Spectroscopic observations of impurities in Tokapole II discharges

Tokapole II is a Tokamak with a four-node poloidal divertor. Impurity concentrations, line radiated power, and impurity behavior have been studied in Tokapole II discharges with two VUV spectrometers, one of which uses a microchannel plate to acquire the VUV spectrum (400-1300 AA) during a single discharge. Oxygen (3-5% of the electron density) is the dominant impurity, and the measured total radiated power is 15-30% of the ohmic input power. Sheath potentials and arcs evidently play a dominant role in the generation of the observed metallic impurities, because the 15-20 eV plasma ions have essentially no sputtering yield. An impurity doping technique has been used to measure the impurity concentrations and the contributions of the various low-Z impurities to the total radiated power. From the time history of the oxygen ions, T e is estimated to be about 100 eV. A model of impurity behavior in a plasma with a magnetic limiter is presented.