Nonlinear evolution of scattering instabilities in laser produced plasmas

Nonlinear theory of scattering processes during interaction of intense electro-magnetic radiation with plasmas is studied. Our studies have concentrated on two instabilities: stimulated Raman (SRS) and Brillouin (SBS) scattering. A theoretical model based on the Zakharov-Maxwell equations is used to describe the evolution of SRS and SBS. The model includes nonlinear processes related to the coupling between Langmuir and ion waves. Numerical results obtained in a homogenous finite plasma have shown that the primary mechanism responsible for saturation of SRS in our theory is the parametric decay instability (PDI) of the resonantly driven Langmuir wave. The different spatial and temporal evolution of SRS and PDI leads to new physics in SRS nonlinear evolution, including disruption of the PDI cascade, localization of Langmuir fields, and burst like behavior of SRS reflectivity.;The nonlinear evolution of the Langmuir and ion acoustic waves provides a mechanism which broadens and enhances the Langmuir and ion acoustic wave spectra. These enhanced fluctuation levels contribute to secondary scattering processes such as enhanced Brillouin, forward Raman, and anti-Stokes forward and backward Raman scattering. Asymptotic saturation levels of stimulated Raman scattering are well approximated by a simple scaling law. This scaling predicts a proportionality of the Langmuir wave amplitude to the PDI threshold with dependence on the intensity and plasma interaction length. Scaling laws for the secondary scattering processes as well as criteria for their strong enhancement are given. The frequency spectra of enhanced Brillouin scattering shows Stokes and anti-Stokes components, where their relative intensities depend on the plasma density. Superthermal electron production is also studied in the asymptotic regime, providing limitations on the interaction length and laser intensity in our model. Our theoretical results are consistent with many experimental observations providing new or alternate explanations for observed results.