Impact of gyro-motion and sheath acceleration on the flux distribution on rough surfaces

As was already observed experimentally, the erosion of tungsten (W) coated graphite (C) tiles in ASDEX-Upgrade exhibit regular erosion patterns on the m rough surfaces who’s origin is not fully understood: Surfaces inclined towards the magnetic field direction show strong net W erosion while surfaces facing away from the magnetic field are shadowed from erosion and may even exhibit net W deposition. This paper presents a model which explains the observed erosion/deposition pattern. It is based on the calculation of ion trajectories dropping through the plasma sheath region to the rough surface with combined magnetic and electrical fields. The surface topography used in the calculations is taken from AFM measurement of real ASDEX-Upgrade tiles. The calculated erosion patterns are directly compared to SEM images of the erosion zones from the same location. The erosion on surfaces inclined towards the magnetic field is due to ions from the bulk plasma which enter the sheath gyrating along the magnetic field lines, while the deposition of W on surfaces facing away from the magnetic field is due to promptly re-deposited W that is ionized still within the magnetic pre-sheath. ∗ Corresponding author ([email protected]) Impact of gyro-motion and sheath acceleration on the flux distribution on rough surfaces2