Current drive and magnetic turbulence experiments with electron cyclotron and lower hybrid waves on the Versator-II tokamak

Abstract

Electron cyclotron (EC) waves were injected into partially lower hybrid 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 ⟨ne⟩ ≈ 8 × 1012 cm-3, the EC waves increased the plasma current and decreased the loop voltage. In contrast, for discharges with ⟨ne⟩ ⩽ 6 × 1012 cm-3, the EC waves decreased the plasma current and increased the loop voltage. This reduction in the plasma current by the EC waves at low density was generally accompanied by an increase in the limiter hard X ray emission and a clamping or reduction in the 70 GHz plasma emission, suggesting that confinement of the suprathermal current carrying electrons was degraded by the EC waves. This enhanced loss of the high-parallel-energy current-carrying LH-driven electron tail may have been caused by EC-wave-induced magnetic turbulence, which was observed to increase during EC wave injection by up to an order of magnitude in the frequency range of 50 to 400 kHz. The observed turbulence levels during EC wave injection and the average parallel energy of the current carrying electrons were higher for the lower density discharges, making the current carrying electrons more susceptible to loss caused by magnetic turbulence. Simple estimates show that the observed EC induced magnetic turbulence can explain the poor observed EC/LH current drive efficiency and its dependence on the thermal electron density.