Absorption and current drive by electron cyclotron waves at upshifted and downshifted resonances in the lower hybrid current drive regime

The linear analysis of the electron cyclotron (EC) wave absorption and current drive in the lower hybrid (LH) current drive regime is performed. To evaluate the absorbed power, the full relativistic dispersion relation is solved using a model function determined by the balance between the collisional and the quasilinear diffusion induced by the LH waves. The current density driven by the EC wave is computed by means of the adjoint method. The absorbed power, the current drive efficiency, and the driven current density are investigated as a function of the propagation angle and of the ratio Y between the electron cyclotron frequency and the frequency of the EC wave. The analysis is centered on the comparison between the downshifted (Y>1) and upshifted (Y<1) resonant interaction with the suprathermal tail. The general features of the results can be explained by means of the investigation of the location of the EC resonant curve in momentum space. The largest absorption and current drive efficiencies are found in the upshifted resonance condition, when the EC wave propagation is directed toward the suprathermal tail.