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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.

The dynamics of mode transition from the electrostatic (E) to electromagnetic (H) mode of a radio-frequency argon inductively coupled plasma (ICP) in atmospheric pressure is investigated, both experimentally and theoretically. High-speed (4500 f/s) camera imaging is performed to investigate the dynamics of E–H discharge mode transition. The temporal plasma loading impedance of Ar inductive discharges at the E–H transition stage is also observed to investigate the transition dynamics. The experimental observations reveal that the formation of the multiple streamerlike electrostatic discharge paths followed by the strong ring-shaped azimuthal discharges leads to ignite the high-pressure (around 1 atm or more) ICPs. The time scale of E–H discharge mode transition is estimated by using proposed models and the estimated results are compared with that of the experimental one. It is found that the estimated results agree well with that of the experimental one.

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.

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.

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.

Plasma edge fluctuations measured with Langmuir probe in relation to the turbulence structure and associated enhanced transport have been studied in the tokamak HYBTOK-II with a variation of the rotating helical magnetic field (RHF) frequency in the range of 5–30 kHz. Edge fluctuations have non-Gaussianity statistics caused by intermittent bursts with time scale of 40–100 μs. The variation in the RHF frequency has a selective effect on the edge turbulent fractal structure 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 RHF frequency dependence of the edge turbulent fractal structure.

A space charge effect on thermoelectron emission current from a tungsten plate has been experimentally investigated in a high density helium plasma. Experimental result shows that the space charge limited current from the tungsten target plate, which is heated by the plasma irradiation, tends to be saturated with an increase in a sheath voltage, which can be predicted by the developed new formula based on the boundary condition of zero electric field on the material surface.

Detailed measurements of rotating helical field (RHF) have been carried out in a small tokamak device HYBTOK-II in order to know the interaction between the tokamak plasma and RHF. The magnetic pick-up coils were inserted deeply inside the plasma to detect the response profile of tokamak plasma to the RHF. It was found that a large relative velocity between the plasma and RHF poloidal rotation velocities gives rise to a strong attenuation of RHF in the plasma because the large screening current flows around the resonance surface against the externally applied RHF. The electrode biasing was applied so as to increase the relative velocity so that the RHF was strongly attenuated in comparison with unbiased case.

Tungsten (W) specimens were irradiated by low energy ( 10 22–23 m −2 s −1 ) helium plasma to investigate the incident ion energy dependence of helium bubble formation. Experimental results indicate the existence of the incident ion energy threshold for the bubble formation. The threshold energy around 15 eV could associate with the surface potential for He ions entering to the W surface.

This study presents experimental results on hydrogen blister formation on powder metallurgy tungsten (PM-W) surface under low energy ( 1021 m−2 s−1) hydrogen plasma irradiation in a divertor plasma simulator-NAGDIS-I. The tungsten samples were exposed to steady-state hydrogen plasma at various sample temperatures and fluences. Hydrogen blister formations are clearly observed on tungsten surface at the surface temperature below 950 K. The blister size is from a few 10 μm to a few 100 μm. It was found that the blister formations obviously depend on the surface temperature and the incident hydrogen ion fluence. No blisters are observed on tungsten surface at the surface temperature above 950 K even if the fluence is high enough.

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.

We achieved a potential well more than fifty times deeper than the plasma electron temperature, down to -0.3 kV with the electrode biasing, by the cross-field radial arcing current of up to 150 A which flows between the small electron-emitting electrode (LaB6) inserted to the plasma center and the stainless-steel wall of vacuum chamber. The empirical scaling of the radial resistance of plasma Rr on the radial arcing current Ib and the toroidal magnetic field BT is found to be Rr ∼ Ib-1.2BT1.0. Structural dynamics of negative potential well formation is observed: the potential dip propagates from the plasma core to the periphery. The ion saturation current also increases, starting from the inner region. The fluctuation frequency in Isat changes abruptly from 200 kHz to 30 kHz in a time of around 0.25 ms after negative biasing.

The spatial and temporal characteristics of the turbulence in the inboard and outboard scrape-off layer (SOL) of tokamak HYBTOK-II have been studied using poloidal probe array. Bursty behaviour with intermittent bursts was observed for both outboard and inboard SOL. In the inboard (high field side), high level of density fluctuations has been observed. The fluctuations in the high field side and low field side are identical in statistics that is non-Gaussian one.

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

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.

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

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.

It has been recognized that the ELMs associated with a good confinement at the edge, such as H-mode, must bring an enormous energy to the divertor target plate through SOL and detached plasmas. The understanding of the ELM energy transport through SOL to the divertor target is rather poor at the moment, which leads to an ambiguous estimation of the deposited heat load on the divertor target in ITER. In the present work the ELM-like plasma heat pulse is generated by rf heating in a linear divertor plasma simulator. Energetic electrons with an energy range 10–40 eV are effectively generated by rf heating in low temperature plasmas with (T e ) ∼1 eV. It is observed experimentally that the energetic electrons ionize the highly excited Rydberg atoms quickly, bringing a rapid increase of the ion particle flux to the target, and make the detached plasmas attached to the target. Detailed physical processes about the interaction between the heat pulse with conduction and convection, and detached recombining plasmas are discussed.

Detailed spectroscopic measurements have been carried out in the linear divertor plasma simulator, NAGDIS-II in order to acquire the accurate evaluation of the electron temperature, which leads to a deeper understanding of the characteristics of detached recombining plasmas. Electron temperature in the detached recombining plasmas was evaluated by two different spectroscopic methods, using continuum emission and a series of line emission from highly excited levels, giving different electron temperatures. The reasons are discussed based on the effects of energetic electrons.

Effects of condensible plasma impurities on the material erosion behaviors have been investigated by improving the previous zero-dimensional model. The new model can describe more precisely the transition from the enhanced erosion due to impurities to reduced erosion with the formation of the impurity film. Model predictions have been proven by the experimental observations in NAGDIS-I. A water-cooled molybdenum target plate was bombarded with argon plasmas into which methane was introduced as carbon impurity. As the flow rate of the methane gas was increased, MoI emission line intensity, corresponding to sputtered particle flux, has been observed to decrease to a great deal.

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.

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

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 order to characterize carbon materials exposed to hydrogen, deuterium and helium plasmas at elevated temperature, we have applied laser Raman spectroscopy. We have found that the exposure to high dense plasma at an elevated temperature, 700–1200 K, does not amorphize the carbon materials. The specimen exposed to the plasma keeps two-dimensional structure orders, though their grain sizes or crystallites are fragmented into very fine ones. Heavier ions irradiation (He) results in more fragmentation than lighter ions (H). Hence the displacement played more important role and the chemical effect of hydrogen which is not appreciable because of spontaneous release of hydrogen at high temperatures. This indicates that hydrogen retention in carbon materials can be significantly reduced at elevated temperature operation. Furthermore the Raman spectroscopy could be used for in situ analysis of carbon based plasma facing materials.

A self-excited vertical oscillation of dust particles in a Coulomb crystal has been observed. It occurs near the plasma-sheath boundary of a dc plasma operated at a low plasma density and gas pressure. The excitation of this spontaneous oscillation is attributed to the finite charging time of particles. As particles move, their charge varies with the local plasma conditions, but with a delay due to the charging time. As particles traverse the vertical electric potential gradient in the sheath, they gain energy. A model of this mechanism is developed to predict the instability threshold and the energy gain.

Formation of the radio-frequency driven sheath and resulting parasitic antenna loading in the ion cyclotron slow wave antennas are studied experimentally in the linear divertor plasma simulator NAGDIS-II. A phased loop antenna array with a poloidal mode of m =0 is used in the present ICRF heating experiment. A large DC voltage drop of about several hundreds volts is induced on the loop antennas with and without Faraday screen during high power RF heating and causes the additional power dissipation ( P sh ) due to the heat flux to the antenna current strap of ions accelerated by the RF driven DC sheath potential. This parasitic antenna loading is measured by calorimetric method and compared with that obtained from the conventional measurement of the antenna voltage and current. When a Faraday screen is employed to reduce the antenna–plasma interaction, P sh becomes much smaller than the radiated RF power. The net antenna loading for ICRF slow wave excitation is evaluated, taking account of the RF sheath dissipation.

The effect of ergodic divertor (ED) on the particle balance in the plasma–wall system is investigated. It is found that in the short timescale 2–3 s at the beginning ED effectively increases the recycling wall area and enhances wall pumping by connecting the field lines to the wall. But in long timescale greater than several seconds ED brings an early termination of wall pumping due to the localization of recycling area around the X-region. A zero-dimensional model reproduces the observed behavior on the particle balance over the long timescale.

Nonlinear dynamic behavior of hot spot formation on tungsten (W) material surfaces is numerically investigated by introducing a transient heat pulse into the local area of the hot W sheet immersed in high heat flux plasma. It is found that the formation and development of hot spot depend not only on plasma parameters (plasma heat flow, plasma potential variation) and the energy and time scale of heat pulse but also sensitively on electron-emission characteristics of the tungsten surface as well as the size of hot spot in a nonlinear way.

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

A high heat flux helium plasma has been used to irradiate TiC-coated graphite samples with a high substrate temperature up to 1800°C. A new technique of pulse biasing on the target with in situ optical spectroscopy gives dynamic erosion characteristics. The emission intensity of TiI spectral line corresponding to sputtered particle flux was observed to decrease as the temperature increases up to 1300°C. This behavior is explained in terms of self-sustaining coverage of the TiC surface with segregated carbon. Over 1300°C, the emission intensity increases with temperature. Pulse biasing gives also remarkable time evolution of TiI line emission intensity, which is strongly temperature-dependent. It is typically characterized by two time scales. These are discussed using the analysis of mass balance equations for carbon atoms. In addition, an initial overshoot of Ti erosion observed at very high temperature is modelled by a stoichiometric change of the TiC layer.

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.