Mitigating ab-Plane Critical Current Density Inhomogeneity in Bulk HTS Rings for the Generation of NMR-Grade Magnetic Fields

(RE)-Ba-Cu-O ring bulks, used as trapped field magnets, show great promise as a compact and lightweight source for generating the magnetic fields required for nuclear magnetic resonance (NMR). The inhomogeneities introduced by the growth process of such bulks pose a challenge to the required field uniformity for NMR. In this study, we model a stack of ring bulks with varying
ab
-plane inhomogeneity and orientation to investigate the impact on the trapped field profile following field cooled magnetization. For a pair of rings, by offsetting the growth sector boundaries (GSBs), the inhomogeneity of the circumferential field at the mid-plane can be reduced by up to 65% compared with aligned GSBs, for less than 0.4% reduction in the central trapped field strength. Using two pairs of rings allows for the generation of a larger homogeneous region at the center of the bore, with a lower peak-peak variation across the measuring volume. The relative harmonic weighting of the field moves to higher orders, requiring more complex shimming coils to correct the residual field.