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

TL;DR

This study investigates how sinusoidal density modulation affects stimulated Raman scattering in inhomogeneous plasmas, revealing conditions for inducing or suppressing SRS and explaining high reflectivity in experiments.

Contribution

It demonstrates that sinusoidal density modulation can induce absolute SRS even with low gain and offers strategies to control SRS through modulation wavelength and amplitude.

Findings

Sinusoidal modulation can induce absolute SRS below the Rosenbluth gain threshold.

Shorter wavelength modulation suppresses SRS inflation.

PIC simulations qualitatively agree with Vlasov simulation results.

Abstract

The influence of sinusoidal density modulation on the stimulated Raman scattering (SRS) reflectivity in inhomogeneous plasmas is studied by three-wave coupling equations, fully kinetic Vlasov simulations and particle in cell (PIC) simulations. Through the numerical solution of three-wave coupling equations, we find that the sinusoidal density modulation is capable of inducing absolute SRS even though the Rosenbluth gain is smaller than {\pi}, and we give a region of modulational wavelength and amplitude that the absolute SRS can be induced, which agrees with early studies. The average reflectivity obtained by Vlasov simulations has the same trend with the growth rate of absolute SRS obtained by three-wave equations. Instead of causing absolute instability, modulational wavelength shorter than a basic gain length is able to suppress the inflation of SRS through harmonic waves. And, the PIC simulations qualitatively agree with our Vlasov simulations. Our results offer an alternative explanation of high reflectivity at underdense plasma in experiments, which is due to long-wavelength modulation, and a potential method to suppress SRS by using the short-wavelength modulation.