Characterization of ionization injection in gas mixtures irradiated by subpetawatt class laser pulses

A Zhidkov,J K Koga,K Huang,M Kando,T Hosokai,N Pathak

doi:10.1103/physrevresearch.2.013216

Abstract

Effects of ionization injection in low and high Z gas mixtures for the laser wake field acceleration of electrons are analyzed with the use of balance equations and particle-in-cell simulations via test probe particle trajectories in realistic plasma fields and direct simulations of charge loading during the ionization process. It is shown that electrons appearing at the maximum of laser pulse field after optical ionization are trapped in the first bucket of the laser pulse wake. Electrons, which are produced by optical field ionization at the front of laser pulse, propagate backwards; some of them are trapped in the second bucket, third bucket and so on. The efficiency of ionization injection is not high, several pC/mm/bucket. This injection becomes competitive with wave breaking injection at lower plasma density and over a rather narrow range of laser pulse intensity.

Highlights

Interest in laser wake field acceleration (LWFA) of electrons [1] has rapidly grown over recent decades owing to notable results achieved by several groups [2,3,4,5,6,7]
We have characterized the effect of ionization injection for the laser wake field acceleration of electrons in gas mixtures using self-consistent particle-in-cell simulations
We performed two-dimensional particlein-cell simulations splitting plasma electrons and ionization electrons, which allowed us to investigate the entire process of electron preacceleration, trapping, and further acceleration

Summary

INTRODUCTION

Interest in laser wake field acceleration (LWFA) of electrons [1] has rapidly grown over recent decades owing to notable results achieved by several groups [2,3,4,5,6,7]. Optical field ionization of inner shell electrons of the high-Z dope in the vicinity of the maximum of the laser pulse field produces a number of low-energy electrons moving with a phase different from that of the laser pulse wake [21] These electrons can be trapped and further accelerated. Separation of ionization injection from the wave breaking injection can be done in rather low-density plasma, which requires high-power laser pulses in order to reach the selffocusing regime for essential electron acceleration. Concentrations and plasma density is investigated via a selfconsistent particle-in-cell simulation with separated postprocessing for wave breaking and ionization electrons This allows for an estimation of the efficiency of the different mechanisms of electron self-injection in gas mixtures

CHARGE STATES IN He-N2 AND He-Ne MIXTURES

PROBING PARTICLES IN PARTICLE-IN-CELL

IONIZATION INJECTION IN LOW-DENSITY PLASMA

Findings

CONCLUSION