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Description
No-Insulation (NI) coil utilizing HTS tape conductor has become a promising option for high-field magnets, capable of delivering both excellent thermal stability and high current density. NI coil can reduce heat generation due to a current behavior that bypasses the defect when a local hot-spot is formed in HTS tape conductor. In the high-field magnet application, a coil structure with large number of bundled HTS tape conductors without insulation between layers is utilized to achieve large current capacity. In recent years, there have been reports of burnouts during power supply shutdowns, even when NI coils were utilized. One of the courses of these burnouts can be a current concentration on particular turn-to-turn contact resistance. In this paper, the authors examined the thermal stability of NI coils with multi-bundled tape conductors under the open-circuit condition. Additionally, an effect of turn-to turn contact resistance distribution on current distribution and thermal stability of the multi-bundled NI coil is performed. This analysis can contribute to the NI coil design with high current capacity and thermal stability. When dealing with a NI coil composed of parallel HTS tapes, the current distribution within the coil becomes intricate. Therefore, the partial element equivalent circuit (PEEC) model is utilized. In this model, every turn within the coil and the corresponding loop section are subdivided and depicted as an electric circuit. This approach enables to conduct a comprehensive analysis on the current and temperature distribution of the NI coil, facilitating a detailed examination of electromagnetic phenomena. The calculation results of current behavior under the power supply shutdown revealed that the circumferential current concentrates in innermost and outermost layers, which results in heat generation in the NI coil. Also, local heat generation and maximum temperature rise due to the power supply shutdown tend to be larger when the number of bundled tape conductors increases. Therefore, the results of this paper contribute to the realization of a coil structure design that can guarantee high thermal stability of NI coil for the application of the high-field magnet.
