New methods for diagnosing and controlling hohlraum drive asymmetry on Nova

Abstract

A novel method to control lowest-order (P2) flux asymmetry in Nova cylindrical hohlraums [E. M. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)] with fixed laser beams is to use a pair of axial gold disks of varying radii to partially block the capsule view of the laser-entrance holes. Some advantages in using axial disks include the prospect for added drive on target, the potential for P4 control when used in tandem with laser pointing, and possibly reduced time-dependent P2(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 is with the use of backlit, low-density (0.3 g/cc) foam surrogate targets. In this scheme, an ablatively driven, inwardly propagating shock is imaged in time using backlighting from an irradiated Ti disk placed outside of the hohlraum. The benefit in using low-density surrogate targets is an amplified shock speed that enables easier detection of both average shock motion (drive) and distortion (flux asymmetry). Experiments and calculations are in excellent agreement over a nearly 10% enhancement in peak drive temperature in the presence of axial gold disks. Measurements of lower-order distortion, P2(t) and P4(t), versus time for several laser pointings (without axial disks) 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 multiple ring, beam-phasing techniques on Nova, as will be required for the National Ignition Facility [J. Lindl, Phys. Plasmas 2, 3933 (1995)], are under development. Current designs indicate an appreciable reduction in P2(t) is possible. Significant control of time-integrated P4 flux asymmetry with appreciable inner and outer ring separation also appears possible.