Inter-Layer Joint of Nb3Sn React&Wind Cables for Fusion Magnets

The Swiss Plasma Center (SPC) has developed a layout of Toroidal Field (TF) coil for EUROfusion DEMO tokamak, based on the reference tokamak baseline of 2015. Each TF coil winding pack is wound with graded Nb3Sn conductors and consists of 12 single layers, connected in series by means of inter-layer graded joints. The design of inter-layer joints takes into account the react-and-wind(R&W) manufacturing technique for fabrication of TF coil winding pack, i.e., the inter-layer joint is prepared with use of two already heat treated Nb3Sn conductors. The development, preparation and test of inter-layer joint at SPC is performed in frame of R&D program for TF coil of EUROfusion DEMO tokamak. The high-grade Nb3Sn TF conductor, operating at 63.3 kA and 12.2 T (T-cs > 6.5 K) was tested at SPC, and afterwards was used for fabrication of inter-layer joint. The developed TF inter-layer joint is an "overlap-type" joint, which can be fit within the dimensions of TF winding pack. Each end of two conductors is copper cladded by a thermal-spraying technique and then bonded together along the surfaces of cladded copper with use of a high-frequency inductor. This paper describes the design of inter-layer joint, process of joint fabrication and test results obtained at SPC at the SULTAN test facility.