An analytical procedure has been developed to model both the current distribution in a cable consisting of many superconducting strands with resistive electrical contact between them and non-uniform contacts to the current supply at the ends. The procedure allows 'steady state' current operating limits to be determined for the cables assuming a distribution of strand contact resistances at the joints. Using the extensive database of operating limits that is now available on short 4-m conductor samples of 40-50 kA multistage cables using Nb3Sn and NbTi strands developed for fusion applications, characteristic distributions of joint contact resistance have been derived for various joint concepts. Many of the conductor samples tested have quenched at currents markedly lower than would be expected from the performance of the individual strands. The model suggests that these 'premature quenches' in short samples are caused by the joint non-uniformity combined with the transverse resistance within the cable, due to the short length available for current transfer. Extrapolated to large coils, the model suggests that all the Nb3Sn and NbTi joints so far demonstrated on short samples would provide an adequate margin against steady state quench, despite a considerable level of non-uniformity in the joint contact resistances, if the present cable transverse conductivity is maintained. (C) 2000 Elsevier Science Ltd. All rights reserved.
