Evaluation of uncertainty in the inductive measurement of critical current densities of superconducting films using third-harmonic voltages

Several techniques can be used for inductive measurement of the critical current density Jc of large-area superconducting thin films used in microwave devices and fault-current limiters. The most popular of these methods employs the third-harmonic voltages V3. We have proposed a standard method using V3 for determining Jc under a criterion of electric field E. Here, the uncertainty in the standard method is evaluated. Since the measured Jc is directly proportional to the magnetic field at the upper surface of the superconducting film, the most significant systematic effect is the deviation of the coil-to-film distance Z1 from the prescribed value. The principal origins of this deviation of Z1 are (1) inadequate pressing of the coil onto the film and (2) ice layers occasionally forming between the coil and the protective polyimide film. If these effects are eliminated, uncertainty of Jc originates mainly from (a) uncertainty of the experimental coil coefficient k′, which is dominated by uncertainty of the transport Jc, and (b) underestimation of the induced electric field E when using a simple Bean model. For a typical DyBa2Cu3O7 film specimen, the relative combined standard uncertainty in the standard method was evaluated as ∼5%. The effect of the film edge on Jc measurements is also described.

► The critical current density Jc of superconducting thin films is a key parameter.
► The most popular method used to inductively measure Jc uses third-harmonic voltages.
► The uncertainty in Jc measurement using third-harmonic voltages has been evaluated.
► The relative combined standard uncertainty in the standard method is less than 5%.