Harmonic decomposition to describe the nonlinear evolution of Stimulated Brillouin Scattering

Overview & Synopsis

An efficient method to describe the nonlinear evolution ofStimulated Brillouin Scattering in long scale-length plasmas ispresented. The method is based on a decomposition of thehydrodynamics variables in long- and short-wavelength components.It makes it possible to describe the self-consistent couplingbetween the plasma hydrodynamics, Stimulated Brillouin Scattering,and the generation of harmonics of the excited ion acoustic wave(IAW). This description is benchmarked numerically and proves tobe reliable even in the case of an undamped ion acoustic wave. Themomentum transferred from the electromagnetic waves to the plasmaions is found to induce a plasma flow which modifies the resonantthree wave coupling between the IAW and the light waves. A novelpicture of SBS arises, in which both IAW harmonics and flowmodification reduce the coherence of SBS by inducing local defectsin the density and velocity profiles. The spatial domains ofStimulated Brillouin activity are separated by these defects andare consequently uncorrelated, resulting in a broad and structuredspectrum of the scattered light and in a temporally chaoticreflectivity.