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Description
No-insulation (NI) coils are known for their high thermal stability and self-protecting features due to turn-to-turn contacts. Parallel co-winding is a promising method to reduce the charging delay of NI coils while maintaining thermal stability. This technique shows significant potential for applications in fusion and other large-scale or high-field magnets. In parallel co-wound NI coils, the potential non-uniform current distribution among parallel superconducting tapes may lead to thermal and mechanical problems. We conducted experiments on small parallel co-wound NI REBCO coils to investigate non-uniform current distribution. The parallel tapes in the input and output sections of the tested coils were separated. A series of Rogowski coils was used to measure the current in each tape. The voltage at different turns within the coils was also measured. In the tested 2-tape and 4-tape co-wound coils, the maximum currents in the parallel tapes during the charging process were 105 A and 60.6 A, respectively, while the average current was 30 A. After the current stabilized, significant non-uniform current distribution remained, with maximum currents of 40.6 A and 59.9 A, respectively. We combined an FEM model based on the T-A formulation with an equivalent circuit model to calculate the current distribution in co-wound coils. The calculated current and voltage results, using the experimental parameters, are consistent with the ramping, sudden discharging, and over-current test results. The calculations also indicated that the current distribution within the coil was also highly non-uniform, primarily influenced by the joint resistance between the superconducting tapes and the terminals. The influence of joint resistance, contact resistance, and other coil parameters on the current distribution was analyzed based on the calculation model and experimental results. Furthermore, the effect of current distribution on local strains considering the screening current effect in high-field magnets was also discussed. This study may contribute to the future development of magnets utilizing parallel co-wound NI coils.
