Multi-beam polarization mixing and its impact on other laser-plasma instabilities in ICF experiments at the National Ignition Facility

Wave mixing between lasers in plasma can lead to power exchange as well as polarization rotation of the overlapping beams. In ICF experiments at the National Ignition Facility (NIF), 96 laser beams with mm-spot-size overlap in a ~mm^3 plasma at the entrance of the “hohlraum” targets, leading to a complex evolution of each beam’s polarization state and power. Calculations including full polarization effects from individual NIF beams can show strong power imbalances between beams after leaving the overlap region, depending on the laser and plasma conditions. The beams calculated to have the highest intensity can subsequently trigger high levels of backscatter as they propagate further inside the target. In particular, stimulated Brillouin scattering (SBS) is of high concern as it is not only deleterious to implosion performance but can also damage optical components in the laser beam line. We will show calculations and experimental observations of high SBS caused by multi-beam interactions and discuss possible mitigation strategies.