Spatiotemporal evolution and nonlinear kinetic simulations of stimulated Brillouin scattering.

摘要

The spatiotemporal evolution of stimulated Brillouin scattering (SBS) in homogeneous plasmas and some aspects of the influence that nonlinear and kinetic effects have on the evolution of SBS were studied.; A one-dimensional analytical linear model based on a fluid description of the plasma was developed initially. It was found that the threshold intensity of the absolute instability and the steady-state spatial growth rate of the convective instability are both independent of the scattering angle. However, the saturation time of the convective instability exhibits a strong inverse dependence on the scattering angle.; The basic model was improved by extending the one-dimensional analysis to include two spatial dimensions and time. In order to assess the effects that the finite size of the laser beam has on SBS, wide and narrow laser-beam geometries were considered. Detailed comparisons were made between the predictions of reduced one- and two-dimensional models, which can be solved analytically, and the results of two-dimensional numerical simulations.; It was found that the linear evolution of SBS was characterized by three parameters: the spatial growth rate in the direction of the Stokes wave, the spatial damping rate of the ion-acoustic wave in the direction of the Stokes wave and the normalized width of the interaction region. SBS can be saturated by the damping or the lateral convection of the ion-acoustic wave, both of which limit the growth of the ion- acoustic wave (directly) and the Stokes wave (indirectly). For most scattering angles the predicted saturation mechanism, and the corresponding saturation time and gain factor agree with the simulation results.; The influence that nonlinear and kinetic effects have on SBS was investigated by performing particle-in-cell (PIC) simulations. The results of these PIC simulations were compared against fluid simulations, and good agreement was obtained for weak laser intensities. When the laser intensity is strong enough, PIC and fluid simulations differ substantially. It was shown that among various saturation mechanisms such as pump depletion, wavebreaking and generation of ion-acoustic wave harmonics, ion-trapping is the mechanism responsible for the observed differences. The SBS reflectivity is shown to depend sensitively on the frequency mismatch between the light wave used to seed the instability and the incident laser.