Broadband Mitigation of the Multibeam Two-Plasmon--Decay and Stimulated Raman Scattering Instabilities*

Laser--plasma instabilities such as cross-beam energy transfer, stimulated Raman scattering (SRS), and two-plasmon--decay (TPD) present a major challenge for laser-driven inertial confinement fusion (ICF). Quantitatively predicting the severity of these instabilities requires a model that captures the complex, 3-D interaction of multiple laser beams, including effects such as speckle, polarization, and bandwidth. Here, we employ the laser plasma simulation environment (\textit{LPSE}) to investigate the multibeam nature and mitigation of these instabilities with broadband lasers for conditions relevant to direct-drive ICF. While multibeam coupling plays a critical role in both absolute TPD and SRS, the coupling for SRS is weaker. The threshold for both instabilities can be increased significantly by using drive lasers with \textasciitilde 1{\%} relative bandwidth. A broadband laser based on optical parametric amplification, with sufficient energy and bandwidth to validate these predictions, is currently in development at the Laboratory for Laser Energetics.