Stimulated scattering in laser driven fusion and high energy density physics experiments

Abstract

In laser driven fusion and high energy density physics experiments, one often encounters a kλD range of 0.15 < kλD < 0.5, where stimulated Raman scattering (SRS) is active (k is the initial electron plasma wave number and λD is the Debye length). Using particle-in-cell simulations, the SRS reflectivity is found to scale as ∼ (kλD)−4 for kλD ≳ 0.3 where electron trapping effects dominate SRS saturation; the reflectivity scaling deviates from the above for kλD < 0.3 when Langmuir decay instability (LDI) is present. The SRS risk is shown to be highest for kλD between 0.2 and 0.3. SRS re-scattering processes are found to be unimportant under conditions relevant to ignition experiments at the National Ignition Facility (NIF). Large-scale simulations of the hohlraum plasma show that the SRS wavelength spectrum peaks below 600 nm, consistent with most measured NIF spectra, and that nonlinear trapping in the presence of plasma gradients determines the SRS spectral peak. Collisional effects on SRS, stimulated Brillouin scattering (SBS), LDI, and re-scatter, together with three dimensional effects, are examined. Effects of collisions are found to include de-trapping as well as cross-speckle electron temperature variation from collisional heating, the latter of which reduces gain, introduces a positive frequency shift that counters the trapping-induced negative frequency shift, and affects SRS and SBS saturation. Bowing and breakup of ion-acoustic wavefronts saturate SBS and cause a dramatic, sharp decrease in SBS reflectivity. Mitigation of SRS and SBS in the strongly nonlinear trapping regime is discussed.