Inductive Soldering of the Junctions of the Main Superconducting Busbars of the LHC

The Large Hadron Collider (LHC) is the next world-facility for the high energy physics community, presently under construction at CERN, Geneva. The LHC will bring into collisions intense beams of protons and ions. The main components of the LHC are the twin-aperture high-field superconducting cryomagnets that will be installed in the existing 26.7-km long tunnel. They are powered in series by superconducting Nb-Ti cables. Along the machine, about 60 000 joints between superconducting cables must be realised in-situ during the installation. Ten thousands of them, rated at 13 000 A, are involved in the powering scheme of the main dipoles and quadrupoles. To meet the requirements of the cryogenic budget, an electrical resistance at operating temperature (1.9 K) lower than 0.6 nΩ has to be achieved. The induction soldering technology was selected for this purpose. After a brief introduction to the LHC project, the constraints and requirements are listed. Then, the applied solution is detailed. The splices of the superconducting cables (Rutherford type) form a multi-interface "composite" which, from the electrical point of view, can be modelled as a pair of equipotential surfaces separated by an equivalent conductor with the resistance localized in the interfaces. A simple model is developed and the total cross-splice resistance is compared with the experimentally observed values. Validation of the resistance of splices and the associated heat dissipation to helium at 1.9 K has been successfully performed in STRING2, the prototype of the LHC full cell. This associated to laboratory measurements confirmed the reliability level of splices initially assumed in the whole LHC budget.