Carbon erosion and a:C-H layer formation at ASDEX Upgrade

During the campaign 2002-2003 a carbon erosion and deposition experiment was performed in the lower divertor of ASDEX Upgrade. A complete divertor cross section with erosion sensitive markers, deposition monitors below the divertor and time resolved measurements by quartz mircobalance monitors (QMB) and Langmuir probes are used. The largest amount of carbon deposition was found at the inner divertor target plates. At the outer divertor, erosion was observed. However, the behaviour at the outer strike point zone is not jet fully understood. The deposition at remote areas is concentrated below the divertor. Neutrals seem to be dominant at the layer formation at remote areas. Additional erosion of deposited layers by ions reduces the layer thickness dramatically at some locations. QMB data show that at the outer divertor, deposition and erosion occur at the same location, depending on the main plasma properties. A parasitic plasma is observed below the divertor.