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Experimental studies on the origin of metal impurities in DIVA are described. Three processes of metal impurity release, i.e. ion sputtering, evaporation and arcing, have been identified. Among these processes, ion sputtering is the dominant process in the quiet phase of the discharge, which is characterized by the lack of spikes in the loop voltage as well as by the fact that no heat flux concentration towards a specific part of the first wall takes place. Moreover, it is demonstrated that a honeycomb structure can decrease the metal impurity release.

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An experimental study of arcing is made, by making use of time-resolved optical and electrical measurements. Photomicrographs are made, and surface features of two parallel rail limiters are examined visually. Arc phenomena are accompanied by X-ray emission from the limiter. Many arc tracks are observed on the limiter surface which is exposed to both runaway electrons and the scrape-off plasma, but hardly any arc is observed on the limiter surface exposed to scrape-off plasma only. The most probable cause of unipolar arcing is the fact that runaway electrons supply a sheath potential sufficient to produce arc tracks on the electrically isolated target or limiter in the tokamak scrape-off layer. This model explains well the unipolar arcing observed in the non-equilibrium phases such as the current rise and end phases as well as during plasma disruptions.

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