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Journal Articles

The Report on the Meeting of the Young Researchers' association of JHPS "Issues following the revision of radiation dose limits for the lens of the eye"

Kataoka, Noriaki*; Nakajima, Junya; Otsu, Saori; Takahashi, Akina; Takamiya, Kei; Umeda, Masayuki; Nishiono, Kanoko*

Hoken Butsuri (Internet), 56(1), p.28 - 31, 2021/03

no abstracts in English

Journal Articles

Report on social communication activities of Young Researchers Association and Students Association of JHPS; Chiba-shi Science Festa

Yamada, Ryohei; Kono, Takahiko; Nakajima, Junya; Hirouchi, Jun; Tsuji, Tomoya; Umeda, Masayuki; Igarashi, Yu*; Koike, Hiromi*

Hoken Butsuri (Internet), 56(1), p.32 - 38, 2021/03

no abstracts in English

Journal Articles

Progress in long-pulse production of powerful negative ion beams for JT-60SA and ITER

Kojima, Atsushi; Umeda, Naotaka; Hanada, Masaya; Yoshida, Masafumi; Kashiwagi, Mieko; Tobari, Hiroyuki; Watanabe, Kazuhiro; Akino, Noboru; Komata, Masao; Mogaki, Kazuhiko; et al.

Nuclear Fusion, 55(6), p.063006_1 - 063006_9, 2015/06

 Times Cited Count:29 Percentile:89.03(Physics, Fluids & Plasmas)

Significant progresses in the extension of pulse durations of powerful negative ion beams have been made to realize the neutral beam injectors for JT-60SA and ITER. In order to overcome common issues of the long pulse production/acceleration of negative ion beams in JT-60SA and ITER, the new technologies have been developed in the JT-60SA ion source and the MeV accelerator in Japan Atomic Energy Agency. As for the long pulse production of high-current negative ions for JT-60SA ion source, the pulse durations have been successfully increased from 30 s at 13 A on JT-60U to 100 s at 15 A by modifying the JT-60SA ion source, which satisfies the required pulse duration of 100 s and 70% of the rated beam current for JT-60SA. This progress was based on the R&D efforts for the temperature control of the plasma grid and uniform negative ion productions with the modified tent-shaped filter field configuration. Moreover, the each parameter of the required beam energy, current and pulse has been achieved individually by these R&D efforts. The developed techniques are useful to design the ITER ion source because the sustainment of the cesium coverage in large extraction area is one of the common issues between JT-60SA and ITER. As for the long pulse acceleration of high power density beams in the MeV accelerator for ITER, the pulse duration of MeV-class negative ion beams has been extended by more than 2 orders of magnitude by modifying the extraction grid with a high cooling capability and a high-transmission of negative ions. A long pulse acceleration of 60 s has been achieved at 70 MW/m$$^{2}$$ (683 keV, 100 A/m$$^{2}$$) which has reached to the power density of JT-60SA level of 65 MW/m$$^{2}$$.

Journal Articles

Long pulse acceleration of MeV class high power density negative H$$^{-}$$ ion beam for ITER

Umeda, Naotaka; Kojima, Atsushi; Kashiwagi, Mieko; Tobari, Hiroyuki; Hiratsuka, Junichi; Watanabe, Kazuhiro; Dairaku, Masayuki; Yamanaka, Haruhiko; Hanada, Masaya

AIP Conference Proceedings 1655, p.050001_1 - 050001_10, 2015/04

For ITER neutral beam system, negative deuterium ion beam of 1 MeV, 40 A (current density of 200 A/m$$^{2}$$) is required for 3600 s. To demonstrate ITER relevant negative ion beam acceleration, beam acceleration test has been carried out at MeV test facility in JAEA. The present target is H$$^{-}$$ ion beam acceleration up to 1 MeV with 200 A/m$$^{2}$$ for 60 s, which beam energy and pulse length are the present facility limit. To extend pulse duration time up to facility limit at high power density beam, new extraction grid has been developed with high cooling capability, which electron suppression magnet is placed under cooling channel. In addition, the aperture size of the electron suppression grid is enlarged from 14 mm to 16 mm and the aperture displacement is modified to reduce collision of negative ion beam on the grid. By these modifications, total grid power loading has reduced from 14% to 11%. As a result, beam acceleration up to 60 s which is the facility limit, has achieved at 700 kV, 100 A/m$$^{2}$$ of negative ion beam without breakdown.

Journal Articles

Operation and maintenance experience from the HTTR database

Shimizu, Atsushi; Furusawa, Takayuki; Homma, Fumitaka; Inoi, Hiroyuki; Umeda, Masayuki; Kondo, Masaaki; Isozaki, Minoru; Fujimoto, Nozomu; Iyoku, Tatsuo

Journal of Nuclear Science and Technology, 51(11-12), p.1444 - 1451, 2014/11

 Times Cited Count:1 Percentile:10.7(Nuclear Science & Technology)

JAEA has kept up a data-base system of operation and maintenance experiences of the HTTR. The objective of this system is to share the information obtained operation and maintenance experiences and to make use of lessons learned and knowledge into a design, construction and operation managements of the future HTGR. More than one thousand records have been registered into the system between 1997 and 2012. This paper describes the status of the data-base system, and provides suggestions for improvement from four experiences: (1) performance degradation of helium compressors; (2) malfunction of reserved shutdown system in reactivity control system; (3) maintenance experiences of emergency gas turbine generators; and (4) experiences of the Great East Japan Earthquake. These experiences are extracted from the system as important lessons learned to be expected to apply for design, construction and operation managements of future HTGR.

Journal Articles

Long-pulse beam acceleration of MeV-class H$$^{-}$$ ion beams for ITER NB accelerator

Umeda, Naotaka; Kashiwagi, Mieko; Taniguchi, Masaki; Tobari, Hiroyuki; Watanabe, Kazuhiro; Dairaku, Masayuki; Yamanaka, Haruhiko; Inoue, Takashi; Kojima, Atsushi; Hanada, Masaya

Review of Scientific Instruments, 85(2), p.02B304_1 - 02B304_3, 2014/02

 Times Cited Count:10 Percentile:49.27(Instruments & Instrumentation)

In order to realize neutral beam systems in ITER whose target is to produce D$$^{-}$$ ion beam of 1 MeV, 200 A/m$$^{2}$$ during 3600s, the electrostatic five-stages negative ion accelerator has been developed at JAEA. To extend pulse length, heat load of the acceleration grids was reduced by controlling the ion beam trajectory. Namely, the beam deflection due to the residual magnetic filter in the accelerator was suppressed with the newly developed extractor with a 0.5 mm off-set aperture displacement. The use of new extractor improved the deflection angle from 6 mrad to 1 mrad, resulting in the reduction of direct interception of negative ions from 23% to 15% of the total acceleration power, respectively. As a result, the pulse length of 130 A/m$$^{2}$$, 881 keV H$$^{-}$$ ion beam has been successfully extended from a previous value of 0.4s to 8.7s.

Journal Articles

Development of negative ion extractor in the high-power and long-pulse negative ion source for fusion application

Kashiwagi, Mieko; Umeda, Naotaka; Tobari, Hiroyuki; Kojima, Atsushi; Yoshida, Masafumi; Taniguchi, Masaki; Dairaku, Masayuki; Maejima, Tetsuya; Yamanaka, Haruhiko; Watanabe, Kazuhiro; et al.

Review of Scientific Instruments, 85(2), p.02B320_1 - 02B320_3, 2014/02

 Times Cited Count:24 Percentile:76.32(Instruments & Instrumentation)

The negative ion extractor for high power and long-pulse operations is newly developed toward the neutral beam injector (NBI) for heating & current drive of future fusion machines such as ITER, JT-60 Super Advanced (SA) and DEMO reactor. The satisfactory cooling capability is designed in the thermal analysis. A negative ion production and a suppression of electrons are experimentally validated for this new extractor. As the results, the negative ion current shows increases by a factor of 1.3 with suppressing the electron current. The beam divergence angle is also maintained small enough, 4 mrad.

Journal Articles

Structural analyses of HV bushing for ITER heating NB system

Tobari, Hiroyuki; Inoue, Takashi; Taniguchi, Masaki; Kashiwagi, Mieko; Umeda, Naotaka; Dairaku, Masayuki; Yamanaka, Haruhiko; Watanabe, Kazuhiro; Sakamoto, Keishi; Kuriyama, Masaaki*; et al.

Fusion Engineering and Design, 88(6-8), p.975 - 979, 2013/10

 Times Cited Count:1 Percentile:12.53(Nuclear Science & Technology)

The HV bushing, one of the ITER NB components, which is to be procured by JADA, is a multi-conductor feed through composed of five-stage double-layered insulator columns with large brazed ceramic ring and fiber reinforced plastic (FRP) ring. The HV bushing is a bulk head between insulation gas at 0.6 MPa and vacuum. The FRP ring is required to sustain the pressure load, seismic load and dead weight. Brazing area of the ceramic ring with Kovar is required to maintain vacuum leak tightness and pressure tightness against the air filled at 0.6 MPa. To design the HV bushing satisfying the safety factor of $$geq$$ 3.5, mechanical analyses were carried out. As for the FRP ring, it was confirmed that isotropic fiber cloth FRP rings should be used for sufficient strength against shear stress. Also, shape and fixation area of the Kovar sleeve were modified to lower the stress at the joint area. As a result, a design of the insulator for the HV bushing was established satisfying the requirement.

Journal Articles

Compensations of beamlet deflections for 1 MeV accelerator of ITER NBI

Kashiwagi, Mieko; Taniguchi, Masaki; Umeda, Naotaka; Dairaku, Masayuki; Tobari, Hiroyuki; Yamanaka, Haruhiko; Watanabe, Kazuhiro; Inoue, Takashi; DeEsch, H. P. L.*; Grisham, L. R.*; et al.

AIP Conference Proceedings 1515, p.227 - 236, 2013/02

 Times Cited Count:9 Percentile:94.55

In a five stage multi-aperture multi-grid (MAMuG) accelerator for the ITER neutral beam injector (NBI), 1 MeV, 40 A D$$^-$$ ion beam is required for 1 hour. However, beamlets are deflected due to (1) magnetic field for electron suppression and (2) space charge repulsion between beamlets, and consequently, cause excess grid heat load. A three dimensional beam analysis has been carried out to compensate the beamlet deflections. This paper shows that the beamlet deflections due to (1) and (2) are compensated by an aperture offset of only 0.6 mm applied to the aperture of 17 mm in diameter in the extractor and by a metal bar attached around aperture area beneath the extractor, respectively. When the metal bar is increased to 3 mm in thickness and installed 30 mm away from the aperture area, the beamlet is steered gently by the weaker electric field distortion. The beam optics was confirmed not deteriorated by those compensations. The presentation also discusses application of these compensation techniques to the ITER design.

Journal Articles

Analysis of electron temperature distribution by kinetic modeling of electron energy distribution function in JAEA 10 ampere negative ion source

Shibata, Takanori; Terasaki, Ryo*; Kashiwagi, Mieko; Inoue, Takashi; Dairaku, Masayuki; Taniguchi, Masaki; Tobari, Hiroyuki; Umeda, Naotaka; Watanabe, Kazuhiro; Sakamoto, Keishi; et al.

AIP Conference Proceedings 1515, p.177 - 186, 2013/02

 Times Cited Count:8 Percentile:94.55

In the neutral beam injector in JT-60SA, one of issues is that negative ion beam is partially intercepted at acceleration grids due to a spatial non-uniformity of negative ion production on large extraction area (0.9$$times$$0.45m$$^{2}$$). Previous experiments showed that fast electrons emitted from filament cathodes are transported in a longitudinal direction by $$mathbf{B} times textrm{grad} mathbf{B}$$ drift and the spatial distribution of electron temperature ($$T_e$$) strongly relates with the non-uniformity. In this study, a three-dimensional electron transport analysis has been developed. Electron temperature in the analysis agreed well with measurements in JAEA 10A ion source. This study clarified that the bias of $$T_e$$ distribution are caused by the following reasons; (1) fast electrons drifted in the longitudinal direction survives near the end wall with energy up to $$E$$ = 25-60 eV and (2) they produces thermal electrons by collision with plasma particles there.

Journal Articles

Vacuum insulation and achievement of 980 keV, 185 A/m$$^{2}$$ H$$^{-}$$ ion beam acceleration at JAEA for the ITER neutral beam injector

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

Plasma Science and Technology, 15(2), p.179 - 183, 2013/02

 Times Cited Count:1 Percentile:5.25(Physics, Fluids & Plasmas)

Vacuum insulation is a common issue for the accelerator and the HV bushing for the ITER NBI. The HV bushing has five-stage structure and each stage consists of double-layered insulators. Hence, several triple points exist around the insulators. To reduce electric field at those points simultaneously, three types of stress ring were developed. In voltage holding test of a full-scale mockup equipped with those stress rings, 120% of rated voltage was sustained and the voltage holding capability required in ITER was verified. In the MeV accelerator, voltage holding capability was not sufficient due to breakdown triggered by electric field concentration at edge and corner on grid components. By extending gap length, 1 MV was sustained in vacuum. Furthermore, with new accelerator grids which compensates beam deflection due to magnetic field and space charge repulsion between beamlets, 980 keV, 185 A/m$$^{2}$$ H$$^{-}$$ ion beam acceleration was demonstrated, which was close to ITER requirement.

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$$^+$$.

JAEA Reports

Safety demonstration test using the High Temperature Engineering Test Reactor (HTTR); Cold test of the loss of forced cooling

Shinohara, Masanori; Yanagi, Shunki; Tochio, Daisuke; Shimazaki, Yosuke; Nojiri, Naoki; Owada, Hiroyuki; Sato, Nao; Sagawa, Hiroshi; Umeda, Masayuki

JAEA-Technology 2011-029, 39 Pages, 2011/12

JAEA-Technology-2011-029.pdf:3.03MB

JAEA plans and performs the safety demonstration test using the HTTR to develop High Temperature Gas Reactor technologies. Cold test of the loss of forced cooling was conducted prior to the safety demonstration test, to check test procedure and plant behavior. Cold test consists of two phases, Phase1, 1 or 2 Vessel Cooling System (VCS) terminates, in the Phase2, all 3 Gas circulators and 1 VCS terminates. Cold test could confirm test process, and obtain data necessary to analysis and 2-dimensional horizontal sectional model analysis was verified to simulate actual measurement value.

Journal Articles

Improvement of voltage holding and high current beam acceleration by MeV accelerator for ITER NB

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

AIP Conference Proceedings 1390, p.449 - 456, 2011/09

 Times Cited Count:2 Percentile:54.73

At JAEA, MeV accelerator has been developed as a proof-of-principle accelerator for ITER NBI. To achieve the acceleration of 1 MeV, 200 A/m$$^{2}$$ beam required for ITER, improvement of the voltage holding capability is essential. Review of voltage holding results ever obtained with various geometries of the accelerators showed that voltage holding capability was about a half of that for ideal small electrode. This is due to local electric field concentration in the accelerators, such as edge and corner between grids and its support structures. Based on these results, accelerator was modified to reduce the electric field concentration by reshaping the support structures and expanding the gap length. After the modifications, voltage holding capability in vacuum was increased from 835 kV to 1 MV. Voltage holding progressed the energy and current to 879 keV, 0.36 A (157 A/m$$^{2}$$).

Journal Articles

Study of beamlet deflection and its compensations in a MeV accelerator

Kashiwagi, Mieko; Inoue, Takashi; Taniguchi, Masaki; Umeda, Naotaka; Grisham, L. R.*; Dairaku, Masayuki; Takemoto, Jumpei; Tobari, Hiroyuki; Tsuchida, Kazuki; Watanabe, Kazuhiro; et al.

AIP Conference Proceedings 1390, p.457 - 465, 2011/09

 Times Cited Count:8 Percentile:88.45

In a five stage multi-aperture and multi-grid (MAMuG) accelerator in JAEA, beam acceleration tests are in progress toward 1 MeV, 200 A/m$$^{2}$$ H$$^{-}$$ ion beams for ITER. The 1 MV voltage holding has been successfully demonstrated for 4000 s with the accelerator of expanded gap length that lowered local electric field concentrations. The led to increase of the beam energy up to 900 keV-level. However, it was found that beamlets were deflected more in long gaps and direct interceptions of the deflected beamlet caused breakdowns. The beamlet deflection and its compensation methods were studied utilizing a three-dimensional multi beamlet analysis. The analysis showed that the 1 MeV beam can be compensated by a combination of the aperture offset of 0.8 mm applied in the electron suppression (ESG) and the metal bar called a field shaping plate with a thickness of 1 mm attached beneath the ESG. The paper reports analytical predictions and experimental results of the MAMuG accelerator.

Journal Articles

Development of full-size mockup bushing for 1 MeV ITER NB system

Tobari, Hiroyuki; Inoue, Takashi; Hanada, Masaya; Dairaku, Masayuki; Watanabe, Kazuhiro; Umeda, Naotaka; Taniguchi, Masaki; Kashiwagi, Mieko; Yamanaka, Haruhiko; Takemoto, Jumpei; et al.

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

High voltage (HV) bushing in the ITER NBI is one of critical components, which acts as a feedthrough for electric power and cooling water from the -1 MV power supply in SF$$_{6}$$ gas to beam source inside vacuum. JAEA has overcome a longstanding issue on manufacturing of a large bore ceramic ring with 1.56 m in diameter as the insulator of the five-stage HV bushing. Joining method of the ceramic and metal flange with thick Kovar plate to form vacuum boundary was also developed. By assembling components, a full-size mockup bushing simulating one stage of the HV bushing was successfully manufactured. In the voltage holding test, the high voltage of 240 kV including the margin of 20 % of a rated voltage was sustained for 3600 s without breakdown, and the voltage holding capability required in ITER was successfully verified.

Journal Articles

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

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$$^{2}$$) 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$$^{2}$$) at perveance matched condition from 796 kV, 0.32 A (140 A/m$$^{2}$$) 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.

Journal Articles

Three-dimensional analysis of beamlet deflection in a MeV accelerator for ITER NBI

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

Plasma and Fusion Research (Internet), 5, p.S2097_1 - S2097_4, 2010/12

An accelerator which generates deuterium negative ion beams of 1 MeV, 40 A for 3600 s is required for the ITER neutral beam injector (NBI). To realize such a high power accelerator, numerical studies have been carried out in parallel to acceleration tests in JAEA. After long pulse acceleration tests up to 30 s, it was found that parts of grid were melt around the grid apertures. In order to investigate how the grids melted, a three dimensional beam analysis was carried out in combination with 2D beam analysis code and gas flow code. The analysis clarified that the beamlet was deflected due to their own space charge repulsion and magnetic field at extraction grid, which led to direct interceptions of the beamlets at the grids. The power loads at the grid by these deflected beamlets were found to be more than 20 kW/cm$$^{2}$$. For next long pulse tests, a new extraction grid with aperture offset and field shaping plate has been designed so as to compensate the beamlet deflections.

Journal Articles

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.

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