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Nishikiori, Ryo; Kojima, Atsushi; Hanada, Masaya; Kashiwagi, Mieko; Watanabe, Kazuhiro; Umeda, Naotaka; Tobari, Hiroyuki; Yoshida, Masafumi; Ichikawa, Masahiro; Hiratsuka, Junichi; et al.
Plasma and Fusion Research (Internet), 11, p.2401014_1 - 2401014_4, 2016/03
One of critical issues for high-energy high-current beam acceleration in ITER and JT-60SA is the high voltage holding which is dominated by vacuum discharges. The past results suggest that vacuum discharge occurs beyond the threshold of the dark current. The dark current can be derived from F-N theory where electric field enhancement factor beta is included. Though, beta could only be evaluated from the experiment previously. Therefore, the method to decide beta without experiment is required. This time dark currents were measured at three different areas to compare beta in different electric field. As a result, the effective electric field 
E, where E is average electric field, were found to be almost constant for different areas although the beta is largely different. By applying E, beta can be evaluated analytically, leading to the analytical prediction of the dark current and voltage holding capability without the measurements.
Kojima, Atsushi; Hanada, Masaya; Tobari, Hiroyuki; Nishikiori, Ryo; Hiratsuka, Junichi; Kashiwagi, Mieko; Umeda, Naotaka; Yoshida, Masafumi; Ichikawa, Masahiro; Watanabe, Kazuhiro; et al.
Review of Scientific Instruments, 87(2), p.02B304_1 - 02B304_5, 2016/02
Times Cited Count：11 Percentile：49.05(Instruments & Instrumentation)Optimization techniques of the vacuum insulation design have been developed in order to realize a reliable voltage holding capability of Multi-Aperture Multi-Grid accelerators for giant negative ion sources for nuclear fusion. In this method, the nested multilayer configuration of each acceleration stage in the MAMuG accelerator can be uniquely designed to satisfy the target voltage within given boundary conditions. The evaluation of the voltage holding capabilities of each acceleration stages were based on the past experimental results of the area effect and the multi-aperture effect on the voltage holding capability. Moreover, total voltage holding capability of multi-stage was estimated by taking the multi-stage effect into account, which was experimentally obtained in this time. In this experiment, the multi-stage effect appeared as the superposition of breakdown probabilities in each acceleration stage, which suggested that multi-stage effect can be considered as the voltage holding capability of the single acceleration gap having the total area and aperture. The analysis on the MAMuG accelerator for JT-60SA agreed with the past gap-scan experiments with an accuracy of less than 10% variation.
Hanada, Masaya; Kojima, Atsushi; Tobari, Hiroyuki; Nishikiori, Ryo; Hiratsuka, Junichi; Kashiwagi, Mieko; Umeda, Naotaka; Yoshida, Masafumi; Ichikawa, Masahiro; Watanabe, Kazuhiro; et al.
Review of Scientific Instruments, 87(2), p.02B322_1 - 02B322_4, 2016/02
Times Cited Count：11 Percentile：49.05(Instruments & Instrumentation)In International Thermo-nuclear Experimental Reactor (ITER) and JT-60 Super Advanced (JT-60 SA), the D
ion beams of 1 MeV, 40 A and 0.5 MeV, 22 A are required to produce 3600 s and 100 s for the neutral beam injection, respectively. In order to realize such as powerful D ion beams for long duration time, Japan Atomic Energy Agency (JAEA) has energetically developed cesium (Cs)-seeded negative ion sources (CsNIS) and electro-static multi-aperture and multi-stage accelerators (MAMuG accelerator) which are chosen as the reference design of ITER and JT-60 SA. In the development of the CsNIS, a 100s production of the H ion beam has been demonstrated with a beam current of 15 A by modifying the JT-60 negative ion source. At the higher current, the long pulse production of the negative ions has been tried by the mitigation of the arcing in the plasma inside the ion source. As for the long pulse acceleration of the negative ions in the MAMuG accelerator, the beam steering angle has been controlled to reduce the power loading of the acceleration grids A pulse duration time has been significantly extended from 0.4 s to 60 s at reasonable beam power for ITER requirement. The achieved pulse duration time is limited by the capacity of the power supplies in the test stand. In the range of 
60 s, there are no degradations of beam optics and voltage holding capability in the accelerator. It leads to the further extension of the pulse duration time at higher power density. This paper reports the latest results of development on the negative ion source and accelerator at JAEA.
Kojima, Atsushi; Hanada, Masaya; Inoue, Takashi; NB Heating Technology Group; Yamano, Yasushi*; Kobayashi, Shinichi*
Journal of the Vacuum Society of Japan, 56(12), p.502 - 506, 2013/12
Voltage holding capability of a large negative ion source for fusion application is experimentally examined, which is characterized by multiple-stage acceleration with multiple-apertures over 1000 on large-area grids of 2 m
for the multiple-beamlet accelerations. From the observation of the vacuum discharge between the grids, it was found that the aperture generated 10 times larger dark current than the flat region and initiated the vacuum discharge associated with the breakdown. As a result, it was found that the sustainable voltages were dominated by not only the surface area but also the number of the apertures. Because these effects were originated in the area effects by weak and strong electric field profiles, these results implied the surface integration of the electric field were the key parameter for the vacuum insulation.
Hanada, Masaya; Kojima, Atsushi; JT-60NBI Group; Yamano, Yasushi*; Kobayashi, Shinichi*
Denki Gakkai Kenkyukai Shiryo, Hoden Kenkyukai (ED-12-35), 6 Pages, 2012/03
Negative ion accelerator for neutral beam injectors for fusion appliaction has large-area acceleration grids of 
1 m and an insulator made of Fiber Reinforced Plastics (FRP), which are extremely larger than those used in the accelerators for industrial and acceleration applications. This paper reports vacuum insulation character its and development for the improvement of voltage holding capability towards JT-60SA.
Kojima, Atsushi; Hanada, Masaya; Hilmi, A.*; Haruyama, Hirochika*; Yamano, Yasushi*; Kobayashi, Shinichi*
Denki Gakkai Kenkyukai Shiryo, Hoden Kenkyukai (ED-12-36), 5 Pages, 2012/03
Vacuum insulation of a large negative ion source for fusion application is experimentally examined, which is characterized by large-area grids with multiple-apertures. The experimental results showed that sustainable voltages were found to be dominated by the electric field profiles generated around the apertures and grid area. These empirical laws are applied to the modification of the ion source.
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：17 Percentile：60.55(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 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.
Kojima, Atsushi; Hanada, Masaya; Tanaka, Yutaka*; Taniguchi, Masaki; Kashiwagi, Mieko; Inoue, Takashi; Umeda, Naotaka; Watanabe, Kazuhiro; Tobari, Hiroyuki; Kobayashi, Shinichi*; et al.
AIP Conference Proceedings 1390, p.466 - 475, 2011/09
Times Cited Count：2 Percentile：53.22(Physics, Atomic, Molecular & Chemical)Voltage holding tests by using JT-60 negative ion source and small electrodes was carried out because JT-60 negative ion source had a critical problem about low voltage holding capability for long time. As a result, the voltage holding capability is decreased with the increase of area where local electric field is generated, as well as the surface area according to existing scaling low about surface area. Therefore, in order to improve the voltage holding without changing the existing accelerator, the voltage holding test was carried out by extending gap lengths of the negative ion source. In order to improve the voltage holding, beam radiation shield needs to be optimized additionally. As a result, the voltage holding has been improved to 500 kV and stabilized. By using this modified ion source, negative ion beams of 500 keV up to 3A has been successfully produced.
Kojima, Atsushi; Hanada, Masaya; Tanaka, Yutaka*; Kawai, Mikito*; Akino, Noboru; Kazawa, Minoru; Komata, Masao; Mogaki, Kazuhiko; Usui, Katsutomi; Sasaki, Shunichi; et al.
Nuclear Fusion, 51(8), p.083049_1 - 083049_8, 2011/08
Times Cited Count：51 Percentile：88.4(Physics, Fluids & Plasmas)Hydrogen negative ion beams of 490 keV, 3 A and 510 keV, 1 A have been successfully produced in the JT-60 negative ion source with three acceleration stages. These successful productions of the high-energy beams at high current have been achieved by overcoming the most critical issue, i.e., a poor voltage holding of the large negative ion sources with the grids of 2 m for JT-60SA and ITER. To improve voltage holding capability, the breakdown voltages for the large grids was examined for the first time. It was found that a vacuum insulation distance for the large grids was 6-7 times longer than that for the small-area grid (0.02 m). From this result, the gap lengths between the grids were tuned in the JT-60 negative ion source. The modification of the ion source also realized a significant stabilization of voltage holding and a short conditioning time. These results suggest a practical use of the large negative ion sources in JT-60SA and ITER.
Kojima, Atsushi; Hanada, Masaya; Tanaka, Yutaka*; Kawai, Mikito*; Akino, Noboru; Kazawa, Minoru; Komata, Masao; Mogaki, Kazuhiko; Usui, Katsutomi; Sasaki, Shunichi; et al.
Proceedings of 23rd IAEA Fusion Energy Conference (FEC 2010) (CD-ROM), 8 Pages, 2011/03
Hydrogen negative ion beams of 490keV, 3A and 510 keV, 1A have been successfully produced in the JT-60 negative ion source with three acceleration stages. These successful productions of the high-energy beams at high current have been achieved by overcoming the most critical issue, i.e., a poor voltage holding of the large negative ion sources with the grids of 
2 m for JT-60SA and ITER. To improve voltage holding capability, the breakdown voltages for the large grids was examined for the first time. It was found that a vacuum insulation distance for the large grids was 6-7 times longer than that for the small-area grid (0.02 m). From this result, the gap lengths between the grids were tuned in the JT-60 negative ion source. The modification of the ion source also realized a significant stabilization of voltage holding and a short conditioning time. These results suggest a practical use of the large negative ion sources in JT-60 SA and ITER.
Tanaka, Yutaka; Ikeda, Yoshitaka; Hanada, Masaya; Kobayashi, Kaoru; Kamada, Masaki; Kisaki, Masashi; Akino, Noboru; Yamano, Yasushi*; Kobayashi, Shinichi*; Grisham, L. R.*
IEEE Transactions on Plasma Science, 37(8), p.1495 - 1498, 2009/08
Times Cited Count：1 Percentile：4.27(Physics, Fluids & Plasmas)Voltage holding capability of the JT-60 negative ion source is limited by surface flashover on the FRP insulator. To improve the voltage holding capability of the ion source, the understanding of the surface flashover is required. In this study, electron energy is estimated by measuring the bremsstrahlung X-ray emitted from an FRP insulator. Energy spectra of X-ray were measured for 3 different positions and compared with those of the vacuum gap between electrodes. Near the anode, X-ray spectrum was dominated by the monoenergetic electron. Near the cathode, spectrum peak shifted to low energy compared with that near the anode. This result showed that a large amount of low energy electrons was generated on the surface of the FRP insulator near the cathode.
Kobayashi, Kaoru; Hanada, Masaya; Akino, Noboru; Sasaki, Shunichi; Ikeda, Yoshitaka; Takahashi, Masahiro*; Yamano, Yasushi*; Kobayashi, Shinichi*; Grisham, L. R.*
IEEE Transactions on Dielectrics and Electrical Insulation, 16(3), p.871 - 875, 2009/06
Times Cited Count：1 Percentile：12.1(Engineering, Electrical & Electronic)Voltage holding capability of a 500kV, 22A three-stage electrostatic accelerator, where large-area grids of 0.28 m and large FRP insulators of 1.8 m in diameter are used, was examined. High voltage was independently applied to each acceleration stage, where the voltage holding capabilities of 130 kV were obtained. To identify whether the breakdowns occur in the gaps between the grids or the FRP insulators, high voltages were applied to the accelerator with and without the grids. Breakdown voltages without grids, i.e., the FRP insulator itself reached 170 kV of design value for each stage. These results show that the breakdown voltage of the accelerator was mainly determined by the gaps between the large-area grids. In this paper, the influence of non-uniform electric field and multi-stage grids on the voltage holding capabilities was also discussed.
Ikeda, Yoshitaka; Hanada, Masaya; Kamada, Masaki; Kobayashi, Kaoru; Umeda, Naotaka; Akino, Noboru; Ebisawa, Noboru; Inoue, Takashi; Honda, Atsushi; Kawai, Mikito; et al.
IEEE Transactions on Plasma Science, 36(4), p.1519 - 1529, 2008/08
Times Cited Count：12 Percentile：41.25(Physics, Fluids & Plasmas)The JT-60SA N-NBI system is required to inject 10 MW for 100 s at 500 keV. Three key issues should be solved for the JT-60SA N-NBI ion source. One is to improve the voltage holding capability. Recent R&D tests suggested that the accelerator with a large area of grids may need a high margin in the design of electric field and a long time for conditioning. The second issue is to reduce the grid power loading. It was found that some beamlets were strongly deflected due to beamlet-beamlet interaction and strike on the grounded grid. The grids are to be designed by taking account of beamlet-beamlet interaction in three-dimensional simulation. Third is to maintain the D- production for 100 s. A simple cooling structure is proposed for the active cooled plasma grid, where a key is the temperature gradient on the plasma grid for uniform D- production. The modified N-NBI ion source will start on JT-60SA in 2015.
Yamano, Yasushi*; Takahashi, Masahiro*; Kobayashi, Shinichi*; Hanada, Masaya; Ikeda, Yoshitaka
Review of Scientific Instruments, 79(2), p.02A524_1 - 02A524_4, 2008/02
Times Cited Count：4 Percentile：23.76(Instruments & Instrumentation)The JT-60U negative-ion-based NBI was designed to have 3-stage electrostatic accelerators with total accelerating voltage of 500kV. In order to hold such high voltage, three Fiberglass Reinforced Plastic (FRP) insulators of 1.8 m inner diameter and 31.5 cm height are used. Currently the hold-off voltage capability of 3-stage FRP insulators has been typically limited to 460 kV without beam acceleration. One of the possible reason is a surface discharges on the FRP insulators. This paper describes measurement results of surface resistivity and volume resistivity under vacuum and atmospheric conditions, secondary electron emission (SEE) characteristics and cathodoluminescence under some keV electron beam irradiation, gas analysis desorbed from the FRP insulator under irradiation of some keV-electron beam were also analyzed.
Yamano, Yasushi*; Kobayashi, Shinichi*; Matsukawa, Makoto
Proceedings of 20th International Symposium on Discharges and Electrical Insulation in Vacuum, p.419 - 422, 2002/07
no abstracts in English
Kobayashi, Shinichi*; Yamano, Yasushi*; Matsukawa, Makoto
Denki Gakkai Kenkyukai Shiryo, p.79 - 84, 2001/11
no abstracts in English
Kobayashi, Kaoru; Hanada, Masaya; Kamada, Masaki; Akino, Noboru; Sasaki, Shunichi; Ikeda, Yoshitaka; Takahashi, Masahiro*; Yamano, Yasushi*; Kobayashi, Shinichi*
no journal, ,
no abstracts in English
Yamano, Yasushi*; Akashi, Keisuke*; Kobayashi, Shinichi*; Tanaka, Yutaka; Kobayashi, Kaoru; Akino, Noboru; Hanada, Masaya; Ikeda, Yoshitaka
no journal, ,
no abstracts in English
Kojima, Atsushi; Tanaka, Yutaka*; Nakano, Shusuke*; Shimizu, Tatsuo; Akino, Noboru; Hanada, Masaya; Yamano, Yasushi*; Kobayashi, Shinichi*
no journal, ,
JT-60 negative ion source suffered from a low voltage holding capability for long time. Recently, the voltage holding was much improved by tuning the gap lengths and minimum lengths between the acceleration grids of the ion source. This improvement is attributed to voltage holding tests of the accelerator. However, in order to design the next ion sources such as ITER and JT-60SA, further understandings and databases of vacuum insulation are required. As for the effect of the electric field, a small electrodes with small aperture which has the locally strong electric field were tested in this time. We found that the maimum electric field did not affect on the breakdown voltage, and large area of the electric filed profile decreased the breakdown voltage.
Kojima, Atsushi; Shimizu, Tatsuo; Akino, Noboru; Hanada, Masaya; Yamano, Yasushi*; Kobayashi, Shinichi*; JT-60NBI Group
no journal, ,
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, we need to have database for the voltage holding capability based on experimental results. Up to now, voltage holding tests were carried out in terms of local electric field and surface area on large acceleration grids. This time, because outgas from FRP insulators has a possibility to affect the voltage holding of the acceleration grids, insulators made of low outgas epoxy was tested. As a result, because low outgas epoxy has small deviation of breakdown voltages, stable sustainment with 1.6 times higher voltage was attained than general epoxy insulators.
