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Abstract:
A numerical study of the focused hog-horn antenna used for the W-band channel of the ASDEX Upgrade frequency modulated continuous wave reflectometry system is performed here. A mode matching code is used to accuretely model wave propagation in waveguide and antenna structures. Then the field distribution (amplitude and phase) in the horn aperture given by this code is used as an input for other codes computing wave propagation in the plasma. Evaluating the received energy in the antenna, we first analyze plasma effects as the nonalignment between the antenna and the plasma equatorial plane. We show in the case of an unfocused antenna that ray-tracing results are in qualitative agreement with full wave calculations. Simulations also confirm that hog-horn antennas allow improving the measurement performances in comparison with unfocused antennas. The role of location of the focusing point of a hog-horn antenna (depending on the antenna design) is then studied. It appears that the focusing point position should be optimized to maximize the energy received in the antenna. Finally, we show that a new design for the W-band antenna on the ASDEX Upgrade broadband reflectometry system could improve the performances of this diagnostic.
