The fabrication and properties of Nb3Sn superconductors by the internal tin process

This paper describes recent progress in the development of Nb₃Sn wires in Switzerland using the internal tin method. The work was carried out in close collaboration between the Fusion Technology Division of CRPP and Swissmetal, with the emphasis on conductors suitable for fusion magnets. The manufacturing process is based on the tubular extrusion of a niobium-copper composite, followed by the cold working of the composite containing a tin core. 19 Nb-Cu-Sn subelements were assembled inside a tantalum diffusion barrier and a copper can and subsequently reduced to wire size by swaging and drawing. The intention was to obtain non-reacted filament sizes of typically 3 μm in 0.8-1 mm wires. The evolution of the design and the manufacturing process are discussed. Experience gained with model conductors has been used to guide the design. The strands have been evaluated with respect to electrical and mechanical properties. In particular, critical current densities in the non-copper area exceeding 750 A/mm² at 12 T and 4.2 K have been achieved