Intensity dependence of electron gas kinetics in a laser corona

Martin Mašek,Karel Rohlena

doi:10.1051/epjconf/20135905013

Abstract

In various experimental situations relevant to the laser fusion, such as plasma near the light entrance holes of hohlraum in the indirect drive experiments or more recently in the shock ignition direct drive a relatively long underdense plasma of corona type is encountered, which is subject to an intense nanosecond laser beam. The plasma is only weakly collisional and thus in the electron phase space a complicated kinetic evolution is going on, which is taking the electron gas fairly far from the thermal equilibrium and contributes to its unstable behaviour. These phenomena impede the absorption and thermalization of the incoming laser energy, create groups of fast electrons and also may lead to a non- linear reflection of the heating laser beam. One of the key processes leading to the electron acceleration is the stimulated Raman scattering (SRS) in its non-linear phase. The SRS in the presence of electron- ion collisions requires a certain threshold intensity above which the mentioned non-dissipative phenomena can occur and develop to the stage, where they may become unpleasant for the fusion experiments. To assess this intensity limit a computational model has been developed based on the Vlasov-Maxwell kinetics describing such a plasma in 1D geometry. At a relatively high intensity of 10 16 W/cm 2 a number of non-linear phenomena are predicted by the code such as a saturation of Landau damping, which is then translated in an unfavourable time dependence of the reflected light intensity and formation of accelerated electron groups due to the electron trapping. The purpose of the present contribution is to map the intensity dependence of this non-linear development with the aim of assessing its weight in fusion relevant situations.

Highlights

In the phase space picture it is seen that the electron plasma wave is not strong enough to trap a large amount of electrons (see Fig. 2(a)), so that the wave-particle interaction does not essentially participate in the temporal evolution of the wave modes
This a ‘second threshold’ connected with a fully non-linear wave-particle collisionless interaction appears. It is characterized by the fast amplitude growth followed by a fast collisionless damping of the resonant electrostatic wave mode as a consequence of strong electron trapping in the electron plasma wave (EPW) troughs leading to a saturation
The simplified version of the numerical simulation should prevent the development of trapped particle instability, the Raman cascading and the forward stimulated Raman scattering (SRS), the following conclusions can still be made: (1) In accordance with the linear theory we found that a strong instability growth and a lower threshold occur in the thinner plasma of the outer corona

Summary

Introduction

In the phase space picture it is seen that the electron plasma wave is not strong enough to trap a large amount of electrons (see Fig. 2(a)), so that the wave-particle interaction does not essentially participate in the temporal evolution of the wave modes. It is characterized by the fast amplitude growth (pumping laser is strong enough) followed by a fast collisionless damping of the resonant electrostatic wave mode as a consequence of strong electron trapping in the EPW troughs leading to a saturation.

Results

Conclusion