The interplay between the kinetic nonlinear frequency shift and the flowing gradient in stimulated Brillouin scattering

Abstract

The effect of the kinetic nonlinear frequency shift (KNFS) on backward stimulated Brillouin scattering (SBS) in homogeneous plasmas and inhomogeneous flowing plasmas is investigated by three-wave coupled-mode equations. When the positive contribution to the KNFS from electrons as well as the negative contribution from ions is included, the net KNFS can become positive at a large electron-ion temperature ratio ZT T e i . In homogeneous plasmas, KNFS can greatly reduce the SBS reflectivity at low or large ZT T e i but has a weak effect on SBS at ZT T e iwhere the positive frequency shifts from electrons almost cancels out the negative shifts from ions. In inhomogeneous plasmas, the net negative frequency shift can enhance the backward SBS reflectivity for the negative gradient of the plasma flowing, and can suppress the reflectivity for the positive case. On the contrary, the net positive frequency can suppress the reflectivity for the negative case of the flowing gradient and enhance the reflectivity for the positive case. This indicates that the SBS in inhomogeneous flowing plasmas can be controlled by changing the sign of the nonlinear frequency shift.

Auto-resonant stimulated Brillouin backscattering in supersonic flowing plasmas by fully kinetic Vlasov simulations

“…However, several authors have pointed out that, in an inhomogeneous flowing plasma, the nonlinear frequency shift may compensate the detuning due to the flow gradient to remain resonant automatically [16][17][18], which resulted in a significantly spatial-dependent growth of IAWs to drastically enhance the SBS reflectivity. It is called as autoresonance [19].…”

Effects of frequency-modulated pump on stimulated Brillouin scattering in inhomogeneous plasmas

“…The Vlasov-Maxwell code called Vlama is used to study the nonlinear effects induced by the frequency-modulated pump. We find that the autoresonance not only occurs in the no-bandwidth case [35,36], but also occurs in the η ∼ 1 case, which is called the multi-locations autoresonance. The SBS reflectivity also increases as η increases to 1, and then decreases with η, which agrees well with three-wave simulations.…”

“…Based on early works [35][36][37][38], there is only one resonant point in an inhomogeneous plasma though the SBS is not sensitive to the nonuniformity of plasma density. The phase mismatch will be larger when the plasma has nonuniform flowing velocity.…”

Influences of sinusoidal density modulation on stimulated Raman scattering in inhomogeneous plasmas

“…In order to obtain numerical solutions, we have developed software that numerically solves equation (1) using Lax-Wendroff scheme [38,39], which we refer to as three-wave simulation below. For simplicity, the damping rates of the three waves are ignored in our three-wave simulations.…”