Energy-Coupling Experiments Using Solid Spheres in the Polar-Direct-Drive Configuration on OMEGA

Energy-coupling experiments in inertial confinement fusion experiments use the measurement of a trajectory in the implosion to infer the efficiency of the drive from the ablation of the outer layers of the target. Tracking the shock trajectory in solid spheres offers the advantage of quantifying energy coupling without the potentially confusing effects of hydro-instabilities that occur in thin-shell implosions. Solid plastic spheres of ~340 μm radius were irradiated with 40 OMEGA beams in the polar-direct-drive geometry with ~14 kJ of energy at an intensity of ~8 × 10¹⁴ W/cm² to launch a spherically converging shock wave. A Fe foil target was driven by 11 beams, with ~4.5 kJ to backlight the shock onto an x-ray framing camera with 40-ps exposure time. The measured shock trajectories will be compared with 2-D radiation-hydrodynamic simulations. Similar experiments that were hydrodynamically scaled up have been performed at the National Ignition Facility to test the scaling arguments going from OMEGA to NIF energies.