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A novel physical vapor deposition method involving electromagnetic acceleration using a set of coaxial electrodes has been developed. In this study, the coaxial ion acceleration method is applied for a diamond-like carbon (DLC) thin film formation. In the developed method, the central electrode made of the deposition material is sputtered by the noble gas plasma current and accelerated toward the deposition chamber. Because the sputtered ions are accelerated by the Lorentz self-force, the ion injection energy can be controlled separately from the plasma temperature. In addition, the gaseous hydrocarbon, which is commonly used for DLC formation, is not required since a noble gas is used as the discharge gas.

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.

Collisional merging experiments of a field-reversed configuration (FRC) plasma at the super-Alfvenic velocity have been conducted in the FAT (FRC Amplification via Translation)-CM (Collisional Merging) device. In the experiments, two FRCs are collided and merged in a confinement section with a quasi-static confinement magnetic field. Therefore, it is necessary to measure the high-frequency pulsed magnetic field superposed on a quasi-stationary signal. The magnetic field is generally measured by a magnetic coil probe in the pulse discharge experiments; however, in such measurements, errors arise in the low-frequency band in the conducted FRC experiments. Therefore, a Hall sensor has been applied for low-frequency magnetic field measurements in the FAT-CM experiments. Calibration of the Hall sensor involves confirming that the sensor has a sufficient response speed and linear characteristics for the magnetic field with a rising time of approximately 240 µs and that its output voltage does not saturate up to a magnetic field of 0.7 T. Combination of the Hall sensor and the magnetic coil probe ensures a comprehensive measurement of the magnetic field in the range of FAT-CM experiments. In this study, accurate magnetic measurements were performed in a collisional merging experiment in the FAT-CM device by using a combined magnetic diagnostic system.

An experimental device to demonstrate additional heating and control methods for a field-reversed configuration (FRC) has been developed. The newly developed device, named FRC Amplification via Translation (FAT), has a field-reversed theta-pinch plasma source and a low-elongation dielectric (transparent quartz) confinement chamber with quasi-static confinement field. In the initial experiments on the FAT device, FRC translation and trapping were successfully demonstrated. Although the typical elongation of the trapped FRC in the confinement region was roughly three, no disruptive global instability, such as tilt, was observed. The FAT device increases the latitude to perform translation-related experiments, such as those concerning inductive current drive, equivalent neutral beam injection effects, and wave applications.

We have developed a compact toroid (CT) injector system for particle refueling of the advanced beam-driven C-2U field-reversed configuration (FRC) plasma. The CT injector is a magnetized coaxial plasma gun (MCPG), and the produced CT must cross the perpendicular magnetic field surrounding the FRC for the refueling of C-2U. To simulate this environment, an experimental test stand has been constructed. A transverse magnetic field of ∼1 kG is established, which is comparable to the C-2U axial magnetic field in the confinement section, and CTs are fired across it. On the test stand we have been characterizing and studying CT formation, ejection/translation from the MCPG, and penetration into transverse magnetic fields.

A new high current type pulsed neutral beam injection device has been developed using washer gun for the plasma source. We extracted the maximum beam current of 3.7A for then acceleration voltage of 10kV successfully. We confirmed from examination of experiment results that aim specifications (15kV, 20A) are enabled by the improvement of some device conditions.

A constructed diagnostic system consisting of a 60-channel set of optical detectors with flexible viewing configurations is realized to investigate three-dimensional magnetohydrodynamic (MHD) motions and the internal structure of a field-reversed configuration (FRC) plasma. The system can detect radiation from the plasma in the wavelength range of 420–820 nm. Optical filters are used to select the wavelength ranges required in the experiment. The sensitivities of all the optical detectors are calibrated using radiation from the FRC plasma at a quiescent phase. Radiation profiles measured by orthogonal viewing configuration of the detectors are shown at three toroidal cross sections. From these profiles, the time evolution of the three-dimensional MHD motion of the plasma is depicted. The radiation profile measured by a one-dimensional viewing configuration yields not only an electron density profile inside the separatrix but also the width of an edge-layer plasma. A bright halo around the edge-layer plasm...

Low frequency f = ⅕ − ⅓fci, (fci: ion gyro frequency in the external field Bw) waves are excited with an antenna which is compatible with a reactor in a plasma with field reversed configuration (FRC). Near and outside the separatrix rs of the FRC plasma, though the applied wave is mainly in compressional mode, azimuthal and radial components are observed in the magnetic field disturbance of the excited wave, which propagate with the dispersion relation consistent with the shear Alfven wave. These disturbances penetrate deep into the FRC plasma across the surface where the wave frequency exceeds local ion gyro frequency and propagate along magnetic lines of force with sound velocity, which behaviour is consistent with the shear Alfven wave with finite temperature correction. Axial magnetic disturbance propagates axially and radially from the antenna across the plasma column.

A repetitively driven compact toroid (CT) injector has been developed for the large field-reversed 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 × 1021 m−3, ~30 eV and 0.5–1.0 × 1019, respectively. In this project, single- and 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.

Experiments on additional heating by neutral beam injection and application of a low frequency wave to a plasma with an extremely high averaged beta value of about 90% - a field reversed configuration (FRC) plasma - are carried out using the FRC Injection Experiment (FIX) apparatus. These experiments are made possible by translating the FRC plasma produced in a formation region of a theta pinch to a confinement region in order to secure better accessibility to heating facilities and to control plasma density. By determining the appropriate injection geometry and the mirror ratio of the confinement region, it became possible to inject a neutral beam with an energy of 14 keV and a current of 23 A into the FRC in a solenoidal confining field of only 0.04-0.05 T. Plasma confinement is improved in this experiment. Ion heating is observed to result from the application of a low frequency (80 kHz, about 1/4 of the ion gyrofrequency) compressional wave. A shear wave, probably mode converted from the compressional wave, is observed to propagate axially.

In this article, the measurement system for the total radiation and electron temperature profiles to be installed in a reversed-field pinch machine, toroidal pinch experiment, RX [TPE-RX, R/a=1.72/0.45 m, Ip

A soft x-ray (SXR) measurement system for tomography analysis on a reversed-field pinch machine. torodial pinch experiment, RX [TPE-RX, R/a=1.72/0.45 m, Ip

A new type pulsed ion beam source was developed by use of washer gun plasma source. It will provide low-cost and maintenance-free ion beam for the neutral beam injector (NBI), eliminating the conventional filament and water cooling systems. Its low-voltage and high-current properties are suitable for the present spherical tokamak experiment.

A compact tomography camera system consisting of a photomultiplier tube, a multislit optical system, and a band-pass interference filter has been developed. The viewing area and spatial resolution can be configured by the arrangement of the slit system. The camera system has been specially designed for self-organized compact torus experiments having strong magnetohydrodynamics events with a submicrosecond time-scale. The developed system has been tested on a field-reversed configuration formed by the field-reversed theta-pinch. Performance evaluation of the system has been performed by comparison to the former optical system.

A compact and repetitively driven plasma source has been developed by utilizing a magnetized coaxial plasma gun (MCPG) for diagnostics requiring deep penetration of a large amount of neutral flux. The system consists of a MCPG 95mm in length with a DN16 ConFlat connection port and an insulated gate bipolar transistor (IGBT) inverter power unit. The power supply consists of an array of eight IGBT units and is able to switch the discharge on and off at up to 10 kV and 600 A with a maximum repetitive frequency of 10 kHz. Multiple short duration discharge pulses maximize acceleration efficiency of the plasmoid. In the case of a 10 kHz operating frequency, helium-plasmoids in the velocity range of 20 km/s can be achieved.

A spatial distribution of toroidal flow in a field-reversed configuration plasma generated by the field-reversed theta-pinch method has been measured by an ion Doppler spectroscopy (IDS) system with a line-spectrum of impurity carbon (CV: 227.2 nm) and a Mach probe. With the IDS system, the axial profile of toroidal flow was observed. The observed results show different time evolutions of toroidal spin-up at around the midplane and end regions of the FRC. The radial profile of toroidal flow has also been observed on the weakly ionized plasma outside the separatrix with the Mach probe method. These results indicate propagation of toroidal momentum to the scrape-off plasma outward from the center.

A new type of economical neutral beam source has been developed by using a single washer gun, pulsed operation, and a simple electrode system. We replaced the conventional hot filaments for arc-discharge-type plasma formation with a single stainless-steel washer gun, eliminating the entire dc power supply for the filaments and the cooling system for the electrodes. Our initial experiments revealed successful beam extraction up to 10 kV and 8.6 A, based on spatial profile measurements of density and temperature in the plasma source. The system also shows the potential to control the beam profile by controlling the plasma parameters in the ion accumulation chamber.

A simple and adjustable tomographic system has been developed for studying the time evolution of the internal structure of a field-reversed configuration plasma. The optical diagnostic system consists of 35 collimators. In each collimator, a plane-convex lens made of BK7 with a focal length of 100mm collimates emissions from the plasma and focuses them onto the end of an optical-fiber tube. The collimator system gives a spatial resolution of 6mm at the center of the 0.256m diameter discharge chamber. The collimated light is transmitted through the optical-fiber tube to a photomultiplier tube (PMT) detector. In this system, the wavelength is selected by a bandpass interference filter with a central wavelength of 550nm and bandwidth of 10nm to eliminate impurities and deuterium-line spectra. The window is verified to be free of impurity lines (Matsuzawa et al., Rev. Sci. Instrum., these proceedings). Thus, only bremsstrahlung is detected by the PMTs. The 35 collimators are arranged on five holders in a fan-...

Deformation of the internal structure of a field-reversed configuration (FRC) was studied using a tomographic reconstruction technique. A simple and configurable tomographic system was developed, with which the time evolution of the FRC internal structure was reconstructed. In the latter phase of equilibrium, a FRC has a well-known global rotational instability with toroidal mode number n=2. It has been believed that elliptical deformation of the FRC allows interaction between the wall and the plasma, which terminates this configuration. However, these experiments revealed the FRC to deform into a dumbbell-like structure before the edge hits the chamber wall, leading to the disruption phase. In addition, an internal shift (toroidal mode number n=1) mode was observed in the equilibrium phase, followed by growth of n=2 rotational instability.

A diagnostic system using optical fibers with high numerical apertures is constructed for investigating a structure of magnetically confined plasma. Since the fiber has no convex lens at its front facet, the radiation from all plasmas included in the viewing angle of the fiber is utilized. First, theoretical forms are derived to determine the structure of the plasma from optical signals of a few fibers arranged axisymmetrically around the plasma. Second, the numerical aperture is measured by a simple method using a circular fluorescent lamp. Third, the theoretical forms are ascertained by a mock-up experiment where a light source representing a model plasma is constructed by a matrix array of straight fluorescent lamps. In this experiment, the effect of a reflected light from a wall of a vacuum vessel is investigated. Finally, the optical system is applied to a field-reversed-configuration plasma. Obtained structures of the plasma for n=1 and 2 toroidal modes are compared with the results of the conventional method using the fibers with convex lenses. The former results agree well with the latter ones in spite of the use of one-sixth the number of fibers in the latter method.

The translating velocity of a field-reversed configuration plasma is controlled using a pulse mirror field. Numerical calculations are carried out, using a flux-conserving moving mirror coil, to simulate the motion of the plasma subject to the pulse field. The calculations suggest that it is possible to stop the plasma smoothly without reflection under the pulse mirror coil when the strength of the mirror is changed depending on the velocity of the plasma. In the experiment, the pulse field is produced by a current flowing in a one-turn coil installed inside a theta-pinch coil. It is demonstrated that the translating plasma passing under the one-turn coil is reflected and stops depending on the strength of the pulse field. The optimum strength of the field to stop the plasma without reflection is in good agreement with the estimation of the calculations.