Effect of n-value on third harmonic voltage method analyzed by finite element method

Abstract

To investigate the correctness of the estimation of the critical current density J c in high- T c superconductor by third harmonic voltage measurement method, the third harmonic voltage V 3 and the distributions of magnetic flux and current in a superconducting film with various n-values are calculated by FEM using the n-value model. When the AC current I 0 cos ω t is applied to a coil, the threshold coil current I th , which is proportional to J c in the film, is determined by so-called off-set method for V 3/ I 0– I 0 curve which is frequently used in the four terminal method. The calculated value of I th is slightly smaller than that of theoretical value by Bean’s model. When the coil current I 0 is smaller than I th , although the current density distribution is almost uniformly distributed as predicted by Bean’s model, the value of current density is smaller than J c . This behavior is different from the prediction of Bean’s model. It is found that the current density is uniform and is equal to J c for high n-value when I 0 is closed to I th . Therefore, it seems to be reasonable to estimate J c by I th in the above method for superconducting films with n-value higher than 20.