Mahajan-Yoshida proposed an idea for confining high-beta plasma utilizing a fast plasma flow in a toroidal direction. In addition, Hasegawa considered a dipole fusion reactor. Here, the authors introduce a “Mini-RT" internal coil device, in which a superconducting coil is levitated in a vacuum vessel and plasma is confined by a dipole magnetic field, to explore a new high-beta plasma for fusion research. The conductor of the floating coil (RC= 0.15 m, IC= 50 kA) is Bi-2223 tape that is cooled by cold helium gas to 20 K. By controlling the coil current of the levitation coil located at the top of the vacuum chamber, a few hours of levitation can be obtained for plasma experiments. It is possible to produce various magnetic configurations with a combination of vertical field coils. Several issues related to interaction with plasma produced around the floating coil have been taken into account for designing the floating coil; for example, clearance of the magnetic surface at the torus inner region, heat load to the floating coil caused by the plasma and so on.
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.