An efficient method to describe the nonlinear evolution of Stimulated Brillouin Scattering in long scale-length plasmas is presented. The method is based on a decomposition of the hydrodynamics variables in long-and short-wavelength components. It makes it possible to describe the selfconsistent coupling between 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 to be reliable even in the case of an undamped ion acoustic wave. The momentum transferred from the electromagnetic waves to the plasma ions is found to induce a plasma flow which modifies the resonant three wave coupling between the IAW and the light waves.A novel picture of SBS arises, in which both IAW harmonics and flow modification reduce the coherence of SBS by inducing local defects in the density and velocity profiles. The spatial domains of Stimulated Brillouin activity are separated by these defects and are consequently uncorrelated, resulting in a broad and structured spectrum of the scattered light and in a temporally chaotic reflectivity.PACS numbers: 52.38. Bv, 52.35.Mw, 42.65.Es * Permanent address : Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester NY 14623, USA 1The description of parametric instabilities in laser-produced plasmas using simple coupled mode equations for three wave interaction is no longer sufficient whenever the longitudinal plasma waves are driven to large amplitudes. Then the nonlinearities of the longitudinal wave can induce detuning with respect to the three wave resonance. This is one of the reasons usually invoked to explain why these simplified models overestimate the scattering levels of Stimulated Brillouin Scattering (SBS). In this article we concentrate on SBS, which is the process by which the incident laser wave couples to an ion acoustic wave (IAW) togive rise to a scattered transverse wave. The generation of the harmonics due to the IAW fluid-type nonlinearity [1, 2, 3, 4, 5] is already known to be able to reduce significantly the SBS reflectivity when compared with the results involving simply a linearized IAW.However, the previous fluid-type models for SBS in Refs. [1, 2, 3, 4], aimed at taking into account the IAW nonlinearity, were incomplete because they did not properly describe the flow modification [6, 7] caused by the incident transverse wave momentum deposition. All the mentioned models [1, 2, 3, 4, 5] also ignored multi-dimensional effects. On the other hand, kinetic effects associated with particle trapping [8] give also rise to a nonlinear IAW frequency shift and therefore modify the SBS nonlinear behavior.In the present Letter, we reconsider the effect of the IAW nonlinearities on SBS by accounting properly for the flow modification caused by SBS. We first derive approximate equations describing simultaneously the plasma hydrodynamics (i.e. the long wavelength density and flow profiles), SBS, and the harmonic g...
The electron kinetic effects are shown to play an important role in the nonlinear evolution of a driven ion-acoustic wave. The numerical simulation results obtained (i) with a hybrid code, in which the electrons behave as a fluid and the ions are described along the particle-in-cell (PIC) method, are compared with those obtained (ii) with a full-PIC code, in which the kinetic effects on both species are retained. The electron kinetic effects interplay with the usual fluid-type nonlinearity to give rise to a broadband spectrum of ion-acoustic waves saturated at a low level, even in the case of a strong excitation. This low asymptotic level might solve the long-standing problem of the small stimulated Brillouin scattering reflectivity observed in laser-plasma interaction experiments.
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