Analysis of contact length distribution of superconducting strands with copper sleeves at cable-in-conduit conductor joints

It is observed that the measured critical currents of cable-in-conduit (CIC) conductors for ITER TF coils become lower than expected due to the unbalanced current distribution that is caused by contact resistance between the strands and the Copper (Cu) sleeves in CIC conductor joints. In order to evaluate the contact length, we identify the three-dimensional positions of all strands in the CIC conductor, and then measure the contact number and lengths of strands that appear on the surface of the cable to contact with the Cu sleeves. It is found that some strands do not appear on the surface of the cable, and the contact lengths are widely distributed with a large standard deviation. We develop a numerical code that simulates strand positions in the CIC, and then compare the analyzed contact strand number and contact length with measured ones. It is found that the results are in agreement, and hence the code can be used to evaluate the contact parameters. After varying the twist pitches of the sub-cables, we show that all strands appear on the cable surface and have contact lengths with small standard deviation. It is found that the twist pitches are a key parameter for optimization of the contact resistance. (author)