High-charge energetic electron beam generated in the bubble regime Baifei Shen ( 沈百飞 ) State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, No. 390, Qinghe Road, Jiading District, Shanghai , China

Outline 1.Motivation 2.High-charge energetic electron beam generated in the bubble regime 3.Generation of highly charged energetic electrons by using multiparallel laser pulses 4.Experiment results and PIC simulation of highly charged energetic electron beams

1. Motivation 1.Fast ignition requires high-charge energetic electron beam 2. Producing sufficiently bright bremsstrahlung X-rays for high-resolution flash radiography of large samples.

0.1n c Fast ignition for laser fusion

High-Resolution X-Ray Radiography Produced by a Laser-Plasma Driven Electron Source

2. High charge energetic electron beam generated in the bubble regime

The maximum wakefield: E max [V/cm] = 0.96 n e 1/2 [cm -3 ] e.g. E max ~ 100 GV/m for n e = cm -3 The maximum wakefield: E max [V/cm] = 0.96 n e 1/2 [cm -3 ] e.g. E max ~ 100 GV/m for n e = cm -3 Electrons can be accelerated by laser Wake-Field characterized by normalized vector potential of laser field 2D PIC simulation for LWFA

8 Electrons are trapped and accelerated in a bubble driven by a short relativistic laser pulse. 上图引自： Scientific America, 41(2006) ； Appl. Phys. B 74, 355 (2002)  Ultra-relativistic intensities~ W/cm 2 ; Pulse length (c  )<Plasma wavelength ( p );  Huge ponderomotive force of the laser pulse blows out electrons forming a void (bubble);  Significant number of electrons are trapped at the stem of the bubble and get accelerated;  Highly mono-energetic and directional electron beam is generated.

How many electrons can be trapped inside a bubble? The bubble is roughly a ball which diameter is about plasma wavelength. There fore, the number of the electrons expelled from the bubble is But we found that if the focal size of the laser is much larger than the plasma wavelength, the transverse size of the bubble is determined by the focal size in stead of the plasma wavelength. Then

Generation of large amount of the energetic electrons with complex bubble structure J. Xu, Baifei Shen et al., New J. Phys. 12 (2010)

The bubble structure changes with the focal size a) w y =8 μm (b) w y =35 μm (c) w y =50 μm

The conversion efficiency is larger than 10% More than 40 nC energetic electrons are generated

Overloading Effect of Energetic Electrons on Wakefield in Bubble Regime PoP 17(10), (2010) Longitudinal extension of the electron beam is limited

The effect of trapped electron on the wake field

Generation of large amount of the energetic electrons with complex bubble structure Relativistic focusing of laser pulse Complex bubble structure J. Xu, Baifei Shen et al., New J. Phys. 12 (2010) The effect of over loading 。 [Phys. Plasmas, 17(10), (2010)]

3. Generation of high charged energetic electrons by using multiparallel laser pulses PHYSICS OF PLASMAS 17, ( 2010)

4. Experiment results and PIC simulation of highly-charged energetic electron beams

Transverse distribution of the accelerated electrons in the absence of the spectrograph magnetic field. Laser focal spot Many electron beams observed in experiment

Experimental results show that the Alfven current limit may be reached duration ： 45 － 50fs power ： 100 － 110TW Focal size ： 35  m Gas density ： 2×10 19 cm -3

More than 20 nC electrons detected in experiment

Non perfect laser pulse used to generate highly charged energetic electron beam

An intense multipeaked laser with a large focal spot can generate highly charged energetic electron bunches.

Acknowledgment Jiancai Xu, Meng Wen, Liangliang Ji, Xiaomei Zhang, Mingwei Liu, Aihua Deng, Yuxing Leng, Ruxin Li, Zhizhan Xu Yuchi Wu, Kegong Dong, Bin Zhu, Yuqiu Gu, Chunye Jiao, Jian Teng, Wei Hong, Zhongqing Zhao, Leifeng Cao Xiaofang Wang M. Y. Yu

Thank you for your attention