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

Nondestructive, AC inductive methods detecting third-harmonic voltages are widely used to measure the distribution of local critical current densities Jc of large-area superconducting films. We have extended this method to determine the electric field versus current density relation by evaluating the dependence of Jc 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 Jc values may be inaccurate when a thick metallic stabilizing layer of Ag or Cu (⩾10 μm) covers the film of (RE)Ba2Cu3O7 (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 Jc 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.