Diffraction Q-factor Research Articles

Stable Excitation of Higher Axial Modes in the Traveling-Wave-Tube Regime in Gyrotron Cavities With Additional Loss Elements

Weakness of the electron-wave coupling in compact low-power terahertz-frequency-range gyrotrons due to relatively low electron beam currents and operation at high cyclotron harmonics results in the use of long-length cavities with high diffraction Q-factors, and, therefore, with a high share of the ohmic loss of the radiated wave power. In this work, we demonstrate a possibility for stable operation at higher axial modes possessing relatively low diffraction Q-factors. Enhanced efficiency of the electron-wave interaction is provided due to the excitation of higher axial modes in the traveling-wave-tube regime.

Terahertz Large-Orbit High-Harmonic Gyrotrons at IAP RAS: Recent Experiments and New Designs

We describe new versions of high-harmonic gyrotrons with axis-encircling electron beams developed on the basis of two (80-keV pulsed and 30-keV continuous wave) experimental setups. Selective operation at the second (0.267 THz) and at the third (0.394 THz) cyclotron harmonics was observed in the 30-keV gyrotron. Cavities with periodic phase correctors, where a far-from-cutoff axial mode with a decreased diffraction Q-factor is excited in a gyrotron-like regime, are designed to improve the operation at the third harmonic, as well as to achieve the fourth-harmonic operation at frequencies up to 0.65 THz. At the pulsed gyrotron setup, a sectioned cavity with a decreased diffraction Q-factor was experimentally tested.

Stability of Excitation of Traveling Waves in Gyrotrons With Low-Relativistic Electron Beams

A number of application require compact low-power gyrotrons with relatively low operating currents. In order to provide the start of such gyrotrons, one should use long-length operating cavities with high diffraction Q-factors, and, therefore, with significant losses of the radiated wave power due to Ohmic heating of the cavity wall. In this paper, we demonstrate that in low-voltage gyrotrons there is a possibility for stable operation at higher order axial modes possessing relatively low diffraction Q-factors. An enhanced efficiency of the beam-wave interaction is provided due to the excitation of such modes in the traveling-wave-tube regime.

Use of Quasiregular Resonator Cavities with Short Phase Correctors in Gyrotrons Operated at Higher Cyclotron Harmonics

We discuss the possibilities of using quasiregular resonator cavities with short irregularities, which ensure correction of the wave phase, in low-power gyrotrons operated at higher cyclotronfrequency harmonics. The use of such phase correctors can help with solving two problems, namely, increasing the selectivity of excitation of a higher cyclotron harmonic and decreasing the diffraction Q-factor of the gyrotron wave excited in an extended cavity.

Optimization of terahertz range gyrotron self-excitation conditions by increasing the lifetime of cyclotron oscillators in low-voltage interaction space

We propose a method of reducing the starting and operating currents of short-wavelength gyrotrons that is based on the application of a decelerating voltage directly to the resonator. The desired effect is achieved due to increasing electron lifetime in the interaction space. At a preset injection current, this circumstance ensures gyrotron self-excitation at a relatively low diffraction Q-factor, which leads to significant reduction in ohmic losses.

Optimal parameters of gyrotrons with weak electron-wave interaction

In low-power gyrotrons with weak electron-wave interaction, there is a problem of determining the optimal length of the operating cavity, which is found as a result of a tradeoff between the enhancement of the electron efficiency and the increase in the Ohmic loss share with increasing cavity length. In fact, this is the problem of an optimal ratio between the diffraction and Ohmic Q-factors of the operating gyrotron mode, which determines the share of the radiated rf power lost in the cavity wall. In this paper, this problem is studied on the basis of a universal set of equations, which are appropriate for a wide class of electron oscillators with low efficiencies of the electron-wave interaction.

Experimental Study of a Gyrotron with a Sectioned Klystron-Type Cavity Operated at Higher Cyclotron Harmonics

We are planning to use extended cavities in order to excite gyrotrons with large electron orbits, which are operated at higher cyclotron harmonics in the terahertz frequency range. This is determined by both the weakness of the electron-wave interaction, and relatively low operating currents. Since the diffraction Q-factor of such cavities is high, a significant part of the highfrequency power produced by the electron beam is lost due to the ohmic loss in the cavity walls. As a way to solve this problem, we proposed a sectioned klystron-type cavity, where an extended length of the electron-wave interaction region can be combined with a relatively low diffraction Q-factor of the system. This work presents the results of the first experiment on a gyrotron with a sectioned cavity, where selective excitation of higher (second and third) cyclotron harmonics was observed in the terahertz frequency range (0.55 and 0.74 THz).

Gyrotron with a sectioned cavity based on excitation of a far-from-cutoff operating mode

A typical problem of weakly relativistic low-power gyrotrons (especially in the case of operation at high cyclotron harmonics) is the use of long cavities ensuring extremely high diffraction Q-factors for the operating near-cutoff waves. As a result, a great share of the rf power radiated by electrons is spent in Ohmic losses. In this paper, we propose to use a sectioned cavity with π-shifts of the wave phase between sections. In such a cavity, a far-from-cutoff axial mode of the operating cavity having a decreased diffraction Q-factor is excited by the electron beam in a gyrotron-like regime.

Experimental Realization of the High-Harmonic Gyrotron Oscillator With a Klystron-Like Sectioned Cavity

High-harmonic terahertz-frequency-range gyrotrons require long-length operating cavities due to both weak electron-wave coupling and relatively low electron currents. Since diffraction Q-factors of such cavities are very high, a great share of the radiated RF power is spent to ohmic losses. A sectioned klystron-like cavity was proposed as a way to combine a long electron-wave interaction region with a relatively low diffraction Q-factor. In this brief, results of the first experiment under the high-harmonic gyrotron with the klystron-like cavity are reported. In this experiment, selective operation of the gyrotron at the second (0.55 THz) and the third (0.74 THz) cyclotron harmonics has been observed.

A millimeter wave relativistic backward wave oscillator operating in TM03 mode with low guiding magnetic field

A V-band overmoded relativistic backward wave oscillator (RBWO) guided by low magnetic field and operating on a TM03 mode is presented to increase both the power handling capacity and the wave-beam interaction conversion efficiency. Trapezoidal slow wave structures (SWSs) with shallow corrugations and long periods are adopted to make the group velocity of TM03 mode at the intersection point close to zero. The coupling impedance and diffraction Q-factor of the RBWO increase, while the starting current decreases owing to the reduction of the group velocity of TM03 mode. In addition, the TM03 mode dominates over the other modes in the startup of the oscillation. Via numerical simulation, the generation of the microwave pulse with an output power of 425 MW and a conversion efficiency of 32% are achieved at 60.5 GHz with an external magnetic field of 1.25 T. This RBWO can provide greater power handling capacity when operating on the TM03 mode than on the TM01 mode.

Electronic Tuning of the Working Frequency of a Gyrotron with an Echelette Structure

We discuss the possibility of developing high-power gyrotrons with electric tuning of operating frequencies on the basis of the use of echelette systems. Along with the high selective properties, such systems are capable of reducing Q-factors of modes so that resonance properties completely disappear, while the electromagnetic-field structure, which ensures the efficient interaction of the field with the electron beam, is retained. A gyrortron with an echelette cavity is described, in which the Q-factor of the working mode was below the minimum diffraction Q-factor and the working frequency was tuned in a wide range.

The reflex gyrotron

A typical problem of low-power short-wavelength gyrotrons is that a too long cavity is required to provide the start of rf oscillations. Therefore, it is attractive to realize a gyrotron with a long enough operating region but relatively low diffraction Q-factor of the operating cavity. In this work, we propose a gyrotron with reflecting of electrons back to the operating cavity. Such a scheme provides significant decrease of the starting current and considerable efficiency enhancement.

RESONANCE INCREASE OF THE TM-EIGENMODE Q-FACTOR IN AN OPEN RESONATOR WITH A SPHERICAL DIELECTRIC INCERTION

A rigorous mathematical model has been constructed for axially-symmetric TM-eigenmodes in an open spherical-mirror resonator with a spherical dielectric insertion. The effect of decreasing (increasing) the eigenmode diffraction Q-factor with changing the geometrical and constitutive parameters of the spherical insertion has been revealed and investigated. The nature of the resonance behavior shown by the eigenmode Q-factor has been established.

Axisymmetric multistage cavity resonators

We study theoretically multistage cavity resonators as well as the gyrotrons manufactured on their basis. Coupled cavities with mode conversion and echelette cavities are particular cases of such devices. The possibilities of reducing the diffraction Q-factor of the operating mode of a cavity, with the preservation of its selective properties and significant increase in the gyrotron radiation power, and provision of high efficiency and acceptable heat ohmic load of the cavity at the first harmonic of the gyrofrequency are considered. Achievable parameters of the gyrotrons at the second and third harmonics with typical electron guns and cryomagnetic systems are analyzed.

High-harmonic gyrotron with sectioned cavity

High-harmonic large-orbit gyrotrons require long-length operating cavities because of both a weak electron-wave coupling and relativeily low electron currents. Since diffraction Q-factors of such cavities are very high, a great share of radiated rf power is spent to Ohmic losses. In this letter, a sectioned klystronlike cavity is proposed as a way to combine a long electron-wave interaction region with a relatively low diffraction Q-factor. In addition, the use of such a cavity makes a relatively broadband frequency tuning possible.

Open Resonators with Conducting Cylindrical Inserts. 2. Resonators with Finite-Length Mirrors

We present the results of experimental studies of controlling the spectrum and Q-factors of modes in an open resonator with a conducting cylindrical insert. The phenomenon of an increase in the diffraction Q-factor is experimentally observed for the fundamental TEM00q mode for two resonant diameters of the conducting cylindrical insert located inside the open resonator. The features of excitation of modes in an open resonator with smooth cylindrical finite-length mirrors and in an open resonator with cutoff bevels at the mirror edges are studied. Prospects for using open resonators with conducting inserts in quasi-optical devices in the millimeter-wave range are discussed.

Open Resonators with Conducting Cylindrical Inserts. 1. Two-Dimensional Model

We present the results of computer simulation of the properties of modes in a two-dimensional open resonator with a conducting cylindrical insert. An increase in the diffraction Q-factor of the fundamental mode by 2–3 orders of magnitude due to the conducting cylindrical insert is found and studied. The prospects of using this phenomenon for controlling the spectrum, frequencies, and Q-factors of excited modes are discussed. The tunability of the open resonator with a conducting cylindrical insert for one (fundamental) mode in a wide frequency range is shown.

Study of a combined millimeter-wave resonator

We present a theoretical investigation and measurements of a multielement open resonator composed of corrugated waveguides and plane semitransparent reflectors. A periodic-transmission-line model is used to analyze the transverse mode structure and the diffraction Q-factor of the proposed resonator. The setup containing the resonator (equipped with Bragg reflectors), mode converters, and an elliptical mirror is employed for measurements in Ka-band. Two- and three-waveguide-section resonators have been studied. The proposed resonators demonstrate good mode selectivity that permits one to utilize them in high-power millimeter-wave sources.

Coaxial resonator for a megawatt 280 GHz gyrotron

To provide the required mode selectivity for a megawatt 280 GHz gyrotron, a coaxial resonator operating in a high order TE mode is considered. Mode discrimination is achieved both by exploring the differences in the transverse structures of the competing modes and investigating a suitable geometry for the coaxial insert. For modes with close eigenfrequencies the associated diffractionQ factors can be widely different in value, thereby ensuring an effective mode selection. In the resonator studied here, the frequency separation between the design mode TE26,10,1 and its nearest competing mode TE20,12,1 is about 0.6% and the ratio of the correspondingQ factors is as high as 6.5. Unlike the coaxial resonator, in the hollow cavity without the inner conductor the fundamental spectrum of eigenfrequencies is more dense, and all TE modes within the frequency interval 271–288 GHz have approximately the sameQ factor.