Non-Maxwellian electron distributions resulting from direct laser acceleration in near-critical plasmas

TL;DR

This study demonstrates that direct laser acceleration in near-critical plasmas produces non-Maxwellian, peaked electron energy distributions, a novel observation with potential applications in advanced particle acceleration and plasma physics.

Contribution

It is the first experimental and simulation evidence showing DLA generates peaked electron spectra instead of broad Maxwellian distributions.

Findings

Non-Maxwellian, peaked electron distributions observed experimentally.

DLA identified as the mechanism producing these spectra.

Potential use of high-density electrons for advanced acceleration applications.

Abstract

The irradiation of few nm thick targets by a finite-contrast high-intensity short-pulse laser results in a strong pre-expansion of these targets at the arrival time of the main pulse. The targets decompress to near and lower than critical densities plasmas extending over few micrometers, i.e. multiple wavelengths. The interaction of the main pulse with such a highly localized but inhomogeneous target leads to the generation of a short channel and further self-focusing of the laser beam. Experiments at the GHOST laser system at UT Austin using such targets measured non-Maxwellian, peaked electron distribution with large bunch charge and high electron density in the laser propagation direction. These results are reproduced in 2D PIC simulations using the EPOCH code, identifying Direct Laser Acceleration (DLA) as the responsible mechanism. This is the first time that DLA has been observed to produce peaked spectra as opposed to broad, maxwellian spectra observed in earlier experiments. This high-density electrons have potential applications as injector beams for a further wakefield acceleration stage as well as for pump-probe applications.