Stimulated Brillouin scattering of the beat wave of two lasers in a plasma.

Stimulated Brillouin scattering of two collinear lasers in a plasma is investigated. Lasers exert a longitudinal ponderomotive force on electrons, imparting them oscillatory axial velocity at the beat frequency. This velocity acts as a driver for parametric excitation of an ion acoustic wave (ω,kvector ) and a noncollinear sideband electromagnetic wave (ω′,kvector ′). The driver velocity vvector 0- couples to the sideband wave to exert a ponderomotive force at (ω,kvector ) on the electrons, driving the ion acoustic wave. The density perturbation of ion acoustic wave beats with vvector 0- to produce a nonlinear current at (ω′,kvector ′), driving the sideband. In the case of finite spot size Gaussian laser beams, the beat wave has a Gaussian profile and excites an ion acoustic wave (ω,kvector ) and a backscattered TM mode (ω′,kvector z′). The growth rate scales as the product of amplitudes of the lasers and maximizes at optimum values of scattering angles. The parametric instability of difference frequency driver is stronger than the sum frequency driver. [ABSTRACT FROM AUTHOR]