Brillouin scattering in an infinite medium is anisotropic, with the threshold of absolute instability is due to the attenuation of the scattered waves. If the collision absorption mechanism prevails, the minimum threshold is observed in backward scattering. For the scattering region, limited in the longitudinal direction (parallel to pump wave propagation direction), the threshold will become greater than in infinite medium due to the appearance of convective losses associated with energy removal by scattered waves. In this paper we consider the scattering of a wide wave beam in the plasma, which size in a transverse direction substantially exceed longitudinal one. It is shown that in this case, the threshold for angularly scattered wave may be lower than the threshold for backward scattering due to the increased time of radiation output for interaction region. This effect has not attracted attention previously. In turn, the decrease of the threshold leads to increasing of loss due to radiation, which is important in plasma heating problems. The results can also be used for plasma diagnostics.
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions
52.38.-r Laser-plasma interactions
$^1$Faculty of physics, Lomonosov Moscow State University
$^2$Faculty of physics, Tajik National University, pr. Rudaki, 17, Dushanbe, 734025, Tajikistan