Numerical Study on Robustness of HTS Coils With Current-Uncontrolled Property Against Electromagnetic and Thermal Disturbance

The high-temperature superconducting (HTS) coils introduced with no-insulation (NI) winding technology or persistent-current mode (PCM), will exhibit uncontrolled current variations under disturbances like AC fields and heat pulses, and the thermal robustness of these coils are ambiguous. Nevertheless, the coil type selection among insulated (INS), NI, PCM, and NI PCM coils is very common during practical designs. In this paper, the electromagnetic-thermal characteristics of the above four coil types are numerically studied and compared, by development of a polygon-anisotropic-resistivity (PAR) three-dimensional finite-element method model. Under transverse AC fields, the NI and NI PCM coils exhibit lower loss and far lower temperature rise, attributed to higher radial thermal conductivity and current bypassing of resistance-concentrated inner and outer turns. Under axial AC fields, the PCM coil generates induced shielding current, yet the NI and NI PCM coils exhibit even higher temperature rise, stemming from elevated superconducting loss caused by induced current concentration on the outermost turns allowed by turn-to-turn pathway, as well as excessive turn-to-turn metallic loss. Meanwhile, the NI PCM coil reduces the skin effect of induced current compared to the NI coil, and exhibit smaller temperature rise. Under a fast and concentrated heat disturbance, the NI and NI PCM coils exhibit substantial reduction of temperature rise compared to the INS and PCM coils. Negative screening current density is observed for NI, PCM, and NI PCM coils, especially on the turns that experience spiral current reduction. The results here provide examples and instructions of coil-type selection for designers.