During high-power operation in JET, a sudden and large increase in the flux of impurities (carbon and beryllium) entering the discharge is often observed shortly after the start of additional heating, especially at the low plasma density required for high ion temperature and fusion yield. This influx leads to a degradation of the plasma performance due to an increase of the impurity concentration and associated deuterium dilution, reduced beam penetration and a reduction in the ion temperature. There is good evidence that these influxes result from a small fraction of the total area of the carbon (beryllium) surfaces in contact with the plasma (tile edges particularly) reaching temperatures where radiation-enhanced sublimation and thermal sublimation for carbon and evaporation for beryllium become important. In this paper we report on measurements of the phenomena observed during inner-wall and carbon or beryllium belt limiter discharges both with and without beryllium gettering.
