Modelling of surface roughness effects on impurity erosion and deposition in TEXTOR with a code package SURO/ERO/SDPIC

The roughness-induced uneven erosion-deposition behaviour is widely observed on plasma-wetted surfaces in tokamaks. The three-dimensional (3D) angular distribution of background plasma and impurities is expected to have an impact on the local erosion-deposition characteristic on rough surfaces. The investigations of C-13 deposition on rough surfaces in TEXTOR experiments have been re-visited by 3D treatment of surface morphology to evaluate the effect of 3D angular distribution and its connection with surface topography by the code package SURO/ERO/SDPIC. The simulation results show that the erosion/deposition patterns and evolution of surface topography are strongly affected by the azimuthal direction of incident flux. A reduced aspect ratio of rough surface leads to an increase in C-13 deposition due to the enhanced trapping ability at surface recessions. The shadowing effect of rough surface has been revealed based on the relationship between 3D incident direction and surface topography properties. The more realistic surface structures used by 3D SURO can well reproduce the experimental results of the increase in the C-13 deposition efficiency by a factor of 3-5 on a rough surface compared with a smooth one. The influence of sheath electric field on the local impact angle and resulting C-13 deposition has been studied, which indicates that the difference in C-13 deposition caused by sheath electric field can be alleviated by the use of more realistic surface structures. The difference in C-13 deposition on smooth graphite and tungsten substrates has been specified by consideration of effects of kinetic reflection, enhanced physical sputtering and nucleation.