Interstrand Resistance and Current Redistribution in a Cable-In-Conduit Conductor (CICC) Termination

The non-homogeneity of the contact resistance distribution in the terminations of large CICC´s leads necessarily to a certain level of current imbalance among the strands in DC operation. The overloaded strands or groups of strands hit first the critical surface in the high field region and a current re-distribution process is thus driven by the initial current sharing voltage to minimize the energy of the system. This process takes place preferably at the electrical connection where the inter-strand resistance is lower. In order to study this phenomenon, a new interstrand resistance experiment is carried out at CRPP on the termination of a NbTi CICC sample. The sample was formerly prepared and tested to determine the contact resistance distribution in the JORDI test facility and it is now re-arranged for this test. Two elements at a time (strands or groups of strands) are selected and fed with a current of the order of 50 A. The voltage between them is sensed and the resistance derived. The results of this sample, together with those related to the Nb₃Sn termination previously tested, allow to evaluate the effectiveness of the current re-distribution in the joints. The comparison of the results of the two terminations allows to judge the impact of the conductor characteristics (e.g. material, coating, layout) on the interstrand resistance and therefore on the current distribution. The effect of the current redistribution on the voltage-current and voltage-temperature characteristics in short length conductor tests is also discussed.