Diagnosing Polar-Direct-Drive Energy Coupling at the National Ignition Facility

A series of experiments was performed at the National Ignition Facility (NIF) to diagnose energy coupling in polar direct drive using 2.1 mm dia. solid sphere targets directly driven by 184 laser beams using ramp pulses and varying the peak intensities between shots from 4 ´ 10¹⁴ W/cm² to 1.2 ´ 10¹⁵ W/cm². The evolution of the target was monitored by illuminating a Cu backlighter foil with the remaining eight beams, producing He_a emission to probe the system. These x rays were captured with a gated x-ray imager (GXI) to yield radiographs, which were analyzed to infer 2-D density profiles of the converging shock as a function of time. The resulting shock and ablation-front trajectories and inferred density profiles were then compared to 2-D radiation-hydrodynamic simulations from the DRACO code¹ using cross-beam energy transfer (CBET) and nonlocal heat-transport models. The results demonstrate slight deviations as the intensity varies and a more-pronounced disagreement in the ablation-front trajectory. An investigation into this discrepancy is ongoing with CBET, nonlocal heat transport, and electron and radiative preheat identified as potential causes. The results will enable the validation and development of a quantitative scaling between OMEGA and the NIF.