Automatic Measurement of the Distribution of Jc and n-Values in Large-Area Superconducting Films Using Third-Harmonic Voltages

Nondestructive, ac inductive methods are widely used to measure critical current densities J_c of large-area superconducting films. However, conventional methods are not accurate because they have not considered the electric-field E criterion of the J_c measurement. In these methods, ac magnetic fields are generated with ac currents I_o cos wt in a small coil mounted just above the film. J_c is calculated from the threshold coil current I_th, at which full penetration of magnetic field is achieved. We recently estimated the induced electric fields in the superconducting film at I_o = I_th, which are proportional to the frequency, and proposed a method to determine the E - J characteristics by measuring I_th at various frequencies. In this paper we propose a standard method to precisely measure J_c with an electric-field criterion by detecting third-harmonic voltages. We also demonstrate a machine that can automatically measure the distribution of J_c 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.