Hohlraum symmetry measurements with surrogate solid targets (invited)

Time-resolved radiographic imaging of low density, solid spherical surrogate targets has been used to provide a time-dependent measurement of drive pressure symmetry in cylindrical hohlraums on both the Nova and Omega lasers. The experiments replace the usual capsule at the center of a gold hohlraum with a sphere of SiO2 foam (rho=0.3 g/cm(3)). The laser generates an x-ray drive inside the hohlraum which does not produce perfectly symmetric drive pressure on a spherical target, giving rise to a distorted shock traveling radially inward. The rarefaction behind the shock generated in this sphere produces a rapid rise in x-ray transmission which is easily detectable experimentally by radiography. The position of this feature may be determined to within a few microns in our experimental setup using a gated x-ray pinhole camera. Time-dependent control of drive symmetry in a hohlraum requires the ability to adjust the laser power as a function of both time and position along the hohlraum axis. We have implemented this control on the Omega laser by delaying sets of beams ("beam staggering'') and on the Nova laser ("beam phasing'') by delivering two independent pulse shapes on inner and outer halves of the Nova beams. The improvement in P-2 pressure symmetry with this control as measured with the foam ball technique was found to be as much as a factor of 3, compared with deliberately mistuned drives. On the National Ignition Facility (NIF), we expect to use this technique as part of the effort to tune drive symmetry to achieve higher gain. We present results from these experiments from both facilities and comparisons with LASNEX simulations. In addition, we discuss the application of the foam ball technique to NIF targets. (C) 1999 American Institute of Physics. [S0034-6748(99)68901-8].