菊池 祐介

Multiple plasmoid reconnection required for the flux closure in the transient-coaxial helicity injection (T-CHI) start-up process has been demonstrated in the Helicity Injected Spherical Torus (HIST) device. Two or three plasmoids are generated after the tearing instability of an elongated Sweet-Parker current sheet during the T-CHI. Here, we report that in the T-CHI start-up plasmas (H, D and He) with the strong toroidal (guide) field (ITF=140 kA), (i) the frequency of regular oscillations of reconnecting magnetic field decreases as the mass number increases, i.e., 250 kHz (H), 150 kHz (D) and 60 kHz (He). (ii) the oscillation propagates radially with 30 km/s (H), 20 km/s (D) and 12-18 km/s (He) from R= 0.25 m at the X-point toward the outboard side. It has been found that the propagation speed agrees with the Alfven speed. Consequently, the plasmoid reconnection could be related to the excitation of Alfven wave, leading to the ion heating in the T-CHI start-up plasmas.

Two discharge modes, α and γ, of a repetitive nanosecond pulsed helium glow discharge at a gas pressure of 10 kPa in the repetition frequency range from 20 to 600 kHz are reported for the first time. The pulsed glow discharge is produced in a pair of parallel plate metal electrodes without insertion of dielectrics. The α mode discharge is volumetrically produced in the electrode gap at a low-repetition frequency, whereas the γ mode discharge is localized at the cathode surface at a high-repetition frequency. At high-repetition frequency, the time interval between voltage pulses is shorter than the lifetime of the afterglow produced by the preceding discharge. Then, the γ mode discharge is maintained by a large number of secondary electrons emitted from the cathode exposed to high-density ions and metastable helium atoms in the afterglow. In the α mode discharge with a low-repetition frequency operation, primary electrons due to gas ionization dominate the ionization process. Thus, a large discharge voltage is needed for the excitation of the α mode discharge. It is established that the bifurcation of α-γ discharge mode, accompanied by a decrease in the discharge voltage, occurs at the high-repetition frequency of ∼120 kHz.

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.

A high-repetition nanosecond pulsed glow discharge plasma at a gas pressure of 1 kPa was generated using a SiC-MOSFET inverter power supply for diamond-like carbon (DLC) film preparation. At a high repetition frequency above 50 kHz, the period of the nanosecond voltage pulse became shorter than the decay time of the afterglow plasma, and many ions and radicals remained in the gap space. The deposition rate was 0.1 mu m/min, which was 5 times higher than that of a conventional plasma CVD process. An increase in hardness to 13GPa and a decrease in hydrogen content in the DLC film were confirmed by increasing the repetition frequency to 200 kHz. (C) 2017 The Japan Society of Applied Physics

The synergistic effects of steady-state and pulsed plasma irradiation to material have been investigated in the device NAGDIS-PG (Nagoya DIvertor Simulator with Plasma Gun). The duration of the pulsed plasma was ∼0.25 ms. To investigate the pulsed plasma heat load on the materials, we developed a temperature measurement system using radiation from the sample in a high time resolution. The heat deposited in response to the transient plasma on a tungsten surface was revealed by using this system. When the nanostructures were formed by helium plasma irradiation, the temperature increase on the bulk sample was enhanced. The result suggested that the amount of absorbed energy on the surface was increased by the formation of nanostructures. The possible mechanisms causing the phenomena are discussed with the calculation of a sample temperature in response to the transient heat load.

The multichannel ion Doppler spectroscopic (M-IDS) system using a fast camera as detectors has been newly developed to measure the spatial profiles of impurity spectral line emissions from plasmas. The M-IDS system developed with eight viewing chords is characterized by the simplest optical systems, a fast sampling frequency, high number of shooting, and a low crosstalk of 3.5% on each chord. In order to verify the usefulness and reliability of the M-IDS system, we have applied it for the start-up plasma produced in the helicity injected spherical torus device. Resultantly, we have successfully measured the radial profiles of Doppler ion temperature T-i.D and Doppler ion velocity v(i.D) of the plasma with a good Gaussian fitting. However, we have found that the sampling frequency is upper limited by 25 kHz because the signal detected with a faster frequency from the CIII impurity line emission is too weak to be analyzed, and thus the sensitivity of the total system needs to be improved. (C) 2017 Wiley Periodicals, Inc. Electron Comm Jpn, 100(2): 68-75, 2017; Published online in Wiley Online Library (wileyonlinelibrary.com).

A new method of synthesis of a-C:H films using a pulsed glow discharge plasma with a mixed gas of helium and nitrogen at sub-atmospheric pressure of 2 kPa was proposed. Characteristics of the pulsed glow discharge plasma produced with a pair of parallel plate electrodes by applying high-repetition bipolar voltage pulses were investigated using optical emission spectroscopy. The ratio of intensities of the 0-0 transitions of the second positive system of nitrogen molecule (337.1 nm) and the first negative system of ionized nitrogen molecule (391.4 nm) was measured to investigate the electric field strength in the discharge plasma. The spatial structure of the plasma emission was similar with that of a typical glow discharge. It was also found that an intensive electric field appeared near the cathode electrode during negative ripple pulse voltages induced by an inductor added between the electrodes and the power supply. A uniform a-C:H film on a silicon substrate with hardness of 7 GPa was successfully prepared with the pulsed glow discharge plasma with the ripple pulse voltage application, where acetylene was used for the precursor gas. The deposition rate is 0.2 mu m/min which is approximately 10 times higher than that of the conventional plasma CVD method under the low gas pressure. It is discussed that a rough estimate of incident ion energy onto the negatively biased substrate using a collisional plasma sheath model indicates the possibility of increase in the incident ion energy that is needed for enhancement of fraction of sp(3) carbon bond due to decrease in the neutral gas pressure from 100 kPa to 2 kPa. (C) 2016 Elsevier Ltd. All rights reserved.

A new technology for obtaining nanostructure on silicon surface for potential applications to optical devices is represented. Scanning electron microscope analysis indicated a grown nanostructure of dense forest consisting of long cylindrical needle cones with a length of approximately 300nm and a mutual distance of approximately 200 nm. Raman spectroscopy and spectrophotometry showed a good crystallinity and photon trapping, and reduced light reflectance after helium plasma exposure. The present technique consists of a simple maskless process that circumvents the use of chemical etching liquid, and utilizes soft ion bombardment on silicon substrate, keeping a good crystallinity. (C) 2016 The Japan Society of Applied Physics

The multi-channel ion Doppler spectroscopic (M-IDS) system using a fast camera as detectors has been newly developed to measure the spatial profiles of impurity spectral line emissions from plasmas. The M-IDS system developed with 8 viewing chords is characterized by the simplest optical systems, a fast sampling frequency, high number of shooting and a low crosstalk of 3.5% on each chord. In order to verify the usefulness and reliability of the M-IDS system, we have applied it for the start-up plasma produced in the Helicity Injected Spherical Torus (HIST) device. Resultantly, we have successfully measured the radial profiles of Doppler ion temperature Ti.D and Doppler ion velocity vi.D of the plasma with a good Gaussian fitting. However, we have found that the sampling frequency is upper limited by 25 kHz because the signal detected with a faster frequency from the CIII impurity line emission is too weak to be analyzed, so that the sensitivity of the total system needs to be improved.

A magnetized coaxial plasma gun (MCPG) device at Univ. of Hyogo was used as a simulation facility for pulse heat loads such as ELMs (Edge Localized Modes) onto plasma-facing materials in magnetically confined fusion devices. In the present study, characterization of the compact toroid (CT) plasmas produced by the MCPG device was performed. Dependences of the CT plasma parameters of line-averaged electron density, ion temperature and flow velocity on the gun discharge voltage and the bias magnetic flux were measured. As a result, it was observed that the pulse length, the line-averaged electron density, and the ion temperature and flow velocity were approximately 200 µs, approximately 6×1021 m-3, approximately 30 eV, and approximately 50 km s-1, respectively. The surface energy density measured with a tungsten calorimeter was approximately 0.4 MJ m-2. Thus, it was shown that the ELM-like pulsed plasma heat loads with the energy density and the pulsed length predicted in the ITER divertor can be produced using the MCPG device.

Energy transfer processes from ELM-like pulsed helium (He) plasmas with a pulse duration of similar to 0.1 ms to aluminum (Al) and tungsten (W) surfaces were experimentally investigated by the use of a magnetized coaxial plasma gun device. The surface absorbed energy density of the He pulsed plasma on the W surface measured with a calorimeter was similar to 0.44 MJ m(-2), whereas it was similar to 0.15 MJ m(-2) on the Al surface. A vapor layer in front of the Al surface exposed to the He pulsed plasma was clearly identified by Al neutral emission line (Al I) measured with a high time resolution spectrometer, and fast imaging with a high-speed visible camera filtered around the Al I emission line. On the other hand, no clear evaporation in front of the W surface exposed to the He pulsed plasma was observed in the present condition. Discussions on the reduction in the surface absorbed energy density on the Al surface are provided by considering the latent heat of vaporization and radiation cooling due to the Al vapor cloud.

<p>A fast two-color pyrometer system was developed to measure the back-surface temperature of thin tungsten materials during plasma-gun generated edge localized mode-like pulsed plasma irradiation. The developed pyrometer system had a time resolution of ∼5 µs and the lowest measureable temperature was ∼1600 K. We observed that the back-surface temperature of the thin tungsten material during the pulsed plasma irradiation reached ∼3280 K. The absorbed energy density and the pulse width of the pulsed heat load estimated by the measured time evolution of the back-surface temperature and 3D heat analyses using ANSYS code were ∼0.52 MJm⁻² and ∼1.6 ms, respectively.</p>

In this study, surface damaged W monoblocks (melting and cracking) by a pulsed plasma gun and an e-beam devices were exposed to cyclic heat loads (simulating normal heat loads and slow transients) and pulsed heat loads (simulating ELMs) to observe the effects of surface damage on surface erosion and heat removal capability. Heat load tests simulating the normal heat load (10 MW/m&lt sup&gt 2&lt /sup&gt , 10 sec, 300 cycles) and the slow transient (∼20 MW/m&lt sup&gt 2&lt /sup&gt , 10 sec, 300 cycles) were performed by the e-beam. The surface morphology changes after the heat load tests were observed using laser sccaning microscopy and FE-SEM. After e-beam irradiation of ∼20 MW/m&lt sup&gt 2&lt /sup&gt , the longitudinal cracks crossing over entire monoblocks appeared on the surfaces of all monoblocks. Recrystallization and additional crack formation were also observed on the surface. However, there was no significant change of heat removal capability. In the additional pulsed heat load test, the energy fluence of 0.042-0.30MJ/m&lt sup&gt 2&lt /sup&gt was applied with pulse numbers of 10&lt sup&gt 3&lt /sup&gt and 10&lt sup&gt 4&lt /sup&gt .The surface morphology changes after laser irradiation were observed using laser scanning microscope. After laser irradiation, the grain ejection occurred above a certain energy fluence (∼25 % of melting threshold).

Surface modifications of toughened, fine-grained, recrystallized tungsten (TFGR W) materials with 1.1 wt.% TiC and 3.3 wt.% TaC dispersoids due to repetitive ELM-like pulsed (similar to 0.15 ms) helium plasma irradiation have been investigated by using a magnetized coaxial plasma gun. No surface cracking at the center part of the TFGR W samples exposed to 20 plasma pulses of similar to 0.3 MJ m(-2) was observed. The suppression of surface crack formation due to the increase of the grain boundary strength by addition of TiC and TaC dispersoids was confirmed in comparison with a pure W material. On the other hand, surface cracks and small pits appeared at the edge part of the TFGR W sample after the pulsed plasma irradiation. Erosion of the TiC and TaC dispersoids due to the pulsed plasma irradiation could cause the small pits on the surface, resulting in the surface crack formation. (C) 2014 Elsevier B.V. All rights reserved.

We have developed a unique experimental device of so-called double plasma gun, which consists of two magnetized coaxial plasma gun (MCPG) devices, in order to clarify effects of vapor shielding on material erosion due to transient events in magnetically confined fusion devices. Two ELM-like pulsed plasmas produced by the two MCPG devices were injected into a target chamber with a variable time difference. For generating ablated plasmas in front of a target material, an aluminum foil sample in the target chamber was exposed to a pulsed plasma produced by the 1st MCPG device. The 2nd pulsed plasma was produced with a time delay of 70 mu s. It was found that a surface absorbed energy measured by a calorimeter was reduced to similar to 66% of that without the Al foil sample. Thus, the reduction of the incoming plasma energy by the vapor shielding effect was successfully demonstrated in the present experiment. (C) 2014 Elsevier B.V. All rights reserved.

Radial profile of externally applied resonant magnetic perturbation (RMP) field with mode numbers of m = 6 and n = 2 in a small tokamak device HYBTOK-II have been investigated using a magnetic probe array, which is able to measure the radial profile of magnetic field perturbation induced by applying RMP. Results of RMP penetration into the plasma show that the RMP decreased toward the plasma center, while they were amplified around the resonant surface with a safety factor q = 3 due to the formation of magnetic islands. This suggests that RMP fields for controlling edge plasmas may trigger some kind of MHD instabilities. In addition, simulation results, based on a linearized four-field model, which agrees with the experimental ones, indicates that the penetration and amplification process of RMP strongly depend on a Doppler-shifted frequency between the RMP and plasma rotation. (C) 2014 Elsevier B.V. All rights reserved.

Effects of humidity and pre-discharges on repetitive partial discharge inception voltage (RPDIV) with twisted enameled wires under bipolar and unipolar impulse voltages have been investigated. The RPDIV under the bipolar impulse voltages have become relatively scattered during the early stage of the voltage sequence in the dry condition. After sufficient number of the voltage sequence, it has reached to a stable state. On the other hand, the RPDIV have reached to a stable state from the beginning of measurement in the humid condition. In the case of the unipolar impulse voltages, no scattered state of the RPDIV has been observed. Surface potential on the enameled wire after the discharge has been measured with a surface potential meter in order to clarify effects of pre-discharges on the RPDIV. As the results, the fluctuation in the RPDIV is associated with the accumulation of the surface charges generated by the discharges.

Characterization of a pulsed plasma formed with a plasma gun device was performed. The averaged electron density was measured with an interferometer system using a Zeeman laser at 633 nm. It was found that the plasma with the density of similar to 10(22) m(-3) was successfully formed. The temporal evolution of the plasma was observed with a fast framing camera. It was found that the shape of the plasma was significantly varied temporally. From the intensity profile, the vertical emission profile was obtained using Abel inversion. The emission profile had a flat top shape with the radius of similar to 20 mm, and gradually decreased with the radius. The penetration behavior of the gun plasma across the magnetic field was observed.

Surface damage of pure tungsten (W), W alloys with 2 wt.% tantalum (W-Ta) and vacuum plasma spray (VPS) Wcoating on a reduced activation material of ferritic steel (F82H) due to repetitive ELM-like pulsed (similar to 0.3 ms) deuterium plasma irradiation has been investigated by using a magnetized coaxial plasma gun. Surface cracks appeared on a pure W sample exposed to 10 plasma pulses of similar to 0.3 MJ m (2), while a W-Ta sample did not show surface cracks with similar pulsed plasma irradiation. The energy density threshold for surface cracking was significantly increased by the existence of the alloying element of tantalum. No surface morphology change of a VPS W coated F82H sample was observed under 10 plasma pulses of similar to 0.3 MJ m (2), although surface melting and cracks in the resolidification layer occurred at higher energy density of similar to 0.9 MJ m (2). There was no indication of exfoliation of the W coating from the substrate of F82H after the pulsed plasma exposures. (C) 2013 Elsevier B. V. All rights reserved.

Damage of fuzzy tungsten surfaces due to a transient plasma load is characterized in terms of mass loss, surface morphology, and optical properties. A single D pulsed (∼0.1-0.2 ms) plasma shot with surface absorbed energy density of ∼1.1 MJ m-2 leads to a mass loss of ∼80 μg, which cannot be explained by physical sputtering. Thus, macroscopic erosion processes such as droplets and dust release as well as arcing are thought to be responsible for the mass loss. In fact, scanning electron microscopy observations reveal the melting of the tips of fuzz and arc tracks. The optical reflectivity of the damaged (melted) surface is measured to be higher than that of an undamaged fuzzy surface (below ∼0.01%). Spectroscopic ellipsometry shows that the refractive index, n, and extinction coefficient, k, increase from n ≈ 1 and k ≈ 0 for an undamaged fuzzy surface with an increase in the degree of damage of fuzz. © 2012 Elsevier B.V. All rights reserved.

Synergistic effects of steady state plasma irradiation and transient heat/particle loads to plasma facing materials are an important plasma surface interaction issue in future fusion devices. To investigate the effects systematically, a new plasma gun device was developed and was combined to the linear plasma device NAGDIS-I. Fundamental characteristics of the plasma gun device are revealed in this paper. The amount of tungsten released from the electrode was measured with a material probe method. Also, the emissions from the impurities were investigated spectroscopically. To measure the plasma density, an interferometer measurement was developed the ion temperature was measured spectroscopically. Moreover, a preliminary irradiation test of the plasma pulse to a tungsten sample was done, and the surface morphology change was shown with the analysis by a scanning electron microscope. © 2013 Elsevier B.V. All rights reserved.

This paper discusses the lifetime evaluation of twisted pairs which was constructed by a surge proof magnet wire. Two kinds of twisted pairs were constructed; one is a conventional twisted pairs and the other is a special twisted pairs which has varnish treatment at both edge sides of it. The both lifetime characteristics were evaluated and the breakdown positions were also investigated. As a result, it was found that the lifetime of the edge parts of usual twisted pairs was about 50% shorter than that of the other positions. It was estimated that the partial discharge activity at the edge part is stronger than that at the other positions, by an ultraviolet light measurement or others. This paper also describes a concept and quality control method for the inverter fed random wound motor which was manufactured by using a surge proof magnet wire.

Deuterium (D) retention in tungsten (W) subjected to pulsed D plasma bombardment (surface absorbed energy density Q similar to 0.5-0.7 MJ m(-2) at similar to 0.3-0.5 ms) has been investigated in a magnetized coaxial plasma gun. A high temperature desorption peak of D(2) appears at similar to 1000-1100 K following transients at similar to 0.5 MJ m(-2). At a higher Q similar to 0.7 MJ m(-2), the total amount of D retained in W is significantly reduced. Nano-sized helium (He) bubbles, created by steady-state He plasma pre-exposure at similar to 573 K, slightly lower D retention, while a significant reduction is observed for a W fuzzy surface formed by steady-state high temperature (similar to 1100 K) He plasma pre-exposure.

Sequential exposures of W surfaces to steady-state and pulsed (similar to 0.5 ms) plasmas have been performed in a linear divertor plasma simulator and a magnetized coaxial plasma gun to investigate effects of D blisters, nano-sized He bubbles, and He-induced W fuzz on surface cracking by pulsed plasma loads. Surface cracks appeared on samples containing D blisters or He bubbles following 10 shots at similar to 0.5 MJ/m(2) per shot, while a mirror-polished sample with no pre-plasma exposure did not exhibit cracks after similar transient exposures. Note that the cracking is limited to the edge region for a sample with D blisters. This means that the energy density threshold for surface cracking is lowered by the existence of D blisters and, especially, He bubbles. On the other hand, it is found that fuzzy surfaces possess a good resistance to surface cracking, although arcing is prone to occur.

Surface damage of carbon fiber composite (CFC) and tungsten (W) due to repetitive ELM-like pulsed plasma irradiation has been investigated by using a magnetized coaxial plasma gun. CX2002U CFC and stress-relieved W samples were exposed to repetitive pulsed deuterium plasmas with duration of similar to 0.5 ms, incident ion energy of similar to 30 eV, and surface absorbed energy density of similar to 0.3-0.7 MJ/m(2). Bright spots on a CFC surface during pulsed plasma exposures were clearly observed with a high-speed camera, indicating a local surface heating. No melting of a W surface was observed under a single plasma pulse exposure at energy density of similar to 0.7 MJ/m2, although cracks were formed. Cracking of the W surface grew with repetitive pulsed plasma exposures. Subsequently, the surface melted due to localized heat absorption. (C) 2010 Elsevier B.V. All rights reserved.

The inactivation of Bacillus atrophaeus spores by a dielectric barrier discharge (DBD) plasma in atmospheric humid air was investigated in order to develop a low-temperature, low-cost, and high-speed plasma sterilization technique. The biological indicators covered with a Tyvek sheet were set just outside the DBD plasma region, where air temperature and humidity as a discharge gas were precisely controlled by an environmental test chamber. The results show that the inactivation of B. atrophaeus spores was found to be dependent strongly on humidity, and was completed within 15 min at a relative humidity of 90% and a temperature of 30 degrees C. The treatment time for sterilization is shorter than those of conventional sterilization methods using ethylene oxide gas and dry heat treatment. The inactivation rates depend on not only relative humidity but also temperature, so that water content in air could determine the generation of reactive species such as hydroxyl radicals that are effective for the inactivation of B. atrophaeus spores. (C) 2011 The Japan Society of Applied Physics

We investigated the characteristics of the partial discharge (PD) with twisted enameled wires by optical emission spectroscopy (OES) in atmospheric humid air. The electric field strength in the gap of the twisted enameled wires during the discharges was estimated from intensity ratio of the spectrum bands of nitrogen molecules obtained by the OES. It was found that the electric field strength was increased when increasing the relative humidity, and the increasing rate was different depending on the temperature. The increase of the electric field strength in the gap of the twisted pair could be related to a reduction of PD inception voltage (PDIV) observed in the experiment. The reduction of the PDIV could be determined by not only dielectric properties of enameled wires such as permittivity and surface conductivity but also changes of the discharge phenomena in the humid air. In this study, effects of the increase of the permittivity of the enameled wire due to the moisture absorption into the insulating layer were quantitatively discussed as one of the candidates to explain the reduction of the PDIV in the humid conditions. On the other hand, the discharge current amplitude was decreased with the increase of relative humidity at high temperature. It was considered that the discharge could be quenched by dissociative electron attachment to the large number of water molecules. The observed PD phenomena with respect to the change of the humidity are very important for quality management of low voltage inverter-fed random wound motors.