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Journal Articles

Vacuum insulation of the high energy negative ion source for fusion application

Kojima, Atsushi; Hanada, Masaya; Hilmi, A.*; Inoue, Takashi; Watanabe, Kazuhiro; Taniguchi, Masaki; Kashiwagi, Mieko; Umeda, Naotaka; Tobari, Hiroyuki; Kobayashi, Shinichi*; et al.

Review of Scientific Instruments, 83(2), p.02B117_1 - 02B117_5, 2012/02

 Times Cited Count:14 Percentile:59.32(Instruments & Instrumentation)

Production of 500 keV, 3 A beams has been successfully achieved in the JT-60 negative by overcoming the low voltage holding of the accelerator. Toward the design of next ion source, database for the voltage holding capability based on experimental results is required and obtained. As a result, the voltage holding capability was found to vary with 67 N power of -0.15 and with 31.7 S power of -0.125 where N is the aperture number and S is the anode surface area. When N = 1100 and S = 2 m$$^{2}$$ are applied to the design of JT-60SA ion source, the factors C are estimated to be 23 and 29, respectively. Therefore, the influence of the local electric field around the apertures is stronger than that of the surface area.

Journal Articles

Voltage holding study of 1 MeV accelerator for ITER neutral beam injector

Taniguchi, Masaki; Kashiwagi, Mieko; Umeda, Naotaka; Dairaku, Masayuki; Takemoto, Jumpei; Tobari, Hiroyuki; Tsuchida, Kazuki; Yamanaka, Haruhiko; Watanabe, Kazuhiro; Kojima, Atsushi; et al.

Review of Scientific Instruments, 83(2), p.02B121_1 - 02B121_3, 2012/02

 Times Cited Count:11 Percentile:51.68(Instruments & Instrumentation)

JAEA has developed the MeV accelerator to demonstrate 1 MeV, 200 A/m$$^{2}$$ H$$^{-}$$ ion beam acceleration required for ITER NBI. A key to realize such a high power accelerator is improvement of voltage holding capability. Based on detailed investigation of the voltage holding characteristics, MeV accelerator was modified to reduce electric field concentration by extending gaps between the grid supports and increasing curvature radiuses at the support corners. After the modifications, accelerator succeeded in sustaining -1 MV in vacuum without beam acceleration. Moreover, beam deflection due to the magnetic field for electron suppression and space charge repulsion was compensated by aperture displacement technique. As the result, beam deflection was compensated and voltage holding during the beam acceleration was improved. Beam parameter of the MeV accelerator was increased to 980 keV, 185 A/m$$^{2}$$, which is close to the requirement of ITER accelerator.

Journal Articles

Effect of non-uniform electron energy distribution function on plasma production in large arc driven negative ion source

Shibata, Takanori; Koga, Shojiro*; Terasaki, Ryo*; Inoue, Takashi; Dairaku, Masayuki; Kashiwagi, Mieko; Taniguchi, Masaki; Tobari, Hiroyuki; Tsuchida, Kazuki; Umeda, Naotaka; et al.

Review of Scientific Instruments, 83(2), p.02A719_1 - 02A719_3, 2012/02

 Times Cited Count:2 Percentile:13.93(Instruments & Instrumentation)

In the NBI for large fusion devices, production of uniform negative ion beam is one of important issues. A physical model is proposed to understand the non-uniformity. It has been qualitatively shown that the non-uniform beam intensity is due to the following process; (1) formation of non-uniform EEDF, (2) localized production of hydrogen atoms/ions (H$$^0$$/H$$^+$$) due to (1), (3) non-uniform flux of H$$^0$$/H$$^+$$ to the PG and (4) localized surface production of negative ions. However, in the past studies, the EEDF was assumed as two temperature Maxwellian distribution from measurements. Thus effects of high energy electrons are not taken into account precisely. In the present research, local EEDF is calculated by the 3D Monte-Carlo kinetic model which takes into account the spatial and magnetic configurations of the real negative ion source. The numerical result show that high energy component of the EEDF enhances the spatial non-uniformity in the production rate of H$$^0$$/H$$^+$$.

Journal Articles

Beam optics in a MeV-class multi-aperture multi-grid accelerator for the ITER neutral beam injector

Kashiwagi, Mieko; Taniguchi, Masaki; Umeda, Naotaka; DeEsch, H. P. L.*; Grisham, L. R.*; Boilson, D.*; Hemsworth, R. S.*; Tanaka, Masanobu*; Tobari, Hiroyuki; Watanabe, Kazuhiro; et al.

Review of Scientific Instruments, 83(2), p.02B119_1 - 02B119_3, 2012/02

 Times Cited Count:9 Percentile:45.39(Instruments & Instrumentation)

In a multi-aperture multi-grid (MAMuG) accelerator of the ITER neutral beam injector (NBI), 1 MeV, 40 A D$$^{-}$$ ion beam is required for 3600 s. Suppression of grid power loading by the direct interception of deflected beamlets is one of the critical issues to realize this accelerator. The beamlets are deflected due to space charge repulsion among beamlets/beam groups and magnetic field. Moreover, the beamlet deflection is influenced by electric field distortion generated by grid supports. To examine such complicated beamlet deflections and design the compensating methods, a three-dimensional beam analysis has been applied to the ITER accelerator. As the simulation model, a 1/4 accelerator model including step/edge of the grid supports is constructed. As results, compensation methods of the beamlet deflection, that it, a metal bar of 1 mm thick around the aperture area, and an aperture offset of 1 mm, were designed.

Journal Articles

First neutral beam injection experiments on KSTAR tokamak

Jeong, S. H.*; Chang, D. H.*; Kim, T. S.*; In, S. R.*; Lee, K. W.*; Jin, J. T.*; Chang, D. S.*; Oh, B. H.*; Bae, Y. S.*; Kim, J. S.*; et al.

Review of Scientific Instruments, 83(2), p.02B102_1 - 02B102_3, 2012/02

 Times Cited Count:20 Percentile:68.57(Instruments & Instrumentation)

The first NB (neutral beam) injection system of the KSTAR tokamak was partially completed in 2010 with only 1/3 of its full design capability, and NB heating experiments were carried out during the 2010 KSTAR operation campaign. The ion source is composed of a JAEA bucket plasma generator and a KAERI large multi-aperture accelerator assembly. Before the beam injection experiments, characteristics of the ion source were investigated. A minimum beam divergence angle was 0.8 $$^{circ}$$. The ion species ratio was D$$^{+}$$:D$$_{2}$$$$^{+}$$:D$$_{3}$$$$^{+}$$=75:20:5. The arc efficiency is more than 1.0 A/kW. In the 2010 KSTAR campaign, the deuterium NB power of 0.7-1.5 MW was successfully injected into the KSTAR plasma with the beam energy of 70-90 keV. L-H transitions were observed within a wide range of beam powers relative to a threshold value. In every deuterium NB injection, a burst of D-D neutrons was recorded, and increases in the ion temperature and the plasma stored energy were found.

Journal Articles

Numerical analysis of surface produced H$$^{-}$$ ions by using two-dimensional particle-in-cell method

Miyamoto, Kenji*; Okuda, Shin*; Hatayama, Akiyoshi*; Hanada, Masaya

Review of Scientific Instruments, 83(2), p.02A723_1 - 02A723_4, 2012/02

 Times Cited Count:8 Percentile:41.82(Instruments & Instrumentation)

The modeling and analysis of a negative ion source is proceeding by using a 2D particle-in-cell simulation. The effect of the H$$^{-}$$ ion production on the plasma grid (PG) surface is investigated. It is shown that with the increase of H$$^{-}$$ ions per time step, the H$$^{-}$$ ion current density is enhanced, while the electron current density decreases with increasing the H$$^{-}$$ production rate on the PG surface. Theseresults agree well with the experimental results observed in typical negative ion sources. Moreover, it is found that plasma quasi-neutrality is held mainly by both H$$^{+}$$ and H$$^{-}$$ ions in the bulk plasma.

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