Stimulated Brillouin Scattering of Outer Beams in Gas-filled Hohlraums

The goal of laser plasma instability (LPI) study in the indirect-drive inertial confinement (ICF) fusion is to minimize its uncertainty during the coupling of laser energy to capsule. Shenguang-III is built to operate at an energy of 180 kJ using 48 UV lasers. Initial 100kJ scale experiments using gas-filled hohlraums exhibits evident beam-to-beam backscatter difference even in the same cone. The focal spot as well as the SRS spectra of multiple beams are studied to elucidate whether the beam condition or the plasma condition might be distinct between different laser beams of the same cone. The SRS spectra suggest the beam-to-beam backscatter difference often correlates with the plasma condition in some shots. Thus a gas-filled hohlraum with one side LEH truncated is employed to study the beam propagation in large, hot and well-defined plasmas. The SRS and SBS backscatter is studied when the outer beam intensity is from 5×10¹⁴ W/cm² to 1.5×10¹⁵ W/cm² while keeping other heaters beams energy constant. The beneficial effects of beam smoothing, the competition of instabilities in large-scale plasmas, the influence of LEH size are also explored in combination with a rad-hydro and LPI simulation.