An improved design for quasi-optical mode conversion of whispering gallery mode gyrotron radiation

Results are reported of a theoretical and experimental investigation of a quasi-optical mode converter for the transformation of whispering gallery mode gyrotron output into a linearly polarized Gaussian like beam. The mode converter consists of a helically cut waveguide launcher, similar to that originally proposed by Vlasovet al, followed by a focusing mirror. Theoretical results using aperture field methods indicate that the length of the waveguide launcher is of critical importance in providing a confined radiation pattern. Experimental results on the radiation pattern were obtained for several launcher lengths using a 0.6 MW, 149 GHz pulsed gyrotron operating in the TE_16,2 mode. Radiation pattern results for the optimum launcher length agree well with theoretical calculations using the Stratton-Chu aperture radiation theory for unperturbed waveguide modes. A mirror focusing in the azimuthal direction was designed by a geometrical optics approach to focus the radiation coming from the launcher. Good focusing with 91.4% efficiency (power in the focused beam divided by gyrotron power) was found experimentally using the combined launcher and mirror with the pulsed gyrotron. These results indicate that quasi-optical antennas are useful for transforming high order, high frequency gyrotron modes into directed beams in free space.     

Calculation of radiation from a quasi-optical reflector antenna for whispering gallery mode

We propose a method to calculate the radiation characteristics of a quasi-optical reflector antenna for whispering gallery mode called Vlasov's antenna. Correction of the shape of the reflector, which was a parabolic cylinder in the original paper by Vlasov et al. (1975), is performed using wave-normal rays. The radiation field is calculated by means of an image source which is obtained with the geometrical optics. Some numerical results show that the radiated beam is gaussian-like in far zone. A scheme of high-power application (500kW, 106·4GHz, TE_12,2) to a beam transmission system is also presented     

回旋管准光辐射器研究与设计

准光辐射器是高功率回旋管准光模式变换器的重要组成部分.本文首先采用几何光学理论分析圆波导准光辐射器,然后根据高斯波束(TEM00模)在辐射器切口处形成的原因,采用耦合波理论设计波纹波导准光辐射器.通过编写程序并进行数值优化完成140GHz,TE28,8模式回旋管波纹波导准光辐射器的设计,结果表明波纹波导辐射器的总长度仅为205.2mm,切口长度为47.2mm,在辐射器螺旋切口Brillouin区内高斯模式标量相关系数大于98%.本文所用的方法也可用于其它频率和模式的回旋管准光辐射器设计.     

High Efficiency Mode Converter for Low-Frequency Gyrotron

A high efficiency quasi-optical (QO) mode converter for high-power, low-frequency gyrotron have been designed and tested. For low-frequency gyrotrons, the scales of the mode converter are comparatively small on the wavelength scale, thus causing significant diffraction losses. Over-1 MW power gyrotron with TE_8,3 cavity at 28 GHz have been developed, which has a high efficiency mode converter designed by the use of numerical methods for launcher optimization. This calculation is sufficiently optimized to maximize the fractional Gaussian content of the far field. The total transmission efficiency from the mode converter to output window is 94.7%. For the experimental result of first tube, the output power of more than 1 MW has been obtained with about 40% efficiency and output burn pattern agrees fairly with the calculated profiles, which imply the design appropriateness. Besides, the frequency dependence for diffraction loss is discussed, and these results give the guiding design principle of the mode converter for high-power, low-frequency and long-pulse gyrotrons.     

三反射镜准光模式变换器的设计与模拟

本文设计了一种用于W 波段边廊模回旋振荡管的三反射镜准光模式变换器。该模式变换器由一个Vlasov 型螺旋 切口辐射器和三面聚焦反射镜构成,可以将回旋管输出的高阶模式高效地转换为准高斯模式。根据矢量绕射理论和物理 光学法编写了模拟仿真程序,并对辐射器和反射镜的空间辐射场进行了计算、优化和分析。计算结果表明：输出波束中 高斯基模的标量相关系数为97.9%,矢量相关系数为95.9%,转换效率达到了81.5%。     

太赫兹宽带Denisov型准光模式变换器的设计分析

研究并设计了一种具有宽频带工作能力的太赫兹准光模式变换器。该准光模式变换器采用具有高效率特性的Denisov 辐射器,工作在TE_6,2模式,用于实现回旋管内的模式变换。由于Denisov 辐射器的参数是影响准光模式变换器宽带性能的主要因素,因此通过对辐射器参数的优化设计,达到增大模式变换器带宽的效果。使用自主开发的准光学模拟程序进行仿真,模式变换器中心频率为94 GHz,带宽达2 GHz。     

Investigation of a Broadband Quasi-Optical Mode Converter for a Multi-Frequency 1 MW Gyrotron

A broadband quasi-optical (QO) mode converter for a multi-frequency gyrotron has been designed and tested at Forschungszentrum Karlsruhe (FZK). The launcher is optimized for the TE_22,8 mode at 140 GHz, but the radiated beams present an almost identically focused pattern for all 9 considered modes between 105 GHz (TE_17,6) and 143 GHz (TE_23,8). Combining with a beam-forming mirror system, which consists of a quasi-elliptical mirror and two phase-correcting mirrors with non-quadratic surface contour, further calculations show that efficiencies of more than 94% have been achieved for converting the rotating high-order cylindrical cavity modes into the usable fundamental Gaussian mode. Low power (cold) measurements show a good agreement with theoretical predictions. This QO mode converter can be used for the broadband operation of a multi-frequency 1 MW gyrotron.     

Experimental Results on a 1.5 MW, 110 GHz Gyrotron with a Smooth Mirror Mode Converter

We present an internal mode converter (IMC) design for a 1.5 MW, 110 GHz gyrotron operating in the TE_22,6 mode. The launcher, designed using the codes Surf3d and LOT, converts the cavity waveguide mode into a nearly pure Gaussian beam. The Gaussian beam output from the launcher is shaped by a series of 4 smooth, curved mirrors to provide a circular output beam with a flat phase front at the gyrotron window. By employing smooth mirrors rather than mirrors with phase correcting surfaces, such an IMC is less sensitive to alignment issues and can more reliably operate with high efficiency. The IMC performance was verified by both cold test and hot test experiments. Beam pattern measurements in each case were in good agreement with theoretical predictions. The output beam was of high quality with calculations showing that the Gaussian Beam content was 95.8 ± 0.5% in both hot and cold test.     

On a self-consistent calculation of the self-modulation instability in gyrotrons operating in whispering gallery modes

A self—consistent calculation of the self—modulation instability in gyrotrons operating in whispering gallery modes is carried out in the cold cavity approximation for satellite modes. The approach is illustrated numerically in the case of the planned operation of the KfK gyrotron in the TE_10,4 mode.     

Design of a quasi-optical system with variable output beam size

A quasi-optical system consisting of a quasi-optical antenna and two ellipsoidal mirrors can convert the gyrotron output (TE₁₅ mode, 354GHz) into a gaussian-like beam. The waist size of the beam produced is adjusted by moving a second ellipsoidal mirror. The second mirror with focal length of 520mm offers a tunable range from 2.0mm to 5.4mm, and those with focal lengths of 1190mm and 2390mm offer other tunable ranges from 5.1mm to 12.9mm and from 12.0mm to 25.1mm, respectively.     

High power mode conversion for linearly polarized HE11 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 HE₁₁, mode satisfies these conditions best. This quasi-optical hybrid mode can be generated from TE_0n gyrotron mode compositions by the two multi-step mode conversion processes: (1) ΣTE_0n to TE₀₁ to TE₁₁ to HE₁₁ or (2) ΣTE_0n 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 ΣTE_0n (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).     

回旋管准光辐射器研究与设计

准光辐射器是高功率回旋管准光模式变换器的重要组成部分.采用几何光学理论分析圆波导准光辐射器,根据高斯波束(TEM00模)在辐射器切口处形成的原因,利用耦合波理论设计波纹波导准光辐射器.通过编写程序并进行数值优化完成140 GHz、TE28,8模式回旋管波纹波导准光辐射器的设计,结果表明波纹波导辐射器的总长度仅为205.2 mm,切口长度为47.2 mm,在辐射器螺旋切口Brillouin区内高斯模式标量相关系数大于98%.本文所用的方法也可用于其它频率和模式的回旋管准光辐射器设计.     

Terahertz Frequency- and Mode-Insensitive Broadband Quasi-optical Converter Antenna System

Recently emerged multimode gyrotron, a high-power broadband terahertz radiator, encounters the challenge of efficiently converting a series of operating whispering-gallery modes (WGMs) into free-space Gaussian beams. To this demand, we propose a frequency- and mode-insensitive antenna capable of broadband multimode converting. For a single mode, to achieve broadband operation, special reflector configuration and large-radius launcher guarantee the system high robustness to frequency-induced wave number variation. Furthermore, for a series of operating WGMs, in order to achieve multimode operation, high-order mode indices guarantees familiar field patterns and ray trajectories. In particular, high-purity Gaussian beams are simultaneously achieved in different WGMs of broad continuous bands, including 351–361 GHz for TE_11,2 mode, 375–385 GHz for TE_12,2 mode, and 398–410 GHz for TE_13,2 mode. The results are verified by both the vector diffraction theory and the method of momentum. This kind of mode converter will promote the development of multimode gyrotrons and other antenna-feeder systems for high-power terahertz applications.     

Rippled wall mode converters for circular waveguide

We present the general theory of rippled wall mode converters. Coupling coefficients for TE and TM waves, of both fixed and rotating polarizations, are calculated. The waveguide ripples considered may be axisymmetric, fluted or helical. We describe in detail a 97% efficient TE₀₄/TE₀₁ converter designed for use with a 35 GHz gyrotron. Cold test results confirm its performance.     

Characteristics of the mode converter of Gyrotron FU CW GII radiating Gaussian beams in both the fundamental and second harmonic frequency bands

Gyrotron FU CW GII, the second gyrotron of the Gyrotron FU CW G-series with an internal quasi-optical mode converter, was recently developed at the Research Center for Development of Far-Infrared Region, University of Fukui (FIR FU) [Y. Tatematsu et al., J. Infrared, Millimeter, Terahertz Waves 35, 169 (2014)]. The design oscillation mode of this gyrotron is TE_8,3 with a corresponding frequency of 393.4 GHz in the second harmonic cyclotron resonance. We observed oscillations of different modes including fundamental harmonic modes in the 200-GHz band upon varying the strength of the magnetic field in the cavity. Radiation patterns corresponding to these modes emitted from the window were Gaussian although the internal mode converter was designed for the TE_8,3 mode. The directions of the radiated Gaussian beams were found to depend on the oscillation modes with a relation similar to that obtained for another gyrotron that was also equipped with a mode converter. This relation is explained based on geometrical optics. Moreover, numerical calculation with the electromagnetic wave code developed in FIR FU confirmed the same relation. The observed features of the Gyrotron FU CW GII show that it can serve as a unique power source with two frequency bands at 200 and 400 GHz. This result provides a way of developing multiple-frequency gyrotrons.     

A quasi-optical mode converter with a bifocal mirror

This paper presents the design procedure of a quasi-optical mode converter to transform any kind of TE_mn mode into a Gaussian wave beam and experimental results obtained in the particular case of the TE₆₄ mode at 110 GHz. The quasi-optical system consists of a helical-cut launcher and a bifocal mirror, which is designed, using the techniques of geometric optics, to focus the radiation of the launcher into a Gaussian focal spot. Such a system was fabricated and tested for the transformation of the TE₆₄ mode. The experimental results showed that about 80% of the power incident in the focal plane is focused into a small Gaussian-like spot of less than 20 mm diameter, while 97% of the power is contained into the main TE₆₄ lobe     

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

This work reports on measurements and calculations (coupled mode equations) on the conversion of circular electric TE_0n 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 TE_0n gyrotron mode mixture to the more convenient TE₀₁ mode for long-distance transmission through overmoded waveguides. Proper matching of the phase differences between the TE_0n 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 TE_0n -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 TE_1n 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.     

A W-band Third Harmonic Gyrotron with an Iris Cavity

The design and experimental results of a W-band gyrotron operating at the third cyclotron harmonic are presented. The gyrotron is designed to operate at the TE₆₁ mode, which is significantly distinct from competing modes. An iris cavity is employed for the purpose of trapping the third harmonic mode more effectively and lowering its start current. In the experiment, the gyrotron is drived by a triode magnetron injection gun (MIG) which can produce a 45 kV, 3 A electron beam. When maximum axial magnetic field is 1.22 T, a single mode third harmonic gyrotron radiation is observed with the frequency of 94.86 GHz. The maximum output power is 5.5 kW, corresponding to an efficiency of 4%. Another third harmonic mode TE₀₂ is also detected at 88.8 GHz, with maximum output power of 1.5 kW.     

High power millimeter-wave TE03 to TM11 mode converters

Transmission of power from 42 ± 0.2 GHz gyrotron (TE₀₃ mode) to tokamak or dummy load requires a set of transmission line components. It includes a set of mode converters that converts circularly unpolarised TE₀₃ mode to polarised HE₁₁ mode. The mode conversion sequence is methodised in two steps; first from TE₀₃ to TE₀₁ mode and then from TE₀₁ to TM₁₁ mode. The proposed mode converters performance depends on the parameters such as waveguide radius, beat wavelength, number of ripples, its perturbation amplitude (η) and bending angle. These parameters are numerically optimised and verified in CST microwave studio. TE₀₃ to TE₀₁ mode conversion is realised by the generation of intermediate mode TE₀₂. The mode conversion length for TE₀₃ to TE₀₂ mode converter is6λ₀, (where λ_b is the beat wavelength of corresponding mode conversion) which converts 99.15% of an incident circular TE₀₃ mode power into TE₀₂ mode power. Next, mode converter converts TE₀₂ mode into TE₀₁ mode with 99.06% efficiency along the optimal length of6λ_b. For TE₀₁ to TM₁₁ mode conversion, two conversion methodologies have been proposed: first using axis arc bend (34.94°) technique to convert TE₀₁ into its analogous degenerate mode with 99.01% conversion efficiency and second using parabolic curve with 90° bend, which gives a conversion efficiency of 97.5%.     

110 GHz gyrotron with a built-in high-efficiency converter

A 110 GHz 1 MW pulse gyrotron has been elaborated. A built-in electrodynamic duct consisting of a new type quasi-optical converter and three matching mirrors transforms operating mode TE_15,4 into a gaussian beam going through the output window. The efficiency of the duct (ratio of the gaussian beam power outside the gyrotron to the total microwave power at the output of the cavity) is about 95%. A system consisting of the gyrotron and an outer mirror transmission line is proved to have efficiency (ratio of the microwave power measured by calorimeter at the end of line to the power of the electron beam) more than 40%.