Impact of critical current fluctuations on the performance of a coated conductor tape

Abstract

Coated conductor (CC) tapes often exhibit critical current, Ic, which fluctuates along the length coordinate, x, leading to an Ic(x) dependence. The need to reduce the price of these tapes does not favor a decrease in the variation of Ic in the near future. Therefore, it is reasonable to develop a method for estimating the possible impact of such fluctuating transport ability on the performance of superconducting devices. Fortunately, providing Ic(x) data together with the delivered conductor is becoming a standard approach among CC tape producers, and in-depth analysis of such data is now possible. Extrapolation of the short sample testing methodology to the case of a device incorporating many meters of CC tape leads to the introduction of an ‘overall critical current’ as a value generating 1 μV cm−1 of electric field over the whole conductor. We show that in the case of Ic(x) fluctuations obeying the Gaussian distribution one can predict the value of the overall critical current from the mean value and the variance of the Ic(x) data set. Deviation from the Gaussian distribution found in real tapes would cause a further reduction of the overall critical current. Conversely, in the case of strong dropouts in the critical current value, a statistical approach is useless and one must analyze the probability of the weakest location developing in a hot spot with a dramatic increase of temperature. Extending the analysis for a single tape to cables and coils results in a rather simple summary: when current sharing is possible, the reduction of overall critical current with respect to its mean value is significantly depressed. This favors the parallel arrangement of tapes with low contact resistance allowing current migration from tape to tape. On the other hand, the insulation of tapes leads to the same overall critical current reduction as observed for a single tape.