Progress understanding how hohlraum foam-liners can be used to improve laser beam propagation through hohlraum plasmas*

The expansion of a laser-heated hohlraum wall can quickly fill the cavity and reduce or prevent propagation of other laser beams into the hohlraum. To delay such plasma filling, ignition hohlraums have typically used a high-density gas-fill or have been irradiated with a short (< 10 ns) laser pulse; the former can cause laser plasma instabilities (LPI), while a short laser pulse limits the design space required to reach symmetric implosions. Foam-liners are predicted to mitigate wall motion in a low gas-fill hohlraum, and so would enable the hohlraum to usefully drive a capsule over a longer duration. On the National Ignition Facility we have been engaged in two types of experiments to study foam-lined hohlraums. The first aims to radiograph the expansion of a foam-lined Au wall in a cylindrical geometry and, using simulation, infer the location of the 1/4 $n_{crit}$ surface. We observe that a 20 mg/cc Ta$_2$O$_5$ foam, 200 µm thick delays the expansion of Au hohlraum wall by 0.5 - 0.7 ns. The second type introduces a Ta$_2$O$_5$ foam-liner into a hohlraum and are designed to measure the effect of the foam-liner on capsule drive.