Intergrade Joints for Nb3Sn cables of high magnetic field magnets

The objective of this thesis is the development of high-field and high-current joints between Nb3Sn cables for superconducting coils. The main fields of application are high energy physics (HEP) and thermonuclear fusion. In this thesis, the focus is on Wind&React (WR) HEP dipole magnets and React&Wind (RW) fusion magnets. These two have in common the design strategy of optimizing the superconductor quantity in the turns of the magnet according to the magnetic field intensity, with the main advantage of a cheaper and smaller coil. This technique is called magnet grading. As a consequence, the magnet is formed by different Nb3Sn cables that have to be connected electrically in series. The joints realize such connections and have to satisfy, in particular, electrical resistance requirements that allow to keep the superconductor temperature under the allowed limit. Nevertheless, the two considered fields of application have different magnet technology and cable, i.e. WR and Rutherford cables for HEP dipole magnets, whereas RW and Cable in Conduit Conductors (CICC) for fusion coils. For this reason, the joint development is split into two parts.In WR HEP magnets, the joint has to be integrated in the dipole head, which has a curved geometry. The first step was the identification of a suitable splicing technique. In this context, ultrasound welding was identified as potential splicing technique applicable before the magnet heat treatment, whereas diffusion-bonding during it and soldering after the coil reaction. These different solutions were experimentally investigated. Several prototypes were designed, electrically tested in SULTAN to assess their electrical resistance and analyzed through modelling, which helped in the interpretation of the obtained results. In the end, the objective is achieved and two different joints are developed, one based on diffusion-bonding and the other on soldering. The bent diffusion-bonded joint prototype has a resistance of 1.04 nΩ at B=10.9 T, T=5 K and ratio between current and critical current I/Ic=0.63. The soldered joint shows R=0.58 nΩ at B=10.9 T, T=5 K and I/Ic=0.54.In RW fusion magnets, opposed to WR coils, splicing can occur only after the heat treatment of the cable. Copper diffusion-bonding is proposed as jointing technique, since cleaner and potentially with higher mechanical strength than soldering. The cables to splice are first coated by copper thermal spray to increase the contact between the surfaces to splice. The proposed set-up takes into account the joint manufacture during in-line winding.A joint prototype was manufactured and tested in SULTAN, demonstrating that the electrical resistance fulfils the requirements (the measured resistance is R=0.48 nΩ at B=8 T, I=63.3 kA and T=5.1 K) and that the operability range of the joint, in terms of current and magnetic field, is wide (at least up to 10.9 T and 63.3 kA). The joint AC losses and behaviour under electromagnetic cyclic loading are illustrated, as such a splice would work in a pulsed fusion machine, the Tokamak. Metallographic analyses of the developed prototype were carried out to provide feedbacks on the joint manufacture.In both applications, i.e. WR HEP magnets and RW fusion coils, recommendations are given to the magnet designer for the implementation of the developed joints in a superconducting magnet.