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For magnets requiring high uniformity, such as those used in MRI and NMR, shimming techniques are applied to correct the magnetic field distribution. These shimming techniques can be divided into two types: shim coils that are powered with current, referred to as active shimming, and iron shims that do not require current, referred to as passive shimming. In active shimming, superconducting shim coils are capable of generating stronger magnetic fields, thereby enhancing the correction effect. Low-temperature superconducting (LTS) NbTi wires are commonly used for this purpose. However, they have the drawback of requiring cryogenic cooling under 4 K and cannot operate in high-field magnets exceeding 10 T.
This study suggests a superconducting active shim coil using REBCO conductors as an alternative to NbTi. While previous research has reported the use of REBCO active shim coils for correcting axial magnetic field harmonics, there are few cases of implementing REBCO shim coils for transverse correction. In this study, a transverse shim coil using REBCO conductors was designed, followed by numerical analysis and liquid nitrogen experiments. The screening current generated in REBCO and their impact on the magnetic field distribution were analyzed, and the magnetic field strength was quantitatively compared at different operating current levels. This research provides insights into future magnetic field correction methods for high-field magnets and identifies the key factors necessary for installing such coils in real magnets.
