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Description
In the development of high-temperature superconductor (HTS) pancake coils, the no-insulation (NI) is a well-known approach that enables self-protection by allowing radial current redistribution during thermal events. This method, while advantageous for mitigating damage during quench, presents significant challenges due to the low contact resistance, which causes a substantial reduction in the azimuthal current responsible for magnetic field generation. This extends the re-energization time after recovery and might also induce quench propagation in multi-coil NI magnet systems. In contrast, metal-insulated (MI) coils, which use a metallic layer as an intermediate insulator, offer higher transverse resistance. This configuration mitigates some of the drawbacks of NI coils by providing a balance between fully insulated and non-insulated designs. However, both NI and MI coils face limitations, particularly in achieving optimal performance during fast discharges and other dynamic operating conditions, where a uniform transverse resistance may not be ideal.
This work investigates the 'graded approach' to transverse resistance in HTS pancake coils. This method is designed to address the NI and MI coils limitations by varying the insulation level across different sections. Three pancake coils were realized and tested: an NI coil, an MI coil, and a graded coil with sectionally varied stainless steel (SS) tape layers to modulate transverse resistance. All coils were tested under the same conditions in liquid nitrogen, undergoing variable current ramps, fast discharges, and overcurrent-induced quenches. Multiple voltage taps and a Hall sensor were implemented in each coil to monitor their respective responses. Additionally, thermo-electrical simulations were used to model and analyze current distribution across the coil turns. The distinctive features of each coil configuration are analyzed through a comparison of the model and test results, highlighting the potential benefits of the graded approach in specific scenarios.
