M Shibuya

National Institute for Fusion Science, Tokitsu-chō, Gifu, Japan

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Publications (42)20.75 Total impact

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    ABSTRACT: Experiments by a four-pin probe and photodetachment technique were carried out to investigate the charged particle flows in the beam extraction region of a negative hydrogen ion source for neutral beam injector. Electron and positive ion flows were obtained from the polar distribution of the probe saturation current. Negative hydrogen ion flow velocity and temperature were obtained by comparing the recovery times of the photodetachment signals at opposite probe tips. Electron and positive ions flows are dominated by crossed field drift and ambipolar diffusion. Negative hydrogen ion temperature is evaluated to be 0.12 eV.
    No preview · Article · Feb 2016 · Review of Scientific Instruments
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    ABSTRACT: Intense hydrogen-negative-ion source development, conducted at National Institute for Fusion Science (NIFS), is reviewed. Presently, the developed negative-ion sources are utilized in the negative-ion-based neutral beam injectors, which are installed to the Large Helical Device, the world’s largest superconducting fusion machine, and the total injection power has achieved 16MW with the energy of 180-190 keV using three injectors with six sources. In the developed negative-ion accelerator with multi-slotted grounded grid, the grid heat load is much reduced due to its high transparency, leading to a high-energy acceleration of a high-current negative ion beam. As a result, one ion source produces 190keV-37A of negative ions for 1.6sec at maximum, corresponding to 340A/m2 of the current density. For further improvement of the negative ion source, plasma characteristics are investigated in the extraction region with a multi-diagnostics system. With the Cs seeding, the H− density increases and the electron density decreases, and, finally, an ion-ion plasma which consists of almost positive and negative ions is observed. The measured negative ion density is not largely decreased toward the plasma grid surface, on which the negative ion is produced. Reduction of the negative ion density is observed by the negative ion extraction, and invasion of the electric field for the negative ion extraction is recognized. Understanding of the negative ion transport in the plasma and the mechanism of the negative ion extraction should contribute to improvement of the source performance.
    No preview · Article · Feb 2013
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    ABSTRACT: The first results of saturation-currents polar distribution, which is measured with a directional Langmuir probe, at the beam extraction region of our caesium (Cs) seeded negative ion source have been reported. The line from maximum to minimum of the distribution tilts by 40° to the normal direction of the plasma grid (PG) surface. The maximum intensity is one order of magnitude larger than the minimum one. Depth distribution of the saturation currents is also measured along the axis of the PG aperture and the middle line between a pair of the axes of nearest neighbor PG apertures. Ionic plasma with quite low electron density is generated within a distance of 10 mm from PG. With a bias voltage lower than plasma potential, the difference between negative to positive saturation currents, which corresponds to electron current, increases rapidly beyond the boundary of ionic plasma. The electron current decreases by a factor of 6 with increasing the bias voltage from 2.8 to 6.4 V.
    No preview · Article · Feb 2013
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    ABSTRACT: Characteristics of negative-hydrogen ion (H−) density in the vicinity of plasma grid (PG) which is a boundary electrode between plasma and beam were experimentally investigated in cesium-seeded H− source. The H− density was measured with Cavity Ring Down method (CRD). Our CRD system has been upgraded from fixed line measurement to movable one which provides a profile measurement of the H− density. The H− density above the PG aperture is lower than that above the PG metal surface, and this density structure become to disappear in further region from the PG surface. The H− density decreases with positive bias voltage where an arc discharge chamber is higher potential than the PG. On the other hand, the H− density does not largely change with negative bias voltage. Reduction of the H− density was observed when a beam extraction voltage is applied. The reduction occurs in the case of lower bias voltage close to plasma potential. The extraction voltage influences H− density to a greater degree than bias voltage in low bias voltage region.
    No preview · Article · Feb 2013
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    ABSTRACT: Electron density measurements of a large-scaled negative ion source were carried out with a surface wave probe. By comparison of the electron densities determined with the surface wave probe and a Langmuir probe, it was confirmed that the surface wave probe is highly available for diagnostic of the electron density in H(-) ion sources. In addition, it was found that the ratio of the electron density to the H(-) ion density dramatically decreases with increase of a bias voltage and the H(-) ions become dominant negative particles at the bias voltage of more than 6 V.
    No preview · Article · Feb 2012 · The Review of scientific instruments
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    ABSTRACT: We report on the characteristics of the electronegative plasma in a large-scale hydrogen negative ion (H(-)) source. The measurement has been made with a time-resolved Langmuir probe installed in the beam extraction region. The H(-) density is monitored with a cavity ring-down system to identify the electrons in the negative charges. The electron-saturation current decreases rapidly after starting to seed Cs, and ion-ion plasma is observed in the extraction region. The H(-) density steps down during the beam extraction and the electron density jumps up correspondingly. The time integral of the decreasing H(-) charge density agrees well with the electron charge collected with the probe. The agreement of the charges is interpreted to indicate that the H(-) density decreasing at the beam extraction is compensated by the electrons diffusing from the driver region. In the plasmas with very low electron density, the pre-sheath of the extraction field penetrates deeply inside the plasmas. That is because the shielding length in those plasmas is longer than that in the usual electron-ion plasmas, and furthermore the electrons are suppressed to diffuse to the extraction region due to the strong magnetic field.
    No preview · Article · Feb 2012 · The Review of scientific instruments
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    ABSTRACT: A Cavity Ring-Down (CRD) system was applied to measure the density of negative hydrogen ion (H-) in vicinity of extraction surface in the H- source for the development of neutral beam injector on Large Helical Device (LHD). The density measurement with sampling time of 50 ms was carried out. The measured density with the CRD system is relatively good agreement with the density evaluated from extracted beam-current with applying a similar relation of positive ion sources. In cesium seeded into ion-source plasma, the linearity between an arc power of the discharge and the measured density with the CRD system was observed. Additionally, the measured density was proportional to the extracted beam current. These characteristics indicate the CRD system worked well for H- density measurement in the region of H- and extraction.
    No preview · Article · Sep 2011
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    ABSTRACT: A millimeter-wave interferometer with the frequency of 39 GHz (lambda = 7.7 mm) was newly installed to a large-scaled negative ion source. The measurable line-integrated electron density (ne•l) is from 2×1016 to 7×1018 m-2, where ne and l represent an electron density and the plasma length along the millimeter-wave path, respectively. Our interest in this study is behavior of negative ions and reduction of electron density in the beam extraction region near the plasma grid. The first results show the possibility of the electron density measurement by the millimeter-wave interferometer in this region. The line-averaged electron density increases proportional to the arc power under the condition without cesium seeding. The significant decrease of the electron density and significant increase of the negative ion density were observed just after the cesium seeding. The electron density measured with the interferometer agrees well with that observed with a Langmuir probe. The very high negative ion ratio of nH-/(ne+nH-) = 0.85 was achieved within 400 min. after the cesium seeding.
    No preview · Article · Sep 2011
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    ABSTRACT: We describe the characteristic of stable beam injection in a neutral beam injector (NBI) for Large Helical Device (LHD) in high injection power of more than 6 MW. In the NBI, it takes a week after starting Cs seeding to finish the pre-injection conditioning. The injection starts with the beam power ~6.2 MW, and the maximum power reaches ~7 MW. The Cs-seeding rate affects the beam stability in such high power injections. By optimizing the rate to 0.65 mg/shot, the success ratio, which is defined as a ratio of actual pulse duration to setting one, increases to 85-90% in the power and energy range of more than 6.2 MW and 185 keV, respectively. The weights of Cs adsorbed on several surfaces in the ion sources of the NBI are measured by means of Inductively-Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), and averaged surface densities are calculated by dividing with the several surface areas. The seeded Cs of 99.5% is condensed in the plasma generator, and very tiny amount of Cs reaches the surfaces of the accelerator grids. This very low amount of Cs on the grids is interpreted that most of the Cs atom evaporated from the inner walls is ionized during the arc discharges, and repelled to the source plasmas by the electrostatic field for H- extraction.
    No preview · Article · Sep 2011
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    ABSTRACT: High-power negative and positive ion-based neutral beam injectors (NBIs) are operated with high reliability in the Large Helical Device (LHD). The total injection power is >20 MW, and such high-power beams are available every 3 min. The high performance of the LHD NBI system has extended the LHD parameter regime to levels equivalent to those obtained in large Tokamaks. Three negative NBIs inject a total power of 16 MW with an energy of 180 keV, which is the world's highest power from a negative NBI system (H-), and one positive NBI (H+) injects 7 MW at 40 ke V. The injection duration can be extended beyond 1 min with reduced power from the negative and positive NBIs, and long-pulse plasmas are successfully sustained with the NBIs. The structure and performance of the LHD NBI system is reviewed.
    No preview · Article · Jul 2010 · Fusion Science and Technology
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    ABSTRACT: The research and development (R&D) activity on the negative ion sources at the National Institute for Fusion Science is described. During the R&D period from 1989 to 1998, intensive experimental investigations were carried out at a test stand with three sizes of ion source, 1/6, 1/3, and full size. Although comprehensive research had been carried out, there remained some problems on the Large Helical Device (LHD) beamlines. To resolve those problems, the ion sources and beamlines have been improved in several successive steps since the beam injection experiment of LHD started. Over the past decade, the injection energy, power, and reliability have been increased; the maximum energy now exceeds the 180-keV design value and the maximum injected powers reach 6.8 and 5.5 MW from beamlines 1 and 2, respectively.
    No preview · Article · Jul 2010 · Fusion Science and Technology
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    ABSTRACT: An additional beamline, BL5, equipped with four positive ion sources will be installed on Large Helical Device (LHD) in 2010. The performance of an ion source which generates 80 keV deuterium and 60 keV hydrogen beams was investigated. The structure of the ion source is based on that of a BL4 ion source on LHD. The main differences between the ion sources for the BL4 and BL5 are the acceleration voltages and the materials of plasma electrodes: copper and molybdenum, respectively. The molybdenum plasma electrode for BL5 has better performance than the copper plasma electrode of BL4. The integrated performance of the ion source for BL5 reached a value equivalent to approximately 58 A in the beam current of hydrogen positive ion at 60 keV in the beam energy.
    Preview · Article · Feb 2010 · The Review of scientific instruments
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    ABSTRACT: Characteristics of multibeamlets are investigated by means of beamlet monitoring technique. The beamlets are extracted from an accelerator with multislot grounded grid and the profiles are observed as infrared images of temperature distributions on a cold isostatic pressed graphite plate exposed by H-beamlets. The optimal horizontal and vertical divergence angles of single beamlet are estimated at 4.1 and 6.1 mrad, respectively.
    Full-text · Article · Feb 2010 · The Review of scientific instruments

  • No preview · Article · Jan 2010

  • No preview · Article · Jan 2010
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    ABSTRACT: A multi-antenna radio-frequency ion source with a Faraday shield is newly tested at a high RF power level in a large area negative ion source of 1/5th scale of the Large Helical Device-NNBI ion source. Inductively coupled dense hydrogen plasmas were generated uniformly over an area of 25×25 cm2 at an RF input power up to 300 kW for a 10 ms pulse duration. A large negative plasma potential for the non-Faraday shielded antenna was remarkably reduced by introducing a Faraday shield. The positive ion saturation current density measured by Langmuir probe reached 148 mA/cm2 at 174 kW around the center of the plasma. The optimal hydrogen filling pressure ranged around 0.13 Pa- 0.4 Pa for the positive ions. Ion beam extraction with a single hole (phi 0.5 cm) extractor has been studied systematically. A maximum H- ion beam current density of 1.6 mA/cm2 was obtained preliminarily. It was confirmed that the plasma profile was controllable by both the number and configuration of the antennas.
    No preview · Article · Mar 2009
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    ABSTRACT: In the large area negative ion source for the LHD negative-ion-(H(-))-based neutral beam system, (I) we used the spectrometer to measure caesium lines in the source plasma during beam shots. (II) With Doppler-shifted measurements, the H(alpha) line at three different locations along the beam as well as the spectrum profile for cases of different plasma grid areas. (III) Caesium deposition monitor with a high speed shutter was tested to measure the weight of the deposited Cs layer. In the observation, cleaner spectra of Doppler-shifted H(alpha) line with only a small level of background light were obtained at a new observation port which viewed the blueshifted light in the drift region after the accelerator of a LHD ion source. Both the amounts of Cs I (852 nm, neutral Cs(0)) and Cs II (522 nm, Cs(+)) in the source plasma light rose sharply when beam acceleration began, and continued rising during a 10 s pulse. It was thought that this was because the cesium was evaporated/sputtered from the source back plate by the back-streaming positive ions. Cs deposition rate to the crystal sensor measured by adjusting the shutter open time was evaluated to be 2.9 nanograms/s cm(2) for preliminary testing. More neutral Cs tended to be evolved in the source after arc discharge. Much Cs could be consumed in a high rate-pulsed operation (such as LHD source).
    No preview · Article · Mar 2008 · Review of Scientific Instruments
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    ABSTRACT: The beam profiles, port-through, rates and injection powers obtained with an improved accelerator with the multislot grounded grid are described. The accelerator has a combination of a steering grid with racetrack shaped aperture and multislot grounded grid to improve the beam optics. The optimal beam optics is obtained at the voltage ratio of 16.5-16.8, and the profiles are well fit by superposing multibeamlets with the divergent angles of 5.0 and 7.2 mrad along the direction parallel to the long and short axes of the slots of grounded grid. By adopting the racetrack shaped steering grid, the port-through rate increases from 34% to 38%, and the maximum injection power reaches 6 MW/187 keV.
    No preview · Article · Mar 2008 · Review of Scientific Instruments
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    ABSTRACT: Large‐scaled hydrogen negative‐ion sources, in which cesium is introduced in the source plasma, have been developed for neutral beam injectors in Large Helical Device, and their operational characteristics are reviewed. For high‐efficient negative ion production, configuration of the magnetic filter field and the cusp magnetic field was optimized, resulting in a high arc efficiency for the negative ion production of 0.23A/kW. With use of a multi‐slotted grounded grid, the gas pressure in the acceleration gap is lowered, leading to reduction of the heat load of the grounded grid. As a result, the voltage holding ability is much improved, and the rated energy of 180 keV is achieved in a short conditioning period of 4 days. The injection power is increased linearly to the 5/2 power of the beam energy and reached 5.7MW with an energy of 184keV, which exceeds the specified value of 180keV–5MW. Beam uniformity has been improved with an individual control of the local arc discharge by adjusting 12‐divided output voltages of the arc and filament power supplies. The injection duration has been extended to 120sec with a reduced power. Spectroscopic measurement has been carried out for the source plasma. The cesium‐ion line is observed in the plasma volume, and, however, the negative ion production is not influenced by the cesium ions in the plasma because the negative ions should be produced on the cesium‐covered plasma grid surface.
    No preview · Article · Aug 2007
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    ABSTRACT: A radial Neutral Beam Injector (NBI) is newly installed on the Large Helical Device (LHD). The aims of the NBI are its usage as a diagnostic NB for charge exchange recombination spectroscopic measurement and using the NB as a heating source for ions in plasmas. A new positive-ion source was developed for this NBI at NIFS. The structure of the cusp field of the source was determined by the numerical code and its performances were verified by experiments. The performances of the developed source fulfill its specification. Especially, the maximum beam current of 102(A) exceeds the requirement of 75(A) about 33(%). The specification of the radial-NBI on LHD and its ion-sources are briefly discussed in Sec.2. The design of cusp-field configuration for the ion-source is shown in Sec.3. The determination of its plasma-electrode thickness is shown in Sec.4. The operation of the source at the NBI are shown in Sec.5. The section 6 is a summary.
    Full-text · Article · Jan 2007