National Ignition Facility Polar-Drive Exploding-Pusher Experiments Improving Performance via Imprint Mitigation

Exploding pushers (XP’s) produced the highest polar-direct-drive (PDD) DT yield (~1.1x10¹⁶) on the National Ignition Facility (NIF) using a ~1.1-MJ pulse illuminating a ~4-mm-diam, 25-mm CH target. Development continues to improve DT yield with an ultimate goal >2x10¹⁶. These NIF PDD-XP targets provide a high-yield neutron source as well as a platform to develop predictive inertial confinement fusion modeling using the 2-D code DRACO. Recent NIF PDD-XP experiments focused on a smaller 3.4-mm-diam target using a DD fill as a surrogate target to keep the illumination intensity similar to the 4-mm-diam targets while increasing shot throughput with its modest facility impact. Shell morphology was improved as predicted by DRACO but the yield was not affected unless the deleterious imprint was included. In addition, the NIF PDD-XP experiments across the larger target diameters (>3 mm) with either DT or DD fills have indicated sensitivity to imprint. Potential yield degradation sources guided the design of subsequent experiments at 3.4-mm scale to assess the impact of imprint mitigation via pulse-shape control as one method to improve performance while improving predictive code capability. The 3.4-mm-scale target results are presented here.