Modeling of the competition of stimulated Raman and Brillouin scatter in multiple beam experiments

Abstract

Multiple laser beam experiments with plastic target foils at the Laboratoire pour L'Utilisation des Lasers Intenses (LULI) facility [Baldis, et al., Phys. Rev. Lett. 77, 2957 demonstrated anti-correlation of stimulated Brillouin and Raman backscatter (SBS and SRS).Detailed Thomson scattering diagnostics showed that SBS always precedes SRS, that secondary electron plasma waves sometimes accompanied SRS appropriate to the Langmuir Decay Instability (LDI), and that, with multiple interaction laser beams, the SBS direct backscatter signal in the primary laser beam was reduced while the SRS backscatter signal was enhanced and occurred earlier in time. Analysis and numerical calculations are presented here that evaluate the influences on the competition of SBS and SRS, of local pump depletion in laser hot spots due to SBS, of mode coupling of SBS and LDI ion waves, and of optical mixing of secondary and primary laser beams . These influences can be significant. The calculations take into account simple models of the laser beam hot-spot intensity probability distributions and assess whether ponderomotive and thermal self-focusing are significant. Within the limits of the model, which omits several other potentially important nonlinearities, the calculations suggest the effectiveness of local pump depletion, ion wave mode coupling, and optical mixing in affecting the LULI observations.

Observation and modelling of stimulated Raman scattering driven by an optically smoothed laser beam in experimental conditions relevant for shock ignition

“…11. Here, however, a clear distinction is visible between the actions of SBS and SRS, which is due to their different time-scale. While SBS grows on times of the same order as the self-focusing, SRS has a much faster growth [51,63], allowing the SRS to adapt very rapidly to ponderomotively induced profile modifications. In this way, SRS (re-)establishes rapidly in the arising filamentary structure, preferentially in the rear of the selffocused speckle in which the plasma density is not (yet) depleted.…”

“…The physics of the saturated regime can be very complex because of the concomitance of numerous non-linear effects in intense speckles. Saturation can be produced by the depletion of the incident flux into the speckle, or by non-linear processes in the coupling process [29,32,[49][50][51][52][53][54] , strongly limiting the amplitude of the plasma wave responsible for the laser light scattering. In our model, R sat denotes, for simplicity, a time-average value, smoothing possible bursty SRS behaviour [55] .…”

Effects of ion trapping on crossed-laser-beam stimulated Brillouin scattering

“…We consider the following steady-state coupled-mode equations for stimulated Brillouin scattering in one spatial dimension: 35,36 v g da 0 /dxϭϪ⌫a 1 a s , ͑7a͒…”

Saturation of stimulated Brillouin backscattering in two-dimensional kinetic ion simulations

“…11 show evidence of both backscatter and forward scatter later in time. The emergence of forward SBS late in the 2D plane wave simulation with † ZT e /T i = 16 raised the possibility that the long wavelength ion waves associated with forward SBS might contribute nonlinear dissipation by means of IAW mode coupling 14,24 that could help to explain the continued absence of SBBS at late times in the 2D simulations. Figure 12a shows the SBBS reflectivity for the 2D plane wave simulation We see from the simulation evidence presented that two-dimensional scattering of the SBBS primary IAW provides a mechanism for relaxing the primary IAW and causing the crash of SBBS.…”