Characterization of radiative losses in TdeV during the impurity control experiments

The total radiated power has been measured under different operating conditions in TdeV. Radiative losses from the bulk plasma in standard discharges, which amount to about 45% of the ohmic input power in the limiter mode of operation and about 25% in the divertor mode of operation, could be reduced by up to 70% by boronizing the chamber walls. In limiter mode, the boronization alters the radiation profile from a flat profile to a hollow one. This change results from elimination of the metallic impurities by masking the exposed metallic surfaces in the chamber with boron. In experiments conducted to control the plasma impurity content by plasma biasing, negative (or positive) biasing can decrease (or increase) the radiated power from the bulk plasma by up to 25% (40%) within the biasing range of about -200V to +300V. This effect is believed to have resulted from the influence of plasma biasing on the particle transport in the scrape-off layer, which in turn alters the impurity source. The observed trend in variations of the impurity content in the main plasma, which is the main source for the radiated power, is reproduced, using a simple 1D transport model.