Design and simulation of X-ray backlit targets for z-pinch hohlraum symmetry experiments

Summary form only given, as follows. Capsule radiation symmetry is a crucial issue in the design of the z-pinch driven hohlraum approach to high-yield inertial confinement fusion. Capsule symmetry may be influenced by the power imbalance of the two z-pinch X-ray sources, and by hohlraum effects (geometry, time-dependent albedo, wall motion). 2-D and 3-D radiosity ("viewfactor") models have been used to design hohlraums that optimize the capsule radiation symmetry for experiments on Z as well as for high-yield targets. The goals of capsule symmetry experiments on Z include commissioning the Z-beamlet backlighter laser and requisite initial imaging detectors, measuring capsule flux asymmetry at the several percent level, and demonstrating understanding of the factors influencing capsule asymmetry in z-pinch hohlraums. Design and simulation efforts have focused on: (a) hohlraum radiation transport models to determine the sensitivity of capsule flux asymmetry to hohlraum geometry, top-bottom pinch power imbalance, and pinch mistiming, (a) 2-mm diameter, 60-/spl mu/m thick plastic shells designed to implode within 10 ns after the secondary hohlraum reaches its peak drive temperature of 75-85 eV, providing a suitable target for initial experiments at lower backlighting energies (2.5-3 keV), (b) 3- to 5-mm diameter thin (20-40 /spl mu/m) Ge-doped plastic shells designed for backlighting with Ti K-shell X-rays to assess symmetry early in the capsule implosion, and (c) uniform density foam spheres (/spl sim/50-100 mg/cc) also designed for Ti backlighting to image the trajectory of the shock/ablation front feature throughout the drive pulse. We present the results of postprocessed 1- and 2-D radiation-hydrodynamics calculations for each type of symmetry target, describing their advantages and disadvantages, and show comparisons with preliminary experimental data.