The most widely used superconductor in the world — and the starting point for fusion magnet design.
Niobium-titanium (NbTi) is a ductile binary alloy (typically Nb–47 wt% Ti) that becomes superconducting below a critical temperature of approximately 9.3 K. It is the most commercially produced superconductor on Earth, used in MRI scanners, particle accelerators, and early fusion experiments. Its excellent mechanical properties — unlike the brittle A15 compounds — make it straightforward to draw into fine-filament wire and wind into complex magnet geometries.1
NbTi has an upper critical field Bc2 of roughly 14.5 T at 0 K and about 10.5 T at 4.2 K, the standard liquid-helium operating temperature. Practical magnets using NbTi are limited to approximately 8–9 T because the critical current density drops steeply as the field approaches Bc2. The alloy is face-centered cubic in its superconducting phase, with flux pinning provided by nanoscale alpha-Ti precipitates formed during carefully controlled heat treatments.2
NbTi is fabricated by melting, forging, and drawing a composite billet of NbTi rods embedded in a high-purity copper matrix. The copper serves as a thermal and electrical stabilizer during quench events. Finished wire contains thousands of NbTi filaments, each 5–50 μm in diameter, twisted to reduce AC losses. Mature industrial processes and decades of production for the MRI market have driven NbTi wire costs below $1 per kA·m at 5 T — an order of magnitude cheaper than Nb3Sn and two orders below REBCO.3
NbTi was used in the poloidal-field coils of several major tokamaks, including JET, EAST, and KSTAR, and it forms ITER's six poloidal-field coils (PF1–PF6), which operate at lower fields (up to 6 T) than the Nb3Sn toroidal-field coils. In modern compact fusion designs targeting fields above 10 T, NbTi is insufficient alone but may still appear in outer poloidal coils, correction coils, and bus bars where field requirements are modest.4
NbTi's legacy in fusion is foundational: it proved that large superconducting magnets could operate reliably in a nuclear environment, paving the way for the higher-field Nb3Sn and REBCO systems now under development.