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

Abstract

Coated conductor (CC) tapes often exhibit the critical current, Ic, that fluctuates along the length coordinate, x, leading to an Ic(x) dependence. Requests for a price reduction do not favor a decrease of Ic variation in the near future. Then it is reasonable to develop a method for estimating possible impact of such fluctuating transport ability on the performance of superconducting devices. Fortunately, providing Ic(x) data together with delivered conductor is becoming a standard approach among CC tape producers and an in-depth analysis of such data is then 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 the overall critical current as the value generating 1 µV/cm of electric field over the whole conductor. We show that in case of Ic(x) fluctuations obeying the Gaussian distribution one can predict the value of 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 further reduction of overall critical current. Conversely, in case of strong dropouts in the critical current value, a statistical approach is useless and one must analyze the probability that the weakest location develops in a hot spot with dramatic increase of temperature. Extending the analysis for single tape to cables and coils resulted in rather simple summary: when a current sharing is possible, the reduction of overall critical current with respect to its mean value is significantly depressed. This favors parallel arrangement of tapes with low contact resistance allowing current migration from tape to tape. On the other hand, insulation of tapes leads to the same overall critical current reduction as observed for a single tape.