7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Experimental studies on the electron acceleration and positron generation inthe interaction of Petawatt femtosecond lasers with gas targets
Z. M. Zhang, Y. C. Wu, W. Hong, X. H. Zhang, Y. Q. Gu and W. M. Zhou
Science and Technology on Plasma Physics Laboratory, Laser Fusion Research Center, China
Academy of Engineering Physics
e-mail (speaker): zmzhang_zju@sina.com

The invention and application of the chirped pulse
amplification technique in short-pulse lasers has led tounprecedented ultra-high laser peak powers. After morethan three decades of development, petawatt (1015W)
class lasers, with pulse durations varying from a few
femtoseconds to several picoseconds, have been
constructed around the world. These ultra-high power
lasers have many applications, such as high-energy ionacceleration, laser-driven electron acceleration, ultrafast
x-rays, and fast ignition.
In 2016, the construction of the ultrahigh-power
laser facility, SILEX-II, of 4.9PW power and 18.6fs
duration, was completed in Laser Fusion ResearchCenter, China Academy of Engineering Physics [1]. Dueto the employment of the complete optical parametricchirped-pulse amplification (OPCPA) technique, theSILEX-II laser has both high peak power and hightemporal contrast, thus providing a good experimental
platform for studying ultra-intense laser plasma physics.
A commissioning experiment was firstly conductedon the SILEX-II laser facility with a peak power of about
1PW and duration of 30fs [2]. A set of comprehensive
diagnostics were set up in order to infer the on-target
laser spot size, laser intensity, and prepulse level of theSILEX-II laser. The experimental results suggest that thelaser can reach an intensity of 5×1020W/cm2 with afocus of 5.8μm (FWHM). The relativistic transparency isobserved to occur for a foil thickness of 20 nm,indicating that the laser also has a high temporal contrast.

The maximum proton energy obtained was about 21MeV.
We have also performed the experimental studies ofelectron acceleration and positron generation on theSILEX-II laser facility [3]. It is observed that MeV
electrons with a high charge of several tens of nC can bewell generated from petawatt femtosecond laserinteracting with a high-density gas jet. Furthermore it isfound that the existence of the gas density down-ramp
region is detrimental to the propagation of thehigh-charge electron beam. This is because anelectrostatic potential will build up in the density
down-ramp region, thus refluxing the energetic electrons.
This effect is clearly unfavorable to the production of
secondary particles. Consequently, by using an integrated
nozzle-converter design to eliminate the density falling
ramp of the gas target such that the electron refluxing isinhibited, we demonstrate a significant enhancement ofpositron yield (up to a factor of 15), finally reaching a
positron yield of 5 × 108sr−1.
References[1] X. Zeng et al., Opt. Lett. 42(10), 2014–2017 (2017).
[2] W. Hong et al., Matter Radiat. Extremes 6, 064401(2021).
[3] Z. M. Zhang et al., Plasma Phys. Control. Fusion 64,095015 (2022).

Figure 1 Experimental setup (left) and the experimental results of the transmitted electrons and positrons generated
from two different gas-converter target configurations, which indicate that the positron yield is greatly inhibited due tothe electron refluxing in the gas density down-ramp region.