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

TL;DR

This study demonstrates that circularly polarized ultraintense short laser pulses can rapidly heat and thermalize electrons in thin copper foils through collisional absorption, a process robust across various laser parameters.

Contribution

It reveals that inverse bremsstrahlung dominates electron heating in collisional regimes at relativistic intensities, highlighting the importance of collisions and polarization in laser-plasma interactions.

Findings

Electrons reach ~3.5 keV temperature rapidly in collisional simulations.

Circular polarization suppresses suprathermal electron generation.

Increasing pulse duration at fixed energy raises electron temperature.

Abstract

The creation of well-thermalized, hot and dense plasmas is attractive for warm dense matter studies. We investigate collisionally induced energy absorption of an ultraintense and ultrashort laser pulse in a solid copper target using particle-in-cell simulations. We find that, upon irradiation by a $2\times10^{20}{\rm\,W\,cm^{-2}}$ intensity, $60{\rm\,fs}$ duration, circularly polarized laser pulse, the electrons in the collisional simulation rapidly reach a well-thermalized distribution with ${\sim}3.5{\rm\,keV}$ temperature, while in the collisionless simulation the absorption is several orders of magnitude weaker. Circular polarization inhibits the generation of suprathermal electrons, while ensuring efficient bulk heating through inverse bremsstrahlung, a mechanism usually overlooked at relativistic laser intensity. An additional simulation, taking account of both collisional and field ionization, yields similar results: the bulk electrons are heated to ${\sim}2.5{\rm\,keV}$, but with a somewhat lower degree of thermalization than in the pre-set, fixed-ionization case. The collisional absorption mechanism is found to be robust against variations in the laser parameters. At fixed laser pulse energy, increasing the pulse duration rather than the intensity leads to a higher electron temperature.