The Effect of Indentation Depth on Performances of Nb3Sn Strands in Cables of ITER Central Solenoid

Abstract

The ITER Central Solenoid (CS) is composed Nb 3 Sn cable-in-conduit conductor. Short twist pitch cable design is appropriate for the CS cables to maintain the current sharing temperature under electromagnetic loading cycles. In the production process of the short twist pitch CS cable, Nb 3 Sn strands, and Cu strands are cabled tightly and compressed to get circular shape. However, these processes induced strong interstrand contact force and the strands deformation. The strand performances are degraded by the severe indentation on the strands at the contact point between the strands, although the strands are deformed before the heat-treatment. Therefore, influence of the indentation on critical current and residual resistive ratio were investigated in two types of bronze-route and an internal-tin Nb 3 Sn strands which are used for the CS cable, in order to determine the threshold of indentation depth in the strands. Also, transverse cross-sectional observation by Electron Probe Micro Analyzer was carried out on the indented strands to investigate the damage in the cross-section. Before August 2016, 37 CS cables were manufactured. On 37 CS cables manufactured before August 2016, numbers of indentations on the Nb 3 Sn strands were investigated to confirm whether there was indentation on the Nb 3 Sn strands which degrades the strand performance.