How to determine critical current density in YBCO tapes from voltage-current measurements at low magnetic fields

摘要

In this paper, an optimization method to determine the magnetic field dependence of the intrinsic critical current density, J_c(B), is introduced. First of all the critical current is measured in various external magnetic fields. Then the J_c(B)-dependence that fits optimally to the measurements is searched for. In these computations the self-field of the sample is taken into account and thereby measurements performed in low external magnetic fields, below 0.1 T, can be exploited. Here the J_c(B)-dependence of YBCO material is described by the Kim model, which is modified to include the anisotropy also. Thus, there are four parameters to search for: zero field critical current density J_(C0), reference field B_0, Kim model exponent alpha and anisotropy scaling factor gamma. The search for these parameters was computationally challenging but computation times were still satisfactory. As examples, the J_c(B)-dependences of two YBCO samples from different manufacturers were found. For both samples, all parameters except B_0 were near each other. For example, J_(c0) values for both samples were about 0.9 X 10~(10) A m~(-2).