A study on an algorithm for finding magnetic center of an asymmetric HTS magnet with 3-axis mapper

High temperature superconducting (HTS) magnets are expected to play a prominent role in future nuclear magnetic resonance (NMR) systems because not only are they an essential item to surpass the current record of NMR frequency, 1 GHz, but also they may even enable an all-HTS NMR device that can be operated without a liquid helium cryogenic environment. The HTS magnets, however, introduce many new challenges as an NMR magnet, including the large high-order harmonic field errors originated from manufacturing errors of the HTS insert, particularly the one comprised of double-pancake (DP) coils. These harmonic errors are dependent on a choice of magnetic center, i.e., a magnetic center can be defined in a way to minimize the harmonic errors in an HTS NMR magnet. This paper proposes a new algorithm to automatically find a magnetic center of an HTS magnet to minimize the target harmonic components after a 'single' field mapping. With a field map given, field gradients are calculated by a harmonic analysis, and then a new magnetic center is determined, using the proposed algorithm, to minimize the target harmonic errors. These steps are recursively repeated until the 'best' magnetic center is defined. A test coil was constructed and 3-D field mapping tests were performed to verify the proposed algorithm. An in-house 3-D field mapping system was used for field mappings during the experiment.