Cutoff Frequencies of a Dielectric-Loaded Rectangular Waveguide With Arbitrary Anisotropic Surface Impedance
TL;DR: In this paper, the cutoff frequencies of TE and TM waveguides with arbitrary wall impedance were derived for a rectangular waveguide bounded by periodic rectangular corrugations along the direction of mode propagation.
Abstract: The coupled transcendental equations describing cutoff frequencies of TE and TM modes are deduced for a rectangular waveguide with arbitrary wall impedance. In the case of TE modes, their generalization is made to a rectangular guiding structure, which possesses both distributed wall impedance and inhomogeneous dielectric loading. Effective impedance is obtained for imperfectly conducting surface incorporating periodic rectangular corrugations along the direction of mode propagation. Circular waveguide bounded by such corrugated surface and rectangular impedance waveguide are considered as numerical examples. For these waveguides, the computations performed to determine the cutoff frequencies of TE guiding modes are validated against the results of the perturbation theory and the finite-element method.
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TL;DR: In this paper, a coaxial metal rod with partial dielectric coating is considered as a means for efficient suppression of all volume competing modes in cavities for second-harmonic gyrotrons operated in whispering gallery modes.
Abstract: A coaxial metal rod with partial dielectric coating is considered as a means for efficient suppression of all volume competing modes in cavities for second-harmonic gyrotrons operated in whispering gallery modes. The rod radius is selected small enough to have only a slight effect on operating mode, which therefore remains insensitive to fabrication tolerances and a misalignment of the coaxial insert. By contrast, for the competing modes such a rod is shown to reduce the effective cavity length, thereby greatly increasing the starting currents. Such a method of mode selection is demonstrated to be more versatile, when compared to that provided by a tapered coaxial conductor. The advantage of a dielectric-coated coaxial insert is illustrated by the example of a cavity for a 100-kW 300-GHz pulsed gyrotron operated in the second-harmonic mode.
16 citations
Cites background from "Cutoff Frequencies of a Dielectric-..."
...surface [13], for example, imperfectly conducting surface [13] or corrugated surface [28], [29]....
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TL;DR: In this paper, the authors investigated wave scattering from a cylindrical cylinder with a tensor impedance surface based on the Lorentz-Mie theory and showed that the surface impedance of the cylinder can be optimized to maximize the scattering cross-section by tailoring the surface surface impedance.
Abstract: Wave scattering from a cylinder with a tensor impedance surface is investigated based on the Lorentz-Mie theory. A practical example of such a cylinder is a subwavelength metallic rod with helical dielectric-filled corrugations. The investigation is performed with the aim to maximize scattering cross-section by tailoring the surface impedance of cylindrical scatterers. For the normally incident TEz and TMz waves the required surface impedance of a subwavelength cylinder can be produced by longitudinal (axial) and transverse (circumferential) corrugations, respectively. It is shown that such corrugations induce superscattering at multiple frequencies, which can be widely tuned with either or both the size and permittivity of dielectric-filled corrugations. In the microwave band, this effect is demonstrated to be robust to material losses and is validated against the full-wave simulations and experiment. For the TEz waves the enhanced scattering from the cylinder is found to have a broad frequency bandwidth, provided that the relative permittivity of corrugations is low or equal unity. In the latter case, the corrugated cylinder acts as an all-metal superscatterer. For such cylinders the near-field measurements are implemented and provide the first experimental evidence of the superscattering phenomenon for all-metal objects. In addition to multifrequency superscattering, the dielectric-filled corrugations are shown to provide multifrequency cloaking of the cylinder under the incidence of the TMz waves. Simultaneous superscattering and cloaking at multiple frequencies distinguishes corrugated cylinders from other known practicable scatterers for potential applications in antenna designing, sensing, and energy harvesting.
16 citations
Cites background from "Cutoff Frequencies of a Dielectric-..."
...Note that the obtained result, if necessary, can be generalized to account for finite conductivity of metallic corrugations [45, 51, 52]....
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...The first one is that the condition k(2) ‖ εk 2 is true, where k(2) ‖ = (kz cos θ + nR −1 sin θ)(2) [32, 52]....
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...It takes the well-known diagonal form in the extreme cases of θ = 0◦ and θ = 90◦, which correspond to the PEC cylinder with longitudinal [47, 48, 51, 52] and transverse [22, 24, 47] rectangular corrugations, respectively....
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15 citations
TL;DR: In this article, the spatial harmonic method is applied to investigate selectivity properties of longitudinally corrugated waveguides for potential application in second-harmonic gyrotrons.
Abstract: Spatial harmonic method is applied to investigate selectivity properties of longitudinally corrugated waveguides for potential application in second-harmonic gyrotrons. The effect of corrugations on frequencies, ohmic losses, and mode conversion of guiding TE modes is studied in details. Numerical results are presented for operating second-harmonic and competing first-harmonic modes of a 0.4-THz gyrotron with corrugated RF structure. It is shown that longitudinal wall corrugations of proper dimensions can ensure increase in ohmic losses and decrease in beam-wave coupling strength for the first-harmonic modes, while their effect on the operating mode is only slight. This demonstrates improved selectivity properties of corrugated waveguides for second-harmonic gyrotrons.
12 citations
TL;DR: In this article, the eigenvalue problem for a longitudinally inhomogeneous waveguide with impedance walls was solved by applying the cross-section method to reduce the problem to an infinite set of ordinary differential equations for amplitudes of basis modes.
Abstract: The eigenvalue problem is solved for a longitudinally inhomogeneous waveguide with impedance walls. The cross-section method is applied to reduce the problem to an infinite set of ordinary differential equations for amplitudes of basis modes. As a numerical example, a tapered metallic cavity of currently available THz gyrotron is considered. The combined effect of mode coupling (conversion) and ohmic wall losses on electromagnetic properties of the gyrotron cavity is considered and discussed.
11 citations
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TL;DR: In this article, a photonic bandgap (PBG) structure is proposed to realize a magnetic surface in the stopband and is used in the waveguide walls to provide magnetic boundary conditions.
Abstract: A novel waveguide using a photonic bandgap (PBG) structure is presented. The PBG structure is a two-dimensional square lattice with each cell consisting of metal pads and four connecting lines, which are etched on a conductor-backed Duroid substrate. This uniplanar compact PBG structure realizes a magnetic surface in the stopband and is used in the waveguide walls to provide magnetic boundary conditions. A relatively uniform field distribution along the cross section has been measured at frequencies from 9.4 to 10.4 GHz. Phase velocities close to the speed of light have also been observed in the stopband, indicating that TEM mode has been established. A recently developed quasi-Yagi antenna has been employed as a broad-band and efficient waveguide transition. Meanwhile, full-wave simulations using the finite-difference time-domain method provide accurate predictions for the characteristics of both the perfect magnetic conductor impedance surface and the waveguide structure. This novel waveguide structure should find a wide range of applications in different areas, including quasi-optical power combining and the electromagnetic compatibility testing.
383 citations
"Cutoff Frequencies of a Dielectric-..." refers background or methods in this paper
...The reason is that such guiding structure can support quasi-TEM modes [9] having widespread applications in the microwave technology....
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...Among them are the field expansion method [15], [16], the finite-element method (FEM) [4], [5], [7], [8] [10], [11], [17], and the finite-difference timedomain method [9], [12], [17], which now find use in various commercial software packages....
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TL;DR: A review of the development and the present state-of-the-art of gyrotrons for controlled thermonuclear fusion plasma applications can be found in this paper, where the first gyrotron was invented, designed and tested in Gorky, USSR (now Nizhny Novgorod, Russia), in 1964.
Abstract: Gyrotrons form a specific group of devices in the class of fast-wave vacuum electronic sources of coherent electromagnetic wave radiation known as electron cyclotron masers (ECMs) or cyclotron resonance masers (CRMs). The operation of CRMs is based on the cyclotron maser instability which originates from the relativistic dependence of the electron cyclotron frequency on the electron energy. This relativistic effect can be pronounced even at low voltages when the electron kinetic energy is small in comparison with the rest energy. The free energy for generation of electromagnetic (EM) waves is the energy of electron gyration in an external magnetic field. As in any fast-wave device, the EM field in a gyrotron interaction space is not localized near a circuit wall (like in slow-wave devices), but can occupy large volumes. Due to possibilities of using various methods of mode selection (electrodynamical and electronic ones), gyrotrons can operate in very high order modes. Since the use of large, oversized cavities and waveguides reduces the role of ohmic wall losses and breakdown limitations, gyrotrons are capable of producing very high power radiation at millimeter and submillimeter wavelengths. The present review is restricted primarily by the description of the development and the present state-of-the-art of gyrotrons for controlled thermonuclear fusion plasma applications. The first gyrotron was invented, designed and tested in Gorky, USSR (now Nizhny Novgorod, Russia), in 1964.
177 citations
"Cutoff Frequencies of a Dielectric-..." refers background in this paper
...For example, such is the case for gyrotrons [20], which always operate near selected cutoff frequencies of the metallic guiding structure....
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TL;DR: In this article, the authors investigated coaxial gyrotron cavities with longitudinal slots on the inner conductor as a means to reduce the number of possible competing modes, and showed that for certain cavity parameters a mode could have its energy concentrated close to the inner conductors.
Abstract: This paper investigates coaxial gyrotron cavities with longitudinal slots on the inner conductor as a means to reduce the number of possible competing modes. In the analytic theory the corrugated surface is treated as a homogeneous impedance surface ("impedance corrugation") to obtain simple formulas for the characteristic equation of the eigenmodes, for the electromagnetic fields and the wall losses. The developed model applies if the number of slots is sufficiently high (cutoff wavelength much larger than the corruption period). The characteristic equation in terms of the ratio C of the outer wall radius to the inner conductor radius is solved numerically to determine a range of eigenvalues and C where the eigenvalue curves are monotonically decreasing. In such a region a cavity having its inner conductor downtapered (radius decreasing toward the cavity output) can be used to reduce the diffractive quality factors of several modes, leaving the working mode undisturbed and without favoring other modes. In addition the electromagnetic field profiles are investigated, and in particular it is shown that for certain cavity parameters a mode could have its energy concentrated close to the inner conductor. As a check on the validity of the theoretical approximations, simulations with the MAFIA code are carried out. These give good agreement with the results of the analytic equations.
151 citations
"Cutoff Frequencies of a Dielectric-..." refers background in this paper
...N of corrugations, then the corrugated walls act on this mode as smooth surface r = R with the averaged surface impedance W̄ = (1 − h/p)η + h/pW [24], [25], [38], [41], [43], where p = 2π R/N is the corrugation period....
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...As the wall conductivity σ tends to infinity (dc = 0), the averaged surface impedance W̄ takes the wellknown form [25], [41], [43] for the PEC cylindrical surface with longitudinal corrugations....
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TL;DR: In this paper, the first W-band gyrotron Traveling-Wave Tube (gyro-TWT) was developed to exploit the 94 GHz atmospheric window for long-range, high-resolution radar applications.
Abstract: Experimental results are presented on the first W-band gyrotron Traveling-Wave Tube (gyro-TWT) developed to exploit the 94 GHz atmospheric window for long-range, high-resolution radar applications. The gyro-TWT is designed to operate in the higher order TE01 mode and is driven by a 100 kV, 5 A electron beam with a pitch angle of v⊥/vz=1 and velocity spread of Δvz/vz=5%. Large-signal simulations predict 140 kW output power at 92 GHz with 28% efficiency, 50 dB saturated gain, and 5% bandwidth. The stability of the amplifier against spurious oscillations has been checked with linear codes. To suppress the potential gyro-BWO interactions involving the TE02, TE11, and TE21 modes, the interaction circuit with a cutoff frequency of 91 GHz has been loaded with loss so that the single-path, cold-circuit attenuation is 90 dB at 93 GHz. A coaxial input coupler with 3% bandwidth is employed with a predicted and measured coupling of 1 dB and 2 dB, respectively. The operating voltage is limited to below 75 kV because of oscillations encountered at higher voltages in this initial embodiment. Preliminary test at Vb=60 kV and Ib=3.7 A yielded 59 kW saturated output power at 92.2 GHz with 42 dB gain, 26.6% efficiency, and a 3 dB bandwidth of 1.2 GHz (1.3%).
127 citations
TL;DR: Absolute instabilities in the gyrotron traveling wave amplifier are investigated with a simulation approach which models electron and wave dynamics in the sever and distributed-loss section.
Abstract: Absolute instabilities in the gyrotron traveling wave amplifier are investigated with a simulation approach which models electron and wave dynamics in the sever and distributed-loss section. Distributed wall losses are shown to be far more effective than the sever in stabilizing these instabilities. Physical interpretations are given and theoretical predictions are verified by a K„-band experiment which achieved 62 kW peak power with 12% bandwidth, 21% efficiency, and 33 dB saturated gain through the use of a mechanically adjustable magnetron injection gun.
123 citations
"Cutoff Frequencies of a Dielectric-..." refers background in this paper
...materials (graphite, lossy ceramics, and carbon colloid) used in waveguides with distributed wall losses [48]–[50]....
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