Alloying Effects on the Upper Critical Field of Nb3Sn Superconductor

Nb 3 Sn conductors are important candidates for high-field magnet applications for Future Protonor Muon Colliders, and they continue to be widely used for many laboratory and NMR magnets. However, the critical current density, J c , of present Nb 3 Sn declines swiftly above 12 − 15 T due to weak, sparse pinning. The present state-of-the-art Ta- and Ti-doped RRP design conductors exhibit upper critical field, H c2 , values of ∼ 24 − 26 T (4.2 K) and J c (4.2 K, 16 T) values of ∼ 1,300 A/mm 2 , significantly below the target J c of 1,500 A/mm 2 (4.2 K, 16 T) established by the FCC design study which serves as the present stretch target for Nb 3 Sn development. To meet this goal requires, at a minimum, greatly enhanced vortex pinning: recent investigations into doping Nb- Ta alloy with Hf and Zr, with and without internal oxidation, have demonstrated grain-refinement routes to higher vortex pinning. A second approach is to significantly enhance H c2 . In this study we have arc-melted multiple compositions of Hf, Zr, Ta and Ti added to Nb to investigate the possibilities of H c2 enhancement through alloying. These new alloys have been successfully drawn to wire size in Cu-Sn composites. H c2 (T) was measured down to 1.9 K and we have found that all alloys show good agreement with the standard WHH fitting procedure without the need to adjust paramagnetic limitation parameter (α) and spin-orbit scattering parameter (λ so ). We found excellent linear dH c2/dT slopes that in principle allow the deconvolution of the electronic specific heat coefficient γ and normal state resistivity ρ so as to better understand alloying effects. So far we have observed that Hf alloying of pure Nb can enhance H c2 (0) by 3 − 4 T to ∼ 28 T, while adding just 1 at.% Hf or Zr into the Nb4Ta base alloy can raise H c2 (0) to ∼ 31 T, while also raising the recrystallization temperature above the usual A15 reaction temperature range 650 ◦ C − 750 ◦ C. The potential for further improvements in Nb 3 Sn properties will be discussed in the context of these results.