Advanced Overmoded Waveguide Components for High-Power Millimetre Wave Applications at 28, 70 and 140 GHz

Improved oversized waveguide components of transmission lines developed for high-power (200 kW) millimetre wave applications at 28, 70 and 140 GHz, as e.g. electron cyclotron resonance heating (ECRH) of plasmas for thermonuclear fusion research with gyrotrons, are described. Axisymmetric narrow pencil beams with well-defined polarization (HE11 mode) are used at openended corrugated waveguide antennas. The HE11 mode can be generated from TE0n output modes of gyrotrons by the multi-step mode conversion processes: (1) TE0n¿TE01¿HE11¿HE11 or (2) TE0n¿TE01¿TM11¿HE11. This paper reprts computations and measurements on mode transducer systems of the first type at 28, 70 and 140 GHz and of the second type at 70 and 140 GHz. In all cases the overall efficiency of the complete mode converter sequence in the desired mode is approximately (91-94)%. The mode purity in the transmission lines is conserved by using optimimized corrugated waveguide bends and diameter tapers with non-linear contours. Highly efficient corrugated mode filters decouple the different waveguide sections.