Determination of Hydrocarbon Impurity Influxes, Photon Efficiencies and Chemical Sputtering Yield in ASDEX Upgrade

The aim of this work is to present recent results of photon efficiency (D/XB) measurements and chemical erosion yields of hydrocarbons in the region of the outer divertor of ASDEX Upgrade. In order to study the influence of plasma edge parameters on the photon efficiencies and therefore on the evaluation of the erosion yield, different hydrocarbons were injected in a localized region of the outer divertor. The emission of radiative species was studied via recently installed new lines of sight which are centred on the gas injector. The photon efficiency D/XB (CH or CD bands) of methane is determined first, via the calibrated gas flux injected during an experiment. These values are compared with a second technique based on evaluation of the photon efficiencies from the spectroscopic measurements of photon flux densities (H gamma or D gamma) which, in their turn, are compared with ion flux densities derived from Langinuir probe measurements. The difference observed between these two methods is related to the fraction of hydrogen that is released as neutral after molecular break-up. The measured fraction is discussed in terms of possible molecular break-up reactions. In a similar way by introducing CH4 or CD4, a dependence of D/XB on the hydrogen mass is observed. The results show an increase of the photon efficiency by a factor of 1.3 to 1.8, in case of CD4 compared to CH4, resulting in an increase of the isotope-effect on the erosion yield derived from spectroscopic measurements. Erosion yields of 1-4% are obtained for the low ion flux density range (Gamma (i) < 10(22) m(-2) s(-1)).