Advantages of Cu-Nb Reinforced Nb$_{\text{3}}$Sn Strands in Large Multi-Stage Stranded Cables With Low-Void Fraction

A large multi-stage stranded cable structure with Nb
$_{\text{3}}$
Sn strands twisted at a short-pitch and compressed to a low-void fraction is a major candidate for Nb
$_{\text{3}}$
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
$_{\text{3}}$
Sn strands (Cu-Nb/Nb
$_{\text{3}}$
Sn 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/Nb
$_{\text{3}}$
Sn strands or conventional non-reinforced ITER-CS strands (Cu/Nb
$_{\text{3}}$
Sn wires), and compression-formed to a low-void fraction below 30%. Cu-Nb/Nb
$_{\text{3}}$
Sn strands extracted from the compacted 4th-stage cables had no degradation of
I
_c
as well as Cu/Nb
$_{\text{3}}$
Sn strands. Destructive investigation of the compacted 4th-stage cables showed Cu-Nb/Nb
$_{\text{3}}$
Sn cables had better manufacturability of the large multi-stage stranded cables with low-void fraction than the conventional Cu/Nb
$_{\text{3}}$
Sn cables. Moreover, the Cu-Nb/Nb
$_{\text{3}}$
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
$_{\text{3}}$
Sn strands for the large multi-stage stranded cables with low-void fraction are discussed.