Temporal characterization of laser accelerated electron bunches using coherent THz

Website: http://loasis.lbl.gov/

Wim Leemansand members of the LOASIS Program

Lawrence Berkeley National Laboratory

BIW 2006May 1-4, 2006

Laser wakefield acceleration

Sprangle et al. (92); Antonsen, Mora (92); Andreev et al. (92); Esarey et al. (94); Mori et al. (94)

d=2 mm

plasmaLWFA: two regimes for bunch production • Large-energy-spread bunch (unchanneled) • Quasi-mono-energetic bunch (channeled)

1. Ionization of gas by laser2. Ponderomotive push of

plasma electrons3. Restoring force from due to

charge separation4. Density oscillation: strong

electric fields (100 GV/m)

10 TW Ti:sapphire 100 TW-class Ti:sapphire Shielded target room

Tool: LOASIS multi-terawatt laser

LOASIS laser system

Three main amplifiers (Ti:sapphire,10 Hz):- Godzilla:

0.5-0.6 J in 40-50 fs (10-15 TW) ===> main drive beam (to date)- Chihuahua:

20-50 mJ in 50 fs ===> ignitor beam250-300 mJ in 200-300 ps ===> heater beam100-200 mJ in 50 fs ===> colliding beam

- T-REX:2-3 J in 30-40 fs ===> capillary experiments

} guiding

Mid 90’s -2003: short pulse laser systems generate electron beams with 100 % energy spread

~10 mrad

e-beam on phosphor screen

100

101

102

103

104

0 10 20 30 40 50

E[MeV]

Detection Threshold

e-beam spectrum

Energy spectrum obtained with a magnetic spectrometer

Modena et al. (95); Nakajima et al. (95); Umstadter et al. (96); Ting et al. (97); Gahn et al. (99);

Leemans et al. (01); Malka et al. (01)

LWFA experiments produce electrons with:

1-100 MeV, multi-nC, ~100 fs, ~10 mrad divergence

Jet

Laser beam

Electron beam

Magnet

PhosphorPhosphor

OAP

Gas Jet

ChargeDetector

Magnet

vacuum

CCD

How short are the bunches ?

•Simulations predict 10-20 fs

•Can we measure them? (Is the linac stable enough?)

• Coherent emission

€

I total(ω)= N+N(N−1)g(k) 2{ }I e(ω)

g(k)= ρ(z)eikzdz−∞

∞

∫

Dominates if z <

Diagnostic relies on coherent transition radiationfrom the plasma-vacuum boundary

Schematic for Transition Radiation

Boundary size

Leemans et al., Phys. Rev. Lett. (2003);Schroeder et al., Phys. Rev. E (2004);Van Tilborg et al., Phys. Rev. Lett. (2006)

Laser-Wakefield Accelerator

Diagnostic implementation:• Use radiated field• Couple out of vacuum chamber

In detail: CTR from Plasma-vacuum boundary

CTR (THz) in spectral and temporal domain

Schroeder et al., Phys. Rev. E (04)van Tilborg et al., Laser Part. Beams (04)van Tilborg et al., Phys. Plasmas, submitted

Intense THz source• 0.01-10 MV/cm at focus (up to 10’s of J in THz pulse)• ‘traditional’ laser-based sources deliver <100 kV/cm

Diffraction function(boundary size )

Form factor

CTR spectrum CTR in time

Single electron TR

Temporal THz measurement: electro-optic sampling

Valdmanis (82); Yariv (88); Gallot (99); Yan (00); Fitch(01); Wilke(02); Berden(04); Cavalieri(05)

Phase shift is proportional to THz field

Electro-Optic measurement of Coherent Transition Radiation yields information on laser accelerated electron beam: < 50 fs bunches

W.P. Leemans et al., PRL2003C.B. Schroeder et al., PRE2004J. Van Tilborg et al., Laser and Particle Beams 2004; PRL 2006

Choice of EO-material affects temporal resolution

• ZnTe vs GaP: • ZnTe cutoff ~ 4 THz • GaP cutoff ~ 8 THz

• CTR based on 50 fs (rms) Gaussian electron bunch

Scanning technique(takes 1.5 hours)

• < 50 fs bunches

•Synchronization

• Charge and bunch stability

Scanning technique provides bunch duration: Resolution limited by crystal properties

Van Tilborg et al., PRL2006, Phys. Plasmas06

Single-Shot Technique for EO detection of THz pulses:Information on every bunch

J. van Tilborg et al., submitted to PRLG. Berden et al., Phys. Rev. Lett. 93, 114802 (2004)

• < 50 fs bunches

• peak E-field of ECTR≈150 kV/cm

3 ps

50 fs

Experiments show double THz pulse

Red curves are double-THz-pulse-based waveforms and spectra

Spectrum AShot A

Spectrum BShot B

Use GaP instead of ZnTe• Higher bandwidth

Observation

•Temporal waveform: double pulse

•Spectral modulation

Why?

• Double bunch e-beam ?

Single-shot 2D EO imaging provides spatial profile of THz beam

5 mm

7 mmShot 2=796 fs

Shot 1=546 fs

Shot 3=1154 fs

Van Tilborg et al., to be published

•Measure 2 D THz profile

• Focused THz beam

• Collimated laser beam

• Step laser beam in time

‘Ray Optics’ approach to analyze spatio-temporal effects of coma

Sho

t 3=

1154

fs

Propagation of a single-cycle pulse through focus

t=0t=+0.3

t=+0.6t=-0.3

t=-0.6

no coma

t=0t=+0.3

t=+0.6

t=-0.3t=-0.6

with coma

‘Ray optics’ model for waveform and spectrum

With coma

No coma

Large spot size, no channel (ZR order of gas jet length)

• RAL/IC: (Mangles et al.)• No Channel: 21019 cm-3

• Laser: 12 TW, 40 fs, 0.5 J, 2.51018 W/cm2, 25 m

• E-bunch: 1.4108 (22 pC), 70 MeV, E/E=3%, 87 mrad

• LOA: (Faure et al.)• No Channel: 0.5-2x1019 cm-3

• Laser: 30 TW, 30 fs, 1 J, 18 m• E-bunch: 3109 (0.5 nC), 170 MeV, E/E=24%,10 mrad

Controlled laser guiding with channel• LBNL: (Geddes et al.)

• Plasma Channel: 1-4x1019 cm-3

• Laser: 8-9 TW, 8.5 m, 55 fs

• E-bunch: 2109 (0.3 nC), 86 MeV, E/E=1-2%, 3 mrad

2004 Results: High-Quality Bunches

Plasma Channel Production: Hydrodynamic Ignitor-Heater in H2 Gas Jet

*

P. Volfbeyn, E. Esarey and W.P. Leemans, Phys. Plasmas 1999C.G.R. Geddes et al., Nature 431, p. 543 (2004), Phys.Rev.Lett. (2005).

Plasma channel

CCD &Spectrometer

2 probe

Interferometer

CylindricalMirror

Heater beam100mJ 250ps

e-

H, He gas jet

Main beam<500mJ >50fs

Pre ionizingBeam 20mJ

Ti:sapphire

At laser power of 8-9 TW: e-beam with %-level energy spread, 0.3 nC, 1-2 mm-mrad

UnguidedBeam profile Spectrum

Guided

Charge~100 pC

2-5 mrad divergence

C.G.R. Geddes et al., Nature 431 (2004); PRL (2005); Phys. Plasmas 2005

-1.000

-0.500

0.000

0.500

1.000

1.500

2.000

2.500

3.000

3.500

4.000

4.500

-4 -3.50 -3.00 -2.50 -2.00 -1.50 -1.00 -0.50 0.00 0.50 1.00

z-vgt

vM

om

en

tum

Phase

Group velocity of laser < speed of light causes particle dephasing which causes momentum bunching

• Dephasing distance:

• Control via density and a0 (laser intensity)

• Optimum acceleration requires Lacc = Ldeph: channel or large ZR

Wake Evolution and Dephasing Yield Low Energy Spread Beams in PIC Simulations

WAKE FORMING

INJECTION

DEPHASINGDEPHASING

Propagation Distance

Lon

gitu

dina

l M

omen

tum

200

0

Propagation DistanceL

ongi

tudi

nal

Mom

entu

m

200

0

Propagation Distance

Lon

gitu

dina

l M

omen

tum

200

0

Geddes et al., Nature (2004) & Phys. Plasmas (2005)

Next step: GeV laser driven accelerator

•

Increasin

g beam

energy:

cm-scale

capillary

discharg

e + 100

TW laser

CapillaryL'OASIS 100 TW laser

€

Wd[GeV] ~ I[W/cm2] n[cm-3]

• Lower density needed: capillary discharges

Plasma injector

3-5 cm

e- beam

1-2 GeVLaser

40-100 TW40 fs

CapillaryTREX

Capillary channel guiding: set-up

Hydrogen based capillary discharge produces suitable density profile for guiding

• 209 m diameter capillary

• 85 mbar initial pressure

• n0 = 8.5x1017 cm-3

• 32 micron matched spot

• Mach-Zehnder interferometer

A. Gonsalves et al., submitted to PRL

CCD

40 TW power guided over > 3 cm

P = 0.1-40 TW in 40 fs, 10 Hz

wx,in=wy,in= 26 m

wx,out=wy,out= 33 m

Output

Input

LOASIS GeV Spectrometer

1GeV

160MeV

40MeV

moderate resolution

Forward view: 0.16 - 1GeV

Interaction point

Yoke

Phosphor

Bottom view: 40-160 MeVhigh resolution(under const.)

Mirr

or a

nd c

amer

as

Capillary

Chamber Shielded mirror and cameras

- Large momentum acceptance (factor 25)

- Maximum resolving energy: ~1.1 GeV

- High resolution (bottom: <1%, forward: 2~4%)Chamber

Beamline

Pole

Up to 1 GeV achieved with 40 TW laser pulses

25 TW

E<0.6 GeV

Q~50-300 pC

40 TW

E< 1.1 GeV

Q~50-100 pC

DATA UNDER PRESS EMBARGO

Summary

• Single shot EO-based methods of CTR THz radiation measures <

50fs laser-wakefield accelerated e-bunches

• Single cycle THz detected, 0.4 MV/cm

• Spatio-temporal coupling from aberrations in imaging can lead to

apparent double bunches

• GeV electron beam generated in 3.3 cm with 40 TW laser pulses

–THz based bunch profile measurements underway

–Novel diagnostics needed with fs and sub-fs resolution for slice

energy spread and emittance

• Next steps are on staging modules towards 10 GeV

Scientists and Techs of LOASIS team

Staff:

Exp’t: C. Geddes, W. Leemans, C. TothTheory: E. Esarey, C. Schroeder, B. Shadwick,Postdocs:E. Michel*, P. Michel, B. NaglerStudents: K. Nakamura, J. van Tilborg, G. Plateau,T. Wolf Techs: D. Syversrud, N. Ybarrolaza

Collaborators:

D. Bruhwiler, D. Dimitrov, J. Cary--TechX CorpT. Cowan, H. Ruhl -- University of Nevada, Reno*

S. Hooker, A. Gonsalves--Oxford University, UKR. Ryne, J. Qiang--AMAC, LBNLR. Huber, R.Kaindl, J. Byrd, M. Martin--LBNLW. Mori--UCLAD. Jaroszynski-University of Strathclyde, UKM. Van der Wiel-TUE, Eindhoven, NLG. Dugan--Cornell UniversityD. Schneider, B. Stuart, C. Barty, C. Bibeau--LLNL