Numerical Evaluation of Transient Thermal Stability of No-Insulation REBCO Pancake Coils With a Noncontact Area Between Turns

We have been developing a no-insulation (NI) REBCO coil for applications in medical cyclotrons for cancer therapy and high-magnetic-field whole-body MRIs. NI-REBCO pancake coil is expected to realize both high current density and thermal stability, which are essentially trade-off relationships. Because electrical insulation is absent in an NI coil, the operating current can bypass toward adjacent turns to avoid the defect area when a local defect occurs. Using this feature, we investigated the possibility of continuous operation even if there is a local defect in a part of the coil windings. However, the contact condition between turns in actual coil windings is not uniform, and there is a possibility of an area that is not completely in contact. Moreover, when winding noncircular coils, e.g., coils for an accelerator, the noncontact area between turns can be large since the straight area is not tensioned. In such a case, even if a local defect occurs, it cannot bypass toward adjacent turns, compromising thermal stability. In this study, we examined the thermal stability of NI-REBCO pancake coils when a local defect occurred using the length, number, and location of noncontact areas between turns as parameters. We also examined the possibility of continuous operation and explored conditions that allow for continuous operation.