Session 7IA - Implosion and Hohlraum II.
INVITED session, Thursday morning, November 14
Grand Ballroom, Adam's Mark
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.