The role of standing wave in the generation of hot electrons by femtosecond laser beams incident on dense ionized target

TL;DR

This paper investigates how standing waves influence hot electron generation during femtosecond laser interactions with dense ionized targets at different intensities, revealing distinct absorption mechanisms and effects of preplasma on electron heating.

Contribution

It uncovers the role of standing wave polarity changes and preplasma in hot electron acceleration mechanisms at high laser intensities.

Findings

Standing wave polarity change triggers electron acceleration without preplasma.

Preplasma enhances stochastic heating and electron temperatures.

High-intensity interactions allow electron re-injection into high-field regions.

Abstract

We demonstrate the differences in hot electron absorption mechanisms dominant in the interaction of femtosecond laser pulse with intensities 1e18 W/cm2 and 1e21 W/cm2 on fully ionized target with steep density profile and preplasma with moderate scale length (3 microns). We show that acceleration of each electron starts at the moment when magnetic component of standing electromagnetic wave changes its polarity in regime without preplasma. In the presence of preplasma the stochastic heating is the dominant absorption mechanism along with the longitudinal electric field. It is observed, that wave's energy is absorbed only if standing wave is already created at the position of electron during the interaction with the pulse with intensity 1e18 W/cm2. In the case with intensity 1e21 W/cm2, part of electrons is pre-accelerated in front of the target before the reflection and following stochastic heating. The presence of preplasma results in electron temperatures close to or even exceeding ponderomotive scaling. At higher intensity, the re-injection of electrons previously repelled by incident wave's ponderomotive force into high-field regions is allowed if standing wave is created.