Measurement Methods for Critical Current Density

Abstract

Chapter 5 covers various measurement methods for the critical current density. The four terminal method with some criterion on the electric field strength or resistivity is commonly used for long wires or tapes. The DC magnetization method with the theoretical prediction of the critical state model is sometimes used for specimens for which the four terminal method can be hardly applied. Although these methods provide an averaged critical current density, Campbell’s method reveals the distribution of local critical current density inside a specimen. An example of large critical current density near the surface region is shown. This method is also useful for analyzing the reversible motion of flux lines discussed in Chap. 3. Other methods using the third harmonic of AC permeability or the fundamental AC susceptibility are also introduced. However, if AC magnetic methods including Campbell’s method are used for superconductors smaller than the pinning correlation length, the critical current densities are seriously overestimated. The reason for the overestimation is discussed, and a method of correction is proposed.