Influence of the Turn-to-Turn Contact Electrical Resistance on the Thermal Stability in Meter-Class No-Insulation REBCO Pancake Coils During a Local Normal-State Transition

In magnetic resonance imaging and nuclear magnetic resonance applications, it is important that the high-temperature superconducting coils provide a high magnetic field in a miniaturized form factor. Our research group considers these requirements to be compatible with no-insulation (NI) coils in which the thickness of the Cu stabilization layer and the turn-to-turn contact electrical resistance are key parameters that enable both high current densities and high thermal stabilities. When the Cu stabilization layer is thin, the current density is impaired, but the thermal stability is improved. If the turn-to-turn contact electrical resistance is very large, then the characteristics of the NI REBCO coil approach the characteristics of conventional insulated coils, and the thermal stability may be degraded. However, when the turn-to-turn contact electrical resistance is large, the excitation delay time is reduced. In this study, we used a combination of the partial element equivalent circuit method and the finite element method to perform a current distribution and thermal analysis of the behavior of NI REBCO coils when a local normal-state transition occurs. We also considered the appropriate turn-to-turn contact electrical resistance that could shorten the excitation delay time as much as possible while still suppressing the formation of hotspots, which is a disadvantage of the NI REBCO coil.