Distinct voltage current characteristics of Nb3Sn strands with dispersed and collective crack distributions

摘要

Two ITER-type Nb3Sn superconductor strands, one prepared with the bronze route and theother with the internal-tin route, were used to investigate the impact of filament cracking on thestrand's transport properties. Careful mechanical polishing allowed unambiguous identificationof the microscopic fractures of filaments caused by axial straining of the strands. Afterapplication of high axial tensile strain, densely and uniformly spaced cracks were observed inthe bronze strand, while fewer but more correlated cracks occurred in the internal-tin strand.Crack initiation was observed in the bronze strand after an applied tensile strain of more than0.8%, while for the internal-tin strand cracks were found already in the unloaded specimen,with further crack growth beyond 0.3% applied strain. With the Pacman strain device, thevoltage—current characteristics at zero applied strain were measured after several successiveapplications of incrementally increasing tensile strain. Distinct dissimilarities in thevoltage—current characteristics were observed between the dispersed and the collective crackdistributions. We also modelled the influence of cracks on the voltage—current characteristics ofthe two strands by considering two limiting cases of the crack behaviour.