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

Abstract

We demonstrate the differences in hot electron absorption mechanisms dominant in the interaction of a femtosecond laser pulse with intensities of 1018 W/cm2 and 1021 W/cm2 on a fully ionized target with a steep density profile and preplasma with moderate scale length (3 μm). We show that acceleration of each electron starts at the moment when the magnetic component of a standing electromagnetic wave changes its polarity in a 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 the standing wave is already created at the position of electron during the interaction with the pulse with an intensity of 1018 W/cm2. In the case with 1021 W/cm2 intensity, the part of the 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 the standing wave is created.