The parametric interaction of two crossed collimated laser beams with ion plasma modes has been studied. The underlying process is a density grating that is created by the two laser beams. The Bragg diffraction that is produced enhances forward stimulated Brillouin scattering (SBS) which results in a time dependent energy exchange between the two beams. A diversity of other SBS processes which depend on the symmetry of driving laser beams are also discussed.
The parametric interaction of a focused Gaussian laser beam with ion plasma modes has been studied by using two dimensional nonparaxial spectral code [Phys. Fluids B 5, 3748 (1993)]. It is shown that the reflectivity of stimulated Brillouin scattering (SBS) from the single laser hot spot is dominated by backscattering. The reflectivity scales with the stationary gain coefficient and very closely follows predictions of a simple three wave coupling model. This is in spite of essentially nonlinear SBS behavior, which develops in the inhomogeneous density channels formed by the ponderomotive force of the beam, where it competes with the self-focusing instability (SFI). For the relatively short focal lengths (f/number<10) the SFI is diffraction limited and evolves until hot spot is focused to a size of few laser wavelengths. The SFI is responsible for the broad and anisotropic angular distribution of the transmitted light.
A single laser hot spot in an underdense plasma is represented as a focused Gaussian laser beam. Stimulated Brillouin scattering (SBS) from such a Gaussian beam with small f/numbers 2-4 has been studied in a three-dimensional slab geometry. It is shown that the SBS reflectivity from a single laser hot spot is much lower than that predicted by a simple three wave coupling model because of the diffraction of the scattered light from the spatially localized ion acoustic wave. SBS gain per one Rayleigh length of the incident laser beam is proposed as a quantitative measure of this effect. Diffraction-limited SBS from a randomized laser beam is also discussed.
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