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 cur-rent 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 condi-tion 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 ac-celerator, 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, compro-mising thermal stability. In this study, we examined the ther-mal 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 pos-sibility of continuous operation and discovered conditions that allow for continuous operation.
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