Magnetization, RRR and Stability of ${\rm Nb}_{3}{\rm Sn}$ Strands With High Sub-Element Number

The magnetization and low field stability were measured on a series of high current-density Nb₃Sn strands with 54 to 198 sub-elements taken from accelerator magnet development programs. The effective filament size, d_eff ,, as determined from the width of the magnetization loop at low field and the extrapolation of the transport critical current from high field, was very close to the sub-element diameter determined by the strand geometry. Self-field corrections were vital for obtaining this agreement; without them, d_eff was overestimated by ~25%. While all strands, even those with d_eff as small as 35 mum, exhibited flux-jumps at low field in magnetization measurements, smaller sub-elements produced smaller flux-jump magnitude and a smaller range of field over which flux jumps occurred, suggesting stability improved with decreasing d_eff. . However, good combinations of the residual resistivity ratio RRR and the critical current density became increasingly difficult to obtain by varying the reaction heat treatment as the sub-element number increased. Further analyses of RRR data indicate that tin contamination of the copper stabilizer is underway even before any Nb₃Sn is formed in strands with high numbers of sub-elements.