Frequency chirp effects on stimulated Raman scattering in inhomogeneous plasmas

Abstract

Previous studies have shown that the use of laser bandwidth may mitigate the growth of stimulated Raman scattering (SRS) in laser plasma interaction experiments, in particular, when the spectrum of the driving (or pump) laser is composed of uniformly distributed frequency components with a well-chosen bandwidth [for example, Luo et al., Phys. Plasmas 29, 032102 (2022); Wen et al., ibid. 28, 042109 (2021); and Follett et al., ibid. 26, 062111 (2019)]. Here, we investigate the effects of frequency chirp in the pump laser on backward SRS in inhomogeneous plasmas, taking into account kinetic effects associated with the nonlinear detuning of the parametric resonance due to high-amplitude electron plasma waves (EPW). Through theoretical considerations and numerical simulations, using a multi-dimensional particle-in-cell (PIC) code, it is shown that positive frequency chirp rates lead to a displacement of the resonance in the plasma profile. For a sufficiently strong positive chirp rate, such that the resonance displacement is faster than the EPW group velocity, the EPWs prove to remain limited in amplitude such that SRS is suppressed. The required frequency chirp rate corresponds to a laser bandwidth of about 1%–2%.

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

“…Recently, it was observed that a negative frequency chirp rate can expand the phase-locking area and thus promote the development of the EPW and SRS. However, this phase-locking can be disrupted and the instability can be suppressed by a positive frequency chirp [37]. Similarly, using a rippled plasma density profile has also been shown to decrease the growth of the SRS instability [38][39][40].…”