Enhanced lower hybrid penetration via intense multi-microsecond pulses

Applying lower-hybrid power in short, intense pulses can overcome Landau damping, allowing penetration into the core of reactor-grade plasmas. We present theoretical description of the absorption which accounts for transient collisional effects as well as nonlinear broadening of the resonant plateau. We show results from ray-tracing calculations which include the nonlinear absorption. We also derive the conditions required for pump depletion by parametric instabilities, and assess density depletion by ponderomotive effects, scattering by low-frequency background fluctuations, and filamentation. Consideration of all of the aforementioned effects as well as potential source availability and launcher requirements leads to the consideration of scenarios based on 5--10 GW 30--100 {mu}s pulses for the ITER Conceptual Design. Experimental tests of the concept can be done by launching waves with high enough parallel wavenumber that the resonant electrons are only moderately far out on the tail of the distribution function. The experiments could entail checking the predicted variation of the penetration with the duration and peak power of the pulses as well as the launcher area. We give sample experimental parameters for the Microwave Tokamak Experiment (MTX), Alcator C-Mod, Versator, and D3-D. 15 refs., 3 figs

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,{pi},0,{pi}) phasing launched up to 1.6 MW of fast wave power with {vert bar}n{sub {parallel}}{vert bar} {approx} 11. This {vert bar}n{sub {parallel}}{vert bar} is suitable for strong electron interaction in ohmic target plasmas (T{sub e} {le} 2 keV). Ion cyclotron absorption was minimized by keeping the hydrogen fraction low ({lt}3%) in deuterium discharges and by operating at high ion cyclotron harmonics ({omega} = 4{Omega}{sub H} = 8{Omega}{sub D} at 1T). The fast wave electron heating was weak for central electron temperatures below 1 keV, but improved substantially with increasing T{sub e}. 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