Determination of photon efficiencies and hydrocarbon influxes in the detached outer divertor plasma of ASDEX Upgrade

Hydrocarbon injection experiments with in situ calibration of spectroscopic signals have been carried out to determine the hydrocarbon flux and the chemical erosion yield under detached plasma conditions in ASDEX Upgrade. A plasma scenario was developed to detach the outer divertor in L-mode and to provide a cold (T-e similar or equal to 1-2 eV) and volume recombining deuterium target plasma. A significant decrease of carbon-related intrinsic photon fluxes, in particular of the CH Gero and of the C-2 Swan band was observed in detachment. Moderate methane/ethene injections were performed through a single gas valve and an increase of carbon-related intrinsic photon fluxes was detected, which can be attributed to the amount of injected molecules. The experimentally determined effective inverse photon efficiencies are slightly larger than the ones deduced in attached plasmas. Nevertheless, the low intrinsic CH and C-2 light observed under detached conditions can still be attributed to a reduction of the hydrocarbon flux of about one order of magnitude. The impact of detachment on the chemical erosion is discussed.