ABSTRACT
Beam spray measurements suggest thresholds that are a factor of ≈2 to 15× less than expected based on the filamentation figure of merit often quoted in the literature. In this moderate-intensity regime, the relevant mechanism is forward stimulated Brillouin scattering. Both weak ion acoustic wave damping and thermal enhancement of ion acoustic waves contribute to the low thresholds. Forward stimulated Brillouin scattering imparts a redshift to the transmitted beam. Regarding the specific possibility of beam spray occurring outside the laser entrance holes of an indirectly driven hohlraum, this shift may be the most concerning feature owing to the high sensitivity of crossed-beam energy transfer to the interacting beam wavelengths in the subsequent overlap region.
ABSTRACT
Solutions to the radiation diffusion equation predict the absorbed energy ("wall loss") within an inertial confinement fusion (ICF) hohlraum. Comparing supersonic versus subsonic solutions suggests that a high metallic foam as hohlraum wall material will reduce hydrodynamic losses, and hence, net absorbed energy by . We derive an analytic expression for the optimal density (for any given drive temperature and pulse-length) that will achieve this reduction factor and which agrees well with numerical simulations. This approach can increase the coupling efficiency of indirectly driven ICF capsules.