Nondestructive inductive measurement of local critical current densities and n-values in coated conductors

Abstract Nondestructive, AC inductive methods detecting third-harmonic voltages are widely used to measure the distribution of local critical current densities J c of large-area superconducting films. We have extended this method to determine the electric field versus current density relation by evaluating the dependence of J c on measurement frequency. This powerful method can also be applied to coated conductors whose critical currents per unit width are larger than 25 kA/m. However, the n -values may be underestimated due to the skin effect and the obtained J c values may be inaccurate when a thick metallic stabilizing layer of Ag or Cu (⩾10 μm) covers the film of (RE)Ba 2 Cu 3 O 7 (abbreviated to (RE)BCO, where RE stands for a rare-earth element). To overcome this problem, a useful method is to limit the measurement frequencies to less than ∼8 kHz. We illustrate the feasibility of this approach with several examples of measurements in YBCO- and GdBCO-coated conductors. We also demonstrate that magnetic-field angle dependent J c measurements are possible for coated conductors. Such measurements are of practical importance in designing superconducting coils and are also useful in investigating the flux pinning mechanism.