Screening Current-Induced Turn-to-turn Contact Status in No-Insulation REBCO Pancake Coils: Effects of Winding Tension, Mandrel, and Overbanding

No-insulation (NI) rare-earth barium copper oxide (REBCO) high-temperature superconducting pancake coils, wound directly from REBCO coated conductor tapes without turn-to-turn insulation, allow current to radially bypass the spiral path via turn-to-turn contacts when the transport current exceeds the critical current. This radial current shunting effectively mitigates the quench risk. However, under Lorentz forces, the initial turn-to-turn contacts may separate, resulting in physical gaps between turns. In particular, the non-uniform distribution of screening currents leads to non-uniform deformation, which further promotes the separation. In this study, a two-dimensional axisymmetric finite element model (FEM) of an NI REBCO pancake coil was developed to investigate screening current-induced (SCI) turn-to-turn contact status. The effects of winding tension, mandrel thickness, and number of overbanding layers on the contact status were examined and discussed. Results show that increasing winding tension or mandrel thickness has only a limited effect on improving turn-to-turn contact in NI coils. In contrast, increasing the number of overbanding layers proves significantly more effective. Therefore, improving turn-to-turn contact in NI coils is best achieved by increasing the number of overbanding layers rather than relying on higher winding tension or greater mandrel thickness. In summary, this study presents the first comprehensive, multi-parameter analysis to evaluate and compare the effects of winding tension, mandrel thickness, and the number of overbanding layers on the contact status in NI REBCO pancake coils. Based on these findings, practical design guidelines are proposed for improving turn-to-turn contact.