harmonics sub-harmonics interference resonanceshellip

When is it acceptable to do so

How much bigger than 1 must a0 be

Coupling sfQED and classical processes

Locally constant field approximation

Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Other constraints on PIC simulations

Size of timestep must be set to avoid multiple scatterings (constraint set by rate of photon emission is strongest)

C P Ridgers et al J Comp Phys 260 273 (2014)

The rates (not cross-sections) for photon and pair production are calculated in an equivalent system of fields with the same instantaneous value of χ

eg a static magnetic field in the ultrarelativistic limit T Erber Rev Mod Phys 38 626 (1966)

Coupling sfQED and classical processes

PIC + Monte Carlo

We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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We assumed a classical background field

Contains sufficient energy that the field is not affected by a (single) QED process

Energy is absorbed by plasmas ndash what happens to the background field

Coupling sfQED and classical processes

QED in intense fields

Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Creation of electron-positron pair transfers energy from background field (at interaction point)

χ 1 photon energy split evenly between electron and positron

For χ 1 most goes to one of the electron and positron

Coupling sfQED and classical processes

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Example photon collides with a single-wavelength linearly-polarized plane EM wave

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
  • Slide 2
  • Slide 3
  • Slide 4
  • Slide 5
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Example photon collides with a single-wavelength linearly-polarized plane EM wave

For χ 1 quantum depletion 2m2ω vs classical a0

2m2ω

For χ 1 quantum depletion m vs classical a0

2m

Beyond the background field approximation

Depletion

Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Example photon collides with a single-wavelength linearly-polarized plane EM wave

Laser absorption is increased (classically) because QED processes introduce new currents

Beyond the background field approximation

Depletion

In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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In QED need to calculate transition rates between initial and final states that contain different numbers of photons

Does the LCFA let us take a simpler approach to calculating the effect of a QED avalanche on the background field

Beyond the background field approximation

Depletion

When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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When does the quantum depletion have a serious effect on the background

Seipt et al PRL 118 154803 2017 for electron-beam laser collision need 10nC and a0 of 1000

Effects on angular distribution at lower intensity

Beyond the background field approximation

Depletion

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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  • Slide 82

Introduction

What are the challenges

QED in strong electromagnetic fields

Coupling QED to classical plasma physics

Experimental observation with currently available laser systems

Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
  • Slide 2
  • Slide 3
  • Slide 4
  • Slide 5
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Positron production

Test bed

Electron-positron pair creation by the collision of high-energy particles with an intense laser pulse

eg wakefield accelerated electrons bremsstrahlung created gamma rayshellip

In the highly non-linear regime

Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

  • Slide 1
  • Slide 2
  • Slide 3
  • Slide 4
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Positron production

Test bed

Interaction can be modelled directly from QED (Di Piazza PRL 117 213201 2016)

Integrated full-scale QED-PIC simulations for wakefield acceleration plus photon and pair creation (Lobet et al PRAB 20 043401 2017)

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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  • Slide 2
  • Slide 3
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Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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  • Slide 2
  • Slide 3
  • Slide 4
  • Slide 5
  • Slide 6
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Positron production

Scaling laws

Can obtain analytical expressions for the photon spectrum the energy of those that pair create and the positron energy

Total number of positrons is given by the integral of the photon spectrum weighted by the pair creation probability

Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Positron production

Scaling laws

Necessary collision parameters

To create 100 positrons from 100 pC of charge need

for a 30fs pulse

Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Positron production

Scaling laws

Synchronisation and overlap between the beams

Beam size effects theory predicts 10500 pairs from collision between 100pC electron beam (2 GeV 10 micron size) and laser pulse I21 = 5 800nm wavelength and 2 micron waist 3D PIC gives 10000

Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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Many theoretical questions remain how to account for back-reaction and multiphoton effects self-consistently in QED

How will the answers to those questions inform simulation development Is the QED-PIC model sufficient

Experimental work is of critical importance ndash and can be done with currently available laser systems

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