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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 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.

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.

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.

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)

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.

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.

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.

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.

Fractal structure of the edge turbulence and enhanced turbulent transport have been studied in the tokamak HYBTOK-II with a variation of the rotating helical magnetic field (RHF) frequency in the range 5–30 kHz. Edge fluctuations have non-Gaussian statistics caused by intermittent bursts with a time scale of 40–100 µs. The variation in the RHF frequency has a selective effect on the fractal structure of edge turbulence and the turbulent flux, demonstrating a selective control of the transport process. A delayed synchronization control of resonant drift wave modes by the RHF is considered as a candidate mechanism to explain the dependence of the effect on the RHF frequency.

Direct observations of tokamak plasma responses to an externally applied rotating helical magnetic perturbation (RHMP) have been performed on the small tokamak device HYBTOK-II in order to clarify the penetration process of the RHMP into tokamak plasmas. Not only electromagnetic but also electrostatic fluctuations have been measured with small magnetic probes and a four-pin Langmuir probe, which were inserted inside the plasma. As regards the observed radial profiles of the RHMP in the plasma, the low Doppler-shifted frequency of RHMP produces the amplification of RHMP in the plasma due to magnetic island formation, while the expected attenuation of RHMP in the plasma has been found in the case of the high Doppler-shifted frequency. It is also found that the observed radial electric fields with the four-pin Langmuir probe have strong dependences on the driving frequency of the RHMP and the toroidal magnetic field. A theoretical calculation of the Alfven wave propagation driven by local helical coils in HYBTOK-II has also been carried out. The observed electric fields are discussed in terms of the Alfven resonance and the growth of magnetic islands.

We have derived an exact theoretical expression for the space-charge limited current from the solid surfaces adjacent to plasmas that is applicable for an arbitrary sheath voltage. Our expression shows that the space-charge limited current tends to saturate with the sheath voltage. This new formula is evaluated by 1-D Particle in Cell (PIC) simulation and experiment, and is in a good agreement with the simulation and experimental results. We have also obtained an analytical equation fitted to the new formula based on conventional Child-Langmuir formula by taking into account a more sophisticated dependence of the electrode potential and the plasma density through the effect of Debye shielding and a sheath expansion due to increased voltage across the sheath.

We propose a modified Child-Langmuir (C-L) equation, which can give space-charge limited electron current from plasma-facing material for not only floating condition but also any sheath voltage. The modified C-L equation has a more sophisticated dependence of the sheath voltage and the plasma density through the effect of Debye shielding than the simply modified C-L equation previously derived by us. The space-charge limited currents given by the modified C-L equation are in good agreement with those calculated by recent theoretical formula within an error of ±6%. The modified C-L formula can be applicable in various plasma fields.

We have investigated the velocity distribution of the intermittent bursty fluctuations in the ion saturation currents observed in the linear divertor plasma simulator, NAGDIS-II. The intermittent bursty signals were analyzed by the wavelet transform with complex Morlet mother wavelet Analyzing a lot of bursts in the ion saturation currents based on the wavelet cross-correlation gives the velocity distribution function. It is positively skewed at the peripheral region of the plasma column, which means the intermittent bursts mainly propagate toward the wall. The averaged velocity depends on the neutral pressure associated with the transition between the attached and detached plasmas.

Penetration processes of rotating helical magnetic perturbation (RHMP) into tokamak plasmas have been studied on a small tokamak device HYBTOK-II for dynamic ergodic divertor experiment preparing in TEXTOR. The radial profiles of RHMP in the plasma have been measured by inserting small magnetic probes into the plasma. The penetration of RHMP has been discussed by using the observed radial profile of RHMP in the plasma based on the growth of magnetic islands (i.e. tearing mode). In the case of the low Doppler-shifted frequency observed from the plasma, the amplification of RHMP in the plasma due to a spatial modification of plasma current has been found. A large Doppler-shifted frequency produces an expected attenuation of RHMP near the magnetic resonance surface due to screening current flow. A decrease of the amplification of RHMP originated from the suppression of the growth of magnetic islands due to a rapid spatial scan of magnetic resonance layer has been found by plasma current oscillation.

The parameters of self-consistent dusty sheaths are investigated using computer simulations of the temporal evolution of one-dimensional slab plasma with two-temperature electrons and dust particles. The evolution is caused by collection of electrons and ions by an electrode (wall) and also by dust particles, which are initially injected into the plasma around the electrode (wall). A peculiarity of the sheaths is a nonmonotonic spatial distribution of the electric potential that causes protection of the electrode (wall) from fast ions. The degree of protection depends on the temperature and density ratios of both electron components.

First, the role of a magnetic divertor in fusion devices is introduced, and it is shown that the energy balance in burning International Thermonuclear Experimental Reactor indicates a strong necessity for reduction of plasma heat load on the divertor target plate. Next, the plasma heat flow to a material surface through sheaths is generally given in terms of the energy transmission factor, in which the energy deposition based on the surface recombination is essential in high-density plasmas. The heat load is independent of any cooling of the plasmas. Then, a variety of interesting characteristics of detached recombining plasmas, which are important in reducing the plasma heat flow, are discussed: generation and structure of plasma detachment, light emission originating from a series of highly excited Rydberg states of atoms, contribution of molecular activated recombination (MAR), transition between conventional electron–ion recombination (EIR) and MAR, relation to dust coagulation processes and the experimental verification of plasma detachment in tokamak fusion devices. Finally, the dynamic behaviour of the detached EIR plasma obtained in the linear divertor plasma simulator NAGDIS-II against the ELM-like heat pulse is shown.

Recombination in a helium and helium-hydrogen plasma was experimentally investigated in the linear divertor simulator NAGDIS-II. To identify the physical mechanisms, electron-ion recombination (EIR) and molecular assisted recombination (MAR), the plasma parameters in the recombining region have been measured. We used spectroscopic techniques on the basis of continuum emission and line emission from highly excited levels due to EIR and Langmuir probes. The dependence of the plasma parameters on the neutral pressure in a pure helium plasma and in a helium-hydrogen mixture shows clearly that recombination is caused by different processes in the two cases. In a pure helium plasma a roll-over and a following decrease of the electron density as well as a monotonic decrease of the electron temperature down to 0.1 eV were observed when increasing the helium pressure. Here EIR is the recombination mechanism. Conversely, when hydrogen is added to the helium plasma, the electron density decreases monotonically while the central electron temperature remains above 1 eV. This particular behaviour in the helium-hydrogen plasma can only be explained by the dominant action of MAR.

Influence of energetic electrons on the light emission from detached recombining plasmas has been investigated by using Collisional-Radiative model, where the rate coefficients were modified by taking electron beam components as the energetic electrons into account. Line emission intensities are changed, strongly depending on the electron beam energy, because the population density distribution is determined by not only recombination processes but also electron impact excitation processes due to the electron beam component such as re-excitation, excitation from meta-stable states and excitation from the ground state. The influence of energetic electrons on the inverse ELMs was also discussed. Furthermore, it is found that bulk electron temperature evaluated with the method of Boltzmann plot tends to decrease dramatically with an increase in the electron beam density.

A Monte Carlo impurity transport code based on object-oriented method has been developed. Edge plasma in fusion devices has a lot of impurity atoms and molecules such as hydrocarbon species, C x H y due to low electron temperature in the edge. Various atomic/molecular processes have been considered in the edge plasma region and we need to determine which process is essential in edge plasma conditions. To take such complicated processes into account in a systematic way, Monte Carlo impurity simulation, which has a strong flexibility in dealing with various atomic and molecular processes, would be useful. By employing the object-oriented method in the simulation code, we can handle arbitrary atomic and molecular processes.

Microwrinkle structures with a pitch of less than 100 nm and up to 600 nm on refractory metals like tungsten (W) and molybdenum, which are irradiated by noble gas ions such as neon and helium, have been studied systematically. The wrinkle formation mechanism is thought to be a buckling of the hard surface layer supported by the soft elastic substrate, which is induced by a penetration of noble gas species from the irradiated surface. Microwrinkle forms on this structure under lateral compressive strain/stress fields coming from thermal constriction on the way to substrate cooling. Such a process should be anticipated when walls in a fusion reactor are attacked by heat pulses such as edge localized modes and/or vertical displacement events, and therefore might be an initial stage of W surface damage.

In order to find the causes of the strong anomaly of current-voltage characteristics of Langmuir probe observed in detached recombining plasmas in a linear divertor plasma simulator, NAGDIS-II, we have investigated plasma resistance along a magnetic field and potential fluctuations in the detached recombining plasmas. Simple calculation on the ratio between the plasma length, at which plasma resistance and resistance of ion sheath formed around a probe tip become equal, and an electron collection length indicates that the evaluation of electron temperature T e becomes inaccurate at T e of less than 0.6 eV when plasma density and neutral pressure are 1.0 x 10 18 m -3 and 10 mtorr, respectively. The potential fluctuation in detached recombining plasmas was found to be so large compared to T e /e, which can also modify the probe characteristics.

A comprehensive investigation has been performed of the static and dynamic behaviour of detached recombining plasmas in the linear divertor plasma simulator NAGDIS-II. For stationary plasma detachment, the transition from electron-ion recombination (EIR) to molecular activated recombination (MAR) has been observed by injecting hydrogen gas into high density helium plasmas. The particle loss rate due to MAR is found to be comparable to that of EIR. Experiments have also been performed by the injection of a plasma heat pulse produced by RF heating into the detached helium plasma to demonstrate the dynamic behaviour of volumetric plasma recombination. Negative spikes in the Balmer series line emission were observed and found to be similar to the so called negative ELM observed in tokamak divertors. Observed Balmer spectra were analysed in detail using the collisional-radiative model. A rapid increase of the ion flux to the target plate was observed associated with the re-ionization of the highly excited atoms generated by EIR.

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.

A full continuous AC tokamak operation with a plasma current of ±600 A and a frequency of 60–100 Hz has been achieved for 1 min in CSTN-AC tokamak device by employing a new invertor power supply system. As an AC toroidal plasma source, it has been used for the studies of deposition of thin films. The duty of AC discharges can be controlled arbitrarily from less than 1 to 100%. A good titanium nitride(TiN) thin-film formation by reactive sputtering is obtained in high-duty AC tokamak with strongly ionized plasmas with a high-charged particle density beyond 10 18 m −3 , in an extremely low gas pressure of the order of 10 −5 mbar. The deposited thin films were analyzed by various surface diagnostics (SEM, EDAX, XPS, RBS).

High-power rf heating in the ion cyclotron range of frequency (ICRF) has been carried out in a divertor plasma simulator NAGDIS-II. The main objective of the present work is to produce high heat flux plasmas for fundamental studies of the divertor plasmas in magnetically confined fusion devices. Waves are excited by phased-loop antenna array in a frequency range between 0.5 and 1.5 MHz. Inverter power supplies using static induction transistors (SIT) are employed to excite ion cyclotron slow waves. In NAGDIS-II a target plasma for rf heating is produced by TP-D-type dc discharges, where the electron density is n e 19 m −3 , and the electron and ion temperature are T e =5–10 eV and Ti=1–10 eV in helium plasmas, respectively. By injecting about 14 kW rf power we have obtained high heat flux plasmas with n e =2×10 19 m −3 , T e ∼20 eV and T i ∼18 eV so far. The plasma heat flux into the end plate is measured by calorimetric method and the power balance of the rf heated plasmas is studied experimentally.

The frequency dependence of the tokamak plasma response to the externally applied rotating helical field was investigated by measuring the radial profile of the perturbation field with small magnetic probes which were inserted in the plasma. The experimental results are discussed, directly comparing with the generally accepted linear theory, and taking into account the E×B drift and diamagnetic effect of the plasma. It was found that the experimental results are in good agreement with the simple linear analysis based on the single helicity approximation in a cylindrical geometry, except for the ergodic region, which comes from the overlapping of several sideband components of the perturbation. It is also found that the radial component of the perturbation field is still amplified inside the resonance surface (r

A detailed experimental investigation of plasma fluctuation and its modification due to radial electric field effects, both its shear and curvature, has been carried out on the Current Sustained Torus IV at Nagoya University [Takamura et al., J. Plasma Fusion Res. 74, 38 (1998)] through measurement with an array of Langmuir probes. The observed statistical dispersion relation indicates a drift-wave-type turbulence. An examination of the radial electric field shear and curvature effects on the fluctuations as well as comparisons to theoretical predictions are presented. The decorrelation of turbulence is found to be influenced by the radial electric field shear in our experiment. A modification of the poloidal correlation length and the two-poloidal-point coherency due to the radial electric field has also been identified. Nonlinear analysis of quadratic mode coupling indicates that the radial electric field can affect the fluctuation amplitudes by modifying the coupling between different spectral componen...

Previous analyse for the interaction between a rotating helical field and the plasma column is mostly limited within vacuum magnetic field model. In the practical situation, the response of plasma to the externally applied helical field, i.e. shielding current near the resonant surface and current profile modification due to island formation, often play important roles. Recently, a strong deviation from a vacuum field model is experimentally observed in CSTN-IV device in Nagoya University. In this work, simulation using a reduced set of resistive MHD equations in cylindrical geometry are carried out and an attempt is made to find the plasma response effects on the externally applied helical field. We show that the deviation from a vacuum model is mainly comes from the perturbed current due to a helical deformation for low S plasma (or for low frequency case). For high S, or high frequency case, the penetrating process is also affected by the shielding current near the resonant surface.

The penetration process of a rotating magnetic perturbation into a rotating tokamak plasma has been investigated, taking account of the mode structure of the perturbations, and using a small magnetic probe in the small research tokamak CSTN-IV. It was observed that the radial component of the perturbation was amplified in the plasma when magnetic islands are formed, while the poloidal component was attenuated but, deep inside the plasma, amplified. Using a resistive MHD code, such modifications of external perturbation were found to be caused by a redistribution of plasma current due to the formation of magnetic islands. Considering the phase velocity of both the perturbation and the intrinsic plasma rotation, it was also found that the sideband component substantially affects the penetration process with magnetic islands forming. Therefore, in dynamic ergodic divertor operation, it is necessary to put the emphasis not only on the frequency of the perturbation fields but also on their direction, mode structure and the radial profile of the intrinsic plasma rotation at the same time.

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.

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.

A strong anomaly of the single Langmuir probe characteristics has been observed in detached recombining plasmas, where the conventional analysis of the probe characteristics gives us a considerably high electron temperature compared with the value determined with the spectroscopic methods. We discuss the reason why such characteristics are obtained in terms of the temperature gradient along the magnetic field associated with the parallel electron transport. Moreover, the validity of the probe measurements in the detached recombining plasmas by using multiple electrodes is also discussed.

A strongly ionized carbon plasma was generated in a tokamak discharge. The electron density of carbon plasma produced by a plasma-assisted arc was found to be about . The carbon ion density was estimated to be about by use of the comparison spectroscopic method. Two-dimensional distributions of carbon ionization states were observed with an ICCD camera. The transport of carbon atoms is not influenced by the toroidal magnetic field, but carbon ions tend to move along the magnetic field.