Measurement of stimulated Raman side-scattering predominance in directly driven experiment

Abstract

Due to its particular geometry, stimulated Raman side-scattering (SRSS) drives scattered light emission in non-usually diagnosed directions, leading to scarce and complex experimental observations. Direct-irradiation campaigns at the SG-II Upgrade facility have measured the scattered light driven by SRSS over a wide range of angles. Typical interaction conditions were as follows: an overlapped laser intensity of 1.2×1015 W cm−2 propagated into a plasma with a density scale length Lnc/4≈250 μm and an electron temperature Te≈2.2 keV. It indicated an emission at large polar angles over a broad azimuthal range, sensitive to the plasma profile, resulting in a loss of about 5% of the total laser energy. Direct comparison with back-scattering measurement, both in the full-aperture back-scattered direction and sampled at smaller polar angles in the same azimuthal plane, has evidenced SRSS as the dominant Raman scattering process. The predominance of SRSS was confirmed by two-dimensional 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.