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

Abstract : This Ph.D. dissertation was conducted in the framework of R&D for magnetic fusion. In Tokamak-type experimental reactors, an ionized gas reaching millions of degrees is confined by high magnetic fields produced by powerful electro-magnets. In order to reduce the thermal dissipation, modern tokamaks use superconducting materials at cryogenic temperatures. These materials can carry large currents without electrical resistance. However, for advanced superconductors, this current-carrying capability is also a function of the mechanical strain state of the material. In the ITER Tokamak, currently under assembling in France, the toroidal field magnets cables are composed of hundreds of Nb3Sn composite superconducting wires sensitive to strain. During machine operation, these cables are submitted to cyclic mechanical loads of electromagnetic and thermal nature. It has been observed that these repetitive loads trigger 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 goal of the Ph.D. thesis is to shed some light on both cable and strand scales by developing a solid numerical electromechanical model to simulate the superconducting cables in operation. This model is meant to identify and understand the main causes of performance degradation as well as to obtain a predictive tool to assess cable behavior for new superconducting cables. During the three years of this Ph.D., both experimental and numerical activities were performed to tackle this issue. A major effort was applied to the development of a numerical model, based on the MULTIFIL code, permitting the mechanical modelling of a Cable-In-Conduit Conductor (CICC), typical of modern cables for fusion reactors. Several upgrades of the model were made to reach a satisfying representativeness of the cable 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 important 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.
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https://hal-cea.archives-ouvertes.fr/tel-03638175
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Submitted on : Tuesday, April 12, 2022 - 9:50:32 AM
Last modification on : Tuesday, April 12, 2022 - 11:20:50 AM

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Rebecca Riccioli. Mechanical modeling of superconducting cables for fusion under cyclic electromagnetic and thermal loads. Engineering Sciences [physics]. Université d'Aix Marseille, 2021. English. ⟨tel-03638175⟩

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