Measurement of Stimulated Raman Side-Scattering Predominance in Directly Driven Experiment

TL;DR

This study demonstrates that stimulated Raman side-scattering (SRSS) is the dominant Raman process in direct-drive experiments, with broad angular emission causing significant energy loss, emphasizing the need for comprehensive angular measurements.

Contribution

The paper provides the first detailed experimental characterization of SRSS angular distribution and confirms its predominance through simulations, highlighting its impact on energy loss assessments.

Findings

SRSS emits at large angles over a broad azimuthal range.

SRSS accounts for about 5% of total laser energy loss.

Simulations confirm SRSS as the dominant Raman scattering process.

Abstract

Due to its particular geometry, stimulated Raman side-scattering (SRSS) drives scattered light emission at non-conventional directions, leading to scarce and complex experimental observations. Direct-irradiation campaigns at the SG-II UP facility have measured the scattered light driven by SRSS over a wide range of angles. It indicated an emission at large polar angles over a broad azimuthal range, sensitive to the plasma profile and laser polarization, resulting in a loss of about 5\% of the total laser energy. Direct comparison with back-scattering measurement has evidenced SRSS as the dominant Raman scattering process. The predominance of SRSS was confirmed by 2D particle-in-cell simulations, and its angular spread has been corroborated by ray-tracing simulations. The main implication is that a complete characterization of the SRS instability and an accurate measurement of the energy losses require the collection of the scattered light in a broad range of directions. Otherwise, spatially limited measurement could lead to an underestimation of the energetic importance of stimulated Raman scattering.