Nb3Al superconductors

Recent progress of Nb₃Al superconductors is described. A large-current conductor of 46 kA at 13 T, produced from a jelly-roll (JR) Nb/Al composite by a direct diffusion-process, is about to be tested as an insert-coil conductor for fusion magnets. The 1 GHz NMR magnet has given a motivation to do an entirely new approach to Nb₃Al processing that allows a dramatic improvement in high-field J_c characteristic. The RHQT process produces a stoichiometric Nb₃Al with fine-grain structures via a bcc supersaturated solid solution and, hence, large J_c over the whole range of magnetic fields. Controlling a heating temperature within a narrow region of extended bcc solid solution ensures the room-temperature ductility of as-quenched wires and homogeneous J_c characteristics of transformed Nb₃Al along the long piece-length. A solenoid coil fabricated by a wind-and-react method, with Cu clad RHQT Nb₃Al conductors, could generate an additional 1.3 T at 2.1 K in a bias field of 21.2 T, and was operated in a persistent-current mode without a bias field. The variants of RHQT process (TRUQ, DRHQ, Cu-added RHQO) improved significantly the high-field J_c without sacrificing the strain tolerance.