This year marks the 50-years anniversary of the discovery of Nb3Sn compound by Professor B.T. Matthias. The first Nb3Sn conductor was fabricated in 1960, and since then it has contributed much for generating fields over 10 T at 4.2 K. A variety of processes have been applied for the fabrication of Nb3Sn. The effect of Cu on the enhancement of Nb3Sn synthesis has yielded a significant breakthrough for the fabrication of multi-filamentary Nb3Sn conductors in industrial scale. Now Nb3Sn conductors are being widely used for NMR, fusion and other high-field facilities as well as for refrigerator-cooled superconducting magnets. Although Nb3Sn conductors have a long history, their high-field performance has shown a pronounced enhancement over the past decade. Further progress may be still expected through the improvement of fabrication parameters.
The progress of Nb3Sn conductor development for fusion power applications at JAERI is described. The superconductor development was carried out in connection with the research and development for four fusion coil development projects: Phoenix, Cluster test, Demo Poloidal Coil and ITER-EDA. In the Phoenix project, the reliable production of multi-filamentary Nb3Sn strands was established during the development work towards realizing a 18 T coil, and an 18 T generation coil (BC-18) with multi-filamentary Nb3Sn strands was successfully operated at 4.2 K for the first time in the world. In the Cluster Test project, a pool-cooled Nb3Sn conductor with a high heat-flux surface, and the first 1-m class bore Nb3Sn coil (TMC) in the world was constructed and operated up to 12 T. In the Demo Poloidal Coil project, which was constructed to demonstrate the possibility of a poloidal coil, a forced-cooled Nb3Sn conductor was developed. The Nb3Sn coil (DPC-EX) constructed with it was pulse-operated up to 7 T in 0.5 s. Finally, in the ITER-EDA project a CICC-type 13 T-46 kA Nb3Sn conductor was developed. A Central Solenoid Model Coil (CSMC) was fabricated using it and successfully operated up to 13 T with 0.4 T/s.