Optimization of critical currents in composite-processed multifilamentary Nb3Sn conductors with Nb-Ti alloy cores

Multifilamentary Nb3Sn conductors with Nb-(1,2,3, 5at%) Ti alloy cores were successfully fabricated, and the relationships among the amounts of titanium addition to the core, core sizes, heat treatment conditions ,Nb3Sn grain structure and the superconducting properties have been studied. The multifilamentary Nb-3Ti/ Cu-7.5Sn conductor shows the highest overall critical current density Jc(overall) at 16 T for all the core sizes of 17 μm-diam, 10 μm-diam and 7.5 μm-diam, after the optimum heat treatment. The optimum heat treatment condition for the multifilamentary Nb-3Ti/Cu-7.5Sn conductor varies slightly with core diameter. The Jc( overall) of multifilamentary Nb-3Ti/Cu-7.5Sn conductor with 10 μm-diam 6,289-cores heat treated at 700°C for 200 hr exceeds 2.5×10⁴A/cm²at 16 T. The amount of residual tin in the Cu-Sn matrix of the multifilamentary Nb-Ti/Cu-7.5Sn conductor after heat treatment decreases with increasing titanium content in the core and decreasing core diameter. The multifilamentary Nb-3Ti/Cu-7.5Sn conductor showed appreciably improved uniaxial strain dependence of critical current Icat 14.5 T, as compared to that for a typical multifilamentary Nb3Sn conductor. The multifilamentary Nb-3Ti/Cu- 7.5Sn conductor developed in this study makes feasible to generate a magnetic field of 16 T.