B. Emmoth

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.

Determination of deuterium surface recombination rates on stainless steel

The recombination rate coefficient kr (molecules · cm2/atoms2 · s), is experimentally determined by measuring the release rate of deuterium to the gas phase, and the quasi-stationary deuterium surface concentration in a stainless steel membrane at elevated temperatures up to about 600°C. In addition, deuterium diffusion through the membrane is also studied with this technique. Results of deuterium recombination rates and diffusion coefficients are presented and compared with earlier measurements.

Wall conditioning of TEXTOR

Abstract We summarize the experience gained over two years on the wall conditioning of TEXTOR by radiofrequency assisted DC glow discharges (RG-discharges). The importance of the contamination history of the wall for the cleaning rate in H2 RG-discharges is underlined. RG-discharges in hydrogen already lead to some sputter erosion of the wall. This is strongly enhanced when small amounts of other gases, e.g. 1% of methane are added. The resulting material transport between liner and limiter is evaluated quantitatively from direct measurements of the energy and fluxes of the impingling species and measured by means of collection probes. The consequences for the case where the materials of limiter and liner differ are discussed. The plasmachemical deposition of carbon via H2/CH4 RG-discharges has been further examined. A homogeneous carbidic deposit results at elevated wall temperature when ~1% of CH4 is added to the hydrogen. Significant amounts of oxygen are removed in the form of CO even from a well-preconditioned wall. The oxygen concentration in the tokamak discharges which follow the carbidization is lowered by a factor of three whereas the carbon content is increased by roughly the same factor and chlorine as plasma impurity is almost completely suppressed. Except for this, the global behaviour of the confined plasma is almost unaffected. This result is compared to our earlier observations.

Deep penetration of deuterium in carbon based substrates exposed to the PISCES-A plasma

Carbon based substrates were exposed to the PISCES-A high flux deuterium plasma, i.e. pyrolytic graphite, a carbon fibre material, the same material coated with a layer of SiC, and a graphite composite containing 30% SiC. The sides of the samples facing the plasma were again analysed after 1½ years of storage. In both analyses, the same amount of retained deuterium was detected, except for the carbon fibre, for which a drop in the front surface concentration of about 35-40% was found. The bulk of the samples was investigated. To analyse the inner parts of the exposed materials, the samples were cleaved; before cleaving, they were covered on all sides by a layer of lacquer to avoid contamination of the new surfaces by deuterium (dust) from the outside of the samples. For carbon fibre, it was found that deuterium had penetrated several millimetres into the material; this is much deeper than what had been expected from previous analyses. In the pyrolytic graphite sample, deuterium penetrated to about 1 mm depth, but the profile was much steeper than that for carbon fibre. For carbon fibre covered by SiC (~ 1 μm layer), no deuterium was found inside the sample, indicating that the SiC layer was impermeable for deuterium. Also the graphite composite sample contained no deuterium.

A computer-controlled particle collection and surface analysis system for tokamak scrape-of-layer analysis

Abstract A microcomputer controlled collection probe system has been designed and is currently in use at the TEXTOR tokamak in Julich. The system enables a time and space resolved determination of the particle flux in five directions in the limiter shadow of the tokamak. Several series of measurements have been performed during the test of different limiter and heating experiments at TEXTOR, as well as for the evaluation of a wall cleaning program. A completing system for the surface analysis of the probes has been constructed at the Research Institute of Physics, Stockholm. By means of a 2 MeV Van de Graaff accelerator, Rutherford back scattering, nuclear reaction analysis and proton induced X-ray emission analysis of the probe surface is possible. With the help of a computer, programmed for automatic target scanning, data acquisition and storage, remote controlled analyses of exposed probes can now be done routinely with this new system. Recently, this analysis station has also been employed for samples exposed in the JET tokamak at Culham. We present here descriptions of the particle collection device as well as the automatic analysis system. Finally, a few examples of results from experiments with plasma exposed samples are presented.

Capture by aerogel—characterization of mobile dust in tokamak scrape-off layer plasmas

The aim of this letter is to demonstrate the feasibility and potential of the novel in situ dust diagnostic method-capture by aerogel targets. Aerogel, a highly porous material with a density of a ...

Impurity redeposition in the SOL of TEXTOR after boronization

Abstract The material redeposited on the collector probe in the SOL of TEXTOR after boronization has been identified as a-C/B : D, i.e. as almost identical with the virgin coating. A conversion factor of 6.3 × 1017 C/cm2 for 100 nm thickness was found which corresponds to a density ρ = 1.26 g/cm2. Erosion of C and B from the coated graphite limiters causes stationary deposition during the discharge and distributions within the SOL. After 200 discharges the ratio B/C= 0.23 is lower than the original (B/C= 1-0.5). Observations made with the collector probe confirm the reduced C and O contamination of the plasma due to boronization. Carbon rates have been determined shotwise after an integrated exposure by AES-depth profiling of the stratified deposit. A deposition rate of 1 × 1016 C/cm2s typically was found at a + 2 cm and ne = 2.8 × 1013cm−3. It increases at lower densities as do the other impurities.

Helium induced surface exfoliation of aluminum and the correlation between flake thickness and ion energy in the range 10 – 80 keV

Abstract Heavy surface erosion phenomena, such as exfoliation and blistering, have been studied by helium implantation of polycrystalline aluminum samples. Bombarding energies in the range 10 – 80 keV have been used, and SEM as well as RBS measurements have been carried out in order to determine the skin thickness of a flake as a function of the implantation energy. The results indicate that swelling is an important effect in regard to the physical thickness of a flake for the ion energies used in this work. Surface deformations have been measured for different target temperatures varying between 300 and 850 K, and it was found that large scale blistering occured at temperatures ⩾670 K. Irradiation fluences that exeeded the critical dose for exfoliation showed that this is not a transient phenomenon for doses up to 3 × 10 18 He / cm 2 .

Oxygen impurity analysis by collector probe measurements in the carbonized textor tokamak

Abstract Rutherford backscattering spectroscopy and nuclear reaction analysis techniques were used to determine the concentration of oxygen, metals and deuterium in the surface region of collector probes exposed to the plasma discharges in the carbonized TEXTOR fusion device. This paper focuses on time- and space-resolved measurements of oxygen collected during 13 tokamak shots on three cylindrical graphite probes placed near to the plasma boundary. Each probe was exposed to several discharges. Generally, the areal concentration of oxygen was found to be 15–25 times higher than that of metals and about 5–8 times lower than that of trapped deuterium. The influence of ICR heating, plasma density and other plasma parameters upon the deposition rate of oxygen and other species are discussed. The possible chemical state of the deposited oxygen is considered.

Deposition and in-depth penetration of deuterium in carbon fibre composites

Deposition, retention and in-depth penetration of deuterium was investigated in carbon fibre composites (CFC). Comparative studies were also performed for isotropic graphite and carbon-silicon (5 or 20% Si) composites. The materials were exposed to fluxes of deuterium in a hollow cathode device and in the TEXTOR tokamak. The irradiated targets were studied by a number of techniques in order to characterize their surface composition and structure. Nuclear reaction analysis, Rutherford backscattering spectroscopy, laser profilometry, crystallographic as well as microscopic methods were applied, and thermally and ion induced release were studied. The studies performed in several time steps revealed a systematic decrease of the deuterium content in the near surface layer (1.5 μm thick) of CFC substrates exposed to the hollow cathode plasma: from 6.8×10 18 cm −2 to 4.18×10 18 cm −2 and 3.76×10 18 cm −2 after 1, 81 and 112 days following the exposure, respectively. Moreover, in these samples a pronounced content of deuterium, even 60 times higher than the background level, was found beneath the exposed surface indicating deep penetration of D atoms through the layered structure of CFC. Thermally stimulated desorption at 1100°C resulted in the release of more than 99.9% of D atoms from the substrate. The disappearance of deuterium was accompanied by topographical changes of the surface layer as observed by scanning electron microscopy.