Impurity generation during intense lower hybrid heating experiments on the Alcator C tokamak

Experiments are underway on the Alcator C tokamak with over 1 MW of RF power injected into the plasma at a frequency of 4.6 GHz to study both heating and current drive effects. During these studies, impurity generation from limiter structures has been observed. The RF induced impurity influx is a strongly nonlinear function of net injected power. For Psub(RF) < 500 kW, only small effects are seen. As Psub(RF) approaches 1 MW, however, sharp increases in impurity influxes and Zsub(eff) are observed. Three different limiter materials have been used during these studies: molybdenum, graphite, and silicon-carbide coated graphite. In each case, the materials of the limiter structure are seen to dominate the increased impurity influx. In a typical case, with Psub(RF) = 1.0 MW, anti nsub(e) = 1.3 x 1014 cm-3, and the SiC coated limiters, Zsub(eff) is seen to increase from 1.5 before the RF pulse to about 4 during the heating. At the same time, central Tsub(e) increases from 2000 to 3000 eV and central Tsub(i) from 1200 to 1800 eV. Similar effects are seen in both H2 and D2 working gas discharges. The contribution to impurity generation of nonthermal electrons, which are produced by the RF, is under investigation. Changes in edge plasma temperature and density, as well as the possibility that the particle transport is affected by the RF, are also being examined. Results of the experiments with the three different limiter materials are compared, and contributions of impurity radiation to the overall power balance are estimated. (orig.)