Unified theory of absolute sidescattering in inhomogeneous plasma

A unified theory of absolute stimulated Raman and Brillouin sidescattering in inhomogeneous plasma is presented. The linearized coupling equations are transformed into a Schroedinger equation in k space and are solved through an eigenvalue problem. The threshold, growth rate, and scattering geometry are obtained for the pump laser with an arbitrary incidence angle relative to the density gradient. Numerical solutions of the k-space coupling equations show good agreement between the theoretical and numerical results. Previous threshold calculations of absolute Brillouin sidescattering [C.S. Liu et al., Phys. Fluids 17, 1211 (1974)] are corrected, and the relationship between forward and backward sidescattering is analysed. This theory, and the determination of regions of absolute instabiliy, is applied to experiments relevant to direct drive inertial confinement fusion. The possibility of observing such instabilities is discussed.