High-power TE11 and TM11 circular polarizers in oversized waveguides at 70 GHz

Elliptical deformation of oversized, smooth-wall circular waveguides can produce choosable elliptical or circular polarization from a linearly polarized TE11 or TM11 mode used as intermediate linearly polarized modes in TEO1 to HE11 mode conversion sequences in electron cyclotron resonance heating (ECRH) of magnetically confined thermonuclear fusion plasmas with high-power gyrotrons. Mode coupling in elliptically distorted overmoded circular waveguides has been studied theoretically and experimentally in order to optimize TE11 (and TM11) polarizers (I.D.=27.79 mm) for the 1 MW/70 GHz long-pulse (3s) ECRH system on the Garching Stellarator W VII-AS. Coupling coefficients for ellipticity coupling of non-degenerate modes are given (coupled-mode differential equations formalism). The polarization converters essentially consist of smooth-wall circular waveguides which are gradually squeezed. A sine-squared function of the length coordinate is used to get an almost elliptical crosssection in the middle and circular cross sections at both ends. Arbitrary elliptical polarization states can be generated introducing an extremely low level (<<1%) of undesired spurious modes. Well defined differential phase characteristics have been achieved.     

TE03 to TE01 mode converters for use with a 150 GHz gyrotron

Two-step high-power TE03-to-TE01 mode transducers (TE02 is the intermediate mode) were designed to transform the TE03 output mode of the 150 GHz KfK gyrotron into the fundamental circular symmetric TE01 mode for low-loss propagation and further conversion into the linearly polarized quasioptical HE11 mode. The advanced perturbation structures of the ruppled-wall mode converters (with input- and output-diameter D=27.8 mm) were optimized by numerically solving the proper coupled-wave differential equations. The overall TE03-to-TE01 conversion efficiency at 150 GHz was calculated to be ₀=99.1% for a long converter (total length L=2.67 m) and ₀=97.8% for a short converter (L=1.07 m); ohmic attenuation is included. The corresponding theoretical bandwidth (for 95%) is ±1.4 GHz and ±2 GHz, respectively. Low-power experimental data are in excellent agreement with these computed values.     

Computer-aided analysis and design of corrugated TE11 to HE11 mode converters in highly overmoded waveguides

A theoretical and experimental study of TE11-to-HE11 mode conversion in highly oversized waveguides is given. The balanced HE11 hybrid mode propagating in a circumferentially corrugated circular waveguide is in many respects ideal for effective electron cyclotron resonance heating of fusion plasmas with high-power gyrotrons. At an aperture antenna it exhibits desirable radiation characteristics such as linear polarization with very low cross polarization and axisymmetric power distribution with low sidelobe levels. This Gaussian-like mode can be generated from a TE11 mode by means of a corrugated mode converter in which the depth of the annular slots is tapered gradually from one-half to one-quarter wavelength. Computer-aided optimization of converter length, shape of corrugations, and of non-linear slot depth variation for 28 GHz- and 70 GHz-transducers was achieved with a scattering matrix formalism employing the modal field expansion technique (Modular Analysis Concept). In both cases the measured conversion efficiency of (98.5±0.5)% and the experimental values of the maximum cross polarization (below –28 dB) are in very good agreement with the predicted values. High-power operation was successfully demonstrated using pulsed 28 GHz- and 70 GHz-VARIAN gyrotrons (200 kW).     

Generation of the Gaussian-like HE11 mode from gyrotron TEOn mode mixtures at 70 GHz

This work reports calculations and measurements on mode converters for the transformation of TEOn gyrotron mode mixtures (primarily TEO2) into the linearly polarized HE11 hybrid mode at 70 GHz. This mode is ideal for quasi-optical launching systems for ECRH of plasmas. Mode transducers with axisymmetric radius perturbations convert the gyrotron TEOn mode composition into the TEO1 mode. Proper matching of the phase differences between the various modes and of the perturbation amplitudes of the several converter sections is required. The TEO1 wave is used for long-distance transmission through smooth-walled overmoded waveguides. A mode converter with constant diameter and periodically perturbed curvature transfers the unpolarized TEO1 mode into the polarized TE11 wave. The experimentally determined TEOn-to-TEO1 conversion efficiency is (98.5±1)% (99% predicted) while the TEO1-to-TE11 transformer has a (94±2)% efficiency (93% theoretically); ohmic losses are included. The Gaussian-like HE11 mode with axisymmetric power distribution and almost no cross polarization is produced in a circumferentially corrugated TE11-to-HE11 mode transducer with a measured conversion efficiency of (98.3±1.5)% (98.5% predicted). The overall efficiency of the complete mode converter system in the desired mode was determined to be (91±2.5)%. High-power operation (200kW, 100ms) has been successfully demonstrated.     

高功率毫米波圆波导TM01—TE11模式变换分析

 在数值研究高功率毫米波TM₀₁—TE₁₁光滑弯曲圆波导模式变换的基础上，对弯曲波导的几何结构进行了优化分析。在模式耦合理论的基础上，优化出了比较好的几何结构，并发现了圆波导半径、弯曲圆波导曲率和频率以及频带宽度之间的变化规律。以此数据设计的模式变换器的转换效率可达99%，带宽超过29.8%。     

8mm高功率过模弯曲圆波导TE01—TM11模式变换

在模式耦合理论的基础上,采用传统的波导轴线常弯曲和改进的波导轴线正弦弯曲方法,对TE01—TM11模式变换器的几何结构进行优化分析,得到了最优几何参量.以波导轴线正弦弯曲结构设计的8mm高功率圆波导TE01—TM11模式变换器的转换效率达到99%,带宽超过32%.同时还研究了蛇形线圆波导TE01—TE11模式变换器 关键词： 圆波导 弯曲波导 模式变换 耦合波方程