The workhorse low-temperature superconductor behind ITER's record-setting magnets.
Niobium-tin (Nb3Sn) is an intermetallic compound with the A15 crystal structure that becomes superconducting below approximately 18.3 K. It is classified as a low-temperature superconductor (LTS) and has been the material of choice for high-field fusion magnets — most prominently the toroidal-field and central-solenoid coils of ITER — because it sustains useful critical-current density at fields up to roughly 23 T, well above the 10 T practical ceiling of NbTi.1
Nb3Sn has an upper critical field Bc2 of approximately 25–27 T at 4.2 K and a critical temperature Tc of 18.3 K, both significantly higher than NbTi. However, the A15 phase is brittle: strain beyond roughly 0.3% irreversibly degrades critical current. This strain sensitivity dominates magnet engineering, requiring wind-and-react or react-and-wind fabrication sequences and careful structural design to limit conductor strain under Lorentz loads.2
Nb3Sn wire is produced by several routes — bronze process, internal tin, powder-in-tube, and restacked-rod process. In each, precursor elements are assembled into a composite billet, drawn to final wire diameter, and then heat-treated at 600–700 °C for hundreds of hours to form the superconducting A15 phase by solid-state diffusion. The resulting wire contains thousands of fine Nb3Sn filaments embedded in a copper or bronze matrix that provides thermal stabilization.3
Nb3Sn enabled the jump from the 5–8 T fields of earlier NbTi-based tokamaks to the 11.8 T toroidal field of ITER. It remains the only LTS material qualified at this scale. However, its field ceiling near 23 T and its need for 4.2 K helium cooling have motivated the fusion community's push toward REBCO HTS, which offers fields above 20 T with greater thermal margin. Nb3Sn will continue to serve in hybrid coil designs, central solenoids, and applications where its mature industrial base and lower per-kilogram cost outweigh its field and temperature limitations.4