The Bronze Processed Nb₃Sn Ultra-Thin Superconducting Wires

We have studied the fabrication of Nb ₃ Sn ultra-thin wire through the bronze process. The starting billet was assembled by 19 Nb rods, a bronze alloy matrix of Cu-14mass%Sn-0.3mass%Ti, Nb diffusion barrier, and the outermost Cu stabilizer. The Cu/non-Cu ratio was designed as 0.89. The outer diameter of the stating billet is 45.2 mm. After the hot extrusion with an extrusion ratio of 9.1, it was cold-drawn from 15.0 mm to 0.05 mm (50 microns) in diameter with intermediate annealing with reel-to-reel process. Eventually, we have obtained over 7,000 m in a piece length without wire-breakages. In addition, we have also demonstrated both 7 and 7/7 stranded cables using 50 microns Nb ₃ Sn wire. The transport critical current, I _c , at 4.2 K and 4 T of both the primary 7 and the secondary 7/7 stranded cables were 14.2 A 102.0 A, respectively. A single ultra-thin wire showed 2.04 A of I _c at 4.2 K and 4 T, thus, measured I _c values of 7 and 7/7 stranded cables become almost equivalent to the calculated values from the I _c of a single wire. In addition, we investigated the bending effect on I _c of the secondary 7/7 stranded cable using the brass fixtures having a different radius. Meantime, we studied the effect of solder impregnation before bending. The I _c of 7/7 stranded cable with solder impregnation showed a dramatic degradation by bending on the radius of 11.5 mm. But the I _c of the cable without solder impregnation did not.