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

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 holeboring (HB) 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 of enhanced ion mobility, the threshold density above which HB 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 HB 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.

Relativistic Vlasov-Maxwell modelling using finite volumes and adaptive mesh refinement

“…Figure 4 shows the distribution function for electrons at time t = 15T after the front of the laser pulse has reached the plasma, for the densities n e = 0.3n c (in the underdense plasma regime), n e = 1.5n c (in the SIT regime) and n e = 2.6n c (in the hole-boring regime). The n e = 0.3n c case is in the regime where the relativistic Raman and modulational instabilities merge [53,54] as evidenced by particle trapping and acceleration, the n e = 1.5n c case develops electron vortices on the front side which is in agreement with dynamics of the SIT regime [43,47,55] and the n e = 2.6n c is identified with characteristics of the hole-boring regime. The Figure also shows the distribution function calculated by the PIC code (grey) at the same instant of time.…”

“…4.1, for semi-infinite plasma -a local density peak is formed at the plasma-vacuum interface, leading to a critical density departing from n eff c [41,42]. The situation is complicated by kinetic effects [43] and ion motion [46,47] and the threshold for transition from the transparent to the opaque regime has to be determined numerically. In particular, when ion motion effects are taken into account one studies the transition between SIT and the so-called hole-boring regime [48][49][50][51][52] in which the ions are accelerated in the charge-separation-induced electrostatic field and the whole plasma-vacuum interface recedes deeper into the plasma.…”

Generation of synchronized high-intensity x-rays and mid-infrared pulses by Doppler-shifting of relativistically intense radiation from near-critical-density plasmas

“…This behavior can be explained by the fact that, as was noted previously [48][49][50][51], induced transparency at a sharp plasma boundary manifests itself in the appearance of electron beams breaking off towards the incident laser radiation. This is accompanied by a deeper penetration of the radiation into the plasma.…”

Fast collisional electron heating and relaxation in thin foils driven by a circularly polarized ultraintense short-pulse laser

“…By contrast, using laser pulses with circular polarization (CP), for which the ponderomotive force does not show high-frequency oscillations, the j × B and vacuum heating mechanisms are essentially suppressed in overdense targets, and so is the fast electron bunch production (and the surface waves induced by them). Still, some fast electrons can be produced with CP if the variation time scale of the laser envelope is not large compared to the laser cycle (Siminos et al 2012(Siminos et al , 2017.…”