Chemical Erosion in TEXTOR-94

In order to study the fluxes and the spatial distribution of hydrocarbons, which are formed by plasma interaction on limiters and walls of TEXTOR, both spectroscopic and mass analysis measurements have been performed. Spectroscopic diagnostic stations on specially designed limiter locks, where an easy replacement of limiters with different materials is possible, allowed the determination of the hydrocarbon production from carbon, silicon and titanium doped (SiC30, RGTi) graphites, B4C as well as carbon coated metal limiters of copper, stainless steel, molybdenum, and tungsten. In addition a special carbon limiter was available, which could externally be heated up to temperatures of 1400 K and, therefore, allowed measurements on a homogeneously hot surface. It was found that the methane formation remained approximately constant up to 1050 K with a yield of about 3.5% and dropped well below 1% at 1350 K. Simultaneously the carbon ion fluxes into the plasma decreased by about 50% for ohmically heated plasmas. The fluxes onto the limiters could be increased by an order of magnitude by inserting the limiter deeper into the plasma, which resulted in a drop of the maximum methane production yield to 1% at 3 1023/(m2 s). Bulk doped graphites as well as Cu- and SS-limiters show a similar methane production behaviour as pure graphites, whereas for Mo- and W-limiters the hydrocarbon formation rate is low. A pronounced isotope effect could not be detected. Photon efficiencies for the CH/D-band emission were calculated with a local erosion and deposition model (ERO-TEXTOR) taking into account the measured plasma boundary parameters and the atomic data from the Langer model. These calculations justify the application of the factors used for the conversion of measured intensities into CH/D4 particle fluxes for TEXTOR limiter conditions.