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 μs 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