Effect of filament diameter and spacing on Jcof Nb3Sn wires in the intermediate field range (10 - 12 T) and at high fields

Commercial binary and Ta alloyed Nb
3
Sn wires (7.5 wt.% Ta) have been investigated in order to optimize J
c
in the intermediate field range (10 - 12 T) and at high fields, i.e. 16 T for the binary and 20 T for the Ta alloyed wire. For this purpose, the diameter of the as-received wires (0.86 mm) was further reduced to different sizes between 0.86 and 0.29 mm. For each diameter, the reaction conditions, i.e. temperature and time, were optimized. The results can be described as follows: a) The A15 layer growth in the Ta alloyed wire is slower than for the binary wire. b) In both wires, there is a cross-over of the J
c
vs. B curves at 14 T when reducing the diameter. The larger sizes have higher J
c
values above 14 T, while the thinner wires are better below 14 T. c) In both wires, an enhancement of the compressive strain, ε up to 0.56%, as well as a substantial decrease of 8
c2
up to 3.3 T was observed after the reduction to 0.29 mm diameter. The drop in B
c2
and thus J
c
at high fields is correlated to an enhanced prestress in the thinner wires, attributed to reduced filament spacings (dispersion hardening). The reason for the enhancement of J
c
below 14 T for thinner wires is still a subject of investigation. The present work shows that the optimization of J
c
for binary and alloyed Nb
3
Sn wires depends on the operational field range and leads to strongly different configuration requirements: For applications at fields B < 14 T, the filament diameter/ spacing ratio should exceed 8 μm/1.5 μm, while for fields below 14 T, the filament diameter- /spacing ratio should be of the order of 1 μm/0.3 μm. Both quantities, filament spacing and diameter, are correlated by the requirement of a maximum A15 content in the wire cross section.