Ultrashort electron source from laser-plasma interaction �

Jiansheng Liu, Aihua Deng*, Ye Tian , Wentao Wang, Cheng

Wang, Ruxin Li, and Zhizhan Xu

State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics (SIOM),

Chinese Academy of Sciences

The Workshop on Ultrafast Electron Sources for Diffraction and Microscopy applications (UESDM 2012) UCLA, Dec 12-14, 2012

*PBPL, Dept. of Physics & Astronomy, UCLA

Outline

3.    Summary �

2.    Electron  source  from  laser-­‐irradiated  solid  target

1.      Quasimonoenerge:c  electron  source  from  cascaded  laser  wakefield  accelerator  (LWFA)  

Laser Wakefield Accelerator (Acceleration gradient = 1011-12V/m) ‏

Radiofrequency Accelerator (Acceleration gradient = 107-8V/m)

Laser Plasma Accelerator, mm～cm scale

LWFAs are compact femtosecond accelerators

Stanford Linear Accelerator, 2 miles

Nature, 431, 538 ( 2004)

Nature, 431, 541( 2004)

Nature, 431, 535 ( 2004) Nature cover (2004.9.30) �

100 MeV-class electron beams were obtained by laser wakefield acceleration (LWFA) in the blowout regime in 2004

∆E/E  ~  10% �

∆E/E  ~  6% �

∆E/E  ~  3% �

1 GeV laser-plasma acceleration achieved with 3 cm-scale capillary at LBNL/Oxford U. �

W. P. Leemans et al., Nature Physics, 2, 696,2006; Physics of Plasmas 14, 056708, 2007

Laser:  a  0    ~  1.46  (40  TW,  37  fs)    Capillary:  D  =  312  µm;    L  =  33  mm � 1 GeV beam, energy spread 2.5 %. �

J. E. Ralph , POP 17, 056709 (2010)

Is there a better way to obtain electron beams with higher energy?

21/3 20

2

4 ( / )3 c

p

E P P mcωω

Δ =

1/3 2/3(1/ )P n∝W. Lu. PRST_AB(2007)

Energy Gain = EW Ld

Contradiction ?

• The plasma density should be as low as possible for obtaining the maximum energy gain

• Electron self-trapping is limited by . Pc= / 4cP P >

e- Laser

Laser

e-

Electron injection and acceleration are successfully separated, controlled, and optimized in different LWFA stages to ensure the efficient coupling between them.

LWFA 2 acceleration �

LWFA 1 injector �

700 750 800 850 9000.0

0.5

1.0

Charge

E (MeV)0 20 40 60 80 100

0.0

0.5

1.0Charge

E (MeV)

An all-optical cascaded laser wakefield accelerator

J.S. Liu et al., Phys. Rev. Lett. 107, 035001 (2011).

800 nm, 40 fs 40-60 TW

∼5.7×1018 cm−3 ~ 2.5 ×1018 cm−3

Quasimonoenergetic electron beam generation

J.S. Liu et al., Phys. Rev. Lett. 107, 035001 (2011).

the first staged LWFA

Siom    Dec.  24,  2010   �

the two-staged LWFA

Maxwellian spectrum

Energy(GeV)

y(m

rad)

0.07 0.1 0.2 0.3 0.5 1

-20

0

205000

10000

15000

100 200 300 4000

0.05

0.1

0.15

0.2

0.25

0.3

Energy(MeV)

dN/d

E(pC

/MeV

)

Energy(GeV)

y(m

rad)

0.07 0.1 0.2 0.3 0.5 1

-20

0

20 0

2

4

x 104

Energy(GeV)

y(m

rad)

0.07 0.1 0.2 0.3 0.5 1

-20

0

2012345x 104

100 200 300 4000

0.2

0.4

0.6

0.8

1

Energy(MeV)

dN/d

E(pC

/MeV

)

100 200 300 4000

0.05

0.1

0.15

0.2

0.25

0.3

Energy(MeV)

dN/d

E(pC

/MeV

)

E=190MeV，Energy  spread  <10%，Charge  ~2pC，Divergence  1mrad �

Energy spread is further reduced to be less than 6% at ~200 MeV

Elaser~ 1.8J

Elaser~ 1.9J

Elaser~ 2J

E=200MeV，Energy  spread  <6%，Charge  ~2pC，Divergence  1mrad �

E=195MeV，Energy  spread  <6%，Charge  ~10pC，Divergence  2mrad �

ΔE/E~6% ΔE/E~6%

Ultrafast  strong  ,ield  laser  interaction  with  solid  target

Incidence laser pulse

Preformed plasma 0.01nc ~ nc

Overdense plasma 10nc ~ 200nc

2. Electron source from laser-irradiated solid target

Blow-­‐off    plasma  

Target Normal

Specular Direction Laser Polarization

Surface Direction

Target normal Quasi-static electric field

 

Fast hot electron

11

Fast electrons generation from front face

Wentao Wang et al. Phys. Plasmas 17, 023108 (2010) Y. Sentoku et al. Phys. Plasmas 6, 2855 (1999)

Surface

Laser Polarization Target Normal

Specular Direction

Incident Laser

Yutong Li et al. Phys. Rev. Lett. 96, 165003(2006)

12

Collimated Laser-Accelerated Electrons

Shigeki Tokita et al. Phys. Rev. Lett. 105, 215004 (2010)

‘Primary   source’:   broad  energy   spectrum   from  100  keV~  1MeV  

‘Optimized   source’:   356 keV ; pulse duration 500fs

Shigeki Tokita et al. Phys. Rev. Lett. 106, 255001 (2011)

energy peak: 200~300 keV the number (L=30mm): 7×1011  sr-­‐1

13

Experimental Setup

²  Contrast ratio is better than 108:1 at 50 ps before the peak of the main pulse

²  Laser intensity: ²  (2×1016-3×1017 W/cm2) ²  Pulse duration 60fs ²  Plasma scale length:0.1-0.5λ

Y. Tian et al., Phys. Rev. Lett. 109, 115002 (2012).

Electron Emission from the Laser-Driven Surface Plasma Wave �

14

Generation of stable collimated electron-beams

Divergence: 146mrad Deflection: 7o Charge 9pC

52 mrad Deflection:4o Charge: 3pC

37 mrad Deflection:3o Charge: 0.3pC

26 mrad Deflection:1.5o Charge: 60 fC

Y. Tian et al., Phys. Rev. Lett. 109, 115002 (2012).

(a-d) Adjusting the focal intensity 3×1017~2×1016 W/cm2 , (e-h) Changing the angle of laser incidence from 34o-82o .

Energy spectrum

Peak Energy: ~100keV

Always close to specular direction

The laser-driven surface plasma wave has a spatial period of λ/cos45o and propagates along the surface at phase velocity of c/cos45o. �

A small amount of electrons escape away from the plasma wave more or less along the specular direction �

The peak electron density appears periodically at the fixed phases when the electric field changes the signs from positive to negative. �

Step 1: Escape away from the surface plasma wave

A “two-step” model is proposed to reveal underlying physics: Electron Emission at Locked Phases from the Laser-Driven Surface Plasma Wave

Spatial distributions of the normal laser electric field En

Spatial distribution of electron density

Momentum distribution of electrons electric field amplitude En(red line) electron density (blue line) at the surface

As an ejected electron escape away from the plasma wave, its trajectory thereafter will abide by the motion of a free electron in the interference field.

When ejected electrons are captured at the phases Φ=2Nπ (Fig. b), the electrons may be deflected to the target normal with a deflection angle Δφ by the ponderomotive force of the laser field. This model reproduces the deflection effect in the experiment. �

Step 2: Deflection by the interference field

A “two-step” model is proposed to reveal underlying physics: Electron Emission at Locked Phases from the Laser-Driven Surface Plasma Wave

x(λ)

y(λ)

4 5 6

4

2

0

-2

-4

-6

6

23

45

0 0.1 0.2 0.3 0.4 0.5 0.6

t(T0 , T

0 =2.67fs)

248.4 as

207.8 as235.2 as

Attosecond scale electron beam duration?

The periodical repetition of the electron emission from the surface plasma wave leads to a pulse train of collimated electron beams with sub-femtosecond duration, i. e. 200 attosecond. �

Spatial distribution of electron density Electron Pulse Train

Summary

Ø Realized the first all-optical cascaded LWFA.

Ø Electrons with Maxwellian spectrum generated form the first LWFA can be accelerated to be a 200MeV electron beam with energy spread of 6% in cascaded LWFA.

Ø Stable collimated electron beams generated from the laser-solid interaction in the direction close to the specular direction.

Ø A“two-step”model is proposed to reveal that fast electron beams are emitted from the surface plasma wave at locked phases with the laser oscillation.