Fabrication and evaluation of a cryostable Nb3Sn superconductor for the Mirror Fusion Test Facility (MFTF-B)

The MFTF-B magnet system requires two 12.5T, 36 cm bore, insert coils. These coils are being constructed with a cryostable Nb
3
Sn conductor manufactured by Furukawa Electric Co. The conductor consists of a core soldered into a cold-worked copper housing, which provides strength. The Nb
3
Sn core is made by a triple extrusion bronze process. A total of 57 lengths, each 295 m long, have been made and tested. We have made extensive tests on this conductor; these tests include critical current, ambient and 4.2 K mechanical property measurements, critical current as a function of tensile strain, and bending tolerance tests. The critical current density was found to be quite anisotropic in this conductor, with J
c
(12T) = 650A/mm
²
for field perpendicular to the conductor wide face, and J
c
(12T) = 500A/mm
²
for field parallel to the conductor wide face. These values are among the highest reported for a production lot of Nb
3
Sn conductor.