Analysis of Contact Length Distribution of Superconducting Strands with Copper Sleeves at Cable-in-conduit Conductor Joints

Abstract

Synopsis: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.

Optimal Pitch Analysis Governing Contact Strand Number and Lengths With Cu Sleeves at CIC Joints

“…The rotating phase angle of th subcable reference line is described as follows, (1) where is an initial phase angle of the reference line at , is the twist pitch of the th subcable. We can analyze all strand positions in the CICC along the longitudinal -direction.…”

“…It is partly explained by the imbalance of the contact resistance among the cables and the copper sleeve at the joint section which mainly induce an unbalanced current distribution; some strands carry more current than others and reach the critical current earlier than others [1]- [4].…”

“…In a first attempt, we identified the three-dimensional positions of all 486 strands of the CICC for LHD OV coil and evaluated the contact lengths between the strands and the copper sleeve [1]. It was proven by this method that the distribution of the contact lengths was not uniform at the joint.…”

Analysis of Contact Lengths of Strands with Cu Sleeves in CICC Joints

“…It is partly explained by the inhomogeneous contact resistance between the strands and the copper sleeve at the joint section. This mainly induces an inhomogeneous current distribution, and some strands reach the critical current earlier than the others [1][2][3][4]. It is important to investigate the contact resistance distribution at the joint of the CICC.…”

“…It is important to investigate the contact resistance distribution at the joint of the CICC. In a first attempt, we identified the three-dimensional positions of all 486 strands of the CICC for Large Helical Device (LHD) Poloidal field (PF) coils [5] and evaluated contact lengths between strands and the copper sleeve [1] for the same lap joint construction as the ITER TF coils. It was proven by this method that the distribution of the contact lengths was not uniform at the joint section.…”