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High Power Laser Science and Engineering, (2020), Vol. 8, e34, 8 pages.

doi:

10.1017/hpl.2020.30

RESEARCH ARTICLE

Gamma-ray generation from ultraintense

laser-irradiated solid targets with preplasma

Xiang-Bing Wang

1,2

, Guang-Yue Hu

1,3

, Zhi-Meng Zhang

, Yu-Qiu Gu

2,4

, Bin

Zhao

, Yang Zuo

, and Jian Zheng

1,4

CAS Key Laboratory of Geospace Environment and Department of Engineering and Applied Physics,

University of Science and Technology of China, Hefei 230026, China

Science and Technology on Plasma Physics Laboratory, Laser Fusion Research Center,

China Academy of Engineering Physics, Mianyang 621900, China

CAS Center for Excellence in Ultra-intense Laser Science (CEULS), Shanghai 200031, China

IFSA Collaborative Innovation Center, Shanghai Jiao Tong University, Shanghai 200240, China

(Received 9 May 2020; revised 28 July 2020; accepted 4 August 2020)

Abstract

In the laser plasma interaction of quantum electrodynamics (QED)-dominated regime, γ-rays are generated due to

synchrotron radiation from high-energy electrons traveling in a strong background electromagnetic ﬁeld. With the aid of

2D particle-in-cell code including QED physics, we investigate the preplasma effect on the γ-ray generation during the

interaction between an ultraintense laser pulse and solid targets. We found that with the increasing preplasma scale

length, the γ-ray emission is enhanced signiﬁcantly and ﬁnally reaches a steady state. Meanwhile, the γ-ray beam

becomes collimated. This shows that, in some cases, the preplasmas will be piled up acting as a plasma mirror in

the underdense preplasma region, where the γ-rays are produced by the collision between the forward electrons and the

reﬂected laser ﬁelds from the piled plasma. The piled plasma plays the same role as the usual reﬂection mirror made

from a solid target. Thus, a single solid target with proper scale length preplasma can serve as a manufactural and robust

γ-ray source.

Keywords: gamma-ray; plasma mirror; preplasma

1. Introduction

Preplasma has an important effect on the interaction

between laser and matter, especially in solid targets, which

produces different laser absorption mechanisms in sub-

relativistic laser states, such as resonance absorption

[

and vacuum heating (the Brunel mechanism)

[

. Based

on these studies, several interesting results have been

obtained recently, such as the Brunel-like mechanism

[

, high

harmonic generation

[

, and vacuum electron acceleration

[5]

With the gargantuan laser powers projected to be realized in

the developing petawatt (PW) facilities ELI in Europe

[

6–9]

and SULF in China

[

10]

, the interaction between lasers and

plasmas is poised to occur in t he ultrarelativistic state

[

11]

. As

is commonly understood, in the quantum electrodynamics

Correspondence to: G.-Y. Hu, University of Science and Technology of

China, Hefei 230026, China. Email: gyhu@ustc.edu.cn

(QED)-dominated regime, when an ultraintense laser

interacts with a target, γ-rays can be generated by

synchrotron radiation arising from high-energy electrons

traveling in a strong, background electromagnetic ﬁeld

[

12, 13]

Previous studies focused on the f unction of the preplasma

before a dense target

[

14, 15]

, scanning different parameters

to obtain t he optimal γ-ray source. The other efﬁcient

method to generate γ-ray ﬂare is to make accelerated

electrons interact with a reﬂected laser, a method called

all-optical Compton backs cattering, in which electrons

may be accelerated by a wakeﬁeld

[

16, 17]

or pondermotive

force

[

18]

and laser is reﬂected by a dense plasma mirror.

In these cases, underdense gas or nanoparticles

[

19]

are

required before the solid targets in t heir plans. The γ-

rays generated in different regimes have lent themselves

to many applications, such as dense matter tomography

[

20]

photonuclear reactions

[21]

, and laboratory astrophysics

[22]

Previous research has shown that an ultraintense laser

terms of the C reative Commons Attribution-NonCommercial-ShareAlike licence (

http://creativecomm ons.org/licenses/by-nc-sa/4.0/), which permits non-

commercial re-use, distribution, and reproduction in any medium, provided the s am e Creative Commons licence is included and the original work is properly

cited. The written permission of Cambridge University Press must be obtained for commercial re-use.

2 X.-B. Wang et al.

interacting with a plasma can emit γ-rays in different

directions, depending on the plasma density

[

11, 12, 23]

and

the corresponding physical mechanisms at play.

As mentioned previously, the preplasma scale length plays

an important role in the ultraintense laser matter inter-

actions, which is the object of the present article. Via

two-dimensional (2D) simulation and theoretical analysis,

we found that the preplasma affects the γ-ray generation

markedly. With increasing preplasma scale length, the angu-

lar distribution of the γ-rays tends to proceed with small

angle. Moreover, the conversion efﬁciency of laser to γ-

ray reaches its highest at an appropriate scale length. This

shows that piled plasma formed in preplasma has similar

effects to the reﬂecting mirror in an all-optical Compton

backscattering scenario. The mirror is usually made of a

solid target and reﬂects the laser. We propose a simple 2D

analytical model to explain the simulation results, which is

in good agreement with our simulation. It indicates that a

single solid target with appropriate preplasma could also be

robust to generating a γ-ray source.

2. Traditional mechanism of γ-ray production in

uniform plasma

The nonlinear QED effect

[

24]

must be considered under

ultraintense laser conditions, which mainly contains two

important processes: γ-ray generation through synchrotron

radiation and electron–positron pair creation by the Breit–

Wheeler (BW) process

[

25]

. These two processes can be sim-

ply described as

−

+mγ

→ e

−

+γ

+nγ

→ e

−

, (1)

where γ

is a laser photon and γ

is a gamma photon.

γ-ray generation is controlled by the parameter:

η =





⊥

+β ×cB|, (2)

which is the ratio of the electromagnetic ﬁeld in the

electron’s rest frame to the Schwinger ﬁeld (E

= 1.3 ×

V/m)

[

26]

, where γ is the Lorentz factor of the relativistic

electron, E

⊥

is the electric ﬁeld perpendicular to the

direction of motion of the electron, β is the electron velocity

normalized by the speed of light c, and B is the magnetic

ﬁeld. When η approaches unity, the QED process must be

considered, especially γ-ray emission.

As can be found in the formula, the motion of the electrons

and the electromagnetic ﬁeld they experience determine

the production of γ-rays. There are four main mechanisms

of γ-ray generation in uniform plasma according to the

density (shown in

Figure 1), which produce different electron

behaviors.

In a comparatively low-density plasma of near-critical

density (n

∼ n

, n

is the critical density), the

laser penetrates the plasma and a proportion of the elec-

trons become trapped in the plasma channel owing to the

combined effects of the radiation reaction and the self-

generated magnetic ﬁeld

[

27]

. Brilliant forward collimated

γ-rays can be generated by relativistic electron bunches

through synchrotron radiation, in a cone with an angle 1/γ

(γ is the Lorentz factor of the electron) around the direction

of electron motion.

When the plasma is no longer transparent (the plasma

density is close to the penetration threshold value n

∼n

in which n

satisﬁes



(1 +a

)



1 +a

−1



−a



Figure 1. Schematic of traditional γ-ray generation mechanisms at uniform plasma with different density (blue backgrounds represent plasma densities;

black circles are electrons; purple represents the gamma photons; small red arrows show the moving direction of the electrons and gamma photons; light red