Ablative energetics of large-capsule low-intensity direct drive implosions at the National Ignition Facility

Recent directly-driven double shell implosion experiments fielded at the National Ignition Facility indicate anomalous heating of the ablator shell resulting in decreased laser ablation pressure with respect to rad-hydro simulations for I~250 TW/cm2 at the capsule surface. Self-emission images during the 6.5 ns laser pulse combined with backlit radiographs of the inner shell near 17ns constrain both the ablation drive and collision driven physics of the double shell target. Additional scattered light data indicate that 2% or less of the 1.1 MJ laser drive energy is not absorbed by the target. Simulations with full laser drive over predict the implosion convergence speed of both outer and inner shells. Artificially reducing the laser drive power (by ~25%) to force a match to the experimental outer shell implosion trajectory results in an inner shell implosion trajectory that is too slow. Only by removing 16% of the laser drive energy and injecting this energy as thermal energy into the outer half of the ablator shell can one simultaneously match the trajectories of both shells. Neither relaxed flux limiting, enhanced electron thermal conduction nor nonlocal heat conduction provide a suitable match to the data. Detailed results and implications of these comparisons will be given.