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 harmonicvac = 1.3152 and with the power density in the focal spot I = 10 20 W/m 2 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.