Masayoshi Y. Tanaka

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.

Collisional energy transfer in two-component plasmas

The friction in plasmas consisting of two species with different temperatures is discussed together with the consequent energy transfer. It is shown that the friction between the two species has no effect on the ion acoustic mode in a quasineutral plasma. Using the Poisson equation instead of the quasineutrality reveals the possibility for an instability driven by the collisional energy transfer. However, the different starting temperatures of the two species imply an evolving background. It is shown that the relaxation time of the background electron-ion plasma is, in fact, always shorter than the growth rate time. Therefore the instability is unlikely to develop. The results obtained here should contribute to the definite clarification of some contradictory results obtained in the past.

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.

Exploration of spontaneous vortex formation and intermittent behavior in ECR plasmas: The HYPER-I experiments

HYPER-I (High Density Plasma Experiment-I) is a linear device that combines a wide operation range of plasma production with flexible diagnostics. The plasmas are produced by the electron cyclotron resonance (ECR) heating with parallel injection of right-handed circularly polarized microwaves of 2.45 GHz from the high-field side. The maximum attainable electron density is more than two orders of magnitude higher than the cutoff density of ordinary waves. Spontaneous formation of a variety of large-scale flow structures, or vortices, has been observed in the HYPER-I plasmas. Flow-velocity field measurements using directional Langmuir probes (DLPs) and laser-induced fluorescence (LIF) method have clarified the physical processes behind such vortex formations. Recently, a new intermittent behavior of local electron temperature has also been observed. Statistical analysis of the floating potential changes has revealed that the phenomenon is characterized by a stationary Poisson process.

High-impedance wire grid method to study spatiotemporal behavior of hot electron clump generated in a plasma

High-impedance Wire Grid (HIWG) detector has been developed to study spatiotemporal behavior of a hot electron clump generated in an electron cyclotron resonance (ECR) plasma. By measuring the floating potentials of the wire electrodes, and generating structure matrix made of geometrical means of the floating potentials, the HIWG detector reconstructs the spatial distribution of high-temperature electron clump at an arbitrary instant of time. Time slices of the spike event in floating potential revealed the growth and decay process of a hot spot occurs in an ECR plasma.

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.

Flow structure formation in an ion-unmagnetized plasma: The HYPER-II experiments

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1, Kasuga-koen, Kasuga, Fukuoka, 816-8580, Japan 九州大学総合理工学府 〒816-8580 春日市春日公園6-1 National Institute for Fusion Science, 322-6, Oroshi-cho, Toki, Gifu, 509-5292, Japan 核融合科学研究所 〒509-5292 土岐市下石町322-6 College of Industrial Technology, Nihon University, 1-2-1, Izumi-cho, Narashino, Chiba, 275-8575, Japan 日本大学 〒275-8575 習志野市泉町1-2-1

Localized intermittent electron flux in an ECR plasma

Intermittent enhancement of local electron flux has recently been observed in a linear electron cyclotron resonance plasma. The aim of this paper is to measure the 2-D distribution of the electron flux, which appears at random positions in the plasma cross section, using intensified charge-coupled device imaging. A series of images of the emission from the Ne 2p1 state is presented; it clearly demonstrates a circular region of enhanced excitation due to increase in the electron temperature.

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.