Collective effect of stimulated Raman side scattering in multibeam processes and its competition with other instabilities relevant to direct-drive scheme

The laser–plasma instabilities are significant challenges remained in inertial confinement fusion experiments. In this paper, we focus on the multibeam nature of stimulated Raman side scattering (SRSS) and its competition with other instability observed in both experiments and numerical simulations. In the recent double-cone ignition (DCI) experiments with four laser beams irradiating a CH planar target, we have observed broadband scattering spectra, indicating the coexistence of SRSS, backward stimulated Raman scattering (BSRS), and two-plasmon decay (TPD). The multiple-beam incidence and symmetrically distributed detectors imply the collective effects of these instabilities. These observations have been reproduced via 2D and 3D particle-in-cell simulations under typical conditions of DCI experiments: single-beam intensity I∼0.6×1015 W/cm2, electron temperature Te∼2 keV, and density scale length at the quarter-critical density Lnc/4≈ 200–400 μm. The mechanism of collective SRSS relies on the common side-scattered light shared by other beams, so for one beam, it forms backward SRSS and forward SRSS for the other. The coexistence of the collectively excited SRSS, BSRS, and TPD is observed. Dependence of competition between these instabilities on the polarization and intensity is also discussed with parameters ranging from direct-drive scheme to shock ignition.