Fast Wave Current Drive and Direct Electron Heating in JET ITB Plasmas
Fast wave current drive experiments have been performed in JET plasmas with electron internal transport barriers produced with LHCD. The central plasma current was difficult to affect, even though the calculated current drive efficiency was fairly high, 0.07A per W absorbed by the electrons. The main reasons are: the strongly inductive nature of the plasma current; the interplay between the fast wave driven current and the bootstrap current, which, due to the dependence of the bootstrap current on the poloidal magnetic field, decreases the bootstrap current as the driven current increases; and parasitic absorption of the waves that decreased the power absorbed by the electrons. The measured difference in the central current density for co and counter current drive is larger than the response expected from current diffusion calculations, suggesting a faster current diffusion than that given by neoclassical resistivity. Effective direct electron heating, comparable to the indirect electron heating with H-minority heating, is found for the dipole phasing of the antennas without producing a significant fast ion pressure and with low impurity content in the divertor plasma even though the single pass damping is only a few percent. For the ±90o phasings producing current drive, with a similar single pass damping, strong degradation of the heating is observed with strong increases in the BeII and CIV line intensities in the divertor. The degradation depends on the phasing of the antennas and increases with reduced single pass damping, consistent with RF-power being lost by dissipation of rectified RF-sheath potentials at the antennas and walls. Asymmetries in direct electron heating, lost power, production of impurities, fast ions and gamma-rays are seen for co and counter current drive that are consistent with differences in the absorption on residual 3He ions due to the RF-induced pinch.