Modeling experiments that probe beam spray thresholds in ICF-relevant plasmas

Recent experiments at the Omega Laser Facility [D. Turnbull et al., Phys. Rev. Lett. 129, 025001 (2022)] have carefully studied the spraying of laser light as it propagates through underdense plasmas of relevance to inertial confinement fusion experiments. In these experiments, frequency redshits of the transmitted light were measured that exceed estimates based on the Dewandre shift caused by the plasma expansion alone. Such redshifts are a signature of forward Brillouin scattering. Here, these experiments are modeled using three-dimensional simulations [pF3D, R. L. Berger et al., Phys. Plasmas 5, 4337 (1998)] that describe the necessary absorption, backscattering, filamentation, and forward scattering of the laser light. The phase plate of the experimental beam is reproduced numerically, allowing for a direct comparison with the experimentally measured transmitted beam spot and frequency. The impact of various bandwidth-based mitigation techniques is explored in anticipation of further upcoming experiments. Simulations using the fully kinetic Vlasov-Fokker-Planck code K2 [M. Sherlock et al., Phys. Plasmas 24, 082706 (2017)] are also shown.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344 and funded by LDRD 22-ERD-042.