Novel High-Entropy Alloys for Inertial Fusion Hohlraums*

The Superaum project is investigating ICF hohlraums made of high-entropy alloys, or arbitrary mixtures of several pure elements.  Benefits include 1) the “cocktail effect” [O. S. Jones et al., Phys. Plasmas 2007] where the Rosseland mean opacity and x-ray drive exceed the weighted average of the elements’; 2) “good” material properties like low oxidation, toxicity, and flaking; 3) high electrical resistivity for use in magnetized hohlraums [J. D. Moody et al., Phys. Rev. Lett. 2022].  We study the x-ray drive through 2D hohlraum modeling with the Lasnex radiation-hydrodynamic code, and a simple 0D hohlraum model based on J. H. Hammer and M. D. Rosen, Phys. Plasmas 2003.  The 0D model allows rapid exploration of a large parameter space to find compositions that maximize x-ray drive, and agrees reasonably with 2D simulations.  Hohlraums made of the “good” high-Z elements Hf, Ta, W, Re, Pt, Au and Bi can give substantially higher x-ray drives than pure Au and the AuTa4 alloy used for magnetized hohlraums.  We have not found any that match pure U.  Elements with higher atomic numbers than Au also improve drive by having lower specific heats.  Adding Th and U (with “bad” material properties) allows for many compositions that exceed pure U.  Rare-earth elements with “bad” properties may also improve the x-ray drive but require more target fabrication work.  We are developing integrated NIF ignition designs to exploit the high x-ray drive compositions, and assessing which ones are acceptable for hohlraum fabrication.