A Single-Ridged Coaxial Hybrid TE011 Coupler with High Mode Purity

We design a single-ridged coaxial hybrid coupler which excites a TE₀₁₁ mode of high mode content in a cylindrical cavity, resonating at 28.2GHz. The coupler consists of a WR-28 rectangular waveguide, a coaxial TE_n11 cavity, and a cylindrical TE₀₁₁ cavity. Both TE₃₁₁ coaxial cavity and TE₄₁₁ single-ridged coaxial cavity are analyzed to examine the TE₀₁₁ mode purity in the central cavity. Mode purity analysis is performed by a field expansion method using Fourier-Bessel orthonormal basis functions. Numerical calculations predict that the TE₄₁₁ single-ridged coaxial cavity excites the TE₀₁₁ mode with mode purity of 98.6%, which is improved by 3% higher compared with the TE₃₁₁ coaxial cavity. Measurements on the single-ridged coaxial coupler show a resonant frequency at 28.078GHz and ohmic and external Qs of 1560, 473 respectively, which are in good agreement with the simulated results of a 3-D finite element electromagnetic code.     

Design of a system converting an output radiation of frequency tunable gyrotron into a gaussian beam

A novel design of a versatile quasi-optical system for conversion of gyrotron radiation into collimated gaussian beams is presented and discussed. The proposed system consists of a quasi-optical antenna, two ellipsoidal mirrors and a spatial filter which truncates the sidelobe radiation. The system is appropriate as a transmission line for frequency tunable gyrotrons operating at TE_0n and TE_1n modes. As an illustration of our approach, we present results which demonstrate the applicability of the developed system for conversion of the radiation generated by the Gyrotron FU IVA. The examples include conversion of four TE_1n mode outputs (TE₁₂, 170 GHz; TE₁₃, 271 GHz; TE₁₄, 372 GHz; TE₁₅, 472 GHz) into gaussian-like beams and three TE_0n modes (TE₀₂, 223 GHz; TE₀₃, 323GHz; TE₀₄, 423 GHz) into bigaussian-like beams.     

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%.     

Evaluation of Temperature-Dependent Effective Material Properties and Performance of a Thermoelectric Module

We devised a novel method to evaluate the temperature-dependent effective properties of a thermoelectric module (TEM): Seebeck coefficient (S _m), internal electrical resistance (R _m), and thermal conductance (K _m). After calculation, the effective properties of the module are converted to the average material properties of a p–n thermoelectric pillar pair inside the module: Seebeck coefficient (S _TE), electrical resistivity (ρ _TE), and thermal conductivity (k _TE). For a commercial thermoelectric module (Altec 1091) chosen to verify the novel method, the measured S _TE has a maximum value at bath temperature of 110°C; ρ _TE shows a positive linear trend dependent on the bath temperature, and k _TE increases slightly with increasing bath temperature. The results show the method to have satisfactory measurement performance in terms of practicability and reliability; the data for tests near 23°C agree with published values.     

Input coupler for Ka band TE021 circular electric mode operation in a gyroklystron amplifier

An input coupler for the TE₀₂₁ circular electric mode in a gyroklystron amplifier through using TE₈₁₁ as a transition coupling mode in a coaxial cavity has been proposed and numerically simulated in a frequency range of Ka-Band with HFSS code. The coupling efficiency of the TE₀₂₁ mode and the suppression for the TE₈₁₁ mode in the main cylindrical cavity are deliberately considered by adjusting the position and size of coupling holes as well as the radius of the drift tubes. The numerical results show that the TE₀₂₁ mode can successfully be excited, and rational coupling efficiency and high mode purity for the TE₀₂₁ mode in the cylindrical cavity can be reached for the engineering application of gyroklystron amplifier study.     

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.     

CMOS prototype image sensor with edge enhancement to compensate for blurring

Small signal analytical analysis and large signal numerical calculations are presented for a gyroklystron in which a low order (TE₀₁₁) mode in the bunching cavity is coupled by the electron beam to a high order (TE₀₄₁) mode in the output cavity. A drift region separates the two cavities. We demonstrate that the probunching of the beam before entering the output cavity not only enhances the interaction efficiency of the TE₀₄₁ mode but also improves the mode stability by suppressing the TE₂₄₁ mode. The gyroklystron may be operated either as an oscillator or as an amplifier by a small variation of the applied magnetic field. The effect of the beam velocity spread on the efficiency is also investigated. It is found that the operation of the gyroklystron is more sensitive to beam velocity spread than that of a gyromonotron. Optimum conditions for a cold beam are not necessarily those for a wam beam.     

A Quasi-Optical System for Converting TE0n Mode Outputs of a Gyrotron into Gaussian Beams

In this paper we present a novel design of a quasi-optical system for conversion of gyrotron outputs into Gaussian-like beams. It consists of a quasi-optical antenna, two focusing mirrors and a filter which removes the side lobes of the beam. The system is appropriate as a transmission line for frequency tunable gyrotrons operating at TE_0n mode. As an illustration of our approach, we present results which demonstrate the applicability of the developed system for conversion of the radiation generated by the Gyrotron FU IVA. The examples include conversion of three TE_0n modes (TE₀₂, 223 GHz; TE₀₃, 323 GHz; TE₀₄, 423 GHz) into Gaussian- like beams.     

Mode selective directional couplers for overmoded waveguide systems

Two types of directional couplers for transverse electric (TE) modes are described: short and multihole couplers, respectively. They selectively pick one mode out of a mode mixture in an overmoded circular waveguide system. Unwanted modes are either statistically kept at low level or are suppressed by destructive interference in the coupling waveguide. Mode selectivity and directivity in multihole couplers oscillate up and down with an increasing number of holes, finally reaching a minimum of approximately 20 dB, unless there are competing modes with rational fractions of the beat wavelength. A multihole coupler for the TE₀₂ mode (28 GHz, 63.4 mm waveguide diameter, 41 holes) and a length of 1.6 m shows a calculated directivity of 68 dB and suppresses the unwanted modes TE₀₁ with 34 dB (24 dB), TE₂₂ with 37 dB (45 dB), and further modes TE_?m (?<5, m<6) with 17 dB to 34 dB in forward direction (figures in parentheses are for unwanted modes propagating in backward direction). A short directional coupler for the TE₀₁ mode (28 GHz, 63.4 mm waveguide diameter) with 16 holes and a length of 230 mm shows a directivity of 55 to 100 dB between 27.9 and 28.1 GHz, suppressing the TE₀₂ mode with 35 to 80 dB, the TE₀₃ mode with 30 to 65 dB, and the TE₂₂ mode with 30 to 70 dB.     

螺旋波纹波导回旋行波管的模式耦合机制

从波导的等效边界条件出发,结合波导的激发方程组,通过数学推导说明了螺旋波纹波导回旋行波管的模式耦合机制。TE_1,1模会在螺旋波纹波导中耦合出TE_-2,1模,并且通过计算说明TE_-2,1模主要和TE_1,1模的空间-1次谐波发生耦合。     

Mode competition in Bragg resonator cyclotron resonance maser experiments driven by a microsecond,intense electron beam accelerator

We report an experimental study on mode competition in a high-g Bragg resonator cyclotron resonance maser (CRM) employing a microsecond, relativistic electron beam. The high quality annular electron beam of 20-80 A was produced through an apertured mask-anode. The high-Q Bragg resonator was designed to excite the TE₃₁, mode CARM oscillation at 18-9GHz. Two configurations of Bragg resonators were used; with ripples half inward and with ripples fully outward with respect to a smooth tubular centre section. Bragg resonator microwave emission was compared to radiation emitted when the electron beam interacted with smooth tubes with the same diameter as the Bragg resonator centre sections. In the present Bragg resonator CRM experiments, three parasitic oscillations were the most serious competing modes, apparently suppressing the TE31 CARM oscillation: TE₂₁, absolute instability, TE₁₁ gyro-BWO, second and third harmonic TE₅₁, mode. When a high current annular beam of 150-200 A was injected into the Bragg resonator, we observed the TE₂₁ absolute instability at B≥6-9kG, and a TE₁₁ gyro-BWO interaction at B≤5-2kG. Experimental evidence on the modes of the absolute instabilities were provided by gas breakdown patterns in fluorescent light tubes and frequency measurements.     

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).     

Millimeter Wave Multi-mode Transmission Line Components

The present paper reports on the recent development of several oversized millimeter wave transmission line components for different applications. The studies include a circular TE₁₁-to-Gaussian beam mode converting horn, a TM₀₁-to-rotating TE₃₁ mode converter, a TE₁₁-mode 90° bend, a series of different HE₁₁-mode transmission line components, a notch filter and a fast laser controlled semiconductor microwave switch.     

RADIATION PATTERN MEASUREMENTS ON A MILLIMETER-WAVE OPEN RESONATOR

Far-field radiation patterns can be used for identifying different kinds of resonant modes in a gyrotron cylindrical open resonator. The operating TE₀₂₁ mode is identified among its closest competitors TE₂₂₁ and TE₆₁₁ by measuring radiation patterns obtained experimentally based on millimeter wave source for exciting the open resonator. A good agreement between experimental and, theoretically predicted values was found.     

MAGICTRAC,a novel method for conversion of whispering-gallery modes into a free-space Gaussian-like beam

A novel device, MAGICTRAC, is described for efficient conversion at millimeter wavelengths of the TE _m,n whispering-gallery mode into a linearly polarized, free-space Gaussian-like beam. MAGICTRAC uses a mode-converting waveguide taper and three mirror optics, one of which incorporates a twist reflector to linearly polarize the output beam. An example design is presented for the TE_15,2 mode at 140 GHz with a calculated efficiency of 96%. Related possible applications include (1) installation of the MAGICTRAC within the vacuum envelope of a gyrotron to separarate the spent e-beam from the generated rf, (2) generation of a whispering-gallery mode by injection of a Gaussian-like beam into the output end, and (3) conversion of TE _m,n modes into TE_0n modes for low-loss transmission in smooth-wall waveguide.     

Efficient Output Mode Converter for 30 GHz Gyroklystron at IAP

An output mode converter for Ka-band multi-MW gyroklystron in the Institute of Applied Physics (IAP) operating in the TE₅₃ mode is suggested. Two variants of the converter, aimed for different applications, are presented: the TE₅₃ to TE₀₁ mode converter with power output along the device axis and the TE₅₃ mode to Gaussian wavebeam quasi-optical converter with a visor. The suggested designs include the built-in electron beam collector. The converters were designed using a new synthesis algorithm, which implies iterative improvement of the waveguide wall shape in order to achieve high efficiency. The calculation results were proven by HFSS simulation and low-power tests of one version of the converter.     

Linear and nonlinear analyses on gyropeniotron in a rectangular waveguide

The dispersion equation of the gyropeniotron using TE_n0 mode in a rectangular waveguide is derived in this paper. Through detailed numerical calculations of the dispersion equation using the TE₂₀ mode, dispersion curves of the peniotron are obtained. Some clear viewpoints are proposed here. In the nonlinear analysis, the model is assumed to be a rectangular cavity with two close ends, constant DC magnetic field and an axis-rotating electron beam. The analytical results of the peniotron using the TE₂₀₁ cavity mode assuming two operating conditions of ω = ω_c, and ω 3ω_c show 51% efficiency for to ω ω_c and 11% efficiency for ω3ω_c.     

Experimental Study of a Slot Antenna Excited by a TE011 Mode Coaxial Cavity

A slot antenna is developed to excite the high harmonic waveguide mode for generating large-area plasmas. This antenna consists of a TE₀₁₁ mode coaxial cavity with the axial slots positioned on equal interval on the inner wall. The waves radiated from those slots can excite the high harmonic mode in the central area. With the azimuthal symmetric wave field of the TE₀₁₁ mode, the number of the slots can be chosen to match the field pattern of the high harmonic mode. In this report, the dispersion relation of the coaxial waveguide, the coupling scheme and the mode competition of the cavity are studied. A method has been successfully developed to suppress the TE₁₂₁ mode which is the most competing mode to the TE₀₁₁ mode.     

Efficient generation of the first waveguide mode in the InGaAs/GaAs/InGaP heterolaser

Generation of the first excited transverse mode (TE₁) in a new InGaAs/GaAs/InGaP diode heterolaser with a thin InGaP layer at the waveguide center was studied. This laser design decreases the competition of the first and fundamental modes and provides TE₁ mode lasing at a threshold current comparable to that of an ordinary laser oscillating at the fundamental TE₀ mode.     

Compact Mode Converter System for the Cold Test of Assembled Gyrotrons

A microwave system is described which allows testing of the quasi-optical mode converter in a gyrotron, which is completely assembled except for the attachment of the electron gun. Test results are presented for a TE_6,2,1 gyrotron, and a similar design for a TE_28,7,1 gyrotron is shown.