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 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.