Studies On The Transport Mechanism Of Superconducting Strand And Mechanical-electro-thermal Performance Of HTS CICC

The superconducting magnet system is an important part of the tokamak device in the International Thermonuclear Experimental Reactor(ITER). The superconducting performance(critical current, critical temperature, etc.) of the strand and cable have significant influence on the safe operation of the magnet system and the tokamak plasma steady flow. In the work conditions(very low temperature, high current, high magnetic field), the magnet is subjected to strong electromagnetic forces, heat load and hydraulic pressure(caused by helium superfluid) which make the strands and cable deformation, vibration and extrusion, leading to the occurrence of superconducting performance obvious degradation. In order to provide a reasonable explanation of degradation mechanisms, we have studied the current flowing patterns, strain dependence, current degradation and electro-mechanical properties of the superconducting strand. In addition, design and research with high superconductivity HTS cable has become a hot area in applied superconductivity field in the world. We will study the mechanical-electro-thermal characteristics by numerical simulation to provide theory support and reference for the optimization design of the HTS cable.Firstly, based on the contact resistance and the effective resistance model, a three-dimensional finite element model of multi-filament composite strand was developed to analyze the current flowing patterns in transverse and longitudinal direction. The AC loss generated within the strand under the time dependent magnetic field was calculated. The influence of the external magnetic field magnitude and length of the twist pitch on AC loss were discussed. The results show that the contact resistance inside the strand has significant effects on the current flowing. The AC loss increases with the magnetic field amplitude and the twist pitch length increase.Secondly, the strain maps were substituted into Maxwell’s equations through a nonlinear electromagnetic constitutive relation of superconductor. Then, the current transport models of the strand in steady-state and transient-state were established, which are suitable for high current, high field work environment. The proposed models were solved using the perturbation method, finite element method respectively. The current density distribution of the strand under the effect of axial strain was obtained. The AC loss was calculated, and the impacts of strain, n-value as well as the magnitude of the transport current on current density were also discussed.Thirdly, we studied the mechanical-electrical coupling behavior of superconducting strand. The mechanical constitutive relations of filament bundles and metal matrix were described with elastic and elasto-plastic axial stress–strain curve, respectively. The I-V characteristic of the metal matrix elements was determined with the Ohmic law, and that of the superconducting filaments followed the n power relation. The stress-strain state was simulated and then implemented in the electromagnetic model, through the scaling law of ITER Nb3 Sn strand. We found that the plasticity of copper and the twist filaments cause the enhanced inhomogeneous strain profile in a strand composite, compared to the strain profile in a strand with untwisted filaments. The combination effect of the tensile force and Lorentz force on the strand was discussed. It can be found Lorentz force combine with the tensile force make the degradation obvious.Finally, a 3D finite element model based on realistic CICC configuration was developed for the mechanical-electro-thermo simulation of the high temperature superconductor(HTS) cable. The finite element method was used to study the electric potential, current density, magnetic flux, heat flux, temperature and thermal stress distribution. The V-I curves of the superconducting tapes were calculated and the results agree well with the experiment results. The results show cable cooling can significantly reduce the stress inside the cable, and current density distributed in superconducting tapes unevenly. So, optimizing the number of superconducting tapes can enhance the current carrying capacity of the cable, and selecting the appropriate metal substrate can reduce stress levels within the cable.Above all, in this dissertation, we carried out a systematic study on the transport mechanism and provided a theoretical basis to predict the performance degradation of the superconducting strand. In addition, the mechanical-electro-thermal behavior of the HTS CICC was studied through a multi-field coupling theoretical model. Some advice and references were provided for optimization and design of the HTS CICC.