Relativistic High-Current Electron-Beam Stopping-Power Characterization in Solids and Plasmas: Collisional Versus Resistive Effects

Abstract

We present experimental and numerical results on intense-laser-pulse-produced fast electron beams transport through aluminum samples, either solid or compressed and heated by laser-induced planar shock propagation. Thanks to absolute K yield measurements and its very good agreement with results from numerical simulations, we quantify the collisional and resistive fast electron stopping powers: for electron current densities of % 8 Â 10 10 A=cm 2 they reach 1:5 keV= m and 0:8 keV= m, respectively. For higher current densities up to 10 12 A=cm 2 , numerical simulations show resistive and collisional energy losses at comparable levels. Analytical estimations predict the resistive stopping power will be kept on the level of 1 keV= m for electron current densities of 10 14 A=cm 2 , representative of the full-scale conditions in the fast ignition of inertially confined fusion targets. In the fast ignition (FI) scheme of inertial confinement fusion, a relativistic electron beam (REB) heats the compressed core and ignites the fusion reactions in a capsule of deuterium and tritium [1]. This REB is generated at the critical density surface, or at the cone tip of a cone-embedded imploded capsule [2] by a high-intensity (% 10 20 W=cm 2 ) and high-energy ($100 kJ) laser. The REB source has a total kinetic energy & 40% of the laser energy [3][4][5] and a mean kinetic energy of 1-2 MeV (to provide an efficient coupling to the dense core). The REB transports energy from the generation region (with density and temperature in the level of a few g=cm 3 and a few eV, respectively) to the high-density ($ 400 g=cm 3 ) and hightemperature ($ 300 eV) core, where it must deliver a minimum of 20 kJ to heat the fuel to thermonuclear temperatures ($ 5-10 keV) [6]. The energy transport efficiency can be limited by such physical processes as collisional or collective energy loss [7], divergence [8,9], filamentation [10][11][12], etc. The energy losses over the highly inhomogeneous electron transport zone should be accurately predicted for a successful full-scale FI design. In particular, the REB stopping power should be limited to a few keV= m over the $100 m standing-off distance between the REB source and the imploded core.The work presented here aims at characterizing the REB stopping power in dense media in underscaled experimental conditions. The measurements are used to benchmark a REB transport code. The tested transport media, ranging from solid to warm dense matter, are much denser than the injected REB, being reasonable to assume an efficient neutralization of the injected current (j h ) by a counterstreaming current (j e ) of background thermal electrons (j h % Àj e ). Under these conditions, the numerical description of the REB transport often uses the so-called hybrid approach, where the incident and weakly collisional electrons are modeled kinetically and the highly collisional return current is described as an inertialess fluid [10,13,14].Most of the REB transport experiments carried out up to now have used solid targets [8,15...

Electron heating by intense short-pulse lasers propagating through near-critical plasmas

“…In the opposite limit of a highly overcritical plasma (  n n e c ), the laser is mainly reflected and/ or absorbed at the plasma surface, where electrons are stochastically heated in the MeV range, with a broad energy distribution [42][43][44][45][46][47][48][49]. The high electron current (~-0.1 50 MA) [6,8,[50][51][52][53][54][55][56] that ensues can then serve as a source of ultrafast (isochoric) target heating, ion acceleration at the target rear or x-ray radiation [57,58].…”

Dielectric function of dense plasmas, their stopping power, and sum rules

“…The problem of stopping power computing for relativistic projectiles has recently arisen due to the reported experiments with protons decelerating from velocities of up to 80% of the speed of light [50] (see also [51]). The importance of the relativistic corrections to the classical asymptotic form (21) of the stopping power as compared to the above intertarget electron-ion correlation contribution was estimated recently in [52].…”

Collimated Propagation of Fast Electron Beams Accelerated by High-Contrast Laser Pulses in Highly Resistive Shocked Carbon

“…1 b) shows sample data as a function of ∆t, from ∆t = 0 ns (REB injection into an unperturbed solid-cold vitreous-C target), up to ∆t = 4 ns (1 ns after shock-breakout time at the inner cone-tip surface; REB generation in an expanding plasma filling the cone volume). The use of the LP laser also for the ∆t = 0 -case allowed the formation of a long CH ablation-plasma that prevented fast electrons from recirculating after their first transit through the Cu layers [13,35]. The simulated density and temperature maps at the corresponding delays are presented on Fig.…”