A." Sagara

H. Uchimura

K. Okuno

K. Toda

M. Kobayashi

M. Miyamoto

M. Sato

N. Ashikawa

Y. "Oya

National Institute for Fusion Science (NIFS-Repository)

     Reduced activation ferritic/martensitic steel (F82H) is a promising structure material of blanket in ITER and a demo reactor, such as FFHR. The blanket structure material is exposed to fuel hydrogen during plasma operation and part of hydrogen is retained in the material. From the point of view of tritium inventory and tritium safety, it is important to investigate hydrogen retention behavior. In addition, helium (He) produced by D-T reaction is implanted into plasma-facing wall in a demo reactor, which should influence the hydrogen retention behavior. In this study, F82H was exposed to plasmas of a mixture of hydrogen and He using Vehicle-1 device at NIFS, and the effect of helium irradiation on hydrogen retention behavior was investigated.    Before plasma exposure, the F82H samples were annealed in vacuum at 873 K. During the plasma exposure in Vehicle, bias voltages of -100 V and -200 V were applied to the samples to implant ions. The plasma density and electron temperature in this experiment were approximately 1010/cm3 and a few eV, respectively. After the plasma exposure, the desorption behavior of retained hydrogen was investigated with thermal desorption spectroscopy. Also, in order to evaluate the relationship of hydrogen desorption behavior and irradiation damages, the change of micro structure caused by the plasma exposure was observed by transmission electron microscope, TEM.    Thermal desorption spectra of H2 after plasma exposures is shown in Fig.1. After exposing to pure hydrogen plasma, two main peaks appeared at 310 and 470 oC, which would be owing to release of hydrogen trapped at naturally existing trap sites, such as grain boundary, point defects and dislocation. In the case of hydrogen plasma exposure followed by He plasma exposure with -100 bias voltage, the spectra was very similar to that of pure hydrogen plasma exposure. However, in both cases of a mixture (H2+He) plasma exposure and hydrogen plasma exposure following by He irradiation exposure with bias voltage of -200 V, desorption rate in the temperature range between 200 and 600 oC significantly increased. The micro structure of F82H after He plasma exposure with bias voltage of -100 V, mixture plasma exposure and H2 plasma irradiation following He pre-irradiation (-200V) are shown in Fig.2. After He plasma exposure (Fig.2 (a)) and mixed plasma exposure (Fig.2 (b)), bubbles with size of 1-5 nm in diameter was observed in depth less than 40 nm. After hydrogen plasma exposure (-100 V) following helium plasma exposure with bias voltage of -200 V, bubbles and voids with size of approximately 15 nm was observed in the depth less than 20 nm, and smaller bubbles were also produced at more deeper regions. The hydrogen trapped in such large bubbles and voids might be responsible for the hydrogen desorption in temperatures between 200 and 600 oC in the TDS spectra (Fig.1) in the case of He pre-irradiation with -200 bias voltage. However, in the case of mixture plasma exposure (Fig.2 (b)), such large bubbles and voids were not observed although hydrogen desorption in temperatures between 200 and 600 oC in TDS spectra (Fig.1) was observed. One of the possible reasons for that is that He could knock-on the retained H to deeper region and the H could be retained in deeper regions, compared to pure hydrogen plasma exposure case.    The result obtained in this experiment indicated that He implantation into F82H can greatly influenced the hydrogen retention behavior in a demo reactor. (a)      (b)     (c)   Figure  2  Micro structure of F82H after (a) He plasma (-100V), (b) H2 and He mixed plasma and (c) pre He plasma (-200V) /H2 plasma irradiation.  Figure 1  Thermal desorption spectra of H2 in F82H.  i) Introduction Tungsten is considered to be a candidate for plasma facing materials (PFMs) in ITER due to higher conductivity and higher melting point. During the plasma operations, carbon, which will be retained in the vacuum vessel as the impurities, will be sputtered by the plasma and will also contaminate the plasma. In addition, the chemical form of hydrocarbons will also influence on the formation of carbon mixed layer on PFMs, leading the enhancement of tritium retention in PFMs. Therefore, the elucidation of reemission behavior of hydrocarbons for tungsten – carbon mixed layer is quite important for tritium safety. In the present study, H2+ was implanted into C+ implanted tungsten as a function of implantation energy and reemission mechanism of hydrocarbons for tungsten – carbon mixed layer was studied.  ii) Experimental   The disk-type tungsten (Allied Material Co. Ltd.) cut from a rod of tungsten under stress-relieved condition was used. The 10 keV C+ was implanted into tungsten with the ion flux of 1.0×1017 C+ m-2 s-1 up to the ion fluence of 1.0×1021 C+ m-2 to prepare the C+ implanted tungsten sample. Thereafter, H2+ was implanted into tungsten with ion flux of 1.0×1018 H+ m-2 s-1 at 673 K to evaluate the chemical sputtering behavior for tungsten. The ion energy of H2+ was varied from 0.6 keV to 3.0 keV. The sputtering particle measurements for the C+ implanted tungsten sample during H2+ implantation was observed directly by a quarupole mass spectrometer.  iii) Results and discussion Figure shows the implantation ion enegry dependence on sputtering hydrocarbons for C+ implanted tungsten. The CH3 was a major hydrocarbon species during 3 keV H2+ implantation, although the emission rate of CH2 was increased compared to that of CH3 during 0.6 keV H2+ implantation. It can be said that the reemission rate of hydrocarbons largely depends on the H2+ implantation energy. For the H2+ implantation with higher energy, most of ion energy would be consumed at the bulk of C+ implanted tungsten. Therefore, liner energy transfer at the surface region for H2+ implantation with lower ion energy should be high compared with that with higher ion energy, leasing to the enhancement of CH2 reemission by phycical sputtering process. The sputtering rate of hydrocarbons during 0.6 keV H2+ implantation was about 3% of that during 3 keV H2+ implantation and large reduction of hydrocarbon reemission was found for 0.6 keV H2+ implantation. In our previous study, the density of tungsten-carbon mixed layer clearly depends on the depth from the surface and tungsten-carbon mixed layer with lower density was extended around the surface region. In the case of H2+ implantation with lower ion energy, most of hydrogen would be implanted into tungsten-carbon mixed layer with lower density. Therefore, the reaction with carbon should be limited, leading to the reduction of hydrocarbon reemission. The hydrogen concentration near surface region was reduced, leasing the reemission of hydrocarbon with lower hydrogen, like CH2.     0.6 30.00.20.40.60.81.0H2+ energy / keV  CH CH2 CH3 CH4Ratio of sputtering particles / -  Fig. Implantation ion energy dependence on hydrocarbon remission for C+ implanted W 343§24. Quantitative Evaluation of Hydrogen Isotope Retention under Complex Ion Irradiation on PFMOya, Y., Okuno, K., Kobayashi, M., Uchimura, H., Toda, K., Sato, M. (Shizuoka Univ.), Miyamoto, M. (Shimane Univ.), Ashikawa, N., Sagara, A.

Quantitative Evaluation of Hydrogen Isotope Retention under Complex Ion Irradiation on PFM

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Quantitative Evaluation of Hydrogen Isotope Retention under Complex Ion Irradiation on PFM

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