Electromagnetic-Induced Quench of an Inner-Shim Coil for a REBCO High-Temperature Superconducting NMR Magnet

A RE(rare earth)Ba2Cu3Ox (REBCO) high-temperature superconducting (HTS) coil is a promising candidate to achieve a nuclear magnetic resonance (NMR) magnet operated at a magnetic field >23.5 Tesla (T), i.e. 1 H NMR frequency of 1 GHz. We developed and tested a 400 MHz (9.39 T) NMR magnet using an REBCO inner coil. The system included a low-temperature superconducting (LTS) shim coil inside the REBCO coil to compensate the large inhomogeneity of the magnetic field. After the magnet was successfully charged, and while the compensation of the magnetic field inhomogeneity was being carried out, the magnet was unexpectedly discharged due to a failure of the chiller used for the DC power supply. During the discharge, the magnetic flux was transferred from the main coil to the inner shim coil, and that coil quenched repeatedly due to the high induced current. After confirming that the coil was not damaged, the magnet was charged again and it was found that several components of the magnetic field inhomogeneity had increased compared to the previous operation. This phenomenon is explained as follows: the induced-current to the shim coil generated a large magnetic field and it added an undesired screening current in the REBCO coil, lead to the increase in the magnetic field inhomogeneity. The results suggest that a system to suppress the induced current is a requirement for a safe magnet operation and homogeneous magnetic field.