Spectroscopic investigation on the impurity influxes of carbon and silicon in the ASDEX upgrade experiment

Emission profiles of carbon and silicon along the heat shield (HS) were measured before and after siliconization of the vessel. Siliconization does not affect the divertor and therefore large changes in impurity concentrations and influxes cannot be attributed to it. A strong transient increase of the silicon concentration was observed immediately after the siliconization lasting for a few discharges. However, no large change was observed in the silicon influx from the HS, excluding it as possible source of the concentration increase. The fast decrease in the silicon concentration can be attributed to strong erosion at the outer limiter. The impurity influxes for C2+ and Si2+ from the HS are estimated and compared with the corresponding core concentrations. The relative contribution to the plasma impurity content from divertor, HS and outer protection limiter is discussed. Zeeman splitting analysis permits one to identify the emitting region of carbon radiation. Anomalously high silicon sputtering yields are measured, which do not however cause high core silicon content.