Does Ion Screening Impact Inverse Bremsstrahlung Absorption?*

Inverse bremsstrahlung absorption (IBA) is the most important mechanism coupling the driver to the target in all mainline approaches to laser-driven inertial confinement fusion. Recent studies of IBA have shown that calculations using the exact quantum-mechanical free-free Gaunt factor derived from a binary-collision model of bremsstrahlung radiation well reproduce absorption measurements in the high-frequency (i.e., low density) limit, although thermal averaging of the Gaunt factor should be performed over the expected super-Gaussian electron distribution function rather than a Maxwellian distribution function to account for the Langdon effect. In the initial measurements of IBA through low-density gas-jet plasmas, a subset of the data using hydrogen gas at densities above 1% of the critical density, n_c, suggested a possible reduction in absorption due to ion screening—a decrease that also seems evident in classical molecular dynamics simulations of IBA. We will present recent experiments designed to provide more stringent tests of the screening physics, including additional hydrogen gas-jet experiments at higher density, as well as spherical solid-target experiments diagnosed using an instrument that compares the transmission along eight unique ray trajectories through the coronal plasma that reach different peak densities ranging from 0.14n_c to 0.96n_c. The results of these experiments should help bolster or refute the screening hypothesis.