Effect of chirp on stimulated-forward Raman scattering in a magnetised plasma with rippled density for fusion applications

Abstract

Stimulated Raman scattering (SRS) is one of the mechanisms limiting power scaling in inertial confinement fusion (ICF). In this work, we demonstrate the effective suppression of SRS by the combined effects of static density fluctuations and an azimuthal magnetic field with a propagating chirped laser pulse. In the presence of an azimuthal magnetic field, chirped laser pulse propagates through a density-rippled plasma and undergoes stimulated-forward Raman scattering (SFRS), resulting in two radially localized electromagnetic sidebands waves and a lower-hybrid wave. Absolute and growing modes saturate due to ion density fluctuations, which then suppress instability growth through mode coupling. The modes modified by the combined effect of chirp and azimuthal magnetic field are effectively damped after saturation. As a result, the overall growth rate of the instability reduces. The comparison of positive and negative chirp demonstrated that when a positive chirp is being used, instability is more effectively suppressed. Based on non-local theory, we have analyzed the growth of the SFRS for positive and negative chirp and estimated it for ICF-relevant parameters and observed the effect of the growth rate.