Stimulated Brillouin scattering and ponderomotive self-focusing from a single laser hot spot

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.

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