Modeling stimulated Brillouin scattering in the underdense corona of a direct drive inertial confinement fusion target

Motivated by recent long scale-length single- and multiple-beam experiments conducted on LLE’s OMEGA laser system [W. Seka et al., Phys. Rev. Lett. 89, 175002 (2002)], we have simulated backward stimulated Brillouin scattering (SBS) in inhomogeneous direct drive plasmas where a critical surface is present at all times. This was achieved by adapting pF3D, a parallel, three-dimensional laser–plasma interaction code developed at the Lawrence Livermore National Laboratory [R. L. Berger et al., Phys. Plasmas 5, 4337 (1998)]. The shortcomings of the paraxial approximation, inherent in pF3D, were identified and circumvented via a practical approach that has led to an improved understanding of the experiments. The results show a qualitative agreement between the simulated and experimental SBS backscattered power reflectivity, with the conclusion that the time dependence of the SBS reflectivity is governed by target hydrodynamics and a quantitative agreement in the blue-shift in the frequency of reflected light. The simulations have also shown a sensitivity of the backscattered reflectivity to the amplitude of the specularly reflected signal of an “opposing” beam.