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

摘要

The knowing the critical current density $J_{C}$ is important to calculate AC losses in superconducting applications. Usually $J_{C}$ 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 $B_{0}(t)$ . 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 $J_{C}$ th- 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.