Coupling structures for lower hybrid current drive experiments have, until now, been smaller than a free space wavelength and have had a correspondingly broad wavenumber spectrum. The paper reports the results of experiments on the PLT tokamak using a 16-waveguide grill (2.2 wavelengths) which produces a very narrow n∥ = k∥c/ω spectrum. Experimental results from the 16-waveguide grill are compared with results from three other PLT grills with less sharply defined n1 spectra. The current drive figure of merit, , is approximately 40% higher for the experiments with the 16-waveguide coupler than for previously reported experiments on PLT, in spite of the larger 'spectral gap'. The experimental results are consistent with the first-pass damping of a large fraction of the launched spectrum.
Waveguide launching of lower hybrid waves