High power mode conversion for linearly polarized HE[sub11] hybrid mode output.

Efficient plasma heating by ECR-wave irradiation requires axisymmetric, narrow, pencil-like millimetre wave beams with well-defined polarization. The linearly polarized gaussian-like HE[sub 11] mode satisfies these conditions best. This quasi-optical hybrid mode can be generated from TE[sub on] gyrotron mode compositions by the two multi-step mode conversion processes: (1) ΣTE[sub 0n] to TEo@ to TE[sub 11] to HE[sub 11] or (2) ΣTE[sub 0n] to TE[sub 01] to TM[sub 11] to HE[sub 11]. The first scheme has the advantage that the converters can all be made without bends, allowing an arbitrary choice and fast change of the polarization plane. The second scheme does not exhibit this advantage, but it is more suitable at very high frequencies (e.g. 140 GHz) because efficient TE[sub 0l]-to-TM[sub 11] transducers can be made considerably shorter than serpentine TE[sub 01]-to-TE[sub 11] mode converters. This paper presents computations on mode converter systems of the first type at 70GHz and of both types at 140GHz (ID = 27·8 mm for 200kW transmission lines). The structure of wall perturbations (phase-matched superposition of 2 or 3 different geometrical periods) in the rippled wall mode converters and the curvature distribution in the bent, smooth-walled TE[sub 01]-to-TM[sub 11] mode transducer were optimized by numerically solving the corresponding coupled-mode differential equations. Computer-aided optimization of circumferentially corrugated mode converters has been achieved with a scattering matrix code employing the modal field expansion technique (modular analysis concept (MAC)). In all cases the predicted overall efficiency of the complete mode converter system from ΣTE[sub 0n] (predominantly TE[sub 02] at 70GHz or TE[sub 03] at 140GHz) to HE[sub 11] in the desired polarization is approximately 95% at 70GHz and 92% at 140GHz (ohmic attenuation is included). Low-power measurements on the conversion efficiency of the various mode transdu... [ABSTRACT FROM AUTHOR]