Reconstruction of current unbalance in full-size ITER NbTi CICC by self-field measurements

Methods have been developed to study the distribution of the transport current among the strand bundles of cable-in-conduit conductors (CICC) by using self-field measurements with Hall probe arrays. The unbalance in the transport current is mainly caused by the unavoidable nonuniformity of the joints and can be a reason for a change in the voltage-temperature characteristic and consequently of the temperature margin. In the present study we focus on the reconstruction of the current unbalance in a full size NbTi CICC tested in the SULTAN test facility. The crucial point in the reconstruction procedure is the proper choice of the reference self-field profile corresponding to a uniform current distribution. To achieve this uniform distribution, the conductors were driven far into the current sharing regime. The self-field profile corresponding to the highest achieved voltage level was taken as a reference. This assumption is supported by the modeling of current-sharing runs in the CUDI-CICC network model. Furthermore, local redistribution effects were observed in the conductors at high currents. The current transfer associated with this redistribution is analyzed as well.