Measurements of X-ray Pre-Heat in NIF Double Shell Capsules*

Hohlraum generated X-rays penetrate the ablator of a double shell capsule and are absorbed in the inner surface of both the ablator and central capsule. Los Alamos has executed two NIF shots to study the impact of preheat.  Both used VISAR in a keyhole geometry to measure the motion of one inner surface. The first of these observed the motion on the inner surface of a beryllium ablator.  Sensitive to the hohlraum gold M-band (~>2keV) penetrating the ablator, the expansion of both pole and equator agreed with HYDRA calculations.   The second experiment observed the inner surface of a tungsten central shell.  Here the calculated velocities, due to harder gold L-band X-rays (~ 9-40 keV) were a factor of several too high.  This expansion responds to ~15-20 keV X-rays, a spectral region calculated without strong lines in the gold L-band.  The entire gold L-band emission or just in this spectral region may be calculated too high.