Diffusion-Bonding Between Strands and Modeling of Splices of Nb3Sn Rutherford Cables

Particle accelerators foresee the use of Nb3Sn in the next generation of dipole magnets. A common design strategy is to grade the coil, i.e., to optimize the quantity of superconductor in the turns with respect to the magnetic field intensity. As a consequence, the magnet winding consists of different Nb3Sn Rutherford cables that must be connected in series through splices having sufficiently low electrical resistance. In previous works, the Swiss Plasma Center designed and tested in the SULTAN facility bent diffusion-bonded splices for wind-and-react dipole magnets to develop an internal splice, i.e., to be embedded in the winding pack. The global splice resistance met the requirement. Such splices consist of hundreds of parallel resistive diffusion-bonded spots, located at the cross-over of the two overlapped Rutherford cables. In this manuscript, samples of single diffusion-bonded spots are manufactured and electrically tested to assess their resistance. The results are in line with the expectations, despite the scattering of the experimental data. Moreover, the network of strands and spots is recreated in an electrical model to interpret the resistance-field characteristic assessed from the measurements of splices between Nb3Sn Rutherford cables.