Competition between the Stimulated Raman and Brillouin Scattering under the Langdon Effect

In this paper, we discuss the influence of the Langdon effect on the competition between stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) instabilities. When the electron density is below a quarter of the critical density, both SRS and SBS can occur simultaneously, requiring careful consideration of the competition between these two instabilities. In practical Laser-Plasma Interaction (LPI) scenarios, the electron distribution function may deviate from the Maxwellian form and transition to a super-Gaussian distribution, necessitating an analysis of how the Langdon effect influences the competition between SRS and SBS. In this study, in order to consider the Langdon effect, numerical methods are employed to incorporate a super-Gaussian electron distribution function, modifying the electron susceptibility to calculate the gain factors (G) of SRS and SBS under different super-Gaussian indices. The respective backscattering shares of SRS and SBS are then estimated using an analytical solution derived from a five-wave coupling model. Moreover, five-wave coupling simulations and Vlasov simulations are used to examine how the Langdon effect impacts the specific coupling dynamics between SRS and SBS under various parameters. The findings reveal that in a specific parameter range, the SBS reflectivity is higher than that of SRS under a Maxwellian electron distribution. However, the Langdon effect significantly reduces the Landau damping of electron plasma waves, while the Landau damping of ion acoustic waves decreases less, resulting in SRS reflectivity exceeding that of SBS.