Dependence of lateral stiffness on magnetic field distribution at field-cooling process of HTS bulk system

Stiffness is one of the important parameters for HTS bulk applications to motor, magnetic bearing in flywheel system and magnetic levitating transporter and so on. We prepared several types of disk-shaped permanent magnets with different diameters for realizing different magnetic field distribution. Since the lateral stiffness depends on not only magnetic field distribution but also the magnitude of magnetic field, we measured the restoring force against lateral disturbance under the condition of the same magnetic flux density on the top surface of HTS bulk. The maximum restoring force of HTS bulk against single permanent magnet increased with the diameter of the permanent magnet and the displacement at the maximum restoring force was almost equal to the diameter of the permanent magnet. We also performed analytical evaluation of restoring force by calculating stored energy within HTS bulk in each bulk location to investigate the relationship between restoring force and magnetic field distribution. Experimental and analytical results implies that slope of magnetic field around field-cooling location is one of the important parameter for achieving large lateral stiffness.