Recent topics in high-field A15 superconductors in Japan

A different type of V3Ga conductors was fabricated in the early stage of high-field A15 conductor development in Japan. Then, Ti-doped Nb3Sn conductors have been developed, and widely used for high-field generation. An increase of Sn concentration in the bronze produces an appreciable progress in the performance of bronze processed (Nb,Ti)3Sn conductors. Nb3Sn wires for an ac application have been also developed through the bronze process. New internal tin(IT) processed (Nb,Ti)3Sn conductors with modified cross-sectional configurations have been produced, which exhibit large Jc in high fields as well as reduced ac loss. Both bronze processed and IT processed (Nb,Ti)3Sn conductors satisfy the ITER magnet specifications. As for new type Nb3Sn conductors, powder core and Jelly Roll processed (Nb,Ta)3Sn wires have been fabricated. Nb3Al wires with improved performance are prepared by rapid heating, quenching and transformation (RHQT) process. The quenched wire with bcc crystal structure can be deformed after the Cu cladding, and finally reacted at 800 °C for the transformation. Recently, stable reel-to-reel RHQT operation has been performed on the wire over a long length. Meanwhile, V-based high-field A15 compounds such as V3Ga may need to be reconsidered for a fusion application, since they have much better radioactive characteristics than those of Nb-based A15 compounds.