Influence of the surface magnetic field of a cylindrical permanent magnet on the maximum levitation force in high-T_c super conductors

摘要

In this paper we present the dependence of the maximum levitation force (F_7~(Max)) of a high-T_c superconductor on the surface magnetic field (B_s) of a cylindrical permanent magnet, based on the Bean critical state model and Ampere's law. A transition point of B_s is found at which the relation between _7~(Max) and B_s changes: while the surface magnetic field is less than the transition point the dependence is subjected to a nonlinear function, otherwise it is a linear one. The two different relations are estimated to correspond to partial penetration of the shielding currents in the interior of the superconductor below the transition point and complete penetration above it, respectively. Furthermore, the influence of the geometrical properties of superconductors on the transition point of B_s is discussed, which shows a quadratic polynomial function between the transition points and the radii and the thickness of superconductors. Some optimum contours of the transition point of B_s are presented in order to achieve large levitation forces.