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 201800, China

E-mail: bfshen @mail.shcnc.ac.cn

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.

0.1nc

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:Emax [V/cm] = 0.96 ne

1/2 [cm-3] e.g. Emax ~ 100 GV/m for ne = 1018 cm-3

Electrons can be accelerated by laser Wake-Field

mW/cm10855.0

2

02219

21

52

20

2

0

I

cm

Iea

e

characterized by normalized vector potential of laser field

2D PIC simulation for LWFA

masselectron :2ee mcmeAa

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~1019-1020W/cm2; 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 2/13

6

1 eep nnQ

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. Then2/1

enQ

Generation of large amount of the energetic electrons with complex bubble structure

J. Xu, Baifei Shen et al., New J. Phys. 12 (2010) 023037

The bubble structure changes with the focal size

a) wy=8 μm (b) wy=35 μm (c) wy=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), 103108(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 pulseComplex bubble structure

J. Xu, Baifei Shen et al., New J. Phys. 12 (2010) 023037 The effect of over loading。 [Phys. Plasmas,

17(10), 103108(2010)]

3. Generation of high charged energetic electrons by using multiparallel laser pulses

PHYSICS OF PLASMAS 17, 103113( 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

Electron energy(MeV)Div

erg

en

ce(m

rad

)

H2-1p5mmjet-75bar-1330us-27100us-0p5mm

height-第七枪

8020107

26

0

-26

Charge density (arb. units)2000 4000 6000 8000 10000

10 20 30 40 50 60 70 800

200

400

600

H2-1p5mmjet-75bar-1330us-27100us-0p5mm

height-第七枪

Energy (MeV)

dN

/dE

(a

rb. u

nits

)

• duration ：• 45 － 50fs• power ：• 100 － 110TW• Focal size ：• 35m• Gas density ：• 2×1019 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