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

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 α and anisotropy scaling factor γ. 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 × 10 10 A m −2 .