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Nakamura, Kazuo*; Jiang, Y.*; Liu, X.*; Mitarai, Osamu*; Kurihara, Kenichi; Kawamata, Yoichi; Sueoka, Michiharu; Hasegawa, Makoto*; Tokunaga, Kazutoshi*; Zushi, Hideki*; et al.
Fusion Engineering and Design, 86(6-8), p.1080 - 1084, 2011/10
Times Cited Count：4 Percentile：33.04(Nuclear Science & Technology)Otsuka, Teppei*; Tanabe, Tetsuo*; Tokunaga, Kazutoshi*; Yoshida, Naoaki*; Ezato, Koichiro; Suzuki, Satoshi; Akiba, Masato
Journal of Nuclear Materials, 417(1-3), p.1135 - 1138, 2011/10
Times Cited Count：2 Percentile：18.56(Materials Science, Multidisciplinary)Nagata, Shinji*; Yamamoto, Shunya; Tokunaga, Kazutoshi*; Tsuchiya, Bun*; To, Kentaro*; Shikama, Tatsuo*
Nuclear Instruments and Methods in Physics Research B, 242(1-2), p.553 - 556, 2006/01
Times Cited Count：7 Percentile：47.67(Instruments & Instrumentation)no abstracts in English
Suzuki, Satoshi; Ueda, Yoshio*; Tokunaga, Kazutoshi*; Sato, Kazuyoshi; Akiba, Masato
Fusion Science and Technology, 44(1), p.41 - 48, 2003/07
Times Cited Count：27 Percentile：85.23(Nuclear Science & Technology)no abstracts in English
Tokunaga, Kazutoshi*; Ezato, Koichiro; Suzuki, Satoshi; Akiba, Masato; Fujiwara, Tadashi*; Yoshida, Naoaki*
no journal, ,
Polycrystalline tungsten samples have been irradiated by 19 keV pulsed hydrogen and helium beam up to 1
10
/m
. Optical reflectivity has been measured with an ultraviolet-visible and near-infrared spectrophotometer, and a Fourier transform-infrared (FT-IR) spectrometer for the wave length between 190 - 2400 nm, 1.4 - 20 
m, respectively. In addition, optical constants have been calculated by results measured using a spectroscopic ellipsometer. Surface morphology has been also examined with a scanning electron microscope. Loss of metallic gloss and remarkable reduction of reflectivity (190-2400 nm) occurs by the hydrogen and helium irradiation of 
1022 He
/m. In particular, helium beam irradiation at a peak temperature 2600 
C causes remarkably decrease of the reflectivity due to fine uneven morphology and bubble formation near-surface. The reduction of the reflectivity is considered to be mainly due to the porous structure with nano-scale hydrogen and helium bubbles.
Tokunaga, Kazutoshi*; Fujiwara, Tadashi*; Ezato, Koichiro; Suzuki, Satoshi; Akiba, Masato; Kurishita, Hiroaki*; Yoshida, Naoaki*
no journal, ,
no abstracts in English
Nakamura, Kazuo*; Matsufuji, Shinji*; Tomoda, Seiji*; Wang, F.*; Mitarai, Osamu*; Kurihara, Kenichi; Kawamata, Yoichi; Sueoka, Michiharu; Hasegawa, Makoto*; Tokunaga, Kazutoshi*; et al.
no journal, ,
no abstracts in English
Nakamura, Kazuo*; Yi, J.*; Liu, X.*; Mitarai, Osamu*; Kurihara, Kenichi; Kawamata, Yoichi; Sueoka, Michiharu; Hasegawa, Makoto*; Tokunaga, Kazutoshi*; Zushi, Hideki*; et al.
no journal, ,
no abstracts in English
Yi, J.*; Nakamura, Kazuo*; Liu, X.*; Mitarai, Osamu*; Kurihara, Kenichi; Kawamata, Yoichi; Sueoka, Michiharu; Hasegawa, Makoto*; Tokunaga, Kazutoshi*; Zushi, Hideki*; et al.
no journal, ,
no abstracts in English
Hirano, Koichiro; Fukuda, Makoto*; Ezato, Koichiro*; Tokunaga, Kazutoshi*
no journal, ,
The beam targets of the J-PARC linac are multiply irradiated by hydrogen negative ion beams with beam energy of 3 MeV. The purpose of this study is to clarify the pulsed heat load conditions that can withstand multiple irradiations when beam targets made of tungsten materials are used. Using an off-line beam test-stand with a 3 MeV hydrogen negative ion beam, multiple irradiation tests were conducted on tungsten materials for ITER, in which temperature changes of heating (3200 degree) and cooling (160 degree) were repeatedly given at a period of 5 Hz. This paper reports on the surface observation of the tungsten materials and the partial pressure measurements by a quadrupole mass spectrometer.
Fukuda, Makoto*; Hirano, Koichiro; Tokunaga, Kazutoshi*; Ezato, Koichiro*
no journal, ,
Tungsten which is used as a plasma-facing material for ITER divertor is expected to receive heat flux under various conditions during ITER operation. Therefore, it is necessary to understand the modification of tungsten surface under various heat load conditions. In this work, as part of the investigation of the repeated short pulse heat load effect on tungsten, the effect of multiple irradiation (3MeV, 5Hz) with a negative hydrogen ion beam was evaluated. As a result, unevenness and cracks were observed on the surface after irradiation, and different surface modification behavior was observed compared with previous experiment which performed with almost constant heat load (10s On and 20-30 s Off as one cycle).
Tokunaga, Kazutoshi*; Nomiyama, Yukino*; Fukuda, Makoto*; Ezato, Koichiro*; Hirano, Koichiro
no journal, ,
The surface modification of tungsten materials exposed by multiple irradiations of a negative hydrogen ion beam is examined to investigate the influence of the repeated short pulse heat loading. Grid-like cracks are formed on the sample surfaces which the surface is perpendicular to the rolling direction. On the other hand, ellipsoidal cracks are formed on the sample surfaces which the surface is parallel to the rolling direction. These results show crack formation is different depending on the orientation of grain boundaries. The surface modification by plastic formation due to the repeated heat loading in the center area surrounded by the cracks is observed. Furthermore, in the case of high heat loading conditions, a number of cone-like protuberances are melted and resolidified, and deep cracks are formed in the vicinity. The peculiar surface such as the deep cracks and the cone-like protuberances are formed.
Fukuda, Makoto*; Tokunaga, Kazutoshi*; Ezato, Koichiro*; Hirano, Koichiro
no journal, ,
Tungsten which is used as a plasma-facing material of the ITER divertor, is expected to receive various type of heat load during ITER operation. Therefore, it is necessary to understand the surface damage of tungsten under various heat load conditions. In this study, as part of the investigation of the effects of short-pulse repetitive heat loads on tungsten, the effects of multiple irradiations of the negative hydrogen ion beam (3 MeV, 5 Hz) on the surface of tungsten were investigated. As a result of the investigation, it was clarified that microcracks were formed on the surface of tungsten exposed to multiple irradiations, and that the microstructure was also changed.
Tokunaga, Kazutoshi*; Nomiyama, Yukino*; Fukuda, Makoto*; Ezato, Koichiro*; Hirano, Koichiro
no journal, ,
It is known that the surface temperature of tungsten (W) divertor during the slow transient at steady state plasma operation in ITER will be higher than that of the recrystallization temperature. Therefore, thermal behavior of recrystallized W(RC-W) exposed the expecting heat loadings during ITER plasma operation is necessary to be investigated. In the present work, RC-W has been exposed to multiple irradiation of negative hydrogen ion beam (3 MeV, 5 Hz) and the effect of repeated short pulse heat loading on surface modification has been investigated. The results have also compared with that of stress-relieved W (SR-W). The area of surface modification of RC-W is larger than that of SR-W in the case of a peak temperature of 1650 degree. The circular cracks along grain boundaries are formed and complex surface modification including line like uneven surface are seen. These modifications of RC-W are different from that of SR-W.
