Measurement and calculation of NIF hohlraum wall albedos
Abstract
We present measurements of the relative and absolute albedos of hohlraum walls made of gold or mixtures of high-Z materials ("cocktails"). The relative albedo measurements were done at radiation temperatures ranging from 100 to 260 eV using gold halfraums with side-by-side patches of gold and cocktail material. The ratio of the spectrally resolved re-emission agrees with calculations using opacities obtained from the super transition array (STA) model, thereby providing corroboration of wall-loss estimates made with this model. The absolute albedo measurements were performed between 70 and 100eV which is the expected range of temperatures during the foot of an indirect-drive temporally-shaped ignition laser pulse. These measurements involved a double hohlraum system in which x-rays generated by a laser-heated gold (primary) hohlraum were used to heat a larger, attached (secondary) hohlraum to a radiation temperature of 70-100 eV. The ratio of the spectrally and temporally resolved radiation flux leaving the secondary hohlraum to that of the primary hohlraum was used to infer the albedo of the secondary hohlraum's wall material. We tested secondary hohlraums of various sizes and materials. We found that the gold and cocktail albedos agree with calculations using the STA opacity model and, in particular, the albedos are higher than we calculate with the XSN average atom model. This is significant because the original NIF laser power balance specifications are based on calculations that used XSN. A larger albedo during the foot of an ignition pulse results in a more symmetric radiation drive on the capsule for a given level of laser power imbalance, which means that there may be more margin for ignition than previously assumed.
- Publication:
-
APS Division of Plasma Physics Meeting Abstracts
- Pub Date:
- October 2003
- Bibcode:
- 2003APS..DPPFO2007J