We have constructed and operated an internal coil device with a high temperature superconductor. Three different types of Ag-sheathed Bi-2223 tapes are employed; i.e., a high critical current tape with a low silver ratio for the main HTS coil, a 0.3wt%Mn-doped one for the persistent current switch, and a 3at%Au-doped one for the coil-leads. Cold gas helium is provided by a GM refrigerator and supplied to the coil through a check valve, and the coil current is directly excited by the external power supply through removable electrodes. It took about 11 hours to cool the coil to 21 K from room temperature, and a nominal cable current of 118 A (overall coil current: 50 kA) was achieved. A decay time constant of the persistent current is a few tens of hours. Plasma experiments in a dipole configuration have been initiated.
Synopsis: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 (R C = 0.15 m, I C = 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.
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