Process Developments in the Fabrication of Depleted Uranium Hohlraums

Research at General Atomics and Lawrence Livermore National Laboratory has been focused on evaluating depleted uranium (DU) hohlraum fabrication over the past 10 years to improve the yield, thereby increasing the availability of DU hohlruams required to support the increased shot rate at the National Ignition Facility. The more straightforward gold (Au) hohlraum fabrication involves four basic steps: mandrel fabrication, electroplating, back machining and milling, and leaching. For Au, the overall fabrication yield of this process approaches 98% [H. Streckert and K. Blobaum, Fusion Sci. Technol., Vol. 63, p. 213 (2013)] Depleted uranium lined hohlraum fabrication, however, requires deposition of a multilayer of thin films after the mandrel fabrication step. These thin film deposition processes have historically proven difficult to execute on a complex cylindrical geometry of a hohlraum, resulting in unacceptable stress-driven delamination, with net yields ranging 20% to 35% [H. L. Wilkens et al., Phys. Plasmas, Vol. 14, 056310 (2007)]. Recent hohlraum design and fabrication process changes, as well as material selections implemented between 2014 and 2016, have improved the fabrication yield to over 60%. These changes are discussed here as well as plans for future improvements.