Advantages of Cu-Nb Reinforced Nb₃Sn Strands in Large Multi-stage Stranded Cables with Low-Void Fraction

A large multi-stage stranded cable structure with Nb ₃ Sn strands twisted at a short-pitch and compressed to a low-void fraction is a major candidate for Nb ₃ Sn conductors for JA-DEMO-TF coils. To realize the conductor, we have proposed the application of Nb-rod-method Cu-Nb reinforced ITER-CS type Nb ₃ Sn strands (Cu-Nb/Nb ₃ Sn wires). Prototype 4 ^th -stage stranded cables simulated the stranded process for the ITER-CS conductor and for the JA-DEMO-TF conductor were fabricated using the Cu-Nb/Nb ₃ Sn strands or conventional non-reinforced ITER-CS strands (Cu/Nb ₃ Sn wires), and compression-formed to a low-void fraction below 30%. Cu-Nb/Nb ₃ Sn strands extracted from the compacted 4 ^th -stage cables had no degradation of I _c as well as Cu/Nb ₃ Sn strands. Destructive investigation of the compacted 4 ^th -stage cables showed Cu-Nb/Nb ₃ Sn cables had better manufacturability of the large multi-stage stranded cables with low-void fraction than the conventional Cu/Nb ₃ Sn cables. Moreover, the Cu-Nb/Nb ₃ Sn triplex had improved current-carrying properties under transverse compression stresses by enhancement of the mechanical properties. In this study, the advantages of applying Cu-Nb/Nb ₃ Sn strands for the large multi-stage stranded cables with low-void fraction are discussed.