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

It is observed that measured critical current of Cable-in-Conduit-Conductor (CICC) for ITER TF coil are lower than expected. This is partly explained an imbalance of the contact resistance at the joint between double pancakes which causes an unbalanced current distribution in a cable and hence during a slow temperature increase some strands reach the critical current earlier than other. In order to estimate the contact resistances, we identify the three-dimensional positions of all strands inside the CICC, and then evaluate the contact parameters such as number and length of the strands which appear on the cable surface and have contact with a copper sleeve. It is observed that many strands do not appear on the surface of the cable, and can lead to unbalanced current distribution. We developed a numerical code to analyze all strand positions in the CICC. We evaluated the contact parameters by using the numerical code, and then compared them with those evaluated from the measured strand positions. It is found that since both results are in good agreement, the numerical code is available for evaluating the contact parameters. By using the code, we can optimize the contact parameters by varying the twist pitches of all staged subcables. The results show that all strands appear at the surface of the cable and have contact with the copper sleeve, and moreover the contact parameters have been improved.