M. Saidoh

Measurement of chemical sputtering yields of various types of carbon

Abstract Measurements of the chemical sputtering during the bombardment of pyrolytic graphite, isotropic carbon and glassy carbon with 0.1–6 keV hydrogen ions have been made in the temperature rarige of room temperature to 700°C. The maximum production rate occurs at 1 keV for the incident energy and 525°C for the target temperature in all types of carbon. Energy and temperature dependences of chemical sputtering of carbon are not affected by the structures of the carbon. The reason is ascribed to radiation damage of the surface of the carbon. The dose dependence of the methane production rate was influenced by the hydrogen concentration in a target prior to bombardment, but the steady rate was obtained after the target was bombarded with protons at a dose of more than 1 × 10 18 H + /cm 2 .

Initial Boronization of JT-60U Tokamak Using Decaborane

A decaborane-based boronization system has been installed in the JT-60U tokamak in order to reduce the influx of impurities during plasma discharges. Boronization has been performed under a glow discharge using a helium-decaborane gas mixture. The properties of the boron films deposited through boronization and the effects of boronization on the tokamak discharges were investigated. It was found that the deposition of a boron layer with high purity was achieved with few impurities other than hydrogen through boronization, and that the present boronization deposited toroidally nonuniform boron film. It was also found that the decaborane-based boronization resulted in good plasma performance similar to that of conventional boronization.

Chemical sputtering yields of titanium carbides

Abstract Chemical sputtering yields of CVD and PVD TiC coatings and sintered TiC bulk have been measured at normal incidence for H+ ions in the energy range between 0.1 and 6 keV and in a target temperature range between room temperature and 700° C. Temperature and sample dependences of the yields are small. The yields are one order of magnitude less than those of graphite and show a maximum at energies around 2 keV for the hydrogen ions. The strong dose dependence of the yields was measured and discussed in view of the surface concentration of carbon. The emphasis in the discussion is given to the importance of enhanced diffusion of carbon due to proton bombardment and of chemisorption of carbon impurities.

A boronization system in the JT-60U tokamak: Application of a new method using a less hazardous substance

Abstract A new type of boronization system has been installed in the JT-60U tokamak. Decaborane B10H14, a material less hazardous than diborane B2H6 for boronization, has been used to deposit a pure boron film on the first wall. The purpose of the boron film is to reduce the levels of impurity contamination in the JT-60U plasmas. The first results indicate that the film produced from decaborane shows a good performance similar to that of layers produced in the conventional way. The new technique thus offers an easier and more convenient way for boronization of the tokamak first wall.

Impurity and particle recycling reduction by boronization in JT-60U

Abstract In JT-60U boronization using decaborane was carried out. Boronization reduced the oxygen impurity in OH discharges and shortened the wall conditioning after the vacuum vessel vent and consequently enabled JT-60U to produce clean plasmas easily except for NB heated plasmas. After boronization, particle recycling was reduced drastically in OH and NB discharges. High confinement plasmas were obtained including high β p mode and H-mode discharges. In the latest boronization part of divertor plates were replaced with B 4 C coated tiles with a B 4 C thickness ∼ 300 μm. After introducing B 4 C divertor tiles, an explosive generation of boron particles from the tiles was observed. By the combined effects of boronization with decaborane and boron generation from B 4 C tiles, oxygen impurity was so low that oxygen line signals were reduced to noise levels after the latest boronization. On the other hand, boron burst from the B 4 C tiles restricted the operation of JT-60U.

Simultaneous ion and gas driven permeation of deuterium through nickel

Abstract The permeation rate of deuterium through nickel exposed to a deuterium ion beam has been measured as a function of temperature (100–1000°C), incident ion flux ( 0.2−1.1 × 10 15 D-atom cm −2 s −1 ), incident ion energy (1–2.5 keV/D-atom) , and deuterium partial pressure. The results are analyzed with a model which is developed to describe ion driven permeation when gas driven permeation coexists; the rate-limiting process of recycling during ion implantation is identified as surface recombination above 400°C and as bulk diffusion below 300°C, and the recombination coefficient is estimated. The model predicts the whole parameter dependence observed. The permeation spike is also investigated.

Deuterium retention of DIII-D DiMES sample

Abstract The deuterium retention property of B4C converted graphite and isotropic graphite exposed to DIII-D deuterium plasma was examined by using a technique of thermal desorption spectroscopy. Major outgassing species were HD, D2 and CD4 in both the graphite and the B4C. In the case of the graphite, the ratios of deuterium desorbed in the forms of HD, D2 and CD4 to the total desorption amount of deuterium were 40%, 27% and 33%, respectively. In the case of the B4C, which was covered by carbon due to redeposition, these ratios were similar to those of the graphite. In a thermal desorption spectrum of deuterium, three desorption peaks appeared in both the graphite and the B4C covered by the redeposition layer. At low temperature region, the desorption rate of deuterium for the B4C covered by the redeposition layer was larger than that of the graphite. From two dimensional distribution of deuterium retention, it was seen that the retained amount at the electron drift side was quite large. The amount at the ion drift side and the edge of inward major radius was also observed to be large. The average retained amount of the graphite was almost the same as that of the B4C covered by the redeposition layer.

Chemical sputtering yield of pyrolytic graphite bombarded with 3 keV H3+ under atomic hydrogen atmosphere

Abstract Effects of atomic hydrogen on chemical sputtering yields of pyrolytic graphite due to bombardment with energetic hydrogen ions have been investigated. Three cases of bombardments were done: bombardment of the virgin target without the atomic hydrogen, bombardment of target with atomic hydrogen and bombardment of target exposed to the atomic hydrogen prior to bombardment. The results show that a difference in yields was not found between them and the effect of the atomic hydrogen was small. The reason is ascribed to the fact that the inactive graphite surface obtained by the atomic hydrogen can be destroyed by the energetic ion impacts.

Tritium retention in graphite inner wall of JT-60U

Abstract In JT-60U, D-D experiments have been performed since July 1991. Although tritium, which is produced by the D-D reaction, is programmed to be exhausted through pumping systems, a part of the tritium is trapped in the graphite inner wall. To evaluate the tritium retention in the graphite wall of JT-60U, several sample tiles were removed from JT-60U in November 1993. The retained tritium distribution in the graphite tiles was measured by means of burning sample pieces, converting the tritium into HTO and scintillation counting the resultant water. The result of this measurement showed that tritium of 1.6 × 1010 Bq still remained in the graphite first wall. This value corresponds to approximately 50% of the tritium generated from July 1991 to October 1993 in JT-60U. The tritium remaining in the divertor tiles was in higher concentration than that in the first wall. In particular, the tritium concentration peaked in the tiles between the strike points. Tritium at the strike points, where an electron or ion strikes directly, was in low concentration.

High heat flux experiment on B4C-overlaid C/C composites for plasma facing materials of JT-60U

High heat flux experiments (5–40 MW/m 2 , 5 s and 550 MW/m 2 , 5–10 ms) in the JAERI electron beam irradiation stand (JEBIS) have been carried out on three kinds (conversion, CVD and LLPS) of B 4 C-overlaid C/C composites, on which B 4 C is overlaid with a thickness of 100–250 μm. Measurements were made with respect to the weight loss, changes of the surface morphology and of the surface atomic composition, and the surface temperature. As a result of these experiments, it is found that B 4 C layers of all samples have no damages except small weight losses up to 12 MW/m 2 heat loads, which are estimated at the divertor tiles of JT-60U in normal plasma operation, and that the conversion method is the best of the three methods applied in the present tests, since no exfoliation has occurred even under the disruption conditions.