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The Particle-In-Cell simulation code PICS has been developed to study plasma in front of a surface with two types of masks, step-type and roof-type. Parameter scans with regard to magnetic field angle, electron density, and mask height were carried out to understand their influence on ion particle flux distribution on a surface. A roof-type mask with a small mask height yields short decay length in the flux distribution which is consistent with that estimated experimentally. A roof-type mask with a large height yields very long decay length and the flux value does not depend on a mask height or an electron density, but rather on a mask length and a biasing voltage of the surface. Mask height also changes the flux distribution apart from the mask because of the shading effect of the mask. Electron density changes the distribution near the mask edge according to the Debye length. Dependence of distribution on parameters are complicated especially for a roof-type mask, and simulation study with various parameters are useful to understand the physical reasons of dependence and also is useful as a tool for experiment studies. Keywords: Sheath, Flux distribution, Particle-In-Cell, Simulation, 2010 MSC: 82D10, 78A35

When tungsten is exposed to helium plasmas, nanostructures are grown on the surface. Field emission properties of the tungsten samples are measured; it is revealed that the field emission was significantly increased as changing the surface morphology by the plasma irradiation. Demonstration of arcing and unipolar arcing on the nanostructured tungsten was conducted in the linear plasma device NAGDIS-II, which simulates the fusion relevant conditions. Arcing can be easily initiated in response to a transient heat load with a pulsed laser, which demonstrates transient heat loads in fusion devices. It is thought that the increase in the field emission and the decrease in the thermal diffusivity near the surface lead to the initiation of arcing. The behavior of the free running arc spots are observed with fast framing camera. Initial behavior of initiated arc spots and an interesting behavior, bifurcation, random motion, etc. are shown.

Recent research in scrape-off layer (SOL) and divertor physics is reviewed; new and existing data from a variety of experiments have been used to make cross-experiment comparisons with implications for further research and ITER. Studies of the region near the separatrix have addressed the relationship of profiles to turbulence as well as the scaling of the parallel power flow. Enhanced low-field side radial transport is implicated as driving parallel flows to the inboard side. The medium-n nature of edge localized modes (ELMs) has been elucidated and new measurements have determined that they carry ~10?20% of the ELM energy to the far SOL with implications for ITER limiters and the upper divertor. The predicted divertor power loads for ITER disruptions are reduced while those to main chamber plasma facing components (PFCs) increase. Disruption mitigation through massive gas puffing is successful at reducing PFC heat loads. New estimates of ITER tritium retention have shown tile sides to play a significant role; tritium cleanup may be necessary every few days to weeks. ITER's use of mixed materials gives rise to a reduction of surface melting temperatures and chemical sputtering. Advances in modelling of the ITER divertor and flows have enhanced the capability to match experimental data and predict ITER performance.

Experimental studies on mesoscale erosion of tungsten surface due to low-energy and high-flux helium/deuterium exposure are systematically described. Following the historical description of the accidental finding of helium bubbles and holes, possible changes in tungsten properties like the A coefficient and the work function of the rmo electron emission, the emissivity and the energy reflection coefficient are discussed in terms of thermal bifurcation. Multi-dimensional parameter area for He bubble formation is identified in the space of tungsten temperature, ion fluence and incident ion energy. In particular, the threshold energy was surprisingly found to be as low as 6 eV, in association with which the formation mechanism is proposed. Finally, the effect of bubbles and holes formation on deuterium retention is quantitatively clarified.

Surface modification of polycrystal tungsten due to low-energy and high-flux helium plasma exposure was investigated. Micron-sized holes and bubbles were formed on the surface at a surface temperature of 2200 K even when the incident ion energy of helium was 10 eV. Hole formation was significantly reduced on the surface when the incident ion energy of helium was around 5 eV and no surface modification was seen at less than 5 eV. Coalescence of micron-sized helium bubbles was observed in the cross section of a W sample.

A radio frequency (rf) inductive discharge in atmospheric pressure range requires high voltage in the initial startup phase and high power during the steady state sustainment phase. It is, therefore, necessary to inject high rf power into the plasma ensuring the maximum use of the power source, especially where the rf power is limited. In order to inject the maximum possible rf power into the plasma with a moderate rf power source of few kilowatts range, we employ the immittance conversion topology by converting a constant voltage source into a constant current source to generate efficient rf discharge by inductively coupled plasma (ICP) technique at a gas pressure with up to one atmosphere in argon. A novel T-LCL immittance circuit is designed for constant-current high-power operation, which is practically very important in the high-frequency range, to provide high effective rf power to the plasma. The immittance conversion system combines the static induction transistor (SIT)-based radio frequency (rf) high-power inverter circuit and the immittance conversion elements including the rf induction coil. The basic properties of the immittance circuit are studied by numerical analysis and verified the results by experimental measurements with the inductive plasma as a load at a relatively high rf power of about 4 kW. The performances of the immittance circuit are also evaluated and compared with that of the conventional series resonance circuit in high-pressure induction plasma generation. The experimental results reveal that the immittance conversion circuit confirms injecting higher effective rf power into the plasma as much as three times than that of the series resonance circuit under the same operating conditions and same dc supply voltage to the inverter, thereby enhancing the plasma heating efficiency to generate efficient rf inductive discharges.

Low-energy and high-flux plasma exposures on tungsten surfaces result in blister/hole formation. Subsequent He plasma exposure at 2,000 K on the blister-rich surface forms a hole structure on the surface, including the blister surface, and some blisters collapse due to recrystallization and bubble formation on the blister skin. Grain ejections were observed after deuterium plasma exposure at 550 K on a W surface on which a hole structure had formed due to He pre-exposure at 1,600 K.

Generation and sustainment of rf induction thermal plasmas in atmospheric pressure range require a high voltage in the initial startup phase and a high rf power during the steady state sustainment phase. The high rf electrostatic filed induced between induction coil conductor wound around a glass discharge tube generates surface electrostatic discharges in which the discharge current flows through the dielectric glass tube. After several hundreds microseconds the electrostatic discharges grow sufficiently to transfer to a volumetric electromagnetic discharge which is maintained by the inductive electric field. During this discharge mode transition, the loading impedance of the induction coil changes significantly. This loading impedance change of the induction coil leads a deviation from a series resonance condition of the LCR output circuit including the induction coil, which strongly affects the output characteristics of a semiconductor rf inverter power supply. In this paper transient response of the induction coil impedance in the initial startup phase of the rf thermal plasma generation is studied experimentally. Dynamic interactions of the rf inverter power supply with generated rf plasmas are also investigated and strong deterioration of the inverter output efficiency is found experimentally. Preliminary frequency tracking experiments show a successful improvement of the inverter output efficiency and absorbed rf power in the generated plasma.

金沢大学理工研究域電子情報学系, The influences of if driving power, neutral gas pressure, and nitrogen seeding on the electrostatic-to-electromagnetic (E-H) mode transition dynamics in radio-frequency argon inductively coupled plasmas (ICPs) in a pressure range of 50-100 kPa are investigated, both experimentally and theoretically. The E-H mode transition dynamics and its characteristic transition time scale are investigated by observing the high-speed imaging (13 500 fps) as well as the temporal change of plasma loading impedance. The experimental results reveal that the E-H mode transition time is not fixed at any operating conditions rather it depends on some important parameters such as the rf driving power, neutral gas pressure, gas type. It is found that the E-H mode transition time depends on the unique parameter Eθ/p; the so-called effective induced electric field, rather than the independent parameter: the if power or neutral gas pressure. It is also found that longer E-H mode transition time is required to ignite the high-pressure Ar-N2 plasmas with a 2.5%-10% N2 seeding than that of pure Ar plasmas with the same operating conditions. The experimental results are compared with that of the recently developed theoretical models, and a good agreement is found between them. © 2004 American Institute of Physics.

This paper will report on the proof-of-principle (POP) experiments conducted to demonstrate reduced wall recycling, using a laboratory-scale test unit, constructed based on the concept of moving-surface plasma-facing component (MS-PFC). In this concept, the moving-surface exposed to edge plasmas in steady state magnetic fusion devices is continuously deposited ex-situ with a getter material, so that particle trapping capabilities can be regenerated prior to the subsequent exposure. In our previous paper, the construction details of the MS-PFC test unit and the first results in the case of titanium gettering was reported, but in the present paper preliminary results in the case of lithium gettering will be presented for comparison. Results indicate that the H α light intensity used as the measure of hydrogen recycling is reduced by ∼6% due to titanium gettering and by ∼12% due to lithium gettering, both at steady state.

In the magnetic fusion community much attention has recently been directed to particle control in the existing confinement experiments, as it is recognized that the core plasma performance can directly be affected by the edge conditions. However, it is still true that there is no clear strategy to cope with the needs for steady-state reactor operation. That is that, due to the saturation nature, wall conditioning such as boronization is not applicable for fuel and impurity particles control in steady-state devices, requiring a start from enabling concepts development. As a possible solution, the concept of moving-surface plasma-facing components (MS-PFCs) was proposed in our previous work and in the present work proof-of-principle experiments have been conducted. In this concept, the plasma-facing surface is mechanically circulated out-of-pile for regeneration of particle capturing capabilities by coating getter films on it. A prototypical MS-PFC test unit has been constructed for these proof-of-principle experiments, employing a rotating copper drum as the moving surface and titanium as the getter material. It has been indicated that, from Hα light intensity data taken in front of the rotating target, relative to no getter cases, approximately 6% reduced hydrogen recycling has been achieved at steady state. In support of these data, a first-order particle balance model predicts a 7% reduction in hydrogen recycling.

To achieve high confinement properties of a tokamak plasma, it is necessary to control the plasma position, current profile, shape of magnetic surface, and so on. In addition, it has been found that a resistive wall mode (RWM) may limit the achievable plasma beta in present tokamak devices. Therefore, it is expected that an active feedback control using external coils is necessary to stabilize the RWM. A power supply for plasma control coils requires an accurate controllability and a fast response against such plasma disturbances. Recent development of high-power fast-switching semiconductors, such as Insulated Gate Bipolar Transistor (IGBT) and MOSFET, improves the temporal response of power supply to a great extent. A small tokamak device, HYBTOK-II, is equipped with IGBT inverter power supplies for Joule and vertical field coils. In this paper a real-time feedback control of the plasma horizontal position has been employed with a Digital Signal Processor (DSP). The experimental results on plasma response with such a feedback control have been compared with analysis of plasma column motion using transfer functions. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 148(1): 1–10, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10295

Heat and particle flux control is one of the most important and difficult problems in steady-state fusion devices. The Dynamic Ergodic Divertor (DED) is an advanced concept in which a rotational perturbed magnetic field is applied to make the tokamak edge magnetic structure stochastic so that it may decrease the heat and particle flux onto the local target in the conventional divertor. This rotational perturbed field is created by two sets of local helical coils (LHC) with a phase difference of 90° in the supplied ac currents. In DED experiments, it is important to study the perturbed magnetic field structure generated by these coil currents. In this paper, a new coil configuration is considered, which has an additional series of return windings so as to improve the mode quality of the rotating helical field (RHF). We obtain the mode structure of RHF using a new Fourier analysis. From this result, we confirm that the parasitic mode of RHF and the coil self-inductance are decreased by the new coil configuration. © 2002 Scripta Technica, Electr Eng Jpn, 139(1): 16–23, 2002; DOI 10.1002/eej.1142

A variety of dynamic behaviors in dusty plasmas is expected under the experimental condition of weak friction with gas molecules. The device ‘‘KAGEROU’’ provides such an environment for dynamic collective phenomena. Self-excited dust oscillations in Coulomb crystals have been observed at low values of plasma density and gas pressure. An instability mechanism was identified to be delayed charging in an inhomogeneous equilibrium dust charge in the sheath. The theoretical growth rate was formulated in relation to the destabilization of a transverse dust lattice wave (T-DLW), which was found to be very sensitive to the presence of a small amount of hot electrons which produces a substantial positive equilibrium charge gradient ∇Q_{d-eq} around the equilibrium position of dust particles in the plasma–sheath boundary. The first experimental observation of a correlated self-excited vertical oscillations in a one-dimensional dust chain indicates a destabilization of T-DLW. The experimental condition is very consistent with the parameter area which predicts numerically an instability of T-DLW.

High-heat-flux plasmas are produced by high-power (∼14 kW) ICRF heating using inverter power supplies in the linear diverter simulator NAGDIS-II. The power flow of radiated RF power is investigated by a calorimetric method. From conventional power calculation using antenna voltage and current, about 70% of the RF power is radiated into the plasma. But the increase of the plasma heat load along the magnetic field by RF heating is about 10% of the RF power. Through this experiment, we find that about half of the RF power is lost at the antenna surface through the formation of RF-induced sheath, and about 30% of the power is lost into the wall of the vacuum vessel through the charge exchange and elastic collision of ions with neutrals. © 2002 Scripta Technica, Electr Eng Jpn, 138(4): 34–41, 2002; DOI 10.1002/eej.1136

In the Current Sustaining Tokamak of the Nagoya University (CSTN)-IV research tokamak system, using a compact 40-kHz pulse width modulation (PWM) inverter power supply, which is controlled through the LabVIEW program, we construct a new tokamak discharge system with multimode including a stable alternating current discharge and a high-repetition high-duty one. These discharge modes can be operated continuously for as long as 60 seconds. The continuous discharge of long duration is able to simulate the important physical and chemical processes of long discharges in fusion devices, in which the heat load to the wall and the particle balance in the plasma-wall system are crucial topics in order to realize a long pulse fusion reactor, like ITER. An ergodic divertor is one of the tools used to control the particle balance and the heat load to the wall. In addition, we installed another inverter power supply to generate a rotating magnetic perturbation for a dynamic ergodic divertor (DED) with the appropriate measurement system so that we could carry out experiments on heat and particle control with DED in prolonged operation. © 2000 Scripta Technica, Electr Eng Jpn, 133(4): 8–18, 2000

Mechanisms for the confinement and the internal structure of an electrostatically coupled dust cloud formed in a dc glow discharge have been investigated from a comparative viewpoint between experimental observations and a simple model. Two kinds of dust clouds with different internal structures are clearly observed, depending on the dispersion of the size distribution of dust particles. The dust cloud can be trapped only in the plasma–sheath boundary area, corresponding to the potential minimum region determined by gravitational and electrostatic forces in the cathode sheath. No dust particles were found deep inside of the sheath, which is consistent with the analysis because the dust particles may be charged positively due to an extreme reduction of the electron density. The internal structure of the electrostatically coupled dust cloud was found to be arranged so that the total potential energy, including the repulsive Coulomb interaction among negative dust particles, may become minimal.

Molecular activated recombination (MAR) has been clearly observed for the first time in a divertor plasma simulator. A small amount of hydrogen gas puffing into a helium plasma strongly reduced the ion particle flux along the magnetic field, although the conventional radiative and three-body recombination processes were quenched. Careful comparison of the observed helium Balmer spectra with collisional radiative atomic and molecular data indicates that the population distribution over the atomic levels with relatively low principal quantum numbers can be well explained by taking the MAR effects into account. [S0031-9007(98)06573-9]

Plasma heat flow to material surfaces through sheaths is studied, taking several key physics factors into account. Electron emission from the surface, which breaks a thermal insulation of the sheath, is studied in both thermoelectron emission (TEE) and secondary electron emission (SEE), in which a correct expression under space charge limited condition is given for arbitrary sheath voltages. Nonlinear thermal bifurcation induced by electron emission is analyzed in the experiment and the theory. The local heat flow was found to be enhanced by a thermal contraction induced by cross-field potential variation in a plasma. An enhancement of SEE of hydrogen-absorbed graphite, and a suppression of SEE by the gyromotion of emitted electrons in obliquely incident magnetic field are identified. The effects of ion reflection on the surface and ponderomotive force are also discussed in terms of energy transmission factor δ. An anomaly of δ in detached recombining plasmas is discussed.

Effective rate coefficients for ionization, line emission and line radiation power of the helium atom in an ionizing plasma are defined, with a collisional radiative model including new recommended excitation rate coefficients, which can be applied for divertor plasmas. Line intensity ratios for electron temperature and density measurements in this model are compared with results by Schweer et al. (J. Nucl. Mater. 196–198 (1992) 174–178) and the differences between the atomic data and the model are discussed.

The importance of the study on particle and energy transport in plasma-wall system for magnetic confinement fusion reactors is described, identifying the trend of research in four basic categories.Academic approaches in edge plasma simulation devices by generating a high heat flux plasma are recommended to complement big tokamak experiments.

It was found in the HYBTOK-Ⅱ tokamak that the negative change of the electric potential of the plasma by a limiter biasing is influenced senseitivity by the magnetic configuration in the SOL, but the positive change is not. The concept of “effective area" is introduced to explain the potential change due to a horizontal shift of plasma column in the negative biasing. Employing such a concept, a simple model for the formation of electric potential in a plasma for both a cold and an electron-emitting limiter is presented. It explains qualitatively the experimental results. The positive biasing was found to enhance the radial electric field at the scrape off layer, and also at the inside, close to the last-closed magnetic flux surface, and seems to increase the particle confinement as measured by an increased line-averaged electron density with decreased Hα and impurity line intensities, along with a stabilization of the magnetic and electrostatic fluctuations.

Dependence of the damping rate of the oscillations of the dust particles levitating in the sheath on the plasma parameters is investigated both theoretically and experimentally. Significant deviations of the damping rate from the values predicted by the Epstein formula are found in the experiment. The delayed charging effect is applied for the theoretical explanation of the experimental results. Qualitative agreement between the theoretical and experimental data is obtained.

Tetrahydroswertianolin (1), a new tetrahydroxanthone glycoside, was isolated from the whole plant of Swertia, MAKINO (Gentianaceae). Its structure was determined by chemical and spectroscopic methods including 2D-NMR. This compound was found to be very effective in immunologically induced liver injury in mice.

Suppression of secondary electron emission from the material surfaces with an obliquely incident magnetic field is demonstrated experimentally in a plasma containing hot electrons.

Summary form only given, as follows. In this paper, the dynamics of E-H discharge mode transition of Ar ICP in atmospheric pressure with a driving frequency range of 0.5/spl sim/1.5 MHz and a moderate rf power of about 2/spl sim/5 kW are investigated experimentally. The transition parameters are calculated by using a proposed model, and the calculated results are compared with that of the experimental results.

Summary form only given, as follows. In several tokamak devices, in order to suppress MHD instabilities, especially classical and neoclassical tearing modes (NTM), the plasma current density profile control have been performed by a ramp-up or ramp-down of plasma current and local plasma heating (or current drive). Recently, the plasma current oscillation experiment has been done to suppress self-generated tearing modes in HT-7 tokamak. The conditions under which the tearing modes were suppressed by the plasma current oscillation are as follows (a) Oscillation amplitude of the plasma current is large enough to move the resonance surface outside the magnetic island, and (b) the oscillation period is faster than the growth time of the tearing modes. In HYBTOK-II tokamak, the magnetic islands are generated by externally applied rotating helical magnetic perturbation (RHMP). We have confirmed that the amplification of RHMP in the plasma is suppressed by the plasma current oscillation. It follows from this result that the growth of magnetic islands is suppressed by the plasma current oscillation.

Fluctuation in detached recombining plasmas has been investigated experimentally in the linear divertor plasma simulator, NAGDIS-II. As increasing neutral gas pressure, floating potential fluctuation of the target plate installed at the end of the NADIS-II device becomes larger and bursty negative spikes are observed in the signal associated with a transition from attached to detached plasmas. The fluctuation property has been analyzed by using Fast Fourier Transform (FFT), probability distribution function (PDF) and wavelet transform. The PDF of the floating potential fluctuation in the attached plasma condition obeys the Gaussian distribution function, on the other hand, the PDF in detached plasma shows a strong deviation from the Gaussian distribution function, which can be characterizedbyflatnessandskewness.Comparisonofthefluctuation properties between the floating potential and the optical emission from the detached plasma has been done based on the wavelet transform to show that a strong correlation between them, which could indicate bursty transport of energetic electrons from upstream to downstream region along the magnetic field.

Saponins in bulbs of a mutant of elephant garlic were investigated, and three new steroidal saponins named yayoisaponins A-C were obtained together with the known dioscin and aginoside. Their structures, including the relative stereochemistry, were elucidated by spectral data interpretation, while the absolute stereochemistry of the sugar moieties was assigned on the basis of a chiral gas chromatographic analysis of the acid hydrolysates. Yayoisaponins A-C and aginoside exhibited not only in vitro cytotoxicity against P388 cells at 2.1 micrograms/ml, but also antifungal activity against Mortierella ramanniana at 10 micrograms/disk.

The n-BuOH extract of Swertia japonica showed a significant hepatoprotective effect on D-galactosamine (D-GalN)/lipopolysaccharide (LPS)-induced liver injury in mice. The activity-guided fractionation led to the isolation of a new tetrahydroxanthone derivative, tetrahydroswertianolin (1), as well as two known iridoids, gentiopicroside (2) and sweroside (3). Their structures were elucidated by spectroscopic methods and chemical reactions. Of the three compounds, 2 and 3 possessed mild hepatoprotective activity at a dose range of 25-50 mg/kg, whereas, 1 exhibited potent activity in a dose-dependent manner. The hepatoprotective effect of tetrahydroswertianolin (1) was stronger than that of glycyrrhizin which was used as a positive control.

745 J. Plasma Fusion Res. Vol.81, No.10 (2005) 745 746 Tungsten is one of the candidates for divertor materials because of its high thermal property and low sputtering yield. However, recent experimental observations have revealed that bubbles and holes are formed on the surface by helium plasma exposure even with an incident ion energy less than the threshold energy of sputtering [1]. The formation of holes and bubbles may lead to degradation of the superior properties of tungsten, especially, its thermal diffusivity. If the thermal diffusivity is degraded, transient heat loads owing to the ELMs (Edge Localized Modes) [2] and disruption may pose serious problems, such as the melting of high-Z materials. In this paper, the transient heat load to the tungsten target during exposure to helium plasmas was demonstrated using pulsed laser irradiation. Although the heat pulse duration was several ns, which is much shorter than the characteristic time of the ELMs (

We investigated the intermittent convective plasma transport in a attached and/or detached plasma condition of the linear divertor plasma simulator, NAGDIS-II. Images taken by a fast-imaging camera clearly show that in attached plasmas, blobs are peeled off the bulk plasma, and propagate outward with an azimuthal motion. In detached plasmas, plasma turbulence observed near the plasma recombining region drives strong intermittent radial plasma transport, which could broaden the radial density profile.

One of the serious concerns for tungsten materials in fusion devices is the radiation defects caused by helium plasma irradiation, while the helium is one of fusion products. Fiber-formed nanostructure is worried to have a possible weakness against the plasma heat flux and may destroy the reflectivity as an optical mirror. In this communication an interesting method for a recovery of such a tungsten surface is shown.

Deepening of floating potential has been observed on the tungsten target plate immersed in high-density helium plasma with hot electron component on the way to nanostructure formation. The physical mechanism is thought to be a reduction of secondary electron emission from such a complex nano fiber-form structure on the tungsten surface.

Oxy-combustion generates a high temperature field (above 3000 K), which is applied to next generation power plants and high temperature industrial technologies because of N2 free processes. However, the combustion temperature is so high that the furnace wall may be fatally damaged. In addition, it is very difficult to control the heat flux and chemical species' concentrations because of rapid chemical reactions. We have developed a new method for controlling the flame by electromagnetic force on this field. In this paper, we experimentally investigated the power coupling between the premixed oxy-combustion with methane and radio frequency (RF) power through the induction coil. By optimizing the power coupling, we observed that the flame can absorb RF power up to 1.5 kW. Spectroscopic measurements also showed an increase in the emission intensity from OH radicals in the flame, indicating improved combustibility.

For the first time, fluctuation characteristics at the high-field side (HFS) and low-field side (LFS) scrape-off-layers (SOLs) in L-mode plasmas of the JT-60U tokamak are compared in detail; the plasma transport is studied using reciprocating Langmuir probes. A statistical analysis based on probability density functions (PDFs) is employed to describe intermittent (nondiffusive) transport in SOL plasma fluctuations. It is found that positive bursty events associated with blobby plasma transport appear frequently at the LFS midplane. Further, the PDF at the LFS midplane is strongly positively skewed, while the PDF at the HFS SOL is close to a Gaussian distribution. The conventional conditional averaging method is improved by using the variable-interval time-averaging method along with it, which enables the precise reconstruction of the burst profile exhibiting a rapid increase and a slow decay. The radial evolution of plasma blobs is analysed by using an autocorrelation function, the short-time Fourier transform and wavelet analysis.

A new technique for the direct measurement of electric field distribution along the plasma column in microwave jet discharges is developed and employed. The technique is based on a servomotor-controlled reciprocating antenna moving along the nozzle axis and plasma column. The measurement technique is applied to a rectangular waveguide-based 2.45 GHz argon and helium plasma jets generated by using the modified TIAGO nozzle at atmospheric pressure with a microwave power of less than 500 W. The measurement has been done with and without igniting the plasma jet in order to investigate the standing wave propagation along the nozzle axis and plasma column. It is observed that the electric field decay occurs slowly in space with plasma ignition than that of without plasma, which indicates the surface electromagnetic wave propagation along the plasma column in order to sustain the plasma jet. This study enables one to design, determine and optimize the size and structure of launcher nozzle, which plays an important role for the stable and efficient microwave plasma generators.

The high-order structure functions have been analyzed to characterize the edge plasma intermittency in fusion devices. The scaling properties of edge turbulence have shown a strong deviation from a prediction of the Kolmogorov's K41 model. The turbulent fluctuations demonstrate a generalized scale invariance and log-Poisson statistics.

Structural bifurcation of microwave-sustained jet discharge at atmospheric gas pressure was found to produce a stable helium plasma jet, which may open the possibility of a new type of high-flux test plasma beam for plasma-wall interactions in fusion devices. The fundamental discharge properties are presented including hysteresis characteristics, imaging of discharge emissive structure, and stable ignition parameter area.

Plasma rotation or its shear is important in the formation of a transport barrier. It is believed that a rotating helical magnetic field generated by a dynamic ergodic divertor (DED) can generate a rotational torque in tokamak plasmas, and therefore control the rotation profiles. To measure the plasma rotations and investigate the effect of DEDs on them, we developed a passive spectroscopic measurement system for the small tokamak HYBTOK-II. A spontaneous toroidal plasma rotation in the co-current direction and poloidal plasma rotation in the electron diamagnetic drift direction have been observed without using a DED. Considerable changes in the plasma flow have been obtained using a DED for reducing toroidal and poloidal rotation velocities near the resonant magnetic surface. The change in the plasma rotation velocity was found to couple with the change in the radial electric field.

In order to estimate the electromagnetic force acting on vessel components during tokamak disruptions, an accurate prediction of the plasma current decay time is necessary. We have verified a current decay model based on a simple series circuit with a plasma resistance and an inductance. The circuit is employed for establishment of a plasma current decay time database using disruptive discharges in a small tokamak HYBTOK-II. An increase in the decay rate of the plasma current during the current quench phase was observed in experiments associated with an increase in the plasma resistance. This experimental result is consistent with the prediction of the model.

Nanosecond laser ablation of tungsten (W) exposed to helium plasma is investigated using optical emission spectroscopy. Submicrometer-sized holes/bubbles are formed on the surface of W when it was exposed to the helium plasma at a sufficiently high temperature (≳1500–1600K). The emissions from a virgin W (before the helium plasma irradiation) cannot be detected when the fluence is