Characteristics of the divertor plasma in neutral-beam-heated ASDEX discharges

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Abstract
Scrape-off plasma parameters have been measured in ASDEX both in the divertor chamber and in the mid-plane, for neutral-injection powers of up to 2.5 MW. Measurements demonstrate the existence of strong inhomogeneities parallel to the field lines and show the dependence of divertor density and temperature on bulk plasma density, heating power and mode of divertor operation (gettered/ungettered, different divertor slit widths, additional gas puffing in the divertor). Three recycling regimes, characterized by different dependences of the divertor density on the bulk plasma density, are identified in agreement with a numerical fluid model of the scrape-off plasma. In the particularly attractive high-recycling regime, divertor temperatures could be kept around 7 eV for the whole range of heating powers tested.