Suppression of Stimulated Brillouin Scattering in Laser Plasma under Conditions of Multifrequency Target Irradiation

Abstract

The conditions under which stimulated Brillouin scattering (SBS) in laser plasma can be partially suppressed are considered. With stimulated scattering suppressed, the fraction of absorbed laser energy can be increased. An analytical theory of stimulated scattering upon interaction of two counterpropagating electromagnetic waves with ion acoustic wave in nonuniformly expanding plasma is presented. Formulas are derived, which allow one to calculate the scattering coefficient and fraction of scattered radiation energy from the specified parameters of laser radiation and plasma. The irradiation of a CH₂ target by a laser pulse is numerically simulated. It is shown that in the multifrequency irradiation mode (with two or more frequencies) the fraction of absorbed laser energy in plasma can be increased due to the partial suppression of stimulated scattering. For an incident Nd-laser beam with a fluence of 5 × 10¹⁴ W/cm² and two close frequencies, the minimum fraction of scattered radiation is 0.87 of that for single-frequency irradiation. With an increase in the number of harmonics n in the beam the scattering suppression efficiency increases: δ_sn/δ_s1 = 0.68 at n = 5 (δ_sn is the scattering fraction in the case of n harmonics); δ_sn/δ_s1 = 0.51 at n = 11. These δ_sn/δ_s1 values have been obtained at Δω_s/ω₀ ~0.5–1.5%, depending on the number of harmonics (Δω_s is the spectral width, ω₀ is the fundamental frequency). At Δω_s/ω₀ > 1.5 the δ_sn/δ_s1 ratios change only slightly. Stimulated scattering is suppressed because of the beatings arising in the laser radiation when adding harmonics with close frequencies. Suppression occurs when the beat wavelength becomes smaller than the characteristic size of variation in plasma velocity.