Predominant contribution of direct laser acceleration to high-energy electron spectra in a low-density self-modulated laser wakefield accelerator

P M King,J L Shaw,A G R Thomas,B M Hegelich,J Hinojosa,A Pak,P Michel,F Albert,K Miller,W Mori,K A Marsh,C Joshi,M Thibodeau,B F Kraus,N Lemos

doi:10.1103/physrevaccelbeams.24.011302

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.

Highlights

Laser wakefield acceleration (LWFA) produces beams of relativistic electrons [1,2] that can be used to generate directional x-rays [3] useful for imaging biological samples [4], laser-driven shock fronts [5] and surface defects in alloys [6]
While the promise of such x-ray sources is evident, optimization of LWFA-driven x-rays with the picosecond-duration lasers available at high energy density science (HEDS) facilities necessitates a detailed understanding of the underlying physics of electron beam generation mechanisms—not in the LWFA regime, but in the self-modulated LWFA (SM-LWFA) regime [15], which is as yet incomplete
In a quasi-blowout regime, the laser pulse was lengthened to overlap with a full plasma period, and electrons in the high-energy tail of the accelerated electron spectrum showed a fork-like splitting when dispersed perpendicular to the laser polarization direction [22,27]

Summary

INTRODUCTION

Laser wakefield acceleration (LWFA) produces beams of relativistic electrons [1,2] that can be used to generate directional x-rays [3] useful for imaging biological samples [4], laser-driven shock fronts [5] and surface defects in alloys [6]. In a quasi-blowout regime, the laser pulse was lengthened to overlap with a full plasma period, and electrons in the high-energy tail of the accelerated electron spectrum showed a fork-like splitting when dispersed perpendicular to the laser polarization direction [22,27] This forklike structure was attributed to DLA through PIC simulations, but the analysis did not include the contribution of the longitudinal field from the focused laser in the DLA process. When the electrons are dispersed orthogonally to the laser polarization direction, a forklike structure [22] characteristic of DLA is observed for electrons with energies above 60 MeV This is the first direct experimental characterization, confirmed by quasi-3D PIC simulations, of DLA in an SM-LWFA in the picosecond, high-energy regime relevant to HEDS experiments

Experimental configuration and methods

Experimental discussion

Simulation configuration

CONCLUSION