Ablative Energetics for High Gain Direct Drive on the National Ignition Facility

We present results from 1.1 MJ direct drive simulations and experiments on NIF where a large 5 mm diameter CH capsule directly driven at a low surface intensity of 250 TW/cm² produces high hydro-efficiency with very low coupling to either laser plasma instabilities (including cross-beam energy transfer (CBET)) and hot electrons. Even though the large capsule is driven using fully defocused (~32 mm) laser beams to achieve the largest laser spots, the laser beam spot to capsule diameter ratio remains small (~0.4). Having laser spots sizes smaller than the capsule diameter eliminates the need for CBET mitigation and increases the absorption of laser energy by the capsule above 95% as confirmed by scattered light diagnostics. By nesting a 1/3 size mid-Z Cr capsule concentrically inside the CH capsule, we can diagnose the residual kinetic energy in outer shell after laser turn-off (which occurs prior to the inter-shell collision) by measure the implosion trajectory of the inner shell using x-ray backlighting. Simulations using full laser drive over-predict the implosion convergence speed of both outer and inner shells. Artificially reducing the laser drive power (by ~25%) to non-physically force a match to the experimental outer shell implosion trajectory results in an inner shell implosion trajectory that is too slow. Only by removing 17% of the laser drive energy and re-depositing this energy as thermal energy into the outer half of the ablator shell can one simultaneously match both trajectories. We hypothesize that ablation pressure is being lost to tangentially directed hydrodynamic forces acting on a rippled ablation front. The impact for wetted-foam ignition designs is assessed.