Changes in impurity radiation during ICRF heating of PLT tokamak plasmas

This paper presents the results of a study of the increase in impurity line radiation in the 15-360 Å region during ICRF heating of PLT plasmas, with emphasis on metallic impurity (Ti and Fe) behaviour. Central titanium and iron densities are given for a variety of ICRF heating experiments; total central metallic impurity concentrations of up to about 0.3% of n_e(0) are observed at the 2.0 MW RF power level. This study shows that the power radiated by these elements is a small (about 10%) fraction of the total input power to the plasma for the present heating efficiency at 2.0 MW RF power. The impurity line brightnesses scale approximately linearly with RF power up to 2.8 MW. The antenna Faraday shields are shown to be the primary source of metallic impurities during both ICRF heating and Ohmic heating only. The impurity content of discharges heated using a single half-turn antenna and a pair of centre-fed antennas (having the same total surface area but half the poloidal extent of the half-turn antenna) is the same at a relatively low RF power of 350 kW, indicating that the impurity influx does not depend on the poloidal length of the antennas (or that the plasma interacts only with a localized area on the Faraday shields).