Modeling the nonlinear saturation of stimulated Brillouin backscatter in laser heated plasmas

After showing that the stimulated Brillouin instability (SBS) is likely to be in a saturated regime under conditions of interest for inertial confinement fusion, two examples of reduced models of nonlinear effects that are included in a fluid model are described. Simulations using a nonlinear damping representing the saturation of the amplitude of acoustic waves in the fluid regime (i.e., weak Landau damping) are compared with experimental measurements done on CO2 plasmas. While good agreement is found between the model and a variety of independent experimental measurements, no simple explanation was found for the very low saturation level (well below the amplitude corresponding to the two-ion-decay instability) that has to be used. In the kinetic regime (i.e., large Landau damping), hybrid-particle-in-cell simulations show that nonlinear frequency shifts induced by trapping saturate SBS. A reduced steady-state model has been shown to be in correct agreement with time-integrated measurements done on Be plasmas. Numerical simulations indicate that a more dynamical model is needed to describe the transient evolution of SBS. (C) 2003 American Institute of Physics.