Ion Bernstein wave antenna loading measurements on the DIII-D tokamak

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)

Analysis of the heating scenarios of the ion Bernstein wave (IBW) experiment in Frascati Tokamak Upgrade

“…A strong change of the antenna coupling due to an increased level of RF power was found in the IBW experiment on DIII-D [49]. For the low operating frequency used in this experiment (30-60 MHz), the typical ponderomotive potential was one order of magnitude higher than the plasma temperature in the scrape-off layer.…”

“…The analysis is performed following Ref. [49]. The parametric dispersion relation is solved for a plasma slab,…”

Observation of parametric decay correlated with edge heating using an ion Bernstein wave antenna on DIII-D

“…have been shown to be critically important in understanding the DIII-D IBW experimental results with respect to the antenna coupling [15] and the absorption mechanisms in the plasma edge, the latter area constituting the subject of the present paper. The non-linear effects have been shown to result in strong edge absorption which was consistent with the lack of observable core heating in these experiments 1111,141.…”

“…Typical SOL plasma parameters in the Ohmic heating phase of a discharge in which edge ion heating was observed were n,(edge) = 2 x 10" cm-3 and T, M Ti % 10 eV. Note that the SOL density was ahnost independent of the line averaged density in the range of discharges used for the IBW experiments, but was very dependent on whether the plasma is inside or outside limited (high edge density) or diverted (low edge density) [15]. of IBW power, no ion tail was observed, indicating a power threshold for ion tail formation.…”

“…A diagram of the antenna is given in Ref. [15]. Each current strap was 41 cm long, 16 cm wide and 1.3 cm thick.…”