Coaxial Waveguide Research Articles

On the theory of electromagnetic interactions of a relativistic electron beam and plasma in a coaxial waveguide in an external magnetic field

Linear electromagnetic interactions of rectilinear electron beams with magnetoactive plasma in a coaxial waveguide are considered. Dispersion equations are derived and field structures of surface plasma waves in the absence of the electron beam are determined. Dispersion equations of beam waves in the absence of plasma waves are derived. Dispersion equations that describe the interaction of plasma and beam waves are derived. Instability increments in regimes of single-particle and collective Cherenkov effects and irregular Doppler effect are calculated.

An X-band overmoded relativistic klystron

An X-band overmoded relativistic klystron is proposed, the operation mode of which is the TM02 mode. The drift tube could not cut off the TM01 mode; isolating the buncher cavity from the input cavity is achieved by introducing a sectional RF lossy material. Microwaves are extracted from the modulated electron beam using a cylindrical waveguide, rather than a coaxial waveguide; thereby, the output structure is significantly simplified. Particle-in-cell simulations show that microwaves with power of 1.28 GW and frequency of 9.30 GHz can be obtained, corresponding to an efficiency of 32% and relative bandwidth of about 8%.

BOUNDARY PROBLEM SOLUTION FOR EIGENMODES IN COAXIAL QUAD-RIDGED WAVEGUIDES

The boundary problem for eigenmodes in coaxial quad-ridged waveguides has been solved by the transverse field-matching technique. The formulas obtained provide possibilities to calculate cutoff wave numbers and electric and magnetic fields distributions of TEM, TE and TM modes in the presence of the ridges either on the inner or on the outer perfectly conducting cylinder. The analysis of the dependences of cutoff wave numbers and electric field distributions convergences on the number of partial modes has been carried out. It has been shown that for calculation of cutoff wave numbers with residual error less than 0.1 % it is enough to utilize 27 partial modes, and for the correct calculation of fields distributions one should utilize more than 30 partial modes.

A parametric convex meshfree formulation for approximating the Helmholtz solution in circular coaxial waveguide

AbstractThe application of convex meshfree approximation to the time‐harmonic electromagnetic wave propagation analysis of a waveguide with non‐convex cross section such as the circular coaxial waveguide remains unsolved. This paper introduces a parametric convex meshfree formulation for the circular coaxial waveguide analysis. The present method reformulates the convex meshfree approximation on the basis of a special parametric space―an extended parametric domain. The new parametric domain ensures a one‐to‐one geometric mapping using the convex meshfree approximation and allows the convex meshfree method to be applied to the oscillatory type of Helmholtz equation for circular coaxial waveguide analysis. Both transverse electric and transverse magnetic mode studies are conducted using the present method, and results are compared with the standard bilinear finite element method. Copyright © 2014 John Wiley & Sons, Ltd.

Structure, surface and broadband impedance spectroscopy of Li4Ti5O12 based ceramics with Nb and Ta

The structural investigation of the Li4Ti5O12 (LTO), Li4Ti4.95Nb0.05O12 (LTO+Nb) and Li4Ti4.95Ta0.05O12 (LTO+Ta) powders was conducted by X-ray diffraction (XRD) in temperature range (300–1270) K. These compounds have cubic spinel-type structure in the investigated temperature range. Ceramic samples were sintered for X-ray photoelectron spectroscopy (XPS) and impedance spectroscopy study. Binding energies of Ti 2p and O1s core levels XPS on the surfaces of the ceramics have been investigated at room temperature. Two and four probe voltmeter–ammeter methods were used for the measurements of electrical impedance in the frequency range 10Hz–1MHz. Coaxial waveguide technique was used for the investigation of impedance spectra in the frequency range from 1MHz to 3GHz. Electrical properties of the ceramics were studied in the temperature interval (300–780) K.

Coaxial multi-layer hybrid plasmonic waveguide at subwavelength scale

A new type of a coaxial multi-layer plasmonic waveguide is proposed. The mode propagation properties are analyzed at the communication working wavelength. Theoretical investigations reveal that the enhanced optical confinement can be achieved in the two low-index dielectric media layers. The mode size can be sub- or deep sub-wavelength scale. The mode propagation loss can be well compensated by replacing the high-index dielectric media with gain material to achieve longer propagation length with better mode confinement. The comparisons of the mode properties between the proposed waveguide and waveguides studied in the published literatures are also considered. These investigations potentially lay the groundwork for the further applications of nanowire type multilayer hybrid structures. This structure could also enable various applications such asnanophotonic waveguides, high-quality nanolasers, and optical trapping and biosensors.

Efficient and Accurate Calculation of the Cutoff Wavenumbers of Coaxial Elliptical-Circular and Circular-Elliptical Metallic Waveguides

In this paper, we propose an efficient method for the calculation of the cutoff wavenumbers of coaxial elliptical-circular and circular-elliptical metallic waveguides. The cutoff wavenumbers are obtained through closed-form expressions making the evaluation efficient, and moreover, very accurate even for large values of the eccentricity of the elliptical boundary. The resulting formulas are free of Mathieu functions, including only simple algebraic expressions with Bessel functions, and are valid for every different value of the indices n and m, corresponding to every higher order TMnm or TEnm mode. The validation of the method is performed by comparing to the general exact solution. The efficiency and accuracy of our method is presented by illustrative examples. Numerical results are given for the cutoff wavenumbers of various higher order modes.

Enhancement of single mode operation in coaxial optical waveguide using DB boundary conditions

In this study, a competent numerical strategy to compute the dispersion of optical waveguides is presented and propagation of electromagnetic waves in a coaxial optical waveguide with DB boundary conditions is instigated. For this intend, cylindrical coordinates are here being used to derive the DB boundary conditions and to obtain field components for the modes. The propagation constant for the waveguide to be studied is determined by solving the Bessel and the modified Bessel functions. The cutoff frequencies for various lower order modes have been calculated and their dispersion characteristics are plotted correspondingly. The behavior of the coaxial optical waveguide under DB boundary conditions is shown to be significantly different from that of coaxial optical waveguide and conventional optical waveguide under traditional or tangential boundary conditions. Finally, the effect of waveguide dimensions on the mode cutoff frequencies and fabrication issues are also addressed.

TEM wave radiation from a dielectric-filled coaxial waveguide with a large circumferential gap on its outer wall

TEM wave radiation from a dielectric-filled coaxial waveguide with a large circumferential gap on its outer wall is investigated rigorously by applying direct Fourier transform and reducing the problem into the solution of a modified Wiener–Hopf equation of the first type. Classical Wiener–Hopf procedure is applied by considering two different approaches for the factorization of the kernel function, and the Wiener–Hopf equation is solved via a set of Fredholm integral equations of the second type. The problem is also analyzed by applying a combination of Fourier transform and simple series representation, and the results are compared numerically. Besides, the effects of radii of the walls, relative permittivity, frequency, and the gap width on the scattered fields are illustrated graphically.

Near-field observation of light propagation in nanocoax waveguides.

We report the observation of propagating modes of visible and near infrared light in nanoscale coaxial (metal-dielectric-metal) structures, using near-field scanning optical microscopy. Together with numerical calculations, we show that the propagated modes have different nature depending on the excitation wavelength, i.e., plasmonic TE11 and TE21 modes in the near infrared and photonic TE31, TE41 and TM11 modes in the visible. Far field transmission out of the nanocoaxes is dominated by the superposition of Fabry-Perot cavity modes resonating in the structures, consistent with theory. Such coaxial optical waveguides may be useful for future nanoscale photonic systems.

Stable 0.3-THz Gyrotron Backward-Wave Oscillator With a Tapered Coaxial Interaction Waveguide

Mode competition is a severe problem in the development of a high-power gyrotron backward-wave oscillator (gyro-BWO) in terahertz (THz) region. To improve the stability of this device, this paper investigates the possibility of using a tapered coaxial waveguide as the interaction structure of a 0.3-THz gyro-BWO. The inner cylinder rarefies the mode density near the operating mode. Moreover, tapering the wall of the outer cylinder suppresses almost all the competing modes and shifts the optimum magnetic tuning range of the operating mode under stable operating conditions. The gyro-BWO is predicted to stably generate a peak power of 17.2 kW with 11.5% efficiency at 0.307 THz, and a continuous tuning bandwidth of 3.4 GHz $({\approx}{1.1\%})$ for a 30-kV, 5-A electron beam with $\alpha=1.0$ and an axial velocity spread of 5%. Compared with the uniform case, the tapered coaxial interaction waveguide enhances both the stability and the frequency tunability of the THz gyro-BWO.

Ku대역 모노펄스 위성추적을 위한 동축구조 도파관 고차모드 커플러 설계

본 논문은 Ku대역 모노펄스 위성추적을 위한 동축구조 도파관 고차모드 커플러를 제안하였다. 제안된 고차모드 커플러는 이동체 탑재용 위성추적시스템에 적용 가능하도록 소형/경량화를 위하여 동축구조의 도파관을 이용하였다. 기본모드 신호 추출을 위해 내측 원형도파관을 이용하였고, 고차모드 신호 추출을 위해 외측 원형도파관에 내어진 4개의 슬롯과 스텝 구조의 직사각형 도파관을 이용하였다. 제안된 고차모드 커플러의 모의실험 결과는 반사손실 및 삽입손실이 약 250 MHz의 대역폭(12.75~13.00 GHz)을 만족하며, 모노펄스 추적에 적용 가능한 기본모드 패턴 및 고차모드 패턴이 만족스럽게 생성된 결과를 확인하였다. 제안된 Ku대역 동축구조 도파관 고차모드 커플러는 고속 정밀추적이 요구되는 모노펄스 위성추적시스템에 이용될 것으로 기대된다. In this paper, a higher-order mode coupler using coaxial-structure waveguide for Ku-band monopulse satellite tracking is proposed. The proposed higher-order mode coupler is built in a coaxial structure for compactness and weight reduction, making it suitable for mobile tracking systems. The inner circular waveguide of coaxial-structure is used to extract the fundamental mode signal and the high-order mode signal is extracted from the four slots of the outer circular waveguide and then transmitted via given stepped rectangular waveguide structure. The simulated results show that proposed higher-order mode coupler covers 250 MHz(12.75 GHz ~ 13.00 GHz) bandwidth with return loss and insertion loss characteristics. The antenna patterns of fundamental mode and higher-order mode applicable to monopulse tracking are generated successfully. Designed higher-order mode coupler using coaxial waveguide structure for Ku-band is expected to be used for high precision monopulse satellite tracking systems.

A two-dimensional modelling study of a coaxial plasma waveguide

We present a two-dimensional (2D) plasma model for a coaxial microwave discharge. The microwave energy at driving frequency of 2.45 GHz is fed into a coaxial configuration in which the plasma acts as an outer conductor in such a way that a spatially extended surface wave is created. This geometry permits large surface treatment, especially if a set of such coaxial lines are used. The 2D model describes the plasma as a two-temperature non-chemical equilibrium fluid sustained by electromagnetic surface waves. We present results for argon as a working gas. The importance of wave–plasma power coupling and electron heat transport is discussed. The radial localization of electron density in the coaxial plasma waveguide is observed and compared with experimental results. Moreover, a parameter study allows one to investigate the dependence of radial contraction of the electron density as a function of pressure in the range 2–8 mbar, and also the dependence of electron density with power input in the range 120–1200 W. In order to verify the model, the modelling results are compared with experimental results for the electron density and temperature and good agreement is found.

Simulation Study of the Relationship between Partial Discharge and Ultrahigh-Frequency Electromagnetic Wave in GIS

When partial discharge occurs in Gas Insulated Switchgear (GIS) with insulation defects, Ultrahigh-Frequency (UHF) electromagnetic wave up to several MHz and GHz will be exited and propagate inside GIS cavity. This study, based on the propagation theory of electromagnetic waves in coaxial waveguide, performs simulation analysis of the relationship between PD pulse form and the exited UHF electromagnetic wave using Finite-Deferential Time-Domain (FDTD) algorithm. First, we study the relationship of partial discharge magnitude and electric field strength of electromagnetic wave. It is found that the changes of partial discharge magnitude have little effect on electric field strength of electromagnetic wave at certain variation rate of PD pulse current. Next, we examine the relationship of variation rate of PD pulse current to electric field strength of electromagnetic wave. It is pointed out that, at a certain partial discharge magnitude, the two are approximately linearly related. Finally, we study the impact of variation rate of PD pulse current on higher mode components. Variation coefficient is used to analyze the proportion of higher mode components in electromagnetic wave. The proportion of higher mode components increases with increasing variation rate of PD pulse current.

Excitation of the Azimuthal Surface Waves in Electron Cyclotron Frequency Range by Rotating Electron Beam in a Coaxial Waveguide

The initial stage of interaction between gyrating beam of electrons, which move along Larmor orbits in a narrow gap between a cylindrical plasma layer and an external screen of a metal coaxial waveguide, and electromagnetic eigenwaves is studied theoretically. These waves are extraordinary polarized; they propagate along the azimuthal angle across an axial external steady magnetic field in the electron cyclotron frequency range. The numerical analysis shows that existence of internal metal conductor affects the magnitude of the growth rate weakly. It narrows the range of effective wavenumbers within which effective wave excitation takes place when compared with the case of absence of this conductor. Excitation of these modes is found to be impossible for resonators with relatively small external radii and low plasma densities when compared with a similar structure completely filled with plasma.

Eigenmodes of coaxial quad-ridged waveguides. Numerical results

The results of numerical investigation of coaxial quad-ridged waveguides’ eigenmodes of two configurations (with ridges on inner or on outer perfectly conducting circular cylinder) for different cross-section dimensions are presented. In particular, dependences of cutoff wave numbers on geometrical dimensions ratios for the first three TE modes and for the first TM mode have been investigated and transversal electric field components distributions for these eigenmodes and for theTEM mode have been obtained. Besides, the optimization of coaxial quad-ridged waveguides has been carried out at antiphase excitation in order to provide maximal single-mode operation frequency band for the first TE mode. As a result two optimal configurations of coaxial quad-ridged waveguides with single-mode operation bandwidth ratios 4.6:1 have been designed. It has been defined that the waveguide with the ridges at the inner conducting circular cylinder has smaller cross-section dimensions at the fixed single-mode operation frequency band. Calculations are conducted utilizing the mathematical models obtained in [1] by the integral equations technique taking correctly into account of singular behavior of the field at ridges’ edges. Reliability of the results obtained is confirmed by the calculations based on finite difference time domain technique, which is implemented in the software package CST Microwave Studio at mesh size λ/100.

Mode selection in an S-band relativistic backward wave oscillator based on a coaxial waveguide

The competition of oscillations in a relativistic backward wave oscillator (RBWO) based on a coaxial waveguide has been studied by theoretical and experimental methods. It is established that the starting current for asymmetric magnetic TEm1 type waves is lower than that for the TEM wave. Predominant excita� tion of the former waves is confirmed by numerical simulations using the PICcode KARAT. In an experi� mental RBWO prototype, the excitation of asymmetric oscillations was suppressed by longitudinal cuts in the central conductor of the coaxial waveguide. As a result, stable generation of the TEM wave has been achieved at a frequency of 1.23 GHz, a peak power of 260 MW, and a 28% efficiency of electronbeampower conver� sion into radiation power for a microwavepulse duration of 33 ns.

Dispersion characteristics of the coaxial interlaced disk-loaded waveguide slow-wave structure

The dispersion equation of the coaxial interlaced disk-loaded waveguide slow-wave structure is derived by the multi-conductor transmission line method. The simulation results by HFSS are in good agreement with the calculation results obtained from the dispersion equation. Influences of structure parameters on dispersion characteristics are discussed. It can be concluded that with the increase of inner conductor and the decrease of the period length, the bandwidth of the slow-wave structure becomes greater. The dispersion characteristics of the coaxial interlaced disk-loaded waveguide and those of the coaxial disk-loaded waveguide are compared. The results show that the coaxial interlaced disk-loaded structure can obtain a wide bandwidth and weak dispersion. This study will be a guide to the research of the coaxial interlaced disk-loaded waveguide slow wave structure used in the traveling-wave tube.

The Exact Absorbing Conditions Method in the Analysis of Open Electrodynamic Structures: Circular and Coaxial Waveguides

The exact absorbing conditions (EAC) have been constructed and used for truncating an unbounded domain of computation in open initial boundary value problems, which describe space-time transformations of electromagnetic waves in axially symmetrical waveguides. The equivalence theorem is proved that gives grounds for rigorous theoretical justification of the EAC-method.

Eigenmodes of coaxial quad-ridged waveguides. Theory

The electrodynamics eigenmodes boundary problems’ solutions for the eigenmodes of coaxial quad-ridged waveguides are presented. These solutions have been obtained by integral equations technique utilizing the proposed system of orthogonal basis functions, which take correctly into account singular behavior of the field at the ridges’ edges. The formulas obtained provide possibilities to calculate cutoff wave numbers and electric and magnetic fields distributions for TEM, TE and TM modes in the presence of the ridges either on the inner or on the outer conducting cylinder. Analysis of the dependence of numerical solutions convergence for cutoff wave numbers on the number of basis functions and partial modes has been carried out. It has been shown that for calculation of cutoff wave numbers with residual error less than 0.1% it is enough to utilize the 7 proposed orthogonal basis functions and the 21 partial modes.