Surface composition and topography of the graphite limiter in ASDEX

After having been in use during about 150 discharges the graphite limiter of the ASDEX tokamak was removed and analyzed by means of SEM, EIXE, FIXE, RBS and Mössbauer Spectroscopy. The surface was seen to be covered by a nonuniform layer of 10¹⁶-10¹⁸ metal at/cm², primarily in the form of droplets with diameters up to a few μm. Simulation experiments suggest that the formation of these droplets is caused by heating. The relative metal concentrations in the droplets are close to those for stainless steel. The Mössbauer spectroscopy analysis indicates, however, that the major part of the metal is present in carbide phases, most probably (Fe, Cr, Ni)₃C. On parts of the limiter the metal coverage shows a sharp maximum at the points nearest the main plasma, flanked by two minima. The minimum on the ion drift side is much broader than the one on the electron drift side. However, these features are not common to all parts of the limiter.