1 MV holding and beam optics in a multi-aperture multi-grid accelerator for ITER NBI

Kashiwagi, Mieko; Taniguchi, Masaki; Kojima, Atsushi; Dairaku, Masayuki; Hanada, Masaya; Hemsworth, R. S.*; Mizuno, Takatoshi*; Takemoto, Jumpei; Tanaka, Masanobu*; Tanaka, Yutaka*; et al.

Proceedings of 23rd IAEA Fusion Energy Conference (FEC 2010) (CD-ROM), 8 Pages, 2011/03

http://www-naweb.iaea.org/napc/physics/FEC/FEC2010/html/node295.htm#62687

At JAEA, a multi-aperture multi-grid accelerator has been developed for the ITER neutral beam system. A target is H ion beam acceleration of 0.5 A (200 A/m) at 1 MeV. In real accelerators, it was found that the voltage holding was about a half of that obtained in an ideal small electrode. After applying necessary gap length and radii of edges of grid supports to lower local electric field concentrations, the accelerator succeeded in sustaining 1 MV for 4000 s. As a result, beam parameters were increased to 879 keV, 0.36 A (157 A/m) at perveance matched condition from 796 kV, 0.32 A (140 A/m) reported in FEC2008. In the beam acceleration, the beamlet deflections due to magnetic field and space charge repulsion caused direct interceptions, that resulted in limitations in the beam energy and current. Compensation of these beamlet deflections has been tested applying aperture offset and field shaping plate, which were examined in a three-dimensional beam analysis.

Heat loading of MeV accelerator grids during long pulse beam operation

Umeda, Naotaka; Mizuno, Takatoshi; Taniguchi, Masaki; Kashiwagi, Mieko; Ezato, Koichiro; Tobari, Hiroyuki; Dairaku, Masayuki; Watanabe, Kazuhiro; Sakamoto, Keishi; Inoue, Takashi

Journal of Plasma and Fusion Research SERIES, Vol.9, p.259 - 263, 2010/08

Long pulse acceleration of ITER class H ion beam has carried out at MeV accelerator. Melts of the acceleration grids were found around grid apertures. To accelerate higher power beam, compensation of the beam deflection and design of a new grid which has high cooling performance is required. In this study, 3D thermal transport analysis was carried out and a new acceleration grid was designed. From the analysis, it was found that the grid temperature exceeded the melting point in a few seconds. To overcome this problem, a new acceleration grid was designed whose cooling channel was drilled near upper surface. This countermeasure is effective not only to reduce the temperature rise but to enlarge the aperture size from 14 mm to 16 mm. From the result of heat analysis, temperature rise of the new grid is greatly reduced than that of the previous grid. It is expected that higher power and longer pulse beam would be accelerated at next test campaign.

Long pulse H ion beam acceleration in MeV accelerator

Taniguchi, Masaki; Mizuno, Takatoshi; Umeda, Naotaka; Kashiwagi, Mieko; Watanabe, Kazuhiro; Tobari, Hiroyuki; Kojima, Atsushi; Tanaka, Yutaka; Dairaku, Masayuki; Hanada, Masaya; et al.

Review of Scientific Instruments, 81(2), p.02B101_1 - 02B101_3, 2010/02

https://doi.org/10.1063/1.3274808

A Multi-Aperture Multi-Grid (MAMuG) accelerator called "MeV accelerator" has been developed for neutral beam injection (NBI) system of ITER. The MeV accelerator succeeded in accelerating 796 keV, 320 mA H ion beam until 2007. However, pulse length was limited to 0.2 s due to un-cooled grids. In the present work, long pulse H ion beam acceleration was performed by the MeV accelerator equipped with water-cooled new grids. The H ion current was increased step by step at certain energy with seeding Cs up to the optimum perveance. At present, pulse length was extended to 5 s for 750 keV, 221 mA (perveance match) and maximum power of 1.01 MJ was achieved (650 keV, 163 mA, 10s). At higher energy and current, pulse length was limited by breakdowns between the grids. This was due to high heat load on A3G and GRG grid by deflection of H ion beam.

3D modeling of the electron energy distribution function in negative hydrogen ion sources

Terasaki, Ryo*; Fujino, Ikuro*; Hatayama, Akiyoshi*; Mizuno, Takatoshi; Inoue, Takashi

Review of Scientific Instruments, 81(2), p.02A703_1 - 02A703_3, 2010/02

https://doi.org/10.1063/1.3273075

In order to develop the large H ion source for future fusion reactors, the uniform production of H ions is one of the important issues. Recently, it has been shown experimentally in JAEA 10A negative ion source that the non-uniformity of the electron energy distribution function (EEDF) inside the source and the resultant non-uniformity of the H production strongly affect the H beam optics. Therefore, modeling of the EEDF and analysis of the spatial non-uniformity of the EEDF is necessary to optimize H ion source and the beam optics. For this purpose, we are developing the 3D3V Monte Carlo modeling of the EEDF in realistic 3D geometry. The code reproduces the spatial non-uniformity of the EEDF observed in the experiments. Our developing code is a powerful tool for the design of the next generation sources.

Behavior of secondary-particles in a MeV-class electrostatic accelerator

Mizuno, Takatoshi; Inoue, Takashi; Taniguchi, Masaki; Kashiwagi, Mieko; Umeda, Naotaka; Tobari, Hiroyuki; Dairaku, Masayuki; Watanabe, Kazuhiro

JAEA-Research 2008-120, 19 Pages, 2009/03

JAEA-Research-2008-120.pdf:5.24MB

In an accelerator for a N-NBI, there are several processes of secondary-particle production such as the collision of H ions with H gas, extraction of H ions from beam plasma, and secondary-electron emission. The secondary particles cause heat load to the NBI components. It is necessary to analyze behavior of them in the accelerator. In this report, the secondary-particle behavior in MAMuG type MeV accelerator at JAEA has been analyzed by EAMCC. In the result, it is clarified that about 40% of H ions extracted from the ion source were lost by the stripping process in the MeV accelerator. More than 90% of the heat load to the intermediate grids was caused by collision of the electrons. A comparison of results obtained from experiments and present analyses showed different tendency in the currents flowing into the 2nd and the 3rd intermediate grids. This is supposed due to H ions extracted from beam plasma as a possible cause of the difference.

The Origin of beam non-uniformity in a large Cs-seeded negative ion source

Hanada, Masaya; Seki, Takayoshi*; Takado, Naoyuki; Inoue, Takashi; Mizuno, Takatoshi*; Hatayama, Akiyoshi*; Kashiwagi, Mieko; Sakamoto, Keishi; Taniguchi, Masaki; Watanabe, Kazuhiro

Nuclear Fusion, 46(6), p.S318 - S323, 2006/06

https://doi.org/10.1088/0029-5515/46/6/S13

The origin of the beam non-uniformity, that is one of the key issues in large Cs-seeded negative ion sources for JT-60U and ITER, was experimentally examined by measuring correlations between the intensity of the H ion beam and plasma parameters such as an electron temperature and plasma density in the JAERI 10 A negative ion source. From the correlation between the beam intensity and the plasma parameters, it was foreseen that the beam non-uniformity was due to the localization of the plasma and/or H0 atoms caused by B x B drift of the fast electron from filaments. The filament position was modified to suppress the B x B drift, and then the spatial uniformity of the beam intensity was examined. By this modification, the root-mean-square deviation of the spatial beam intensity from the averaged value deceased to a half of that before modification while the beam intensity integrated along the longitudinal direction was kept to be constant. From this result, it was confirmed that one of the origin of the beam non-uniformity was caused by plasma localization.

Numerical analysis of the spatial nonuniformity in a Cs-seeded H ion source

Takado, Naoyuki*; Hanatani, Junji*; Mizuno, Takatoshi*; Kato, Kyohei*; Hatayama, Akiyoshi*; Hanada, Masaya; Seki, Takayoshi; Inoue, Takashi

Review of Scientific Instruments, 77(3), p.03A533_1 - 03A533_3, 2006/03

https://doi.org/10.1063/1.2172356

Surface production and transport process of H ions are numerically simulated to clarify the origin of H beam non-uniformity. A three-dimensional transport code using Monte Carlo method has been applied to productions of H atoms and H ions in a large negative ion source under the Cs seeded condition. The results show that a large fraction of hydrogen atoms are produced in a high electron temperature region. This leads to a spatial non-uniformity of H atom flux to the plasma grid where H atoms capture electrons and converted to H ions. In addition, most surface-produced H ions are extracted even through the high electron temperature region without destruction.

Numerical analysis of plasma spatial uniformity in negative ion sources by a fluid model

Mizuno, Takatoshi*; Kitade, Yuki*; Hatayama, Akiyoshi*; Sakurabayashi, Toru*; Imai, Naoki*; Morishita, Takatoshi; Inoue, Takashi

Review of Scientific Instruments, 75(5), p.1760 - 1763, 2004/05

https://doi.org/10.1063/1.1695622

Spatial non-uniformities of extracted negative ion beam were observed experimentally in tandem-type negative ion sources. To improve the beam uniformity, it is important to analyze the plasma profile in the ion source including magnetic filter effect. In the filter region, Lorentz force is important for both ions and electrons. However, their dynamics are completely different, i.e. electrons are magnetized and ions are not magnetized. Then, the system of two-dimensional two-fluid model equations is solved simultaneously to obtain self-consistent profiles of the plasma parameters. The result shows that a possible cause of spatial non-uniformity is the ion flow rather than ExB drift motion of electrons. This flow of ions is caused by synergetic effect of the force by electric field, Lorentz force and inertia force. To verify the results above and more quantitative comparisons with experiments, full 3D analysis is needed, because the electron loss along the field line is important for the plasma potential and the electric field in the filter region. Full 3D analysis is now in progress.

Improvement of beam uniformity in a Cs-seeded large negative ion source

Seki, Takayoshi; Hanada, Masaya; Tobari, Hiroyuki; Inoue, Takashi; Kashiwagi, Mieko; Taniguchi, Masaki; Watanabe, Kazuhiro; Sakamoto, Keishi; Takado, Naoyuki*; Mizuno, Takatoshi*; et al.

no journal, , 

no abstracts in English

Numerical analysis of secondary-particle behavior in MeV-class accelerator for ITER NBI

Mizuno, Takatoshi; Inoue, Takashi; Taniguchi, Masaki; Kashiwagi, Mieko; Umeda, Naotaka; Tobari, Hiroyuki; Watanabe, Kazuhiro; Dairaku, Masayuki; Sakamoto, Keishi

no journal, , 

no abstracts in English