Characterisation and formation of a-C: D layers below the divertor of ASDEX upgrade

Underneath the ASDEX Upgrade divertor two different types of amorphous hydrocarbon layers were found. Transparent soft layers with a typical ratio of D/C approximate to 1 and brownish hard layers with a ratio D/C approximate to 0.5. The amount of carbon deposited at the inner divertor is a factor approximate to 3 larger than at the outer divertor. Typical 0.3% of the total deuterium input during one experimental campaign are found in these layers in the divertor and only about 8 (.) 10(5) of the D input in the pumping ducts. From the deposition pattern it is concluded, that the layers are mainly build up by species with high sticking probability (0.1 < beta < 0.9). Neutral collisions and a parasitic plasma below the divertor are involved in the layer formation. Discharge resolved measurements of the layer deposition reveal a proportionality of the layer growth on the neutral density below the divertor.