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The plasma experiment apparatus S-RT (Superconducting Ring Trap) is planned for the purpose of high beta plasma confinement research in the University of Tokyo. As a preceding step, Mini-RT, which is the size reduction version of S-RT, has been constructed as a joint research of NIFS, the University of Tokyo, and Kyushu University. In this experiment a magnetic-levitation coil (floating coil) operated in persistent current mode has to levitate for 8 hours in the plasma vacuum vessel. The HTS floating coil wound with a Bi-2223 tape has a diameter of 300 mm and an electromotive force of 50 kA. Since any refrigerant cannot be fed to the coil during the plasma experiment, the coil is designed so that the temperature rise after 8 hours of levitation is less than 40 K with the specific heat of the coil and radiation shield. At the end of the daily plasma experiment, the coil will be drawn down to the maintenance location at the bottom of the plasma vacuum vessel, and it will be re-cooled to 20 K.