Modeling and experimental analysis of cable-in-conduit superconductor joints for fusion magnets

This PhD dissertation investigates the critical role of cable-in-conduit conductor (CICC) joints in the ITER magnet system, a key component of the experimental tokamak aiming for sustainable thermonuclear fusion. The magnet system comprises various coils that rely on advanced NbTi and Nb3Sn CIC superconductors. Given the significant size of the coils and the limited production lengths of the superconductors, electrical joints are indispensable for connecting unit lengths of the cables, then are essential for reliable ITER operation.Thorough understanding and qualification tests are mandatory parts of the ITER magnet R&D program. This research primarily focuses on the Poloidal Field (PF) joints, analyzing their electromagnetic and thermal performance through experimental tests and numerical simulations. Experimental tests at the SULTAN facility and the University of Twente often face limitations in replicating real operational conditions. Therefore, precise numerical simulations using the JackPot-AC/DC model, which provides strand-level analysis, are crucial. This model evaluates the performance of the cables and joints, aiding in optimizing them for real operational conditions.Firstly, the dissertation establishes key practical scaling laws for Nb3Sn strands operating in low magnetic fields and accurately calculates the hysteresis and coupling losses of NbTi strands. These findings significantly enhance performance predictions for ITER coils. Extensive investigation into contact resistances within PF joints reveals that inter-strand, inter-petal, and strand-to-sole resistances critically influence joint performance. This essential data supports the JackPot-AC/DC simulations, clarifying the dedicated influence of experimental and actual operational conditions.Secondly, the analysis of unexpected nonlinear voltage-current characteristics observed during DC tests reveals the effect of electromagnetic force on the joints, providing insights for non-destructive examination of joint samples. Design modifications are proposed to mitigate induced currents and improve thermal stability, showing promising results in both simulations and experimental validations.Thirdly, this dissertation offers a comprehensive analysis of the electromagnetic and thermal stability of ITER’s superconducting joints, contributing to their safe and reliable operation under various plasma scenarios. Additionally, an effective method is proposed to evaluate thermal stability with considerable accuracy and efficiency, benefiting the study of magnets. The findings support current ITER operations and inform the design of future CICC-based joints, advancing the development of fusion energy