Abstract
Antenna loading measurements carried out during high power ion Bernstein wave (IBW) heating experiments on the DIII-D tokamak indicate that efficient, direct coupling to the IBW at ω ≲ 2ωci as predicted by linear coupling theory did not occur. Whereas strong peaking in the loading resistance near cyclotron harmonics is predicted for high edge densities (ω < ωLH|edge), the observed loading resistance is nearly independent of the toroidal magnetic field. The loading anomaly can be explained by invoking the ponderomotive force which decreases the edge density immediately in front of the antenna, thus allowing coupling to the cold plasma lower hybrid wave (LHW). A linear LHW coupling code including the modified density profile due to the ponderomotive force reproduces the measured dependence of antenna loading on toroidal magnetic field, edge density, antenna frequency and antenna phasing. Further evidence for the ponderomotive force is obtained from reactive loading measurements which indicate that the plasma is pushed away from the antenna as the radiofrequency power level is increased. The results indicate that the lack of central ion heating observed during DIII-D IBW experiments may be due to a lack of efficient mode transformation from the coupled LHW to a centrally propagating IBW, possibly as a result of nonlinear mechanism(s)
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