Automatic Measurement of the Distribution of $J_{\rm c}$ and n-Values in Large-Area Superconducting Films Using Third-Harmonic Voltages
Nondestructive, ac inductive methods are widely used to measure critical current densities Jc of large-area superconducting films. However, conventional methods are not accurate because they have not considered the electric-field E criterion of the Jc measurement. In these methods, ac magnetic fields are generated with ac currents Io cos wt in a small coil mounted just above the film. Jc is calculated from the threshold coil current Ith, at which full penetration of magnetic field is achieved. We recently estimated the induced electric fields in the superconducting film at Io = Ith, which are proportional to the frequency, and proposed a method to determine the E - J characteristics by measuring Ith at various frequencies. In this paper we propose a standard method to precisely measure Jc with an electric-field criterion by detecting third-harmonic voltages. We also demonstrate a machine that can automatically measure the distribution of Jc and the n-values of the power-law E - J characteristics. We show several examples of measurements of co-evaporated YBCO films and low-cost metal organic deposition YBCO films, which were used in switching experiments for resistive fault-current limiters.