Joints for large superconducting conductors

Large fusion magnets call for high-current conductors (up to 60 kA). This has been achieved by the cable-in-conduit conductor concept. The connection of these conductors has to take into account several demanding boundary conditions: a large number of strands (around 1000), a low resistance at high current (1–2 nΩ), low losses in pulsed field operation, Nb3Sn heat treatment, helium tightness control, limited available space. A conceptual design was developed by the CEA, based on the connection of two separate twin boxes clamped together, according to the lap-joint concept. These boxes are machined out of an explosive bonding plate (jacket material/copper), and the electrical connection is achieved by tin–lead soldering of the facing copper soles. After qualification of the explosive bond and validation of the joint concept in the laboratory, the technology was transferred to the industry within the framework of the manufacture of the ITER Toroidal Field Model Coil (TFMC). In addition, three full-size joint samples (FSJS), relevant to different jacket materials and joining techniques, were manufactured by the industry and successfully tested in the SULTAN facility at CRPP, Villigen. The paper reports on the results of the laboratory tests, describes the transfer of technology to industry, and lastly presents some typical experimental results.