Mechanical modelling of superconducting cables for fusion under cyclic electromagnetic and thermal loads

In Tokamak-type experimental fusion reactors, an ionized gas reaching millions of degrees is confined by high magnetic fields produced by electro-magnets. To reduce the thermal dissipation, modern tokamaks use superconducting materials at cryogenic temperatures that can carry large currents without electrical resistance. However, for advanced superconductors, this current-carrying capability is a function of the mechanical strain state of the material. In the ITER Tokamak the toroidal field magnets cables are composed of hundreds of strain-sensitive composite superconducting wires. During operation, these cables are submitted to cyclic electromagnetic and thermal mechanical loads triggering a gradual but steady decrease of the electrical performance of the cable. Up to now, the exact mechanisms relying this macroscopic loss of electrical performance to the local strain state of the superconducting wires are still partially unknown. This issue is extremely complex because of its multi-scale and multi-physics nature. The Ph.D. goal is to identify and understand the main causes of performance degradation as well as to obtain a predictive tool to assess superconducting cables behavior by developing a solid numerical electromechanical model to simulate the superconducting cables in operation. In parallel, the experimental activities focused on the mechanical characterization of Nb3Sn wires under cyclic compressive and tensile stresses, at both room and cryogenic temperature. Thanks to these test campaigns, specific experimental protocols were developed and behaviors and trends about cyclic loading of superconducting wires were identified. Finally, the modelling of complete cables under representative loading permitted a new interpretation of the mechanisms driving the electrical performance degradation in the ITER TF magnet conductors. Moreover, parametric studies demonstrated the impact of certain design parameters of the cables on their global mechanical behavior. This opened the way to studies of cables from other and new fusion projects, thus demonstrating the versatility of the model developed.