On the formation of a-C:D layers and parasitic plasmas underneath the roof baffle of the ASDEX Upgrade divertor

The key problem for the use of carbon in a future fusion device is the formation of tritium containing a-C:H layers. ASDEX Upgrade offers the possibility to investigate these layers at ITER relevant divertor conditions. Long term probes show, that the layer growth under the structure of the new divertor (Div IIb) is very similar to Div II. Additional quartz microbalance monitors offers measurements on the layers growth on a shot-to-shot base. The layers are found to grow continuously during the campaign. Using similar shots, a proportionality of the growth rate to the divertor neutral pressure is found. A Langmuir probe, installed below the divertor structure, measured a strongly variable plasma with electron densities up to 1×1018 m−3. The density of this parasitic plasma is correlated to the divertor radiation and neutral density, which point to photo ionisation or photo emission as the origin of the plasma.