2019 Volume 60 Issue 4 Pages 574-582
In four probe sensing for measurement of voltage-current curve of superconducting layer-coated tapes, it has been reported that, when a large accidental defect is included in the region between the voltage probes, the critical current value estimated with the one micro-volt/cm criterion is low when the voltage probe spacing is small but it becomes higher when the spacing is larger. In the present work, a simulation study was carried out to reproduce the feature stated above and to describe the dependence of critical current value on the voltage probe spacing under the co-existence of a large crack and multiple small cracks. In simulation, the current shunting model at cracks and a Monte Carlo simulation method were applied to a model superconducting tape, which was composed of one local section with a large crack and multiple local sections with small cracks of different size from each other. The experimentally observed increase in critical current with increasing voltage probe spacing in the large defect included tape was reproduced well by the present simulation. Then, it was shown that the increase in critical current of the large crack-included region with increasing voltage probe spacing is caused by the increase in the ligament part-transported current and shunting current at the large crack due to the increase in critical voltage for determination of critical current. Furthermore, it was shown that the upper and lower bounds of the critical current of the region, in which a large crack and multiple small cracks exist, can be derived from the voltage-current curve of the section with a large crack, and the critical current of the region is given by the upper bound for a given size of the largest crack when the difference in size between the large crack and multiple small cracks is large.