Direct electron heating by 60 MHz fast waves on DIII-D

Efficient direct electron heating by fast waves has been observed on the DIII-D tokamak. A four strap antenna with (0,π,0,π) phasing launched up to 1.6 MW of fast wave power with |nparallel| ∼ 11. This |nparallel| is suitable for strong electron interaction in ohmic target plasmas (Te ≤ 2 keV). Ion cyclotron absorption was minimized by keeping the hydrogen fraction low (<3%) in deuterium discharges and by operating at high ion cyclotron harmonics (ω = 4ΩH = 8ΩD at 1T). The fast wave electron heating was weak for central electron temperatures below 1 keV, but improved substantially with increasing Te. Although linear theory predicts a strong inverse magnetic field scaling of the first pass absorption, the measured fast-wave heating efficiency was independent of magnetic field. Multiple pass absorption of the fast waves appears to be occurring since at 2.1 T nearly 100% efficient plasma heating is observed while the calculated first pass absorption is 6% to 8%. The central electron temperature during fast wave heating also increased with magnetic field. The improved electron heating at higher magnetic fields may be due in part to a peaking of the ohmic plasma current and the ohmic electron temperature profiles. Centrally peaked deposition profiles were measured by modulating the fast wave power at 10 Hz and observing the local electron temperature response across the plasma. 11 refs., 10 figs