Coaxial Cavity Research Articles

An indirectly coupled open-ended resonator applied to characterize dielectric properties of MgTiO3–CaTiO3 powders

The effect of CaTiO3 addition on the complex permittivity of MgTiO3 powder was characterized with an open-ended coaxial cavity resonator in the range of 2.12–3.66 GHz. Permittivities and loss tangents of (1 − x)MgTiO3-xCaTiO3 composite powders with x of 0, 2, 5, and 10 mol. % were measured and compared to theoretical values. Inclusion permittivities and dielectric loss tangents were determined by using Bruggeman symmetric and Looyenga mixing rules and a general mixing model. Additions of CaTiO3 resulted in a clear increase in inclusion permittivities from 13.4 up to 14.9 and in loss tangents from 7.1 × 10−3 up to 8.5 × 10−3. Comparison with the theoretical loss tangent values and quantitative determination of CaTiO3 molar ratios by using measured loss tangents and a general mixing model gave a good correlation. The characterization method was proved to be capable of detecting dielectric changes of MgTiO3–CaTiO3 composite powder and of quantifying the amount of additional substances. This information can be exploited, for example, in the analysis and quality control of different composite powders.

Note: Multi-pass Thomson scattering measurement on the TST-2 spherical tokamak.

In multi-pass Thomson scattering (TS) scheme, a laser pulse makes multiple round trips through the plasma, and the effective laser energy is enhanced, and we can increase the signal-to-noise ratio as a result. We have developed a coaxial optical cavity in which a laser pulse is confined, and we performed TS measurements using the coaxial cavity in tokamak plasmas for the first time. In the optical cavity, the laser energy attenuation was approximately 30% in each round trip, and we achieved a photon number gain of about 3 compared with that obtained in the first round trip. In addition, the temperature measurement accuracy was improved by accumulating the first three round trip waveforms.

Frequency dependence of the microwave surface resistance of MgB2 by coaxial cavity resonator

We report on the microwave (mw) properties of a cylindrical MgB2 rod prepared by the reactive liquid Mg infiltration technology. The MgB2 rod, 94.3mm long, is used as inner conductor of a coaxial cavity having a Cu tube as external conductor. By analyzing the resonance curves of the cavity in the different resonant modes and at different temperatures, we have determined the temperature dependence of the mw surface resistance, Rs, of the MgB2 material, at fixed frequencies, and the frequency dependence of Rs, at fixed temperatures. Our results show that the Rs(f) curves follow a fn law, where n decreases on increasing the temperature, starting from n≈2, at T=4.2K, down to n≈0.7 at T⩾Tc. The double-gap nature of MgB2 manifests itself in the presence of a wide low-T tail in the Rs(T) curves, which can be ascribed to the quasiparticles thermally excited through the π gap even at relatively low temperatures.

Design of the RF Circuit for a Coaxial Cavity High-Power Multiple-Beam Klystron

This paper presents the design methodology and detailed design of the interaction circuit for use in an L-band 10-MW MBK, which operating in the fundamental mode at a center frequency of 1.3 GHz. Coaxial cavities are adopted to give adequate space for the use of multiple large cathodes, which has a low cathode loading and thereby facilitate a long lifetime. The emphasis for the present design is on efficiency and high-peak output power. The initial design was carried out using the large signal code KLY. To calculate the beam-wave interaction characters conveniently with cavities present, we developed a 2-D single-beam equivalent design methodology, in which the multitunnel coaxial cavities are replaced by the conventional single-tunnel cylindrical cavities with transformed RF parameters. The 3-D PIC simulations with actual coaxial cavity geometries are carried out at last to check on the large signal and the 2-D PIC simulations. The 3-D simulation results indicate that the equivalent design methodology is effective. The final RF circuit consists of six coaxial cavities including a second-harmonic cavity to reduce the device length. The circuit has a predicted gain of 50 dB at a peak pulsed output power of 10.4 MW with a corresponding electronic efficiency of more than 68%. The design and test results of the output system with two windows are also presented.

Recent Advances in the Worldwide Fusion Gyrotron Development

Progress in the worldwide development of high-power gyrotrons for magnetic confinement fusion plasma applications is presented. Gyrotron oscillators are used for electron cyclotron heating, electron cyclotron current drive, stability control, and plasma diagnostics. After technology breakthroughs in the research on gyrotron components in the 1990s, significant progress has been achieved in the 2000s, in the field of long-pulse and continuous wave (CW) operation for a wide range of frequencies. Currently, the development of 1-MW-class CW gyrotrons for the tokamak ITER (170 GHz), the stellarator Wendelstein 7-X (140 GHz), and the tokamaks DIII-D and JT-60SA (110 GHz) has been very successful in EU, Japan, Russia, and USA. The Japan 170-GHz ITER gyrotron holds the energy world record of 2.88 GJ (0.8 MW at 1-h pulse duration). For this progress in the field of high-power long-pulse gyrotrons, innovations such as the realization of high-efficiency stable oscillation in very high-order cavity modes with low ohmic losses, the use of single-stage depressed collectors for energy recovery (efficiency enhancement and simpler power supplies), highly efficient internal quasi-optical (q.o.) mode converters (low level of internal stray radiation), and synthetic diamond windows have essentially contributed. The total tube efficiencies are around 50% and the purity of the linearly polarized fundamental Gaussian output mode is 97% and higher. Power modulation technologies for stabilization of neoclassical tearing modes have proceeded. Future prospects of advanced high-power fusion gyrotrons are in the areas of two- and three-frequency gyrotrons, fast step-wise frequency tuneability, higher unit power (coaxial cavities), and higher frequencies for more efficient plasma stabilization and noninductive current drive as well as reliability, availability, maintainability and inspectability for next step fusion power stations. The GYCOM step-tuneable 1-MW gyrotron for ASDEX Upgrade employing a broadband travelling-wave-resonator window (with two diamond disks) operates at 105, 117, 127 and 140 GHz. The EU 170 GHz coaxial-cavity gyrotron prototype achieved in millisecond pulses the power of 2.1 MW at 46% efficiency and 96% Gaussian mode purity. A new power record of second harmonic oscillation has been achieved in the subterahertz band (83 kW/389 GHz/3 ms) in Japan for application to collective Thomson scattering diagnostics. A fundamental frequency 670-GHz gyrotron with pulsed magnet generated 210 kW in 20-30- μs pulses at 20% efficiency (collaboration of institutions in Russia and USA).

A New Type of Output Circuit with Coaxial Compound Cavity Loaded with a Filter

该文介绍了一种适用于S波段宽带多注速调管的新型滤波器加载同轴复合腔输出回路。根据耦合谐振腔理论对该输出结构进行了理论分析并给出其等效电路；采用3维电磁场仿真软件CST-MWS进行计算模拟，研究了输出腔间隙的场分布均匀性及输出回路间隙阻抗的频率特性。研究结果表明，采用该滤波器加载同轴复合腔输出回路不仅可以极大地降低多注速调管总体对高阴极发射电流密度的要求，而且在保证多注速调管128 MHz/dB带宽的同时输出腔间隙处的轴向电场有较好的均匀性。

Klystron Output Circuit of High Order Transverse Magnetic Mode in Coaxial Cavity Loaded with Waveguide Filter

摘要: 该文设计了工作于高阶横磁TM310模式单间隙封闭圆柱同轴腔。在输出矩形波导内设计了横向电感滤波膜片的新型结构。用模拟场分析法计算了腔内6个漂移管中心处间隙阻抗平均值的频率特性。模拟和计算发现，该结构模式输出回路的间隙阻抗平均值随频率的变化出现双峰曲线。这表明，采用滤波膜片波导的输出带宽比采用空波导情形的带宽有明显的增加。 关键词: 多注速调管 / 同轴谐振腔 / 高阶横磁模 / 间隙阻抗 / 输出带宽 / 滤波器加载输出回路

Determination of complex permittivity of surfactant treated powders using an open-ended coaxial cavity resonator

An indirectly coupled open-ended coaxial cavity resonator was used to detect the effect of stearic acid surface treatments on the dielectric properties of powders in the frequency range 2.17–3.79GHz. The Bruggeman symmetric and Looyenga classical mixing rules and a general mixing model were used to determine changes in permittivity and loss tangent of the inclusions. Surfactant processed powders with stearic acid 0.05–1.36vol.% induced only a small change in inclusion permittivity but caused a notable change in dielectric losses. The dielectric loss tangents of SiO2 and Al2O3 increased from 2×10−4 to 4×10−4 and 5×10−3 to 6.1×10−3, respectively. However, a significant decrease in dielectric loss tangent, from 6.2×10−3 to 3.8×10−3, was noted with surfactant treated ZrO2. The investigated novel characterization method proved to be able to detect even very slight changes in dielectric properties to be utilized in various types of powdery material characterization, e.g., in composite applications and material quality monitoring.

A Modified Meshless Local Petrov–Galerkin Applied to Electromagnetic Axisymmetric Problems

A modified meshless local Petrov-Galerkin for an electromagnetic axisymmetric problem is presented in this paper. The method uses the shape functions generated by the radial point interpolation method with a modified T-scheme to select the support nodes, and also a new and malleable strategy to determine the test domains. The convergence of the method is evaluated using a coaxial cavity problem and it is compared with the finite-element method for two different meshes: one with a good quality mesh and another partially composed to bad quality elements. The total execution time using both methods is also compared.

Microwave Response of Coaxial Cavities Made of Bulk Magnesium Diboride

We report on the microwave (mw) properties of coaxial cavities built by using bulk MgB2 superconductor prepared by reactive liquid Mg infiltration technology. We have assembled a homogeneous cavity, by using an outer MgB2 cylinder and an inner MgB2 rod, and a hybrid cavity by using an outer copper cylinder and the same MgB2 rod as inner conductor. By the analysis of the resonance curves, in the different resonant modes, we have determined the microwave surface resistance, Rs, of the MgB2 materials as a function of the temperature and the frequency, in the absence of DC magnetic fields. At T = 4.2 K and f = 2.5 GHz, by a mw pulsed technique, we have determined the quality factor of the homogeneous cavity as a function of the input power up to a maximum level of about 40 dBm (corresponding to a maximum peak magnetic field of about 100 Oe). Contrary to what occurs in many films, Rs of the MgB2 material used does not exhibit visible variations up to an input power level of about 10 dBm and varies less than a factor of 2 on further increasing the input power of 30 dB.

Study on the empirical design of open‐ended coaxial cavity resonator

AbstractThis article is a study of an empirical design of an open‐ended coaxial cavity resonator. The coaxial cavity resonator can be designed using the coaxial transmission‐line theory. It can be done by using the radius of the inner conductor, the inner radius, and the length of the resonator. However, the basic coaxial transmission‐line theory can be seen that the characteristics of the resonant frequency and the Q value are varied by the change of length, regardless of the value of radius of the inner conductor and inner radius of the resonator. We find out the impact of radius of the inner conductor, inner radius of the resonator, and the length of the resonator parameter and propose the optimized empirical resonator design method by reducing the error between the theoretical value and the design value. High frequency structure simulator was used for simulation and the material of the resonator was designed with a copper. Based on the simulation, several resonators are fabricated by the size of 14 mm for the radius of inner conductor, 2, 5, and 10 mm, respectively, for the inner radius of resonator, and 8.5 mm for the length of the resonator. The resonant frequencies of the produced resonators were measured at 6.1, 5.7, and 6.5 GHz, respectively. According to the result of simulation and measurement, we know that we can design the relatively exact open‐ended coaxial cavity resonator by applying the basic coaxial transmission‐line theory directly when the length of the resonator is less than 10 mm, and adding the correction factor of 0.5 GHz to the calculated resonant frequency in case of more than 10 mm of the length of the resonator. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:606–610, 2014

A MULTI-EVANESCENT-MODE COAXIAL CAVITY RESONATOR

This paper presents a novel miniaturized multi-evanescent-mode resonator. The resonator is achieved with a coaxial cavity. This coaxial cavity essentially has a direct short connection at one end and is connected with several lumped capacitances at the other end. The key technology of the resonator is the usage of multiple evanescent-modes of TM (transverse magnetic wave) modes. Due to the combined efiects of the evanescent mode and multiple modes, the size of the resonator is greatly reduced. In this paper, the theory of resonator is discussed in detail. To verify the correctness of operation, the resonator is used in experimental measurements conducted to realize a third-order band- pass fllter based on SIW (substrate-integrated waveguide) technology. The measured results are found to agree with the theoretical values.

Analysis and calculation of a 170 GHz megawatt-level coaxial gyrotron

Gyrotrons are the most promising microwave source devices that can be used in the International Thermonuclear Experimental Reactor, but there are many difficulties to be solved in study and design of gyrotrons to meet the requirements. In this paper, the beam-wave interactions of a 170 GHz megawatt-level smooth-wall coaxial gyrotron are studied numerically. In order to attain high efficiency and stable radiation, TE31,12 mode that lies in a relative sparse spectrum is selected as the operating mode, and the beam-wave coupling coefficient and start oscillation current are calculated by a set of source codes developed by Matlab. Taking into account the electronic velocity spread and cavity wall resistivity, and based on a single-mode approximation, the optimization design and simulation of beam-wave interaction of a 170 GHz megawatt smooth-wall coaxial gyrotron have been fulfilled. The relationships between efficiency and magnetic field, and the voltage, current, taper angle of insert, and other parameters are presented. Results show that the voltage and magnetic field have great influence on efficiency; however, the current and velocity spread do change slightly, thus reduce the requirements of electron gun design. In addition, the optimized taper angle of insert and coaxial cavity geometry parameters can improve the efficiency, reduce the impact of velocity spread on efficiency, and can achieve an electronic efficiency around 50% and an output power 1.7 MW.

Design of ultrasmall plasmonic coaxial lasers on Si

An ultrasmall plasmonic coaxial laser made of metal-semiconductor-metal on a sil- icon substrate through an interlayer bonding was designed. From the effective refractive indices and the transparent material gain, the nanoscale structural dimensions with both the radius and the width at 80 nm for the coaxial plasmonic waveguide were decided. The influence of the interlayer bonding material on the optimization of resonant wavelength and Q-factor was evalu- ated. A three-dimensional body-of-revolution finite-difference time-domain method was used to show that a coaxial cavity with a SiO2 interlayer can laze at around 1480-nm wavelength with a net optical threshold power density of about 800 W∕cm 2 and a subwavelength mode volume of 0.014ðλ∕2nÞ 3 . This nanolaser on silicon platform will benefit those working on nanophotonic integrated circuits. © 2013 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10.1117/1 .JNP.7.070598)

Study of a Coaxial Gyrotron Cavity With Improved Mode Selection

The resonant frequency, the diffractive quality factor, and the mode competition in the coaxial gyrotron cavity with a tapered inner rod have been investigated. The resonant frequency and the diffractive quality factor can be adjusted by changing the tilt angle and the radius of the tapered inner rod. When the inner rod radius is far smaller than the caustic radius of the given mode, the resonant frequency and the diffractive quality factor are hardly affected by the tilt angle of the tapered inner rod; the resonant frequency increases slightly but the diffractive quality factor increases noticeably with the increase of the tilt angle of the tapered inner rod when the inner rod radius is a little smaller than the caustic one; the resonant frequency and the diffractive quality factor decrease with the increase of the tilt angle of the tapered inner rod when the inner rod radius is larger than the caustic one. The mode competition can be improved with an appropriate design of the tilt angle and the radius of the tapered inner rod in a coaxial gyrotron cavity with a tapered inner rod.

Ohmic Losses in Coaxial Cavity Gyrotron with Outer Corrugation

Ohmic losses of a coaxial cavity gyrotron with outer corrugation are investigated. The results show that the averaged ohmic loss densities of the inner and outer conductors have similar changes along with the axial direction of the gyrotron; whereas averaged ohmic loss densities of the outer conductor are more than the inner conductor; the outer slot depth and width cause greatly the averaged ohmic loss densities of the corrugation bottom and the corrugation period of outer conductor, and averaged densities of ohmic losses on the inner conductor are almost unaffected.

A short-ended CRLH coaxial cavity resonator with zeroth-order resonance

This paper introduces a coaxial cavity resonator structure with a new configuration based on the composite right/left-handed transmission line (CRLH–TL). In order to create the CRLH–TL, the shunt inductance coils and the series capacitance gaps are added to a conventional half-wavelength short-ended coaxial cavity resonator. The final resonator is composed of three periodically connected CRLH unit cells. Through the adjustment of the diameter of the inserted coils and the gaps of created capacitances, the proposed resonator was able to support the desired zeroth-order resonance (ZOR) in the balanced mode. In order to verify the applicability of the proposed resonator to the CRLH metamaterial, a dispersion diagram and the constitutive parameters of a unit cell were determined using three-dimensional EM simulation. The resonant frequencies and quality factors associated with each resonance modes were then computed through eigenmode analysis for the overall resonator formation. In addition, the electromagnetic field distributions of the resonator at the ZOR frequency have been described and discussed. These results enable us to validate that the proposed CRLH coaxial cavity resonator can be utilized as a zeroth-order resonator based on the CRLH–TL structure. As a result the proposed resonator has a 69% reduction in size compared to traditional short-ended coaxial cavity resonators.

An Ultrahigh-order-mode, Higher-harmonic Coaxial Gyrotron Oscillator in Sub-terahertz Wave Range

A coaxial cavity gyrotron oscillator at a frequency of 0.34 THz is studied, which operates with a quite low magnetic field of 4.55 Tesla at the third cyclotron harmonic of the ultrahigh-order mode TE43,4. Properly choosing the depth of the longitudinal corrugations on the inner rod and optimizing the electron-beam position significantly suppress the mode competition. Nonlinear multimode simulations show the feasibility of the single-mode operation with an output power of 163 kW by using an electron beam with a voltage of 70kV and a current of 30A, which corresponds to an interaction efficiency of 9.2 % with maxim density of ohmic losses 2.9 kW/cm2.

Miniaturised double‐coaxial cavity resonator and duplexer with ZOR characteristic using air‐core coils

A novel method of miniaturising the size of a coaxial cavity resonator by utilising zeroth-order resonance (ZOR) characteristics is presented. This miniaturisation has been achieved through the introduction of air-core coils inside a conventional coaxial cavity resonator. This structure possesses ZOR characteristics, which results in the miniaturisation of the resonator while still exhibiting a reasonable high unloaded Q factor. A duplexer was designed and fabricated using the proposed resonator. The measurement results verified that a 30% reduction in size was achieved compared with a traditional duplexer.

한쪽 면이 열린 동축 공동 공진기를 이용한 빗물 감지에 관한 연구

본 논문은 한쪽 면이 열린 동축 공동 공진기를 이용하여 빗물의 양을 선형적으로 감지하는 레인 센서에 관한 연구이다. 공진기의 열린 면에 놓이는 유전체의 양과 특성에 따라 공진 주파수와 손실이 달라지는 원리를 이용하여 공진기를 자동차 앞유리 표면에 떨어진 빗물의 양을 측정할 수 있는 센서로 활용할 수 있음을 보여준다. 공진기 양 벽면에 입력과 출력 포트를 구성하고, 입출력 커플링 프로브는 'ㄱ'자 형으로 구성한다. 공진기 내부도체 반지름을 2 mm, 5 mm, 10 mm로 변화시켜 빗물의 반응을 시뮬레이션으로 확인한 결과, 내부 도체 반지름이 5 mm일 때 공진 주파수가 비교적 선형적으로 민감하게 변화하여 빗방울의 양을 감지할 수 있는 공진기에 가장 적합함을 확인할 수 있었다. 시뮬레이션 결과를 바탕으로 제작된 공진기는 빗방울의 양이 변함에 따라 공진 주파수가 3.55 GHz부터 3 GHz까지 선형적으로 변하였고, Q값은 42.38부터 24.3까지 변화하는 것을 확인하였다. 따라서 본 논문에서 설계한 공진기의 구조가 공진 주파수를 측정 파라미터로 이용하여 빗물의 양을 선형적으로 감지하는 레인 센서로 적용할 수 있음을 보여주었다. This paper is a study of a rain sensor using an open-ended coaxial cavity resonator which senses the amount of rain drops linearly. It shows that it will be used as a sensor to sense the amount of rain dropped on the windshield of an automobile based on the principle of varied resonant frequency and the loss according to the amount and characteristics of an dielectric lied on the open side of a resonator. The input and output ports are built in the both sides of the resonator and the input and output coupling probes are formed like 'ㄱ' shape. The response of rain drops were simulated by the radius of inner conductor of 2 mm, 5 mm, and 10 mm respectively and it showed that the raindrop was sensed most linearly and sensitively when the radius of inner conductor is 5 mm, We have measured that the resonant frequency have varied from 3.55 GHz to 3 GHz and the Q value have varied from 42.38 to 24.3 according to the variation of rain drop amount on the fabricated resonator. Therefore, it shows that the designed resonator can be applied as a rain sensor that measures the amount of rain drops linearly by using the resonant frequency as a measurement parameter.