Cable Rotation and Twist Pitch Variation for ITER TF Conductor in China

Toroidal field (TF) coil is made of cable-in-conduit conductor (CICC) with an operating current of 68 kA. In total, 900 Nb₃Sn strands and 522 copper strands are cabled around the central spiral and then wrapped with stainless-steel tape of 0.1-mm thickness. The cable is then inserted into a jacket under tensile force that is increasing with length of insertion. Insertion and compaction are key technologies injacketing. The tensile force can lead to rotation of a cable, resulting into an increase in a twist pitch. Understanding the twist pitch variation is very important; in particular, the twist pitch of a cable inside a CICC strongly affects properties such as ac loss. In this paper, we present the results of a Chinese TF conductor during manufacture. A rotation sensor and a straight line on the cable surface were used to measure the rotation during insertion and compaction. X-ray was used to check the changed twist pitch during compaction. Short samples were subjected to destructive examination to assess the exact twist pitch. The test results show that the Chinese TF conductor has a slight rotation and very small twist pitch variation during conductor manufacture.