Carbon layers in the divertor of ASDEX Upgrade

One of the main disadvantages of carbon as first wall material in a fusion device is the co-deposition of hydrogen with the eroded carbon. These layers will contain a significant amount of the tritium inventory of a fusion reactor. After venting brownish layers and flakes were found under the divertor structure of ASDEX Upgrade. First investigations were made on these flakes. Due to the complicated structure beyond the divertor, shadowing effects occur indicating that the brown layers are deposited by ionised particles. The flakes were analysed using SEM and ion beam techniques. Two different types of hydrocarbon layers were found: The brownish hydrogen poor layer (D/C = 0.4) and transparent hydrogen rich layer (D/C = 1) The total amount of carbon beyond the divertor could be estimated to 1.5 g, deposited in 3000 s of plasma discharges. First measurement of the layer growth using a quartz crystal microbalance instrument is presented.