INFLUENCE OF AN APPLIED MAGNETIC-FIELD ON SHIELDING CURRENT PATHS IN A HIGH T-C SUPERCONDUCTOR

The influence of an applied magnetic field on shielding current paths in a bulk high T-c superconductor was investigated. This issue is very important for the quantitative analysis of levitation force, since th e distortion and localization of the current paths have been found to be a major source of discrepancy between computed and experimental res ults. Furthermore, it has been speculated that the current paths vary, depending on the applied magnetic field. The magnetic field due to su perconducting currents was measured by a Hall sensor when a permanent magnet was positioned at different gaps from a superconductor. The cur rent paths were then reconstructed from the measured field data by an inverse analysis. A genetic algorithm, which is a robust probabilistic optimizing method, was used for the inverse analysis. It was found th at the current paths merged into larger loops as the permanent magnet was retreating from the superconductor. It implies that a stronger app lied magnetic field locally degrades the current density (J(c)) and th e shielding current paths are localized by these low-J(c) areas which act as insulating boundaries.