Modelling of local carbon deposition from methane and ethene injection through graphite and tungsten test limiters in TEXTOR

The 3D Monte Carlo code ERO has been used to simulate dedicated TEXTOR experiments in which methane (CH4)-C-13 and ethene (C2H4)-C-13 were injected into the plasma through graphite and tungsten spherical limiters to investigate local carbon transport and deposition. Spectroscopy was used to follow the observable hydrocarbon break-up products, and the local C-13 deposition efficiency was measured by post-mortem surface analysis. The experimental observations showing the dependence of the C-13 deposition on substrate and gas have been modelled. The main uncertain input parameters for the modelling, as C-12 concentration in the background plasma, effective sticking coefficient for hydrocarbons and enhanced erosion of redeposited carbon, have been defined by benchmarking between modelling and experiment. With the same set of parameters, the modelled C-13 deposition efficiencies are in good agreement with the experimental ones (0.8-2.1%). Also, the observed distribution of C-13 deposition on the limiter surfaces is well reproduced by modelling. The higher amount of C-13 deposition on graphite compared with tungsten limiter is mainly due to the higher reflection of carbon on tungsten and the higher physical sputtering of carbon film on top of the tungsten compared with graphite. The larger amount of C-13 deposition for (C2H4)-C-13 instead of (CH4)-C-13 injection can be explained by more C-13 returning to the limiter surface from (C2H4)-C-13 injection than (CH4)-C-13 injection, which is mainly determined by the different masses and reaction rate coefficients.