Levitation forces and relaxation properties of a high-Tc superconductor in varying external magnetic fields

Abstract

Because the intensity and distribution of the external magnetic field are limited, the levitation characteristics of a high-Tc superconductor (HTS) are also remarkably restricted on the present magnetic levitation system. Based on the unique features of the magnetic field distribution and gradient of the superconducting magnet (SM), the levitation forces of the HTS in a gradient varying low magnetic field and an intensity considerable varying magnetic field of the SM were investigated in this paper. The results showed that the force and the hysteresis loss of the bulk both increased with increasing the external magnetic fields, when the HTS was placed in the gradient varying magnetic field. The influence of the moving times on the levitation force could be neglected, with increasing the HTS's moving times. Furthermore, the direction variation of the induced shielding current (Is) in the HTS which was kept in the intensity considerable varying magnetic field directly resulted in the positive and negative values of the force, so that the forces measured in the field increase and decrease process exhibited an approximate anti-symmetrical character. When the full penetration behavior happened in the HTS, the variation tendency of the force with the external magnetic field displayed two different segments, whatever in the field increase or decrease process. In the aspect of the relaxation properties, it can be found that the magnetic field (Bz) and the magnetic field gradient (dBz/dz) of the SM were bigger, the attenuation of the forces were larger. The field-cooling (FC) method was beneficial to improve the relaxation performance of the HTS.