Direct laser acceleration of electrons in high-Z gas target and effect of threshold plasma density on electron beam generation

TL;DR

This study investigates how the threshold plasma density affects laser-driven electron acceleration in nitrogen and helium-nitrogen gas jets, revealing optimal conditions for generating high-energy, quasi-monoenergetic electron beams.

Contribution

It demonstrates the influence of helium doping on threshold plasma density and electron beam quality, supported by experimental data and 2D PIC simulations.

Findings

Relativistic electron beams generated at specific threshold densities.

Optimal helium fraction yields higher energy and better beam quality.

Threshold density increases with helium doping.

Abstract

An experimental study of laser driven electron acceleration in N2 and N2-He mixed gas-jet target using laser pulses of duration ~60-70 fs is presented. Generation of relativistic electron beam with quasi-thermal spectra was observed at a threshold density of ~1.6x1018 cm-3 in case of pure N2, and the threshold density was found to increase with increasing doping concentration of He. At an optimum fraction of 50% of He in N2, generation of quasi-monoenergetic electron beams was observed at a comparatively higher threshold density of ~2x1018 cm-3, with an average peak energy of ~168 MeV, average energy spread of ~21%, and average total beam charge of ~220 pC. Electron acceleration could be attributed to the direct laser acceleration as well as the hybrid mechanism. Observation of an optimum fraction of He in N2 (in turn threshold plasma density) for comparatively better quality electron beam generation could be understood in terms of the plasma density dependent variation in the dephasing rate of electrons with respect to transverse oscillating laser field. Results are also supported by the 2D PIC simulations performed using code EPOCH.