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

Abstract

Traditionally, the critical current, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> , 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 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> along the conductor length, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> , 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, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c_,ovrl</i> , has been evaluated from the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c(x</i> ) data. However, the dissipation in a non-uniform conductor with fluctuating critical current concentrates in the “weak spots” with the lowest <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> values. Then, depending on the conditions of heat removal, thermal runaway in some location could happen before reaching <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c_,ovrl</i> 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, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_tr</i> , at which the weakest location in each of the analysed conductors would convert to a “hot spot” with rapid increase of temperature. Then, comparing <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_tr</i> to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c_,ovrl</i> 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.