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.
A magnetically-levitated superconducting coil device, Mini-RT, has been constructed using a high temperature superconductor (HTS) for the purpose of examining a new magnetic confinement scheme of high-beta non-neutral plasmas. The floating coil is wound with silver-sheathed Bi-2223 tapes, and it is operated in the temperature range of 20-40 K. A number of studies and experiments were carried out in order to realize the necessary system. One of them was to demonstrate magnetic levitation using a miniature HTS floating coil having a diameter of 80 mm. The coil was fabricated using Bi-2223/Ag tapes of 12 m and excited by field cooling with liquid nitrogen. The magnetic levitation was examined using a real-time feedback control system with laser displacement gauges. Additionally, a persistent current switch (PCS) has been developed using Bi-2223/Ag tapes of 21 m, and a prototype HTS-PCS was tested in a cryostat. After construction of the floating coil and HTS-PCS for the Mini-RT device was completed, excitation tests were carried out in the cryostat and the basic properties up to the nominal operation condition were examined.
A magnetically levitated superconducting coil device, Mini-RT, has been constructed using a high temperature superconductor (HTS) for the purpose of examining a new magnetic confinement scheme of high-beta non-neutral plasmas. The floating coil and persistent current switch (PCS) are wound with silver-sheathed Bi-2223 tapes, and they are operated in the temperature range of 20-40 K. After the basic properties of the main coil and HTS-PCS were examined in a liquid helium cryostat, the coil was installed into the vacuum chamber of the Mini-RT device. The HTS floating coil was then cooled by helium gas using GM cryo-coolers equipped with detachable transfer tubes. The excitation tests of the coil were carried out by supplying current from an external DC power supply through the detachable current feeder terminals as the HTS-PCS was turned off. The nominal operation condition was examined by overcoming many difficulties and the persistent current mode was tried by turning on the HTS-PCS. The decay time-constant was evaluated by maintaining the coil temperature for up to four days and the results were compared with calculations. The first magnetic levitation was also examined.
Plasma is produced using a 2.45 GHz microwave in a Mini-RT internal coil device, where a high-temperature superconductor is employed. The radius of the internal coil is 0.15 m and its weight is 16.8 kg. The maximum coil current is 50 kA, and the typical magnetic field strength at the internal coil is 0.1 T. First plasma experiments were carried out using a mechanically supported coil, and a typical plasma density in the range of 1016 m-3 was produced. When the internal coil current is decreased to less than 10 kA, plasma cannot be produced because of the disappearance of the electron cyclotron resonance layer of the 2.45 GHz microwave. Concerning to the levitation experiments of the internal coil, the coil position is monitored with laser sensors, and the levitation coil current is feedback-controlled. The HTS coil has been levitated for a period of one hour with an accuracy of 10 micrometers. Plasma production with a floating coil has been successfully initiated.