CURRENT DRIVE AND MAGNETIC TURBULENCE EXPERIMENTS WITH ELECTRON-CYCLOTRON AND LOWER-HYBRID WAVES ON THE VERSATOR-II TOKAMAK

Electron cyclotron (EC) waves were injected into partially lower hybri d current driven (LHCD) discharges on the Versator-II tokamak to test the theoretically predicted current drive synergism between EC and LH waves. For discharges with line averaged thermal electron density (n(e )) approximate to 8 x 10(12) cm(-3), the EC waves increased the plasma current and decreased the loop voltage. In contrast, for discharges w ith (n(e)) less than or equal to 6 x 10(12) cm(-3), the EC waves decre ased the plasma current and increased the loop voltage. This reduction in the plasma current by the EC waves at low density was generally ac companied by an increase in the limiter hard X ray emission and a clam ping or reduction in the 70 GHz plasma emission, suggesting that confi nement of the suprathermal current carrying electrons was degraded by the EC waves. This enhanced loss of the high-parallel-energy current-c arrying LK-driven electron tail may have been caused by EC-wave-induce d magnetic turbulence, which was observed to increase during EC wave i njection by up to an order of magnitude in the frequency range of 50 t o 400 kHz. The observed turbulence levels during EC wave injection and the average parallel energy of the current carrying electrons were hi gher for the lower density discharges, making the current carrying ele ctrons more susceptible to loss caused by magnetic turbulence. Simple estimates show that the observed EC induced magnetic turbulence can ex plain the poor observed EC/LH current drive efficiency and its depende nce on the thermal electron density.