Limitation of Current Transport in Coated Conductors: Statistical Fluctuations or Weak Spots?

Traditionally, the critical current,
I_c
, of a superconducting wire, considered as the limit of DC operation, is experimentally verified on a sample few centimeters long. This approach could fail in case of high-temperature superconductor wires, where significant changes of
I_c
along the conductor length,
x
, are commonly observed. Then, the quantity that could replace the uniform critical current as the entry data in designing of a superconducting device is to be found. We have analysed the properties of 6 coated conductor tapes provided by 4 different manufacturers. First, the overall critical current of the full tape length,
I_c,ovrl
, has been evaluated from the
I_c(x
) data. However, the dissipation in a non-uniform conductor with fluctuating critical current concentrates in the “weak spots” with the lowest
I_c
values. Then, depending on the conditions of heat removal, thermal runaway in some location could happen before reaching
I_c,ovrl
during conductor testing or operation. Taking into account the conductor architectures and assuming the cooling in liquid nitrogen bath, we computed the thermal runaway current,
I_tr
, at which the weakest location in each of the analysed conductors would convert to a “hot spot” with rapid increase of temperature. Then, comparing
I_tr
to
I_c,ovrl
one can identify which of these two values is lower, i.e., represents the upper limit of transportable current. We discuss how the result of such analysis would be influenced by a metallic stabilisation that exhibits significant impact on cooling.