Construction and Operation of an Internal Coil Device, RT-1, with a High-Temperature Superconductor

Abstract

An internal coil device called Ring Trap-1 (RT-1) has been constructed to explore an innovative concept for a high-beta plasma based on a new relaxation theory. A high-temperature superconductor (HTS) Bi-2223 tape is employed for the internal coil of RT-1. The coil is cooled to 20 K with helium gas supplied by G-M refrigerators, and charged to a magnetomotive force of 250 kA using an external power supply. For these cooling and charging methods, we have developed several innovative techniques such as a demountable transfer tube system, persistent current switch, detachable electrode, and others. In addition, we have paid much attention to the deterioration of the HTS tape during the fabrication of the internal coil. As a result, we have demonstrated that the decay of the persistent current of the internal coil is ∼1% during 8 h. The internal coil is lifted with a levitation coil located at the upper region of the vacuum vessel. The coil position monitored with laser sensors is feedback controlled through the regulation of the levitation coil current. Stable levitation for a few hours has been demonstrated for various plasma experiments.

Measurement of the Density Profile of a Toroidal Non-neutral Plasma with a Wall-Probe Array

“…We used three wall probes and estimated the confinement regions of the plasma for the typical cases of (1) during electron injection, (2) just after the electron supply ended, and (3) just before the confinement ended. During the electron gun operation, the outer edge of the electron confinement region approximately agreed with the magnetic separatrix.…”

“…Because of the asymmetry of the magnetic surfaces generated by the combination of the levitated and levitation coils, walls N2 and N3 rather than N4 were close to the plasma, and relatively small signals were observed on N4. To evaluate the relationship between the plasma shapes and the observed E r on N2-N3, (Q N2 /Q N4 ) 2 + (Q N3 /Q N4 ) 2 were calculated with variouos values of r 0 . As shown in Fig.…”

“…The Ring Trap 1 (RT-1) device is a "laboratory magnetosphere" that produces a magnetospheric dipole field configuration by a levitated superconducting coil [1,2]. A pure electron plasma can be confined in RT-1; the density is of the order of 10 11 m −3 , and the confinement time is longer than 400 s [3].…”

“…4. Figure 1 shows the poloidal cross section of RT-1 [1,2]. Inside the chamber, RT-1 has a high-temperature superconducting coil with a Bi-2223 wire rod.…”

“…A more detailed description of the RT-1 device and the superconducting levitated coil system can be found in Ref. [2]. The vacuum chamber had an inner diameter of 2 m and was evacuated to a base pressure of 7 × 10 −7 Pa by two turbomolecular pumps.…”