Cable Untwisting and Final-Stage Twist Pitch Measurements of ITER Toroidal Field Cable-in-Conduit Conductors

High performance magnetics (HPM) is jacketing toroidal field (TF) Nb₃Sn cable-in-conduit conductors (CICCs) for Oak Ridge National Laboratory as part of the United State´s contribution to the international ITER fusion experiment. HPM has been contracted to jacket a total of 8.0 km of CICC weighing 72 metric tons and consisting of nine 770-m Nb₃Sn superconducting cables, one 770-m copper cable, two 100-m Nb₃Sn cables, and one 100-m copper cable. The three 100-m cables and one 770-m copper cable are lengths used to qualify the procedures prior to jacketing the nine Nb₃Sn production lengths. The measured final-stage twist pitch of the 770-m copper CICC was as high as 525 mm at the leading end of the inserted cable compared with the specified allowable twist pitch of 420 mm ± 20 mm. To further understand the elongated twist pitches, two novel measurements were developed and implemented at HPM. A cable rotation sensor was built and installed at the head of the cable during insertion into the jacket to measure the untwisting as a function of inserted cable length, and a nondestructive evaluation technique using thermal imaging was used to measure the final-stage twist pitches along the length of the compacted conductor. The standard method of measuring the twist pitch is by destructively dissecting the CICC, where the jacket is removed from the CICC and the twist pitch measured by visual inspection.