Review of Drive Symmetry Measurement and Control Experiments on the Nova Laser System

Symmetric radiation drive is required for achieving ignition in laboratory experiments. Over the last two years, a concerted series of drive symmetry experiments have been performed on the Nova laser system. The goals of this work were to develop measurement techniques and to apply them to symmetry variation and control experiments. The emphasis in this initial work has been on time integrated measurements (integrated over the laser drive pulse). We have also begun work on methods for time resolved measurements. Most of our work used the symmetry signature impressed on the compressed core of a capsule imploded in a hohlraum (cylindrical canister) environment. X-ray imaging of this core provides a mapping that can be compared with theoretical modeling and related to a specific amount of drive asymmetry. This method is indirect and we have taken great care in understanding the formation of the symmetry signature and in its comparison with simulations. A review of drive symmetry measurement and control experiments is presented, including data from time integrated and time resolved measurements; these measurements are also compared to modeling. Under carefully controlled conditions results from symmetry measurements (and from other auxiliary measurements) are reproducible, and indicate that aspects of implosions symmetry can be controlled.