X-ray conversion efficiency in vacuum hohlraum experiments at the National Ignition Facility

X-ray fluxes measured in the first 96 and 192 beam vacuum hohlraum experiments at the National Ignition Facility (NIF) were significantly higher than predicted by computational simulations employing XSN average atom atomic physics and highly flux-limited electron heat conduction. For agreement with experimental data, it was found that the coronal plasma emissivity must be simulated with a detailed configuration accounting model that accounts for x-ray emission involving all of the significant ionization states. It was also found that an electron heat conduction flux limit of f = 0.05 is too restrictive, and that a flux limit of f = 0.15 results in a much better match with the NIF vacuum hohlraum experimental data. The combination of increased plasma emissivity and increased electron heat conduction in this new high flux hohlraum model results in a reduction in coronal plasma energy and, hence, an explanation for the high (similar to 85%-90%) x-ray conversion efficiencies observed in the 235 < T-r < 345 eV NIF vacuum hohlraum experiments. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4704795]