NEW METHODS FOR DIAGNOSING AND CONTROLLING HOHLRAUM DRIVE ASYMMETRY ON NOVA

A novel method to control lowest-order(P-2) flux asymmetry in Nova cyl indrical hohlraums [E, M. Campbell er al., Rev. Sci. Instrum. 57, 2101 (1986)] with fixed laser beams is to use a pair of axial gold disks o f varying radii to partially block the capsule view of the laser-entra nce holes. Some advantages in using axial disks include the prospect f or added drive on target, the potential for P-4 control when used in t andem with laser pointing, and possibly reduced time-dependent P-2(t) flux asymmetry swings at early time. Neutron-based diagnostics have pr ovided some suggestion of increased drive, but a more direct measure o f drive enhancement is with the use of backlit, low-density (0.3 g/cc) foam surrogate targets. In this scheme, an ablatively driven, inwardl y propagating shock is imaged in time using backlighting from an irrad iated Ti disk placed outside of the hohlraum. The benefit in using low -density surrogate targets is an amplified shock speed that enables ea sier detection of both average shock motion (drive) and distortion (fl ux asymmetry. Experiments and calculations are in excellent agreement over a nearly 10% enhancement in peak drive temperature in the presenc e of axial gold disks. Measurements of lower-order distortion, P-2(t) and P-4(t), versus time for several laser pointings (without axial dis ks) using this technique have also been carried out and show good agre ement between experiment and simulations. Efforts to further control t ime-dependent flux asymmetry using multiple ring, beam-phasing techniq ues 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 P-2(t) is possible, Sign ificant control of time-integrated P-4 flux asymmetry with appreciable inner and outer ring separation also appears possible. (C) 1997 Ameri can Institute of Physics.