This paper presents improvement of a novel levitation system in which soft magnetic material can be levitated by high-temperature superconductor (HTS). The levitation system consists of two permanent magnets, HTS samples of Dy 1Ba2Cu3Oy (DBCO), and movable yoke with cylindrical parts to which trapped flux in the HTS is gathered and produces an attractive force. The attractive force generally increases with decreasing air gap, but it decreases in a certain air gap called "positive stiffness air gap," which causes positive stiffness to achieve stable levitation of the movable yoke. In this work, we propose closed magnetic circuit using ferromagnetic yoke to enhance the magnetic force and investigate the relationship between attractive force and air gap with different thicknesses of the HTS and different diameters of cylindrical parts. By our proposing closed magnetic circuit, we obtain maximum levitation force of 85 N and stiffness of 4.5 N/mm. The tradeoff between the levitation force and positive stiffness air gap with the thickness of the HTS indicates that there is optimal thickness of HTS for the practical levitation system.
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