The gold bubble feature seen in NIF ignition hohlraums and its 8-fold symmetry

At the National Ignition Facility (NIF), a fuel capsule is imploded by an x-ray drive created by 192 laser beams heating a gold hohlraum. The beams enter in openings at the ends of the hohlraum in four cone angles, the outer cones (50$^{\mathrm{o}}$ and 44.5$^{\mathrm{o}})$ and the inner cones (30$^{\mathrm{o}}$ and 23.5$^{\mathrm{o}})$. The region where the outer cones hit the hohlraum wall ablates wall material radially into the hohlraum, producing a gold ``bubble''. The region where the inner beams hit the hohlraum wall is prevented from ablating by material blowing off from the capsule. Recent ``ViewFactor'' experiments have used a truncated hohlraum (one side cut off at 40{\%} of standard length) to study hohlraum performance. Hard x-ray (\textgreater 3 keV) images taken from this open end show that the gold ``bubble'' has an 8 fold symmetry corresponding to the 50$^{\mathrm{o}}$ beam geometry. This different behavior of the 50$^{\mathrm{o\thinspace }}$and 44.5$^{\mathrm{o}}$ beams is actually visible in the hard x-ray images of the standard hohlraum targets. These latter images are used to study variations in the bubble as a function of laser pulse shape, gas fill, hohlraum length, and energy transfer.