Homogeneity Study for the NHMFL Series Connected Hybrid Magnet System

The first Series Connected Hybrid (SCH) Magnet presently being designed and built at the NHMFL has targeted 36 T with 1 ppm maximum inhomogeneity over a 10-mm-diameter spherical volume at the midplane of a 40-mm clear bore for scientific users interested in medium resolution NMR studies. The SCH comprises a 13T superconducting outsert built with Nb₃Sn cable-in-conduit conductors, a 23 T resistive insert using Florida-Bitter technology, and water-cooled resistive shims providing up to 90 ppm correction. The insert and outsert are connected electrically in series. Inhomogeneity predictions for the unshimmed SCH were made using various finite element analyses on a preliminary SCH design. Manufacturing tolerances were benchmarked by performing a sensitivity analysis and comparing the finite element analyses results to inhomogeneity measurements on the Keck and Cell 2 magnets, two NHMFL magnets, which represent the state-of-the-art for producing high homogeneity fields with resistive Florida-Bitter technology. Next, the shim terms from the measured and predicted 3-D field maps were analyzed with an NHMFL in-house code, which provided the inhomogeneity specification of the unshimmed SCH. Finally, a design of the SCH water-cooled resistive shims was developed based on all the measured and calculated data including additional engineering safety factors. This analysis is an essential step for achieving the targeted overall performance of a record 1 ppm inhomogeneity at 36 T.