Predominant Contribution of Direct Laser Acceleration to High-Energy Electron Spectra in a Low-Density Self-Modulated Laser Wakefield Accelerator

TL;DR

This study demonstrates that direct laser acceleration (DLA) significantly contributes to high-energy electron spectra in a low-density self-modulated laser wakefield accelerator, supported by experimental observations and detailed simulations.

Contribution

It reveals the dominant role of DLA in producing high-energy electrons, a novel insight supported by experimental data and advanced simulations.

Findings

High-energy electrons (>60 MeV) mainly gain energy from DLA.

Two-temperature electron spectra are observed and reproduced in simulations.

DLA electrons lose energy to the laser's longitudinal field, affecting net energy gain.

Abstract

The two-temperature relativistic electron spectrum from a low-density ($3\times10^{17}$~cm$^{-3}$) self-modulated laser wakefield accelerator (SM-LWFA) is observed to transition between temperatures of $19\pm0.65$ and $46\pm2.45$ MeV at an electron energy of about 100 MeV. When the electrons are dispersed orthogonally to the laser polarization, their spectrum above 60 MeV shows a forking structure characteristic of direct laser acceleration (DLA). Both the two-temperature distribution and the forking structure are reproduced in a quasi-3D \textsc{Osiris} simulation of the interaction of the 1-ps, moderate-amplitude ($a_{0}=2.7$) laser pulse with the low-density plasma. Particle tracking shows that while the SM-LWFA mechanism dominates below 40 MeV, the highest-energy ($>60$ MeV) electrons gain most of their energy through DLA. By separating the simulated electric fields into modes, the DLA-dominated electrons are shown to lose significant energy to the longitudinal laser field from the tight focusing geometry, resulting in a more accurate measure of net DLA energy gain than previously possible.