Progress in Coating Multi-Layered Cocktail Hohlraums

The process of making multi-layered depleted uranium (DU) and gold "cocktail" hohlraums is being developed in a sputter-coater designed and assembled at General Atomics. These elements have been chosen to increase the hohlraum wall albedo, targeting the composition that results in the highest hohlraum efficiency. Rather than co-sputtering the cocktail constituents as was done previously, the approach taken in this work is to sputter alternating multiple thin layers of DU and gold. The intended outcome of creating a multi-layered structure is to encapsulate the DU in gold, thus reducing or perhaps preventing rapid oxidation of uranium, a known problem in the co-sputtered materials. Residual stress in coatings has been reduced to sufficiently low levels by optimizing deposition pressure allowing fabrication of free-standing cylinders and foils. Characterization of this type of sandwich material is difficult due to the fact that the cocktail region consists of buried interfaces, though promising results from Auger depth profiling show that the materials have sufficiently low oxygen content (4 at. %) as well as the targeted composition.