E. Vietzke

Boronization in textor

Abstract The liner and limiters of TEXTOR have been coated in situ with a boron containing carbon film using a RG discharge in a throughflow of 0.8 He + 0.1 B2H6 +0.1 CH4. The average film thickness was 30–50 nm, the ratio of boron and carbon in the layer was about 1:1 according to Auger Electron Spectroscopy. Subsequent tokamak discharges are characterized by a small fraction of radiated power ( OVI/ n e of the OVI intensity normalized to the averaged plasma density n e decreases by more than a factor of four. The decrease in the oxygen content manifests itself also as a reduction of the CO and CO2 partial pressures measured during and after the discharge with a sniffer probe. The carbon levels are reduced by a factor of about two as measured by the normalized intensity CII/ n e of the CII line and via the ratio of the C fluxes and deuterium fluxes measured at the limiter (CI/Dα). The wall shows a pronounced sorption of hydrogen from the plasma, easing the density control and the establishment of low recycling conditions. The beneficial conditions did not show a significant deterioration during more than 200 discharges, including numerous shots at ICRH power levels > 2 MW.

Chemical erosion of amorphous hydrogenated carbon films by atomic and energetic hydrogen

The chemical erosion of amorphous hydrogenated carbon films by thermal and energetic hydrogen (deuterium) impact has been studied using atomic and ion beam techniques. The samples were produced during the routine carbonization procedure in TEXTOR as well as in a simulation vessel. In the reaction of thermal hydrogen atoms with these films the radical CH3 is formed accompanied by a wide spectrum of higher hydrocarbons. The overall erosion rate at 200°C is 4 × 10−2 eroded C/H and the temperature dependence of the erosion is similar to that of the reaction of hydrogen ions with graphite. Only few mixed molecules are formed when deuterium instead of protium atoms are used in the reaction on hydrogenated carbon films. Energetic hydrogen ions react with these films forming predominantly CH4 with smaller contributions of higher hydrocarbons. The temperature dependence of the erosion is similar to that of the reaction with thermal atoms. The overall reaction rate is only about twice that of thermal hydrogen exposure. These results agree reasonably well with measurements on the erosion of carbonized TEXTOR surfaces by particles from a RG-glow discharge in hydrogen.

Hydrocarbon formation in the reaction of atomic hydrogen with pyrolytic graphite and the synergistic effect of argon ion bombardment

The reactions of atomic hydrogen with pyrolytic graphite have been investigated by an atomic beam technique. Up to temperatures of 800 K atomic hydrogen reacts with graphite forming CH4 and C2-compounds with a reaction probability of about 2 × 10−4. Above 1000 K no hydrocarbons have been found. A hydrogen atom exposure of the graphite with simultaneous bombardment by Ar+ ions drastically enhanced the hydrocarbon formation up to a factor of 100 exhibiting a characteristic temperature behaviour with a maximum at about 800 K. As main reaction product CH3 is formed together with some CH4 and C2-compounds. When the Ar+ beam is turned off the enhancement of the reaction probability is only slowly decreasing with further hydrogen bombardment. In contrast to ion bombardment no significant influence of simultaneous electron irradiation on hydrocarbon formation has been observed.

Differences in the CH3 and CH4 formation from graphite under bombardment with hydrogen ions and hydrogen atoms/argon ions

Abstract In the surface reactions on graphite induced by atomic hydrogen/ energetic ion irradiation the predominant reaction product is the radical CH3 whereas with H2+ bombardment mainly CH4 is formed. In both cases the temperature dependence is identical, indicating the same rate-determining steps in the reaction path. From the mass spectra of the reaction product on graphite by H0/D2+ irradiation it is concluded that the reaction by energetic hydrogen ions occurs at the inner surfaces whereas with irradiation with H0/energetic ions CH3 is formed at the surface. Both types of reaction are additive and do not mix. On the basis of these results a reaction mechanism is proposed.

Thermal desorption of hydrogen and various hydrocarbons from graphite bombarded with thermal and energetic hydrogen

Abstract The thermal release of hydrogen (deuterium) and hydrocarbons has been investigated after exposing pyrolytic graphite to thermal and energetic hydrogen under various conditions. After exposing graphite to thermal hydrogen atoms, the retained hydrogen desorbs in the subsequent heating in two different desorption processes whereby the radical CH3, C2Hx and C3Hx species desorb together with the hydrogen molecule in the first desorption process. Absolute amounts of the hydrocarbons formed reach about 0.25 of the amount of desorbed H2 and follow a similar fluence dependence as the H2 desorption. The simultaneous bombardment of the graphite surface with energetic inert ions (Ar−) and with thermal hydrogen atoms increases the retention of hydrogen and changes the desorption spectrum of the hydrogen whereas the hydrocarbon desorption spectrum remains unchanged. In the case of bombarding graphite with energetic hydrogen ions (investigated for EH > 350 eV) the desorption of C2 and C3 type hydrocarbons is negligible compared to that of methane. The methane desorption itself is characterized by a sharp fluence dependence and by the formation of CH4 molecules.

Enhancement of the sputtering yield of pyrolytic graphite at elevated temperatures

Abstract The temperature dependence of the yield as well as the composition of the sputtered particles during bombardment of pyrolytic graphite by 5 keV Ar + ions has been measured up to temperatures of 2200 K using a mass-spectrometric technique. An attempt was made to identify the sputtered as well as thermal sublimed species. We looked for species C 1 to C 4 . It has been found that above 1000 K the sputtering yield increases drastically with increasing bulk temperature and exceeds at 2200 K the low temperature value by a factor of about 40. At this temperature the thermal sublimation yield is still more than one order of magnitude smaller than the sputtered one. The sputtered particles consist predominantly of C 1 with a fraction of C 2 and C 3 , which is much smaller than in the case of thermal sublimation. The results seem to be important for possible applications of graphite in fusion devices since structural parts of graphite may exceed the temperature of 1000 K.

The reaction of energetic O2+, thermal O2, and thermal O2/ar+ on graphite and the use of graphite for oxygen collector probes

The bombardment of graphite with energetic oxygen leads initially to a retention of oxygen within the graphite and subsequently to a reemission of CO and CO2, increasing continuously with further implantation. After saturation of the graphite surface layer with oxygen, the reemission of CO and CO2 reaches a steady state in which the implanted oxygen ion reacts to CO or CO2 resulting in a chemical erosion yield close to 1. Since physical sputtering occurs simultaneously, the total erosion yield for graphite is nearly 1 for low energy oxygen and greater than 1 for energies above 500 eV. n nBy a subsequent heating of the targets the retained oxygen is released completely in the form of CO and CO2. Pyrolytic graphite collects nearly all the implanted oxygen until a critical irradiation dose is reached beyond which the retention behaviour decreases. This critical dose depends on the oxygen energy and on the irradiation temperature. These results were applied in developing a new method for measuring the oxygen impurity flux in the scrape-off region of fusion machines as demonstrated by exposed probes in TEXTOR under various discharge conditions.

The enhanced sputtering yield of graphite at elevated temperatures: The energy of the released carbon atoms

Abstract The velocity distributions of carbon atoms released by argon ion bombardment of graphite have been measured using a time-of-flight method. As expected, at room temperature we observe a velocity distribution of the sputtered carbon atoms due to a collision cascade mechanism. At elevated temperatures, the velocity distributions cannot be interpreted by a collision cascade mechanism even though a low surface binding energy is assumed. They can be explained by thermal desorption with a Maxwell-Boltzmann distribution near the bulk temperature.

Properties of carbonization layers relevant to plasma-surface-interactions

Abstract Thin carbonaceous films have been prepared on samples of stainless steel, Inconel 600, Inconel 625 and silicon by carbonization from CH 4 -H 2 mixtures. They were investigated by transmission electron microscopy, nuclear reaction and backscattering techniques and thermal desorption. The films are semi-transparent, amorphous and homogeneous down to ~10 A. Their composition is H/C = 0.4 ± 0.1 and independent of the substrate. The density is 1.4 g cm −3 , the average C-C atomic distance ~2.5 A. The films release H 2 and about 3–10% CH 4 upon heating. The desorption of CH 4 occurs in a well defined peak around 500°C, whereas H 2 is relased up to 1100°C. The films turn black in colour after the CH 4 has been released, possibly due to a transformation from the initial amorphous into a graphitic structure.

Surface modification due to hydrogen-graphite interaction

Abstract Energetic hydrogen impact on graphite leads to the formation of a hydrogen-saturated graphite layer with a hydrogen concentration of about 0.4 H/C at room temperature. Observations on optical and electrical properties of these layers and on the nature of the carbon-hydrogen bonds are summarized. The chemical erosion of graphite due to interaction with hydrogen species is reviewed with respect to possible conclusions about the surface structure after hydrogen impact. New results on physical sputtering and chemical reactivity by thermal hydrogen attack on hydrogen-saturated graphite layers are reported. Finally, the properties of hydrogen-saturated graphite layers are compared with those of amorphous hydrogenated carbon films as obtained by carbonization of tokamaks.