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The current uniformity among the strands in large cabled conductors is affected mostly by the contact resistance distribution and the interstrand resistance in the electrical terminations. While the contact resistance distribution between the strands and the copper of the termination affects the current distribution among the strands, the resistance between the strands in the termination (inter-strand resistance) has an effect on the re-distribution of the current from the most to the less loaded strands. The contact resistance distribution of an ITER conductor termination is measured in the JORDI (JOint Resistance Distribution) facility at CRPP. The sample is prepared starting from a section of Nb3Sn Cable in Conduit Conductor (CICC) and the joint is assembled following the procedure ("solder dipped") used for most of the ITER qualification samples tested in the SULTAN facility. The cable is opened into 150 groups of strands ("elements") and each element is connected to low temperature shunt resistors which split the current from a 10 kA power supply. The voltage drop is sensed between each element and the contact surface of the joint made equi-potential by a superconducting soldering alloy. The resistance of each channel is deduced from the measured voltage drop. The resistance distribution measured under imposed current uniformity allows deducing the current distribution in normal operation, namely under imposed voltage. The same sample is then adjusted to measure the inter-strand resistance. Two strands at the time are fed with a current of the order of 100 A, the voltage between the selected strands is sensed and the equivalent resistant deduced. The results of both measurements are compared with those obtained in previous experiments, carried out on different conductor layouts.
Date of Publication: June 2010