High power mode conversion for linearly polarized HE11hybrid mode output

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Abstract
Efficient plasma heating by ECR-wave irradiation requires axisymmetric, narrow, pencil-like millimetre wave beams with well-defined polarization. The linearly polarized gaussian-like HE11, mode satisfies these conditions best. This quasi-optical hybrid mode can be generated from TE0n gyrotron mode compositions by the two multi-step mode conversion processes: (1) ΣTE0n to TE01 to TE11 to HE11 or (2) ΣTE0n to TE01 to TM11 to HE11. 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. 140GHz) because efficient TE01-to-TM11 transducers can be made considerably shorter than serpentine TE01-to-TE11 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 TE01-to-TM11 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 ΣTE0n (predominantly TE02 at 70GHz or TE03 at 140GHz) to HE11 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 transducers are in excellent agreement with the predicted values. High-power operation has been successfully demonstrated using a pulsed 70GHz gyrotron (200kW, 100ms).

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