The lower hybrid (LH) power deposition and the current drive (CD) efficiency were assessed by the application of modulated LH power. Density and magnetic field scans were performed and the response of the electron temperature provided by the available electron cyclotron emission diagnostic was investigated by means of fast Fourier transform analysis. An innovative technique based on a comparison between modelled and experimental data was developed and used in the study. The LH waves are absorbed by fast electrons with energies of a few times the thermal one, causing a modification in the electron distribution function (EDF) by creating a plateau in the parallel direction. The phase of the temperature perturbations, ϕ, as well as the ratio between the amplitudes of the third and the main harmonics, δTe3/δTe1, are found to be strongly affected by the plateau of the EDF as the broader the plateau the larger |ϕ|, (ϕ < 0), and the smaller δTe3/δTe1 are. Transport and Fokker–Planck modelling was used to support this conclusion as well as to interpret the experimental data and hence to assess the LHCD efficiency and deposition profile. The results from the analysis are consistent with broad off-axis LH power deposition profile. For densities between 1 × 1019 and 4 × 1019 m−3, which is the accessibility limit at the highest magnetic field discharges, a gradual shift of the maximum of the power deposition to the periphery and a degradation of the CD efficiency was observed.
Heating and current drive by electron cyclotron waves in JT-60U