Development of internal tin Nb3Sn conductor for fusion and particle accelerator applications

Performance improvements are needed for large scale applications of Nb3Sn, such as ITER or LHC upgrades. The highest critical current density (J(c)) values are achieved in distributed-barrier strand made by the Restacked Rod Process, which can reach 12 T, 4.2 K J(c) values of 3000 A/mm(2), with high residual resistivity ratio (RRR) values. For purposes of accelerator magnet stability, it is desirable to combine high J(c) with a small effective filament diameter (D-eff). Initial experiments show reducing D-eff from 80 mu m to 40 mu m leads to a 10% reduction in J(c). For fusion applications, a single-barrier design with well-spaced filaments is used to achieve the low hysteresis losses that are required. The status of our fusion strand development program is presented, including results for strand made using Nb-47 wt%Ti rods to supply Ti dopant. Such strands can reach 12 T, 4.2 K J(c) > 1000 A/mm(2), with losses < 1000 mJ/cm(3).