Field Mapping and Automated Shimming of an HTS Magnet by “Internal” Active Shim Coils Located in the Bore of the Magnet

In order to obtain the target field homogeneity of a nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) magnet, commonly used are active shim coils, i.e., axial and radial, that typically locate radially outside of the magnet. In a high-temperature superconducting (HTS) magnet, however, we reported a “strong” hysteresis in the field-to-current ratio of an external (placed radially outside the HTS magnet) active shim coil by the screening-current-induced fields (SCFs). This nonlinear behavior of an external active shim due to the SCF is one of the major technical challenges to achieve the target field homogeneity of an HTS NMR or MRI magnet. In this paper, we constructed and operated active shim coils, i.e., two axial ( Z1 and Z2) and two radial (X and Y) , that were installed inside of an HTS magnet to investigate effectiveness of internal shim coils. The HTS magnet, wo…