Inhomogeneous Copper Diffusion in NIF Beryllium Ablator Capsules

The National Ignition Facility point design uses a five-layer capsule to modify the X-ray absorption in order to achieve optimized shock timing. A stepwise copper dopant design defines the layer structure; however, the as-deposited Cu distribution is significantly altered during the CH mandrel removal by pyrolysis. The changes are significant: (a) Cu diffuses on average several microns, a distance more than an order of magnitude larger than predicted from the bulk diffusion data, and (b) the Cu distribution, as a result of diffusion, is highly heterogeneous, introducing a local variation of [approximately]0.06 at. % near the original layer interface. In this study, we developed quantitative techniques to measure Cu diffusion and explored its correlation to beryllium microstructures. Plausible diffusion mechanisms and mitigation methods will be discussed. These findings will enable more accurate evaluation of the expected target performance.