Behaviour of light impurities in beam-heated JT-60 plasmas with hot graphite walls

The behaviour of light impurities in beam-heated JT-60 plasmas, after changing the limiter and armour tile material from TiC coated molybdenum and inconel to graphite, is discussed. In limiter discharges with high power neutral injection of greater than 12 MW and small gas puff rate, it is generally observed that the influx of carbon increases during the neutral beam phase. This phenomenon is delayed when the base temperature of the vacuum vessel is low. If the heat load to the wall is taken to be 6 MW/m2 then it is estimated that the increased carbon influx occurs at a wall temperature of 800 K. This temperature coincides with that for maximum chemical sputtering. The strongly rising influx of carbon is therefore attributed to chemical sputtering. (orig.)