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 ¹³CH₄ and ethene ¹³C₂H₄ 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 ¹³C deposition efficiency was measured by post-mortem surface analysis. The experimental observations showing the dependence of the ¹³C deposition on substrate and gas have been modelled. The main uncertain input parameters for the modelling, as ¹²C 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 ¹³C deposition efficiencies are in good agreement with the experimental ones (0.8-2.1%). Also, the observed distribution of ¹³C deposition on the limiter surfaces is well reproduced by modelling. The higher amount of ¹³C 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 ¹³C deposition for ¹³C₂H₄ instead of ¹³CH ₄ injection can be explained by more ¹³C returning to the limiter surface from ¹³C₂H₄ injection than ¹³CH₄ injection, which is mainly determined by the different masses and reaction rate coefficients.