Numerical analysis of over-banding effects on screening-current-induced strain and turn-to-turn contact in no-insulation REBCO pancake coils

In operational high-field magnets, no-insulation (NI) rare-earth barium copper oxide (REBCO) pancake coils are subjected to severe hoop tensile stresses generated by strong Lorentz forces. Moreover, screening currents intensify the non-uniform distribution of these stresses, which can further threaten the mechanical stability of NI coils and lead to inter-turn separation. Over-banding is an effective reinforcement method for NI coils. This paper presents a finite element analysis of the effects of over-banding on the screening-current-induced (SCI) hoop strain and turn-to-turn contact in NI coils, with the aim of providing practical guidance for the implementation of over-banding to enhance mechanical stability and preserve full turn-to-turn contact. The analysis demonstrates that increasing the number of over-banding turns reduces the maximum SCI hoop strain and the inter-turn separation in NI coils. However, both beneficial effects exhibit saturation characteristics. Merely increasing the number of over-banding turns has limitations, making it insufficient to achieve a full turn-to-turn contact. Therefore, applying sufficient winding tension to the over-banding tape is essential for ensuring full contact in practical NI REBCO pancake coils.