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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). 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 boronised 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 the low temperature, high recycling divertor of a future reactor.

Additional information

Authors: KALLENBACH A, Max-Planck-Institut für Plasmaphysik, Garching bei München (DE);NEU R, Max-Planck-Institut für Plasmaphysik, Garching bei München (DE);POSCHENRIEDER W, Max-Planck-Institut für Plasmaphysik, Garching bei München (DE);ASDEX UPGRADE TEAM, Max-Planck-Institut für Plasmaphysik, Garching bei München (DE)
Bibliographic Reference: Article: Nuclear Fusion, Vol. 34 (1994) No. 12, pp. 1557-1565
Record Number: 199510218 / Last updated on: 1995-02-21
Original language: en
Available languages: en
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