Kinetic and finite ion mass effects on the transition to relativistic self-induced transparency in laser-driven ion acceleration

TL;DR

This paper investigates how ion motion and finite ion mass influence the transition to relativistic self-induced transparency in laser-plasma interactions, revealing new regimes and dependencies that affect ion acceleration efficiency.

Contribution

It introduces a detailed analysis of kinetic effects on transparency transition, highlighting the role of ion dynamics and laser profile in ion acceleration.

Findings

Ion motion suppresses electron escape, affecting transparency transition.

Transition threshold depends on laser profile and ion charge-to-mass ratio.

A new dynamic transition regime enhances ion acceleration performance.

Abstract

We study kinetic effects responsible for the transition to relativistic self-induced transparency in the interaction of a circularly-polarized laser-pulse with an overdense plasma and their relation to hole-boring and ion acceleration. It is demonstrated using particle-in-cell simulations and an analysis of separatrices in single-electron phase-space, that ion motion can suppress fast electron escape to the vacuum, which would otherwise lead to transition to the relativistic transparency regime. A simple analytical estimate shows that for large laser pulse amplitude $a_0$ the time scale over which ion motion becomes important is much shorter than usually anticipated. As a result, the threshold density above which hole-boring occurs decreases with the charge-to-mass ratio. Moreover, the transition threshold is seen to depend on the laser temporal profile, due to the effect that the latter has on electron heating. Finally, we report a new regime in which a transition from relativistic transparency to hole-boring occurs dynamically during the course of the interaction. It is shown that, for a fixed laser intensity, this dynamic transition regime allows optimal ion acceleration in terms of both energy and energy spread.