Long-term erosion and deposition studies of the main graphite limiter in TEXTOR

Erosion and deposition behaviour of the TEXTOR main toroidal belt limiter ALT-II has been studied for two generations of limiter tiles with different surface shapes. Two specially prepared tiles with marker holes were subsequently exposed during experimental campaigns with a total plasma duration of >7000 s and a surface averaged background ion fluence exceeding 2×1025 m−2. The tile surface relief was measured by optical profilometry and SIMS before and after the exposure. The highest surface erosion of >10 μm on both tiles was observed in regions with an angle of incidence of the magnetic field α of gsim1°, whereas in the regions with α~0.1° both net-erosion and net-deposition were observed depending on the tile shaping. The marker holes were always deposition dominated. Modelling by the ERO code explains qualitatively the surface erosion and deposition behaviour in terms of background flux dilution for shallow angles of incidence.