S. Lindig

Blister formation of tungsten due to ion bombardment

Blisters formed at tungsten surfaces due to deuterium ion bombardment have been studied systematically in the energy range 100 eV to 1 keV. The bombardment with I keV D+ at room temperature (RT) shows that the blister size increases and the number decreases with the deuterium fluence from 1 X 10(19) to 1 X 10(21) D+/cm(2). No blisters are found at elevated temperatures between 600 and 800 degreesC. For bombardment with an energy as low as 100 eV, blisters are observed at the high fluence of 1 x 10(21) D+ /cm(2). The blister size increases and the number decreases with the bombardment energy. Combined with scanning electron microscopy (SEM) ion beam depth profiling measurements have been used to investigate the effect of blister formation on the trapping behavior of deuterium in tungsten. Double implantations, where 4 keV He+ and 100 eV D+, respectively, were injected in W prior to the bombardment of I keV D+ show a pronounced increase of deuterium retention and blister disappearance. Possible mechanisms are proposed to describe the observed phenomena

Synthesis of diamond fine particles on levitated seed particles in a rf CH4/H2 plasma chamber equipped with a hot filament

The first successful growth of diamond layers on levitated seed particles in CH4/H2 plasma is presented. The particles were grown in a rf CH4/H2 plasma chamber equipped with a tungsten hot filament. The seed diamond particles injected in a plasma are negatively charged and levitated under the balance of several forces, and diamond chemical vapor deposition takes place on them. The SEM images show that the crystalline structures are formed after the coagulation of islands. The micro-Raman spectroscopy of the particle grown after several hours shows the clear peak assigned to diamond.

Subsurface morphology changes due to deuterium bombardment of tungsten

Recrystallized polycrystalline tungsten was exposed to a deuterium plasma beam with high flux (1022 D m−2 s−1) and low energy (38 eV D−1) to fluences up to 1027 D m−2. The sample temperature was varied between 320 and 800 K. The three-dimensional morphology of blister-like structures and the grain orientation were investigated by scanning electron microscopy combined with focused ion beam cross-sectioning and electron backscattering diffraction. Cracks with distorted areas ( 480 K) were observed beneath the surface. The surface blister-like structures and the defects underneath are correlated along crystallographic orientation of the W grains in accordance to the low-indexed slip system {110}111. The defects are mobile and accumulate under deuterium loading. Samples exposed near room temperature do not form such large cavities by subsequent heating up to 1300 K. Deuterium bombardment above 700 K does not lead to blister-like structures.

Surface morphology and deuterium retention in tungsten exposed to low-energy, high flux pure and helium-seeded deuterium plasmas

Blistering and deuterium retention in re-crystallized tungsten exposed to low-energy, high flux pure and helium-seeded D plasmas to a fluence of 1027?D?m?2 have been examined with scanning electron microscopy, thermal desorption spectroscopy, and the D(3He,p)4He nuclear reaction at 3He energies varied from 0.69 to 4.0?MeV. In the case of exposure to pure D plasma (38?eV?D?1), blisters with various shapes and sizes depending on the exposure temperature are found on the W surface. No blisters appear at temperatures above 700?K. The deuterium retention increases with the exposure temperature, reaching a maximum value of about 1022?D?m?2 at 480?K, and then decreases as the temperature rises further. Seeding of 76?eV He ions into the D plasma significantly reduces the D retention at elevated temperatures and prevents formation of the blisters.

Improvement of the thermo-mechanical properties of fine grain graphite by doping with different carbides

The possibilities for optimization of doped fine grain graphites with high thermal conductivity and high thermal shock resistance are demonstrated at laboratory scale. A mixture of MCMB powder and different carbides (B4C, TiC, VC, ZrC and WC) was used as starting material. VC acts as catalyst of the graphitization at the lowest temperature, and ZrC is the most effective catalyst of all investigated carbides. A direct proportionality between the mean crystallite height, Lc, and the thermal conductivity at room temperature was found for all materials except for the B4C- and the ZrC-doped graphites. With increasing graphitization temperature the open porosity of all doped materials becomes gradually closed, suggesting the existence of a diffusion mechanism responsible for both the catalytic effect and the closing of the open porosity. The addition of carbides does not strongly influence the mechanical properties of pure graphite. A high ratio flexural strength to Young’s modulus was achieved.

Deuterium inventory in the full-tungsten divertor of ASDEX Upgrade

The deuterium inventory in tungsten-coated divertor tiles used during the first full-tungsten plasma-facing wall phase of ASDEX Upgrade was measured by various methods of analysis. The D inventory in the inner divertor was still dominated by codeposition with residual carbon, whereas it was dominated by trapping in the thicker vacuum plasma sprayed tungsten layers at the outer divertor. The total inventory in the divertor area decreased by a factor of 5–10 compared with the period of carbon-dominated plasma-facing wall.

Development of tungsten coated first wall and high heat flux components for application in ASDEX Upgrade

In the tokamak experiment ASDEX Upgrade, the investigation of tungsten as a first wall material is an ongoing research project. In a step-by-step strategy, the tungsten covered surface area is increased from campaign to campaign. For this purpose an industrial-scale method for coating graphite with micrometer tungsten films had to be identified. Test coatings deposited by magnetron sputtering and by plasma-arc deposition were compared. By X-ray analysis it was found that sputter-deposited coatings suffer from high compressive stress (1.7 GPa). This leads to delamination when a film thickness of about 3 μm is exceeded. For arc-deposited coatings, a compressive stress value of 0.5 GPa was determined and no delamination occurred up to the maximum film thicknesses investigated, i.e. 10 μm. Upon thermal loading, none of the arc-deposited coatings failed up to the melting condition, while one sputter-coating delaminated. First results on similar investigations employing CFC substrates are presented.

Oxidation behaviour of silicon-free tungsten alloys for use as the first wall material

The use of self-passivating tungsten alloys as armour material of the first wall of a fusion power reactor may be advantageous concerning safety issues. In earlier studies good performance of the system W?Cr?Si was demonstrated. Thin films of such alloys showed a strongly reduced oxidation rate compared to pure tungsten. However, the formation of brittle tungsten silicides may be disadvantageous for the powder metallurgical production of bulk W?Cr?Si alloys if a good workability is needed. This paper shows the results of screening tests to identify suitable silicon-free alloys with distinguished self-passivation and a potentially good workability. Of all the tested systems W?Cr?Ti alloys showed the most promising results. The oxidation rate was even lower than the one of W?Cr?Si alloys, the reduction factor was about four orders of magnitude compared to pure tungsten. This performance could be conserved even if the content of alloying elements was reduced.

Impact of gyro-motion and sheath acceleration on the flux distribution on rough surfaces

As was already observed experimentally, the erosion of tungsten (W) coated graphite (C) tiles in ASDEX-Upgrade (AUG) exhibits regular erosion patterns on the micrometre rough surfaces whose origin is not fully understood: surfaces inclined towards the magnetic field direction show strong net W erosion while surfaces facing away from the magnetic field are shadowed from erosion and may even exhibit net W deposition. This paper presents a model which explains the observed erosion/deposition pattern. It is based on the calculation of ion trajectories dropping through the plasma sheath region to the rough surface with combined magnetic and electrical fields. The surface topography used in the calculations is taken from atomic force microscope measurement of real AUG tiles. The calculated erosion patterns are directly compared with secondary electron microscopy images of the erosion zones from the same location. The erosion on surfaces inclined towards the magnetic field is due to ions from the bulk plasma which enter the sheath gyrating along the magnetic field lines, while the deposition of W on surfaces facing away from the magnetic field is due to promptly re-deposited W that is ionized still within the magnetic pre-sheath.

Qualification of tungsten coatings on plasma-facing components for JET

This contribution summarizes the work that has been performed to establish the industrial production of tungsten coatings on carbon fibre composite (CFC) for application within the ITER-like Wall Project at JET. This comprises the investigation of vacuum plasma-sprayed coatings, physical vapour deposited tungsten/rhenium multilayers, as well as coatings deposited by combined magnetron-sputtering and ion implantation. A variety of analysis tools were applied to investigate failures and oxide and carbide formation in these systems.