M Shibuya

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

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Publications (39)17.34 Total impact

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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.
    02/2013;
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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.
    02/2013;
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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.
    02/2013;
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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.
    The Review of scientific instruments 02/2012; 83(2):02B113. · 1.52 Impact Factor
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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.
    The Review of scientific instruments 02/2012; 83(2):02B116. · 1.52 Impact Factor
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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.
    09/2011;
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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.
    09/2011;
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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.
    09/2011;
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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.
    The Review of scientific instruments 02/2010; 81(2):02B117. · 1.52 Impact Factor
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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.
    The Review of scientific instruments 02/2010; 81(2):02B116. · 1.52 Impact Factor
  • 01/2010;
  • 01/2010;
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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.
    03/2009;
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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).
    Review of Scientific Instruments 03/2008; 79(2 Pt 2):02C105. · 1.60 Impact Factor
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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.
    Review of Scientific Instruments 03/2008; 79(2 Pt 2):02C107. · 1.60 Impact Factor
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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.
    08/2007;
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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.
    01/2007;
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    ABSTRACT: A negative-ion-based neutral beam injection (NBI) system has been operated reliably in the Large Helical Device (LHD) since it was operational in 1998. The injection power achieved is 13.1 MW with three injectors. In one injector with modified ion sources with the multi-slotted grounded grid, the injection power reached 5.7 MW with an energy of 184 keV, both of which exceed the designed values of 180 keV -5 MW. The individual control of the arc-discharge with the divided arc and filament power supplies is effective to improve the beam uniformity. The injection duration is extended to 120 sec with a reduced power of 0.2 -0.3 MW using one ion source with the cooled plasma grid. The performance of the negative-NBI in LHD is reviewed with regard to the progress of the negative ion sources.
    Nuclear Fusion 05/2006; 46(6). · 2.73 Impact Factor
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    ABSTRACT: The results on high power injection with the neutral beam injection (NBI) system for the Large Helical Device (LHD) are reported. The system consists of three beam-lines with two hydrogen negative ion (H− ion) sources installed in each beam-line. In order to improve the injection power, a new beam accelerator with a multi-slot grounded grid (MSGG) has been developed and applied to one beam-line. Using the accelerator, a maximum power of 5.7 MW was achieved in 2003 and 2004, and the maximum energy of 189 keV was reached. The power and energy exceeded the design values of the individual beam-line for LHD. The other beam-lines also increased their injection power up to about 4 MW, and the total injection power of 13.1 MW was achieved with three beam-lines in 2003. Although the accelerator had an advantage in high power beam injection, extracted beams expand in the short side direction of the ground-grid slot. The disadvantage has been resolved by modifying the aperture shapes of the steering grid.
    Plasma Science and Technology 04/2006; 8(1):24. · 0.51 Impact Factor
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    ABSTRACT: The velocity spectra of the negative-ion-(H−) based neutral beams are studied in high-performance large-area ion sources during injection into large helical device fusion plasmas. We are conducting systematic observations in standard neutral beam injection to correlate beam spectra with source operating conditions. Almost all of the transmitted beam power was at full acceleration energy ( ∼ 170 keV). The small stripping beam component which was produced in the extraction gap was evaluated to be about 9%–22% by amplitude of the measured spectra for the sources in beam lines 1 and 2. H− production uniformity from the spectrum profile was 86%–90% for three sources. For the longest pulse injection during 74 and 128 s, a full energy component tended to decrease with time, while the accelerator gap stripping tail tended to increase slightly with time, which is attributed to beam-induced outgassing in the accelerator. A higher conductance multislot ground grid accelerator appeared to show little growth in the accelerator gap beam stripping during long pulses compared to the conventional multiaperture ground grid. The beam uniformity appeared to vary in part with the Cs uniformity on the plasma grid.
    Review of Scientific Instruments 03/2006; 77(3):03A538-03A538-4. · 1.60 Impact Factor