The Formation of Plateau on the Electron Distribution in the Presence of SRS Due to a Trapped Particle Instability

The behaviour of electron gas in a laser plasma corona is studied in the presence of stimulated Raman scattering. The 1D Vlasov-Maxwell model describing plasma relevant to the experiment PALS (Prague Asterix Laser System), where the nanosecond iodine laser with the wavelength of first harmonic λvac = 1.3152 and with the power density in the focal spot I = 10 W/m is in operation. For the solution of Vlasov equation for the electron distribution function a transform method with the Fourier expansion in spatial coordinate and with the Hermite expansion in the velocity space is used. For the numerical stabilization a small collision term is added to the Vlasov equation keeping its value realistic for the condition relevant to the PALS experiment. The dominant wave modes in our model are both the backward (SRS-B) and the forward (SRS-F) Raman scattering, each of them accompanied by the forward going electron plasma wave. These waves interact with the electrons in the plasma, trap and accelerate them. The temporal evolution in the phase space is studied in detail focusing on the influence of the SRS-B plasma wave interaction with plasma electrons. Several mechanisms were identified such as the SRS-B plasma wave spectral broadening due to a trapped particle instability or the formation of an electrostatic quasi-mode by non-linear non-resonant interaction of SRS-B and SRS-F plasma waves. The results are visualized by the behaviour of the electron distribution function, evolution of the resonant SRS-B electrostatic wave, and by the electrostatic spectrum at the moment when the wave modes growing due to the trapped particle instability and due to the non-resonant SRS-B and SRS-F plasma wave interaction are fully developed. Introduction In a few last decades experimental and theoretical studies of nanosecond intense laser beam propagation through the plasma corona became very intense in order to understand what processes influence the formation of plasma corona. This knowledge is very important for numerous experimental situations including the thermonuclear fusion experiments and production of highly charged ion and electron beams. This paper is devoted to the theoretical study of stimulated Raman scattering of impinging laser beam. During the propagation through a well underdense plasma in a self-generated corona, the high-amplitude longitudinal electron plasma wave and the transverse electromagnetic wave are resonantly generated. In a 1D geometry two scattering processes are possible: forward and backward, where in both the processes the electron plasma wave is propagating in the direction of impinging laser beam. Thus in the case of the Raman backscattering a part of laser energy does not reach the critical surface and it is carried out by the scattered electromagnetic wave so that it cannot be used for the target heating. This fact makes trouble in experiments with indirect driven laser fusion, where a capsule with thermonuclear fuel is inside the hohlraum made of a high-Z material, which converts the light of laser beams to the soft X-rays compressing the fuel capsule. But during the experiment a well underdense plasma is generated inside the hohlraum so the parametric processes like the Raman back-scattering can take place in the light entrance hole, thus significantly decreasing the efficiency of laser light conversion [Lindl, 1995]. On the other hand, the generation of high-amplitude electrostatic wave during the stimulated Raman scattering enable electron trapping and acceleration. These electrons then speed up the expansion of the plasma corona and thus contribute to the enhanced charge freezing effect [Rohlena et al., 1996]. The trapped particles in addition cause the secondary instability, first described by Kruer et al. [1969] so-called “Trapped particle instability” (TPI), which is responsible for a significant spectral broadening of the backward Raman daughter electrostatic wave. Due to this instability sidebands with a lower wave number grow preferentially mainly because of a higher Landau damping rate of the wave modes with WDS'06 Proceedings of Contributed Papers, Part II, 173–178, 2006. ISBN 80-86732-85-1 © MATFYZPRESS