Ultrashort electron source from laser-plasma interaction

Transcription

1 The Workshop on Ultrafast Electron Sources for Diffraction and Microscopy applications (UESDM 212) UCLA, Dec 12-14, 212 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 *PBPL, Dept. of Physics & Astronomy, UCLA

2 Outline 1. Quasimonoenerge:c electron source from cascaded laser wakefield accelerator (LWFA) 2. Electron source from laser- irradiated solid target 3. Summary

3 LWFAs are compact femtosecond accelerators Radiofrequency Accelerator (Acceleration gradient = V/m) Laser Wakefield Accelerator ( V/m (Acceleration gradient = Stanford Linear Accelerator, 2 miles Laser Plasma Accelerator, mm~cm scale

4 1 MeV-class electron beams were obtained by laser wakefield acceleration (LWFA) in the blowout regime in 24 E/E ~ 1% Nature, 431, 541( 24) E/E ~ 6% E/E ~ 3% Nature cover (24.9.3) Nature, 431, 535 ( 24) Nature, 431, 538 ( 24)

5 1 GeV laser-plasma acceleration achieved with 3 cm-scale capillary at LBNL/Oxford U. Laser: a ~ 1.46 (4 TW, 37 fs) Capillary: D = 312 µm; L = 33 mm 1 GeV beam, energy spread 2.5 %. W. P. Leemans et al., Nature Physics, 2, 696,26; Physics of Plasmas 14, 5678, 27

6 Is there a better way to obtain electron beams with higher energy? Energy Gain = E W L d 4 ω Δ E = ( P/ Pc ) mc 3 ω 2 2 p 1/3 2 1/3 2/3 P (1/ n) W. Lu. PRST_AB(27) Contradiction? J. E. Ralph, POP 17, 5679 (21) The plasma density should be as low as possible for obtaining the maximum energy gain Electron self-trapping is limited by P/ P > 4. P c = c

7 An all-optical cascaded laser wakefield accelerator cm 3 ~ cm 3 8 nm, 4 fs 4-6 TW Laser e - e - Laser LWFA 1 injector Charge E ( MeV) LWFA 2 acceleration Electron injection and acceleration are successfully separated, controlled, and optimized in different LWFA stages to ensure the efficient coupling between them. J.S. Liu et al., Phys. Rev. Lett. 17, 351 (211). Charge.5 E ( MeV)

8 Quasimonoenergetic electron beam generation the first staged LWFA the two-staged LWFA Maxwellian spectrum Siom Dec. 24, 21 J.S. Liu et al., Phys. Rev. Lett. 17, 351 (211).

9 Energy spread is further reduced to be less than 6% at ~2 MeV y(mrad) y(mrad) y(mrad) Energy(GeV) x x E laser ~ 1.8J E=19MeV,Energy spread <1%, Charge ~2pC,Divergence 1mrad E laser ~ 1.9J E=2MeV,Energy spread <6%, Charge ~2pC,Divergence 1mrad E laser ~ 2J E=195MeV,Energy spread <6%, Charge ~1pC,Divergence 2mrad dn/de(pc/mev) ΔE/E~6% ΔE/E~6% Energy(MeV) Energy(MeV) Energy(MeV)

10 2. Electron source from laser-irradiated solid target Ultrafast strong,ield laser interaction with solid target Incidence laser pulse Preformed plasma.1n c ~ n c Overdense plasma 1n c ~ 2n c Blow- off plasma Target normal Quasi-static electric field Target Normal Fast hot electron Laser Polarization Specular Direction Surface Direction

11 Fast electrons generation from front face Wentao Wang et al. Phys. Plasmas 17, 2318 (21) Target Normal Laser Polarization Y. Sentoku et al. Phys. Plasmas 6, 2855 (1999) Incident Laser Specular Direction Surface Yutong Li et al. Phys. Rev. Lett. 96, 1653(26) 11

12 Collimated Laser-Accelerated Electrons Primary source : broad energy spectrum from 1 kev~ 1MeV Shigeki Tokita et al. Phys. Rev. Lett. 15, 2154 (21) Optimized source : 356 kev ; pulse duration 5fs energy peak: 2~3 kev the number (L=3mm): sr - 1 Shigeki Tokita et al. Phys. Rev. Lett. 16, 2551 (211) 12

13 Electron Emission from the Laser-Driven Surface Plasma Wave Experimental Setup Y. Tian et al., Phys. Rev. Lett. 19, 1152 (212). ² Contrast ratio is better than 1 8 :1 at 5 ps before the peak of the main pulse ² Laser intensity: ² ( W/cm 2 ) ² Pulse duration 6fs ² Plasma scale length:.1-.5λ 13

14 Generation of stable collimated electron-beams Divergence: 146mrad Deflection: 7 o Charge 9pC 52 mrad Deflection:4 o Charge: 3pC Always close to specular direction 37 mrad Deflection:3 o Charge:.3pC 26 mrad Deflection:1.5 o Charge: 6 fc (a-d) Adjusting the focal intensity ~ W/cm 2, (e-h) Changing the angle of laser incidence from 34 o -82 o. Energy spectrum Peak Energy: ~1keV Y. Tian et al., Phys. Rev. Lett. 19, 1152 (212). 14

15 A two-step model is proposed to reveal underlying physics: Electron Emission at Locked Phases from the Laser-Driven Surface Plasma Wave Step 1: Escape away from the surface plasma wave The laser-driven surface plasma wave has a spatial period of λ/cos45 o and propagates along the surface at phase velocity of c/cos45 o. Spatial distributions of the normal laser electric field En Spatial distribution of electron density A small amount of electrons escape away from the plasma wave more or less along the specular direction Momentum distribution of electrons electric field amplitude En(red line) electron density (blue line) at the surface The peak electron density appears periodically at the fixed phases when the electric field changes the signs from positive to negative.

16 A two-step model is proposed to reveal underlying physics: Electron Emission at Locked Phases from the Laser-Driven Surface Plasma Wave Step 2: Deflection by the interference field 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.

17 Attosecond scale electron beam duration? y(λ) x(λ) Spatial distribution of electron density t(t, T =2.67fs) 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. 2 attosecond as as 27.8 as Electron Pulse Train

18 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 2MeV 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.