The use of a non-neutral plasma confinement device with a floating internal coil has been planned for the purpose of high-beta plasma confinement research at the University of Tokyo. A device known as the Mini-RT (Miniature Ring Trap) has been constructed as a joint research project between the University of Tokyo, NIFS and Kyushu University. In the experiment, a magnetic-levitation coil (floating coil) operated in a persistent current mode is levitated for 8 hours in a plasma vacuum vessel. The high-temperature superconducting (HTS) floating coil is wound with Bi-2223 tape, and has a diameter of 300 mm and an electromotive force of 50 kA. Since 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 as the result of considering the specific heat of the coil and incorporating a radiation shield. At the end of the daily plasma experiment, the coil is drawn down to the maintenance location at the bottom of the plasma vacuum vessel, and is re-cooled to 20 K. The engineering design points of the Mini-RT, such as the HTS floating coil, HTS persistent current switch (PCS), cooling system and excitation scheme are summarized.
