Kohei Ogiwara

Experimental studies on ion acceleration and stream line detachment in a diverging magnetic field

The flow structure of ions in a diverging magnetic field has been experimentally studied in an electron cyclotron resonance plasma. The flow velocity field of ions has been measured with directional Langmuir probes calibrated with the laser induced fluorescence spectroscopy. For low ion-temperature plasmas, it is concluded that the ion acceleration due to the axial electric field is important compared with that of gas dynamic effect. It has also been found that the detachment of ion stream line from the magnetic field line takes place when the parameter |f(ci)L(B)∕V(i)| becomes order unity, where f(ci), L(B), and V(i) are the ion cyclotron frequency, the characteristic scale length of magnetic field inhomogeneity, and the ion flow velocity, respectively. In the detachment region, a radial electric field is generated in the plasma and the ions move straight with the E×B rotation driven by the radial electric field.

High resolution laser induced fluorescence Doppler velocimetry utilizing saturated absorption spectroscopy

A high resolution laser induced fluorescence (LIF) system has been developed to measure the flow velocity field of neutral particles in an electron-cyclotron-resonance argon plasma. The flow velocity has been determined by the Doppler shift of the LIF spectrum, which is proportional to the velocity distribution function. Very high accuracy in velocity determination has been achieved by installing a saturated absorption spectroscopy unit into the LIF system, where the absolute value and scale of laser wavelength are determined by using the Lamb dip and the fringes of a Fabry-Perot interferometer. The minimum detectable flow velocity of a newly developed LIF system is +/-2 m/s, and this performance remains unchanged in a long-time experiment. From the radial measurements of LIF spectra of argon metastable atoms, it is found that there exists an inward flow of neutral particles associated with neutral depletion.

Self-Calibrated Measurement of Ion Flow Using a Fine Multihole Directional Langmuir Probe

A fine multihole directional Langmuir probe (FM-DLP) has been developed to measure ion Mach number and tested in an electron cyclotron resonance (ECR) plasma. It is found that the FM-DLP can measure the ion Mach number with the same method used for a conventional directional Langmuir probe (DLP). Moreover, the sensitivity of the FM-DLP is almost twice as high as that of the conventional DLP by changing the aspect ratio of the hole that collects ion saturation current. It is also found that the electron saturation current of the FM-DLP is markedly reduced to the level of ion saturation current; thus, the current–voltage characteristics of the FM-DLP become similar to those of an emissive probe, which suggests the emissive-probe-like function of the FM-DLP. We have demonstrated that the FM-DLP can measure the plasma potential, which enables us to determine the calibration factor without other diagnostic tools. Therefore, it is concluded that the FM-DLP has a self-calibration capability for ion flow measurement.

Simulation of Effective Production of Very High Frequency Hydrogen Plasma Using a Balanced Power Feeding Method

The characteristics of a VHF hydrogen plasma produced by a balanced power feeding method were examined by using a two-dimensional Monte Carlo fluid hybrid model. The simulation results showed that the electron density is maximum at a certain pressure between the discharge electrodes and significantly decreases outside the electrodes at a high gas pressure. The power absorption efficiency between the electrodes was improved by increasing the gas pressure. In addition, the plasma was produced efficiently between the electrodes at low applied voltages.

Observation of Ion Stream Line Detachment and Onset of Azimuthal Rotation in a Diverging Magnetic Field

The ion flow structure in a diverging magnetic field is measured in a steady-state electron cyclotron resonance plasma. It has been observed that stream line detachment takes place when the nonadiabaticity parameter of ions becomes the order of unity. In the detachment region, the plasma starts an azimuthal rotation, and the energy conservation given by the 1-D model is no longer applicable.

Characteristic of VHF plasma produced by balanced power feeding

The characteristics of a VHF hydrogen plasma produced by a balanced power feeding method were examined using a two-dimensional hybrid model. The simulation results showed that the electron density peaks at a certain pressure inside the discharge electrodes and significantly decreases outside the electrodes for high gas pressure. In addition, the power absorption efficiency inside the electrodes was improved by increasing the gas pressure. On the other hand, the plasma was produced within the electrodes for low applied voltages.

Lamb-Dip Laser-Induced Fluorescence Spectroscopy for Measuring Magnetic Field in a Plasma

We have developed a Lamb-dip laser-induced fluorescence (LIF) system to precisely measure the local magnetic field strength in a plasma. Utilizing the hole burning effect, we made sharp dips on the LIF spectrum as the frequency markers and accurately determined the Zeeman splitting by reading the frequency interval of the dips. The method is valid even in the conditions where the Doppler broadening of the LIF spectrum is larger than the Zeeman shift. The newly developed LIF system is capable of determining a magnetic field strength on the order of 10-4 T. It has been demonstrated that the Lamb-dip LIF system can successfully reproduce a very small field inhomogeneity in the HYPER-I linear device.

Vortex Formation in a Plasma Interacting with Neutral Flow

Recently, it has been observed that there exists a class of vortices which rotates in the opposite direction to E×B drift (referred to as anti‐E×B vortex). This result suggests that a predominant force other than electric field is acting on ions. It is found that momentum transport and resultant force generation through the interaction between ions and neutral flow play an essential role on anti‐E×B vortex formation. The existence of inward neutral flow, which drives the ions in the anti‐E×B direction, has been confirmed using a newly‐developed high‐resolution laser induced fluorescence (LIF) spectroscopy system.

Study of spatial profiles of capacitively coupled VHF H2 plasma by simulation

The spatial profiles of a VHF H2 plasma (60 MHz) for different discharge gap distances were examined at pressures of 66.7 and 133.3 Pa by two-dimensional simulations using the plasma hybrid code. The electron density had a peak profile, and the maximum density depended on both the discharge gap distance and the pressure. A high-electron-density plasma with a low-electron temperature of approximately 1 eV was predicted by simulation at discharge gap distances of 15 and 20 mm. The plasma potential profile was composed of a plateau at the center and sharp slopes at the two sides. The axial profiles of the H+, H2 +, and H3 + densities were calculated for the discharge gap distances of 10, 15, and 20 mm. It was found that the dominant ion species was H3 + except near the discharge electrode and the H2 + density near the discharge electrode was not negligible compared with the H3 + density at 66.7 Pa.

Asymmetry of velocity distribution function and inhomogeneity-induced flow associated with neutral depletion structure in an ECR plasma

A neutral depletion structure with strong inhomogeneity in the radial direction has been observed in an electron cyclotron resonance plasma. We have measured the velocity distribution function of neutrals with a high resolution laser-induced fluorescence system and examined the relationship between asymmetry of distribution function and flow induced by inhomogeneity. It has been revealed that the third order moment of distribution function, that is, skewness, is proportional to the inhomogeneity-induced flow, and a simple relation between the skewness and the normalized flow velocity has been obtained and confirmed in the experiment.