The Rayleigh-Taylor instability is an important limitation in ICF capsule designs. Significant work both theoretically and experimentally has been done to demonstrate the stabilizing effects due to material flow through the unstable region. The experimental verification has been done predominantly in planar geometry. Convergent geometry introduces effects not present in planar geometry such as shell thickening and accelerationless growth of modal amplitudes (e.g.\ Bell-Plesset growth). Amplitude thresholds for the nonlinear regime are reduced, since the wavelength of a mode m decreases with convergence \lambda \sim r/m, where r is the radius. We have investigated convergent effects using an imploding cylinder driven by x-ray ablation on the NOVA laser. By doping sections of the cylinder with high-Z materials, in conjunction with x-ray backlighting, we have measured the growth and feedthrough of the perturbations from the ablation front to the inner surface of the cylinder for various initial modes and amplitudes from early time through stagnation. Mode coupling of illumination asymmetries with material perturbations is observed, as well as phase reversal of the perturbations from near the ablation front to the inner surface of the cylinder. Imaging is performed with an x-ray pinhole camera coupled to a gated microchannel plate detector. \par
In collaboration with C. W. Barnes, J. B. Beck, N. Hoffman (LANL), D. Galmiche, A. Richard (CEA/L-V), J. Edwards, P. Graham, B. Thomas (AWE). \par
^**This work was performed under the auspices of the U.S. Department of Energy by the Los Alamos National Laboratory under Contract No. W-7405-ENG-36.
[7IA.02] New Methods for Controlling and Diagnosing Hohlraum Drive Asymmetry on Nova
Peter Amendt (Lawrence Livermore National Laboratory)
Efforts are underway to diagnose and control x-ray drive asymmetry in Nova hohlraums. Controlling time-integrated, lowest-order P_2 flux asymmetry on target is conventionally accomplished by displacing the laser beams outward or inward along the hohlraum symmetry axis. A novel method to control P_2 with fixed laser beams is to use a pair of axial gold discs of varying radii to partially block the laser-entrance-hole (LEH) from the view of the capsule. Some advantages in using axial discs include the prospect for added drive on target, the potential for P_4 control when used in tandem with laser pointing, and possibly reduced time-dependent P_2(t) flux asymmetry swings at early time. Neutron-based diagnostics have provided some suggestion of increased drive, but a more direct measure of drive enhancement on target is with use of backlit, low-density (0.3 g/cc) foam balls. In this scheme, the ablatively-driven, inwardly propagating shock is imaged in time using backlighting from an irradiated Ti disc placed outside of the hohlraum. The benefit in using low-density surrogate targets is an amplified shock motion which enables easier detection of both average shock motion (drive) and distortion (flux asymmetry). Experiments and calculations are in excellent agreement over a nearly 20 eV enhancement in peak drive in the presence of axial gold discs. Measurements of lower-order distortion, P_2(t) and P_4(t), versus time for several laser pointings using this technique have also been carried out and show good agreement between experiment and simulations. Efforts to further control time-dependent flux asymmetry using beam-phasing techniques on Nova, as will be required on the National Ignition Facility, are under development. Current designs indicate a nearly factor of three reduction in P_2(t) variations and significant control of time-integrated P_4 flux asymmetry with modest cone separation.