Numerical Evaluation of the Transient Thermal Stability of No-Insulation Pancake Coils Wound With a REBCO-Coated Conductor With Some Defects

We have been developing large-diameter (m-class) no-insulation (NI) REBCO coil systems for applications in areas, such as medical cyclotrons and high-magnetic-field whole-body magnetic resonance imaging (MRI). NI coil technology can achieve both high thermal stability and current density, which have an in-verse relationship. Our previous studies revealed that the application of NI winding technology can allow a coil to continuously operate even when some parts of the coil windings are degraded. Accordingly, by applying the NI winding technology, the use of REBCO-coated conductors with defects or degradation, that is, coil production that allows for variations in conductor characteristics, can result in reduced costs. In this study, the transient thermal stability of NI pancake coils wound with a REBCO-coated conductor with one or more defects was analyzed and evaluated numerically. The conditions for stable operations in a coil with defects were clarified using turn-to-turn contact electrical resistance and I_OP/I_C ratios as parameters. In addition, we evaluated the possibility of establishing a monitoring technology to detect the state of health of the coil by observing changes in the voltage at both ends of the coil to see if any new deterioration has occurred.