The origin of carbon and oxygen impurities observed in the bulk plasma of the ASDEX Upgrade tokamak is investigated by means of particle influx measurements using spectroscopy in the visible spectral range and impurity density measurements using spectroscopy in the soft X ray range. A number of ohmically heated discharges are evaluated and statistically analysed to reduce the influence of the individual experimental scatter. The interpretation of the spectroscopic measurements is supported by mass spectrometry, which allows the partial fluxes of the various molecular species to be resolved. The measured fluxes and concentrations reveal a consistent picture: for low electron densities and impure wall conditions, CO recycling in the main plasma chamber is the dominant source of carbon and oxygen, leading to similar concentrations of these species in the bulk plasma. At higher densities or with freshly boronized walls, the oxygen concentration is significantly reduced to 0.05 of the electron density. The corresponding reduction of carbon saturates at a level of about 0.3. This behaviour is attributed to chemical erosion due to hydrocarbon formation and desorption with a yield of a few per cent of the hydrogen flux. The resulting carbon ion flux is reduced owing to wall redeposition, but still amounts to about 0.45 of the hydrogen flux. This level imposes severe limitations on the use of graphite in a low temperature, high recycling divertor of a future reactor
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