Optimizing foam-liner performance to improve laser beam propagation in hohlraums

Hohlraum wall expansion in low gas-fill hohlraums can reduce or prevent propagation of other laser beams into the hohlraum. To avoid such filling ignition hohlraums have typically been filled with a high density gas or irradiated with a short (< 10 ns) laser pulse. Foam-liners are predicted to mitigate wall motion in a low gas-fill hohlraum with little LPI, and so would enable the hohlraum to symmetrically drive a larger capsule over a longer duration providing a pathway to higher yield.

On the National Ignition Facility we have been engaged in a a series of experiments to study foam-lined hohlraum expansion. Through radiography of the expanding foam-lined Au wall in a cylindrical geometry we can compare the performance of foams of different material, density and thickness, using simulations to estimate the location of the ¼ n_crit surface. We will discuss results from experiments that compare Au and Ta₂O₅ foams at densities between 10 and 40mg/cc. These demonstrate how the interplay between the expanding foam plasma and the shock reflected by the hohlraum wall is critical in optimizing the foam-liner parameters. LLNL-ABS-753541