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/mm², 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 mum to 40 mum 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/mm², with losses < 1000 mJ/cm³.