Laser-Direct-Drive Energy Coupling at 4×10¹⁴ W/cm² to 1.2×10¹⁵ W/cm² from Spherical Solid Plastic Implosions at the National Ignition Facility

Laser-direct-drive energy-coupling experiments are being conducted at the National Ignition Facility (NIF) using a spherical, solid plastic target with a diameter of 2.1 mm. The targets are irradiated with 184 NIF laser beams in a polar-direct-drive (PDD) geometry. A series of shots was performed by varying the peak intensity in the range of 4×10¹⁴ W/cm² to 1.2×10¹⁵ W/cm² with a 7-ns-long shaped pulse. The trajectory of the spherically converging shock wave was recorded using ~100-ps gated, x-ray backlighting at 8.4 keV. In addition, the arrival time of the shock in the center of the sphere was measured from the x-ray flash created by the shock collapse with gated x-ray imagers. Solid spheres offer the advantage for quantifying energy coupling without the challenges from hydrodynamic instabilities of imploding shells. Good agreement is obtained of the measured shock trajectories with the trajectories from 2-D radiation-hydrodynamics simulations from the DRACO code using CBET and nonlocal heat-transport models. Similar experiments have been performed on OMEGA (configured for PDD) with an energy of 13 kJ to test the scaling arguments of PDD implosions from OMEGA to NIF energies.