Identification and analysis of stimulated Raman side scattering in OMEGA EP experiments

Recent long-scale-length spherical target experiments, performed on the OMEGA EP laser, are analyzed for stimulated Raman side-scattering (SRSS). This work has been motivated by results obtained at the National Ignition Facility (NIF), relevant to directly-driven inertial confinement fusion (ICF)^1,2. These NIF experiments have shown SRSS to be an important process, possibly responsible for the observed hot electron generation. Our model, based on ray tracing, is able to explain the time-dependent scattered light spectra from the OMEGA EP experiments: It identifies SRS side-scatter and near backscatter from portions of each incident beam where the scattered electromagnetic is generated in the direction parallel to contours of constant density. These processes are shown to be the origin of the major spectral features observed in the experiment. The nature of SRSS instability (temporal versus spatial growth) is discussed. It is suggested that the OMEGA EP platform could provide a good surrogate in which to develop SRSS mitigation strategies.

[1] M. Rosenberg et al., Phys. Rev. Lett. 120, 055001 (2018).

[2] P. Michel et al., Phys. Rev. E99, 033203 (2019).