High-power millimetre-wave mode converters in overmoded circular waveguides using periodic wall perturbations

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Abstract
This work reports on measurements and calculations (coupled mode equations) on the conversion of circular electric TE0n gyrotron mode compositions (TE01 to TE04) at 28 and 70 GHz to the linearly polarized TE11 mode by means of mode converter systems using periodic waveguide wall perturbations. Mode transducers with axisymmetric radius perturbations transform the TE0n gyrotron mode mixture to the more convenient TE01 mode for long-distance transmission through overmoded waveguides. Proper matching of the phase differences between the TE0n modes and of lengths and perturbation amplitudes of the several converter sections is required. A mode converter with constant diameter and periodically perturbed curvature transfers the unpolarized TE01 mode into the TE11 mode which produces an almost linearly polarized millimetre-wave beam needed for efficient electron cyclotron resonance heating (ECRH) of plasmas in thermonuclear fusion devices. The experimentally determined TE0n -to-TE01 conversion efficiency is (98 ± 1)% at 28 and 70 GHz (99% predicted) while the TE01-to-TE,, converter has a (96 ± 2)% conversion efficiency at 28 GHz (95% predicted) and (94 ± 2)% at 70 GHz (93% predicted) with ohmic losses included in each case. This paper also presents theoretical and experimental results on the two-step TE16-to-TE12-to-TE11 mode conversion at 28 GHz by means of two periodically rippled-wall mode converters. The conversion efficiencies achieved are almost 92% and 95%, respectively. Similar converters might be used for transformation of the output modes of future high-frequency TE1n gyromonotrons or 10 GHz gyro-klystron amplifiers into the TE11 mode, which in turn can then be transformed by circumferentially corrugated or dielectrically coated mode transducers into the perfectly linearly polarized quasi-optical HE11 hybrid mode. The efficiency of periodically modulated wall mode-converters can be considerably improved by proper re-matching of the phase difference between the two converted modes within the converter.

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