Particle-In-Cell Simulations of Stimulated Raman Scattering for Parameters Relevant to Inertial Fusion Energy

Using the electromagnetic Particle-In-Cell (PIC) code OSIRIS, stimulated Raman scattering (SRS) is simulated in 1 and 2D under conditions relevant to Inertial Fusion Energy (IFE) and the National Ignition Facility (NIF). In this regime, the plasma wave has a klambda D ≳ 0.3 (k is the plasma wave wavenumber and lambda D is the plasma Debye length) and kinetic effects are important even for small amounts of growth. OSIRIS simulations show that inflation, frequency shifts, sideband instabilities, beam modes, pump depletion, plasma wave convection, plasma length, and ion motion all play a role in SRS dynamics. While each nonlinearity is a subject in its own right, an array of simulations are used to study the dynamics of SRS behavior as a whole.;A comprehensive picture of the onset, saturation, and recurrence of SRS is presented. The onset is shown to depend on the convective gain length in the strongly damped regime. Even though SRS is below the absolute threshold, it is shown to grow at the undamped absolute growth rate due to the effect of trapped particles. Saturation is shown to depend on both the plasma wave's nonlinear frequency shift and pump depletion, with sidebands and beam modes growing significantly only after saturation. Following saturation, plasma waves are shown to convect as a packet which Raman scatters at a shifted frequency, with recurrence depending on the nonlinear packet speed, shifted frequencies, and pump depletion.;The 1D simulations indicate that total time-averaged reflectivities less than 1% require the ratio of the speckle length to the convective gain length to be ≲ O(1). The total time-averaged reflectivity is shown to be lower when the ratio of the growth rate to the detuning rate due to the nonlinear frequency shift is lower ( ≲ O(1)) and lower for shorter simulated speckle lengths. Instantaneous reflectivity levels are shown to increase in time when multiple plasma packets exist simultaneously within the simulated space. 2D reflectivity is shown to be lower in comparison with 1D due to transverse localization of the plasma wave, but the same dependence of the instantaneous reflectivity on the nonlinear frequency shift and plasma packet effects is shown. The results indicate that mesoscale models that incorporate kinetic effects must include the effects of plasma packets and a nonlinear frequency shift, albeit the frequency shift is shown to be larger than theoretically expected.