On the stimulated Raman sidescattering in inhomogeneous plasmas: revisit of linear theory and three-dimensional particle-in-cell simulations

Abstract

Stimulated Raman sidescattering (SRSS) in inhomogeneous plasma is comprehensively revisited on both theoretical and numerical aspects due to the increasing concern of its detriments to inertial confinement fusion. Firstly, two linear mechanisms of finite beam width and collisional effects that could suppress SRSS are investigated theoretically. Thresholds for the eigenmode and wave packet in a finite-width beam are derived as a supplement to the theory proposed by Mostrom and Kaufman (1979 Phys. Rev. Lett.42 644). Collisional absorption of SRSS is efficient at high-density plasma and high-Z material, otherwise, it allows emission of sidescattering. Secondly, we have performed the first three-dimensional particle-in-cell simulations in the context of SRSS to investigate its linear and nonlinear effects. Simulation results are qualitatively agreed with the linear theory. SRSS with the maximum growth gain is excited at various densities, grows to an amplitude that is comparable with the pump laser, and evolutes to lower densities with a large angle of emergence. Competitions between SRSS and other parametric instabilities such as stimulated Raman backscattering, two-plasmon decay, and stimulated Brillouin scattering are discussed. These interaction processes are determined by gains, occurrence sites, scattering geometries of each instability, and will affect subsequent evolutions. Nonlinear effects of self-focusing and azimuthal magnetic field generation are observed to be accompanied with SRSS. In addition, it is found that SRSS is insensitive to ion motion, collision (low-Z material), and electron temperature.

Time evolution of stimulated Raman scattering and two-plasmon decay at laser intensities relevant for shock ignition in a hot plasma

“…Considering light refraction across the density distribution obtained by CHIC simulations, SRSS light was emitted at an angle , in agreement with the PIC simulations of Xiao et al. [20] . The side-scattered EPWs excited at the lower densities ( , ) correspond to for a plasma temperature , as expected at times near the laser peak; this suggests that SRSS, as BRS, is limited at lower densities by Landau damping.…”

“…According to the spectra, SRSS was driven at densities 0.08-0.17 n c , with a maximum growth rate around n e ∼ 0.12 n c (Figure 4). Considering light refraction across the density distribution obtained by CHIC simulations, SRSS light was emitted at an angle 65 • < θ < 90 • , in agreement with the PIC simulations of Xiao et al [20] . The side-scattered EPWs excited at the lower densities (λ S RSS ≈ 2000 nm, k e = 1.12 ω 0 /c) correspond to k e λ D ≈ 0.3 for a plasma temperature T e ≈ 4 keV, as expected at times near the laser peak; this suggests that SRSS, as BRS, is limited at lower densities by Landau damping.…”

“…This value is approximately six times lower than the average intensity at the laser peak, and much lower than the local intensity expected from filamentation and in high-intensity speckles. For calculating the SRSS threshold, as pointed out by Xiao et al [20] , we have to account for the finite beam size that limits the instability gain growing in the transverse direction. This mechanism results in a threshold much higher than the classical one and can explain why SRSS is seldom observed in the experiments.…”

“…Despite that, very few experiments have shown evidence of SRSS, which was usually explained by the large collisional absorption of side-scattered light. A recent work by Xiao et al [20] revisited the theory of SRSS, including the effect of the laser beam width on the threshold of the instability, which could explain the scarcity of experimental evidence of SRSS. According to Xiao et al, SRSS, preferentially driven at low plasma densities where collisional absorption is low, can however be strong and compete with backward SRS at SI intensities, resulting in the scattering of a significant amount of laser energy.…”

“…A recent work by Xiao et al. [20] revisited the theory of SRSS, including the effect of the laser beam width on the threshold of the instability, which could explain the scarcity of experimental evidence of SRSS. According to Xiao et al.…”