Efficiency of lower hybrid current drive at 2.45 GHz in ASDEX

The experimental lower hybrid current drive efficiency at zero electric field is found to agree with theory if the spectrum of the lower hybrid waves is fully accessible to the central region of the plasma. For broad or very low parallel wave number spectra, N∥, for which the plane wave accessibility condition is not fully satisfied, the observed efficiency is higher than predicted. At non-zero electric field, the measurements agree with theories for an electric field dependent efficiency. The model can also be applied to experiments in which a symmetric spectrum of waves is launched, or to cases where the current is driven opposite to the DC electric field if runaway electrons can be neglected. A shift of the deposition zone by superposition of a low N∥ current drive spectrum and a high N∥ symmetric spectrum does not change the current drive efficiency.