Development of an Intra-Layer No-Insulation (LNI) REBCO Coil Implemented With a Resistance-Controlled (RC) Interface

The intra-layer no-insulation (LNI) method achieves self-protection characteristics against quenching in a layer-wound REBCO coil, owing to a current bypass function associated with the copper sheet inserted in each layer. The self-protection is strongly influenced by the electrical contact resistivity (ρ_ct) of the interface between conductors and copper sheets. In particular, a coil requires a high ρ_ct value, typically of the 100 mΩcm²-class. However, the ρ_ct value of a coil substantially changes due to external disturbances such as thermal cycles and electromagnetic forces. Therefore, it is vital to develop a technology to form an interface that provides a desired ρ_ct value and that is stable against external disturbances. In the present work, we developed a basic method to form a resistance-controlled (RC) interface with a layer thickness as thin as ∼30 μm, using epoxy resin mixed with conductive and insulating fillers, which respectively act as current bypassing paths and spacers. A certain value of the mixing ratio of the conductive fillers provides a target ρ_ct value from a wide range from 1 mΩcm² to 1,000 mΩcm². An RC interface-implemented LNI-REBCO coil exhibits a stable ρ_ct value under thermal cycles and self-protection against an overcurrent quench in liquid nitrogen.