Stabilization and coil performance of RHQT-processed Nb3Al conductors

A Nb₃Al superconductor based on jelly-rolled (JR) Nb/Al composites of high performance can be fabricated by rapid-heating and quenching of a wire of such composites with subsequent transformation-annealing to form the Nb₃Al phase (RHQT process). Since the conductor thereby is heated up to 2000°C, far above the melting point of Cu, the Cu stabilizer needs to be externally incorporated by a mechanical-cladding method between quenching and annealing. Such an appropriate deformation promotes the transformation from a disordered bcc phase, and thereby enhances the critical current density J_c. A solenoid coil was fabricated by a wind-and-react method: the Cu-clad conductor (1.61^w×0.71^t mm²) with Al₂ O₃ yarn insulation was wound into a coil (19.7/40.8^φ×49.7^H mm), transformed at 800°C, and impregnated with beeswax. The resultant coil, while carrying a current of 179 A at 2.1 K in a superconducting back-up field of 21.2 T, generated an additional 1.3 T. The total magnetic field of 22.5 T is the highest record ever for a metallic superconducting coil