Preliminary Tests of Soldered and Diffusion-Bonded Splices Between Nb3Sn Rutherford Cables for Graded High-Field Accelerator Magnets

High-field accelerator magnets of the next generation will require coil grading in order to decrease the amount of superconductor and hence the magnet cost. The joints between Nb₃Sn conductor grades must have low electrical resistance (<;1 nΩ) and reasonably good mechanical strength. The joints are classified as “Wind & React” because they will be assembled during the coil winding, prior to the heat treatment. Various techniques have been initially considered for the preparation of joints between Nb₃Sn Rutherford cables, including soldering, diffusion bonding with and without interleaved copper foil, electroplated cables (to “smooth” the surface of the cable overlap), and crimped cables. Initial investigations included examinations of cross-sections (cut by electro-erosion). In a second stage, joint samples are prepared and assembled for test in the SULTAN test facility, with background field up to 11 T and operating current up to 14 kA. Besides the resistance test results, the selection of the most promising joint layout will also consider the restriction imposed by the coil manufacturing, e.g., available space for assembly, access after heat treatment, applicable quality control, etc.