Plasma-wall interactions represent an important aspect in operation of fusion devices. Hyperthermal plasma particles may collide with solid surfaces such as limiters or divertors eroding the material by chemical and physical processes. Earlier data on chemical erosion of graphic material under the effect of hydrogen discharge (simulating conditions of the ITER divertor) showed production of considerable amounts of methane. Importance of collisions of slow (1-50 eV) molecular, namely hydrocarbon ions with parts of divertor surfaces (carbon, tungsten) and at the same time lack of such data has been repeatedly emphasised. With this motivation, interaction of low energy ions with carbon surfaces was investigated in the J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences, Prague as a part of studies aimed at extending the manifold of atomic and molecular data for edge plasmas.

The following ions were under studies: CHn +(n=3-5), C2Hm +(m=2- 5), C3Hp + (p=3-8) and their D and 13C isotopic variants, incident energies 10- 50 eV. The survival probability of these ions on the surface ranged from 0.3% (CH4 +) to about 10% (CH5 +) for incident angle 30° with respect to the surface and decreased steeply with increasing incident angle (about 5-10 smaller for 50°).

The interaction of hydrocarbon ions (see box) striking a carbon surface (Highly Ordered Pyrolytic Graphite, samples of Tokamak tiles) under a preselected angle was investigated in beam scattering experiments using mass spectrometrictechniques. These experiments showed clearly that, at room temperature, the carbon surface was covered by a hydrocarbon layer (presumably backstreaming of cracked hydrocarbons from the vacuum system). Indeed, it is well known that the hydrocarbons form a stable surface coverage under most conditions, and are very difficult to avoid even in nonfusion high vacuum systems. If the studied surface was heated to above 600°C, the product ions were only undissociated projectile ions and their fragmentation products resulting from inelastic collisions with the carbon surface. A mass spectrum of product ions for collisions of the CD4 + projectile (energy 31.2 eV) on a heated surface is shown in the figure, upper part. If the surface was at room temperature, both fragmentation and chemical reactions of the projectile ion with the surface material were observed (see figure, lower part), which proves the presence of the hydrocarbon layer. The chemical reactions were (a) transfer of H-atom from the surface material, formation of a protonated projectile (CD4H+) and its fragmentation products (red peaks in the figure) (b) carbon-chain build up reaction (green and blue peaks) in an interaction of the projectile ion with the terminal CH3- groups of the surface material.

These results not only allow for more detailed understanding of the plasma-wall interaction, but also can motivate for diagnostics applications. For example, removal of the hydrocarbon layer by heating above 600°C could be sensitively checked by the above mentioned facile H-atom transfer reaction.

Article provided by: Dr. Zdenek Herman J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences,Prague
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