Internal Tin ${\hbox {Nb}}_{3}{\hbox {Sn}}$ Conductors Engineered for Fusion and Particle Accelerator Applications

The critical current density (J c ) of Nb 3 Sn strand has been significantly improved over the last several years. For most magnet applications, high J c internal tin has displaced bronze process strand. The highest J c values are obtained from distributed barrier strands. We have continued development of strands made with Nb-47 wt%Ti rods to supply the dopant, and have achieved J c values of 3000 A/mm 2 (12 T, 4.2 K). Such wires have very good higher field performance as well, reaching 1700 A/mm 2 at 15 T. To reduce the effective filament diameter in these high J c strands, the number of subelement rods incorporated into the final restack billet has been increased to 127 in routine production, and results are presented on experimental 217 stacks. A new re-extrusion technique for improving the monofilament shape is also described. For fusion applications such as ITER, we have developed single-barrier internal tin strands having non-Cu J c values over 1100 A/mm 2 (12 T, 4.2 K) with hysteresis losses less than 700 mJ/cm 3 over non-Cu volume. The J c -strain behavior of such composites is also presented.