Demonstration of symcaps to measure implosion symmetry in the foot of the NIF scale 0.7 hohlraums

摘要

Implosions using inertial confinement fusion must be highly symmetric to achieve ignition on the National Ignition Facility. This requires precise control of the drive symmetry from the radiation incident on the ignition capsule. For indirect drive implosions, low mode residual perturbations in the drive are generated by the laser-heated hohlraum geometry. To diagnose the drive symmetry, previous experiments used simulated capsules by which the self-emission X-rays from gas in the center of the capsule during the implosion are used to infer the shape of the drive. However, those experiments used hohlraum radiation temperatures higher than 200 eV (Hauer et al., 1995; Murphy et al., 1998a, 1998b) with small NOVA scale hohlraums under which conditions the symcaps produced large X-ray signals. At the foot of the NIF ignition pulse, where controlling the symmetry has been shown to be crucial for obtaining a symmetric implosion (Clark et al., 2008), the radiation drive is much smaller, reducing the X-ray emission from the imploded capsule. For the first time, the feasibility of using symcaps to diagnose the radiation drive for low radiation temperatures, <120 eV and large 0.7 linear scales NIF Rev3.1 (Haan et al., 2008) vacuum hohlraums is demonstrated. Here we used experiments at the Omega laser facility to demonstrate and develop the symcap technique for tuning the symmetry of the NIF ignition capsule in the foot of the drive pulse.