Beam Spray Thresholds in ICF-Relevant Plasmas
D. Turnbull, J. Katz, D. E. Hinkel, P. Michel, T. Chapman, L. Divol, Eugene Kur, Steve MacLaren, A. L. Milder, M. D. Rosen, A. Shvydky, G. B. Zimmerman, D. H. Froula
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.