Advances in Nb3Sn strand for fusion and particle accelerator applications

Nb₃Sn conductor made by the internal tin route is the material of choice for the highest field superconducting magnets. These include systems ranging from solenoids used in 900MHz NMR and 20 T laboratory magnets, to large-scale applications such as ITER and possible LHC upgrades. We present our latest results on internal tin strands having critical current density (J_c) values of 3000 A/mm² (4.2 K, 12 T), as it relates to such magnet systems. One obstacle to wider use of internal tin strand is the relatively small billet size, typically limited to 50 kg or less. As part of the R&D for the U.S. High Energy Physics National Conductor Program, we have developed a method of scaling up the distributed barrier internal tin process to billet sizes several times larger. In the past year we have successfully produced a high J_c distributed barrier strand made entirely by hot extrusion. Results are also presented on a new method of supplying Ti dopant for the Nb₃Sn that does not rely on alloying the Sn cores, matrix Cu, or the Nb filaments directly. Material made with this new doping method reached a J_c (4.2 K, 12 T) value of 2500 A/mm². Finally, the state of development of composites having lower AC losses is described. Such conductors are being developed for possible future fusion applications including ITER.