Stimulated Raman scattering in the degenerate regime

TL;DR

This paper investigates stimulated Raman scattering in plasma's degenerate regime, revealing unique electrostatic modes, electron acceleration, and implications for laser energy loss at high intensities through theoretical and simulation studies.

Contribution

It introduces a theoretical model for degenerate SRS, demonstrating its distinct electrostatic mode and validating findings with particle-in-cell simulations.

Findings

Degenerate SRS occurs at plasma density > 0.25n_c with high laser amplitude.

Electrostatic mode has a constant frequency around half of the incident light.

Degenerate SRS produces super hot electrons and causes significant laser energy loss.

Abstract

Stimulated Raman scattering (SRS) in plasma in the degenerate regime is studied theoretically and numerically. Different from normal SRS with the non-degenerate eigen electrostatic mode excited, the degenerate SRS is developed at plasma density $n_e>0.25n_c$ when the laser amplitude is larger than a certain threshold. To satisfy the phase-matching conditions of frequency and wavenumber, the excited electrostatic mode has a constant frequency around half of the incident light frequency $\omega_0/2$, which is no longer the non-degenerate eigenmode of electron plasma wave $\omega_{pe}$. Both the scattered light and the electrostatic wave are trapped in plasma with their group velocities being zero. Super hot electrons are produced by the degenerate electrostatic wave. Our theoretical model is validated by particle-in-cell simulations. The SRS driven in this degenerate regime is an important laser energy loss mechanism in the laser plasma interactions as long as the laser intensity is higher than $10^{15}$W/cm$^2$.