Characterisation of deposited hydrocarbon layers below the divertor and in the pumping ducts of ASDEX Upgrade

The growth of codeposited layers has been studied with long term samples below the divertor IIb and in a pump duct of ASDEX Upgrade from March to August 2001. The composition of redeposited layers and their optical properties were analyzed with ion beam techniques and ellipsometry. The deposition in the sub-divertor area showed a complicated deposition pattern with a maximum deposition of about 1.3 μm. All deposits form soft hydrocarbon layers which consist mainly of deuterium and carbon with D/C from 0.7 to 1.4. Only a small deposition was observed in the pump duct, with a maximum of about 2.5×1015 D-atoms/cm2 at the duct entrance. The observed deposition pattern in the duct is compared with simulation calculations assuming neutral hydrocarbon radicals as precursors for film deposition. The deposition pattern can be explained by two different radical species with surface loss probabilities β<10−3 and 0.1⩽β⩽0.9. The most likely species are CD3, C2D5 and C2D3 radicals.