DC Performance Results Versus Assessment of ITER Main Busbar NbTi Conductors

Four ITER main busbar (MB) conductor samples were tested in the SULTAN test facility (Centre de Recherches en Physique des Plasma, Ecole Polytechnique Fédérale de Lausanne, Switzerland) between 2011 and 2013. The MB conductors are NbTi-based cable-in-conduit conductors (CICCs), and they will become part of the feeder system of the ITER magnets. The measured dc performance of the four MB samples varied significantly, supposedly depending on the design of the bottom terminations. Two out of three samples with a U-bend box, made of a continuous conductor section, exhibit approximately 0.5 K lower current-sharing temperature Tcs than the sample with a solder-filled bottom joint, consisting of two straight conductor sections. We assess the theoretically expected Tcs performance of the MB conductor based on the characterization of individual NbTi strands, on the ITER NbTi scaling law, and on the magnetic field distribution across the cable cross section. The magnetic field in a SULTAN sample consists of three components, namely, the background SULTAN field, the self-field generated by the current in the conductor under test, and the magnetic field generated by the current in the return conductor. Taking into account all the three components, we calculate the average electric field in the cable and determine Tcs as in the experiment, namely, as the temperature at which the electric field reaches the critical value of Ec=0.1 μV/cm. The theoretically assessed Tcs confirms that the MB sample with solder-filled joint behaves as expected.