3-D field mapping and active shimming of a screening-current-induced field in an HTS coil using harmonic analysis for high-resolution NMR magnets

A screening-current-induced field (SCF) in a superconducting magnet is now considered as one of the major technical challenges for HTS (high-temperature superconducting) magnets, particularly wound with tape-shape conductors such as Bi2223 and YBCO, to be used for NMR (nuclear magnetic resonance) applications where a ppb (part-per-billion) level of field homogeneity is required. To date, many attempts to characterize the SCF and minimize its impact on field homogeneity have been undertaken, but it is still one of bottlenecks for HTS NMR magnets. In general, magnetic fields generated by an NMR magnet are expressed by the spatial harmonics in order to systematically characterize the field errors and obtain a target field homogeneity by shimming. Thus, it is important to analyze the spatial harmonics of the SCF in an HTS magnet for its application to a high-resolution NMR device. In this paper, magnetic fields of an HTS coil were measured within a target space using an in-house 3-D field mapper, and their spatial harmonics were obtained using associated Legendre equations. Also, the temporal behavior of the SCF was investigated by continuous field mappings and their analyses. Finally, a radial shim coil, ZY, was designed, constructed, and operated with the HTS test coil, and the hysteresis and the temporal behavior of the SCF during shimming were also studied. With a shim coil current, the ZY gradient of the SCF was reduced by 70%.