Reducing parametric backscattering by polarization rotation

TL;DR

This paper demonstrates that slow rotation of laser polarization significantly reduces backscattering in underdense plasmas, with simulations and analytical models showing a fivefold decrease in reflectivity, relevant for inertial confinement fusion.

Contribution

It introduces a novel polarization rotation technique to reduce parametric backscattering, supported by particle-in-cell simulations and fluid-model analytical estimates.

Findings

Polarization rotation reduces backscatter reflectivity by a factor of 5.

Analytical models agree with simulation results.

The method maintains constant laser intensity over time.

Abstract

When a laser passes through underdense plasmas, Raman and Brillouin Backscattering can reflect a substantial portion of the incident laser energy. This is a major loss mechanism, for example, in employing lasers in inertial confinement fusion. However, by slow rotation of the incident linear polarization, the overall reflectivity can be reduced significantly. Particle in cell simulations show that, for parameters similar to those of indirect drive fusion experiments, polarization rotation reduces the reflectivity by a factor of $5$. A general, fluid-model based, analytical estimation for the reflectivity reduction agrees with simulations. However, in identifying the source of the backscatter reduction, it is difficult to disentangle the rotating polarization from the frequency separation based approach used to engineer the beam's polarization. Although the backscatter reduction arises similarly to other approaches that employ frequency separation, in the case here, the intensity remains constant in time.