The Influence of Bending Strain on the Critical Current of ${\rm Nb}_{3}{\rm Sn}$ Strands With Different Filament Twist Pitch

The effect of bending strain on the transport properties of ${\rm Nb}_{3}{\rm Sn}$ strands has been the object of several investigations in the last years. We report on the performances of internal tin ${\rm Nb}_{3}{\rm Sn}$ strands with different filament twist pitches, pre-compressed by swaging into thin stainless steel tubes before the reaction heat treatment and subject to pure bending strain. We have previously reported on the comparison between the performances of the technological OST-Dipole strand (TW-strand) with those of the same strand, but with untwisted superconducting filaments (UNTW-strand). The critical current, as well as the n-index of the samples with twisted filaments gradually degraded with the applied bending strain, while an enhancement of the performances has been observed on UNTW-strands. Here, results on further experimental measurements carried out on OKSC internal tin strands with different filament twist pitches are discussed, in order to clarify some aspects of the current transfer process within the strands. Finally, we carried out an analysis through the second derivative of the V-I curve, which 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.