Distinct voltage–current characteristics ofNb3Sn strands with dispersed and collective crack distributions

Abstract

Two ITER-type Nb3Sn superconductor strands, one prepared with the bronze route and the other with theinternal-tin route, were used to investigate the impact of filament cracking on thestrand’s transport properties. Careful mechanical polishing allowed unambiguousidentification of the microscopic fractures of filaments caused by axial straining ofthe strands. After application of high axial tensile strain, densely and uniformlyspaced cracks were observed in the bronze strand, while fewer but more correlatedcracks occurred in the internal-tin strand. Crack initiation was observed in thebronze strand after an applied tensile strain of more than 0.8%, while for theinternal-tin strand cracks were found already in the unloaded specimen, with furthercrack 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 collectivecrack distributions. We also modelled the influence of cracks on the voltage–currentcharacteristics of the two strands by considering two limiting cases of the crackbehaviour.