Demonstration of implosion symmetry in NIF scale 0.7 Hohlraums

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 hohlraurn geometry. To diagnose the drive symmetry, previous experiments used simulated capsules by which the selfemission x-rays from gas in the center of capsule during the implosion are used to infer the shape of the drive. However, those experiments used hohlraurn radiation temperatures higher than 200 eV (Hauer et al., 2007, Murphy et aI., 1998a, Murphy et al., 1998b) with small NOVA scale hohlraurns under which conditions the symcaps produced large x-ray signals. During the foot of the NIF ignition pulse where controlling the symmetry has been shown to be crucial for obtaining a symmetric implosion (Clark et aI., 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 scale NIF Rev3.1 (Haan et al., 2008) vacuum hohlraurns 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.