Mode Cylindrical Cavity Research Articles

Analysis and Control of Acoustic Modes in Cylindrical Cavities with application to Direct Field Acoustic Noise (DFAN) Testing.

It is well known that acoustic cavities have frequencies at which certain free-response 'modes' of propagation respond especially strongly. In the absence of significant damping, these cause peaks of high SPL in the frequency response as well as spatial non-uniformity and temporal ringing. The spatial non-uniformity is especially problematic since it means the room cannot be 'EQ'd' to compensate, since the SPL is different in different positions. This phenomenon has been studied extensively in the room acoustics literature and various strategies for mitigation proposed. Many of these make use of the theoretical mode shapes for a cuboid with a rigid boundary condition, since this is a common shape of room and a reasonable approximation for a solidly constructed wall. But modes exist for other shape spaces too. Of particular interest is the cylindrical cavity that is formed when large enclosing arrays of loudspeakers are used to perform high-intensity acoustic tests on space hardware. These possess problematic modes that can cause over-testing in some positions and under-testing in others. In this work, it is investigated how a simple FEM simulation can compute Q-factors for these modes and identify which will be problematic. How this might inform control system design is discussed.

Lift-Off and Tilt Effect in Microwave Surface Resistance Measurement Using TE011 Mode Cylindrical Cavity

Measurement of surface resistance at microwave or higher frequency bands is needed for applications, such as accelerators, microwave systems, and integrated circuits. Cylindrical cavity resonators operating at TE₀₁₁ mode have been applied in surface resistance measurement for decades. It is common that samples under test are usually used as one of the cavity&#x2019;s endplates and contact the cylinder side wall directly, which cannot be regarded as noncontact measurement. In this article, through quantitative research on lift-off effect of samples under test, we explore the possibility of realizing noncontact surface resistance measurement by intentionally introducing an air gap (ranging from &#x007E;0 to 10 mm) between the samples and the cavity side wall. From the results of both electromagnetic simulations and experiments, it can be concluded that by taking care of the sample&#x2019;s tilt effect, surface resistance can be measured in a noncontact manner. Measurement results of polished metal plates with electrical conductivity ranging from &#x007E;2 to 58 MS/m (measured with standard eddy current method) show that the relative measurement error is expected to be less than 10&#x0025;. Compared with related publications, the focus of this work is put on the lift-off effect and a larger range of air gap is introduced. The obtained results indicate that the air-gap size may be an additional consideration in the design of surface resistance measurement system for special applications, such as temperature-dependent measurements in vacuum chamber, <i>in situ</i> diagnosis tools.

RF behavior of a 35 GHz conventional cavity gyrotron using multimode analysis and PIC simulation

ABSTRACT In this paper, the RF behavior of a 35 GHz, TE 04 mode cylindrical cavity gyrotron is investigated using the nonlinear multimode theory and PIC code “CST Particle Studio”. A code is developed to realize the beam-wave interaction mechanism using time-dependent multimode theory. The cold cavity behavior is observed for the cavity operation in the designed TE 04 mode and 35 GHz frequency. The magnetic field profile is kept uniform along the length of the interaction structure. The output power is estimated for the designed mode as well as all nearby competing modes using the multimode analysis and PIC simulation. This yields the output power more than 200 kW with 33% efficiency in the designed TE 04 mode, whereas the smaller output power in the nearby competing modes. Power sustainability is also observed for the stable operation of the device. The effects of beam velocity spread and detuning of magnetic field on its performance are also observed.

Starting currents of modes in cylindrical cavities with mode-converting corrugations for second-harmonic gyrotrons

A self-consistent system of equations (known as single-mode gyrotron equations) is extended to describe the beam-wave interaction in a cylindrical gyrotron cavity with mode-converting longitudinal corrugations, which produce coupling of azimuthal basis modes. The system of equations is applied to investigate the effect of corrugations on starting currents of the cavity modes. For these modes, eigenvalues, ohmic losses, field structure, and beam-wave coupling coefficients are investigated with respect to the corrugation parameters. It is shown that properly sized mode-converting corrugations are capable of improving the selectivity properties of cylindrical cavities for second-harmonic gyrotrons.

Low‐profile ring slot SIW antenna based on higher‐order cylindrical cavity modes

A novel ring slot substrate-integrated waveguides (SIW) antenna is presented based on higher-order cylindrical cavity mode. First, TM02 is used to realise a conical beam antenna. Electric field distribution on slot aperture generates four pairs of magnetic monopoles, each rotated 90° with respect to the centre of the cavity, resulting in a conical beam. By applying an offset to the feed point, the loading effect of slots generates a hybrid mode, which is a combination of TM31 and TM12. The hybrid mode aligns the magnetic monopoles; thus, a high-gain antenna with broadside radiation can be realised. The measured results show a 10.5% impedance bandwidth, 11.5 dBi gain, and 90% radiation efficiency. The proposed antenna is low-profile and compact with a planar structure that could be integrated with other planar circuits; therefore, it could have many practical applications in communication systems.

Design of a rectangular waveguide to cylindrical cavity mode launcher for TE011mode with maximum quality‐factor

Design of a rectangular waveguide to cylindrical cavity mode launcher for TE₀₁₁mode with maximum quality‐factor

Electrons acceleration in a TE113 cylindrical cavity affected by a static inhomogeneous magnetic field

The relativistic dynamics of an electron accelerated in a cylindrical cavity mode TE113 in the presence of a static inhomogeneous magnetic field is studied. This type of acceleration is known as Spatial AutoResonance Acceleration (SARA). The magnetic field profile is such that it keeps the phase difference between the electric microwave field and the electron velocity vector within the acceleration phase band. We study the dynamic of the electron through simulations of the relativistic Newton-Lorentz equation. Numerical experiments with TE113 cylindrical microwave fields of a frequency of 2.45GHz and an amplitude of the order of 7kV/cm show that it is possible accelerate the electrons up to energies of the order of 300keV. This energy is about of 30% higher than those obtained in previous studies by using the TE112 mode.

Higher order reentrant post modes in cylindrical cavities

Reentrant cavities are microwave resonant devices employed in a number of different areas of physics. They are appealing due to their simple frequency tuning mechanism, which offers large tuning ranges. Reentrant cavities are, in essence, 3D lumped LC circuits consisting of a conducting central post embedded in a resonant cavity. The lowest order reentrant mode (which transforms from the TM010 mode) has been extensively studied in past publications. In this work, we show the existence of higher order reentrant post modes (which transform from the TM01n mode family). We characterize these new modes in terms of their frequency tuning, filling factors, and quality factors, as well as discuss some possible applications of these modes in fundamental physics tests.

Compressive sensing based spherical harmonics decomposition of a low frequency sound field within a cylindrical cavity.

A low frequency sound field within a cylindrical cavity can be well approximated by a sparse set of Fourier-Bessel series in a spherical coordinate system. The approximation accuracy can be guaranteed as long as the series coefficients are well estimated by use of spherical microphone arrays (SMA). Conventional methods like spherical Fourier transform and Helmholtz equation least square require a large number of sensors, and it is difficult to estimate the high order coefficients in the presence of sensor noise. To cope with these issues, compressive sensing (CS) is utilized and compared with the conventional methods. In this study, the estimate of the high order coefficients from contaminated measurements is examined, and the effect of modal behavior is analyzed. Numerical simulations show that sensors can be significantly saved especially for high order cavity modes due to the low sparsity of coefficients which is also stable as the modal frequency increases, and the overall accuracy is also improved. The optimal SMA configuration for the use of CS is studied. A large SMA with omni-microphone deployed by the Fliege sampling scheme is suggested. The CS results are finally validated through the good simulated reconstructions of cylindrical cavity modes.

An all‐dielectric band‐stop frequency‐selective surface

ABSTRACTIn this paper, an all‐dielectric band‐stop frequency‐selective surface (FSS) is proposed for X‐band applications. The unit particle of the FSS consists of a thick upper hexagonal prism part and a thin lower hexagonal prism part for monolithic construction. The proposed FSS has a planar array of hexagonal ceramic prisms with a honeycomb‐shaped structure. The proposed FSS has three resonant dips. These are combined to establish a −10 dB fractional bandwidth (FBW) with a broad stop‐band of 31.8% (center frequency of 9.75 GHz). To demonstrate the resonant characteristic modes, cylindrical cavity mode analysis based on cylindrical wave functions is used. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 59:56–60, 2017

Acoustic Scattering of 3D Complex Systems Having Random Rough Surfaces by Scalar Integral Equations

Abstract We propose a numerical surface integral method to study complex acoustic systems, for interior and exterior problems. The method is based on a parametric representation in terms of the arc’s lengths in curvilinear orthogonal coordinates. With this method, any geometry that involves quadric or higher order surfaces, irregular objects or even randomly rough surfaces can be considered. In order to validate the method, the modes in cubic, spherical and cylindrical cavities are calculated and compared to analytical results, which produced very good agreement. In addition, as examples, we calculated the scattering in the far field and the near field by an acoustic sphere and a cylindrical structure with a rough cross-section.

Linear analysis of 170 GHz cylindrical and confocal gyrotron

In this paper, the mode density, coupling coefficient, eigenfrequencies, quality factors and starting currents of a 170 GHz cylindrical and confocal gyrotron are studied based on the linear theory. Results show that the mode density in the confocal cavity greatly decrease due to the diffraction properties. Modes in the confocal cavity have lower quality factors than the modes in cylindrical cavity, hence the starting currents are higher than the latter. The confocal gyrotron is more stable than the cylindrical gyrotron when operating at large current. Meanwhile, through altering the mirror width, quality factors together with the starting current of the confocal modes are adjustable.

Low frequency sound spatial encoding within an enclosure using spherical microphone arrays.

In a spherical coordinate system, interior sound field can be expressed in terms of a series of Fourier-Bessel expansions. The process that obtains the expansion coefficients by use of a microphone array (e.g., a spherical microphone array) is called spatial encoding. Until now spatial encoding has mainly been examined in a free field or a diffuse field which can be modeled as a sum of plane waves. For spatial encoding within an enclosure at low frequencies, special challenges would be encountered in two aspects. First, the expansions are influenced by array configurations. Second, an acoustic mode based model instead of a plane wave based one should be considered. This study focuses on these challenges. Different kinds of array configurations were compared specifically at low frequencies, and the spatial encoding for the cylindrical cavity modes was investigated. It was found that the spherical array with cardioid microphones was optimal when kr<1, the cavity modes can be effectively represented by only a sparse subset of expansion coefficients and a good reproduction can be achieved even outside the spherical valid region, which demonstrates an effective alternative way to describe the cylindrical cavity modes and can be implemented efficiently in practice.

Dynamic Measurement of Dielectric Properties of Materials at High Temperature During Microwave Heating in a Dual Mode Cylindrical Cavity

A microwave cavity and heating system for microwave processing and in situ dynamic measurements of the complex permittivity of dielectric materials at high temperatures ( $\sim {\hbox{1000}} ^{\circ}{\hbox{C}}$ ) has been developed. The method is based on a dual-mode cylindrical cavity where heating and testing are performed by two different swept frequency microwave sources. A cross-coupling filter isolates the signals coming from both sources. By adjusting the frequency bandwidth of the heating source and the level of coupling to the cavity, an automatic procedure allows for the establishment of a desirable level of heating rate to the dielectric sample to reach high temperatures in short cycles. Dielectric properties of materials as a function of temperature are calculated by an improved cavity perturbation method during heating. Accuracy of complex permittivity results has been evaluated and an error lower than 5% with respect to a rigorous analysis of the cavity has been achieved. The functionality of the microwave dielectric measurement system has been demonstrated by heating and measuring glass and ceramic samples up to ${\hbox{1000}} ^{\circ}{\hbox{C}}$ . The correlation of the complex permittivity with the heating rate, temperature, absorbed power, and other processing parameters can help to better understand the interactions that take place during microwave heating of materials at high temperatures compared to conventional heating.

Electric Field Homogeneity Optimization by Dielectric Inserts for Improved Material Sensing in a Cavity Resonator

A TM010 mode cylindrical cavity is employed as the main sensing element to characterize variable density compositions of material streams, for example, paper, textile fiber, or plastic composites. Through-passage holes at both end plates in the middle of the resonator frame provide for the material flow, hence forming a cylindrical channel region that is isolated from the cavity by a thin dielectric insert tube to avoid contamination of the cavity. In relation to the cavity dimensions, the hole size is relatively large to accommodate material flow variations in volume or mass density. The large through-hole geometric feature causes the electric field uniformity to be compromised within the channel region. As a consequence, unwanted frequency shift responses occur due to lateral movements in the material stream. A study related to the electric field homogeneity of a cylindrical cavity is presented. It is shown that the field homogeneity can be improved by placing a tubular dielectric insert inside the cavity. The main aim is to optimize the shape profile and select a feasible material for the dielectric insert. The evaluation of the electric field homogeneity is based on the measure of electric field variations within the channel. Dielectric inserts with conical profile are presented as an improved alternative to cylindrical tubes for uniform field homogeneity inside the sensing region.

Design and simulation of a C-band SLED with mode converter

The transient response analysis of the SLED based on the equivalent circuit is described. Then, a C-band SLED using TE0,1,15 mode cylindrical cavity with TE10-TE01 mode converter has been designed. According to the main RF parameters of the accelerator, the coupling coefficient is optimized to obtain the maximum multiplication factor. The key components of the pulse compressor include a 3 dB directional coupler, a TE10-TE01 mode converter, and a cylindrical cavity, which are simulated and optimized using 3D electromagnetic field simulation software. In addition, the function defining the relation between the coupling factor and aperture size is derived by a mathematical fitting method.

Theory and Experiment of a W-Band Tunable Gyrotron Oscillator

A gyrotron capable of both frequency and power tuning is a promising coherent millimeter-THz wave source. A self-consistent nonlinear theory is applied to investigate the electron cyclotron interaction between electron beam and wave modes of axial nonfixed profiles in an extended W-band <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">${\rm TE}_{01}$ </tex-math></inline-formula> mode cylindrical cavity. It is revealed that tuning the magnetic field strength can excite electron cyclotron resonances on forward wave, backward wave, and even simultaneous on both waves, which makes the system operate under distinctive states, namely the gyrotron backward wave oscillation state and the gyromonotron state. In this paper, a W-band prototype gyrotron oscillator based on an extended cylindrical waveguide cavity is built, and the experiment test indicates that the system starts oscillation in a relative wide range of the operation parameters. The measured frequency spectrum reveals the system iteratively switches between the lower order instability axial modes, and it operates under nonstationary oscillation states. The experimental measurement of highest output power <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">${\sim}{\rm 8}~{\rm kW}$ </tex-math></inline-formula> is consistent with the theoretical predictions. An optimized gyrotron circuit with efficiency exceeding 20% and tunable bandwidth over 10 GHz is also presented. The free oscillation behaviors revealed in this paper provide interesting guidance for developing tunable gyrotrons in millimeter-THz wave range.

Design and simulation of a C-band pulse compressor

The design and optimization procedure of a pulse compressor is presented. A C-band (5712 MHz) pulse compressor using a TE0,1,15 mode cylindrical cavity with dual side-wall coupling irises has been designed. Also the coupling coefficient, position of the short plane and size of the bottom groove have been optimized by using HFSS.

Mode Counting in Rectangular, Cylindrical, and Spherical Cavities With Application to Wireless Measurements in Reverberation Chambers

The accuracy of wireless measurements in a reverberation chamber depends on the mode density. Reverberation chambers are normally rectangular in shape, but could also be made cylindrical or spherical. In this paper, modes in classical rectangular, cylindrical, and spherical cavities are counted over large frequency ranges in order to determine the number of modes over a specific excitation bandwidth as a function of frequency, i.e., mode density. The cavities are then judged by these mode densities for applications to reverberation chambers.

Growth of Si0.75Ge0.25 alloy nanowires in a separated H-field by microwave processing

This paper presents a rapid and novel technique of growing nanowires of Si0.75Ge0.25 alloy at 900 °C in less than 10 min by processing the constituents in a TE011 single mode cylindrical resonant cavity, operated at 2.45 GHz and ∼300 W. The microstructural, crystal structural, and x-ray photoelectron spectroscopic studies of the nanowires have been carried out to establish their dimensions, crystallographic structure, and composition, respectively. It is proposed that the growth of nanowires is due to electromagnetic field assisted morphological transformation.