Intra wire resistance and strain affecting the transport properties of Nb₃Sn strands in cable-in-conduit conductors

The aim of this thesis work is to arrive at a deeper understanding of the effects of strain and filament fracture on the electric properties of superconducting strands and cable-in-conduit conductors that are subjected to various mechanical loads. Since inter-filamentary current redistribution plays a central role in such analysis, first an extensive database of intra-wire resistance is compiled for a comprehensive range of state-of-the-art technical superconductors. Then the electrical and mechanical properties of Nb3Sn strands are coupled with the aid of 3D numerical models, which allows predicting the voltage-current relation of the wires under combined axial and bending strain. Also filament fracture at high strain levels is incorporated in the models and its effect on the AC losses is characterized in various types of Nb3Sn strands. This approach yields a convenient set of polynomial expressions for the critical current and n-value of the strands under different loading scenarios, which are readily implemented in combined electric and mechanical full-size cable-in-conduit conductor models. Using these tools, a crucial performance determining effect of strain and filament fracture, namely the degradation of the current sharing temperature in cable-in-conduit conductors, is analyzed. It is demonstrated how magnetization measurements might serve as a non-destructive diagnostic tool to assess micro-structural damage in the cables.