Advances in superconducting strands for accelerator magnet application

Considerable advances have recently been obtained in the critical current densities J_c of Nb₃Sn based superconductors - the prime candidates for the next generation of superconducting accelerator magnets. The non-Cu critical current densities now approach 3000 A/mm² at 12 T and 4.2 K in engineering quality strand. The design of new strands minimizes the amount of Cu in the package from which the Nb₃Sn is formed and increases the Sn level beyond that required to simply achieve A15 stoichiometry. The result is an A15 layer that is significantly more uniform than earlier generations of wire, both chemically and microstructurally, and wires that significantly surpasses previous Nb₃Sn strands in layer critical current density and in the specific grain boundary pinning force. Remarkably, these developments have been achieved in internal Sn based strands manufactured using both the modified jelly-roll technique with Nb-Ti alloy and the rod-in-tube approach with Nb-Ta alloy. The rod-in-tube approach is particularly exciting because it offers greater manufacturing flexibility. Advances have also been made in strand designs that offer the potential to reduce the large effective filament diameters, which are an issue with these new high-J_c strands. We review the latest developments in Nb₃Sn superconductors and compare their performance and potential with other round-wire high-field superconductors.