Electromagnetic-mechanical analysis of parallel-wound no-insulation HTS coils: current-imbalance-induced mechanical response

Abstract Parallel-wound no-insulation (PW-NI) high-temperature superconducting coils fabricated from stacked tapes significantly reduce charging delay while maintaining thermal stability. However, the PW-NI architecture leads to current imbalance, while the resulting mechanical response and the mechanism of screening-current-induced hoop stress specific to this configuration remain unclear. This study investigates the electromechanical behavior driven by current imbalance during charging and fast discharge, as well as the hoop strain induced by screening currents under a high background field. A multi-physics simulation model was developed by integrating the T–A formulation, an equivalent circuit network, and a discrete mechanical model. A three-tape PW-NI prototype coil was fabricated, and the electromagnetic component of the proposed model was validated through experiments conducted under various current ramping rates. The mechanical component was validated against experimental hoop strain measurements reported in the literature. Results indicate that non-uniform currents among PW tapes lead to hoop stress overshoots during charging. Under a high background field, the PW-NI coil induces localized screening current loops that concentrate near the edges of the stacked tapes, generating repulsive forces that widen the inter-turn gaps. During fast discharge, a large amount of coupling current is generated, shifting the repulsive forces to the intra-turn region and driving the expansion of intra-turn gaps. Furthermore, the effects of terminal resistance and contact resistivity on current imbalance and the resulting hoop stress were investigated. A semi-analytical expression was derived to rapidly estimate the maximum current difference between PW tapes, serving as an efficient surrogate model for parametric optimization. These results provide theoretical guidance for the design and application of PW-NI coils in large-scale magnets.