The key role of twist pitch and mechanical pre-treatment in the transport properties of Nb3Sn wires subject to bending strain

In modelling the transport properties of multi-filamentary Nb3Sn strands, the knowledge of the geometrical parameters of the superconducting filaments and the electrical and mechanical properties of the different materials composing the wire are required. In particular, the filament twist pitch and the transverse resistivity between filaments have a crucial role in the definition of the current transfer length and, consequently, in the simulation of the transport performances of superconducting wires subject to mechanical loads, as in cable-in-conduit conductors (CICC) for fusion magnets during operation. We have measured the critical current and the n-index of internal tin Nb3Sn wires with different values of the filament twist pitch, having inserted the strand into a stainless steel jacket and under the application of pure bending strain. Results show that the degradation of the transport properties is affected by the twist pitch value and, in the limit case of non-twisted filaments, even an improvement is observed in presence of bending. Moreover, the reversibility of critical current after relaxing the mechanical load has also been checked and an improvement of the performances has been observed after pre-bending applications, presumably due to the strain relaxation. In addition, the differential analysis through the second derivative of the V–I curve evidenced a peaked critical current distribution for the UNTW-strands, while TW-strands under bending showed a higher degree of non-homogeneity, proven by broader distributions.