The index n of the voltage-current curve, in the characterization and specification of technical superconductors

The attention of the fusion magnet specialists has been drawn in the last years to the volt-ampere characteristic (VAC) of the large cable-in-conduit conductors (CICC), as early voltage was observed in conductor and model coil tests, far away from the expected critical current. In this paper, the studies and models developed in the last decades for the VAC of bulk material and multifilament strands are shortly reviewed. The practical role of the index n of the VAC, both at the industry and in the magnet design is discussed for composite strands and for cabled conductors. Finally, few flashes of experimental results on the VAC of large CICC, all measured in the SULTAN test facility, are presented and discussed, with recommendation for test, QA and design for the fusion conductors