The Raman gap and collisional absorption

TL;DR

This paper investigates the long-standing Raman gap in laser-plasma experiments, proposing that collisional damping causes the gap, supported by linear analysis, PIC simulations, and experimental comparisons.

Contribution

It introduces a linear theory linking collisional damping to the Raman gap and validates it with PIC simulations and experimental data.

Findings

The Raman gap can be explained by collisional damping effects.

The theory predicts the gap's width based on plasma parameters.

Simulation results agree with the theoretical predictions.

Abstract

One of the long-standing puzzles observed in many laser-plasma experiments is the gap in the Raman backscattering spectrum. This gap is characterized by the absence of backscattered light between some critical wavelength and twice the incident laser wavelength. The latter is associated with the absolute Raman instability from the quarter-critical density surface. Supported by particle-in-cell (PIC) simulations, it is suggested that the gap can result from the collisional damping of the backscattered light. A linear analysis of the competition between the Raman growth rate and the damping rate in a non-homogenous plasma predicts the gap's existence and width as a function of the system's parameters. The theory is compared with the PIC simulations and past experiments.