The Effect of Strand Bending on the Voltage-Current Characteristic of $\hbox {Nb}_{3}\hbox {Sn}$ Cable-In-Conduit Conductors

The primary issue facing Nb₃Sn cable-in-conduit conductors (CICCs) has been degradation, often observed during electromagnetic cycling, in the current sharing temperature, n-index, and critical current, in respect to measured strands values, due to strain effects. In the last years, work performed mostly relating to fusion magnet technology has led to a better understanding of the parameters required to improve the constraints imposed by the brittle nature of the Nb₃Sn filaments, such as cables low void fraction and long twist pitch sequence. On the other hand, experimental campaigns on bent Nb₃Sn wires, pre-compressed into a stainless-steel jacket, have shown that an appreciable decrease in the n-index values already occurs at the strand level, well below the irreversible mechanical load regime for filament breakage. This result is explained, with the support of simulation results, taking into account the broadening of the critical current distribution on wires cross section due to the presence of the jacket and to bending strain. Considering the CICC´s layout impact on their overall performances, the effective resemblance between pre-compressed bent wires and strands inside cables is emphasized, and an innovative interpretation of cabled conductors test results is given, suggesting a tool to predict their performances in terms of the n-index versus critical current relation of constituting strands, characterized under simultaneous pre-compression and bending strain.