Novel Applications of Inductive Method for Measuring Critical Current Density

The inductive method for measuring the critical current density in a high-temperature superconducting (HTS) sample has been investigated numerically. In the method, the magnetic field is generated by a small coil placed just above an HTS sample. While applying an ac current I(t)=I₀sin2πft in the coil, the third-harmonic voltage V₃sin(6πft+θ₃) induced in the coil is measured. For the purpose of numerically reproducing the inductive method, a numerical code has been developed on the basis of the finite element method and the backward Euler method. By means of the code, I₀-V₃ curves are determined for an HTS bulk or for double-sided HTS films on a substrate. The results of computations show that, for an HTS bulk, the accuracy of the inductive method is drastically affected by the coil position, the coil radius and the frequency. In addition, it is also found that, for double-sided films, the accuracy is significantly influenced by a substrate thickness.