D. Abraimov, G. Bradford, J. Bang, L.D. Cooley, F. Kametani, D.C. Larbalestier, H. Mata, Y. Oz, V. Phifer
National High Magnetic Field Laboratory, Florida State University,
Keywords: fusion, superconducting magnets, superconductors, failure, testing
Summary:Designs for compact fusion reactors envision magnets operating near 20 T at temperatures around 20 K, a parameter space that cannot be filled by the conductor types used for ITER. Much emphasis has been given to using REBCO (Rare-Earth or yttrium barium cuprate) high-temperature superconducting tapes that are stacked, wrapped, or co-wound into cables and coils. Here, we describe relevant findings from research programs in the Applied Superconductivity Center and the National High Magnetic Field Laboratory where REBCO coils have been pushed to 45.5 T as test devices and 32 T as user magnets. We perform extensive characterizations of the conductor and cables incoming from manufacturers prior to coil winding and testing, including characterizations as a function of field angle and temperature up to 31 T. Coil testing is often intended to push toward stress and fatigue limits to uncover unknowns, and a comprehensive de-construction and re-characterization of conductors provides post-mortem information traceable back to the as-received conductors. In many regards, the depiction of REBCO as a smooth, rectangular, uniform monolith is an abstraction that can mislead engineers about the risks that come with the manufactured conductor, such as slitting defects, edge cracks, variations in properties due to variations in microstructure and nanostructure, copper plating difficulties, environmental effects, and cabling artifacts. Moreover, REBCO sourced from different manufacturers is not interchangeable for the purposes of our programs, and no manufacturer yet produces through-run and run-to-run uniformity like that of ITER conductors. We discuss how a publicly-funded program of substantial size can augment what is presently being achieved under INFUSE to improve manufacturing and the fusion conductor supply chain.