A New Direct Magnetic Method for Determining ${\rm J}_{\rm C}$ in Bulk Superconductors From Magnetic Field Diffusion Measurements

The knowing the critical current density J_C is important to calculate AC losses in superconducting applications. Usually can be obtained from magnetization measurements or electric measurements based on global quantities as the magnetic flux or the voltage. In this paper a quick and direct magnetic method for determining J_C is proposed. It is based on direct measurements of local magnetic field in the gap between two bulk HTS pellets. Field penetration measurements were carried out on HTS pellets at 77 K by applying increasing axial magnetic fields with a quasi constant sweep rate. This determination of J_C is theoretically based on Bean model. J_C is deduced from the complete penetration magnetic field B_P. B_P is deduced from the delay T_P between the applied magnetic field B_a(t)and the magnetic field at the center between the two pellets B₀(t). Numerical calculations allow deducing J_C more precisely from theoretical calculations and measurements. The numerical calculations are made with the power law E = E_C(J/J_C)ⁿ. For the determination of J_C the influence of the gap due to Hall probe sensor and the applied magnetic field rise rate are taken into account. The influence of thermal is also studied.