Investigation of erosion and deposition on wall components of TEXTOR-94

The paper describes in the first part the formation of carbon flakes up to 10-20 μm thickness (average growth rate 2 nm/s) on the graphite tiles of the toroidal belt limiter. This occurred as a consequence of a slight change of the geometry and turned parts of the surface area from net erosion into net deposition zones. The possible influence of the morphology on this behaviour is discussed in the second part by means of an erosion experiment where the gradual disappearance of a boron substrate could be discriminated from the simultaneous carbon deposition on the surface. The two counter-acting processes co-exist within 10-30 μm distance and lead to an extremely non-uniform carbon deposition even in net erosion zones. The carbon agglomeration coincides with surface imperfections, e.g. grooves, but agglomeration by temperature enhanced mobility is not excluded. The changeover from net deposition to net erosion averaged over larger distances can still be observed and is due to the hydrogen and carbon fluxes in the SOL. This is confirmed by Monte-Carlo code calculations. (orig.)