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Black silicon formed by helium plasma irradiation has attractive surface morphologies in terms of solar cell fabrication, because it has good photon absorption property across the solar spectrum. It also does not need any masking process, which is sometimes employed in the fabrication of pyramid-like microstructures. Black silicon formed by helium plasma irradiation has potential not only for solar cell applications, but also in the fields of photonics, optics, and ultrasonic wave generation. Here, the nano-cone structure formation process, detailed observations of surface morphologies with TEM as well as SEM, the optimal He irradiation conditions, surface reflectivity over UV–visible-NIR wavelengths, and scattered light enhancement with Raman spectroscopy in addition to X-ray diffraction measurements were studied. Investigation of electronic properties is reported, including good resistivity of the Si black surface without an ITO electrode.

Fiber-form nanostructures grown on tungsten (W) surfaces with helium (He) plasma irradiation exhibit an excellent thermal radiative property at low sample temperatures up to 1200 °C. To maintain this radiative property at higher temperatures, the recovery to undefected surface and annealing effect of nanostructured W surfaces need to be investigated while focusing on the dopant effect. TFGR-W-1.1%TiC/H (toughing, fine-grained recrystallized tungsten with TiC dispersoids) was experimentally found to have the best characteristics among the tested doped W samples. However, it was noted and discussed that a further improvement at higher temperatures needs an additional surface treatment. Possibilities of maintaining complicated structures with nano-scale lengths are discussed in terms of dopant pinning effect and self-diffusion on tungsten surface.

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

Growth mechanisms of complex tungsten (W) fuzzy nanostructure on helium-plasma-irradiated W surface have been extensively studied since the discovery of the phenomenon in 2006. However, the growth process of complicated W nano-fibers from the atomistic and crystal structure standpoints is not well understood. In this study, the X-ray diffraction (XRD) and transmission electron microscopy (TEM) were performed to investigate complexity of nano-fibers as well as their crystallinity. Small-angle-incident XRD on the substrate with fuzz and He-defected bulk surface was used to determine grain distribution among contributions of different crystal orientation families, showing the difference in grain distribution of surface fuzz layer and the basing W bulk surface with helium defects compared with the distribution of pristine powder metallurgy W (PM-W). Peak shifts and broadenings of X-ray diffracted spectra with respect to pristine PM-W substrate and TEM observations were discussed in terms of crystallinity and He-bubbles induced strain. It is speculated that the grain distribution of the He-defected W with nano-fibers would be very similar to that of bulk α-phase W globally. Moreover, the scraped-off fuzz aggregate without basing plate makes it possible to conduct XRD using a thin glass capillary tube. The grain distribution of pure fuzz aggregate was found to be very similar to natural α-phase W with intensification of high grain orientation modes. The process of such high modes may bring morphological complexity to nano-fibers because grain boundaries play a role of turning surfaces along nano-fibers.

The radiative cooling capability of tungsten with a fiber-form nanostructure is evaluated precisely using an electron-beam power system. It is found that the total emissivity of black nanostructured tungsten is very close to 1.0, meaning an almost complete black body. The discrepancies in the determination of the total emissivity between the electron-beam method and a fixed biasing procedure are discussed in terms of the power transmission through the plasma sheath, focusing on the ion energy reflection and cooling associated with electron emission.

The initial stage of fiber-form nanostructure growth on metal surface with helium plasma irradiation is illustrated, taking recent research knowledge using a flux gradient technique, and including loop-like nano-scale structure as precursors. The growth mechanism of fibers is discussed in terms of the shear modulus of various materials that is influenced by the helium content as well as the surface temperature, and the mobility of helium atoms, clusters and/or nano-bubbles in the bulk, loops and fibers. This model may explain the reason why tantalum does not provide fiber-form nanostructure although the loop-like structure was identified. The model also suggests the mechanism of an existence of two kinds of nanostructure of titanium depending on surface temperature. Industrial applications of such nanostructures are suggested in the properties and the possibilities of its growth on other basic materials.

The generation of tungsten fuzzy nanostructure by exposure to helium plasma is one of the important problems for the use of tungsten material as divertor plates in nuclear fusion reactors. In the present paper, the formation mechanisms of the helium bubble and the tungsten fuzzy nanostructure were investigated by using several simulation methods. We proposed the four-step process which is composed of penetration step, diffusion and agglomeration step, helium bubble growth step, and fuzzy nanostructure formation step. As the fourth step, the formation of the tungsten fuzzy nanostructure was successfully reproduced by newly developed hybrid simulation combining between molecular dynamics and Monte-Carlo method. The formation mechanism of tungsten fuzzy nanostructure observed by the hybrid simulation is that concavity and convexity of the surface are enhanced by the bursting of helium bubbles in the region around the concavity.

Power transmission factor (PTF) through the sheath of deuterium plasmas with hot electron component on virgin as well as nanostructured tungsten due to helium pre-irradiations are studied experimentally and compared with a PTF theory. PTF for nanostructured tungsten is larger than that for normal one, the reasons of which are discussed in terms of surface characteristics.

Power transmission factors (PTF) through the sheath of helium plasmas with hot electron component against two kinds of tungsten wall, virgin as well as nanostructured one due to helium irradiation are studied theoretically and experimentally. In the present theory the ion-induced electron emission in addition to secondary electron emission is taken into account, contributing to the cooling of the wall. Some discussions on the comparison between the measurement and the modified PTF theory are given. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

The physical properties of tungsten with nanostructure on its surface are investigated focusing on the thermal radiation and cooling characteristics. First, direct evidence of substantial W surface cooling has been clearly shown with use of a very thin thermocouple inserted into W target, which solves an uncertainty associated with a radiation thermometer. Second, the above measurements of W surface temperature make it possible to estimate quantitatively the total emissivity from which we may evaluate the radiative power through the Stefan–Boltzmann equation, which is very important for mitigation evaluation of a serious plasma heat load to the plasma-facing component.

One of the serious concerns for tungsten materials in fusion devices is the radiation defects caused by helium plasma irradiation since helium is a fusion product. The fiber-formed nanostructure is thought to have a possible weakness against the plasma heat flux on the plasma-facing component and also may destroy the reflectivity of optical mirrors. In this paper an interesting method for the recovery of such tungsten surfaces is shown. The recovery process depends on the grade and manufacturing process of tungsten materials.

A unique plasma-generation device called the “Aichi Institute of Technology-Plasma Irradiation Device” has been developed for plasma–wall interaction (PWI) studies toward realization of a fusion reactor. It has power-saving characteristics as well as compactness due to the employment of a multicusp magnetic field configuration instead of a strong axial magnetic field. Helium as well as argon plasmas are generated, showing an outstanding property of production of the hot-electron component which can be controlled by changing the working gas pressure. Utilizing such outstanding properties, some typical studies on PWI are introduced. An azimuthally asymmetric confinement depending on the direction of magnetic field lines, particularly for energetic electrons, has been found. Some discussions on the physical mechanisms are given. IEEJ Trans 2011 DOI: 10.1002/tee.21801

Helium plasma irradiation and electron heating experiments were conducted using tungsten in the divertor simulator NAGDIS-II. Helium plasma irradiation to tungsten led to the formation of nanostructures on the surface, while the nanostructures were annihilated after the potential of the specimen was changed to positive for several 10 min so that electrons irradiated the sample without ion irradiation. The specimens were analyzed in detail by transmission electron microscope with the help of focused ion beam technique. It is revealed that the helium nano-bubbles still remained even after the nanostructures were disappeared from the surface. Porosity of the nanostructured tungsten was measured from the TEM images.

Behaviors of an arc spot ignited on a tungsten electrode in response to a transient heat load are investigated in the linear divertor simulator NAGDIS-II using a pulsed laser as a transient heat source. Tungsten neutral emission from the arc spot is observed with an image intensified fast framing camera. When arcing is initiated, the tungsten emission continues much longer than the laser pulse width of ∼0.6 ms. The ignition of arcing becomes much easier compared to clean surface when the tungsten surface is exposed to the helium plasma and morphology is slightly changed.

The effects of temperature excursion on the helium defects of tungsten surface have been investigated by using compact plasma device AIT-PID (Aichi Institute of Technology – Plasma Irradiation Device). An initial stage of bubble formation has been identified with an order of smaller (sub-micron) bubbles and holes than those in the past in which the micron size is the standard magnitude. The radiation cooling has been detected when a blacking of tungsten surface coming from nanostructure formation is proceeding due to an increase in the emissivity. The temperature increase to the domain (∼1600 K) in bubble/hole formation from that in nanostructure formation has been found to bring a constriction in diameter and a reduction in length of fiber-form nanostructure.

In this article, we have analyzed the measured probe characteristics of microwave-sustained Ar and He plasma jets at atmospheric pressure using the high-pressure probe theory developed by Talukder et al. in order to determine the generated plasma parameters (electron temperature, Te, electron density, Ne, plasma potential, ϕp). The calculation shows some unusual temperature profiles (both axial and radial) as well as provides some over estimation of electron temperature measured from the distorted I-V characteristic due to a smaller electron to ion saturation current ratio coming from the limited electron saturation current. In order to overcome these limitations, the measured probe characteristics are also analyzed by using the asymmetric double probe theory developed by our group, which provides reasonable value of plasma parameters. The influence of microwave power and gas flow rate on the plasma parameters are also investigated (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Clear footprint of an arc spot, which is formed after arc spot runs on a fiberform nanostructured tungsten, reveals the detailed structure and motion of arc spot in a magnetic field. Fine inner structure of an arc trail is observed, exhibiting that many sub-arc spots existed inside the arc spot, with forming a group. The sub-arc spots move randomly but they globally move to some direction, which is affected by the axial and parallel magnetic field with respect to the specimen. The trails are analyzed by using box-counting method and fractality of the arc trails in magnetic field is discussed. It is shown that the nanostructured metal can clearly record the footprint of the arcs, similar as the man's footprints in the snow, and be a powerful tool for basic arc experiments.

This overview describes recent studies of nuclear fusion by using divertor plasma simulators generating high heat flux plasma. The experimental results regarding plasma-material interaction related to tungsten and carbon dusts, and detached recombining plasma are reviewed. © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Detached deuterium recombining plasma has been generated in the toroidal divertor simulator. The electron temperature (0.1–0.4 eV) and density (∼10 18 m −3 ) in the detached plasmas were evaluated with a spectroscopic method using a series of deuterium Balmer line emission from highly excited levels and the Stark broadening of D(2–12). We have investigated the role of volume plasma recombination through Electron–Ion Recombination (EIR) and Molecular Activated Recombination (MAR) processes. Moreover, the carbon erosion in the detached deuterium plasma has been studied with a weight loss method. It is found that deuterium neutrals generated by EIR process could have strong influence on the carbon chemical erosion.

Simulated experiments to produce carbon dust particles with cauliflower structure have been performed in a liner plasma device, NAGDIS-II by exposing high density deuterium plasma to a graphite sample (IG-430U). Formation of carbon dust depends on the surface temperature and the incident ion energy. At a surface temperature 600–700 K, a lot of isolated spherical dust particles are observed on the graphite target. The internal structure of an isolated dust particle was observed with Focused Ion Beam (FIB) system and Transmission Electron Microscope (TEM) in detail. FIB analysis clearly shows there exist honey-combed cell structure with thin carbon walls in the dust particle and the dust particle grows from the graphite surface. TEM image also shows that the dust particle is made of amorphous carbon with crystallized grains with diameters of 10–50 nm.

Oxy-combustion generates a high-temperature field of more than 3000 K and represents a nitrogen-free process. In order to apply oxy-combustion to next-generation power plants and high-temperature industrial technology this process has been enhanced by a new method, referred to as ‘plasma-assisted combustion’. This method uses to control the oxy-combustion flame with the radio-frequency electromagnetic force acting on the hightemperature field of the oxy-combustion flame. In order to establish the plasma-assisted combustion technique it is necessary to assess the influence of the electromagnetic field on the flame parameters, i.e. to compare the main plasma properties (conductivity, electron density and temperature) of the oxy-combustion flame with and without the assistance of the electromagnetic field. For that purpose we have developed a reciprocating double probe method to measure the plasma properties of the oxy-combustion flame. In this paper we present (i) the experimental part (ii) the model that enables to estimate the electron temperature and density from the conductivity derived from the double probe measurements and (iii) measurements without the applied electromagnetic force. At the centre of the flame, the measured electrical conductivity is (1.3 ± 0.7) × 10–4 S/m which gives an electron temperature of 2800 ± 100 K and an electron density of (1.2 ± 0.5) × 1015 m–3. The electron temperature in oxy-combustion is equivalent to the adiabatic flame temperature. The equilibrium adiabatic flame temperature in the oxy-combustion of methane is estimated to be approximately 3100 K, which is in good agreement with the electron temperature measured in this study. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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 multifractal statistics. The generalized scale invariance has been observed by involving the Extended Self-Similarity hypothesis. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Simulations of the helium radiative properties carried out with the recently developed multi-level meta-stable resolved collisional-radiative code SOPHIA discovered new unique emission lines to analyze optically thick divertor plasmas relevant for ITER. The comparison of their time dependent line emission obtained from the NAGDIS-II plasma simulator experiments with time dependent temperature probe measurements shows a strong correlation. This indicates that line intensity fluctuations can be transformed to the important quantities of density and temperature fluctuations. A transformation method based on integral line intensity ratios which can be recorded with high time resolution is discussed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effects of a transient heat load on tungsten damaged by helium plasma irradiation have been investigated using a ruby laser with long pulse duration in the divertor simulator NAGDIS-II (Takamuraet al2002Plasma Sources Sci. Technol.11A42). The pulse width of the ruby laser was ∼0.6 ms, which is close to that of the expected heat load accompanied by type-I edge localized modes (ELMs) in ITER operation. Helium holes/bubbles, which were formed in the surface region of powder metallurgy tungsten due to the exposure to the helium plasma, disappeared after the laser pulse irradiation to the tungsten surface with sufficient pulse energy. The results indicated that the transient heat loads similar to those expected by ELMs will mitigate damages such as bubbles and holes produced by helium irradiation. When a vacuum plasma sprayed tungsten coating on graphite was exposed to the helium plasma, the surface was covered with arborescent nanostructured tungsten containing many helium bubbles inside the structure. Melting traces were found on the surface after the laser pulses irradiated the surface even though the pulse energy was lower than that for melting bulk tungsten. A numerical temperature calculation of the sample suggested that the effective thermal conductivity near the surface dramatically decreased by several orders of magnitude due to the formation of nanostructured tungsten.

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.

An integrated view of disruption dynamics on internal plasma and associated electromagnetic structures with a high time resolution is presented in a systematic way. The main results are as follows: (i) observation of electron temperature oscillation due to rotation of magnetic islands of m(poloidal)/n(toroidal) = 3/2 mode numbers before the current quench, (ii) finding of rapid pump-out (~20 µs) of plasma current and particles from the central core region to the edge just at the current quench start time and then returning to a peaked profile during the following slow decay phase and (iii) a new proposal for the evaluation of current quench time and the electron temperature dependence of the current quench time, the so-called τ/S scaling.

Dynamic behaviour of plasma-wall interactions has been investigated by using triple probe and H α line emission of disruptive plasma in small tokamak HYBTOK-II. As the features of the disruption discharge of HYTOK-II, it was found that the waveform of plasma current quench has two phases of the first slow and the subsequent fast decays. The interaction between plasma and wall was found to be largest during fast decay phase in the course of whole discharge. The increase of plasma-wall interactions is thought to be caused by both of convective pump-out of the plasma particles in the core region and the rapid movement of plasma to the inner wall at the start time of two decay phases.

We have investigated the influence of low energy helium and hydrogen/deuterium plasma irradiation to tungsten or tungsten coated graphite samples. The experimental conditions of helium hole/bubble formation and hydrogen blister formation were summarized. The tungsten coated graphite exposed to a helium plasma shows different surface modification from those of bulk tungsten. We also demonstrated a transient heat load onto a damaged tungsten surface due to a helium plasma exposure by a laser-beam irradiation. Simultaneous helium plasma and laser pulse irradiations lead to a further roughing of the tungsten surface.

A new toroidal divertor plasma simulator, NAGDIS-T (NAGoya DIvertor plasma Simulator with Toroidal magnetic configuration) has been constructed, in which the toroidal plasma is generated by RF Joule and/or DC discharge. In order to obtain high density deuterium plasmas relevant to divertor conditions in fusion devices, the optimum condition of RF ohmic discharge was investigated by varying gas pressure, driving frequency and RF transformer turn ratio. The experimental result was compared with the simple numerical modeling.

Toroidal simulator for boundary plasma studies may have a substantial potential in plasma and impurity transport under an open magnetic field configuration and plasma-surface interactions under a grazing incident magnetic field line to the wall of the plasma-facing component in fusion devices. NAGDIS-T is the toroidal version in NAGDIS-series plasma simulator with RF Joule heating for toroidal plasma generation. A simple numerical analysis was employed to predict the plasma parameters in this new machine, considering the particle and energy balance equations. It was found that the gas pressure dependence of the plasma density is very similar to that obtained in the preliminary experiment with moderate RF power. The analysis predicts a plasma density exceeding 10 19 m -3 by injecting the RF power of more than 10 kW which will be available soon.

We have investigated a relation between the edge magnetic structure of the Large Helical Device (LHD) and the fluctuation properties of the ion saturation currents I sat measured by a Langmuir probe array (16 channels) embedded in an outboard divertor plate. The edge magnetic structure is calculated by KMAG and Watanabe codes. The fluctuation properties are analyzed based on the probability distribution function(p.d.f.) and complex Morlet wavelet decomposition. Large positive skewness was obtained from I sat measured at a low field side of a divertor leg and negative skewness was observed at striking point of the divertor leg, where the connection length of magnetic line of force is long. The fluctuation characteristics has very weak dependence of plasma density and its gradient. Waiting time statistics was analyzed to indicate that the waiting time statistics is not a Poisson process and there is no clear signature of SOC paradigm.

The edge plasma turbulence in the T-10 tokamak, the HYBTOK-II tokamak, the linear machine NAGDIS-II and the Large Helical Device have been studied. The fluctuations in the plasma density have been analysed in terms of the multifractal formalism revisited with wavelets.In most of the cases considered, the edge density fluctuations demonstrate multifractal statistics, i.e. the scaling behaviour of the absolute moments is described by a convex function with non-trivial self-similarity properties. The multifractality factor defined in the multiplicative cascade model is a relevant parameter for characterizing the edge plasma turbulence. The multiplicative cascade process has a 'coarse' time scale T ~ 50?200?ms iterating towards finer scales. This time scale is referred to as an integral correlation scale. The self-similarity parameters have been observed to depend on the edge plasma condition. The correlation and transport properties have been analysed regarding the multifractality parameter. The diffusion transport coefficient in an edge plasma is not a trivial function of multifractality parameters.

An experiment with rotating helical magnetic fields (RHMF) in the HYBTOK-II tokamak is reported. The effect of the RHMF rotation with a frequency of 5 kHz on the poloidal rotation of plasma density perturbation was studied by a movable multi-pin Langmuir probe. The wavelet technique has been used to study the poloidal rotation of the bursts. During the RHMF operation the poloidal direction of the plasma density perturbation rotation coincides with the direction of the RHMF rotation. The effect of the RHMF is found to be essential up to minor radius r = 6 cm.

The possibility of variation of the fraction of the energetic electron component in a magnetically confined diffusion argon plasma with a bi-Maxwellian electron energy distribution function (EEDF) created in the target part of a double plasma device by changing the anode voltage is demonstrated. The growth of fraction of the energetic electrons with a decrease of the anode voltage while the separation grid potential is kept constant is observed. An analysis of the loss rates of cold and hot electrons is given. Mechanisms of the formation of the bi-Maxwellian EEDF in the magnetically confined diffusion plasma are discussed.

Low-energy deuterium (D) plasma exposure on tungsten (W), which is an important material for ITER, results in blister formation on the surface. Blister formation increases both micron-sized dust production and D retention. Blister formation depends greatly on surface pre-treatment. Deuterium plasma exposure on mirror-finished powder metallurgy W at 500 K for 3 h forms a blister with a diameter of a few hundred micrometres on the surface. Blister formations on the mechanically-polished and helium-pre-exposed surfaces are drastically suppressed. Deuterium retention is also reduced on both the mechanically-polished surface and the helium-pre-exposed surface compared with that on the mirror-finished surface. The suppressive effect of blister formation on the mechanically-polished surface is maintained for 50 h. The size of blisters and D retention on mirror-finished surface exposed for 50 h increases by some degrees (500 µm, 7 × 1020 m−2) compared with that on mirror-finished surface exposed for 3 h (200 µm, 5 × 1020 m−2), but is not proportional to the exposure time. The saturation level of D retention even on blister-rich surfaces seems to be lower than an order of 1021 D2 m−2.

A diagnostic method for the determination of the neutral helium diffusion coefficient in the edge of magnetically confined fusion plasmas is developed on the basis of emission spectroscopy and population kinetics. The new collisional-radiative numerical code SOPHIA showed that intercombination transitions are extremely important to describe the qualitative and quantitative behaviour of the line emission and the population flow in diffusive plasmas. Corresponding simulations relevant for edge plasma conditions and the NAGDIS-II plasma simulator are discussed.

Deuterium retention in tungsten increases substantially when it is pre-exposed to low-energy He plasma at a high temperature (1600 K), while it decreases when pre-exposed at a low temperature (700 K). The increase of deuterium retention could be attributed to the submicron-sized He bubbles and secondary interstitial defects formed on the tungsten surface. Occupation of trap sites by He atoms during the He pre-exposure at low temperature could decreases the effective number of available trap sites for deuterium atoms, which would decreases the deuterium retention on the tungsten surface.

Formation of carbon dust particles on a graphite surface irradiated by a high-density hydrogen plasma and a mixed hydrogen/helium plasma has been investigated in the NAGDIS-II linear divertor plasma simulator. Carbon dust particles are generated at the center region of the graphite surface facing the high-density plasma. No dust particles are formed in the peripheral region. These observations indicate that the re-deposition process of the hydrocarbon ions due to a strong plasma flow at the center of the plasma column is one of the key factors determining the dust growth as well as chemical sputtering.

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.

A systematic study on the formation mechanism of micron-sized He bubbles and holes in powder metallurgy tungsten due to helium ion irradiation with an ion energy below 30 eV and a particle flux above 1022 m−2 s−1 has been performed in the linear divertor plasma simulator NAGDIS-II. Holes are formed with incident helium ion energy above 5 eV, which could be related to the surface barrier potential energy for He penetrating into tungsten. Tungsten surface temperature strongly influences the number and size of hole. Above 1600 K, bubbles and/or holes with several hundreds nano-meter diameter appear on the tungsten surface. Single crystal tungsten, which has much fewer intrinsic defects than powder metallurgy tungsten, was also irradiated by He plasmas. There is no qualitative difference in the hole formation between the two grades of tungsten. Bubble and hole formation mechanisms are discussed based on the experimental results.