Effect of Length Scale on Critical Current Measurement in High Temperature Superconductor Wires

Measured variations in the local critical current (Ic) along the length of second-generation high-temperature superconductor wires depend surprisingly strongly on the voltage measurement gauge length. Longer gauge lengths can give the appearance of higher Ic uniformity than is actually present, and understanding this gauge length effect is essential for insuring that second-generation wires meet commercial specifications. The effect can be understood from the moderate index value n, which characterizes the voltage-current relationship (V ~ In) and is often found to be in the same range for regions of lower as well as higher critical current. In this paper, we present data demonstrating the gauge length dependence of the measured minimum local Ic and discuss a simple model based on earlier work of Friesen and Gurevich, which explains the effect and highlights the importance of measuring long length wires with short gauge lengths.