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 13CH4 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.