Regarding CO 2 /O 2 -based mixture as an arc quenching gas in high-voltage circuit breakers, the predominant component species present in high numbers at temperatures of 300-20 000 K are determined in consideration of addition of CF 3 CFCH 2 (hereinafter written as C 3 H 2 F 4 ) gas to the CO 2 /O 2 -based mixture. In the determination, not only are 87 gaseous species but also condensed-phase carbon (hereinafter written as C(c)) taken into account as component species. The evaluation results reveal that CO 2 , O 2 , CF 4 and HF in gaseous phase behave as the predominant component species at 300-1000 K for lower content additive-gas, whereas the species C(c) is also generated as the predominant component at the same temperature for higher content additive-gas.Subsequently, reactions of chemical elements C, O, F and H to form the above-described chemical species are successfully expressed using CO 2 , O 2 and C 3 H 2 F 4 contents as stoichiometric coefficients. Expressing the reactions furthermore enables us to formulate the prevent condition of the C(c) generation as a function of O 2 content X O2 and C 3 H 2 F 4 content X CHF . Similar formulations are successfully performed for CO 2 /O 2 /C4-FN and CO 2 /O 2 /C5-FK mixtures.

A repetitively driven compact toroid (CT) injector has been developed for the large fieldreversed configuration (FRC) facility of the C-2/C-2U, primarily for particle refueling. A CT is formed and injected by a magnetized coaxial plasma gun (MCPG) exclusively developed for the C-2/C-2U FRC. To refuel the particles of long-lived FRCs, multiple CT injections are required. Thus, a multi-stage discharge circuit was developed for a multi-pulsed CT injection. The drive frequency of this system can be adjusted up to 1 kHz and the number of CT shots per injector is two; the system can be further upgraded for a larger number of injection pulses. The developed MCPG can achieve a supersonic ejection velocity in the range of ~100 km s −1 . The key plasma parameters of electron density, electron temperature and the number of particles are ~5 × 10 21 m −3 , ~30 eV and 0.5-1.0 × 10 19 , respectively. In this project, singleand double-pulsed counter CT injection fueling were conducted on the C-2/C-2U facility by two CT injectors. The CT injectors were mounted 1 m apart in the vicinity of the mid-plane. To avoid disruptive perturbation on the FRC, the CT injectors were operated at the lower limit of the particle inventory. The experiments demonstrated successful refueling with a significant density build-up of 20-30% of the FRC particle inventory per single CT injection without any deleterious effects on the C-2/C-2U FRC.

A magnetized coaxial plasma gun (MCPG) is utilized to generate a compact toroid (CT). An MCPG-type CT injector had been developed as a particle refueling system for C-2/C-2U field-reversed configuration (FRC) plasmas. To inject CTs repetitively for a long-lived plasma, the injector has been upgraded. Iron-core bias coil system has been adopted to generate stationary bias magnetic field. Typical MCPG systems use excess neutral gas to produce a breakdown; therefore, the excess gas tends to flow into the confinement vessel and cool off the edge plasma as well as the target plasma. This negative effect is more serious for repetitive CT injection so that a pre-ionization (PI) system is required to reduce initial gas amount. By injecting the initial plasma using the PI system, amount of the neutral gas for the injector can be reduced. The combination of these systems also expands operating range of the injector. By moving the iron-core bias coil, the radial magnetic field can be controlled. The PI system can easily produce breakdown; therefore, the MCPG can be operated at lower gas pressure, reduced by approximately 40 %. The optimum CT has higher velocity (>100 km/s) and ion temperature (>70 eV), increased by more than 40 %.

In order to investigate the collisional merging process of field-reversed configurations (FRCs), the FAT device has recently been upgraded to FAT-CM, consisting of two field-reversed theta-pinch (FRTP) formation sections and the confinement section. Collisional merging of the two FRCs causes a conversion of the kinetic energy to mostly thermal ion energy, resulting in an increase of the ion pressure that greatly expands the FRC size/volume. This increase of the FRC size is observed by magnetic diagnostics in the confinement region, leading to an increase in the excluded flux; on a side note, these characteristics/phenomena have also been observed in C-2/C-2U experiments at TAE Technologies. The process of FRC formation, translation and collisional merging in FAT-CM has been simulated by Lamy Ridge, 2D resistive magnetohydrodynamics code, in which the same phenomenon of the excluded-flux increase via FRC collisional merging has been observed. Simulation results also indicate that there is an importance of the external magnetic field structure/profile in the confinement region, clearly affecting the FRC merging. Steeper magnetic field gradient by a strong mirror field appears to suppress the axial expansion of collided FRCs and lead a merged FRC to higher temperature.

We have been conducting compact toroid (CT) collision and merging experiments by using two magnetized coaxial plasma guns. As is well known, an actual CT/plasmoid moves macroscopically in a confining magnetic field. Therefore, three-dimensional measurements are important in understanding the behavior of the CTs. To observe the macroscopic process, we adopted a fast-framing camera (ULTRA Cam HS-106E) developed by NAC Image Technology. The characteristics of this camera are as follows: a CCD color sensor, capable of capturing 120 images during one sequence with a frame rate of up to 1.25 MHz. Using this camera, we captured the global motion of a CT inside the magnetic field and the collision of two CTs at the mid-plane of the experimental device. Additionally, by using a color sensor, we captured the global change in the plasma emission of visible light during the CT collision/merging process. As a result of these measurements, we determined the CT’s global motion and the changes in the CT’s shape and visible emission. The detailed system setup and experimental results are presented and discussed.