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.