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.     

Some perspectives on the use of powerful gyrotrons for the electron-cyclotron plasma heating in large tokamaks

Conclusion The main problems for the further development of high efficiency powerful gyrotrons are obviously the elaboration of electron guns with a large current density and the ensuring of a stable single-mode operation in resonators with a large cross-section. For the increase in microwave pulse energy (with a pulse length of the order of the time of plasma confinement in large tokamaks) the investigation of the possibilities to diminish heating loads in the resonator and output window of a gyrotron is necessary.     

Are Coaxial Super Power Gyrotrons Feasible?

Limits on the power generated by coaxial gyrotrons are investigated. It is shown that among all the operating modes suggested recently for coaxial super power gyrotrons only modes with azimuthal index m lower than about 44 pass the spatial stochasticity test. Modes with higher azimuthal indices cannot be used as operating modes because gyrotron oscillations become chaotic in the azimuthal direction and efficiency is very low. It is argued that there exists a natural upper limit on power generated by gyrotrons which is about 4 MW.     

Large-orbit Subterahertz and Terahertz gyrotrons

We review briefly the main ideas and achievements in the field of physics related to shortwavelength large-orbit gyrotrons, in which the coupling of electrons with the working mode and the discrimination of parasitic modes in the case of resonance at the high cyclotron harmonic are more efficient compared with conventional gyrotrons. The results of studying a new large-orbit gyrotron with moderate electron energies of 50–80 keV and comparatively low magnetic fields of 10.5–14 T are presented. In this gyrotron, high-power single-mode generation was obtained at the second and third cyclotron harmonics in the frequency range 0.55–1.00 THz. The prospects of development and application of short-wavelength large-orbit gyrotrons are discussed.     

Gyrotron traveling wave amplifier with out-ridged waveguide

The out-ridged waveguide is proposed for the use of gyrotrons operating at higher harmonics of electron gyrofrequency. Eigenvalues and the associated field components of the out-ridged waveguide are obtained with Ritz-Galerkin method. The prominent advantages of the out-ridged waveguide over the existing waveguide structures used in gyrotrons include the simplicity in manufacture, freedom from local modes, good separation of lower order modes, high power handling ability, and good beam-field coupling at higher harmonics of the electron gyrofrequency. Gain-frequency functional curves of the gyrotron traveling wave amplifiers with the out-ridged waveguide and some other waveguide structures are computed numerically by employing the general analytical results developed in [15].     

Design and Cold Tests of Asymmetric Quasioptical Resonators

This paper presents the design and test of asymmetric quasioptical resonators for gyrotron applications, where most of the diffraction output coupling comes from the smaller of two mirrors. A computer code based on scalar diffraction theory is used to calculate the resonator losses and the electric field near the mirrors. Resonators with mirror separations approaching confocal have reduced side lobes in the diffraction patterns, which should allow for better power transmission and focusing. Cold tests have been performed at 94 and 141 GHz to measure the quality factor of the asymmetric resonators as a function of mirror separation and mirror misalignment. Typical resonator parameters include 65 cm mirror separation, 1-3% output coupling and 85% of the losses from the small mirror. A half-symmetric resonator with one flat and one curved mirror has also been investigated. The calculated values of Q are in reasonably good agreement with the cold test measurements.     

General theory and design of microwave open resonators

Based on the analysis of wave propagation in a waveguide with slowly varying cross-section, a general theory is developed for microwave open resonators with rotational-symmetric waveguides; this geometry has been extensively used in the gyrotrons. The theoretical analysis is carried through under the most general conditions, that is the cross-section radius r(z) of the resonator is a slowly varying arbitrary function of z and the resonator is open. The general expressions for the resonant frequencies and the Q have been derived. A design method for this kind of open resonator has been proposed. Calculation results for two practical resonators are presented.     

Analysis of open resonators

A multiple reflection approach is employed to examine the properties of the lowQ open resonators commonly used in high power gyrotrons. The resonant frequency, diffractionQ, and RF field profile are derived in closed forms in the lowQ limit. Formation of the eigenmode in the resonator is shown as the result of constructive wave interference. The lower limit ofQ in the context of a conventional definition ofQ is derived.     

On one mechanism of spurious oscillations in high-power gyrotrons

One of the problems encountered in the design of high-power gyrotrons is the need to ensure stable single-mode generation. The conditions of spurious self-excitation in a gyrotron resonator have been studied fairly completely, and methods for its suppression have been developed (see Petelin [1] and Nusionvich [2], for example). At the same time, stable gyrotron operation can also be disrupted by the generation of spurious oscillations in the transition region between the electron gun and the resonator, where the electromagnetic field is nonuniform. A theoretical basis for this assumption is provided by results [1, 3, 4] indicating a fairly high efficiency of interaction of a high-frequency field with an electron current guided by a nonuniform magnetostatic field.Institute of Applied Physics, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 11, pp. 1458–1472, November, 1994.     

I/f noise in systems showing stochastic resonance

Stochastic resonator systems with input and/or output 1/f noise have been studied. Disordered magnets/dielectrics serve as examples for the case of output 1/f noise with white noise (thermal excitation) at the input of the resonators. Due to the fluctuation-dissipation theorem, the output noise is related to the out-of-phase component of the periodic peak of the output spectrum. Spin glasses and ferromagnets serve as interesting examples of coupled stochastic resonators. A proper coupling can lead to an extremely large signal-to-noise ratio. As a model system, a l/f-noise-driven Schmitt trigger has been investigated experimentally to study stochastic resonance with input 1/f noise. Under proper conditions, we have found several new nonlinearity effects, such as peaks at even harmonics, holes at even harmonics, and 1/f noise also in the output spectrum.     

Effect of background plasma on electromagnetic properties of coaxial gyrotron cavity

We analyze the effect of the background plasma on the electromagnetic properties of coaxial resonators with a smooth and a corrugated inner conductors used in high-power gyrotrons. It is shown that the plasma produces different effects on the modes with different signs of the azimuthal index, leading to a decrease or an increase in the resonance frequencies. A modification of the distributions of electromagnetic fields and the electromagnetic energy density by the background plasma occurs in such a way that Ohmic losses decrease both on the inner and on the outer conductors. In the case of a smooth inner conductor, this is due to a decrease in the field strengths on the surface of the conductors. If the inner conductor is corrugated, the background plasma leads to an increase in the field strengths on its surface. Nevertheless, the relative Ohmic loss power decreases due to an increase in the energy density in the resonator (which is also caused by the background plasma). Calculations were mainly performed for a coaxial resonator of a gyrotron operating on the TE34.19 mode (Karlsruhe, Germany).     

Experimental study of a gyrotron operated at the second gyrofrequency harmonic with the single-stage energy recovery

We obtain an output efficiency of 60%, which is record-breaking for gyrotrons operated at the second gyrofrequency harmonic, for an output power of 6 kW in the continuous-wave regime. The increase in the efficiency is achieved by using single-stage energy recovery in a gyrotron with an electrically insulated collector. Experimental results confirm the validity of calculation of the electron-beam energy spectra in the gyrotron collector region. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 10, pp. 850–854, October 2008.     

Numerical Study of Mode Competition in Coaxial Cavity Gyrotrons with Corrugated Insert

A time-dependent self-consistent multimode code which allows to model the interaction between the modes of a coaxial cavity gyrotron operating at the fundamental and/or at higher harmonics of the cyclotron frequency is presented. The Method of Moments (MoM) is used to obtain a rigorous solution of the eigenvalue problem for a coaxial waveguide with corrugated insert. It is shown that the use of the Surface Impedance Model for the calculation of modes operating at higher harmonics leads to incorrect results in high-power coaxial cavity gyrotrons. However, the numerical simulations based on the MoM agree well with the available experimental results.     

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 TE_26,10,1 and its nearest competing mode TE_20,12,1 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.     

Nonlinear dynamics of planar gyrotrons with transverse diffraction coupling of radiation

We propose that the planar geometry of the interaction space and transverse (relative to the direction of translatory motion of electrons) power extraction be used for increasing the integrated radiation power of short-wavelength gyrotrons. The advantage of such a scheme over the traditional cylindrical geometry of gyrotrons is the possibility to ensure coherence of radiation for a large superdimensionality factor due to diffraction mechanism of mode selection in the open transverse coordinate and locking of radiation from various fractions of the ribbon-shaped polyhelical electron beam by transverse energy fluxes. Simulation of the non-linear dynamics of a planar gyrotron demonstrates the possibility of attaining the output power of hundreds of kilowatts at frequencies of up to 1 THz. A further increase in the output power to the megawatt level can be attained by using resonators with a nonuniform profile, for which the gap between the plates increases from the center to the periphery. Such a configuration is analogous to unstable optical resonators used in some powerful lasers.     

Resonance and band filtration on the basis of two-phase crystal-like structures

Two-phase crystal-like structures for the resonance and band filtration of signals are proposed. It is shown that the selectivity of a two-phase resonator is higher than that of a traditional half-wave one. Implementation of band filtration by coupled resonators is considered. Experimental and calculated characteristics of a band filter on the basis of a two-phase electromagnetic crystal are presented.     

Low power measurements of the quality factor of an open resonator with stepped mirrors

Open resonators in which both mirrors have a central step have been predicted to exhibit mode selection properties. This is advantageous for the design of quasi-optical gyrotrons. Quality factor measurements of a 36 cm resonator at 120 GHz are presented and compared with theory. Although the effect of the step is clearly seen, the quality factor Q of the resonator is lower than that predicted.     

Multimirror quasi-cylindrical cavity resonators for frequency-tunable gyrotrons

We consider the possibility to create a gyrotron with smooth mechanical frequency tuning on the basis of multimirror quasi-optical resonators. The two-dimensional problem of finding igenmodes at the open quasi-cylindrical resonator of such a type is solved. Parameters of the gyrotron based on such a device, namely, its efficiency and starting current, are estimated.     

Operation of a 32 GHz gyrotron

The design and operation of a 32 GHz pulsed gyrotron are reported. The device is step-tuned between the TE_1,2 (24.16 GHz) and TE_0,2 (31.78 GHz) modes with cathode voltages ranging from 30 to 40kV and beam current up to 5.0A. Experimental frequencies are in close agreement with the self-consistent calculated values and in the TE_2,2 resonator mode an output peak power of 6kW corresponding to an 18% efficiency was measured by using a fast response calorimeter with a thermal sensitivity of 0.1°C/Wmin.     

Piezoelectric resonators with mechanical damping and resistance in current conduction

A novel design method for high Q piezoelectric resonators was presented and proposed using the 3-D equations of linear piezoelectricity with quasi-electrostatic approximation which include losses attributed to mechanical damping in solid and resistance in current conduction. There is currently no finite element sofware for estimating the Q of a resonator without apriori assumptions of the resonator impedance or damping. There is a necessity for better and more realistic modeling of resonators and filters due to miniaturization and the rapid advances in frequency ranges in telecommunication. We presented new three-dimensional finite element models of quartz and barium titanate resonators with mechanical damping and resistance in current conduction. Lee, Liu and Ballato’s 3-D equations of linear piezoelectricity with quasi-electrostatic approximation which include losses attributed to mechanical damping in solid and resistance in current conduction were formulated in a weak form and implemented in COMSOL. The resulting finite element model could predict the Q and other electrical parameters for any piezoelectric resonator without apriori assumptions of damping or resistance. Forced and free vibration analyses were performed and the results for the Q and other electrical parameters were obtained. Comparisons of the Q and other electrical parameters obtained from the free vibration analysis with their corresponding values from the forced vibration analysis were found to be in excellent agreement. Hence, the frequency spectra obtained from the free vibration analysis could be used for designing high Q resonators. Results for quartz thickness shear AT-cut and SC-cut resonators and thickness stretch poled barium titanate resonators were presented. An unexpected benefit of the model was the prediction of resonator Q with energy losses via the mounting supports.