Abbas Nikroo | 19-FS-067
Current ablators for inertial confinement fusion (ICF) suffer from either finite grain structure, which may imprint on the densifying shocks, or low density, which leads to thicker shells and longer pulse lengths, making hohlraum symmetry control more challenging. Beryllium carbide offers another choice for the ablator material, with desirable characteristics in grain structure, density, and compressibility factor (Z). The microstructure approaches the highly uniform amorphous regime, with approximately 25-nanometer grain size material obtained via reactive sputter deposition. This low-Z material has densities reported between 1.9-2.4 grams/cubic centimeter.
In this feasibility study, we evaluated reactive sputter deposition of these films in an existing chamber, using beryllium sputter targets as the beryllium source and adding methane gas to allow the in situ reaction to process beryllium carbide. We were able to make a variety of films, including spheres, using various relative flows of methane gas to change film stoichiometry. Because of the small active area of the available system, the stoichiometry changed rapidly versus position. Therefore, various parts of the deposited beryllium carbide films were susceptible to in-situ stress and oxidation upon exposure to air, producing fine beryllium and beryllium hydride particles. Although the unexpected beryllium compound particle formation and resulting safety constraints prevented the planned metrology and fine tuning of the deposition technique, this work highlights the potential difficulty of using beryllium carbide as an ablator material for ICF.
This study supports Lawrence Livermore National Laboratory's core competencies in high-energy-density science and advanced materials and manufacturing, including complex fabrication processes that occur in inertial confinement fusion research.