Advantages of Cu-Nb Reinforced Nb₃Sn Strandsin Large Multi-Stage Stranded CablesWith Low-Void Fraction

A large multi-stage stranded cable structure withNb3Sn strands twisted at a short-pitch and compressed to a low-void fraction is a major candidate for Nb3Sn conductors for JA-DEMO-TF coils. To realize the conductor, we have proposed the application of Nb-rod-method Cu-Nb reinforced ITER-CS typeNb(3)Sn strands (Cu-Nb/Nb3Sn wires). Prototype 4th-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/Nb3Sn strands or conventional non-reinforced ITER-CS strands (Cu/Nb3Sn wires), and compression-formed to a low-void fraction below 30%. Cu-Nb/Nb3Sn strands extracted from the compacted 4th-stage cables had no degradation of I-c as well as Cu/Nb3Sn strands. Destructive investigation of the compacted 4th-stage cables showed Cu-Nb/Nb3Sn cables had better manufacturability of the large multi-stage stranded cables with low-void fraction than the conventional Cu/Nb3Sn cables. Moreover, the Cu-Nb/Nb3Sn 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)Snstrands for the large multi-stage stranded cables with low-void fraction are discussed.