AC loss problem in high-T_c bulk superconductors

Smooth voltage-current characteristics and very low thermal conductivity of high temperature superconductors (HTS) materials lead to the pronounced difference of the actual AC losses from the values predicted by Bean's critical state model. To calculate AC losses in HTS, the approach is suggested which takes into account actual voltage-current characteristics of HTS and heating due to AC losses. This approach is based on the ideology of the critical state model with an effective critical current density depending on the local magnetic field and the local temperature. For low magnetic fields, when the heating can be negligible, the analytical solution for AC losses is obtained. The theoretical results explain experimentally observed dependencies of AC losses on the frequency and amplitude of the magnetic field. It is shown that a small variation of the approximation parameters for the voltage-current characteristic of HTS leads to the pronounced change of the calculation result.