Coated conductor (CC) tapes with functional REBCO (RE = Y, Gd, Er,…) layer exhibit unprecedented capability of transporting large amounts of electricity. Critical current, I_c, could reach e.g. 1000 A in a tape 12 mm wide when cooled down to 77 K. However, checking this property along the tape length quite often reveals a fluctuation that in certain locations results in I_c value reduced to, say, 900 A. It is then not obvious which value should be considered as the limitation for a steady long term transport of electrical current.

We have tested two different approaches in tackling the problem: in case of fluctuations described by a narrow normal distribution, i.e. with standard deviation approximately less than 5% of the mean I_c value, a statistical analysis allows to predict the “overall critical current”. It is determined as the value generating on a long length the total dissipation identical with the one expected at conventional 1 µV/cm in case of a homogeneous conductor. However in case of “un-statistical” dropouts of critical current a different analysis is necessary. For this purpose we have investigated the evolution of temperature in the location with reduced critical current. It was found that at surpassing the local I_c, this temperature could stabilize at an elevated value, when the dissipation is balanced by cooling capability of the environment. Nevertheless there will be always the value of transported current that triggers local thermal runaway and creation of a hot spot. Analytical formulas were developed describing the influence of tape architecture and quality of cooling on the process of a hot spot creation that provide clear indications for improvement of tape stability. Experiments have been performed that confirm the validity of theoretical predictions.