Fabrication of high-field Nb3Sn conductors from Sn-Ta/Nb composites

The mechanism of thick (Nb,Ta)₃Sn layer synthesis in the Ta-Sn core/Nb (Nb-Ta) sheath composite is studied. The diffusion of Nb from the sheath to the core seems to play a key roll to enhance that of Sn from the core to the sheath synthesizing (Nb,Ta)₃Sn layer. A simplified powder core process, in which Ta + Sn mixed powder is used instead of Ta-Sn reacted powder as the core, does not degrade the high-field performance of the (Nb,Ta)₃Sn conductor. A finer dispersion of Ta in the core is found to improve the J_c of the conductor below 23 T. New Jelly Roll processed (Nb,Ta)₃Sn wires are fabricated using Sn-Ta sheets richer in Sn. The JR wires exhibit attractive high-field performance, and a small amount of Cu addition to the Sn-Ta sheet effectively decreases the reaction temperature.