Overcurrent behaviour and dynamic contact resistance of a semi-insulation HTS coil via plasma-deposited film

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Abstract REBCO tapes are among the most promising high-temperature superconducting (HTS) materials for high-field applications. Their practical implementation typically involves wound coils, where the turn-to-turn insulation strategy significantly influences the electromagnetic response and self-protection behaviour. Conventional insulated coils provide well-defined current paths and fast magnetic-field responses, but they lack effective bypass-current channels. In contrast, no-insulation (NI) coils allow radial current bypass through turn-to-turn contacts, which improves self-protection by reducing local hot-spot growth. However, this radial current-sharing path also causes magnetic-field charging delay and makes the coil response sensitive to the turn-to-turn contact state. Therefore, a tunable turn-to-turn interface is expected to balance electromagnetic controllability and self-protection capability. In this study, we proposed HTS coils wound with atmospheric-pressure plasma jet (APPJ)-deposited film tapes. This plasma-deposited film may act as a semi-insulation turn-to-turn contact layer and introduces a condition-dependent effective contact resistance. During normal charging, the central magnetic field follows the transport current closely, indicating that radial current flow is suppressed. Under overcurrent tests, reproducible voltage disturbances and recoverable central-field responses are observed, suggesting a recoverable redistribution of transport current through radial contact paths in the plasma-film layer. Furthermore, a lumped magnetic-diffusion model was used to estimate the global effective contact-resistance scale, and a distributed radial-shunt model was then used to compare representative weak-contact distributions. The result demonstrates that spatially separated multi-site weak-contact regions are more consistent with the observed transient voltage spike and rapid recovery than a single weak region or a continuous radial channel. These findings suggest that the plasma film serves as a tunable semi-insulation turn-to-turn interface, providing a possible route to improve current distribution behaviour and self-protection capability in HTS coils.

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