Fast-wave Devices Research Articles

Development of simulation tools for numerical investigation and computer-aided design (CAD) of gyrotrons

As the most powerful CW sources of coherent radiation in the sub-terahertz to terahertz frequency range the gyrotrons have demonstrated a remarkable potential for numerous novel and prospective applications in the fundamental physical research and the technologies. Among them are powerful gyrotrons for electron cyclotron resonance heating (ECRH) and current drive (ECCD) of magnetically confined plasma in various reactors for controlled thermonuclear fusion (e.g., tokamaks and most notably ITER), high-frequency gyrotrons for sub-terahertz spectroscopy (for example NMR-DNP, XDMR, study of the hyperfine structure of positronium, etc.), gyrotrons for thermal processing and so on. Modelling and simulation are indispensable tools for numerical studies, computer-aided design (CAD) and optimization of such sophisticated vacuum tubes (fast-wave devices) operating on a physical principle known as electron cyclotron resonance maser (ECRM) instability. During the recent years, our research team has been involved in the development of physical models and problem-oriented software packages for numerical analysis and CAD of different gyrotrons in the framework of a broad international collaboration. In this paper we present the current status of our simulation tools (GYROSIM and GYREOSS packages) and illustrate their functionality by results of numerical experiments carried out recently. Finally, we provide an outlook on the envisaged further development of the computer codes and the computational modules belonging to these packages and specialized to different subsystems of the gyrotrons.

The Gyrotron at 50: Historical Overview

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.

Linear Analysis of a Cyclotron Autoresonance Maser (CARM) Operating in a Transverse Magnetic Mode

In the fast-wave devices like gyrotron, gyro-peniotron and cyclotron autoresonance maser (CARM) that generate millimeter and sub-millimeter waves, the transverse dimensions of the resonator and the output cylindrical waveguide become small. In order to prevent loss of electrons and thermal loading of the rf structure, the electron beam must be kept relatively far from the walls. The latter requirement demands smaller transverse dimensions of the helical electron beam as well. In this paper linear formulation of a CARM operating in a general transverse-magnetic (TM) mode is derived, and a detailed analysis of the influences of the parameters is presented for the TM1,1 mode CARM. It is found that, compared to the TE1,1 mode which is often employed in gyrotron traveling wave tube (gyro-TWT) and CARM experiments, the TM1,1 mode has a greater eigen value and consequently leads to a greater waveguide radius for a given cutoff wave number, and also, allows the electron beam to be settled close to the waveguide axis to have a small transverse dimension. Results show that a TM-mode CARM can reach high power and ultrahigh gain, just as a TE-mode CARM or a TE-mode gyro-TWT does.

Microwave Excitation by a Constrained Large-Orbit Electron Beam—A Unified Dispersion Relation for Slow- and Fast-Wave Devices

A unified linear dispersion relation that describes both slow- and fast-wave devices excited by a constrained large gyration orbit (LO) monoenergetic electron beam of infinitely small thickness has been derived and studied numerically. Beam electrons in a sufficiently long sinusoidally corrugated metal slow-wave structure are assumed completely neutralized by the background ions in equilibrium state. An exact dispersion relation of an LO backward-wave oscillator that can reasonably describe instabilities in the slow-wave device region has been obtained. A parameter a, defined as a ratio of the transverse to the longitudinal component of the electron velocity, is found to have a critical value above which the excitation of a nonaxisymmetric quasi-TE/sub 11/ mode caused by the fast cyclotron instability dominates the conventional Cherenkov instability. However, for an SWS having infinitely small amplitude of corrugation, radiation with w W is altogether suppressed; instead, an alternate mechanism, namely, Cherenkov instability in the azimuthal direction with w<W found first in the current paper leads to the excitation of microwaves. This suggests that the radiation from some previous CRM experiments with a high current density electron beam neutralized by ions might have been caused not by CRM instability but by the present Cherenkov instability in the azimuthal direction.

Vacuum electronics

This paper explores the recent history and diversity of this remarkable technology, with emphasis on recent advances in the more traditional device types (traveling-wave tube and klystron), as well as more recent innovations such as the microwave power module, inductive output amplifier, fast-wave devices, ultrahigh-power sources, and RF vacuum microelectronics. These advances can be credited to a combination of device innovation, enhanced understanding gained through improved modeling and design, the introduction of superior materials and sub-assembly components and the development of advanced vacuum processing and manufacturing techniques.

Anomalous free electron laser interaction

Free electron lasers (FELs) are considered, typically, as fast wave devices. The normal FEL interaction satisfies the tuning condition ω≅( k z + k W) V z , where ω and k z are the em-wave angular frequency and longitudinal wave number, respectively, V z is the electron axial speed, and k W is the wiggler periodicity. This paper presents an anomalous FEL interaction, which may occur in slow-wave FELs (i.e. loaded by dielectric or periodic structures). The anomalous FEL effect presented here satisfies the tuning condition ω≅( k z − k W) V z , and it resembles the anomalous effect in slow-wave cyclotron resonance masers. A necessary condition for the anomalous interaction is ω/ k z < V z (i.e., the em-wave phase velocity should be slower than the electron beam). The paper presents a preliminary experimental result demonstrating the anomalous FEL effect in a stripline dielectric-loaded FEL experiment. A linear Pierce equation is applied to describe both the anomalous and normal FELs in the same framework. The paper is concluded with a conceptual discussion.

Free-electron lasers: vacuum electronic generators of coherent radiation

Vacuum electronic sources have shown marked improvement since the invention of the magnetron before World War II, and dramatic increases in both average powers and frequencies have been achieved. Of course, many of these gains have been achieved by the development of different devices. The typical development pattern for a given device exhibits and initial period of rapid improvement followed by a plateau determined by technological or physical limitations on the concept. Slow wave devices such as magnetrons and/or klystrons operate efficiently at frequencies up through X-band or somewhat higher. Helix or coupled cavity traveling-wave tubes are used for various applications at frequencies ranging up through W-band. At still higher frequencies, fast wave devices such as gyrotrons and free-electron lasers are required for high-power operation. The free-electron laser concept is unique in that the mechanism is applicable across the entire electromagnetic spectrum; free-electron lasers have been built at wavelengths from microwaves through the ultraviolet, and plans are under development for X-ray systems. Our purpose in this paper is to describe the principal directions of free-electron laser research at the present time. To this end, we first give a brief tutorial of the physics underlying the concept and then describe the principal development paths under way.

Fast-Wave Analysis of an Inhomogeneously-Loaded Helix Enclosed in a Cylindrical Waveguide

The inhomogeneously-loaded helix enclosed in a metal cylindrical waveguide operated in the fast-wave regime is field-analyzed using the tape-helix model. The inhomogeneous discrete dielectric supports have been modeled by azimuthally smoothing them out into a number of continuous, homogeneous dielectric tube regions of appropriate effective relative permittivity values determined by the relative volumes occupied by the supports in these regions. While accounting for the radial inhomogeneity, the nonuniformity of the radial propagation constant over the structure cross section is also taken into consideration. Further, structured expressions amenable to easy computation are presented with the help of the relevant recurrence relations, and a method is suggested to use them in an iterative cycle for the desired convergence accuracy. The dispersion relation developed, as a special case, passes on to that obtained elsewhere for a simpler structure in which the dielectric supports for the helix was not considered. It is observed that the dielectric-support parameters become more effective in controlling the dispersion curve than the bare helix parameters. It is also found that the effect of the controlling parameters such as the helix pitch angle on the phase velocity and interaction impedance of the structure becomes less pronounced as the helix is brought closer to the waveguide wall. The presence of the dielectric in the structure increases the value of the interaction impedance to a reasonable value over the entire operating frequency band. The results in terms of the structure parameters, namely, helix pitch angle, helix-support permittivity, helix-to-waveguide wall separation and structure inhomogeneity presented here are rather general, and can be used for the structure design and performance evaluation as well as for the control of any space harmonic of interest in a fast-wave device such as the gyro-traveling-wave amplifier.

Fast-Wave Analysis of an Inhomogeneously- Loaded Helix Enclosed in a Cylindrical Waveguide

The inhomogeneously-loaded helix enclosed in a metal cylindrical waveguide operated in the fast-wave regime is field-analyzed using the tape-helix model. The inhomogeneous discrete dielectric supports have been modeled by azimuthally smoothing them out into a number of continuous, homogeneous dielectric tube regions of appropriate effective relative permittivity values determined by the relative volumes occupied by the supports in these regions. While accounting for the radial inhomogeneity, the nonuniformity of the radial propagation constant over the structure cross section is also taken into consideration. Further, structured expressions amenable to easy computation are presented with the help of the relevant recurrence relations, and a method is suggested to use them in an iterative cycle for the desired convergence accuracy. The dispersion relation developed, as a special case, passes on to that obtained elsewhere for a simpler structure in which the dielectric supports for the helix was not considered. It is observed that the dielectric-support parameters become more effective in controlling the dispersion curve than the bare helix parameters. It is also found that the effect of the controlling parameters such as the helix pitch angle on the phase velocity and interaction impedance of the structure becomes less pronounced as the helix is brought closer to the waveguide wall. The presence of the dielectric in the structure increases the value of the interaction impedance to a reasonable value over the entire operating frequency band. The results in terms of the structure parameters, namely, helix pitch angle, helix-support permittivity, helix-to-waveguide wall separation and structure inhomogeneity presented here are rather general, and can be used for the structure design and performance evaluation as well as for the control of anv space harmonic of interest in a fast-wave device such as the gyro-traveling-wave amplifier.

Resonant Enhancement of Radiation from a Backward Wave Oscillator Utilizing Large Diameter Corrugated Metal Structure

An experiment on a backward wave oscillator (BWO) utilizing a large diameter corrugated metal wall slow wave structure (SWS) has been presented. Measurements are made above and below the starting beam energy for conventional BWO operation. For the latter case, an excited noise-level backward wave at 20 GHz (TM 01 mode) is coupled at the ends of the SWS to a forward anomalous Doppler shifted electron cyclotron (EC) beam mode which amplifies the original oscillation by an amount of 50 dB. Resultant microwave output powers of 200 kW with 4.5% electronic efficiency are observed near the condition of resonant magnetic field for an electron beam with energy 35 keV. Unlike the well-known fast wave devices, our slow wave EC device is simple to operate utilizing an electron beam with dominant parallel velocity in a uniform magnetic field.

Chaos and gyrotron-type of interaction

The theory of chaotic behaviour of dynamical systems has been used to investigate the basic features of interaction in fast-wave devices such as gyrotrons. A simple model of the interaction process seems to indicate that gyrotrons readily resist the onset of chaotic behaviour even at large amplitudes of oscillations. >

Microwave and Millimeter-wave Vacuum Tube Electron Devices: Overview and State-of-the-art

In this paper is given an overview and state-of-the-art of microwave and millimeter-wave vacuum tube electron devices. The classification of such devices, and the general principles governing slow-wave and fast-wave devices, have been given. Though the conventional types of microwave tubes are briefly mentioned, the high-power microwave tubes whose production has advanced very rapidly in recent years, have been stressed. Some of these new types of high-power microwave electron devices are cyclotron masers (CRMs) relativistic slow-wave microwave devices and plasma microwave devices.

Electromagnetic waves: microwave components and devices

Part 1 Electromagnetic waves: Maxwell's equations electromagnetic waves in non-conducting media energy flow in an electromagnetic wave electromagnetic waves in conducting materials propagation of waves in plasmas polarization of waves propagation in gyromagnetic media boundary conditions. Part 2 Waves guided by perfectly conducting boundaries: TEM transmission lines reflection of waves by a conducting plane transverse electric waves transverse magnetic waves propagation in a rectangular waveguide power flow in rectangular waveguide higher-order modes in a rectangular waveguide other waveguides. Part 3 Waves with dielectric boundaries: reflection of waves by a dielectric boundary total internal reflection the Brewster angle dielectric waveguides mono-mode and multi-mode optical fibres radomes, windows and optical blooming quasi TEM waveguides non-TEM waveguides. Part 4 Waves with imperfectly conducting boundaries: waves incident normally on a conducting surface transmission through a thin conducting sheet electromagnetic screening waves incident obliquely on a conducting surface losses in transmission lines and waveguides microwave attenuators microwave loads. Part 5 Antennas: magnetic vector potential retarded potentials small electric dipole the reciprocity theorem small magnetic dipole half-wave dipole dipole arrays radiation from apertures slot antennas phased array antennas. Part 6 Coupling between wave-guiding systems: discontinuities broadband matching techniques coupling without change of mode coupling with change of mode coupling by apertures effect of holes in screens on screening effectiveness waveguide directional couplers distributed coupling. Part 7 Electromagnetic resonators and filters: transmission-line resonators cavity resonators effect of resonance on screened enclosures dielectric resonators Fabry-Perot resonators filter theory transmission-line filters waveguide filters optical filters. Part 8 Ferrite devices: microwave properties of ferrites resonance isolators phase shifters and circulators junction circulators Faraday rotation devices edge mode devices YIG filters. Part 9 Solid state microwave devices: semiconductor materials diodes transistors detectors and mixers switches oscillators amplifiers monolithic microwave integrated circuits LEDs and laser diodes. Part 10 Vacuum devices: space-charge waves Klystron amplifiers slow-wave structures travelling wave tubes crossed-field tubes fast-wave devices electron accelerators. Part 11 Microwave measurements: measurement of frequency measurement of power measurement of gain and loss measurement of return loss (part contents).

Status review of research on millimeter-wave tubes

Devices employing electron beam-electromagnetic wave interaction are classified in accordance with the phase velocity of the wave. Some general principles are stated governing the beam size and position applicable to all types of slow-wave interaction and for various types of fast-wave devices such as the gyrotron, the free-electron laser, and their derivatives. The quality of the electron beam, its energy, and the magnetic field strength required are indicated for each type of interaction. The characteristics of the various devices are compared in the context of achieving high-power operation at millimeter wavelengths. Some recent experimental research achievements in the U.S. and the U.S.S.R. are listed to indicate the current state of the art. Finally, areas that appear fruitful for further work are recommended.

AC space charge effects in gyrotrons†

This paper presents a fully self-consistent small signal theory of TEom and TEom gyrotrons. In particular, a reduced plasma frequency ϖp, and a plasma reduction factor It as applicable to those devices are derived, thus describing fully the beam behaviour in gyro-klystrons. The lasing mechanism taking place in fast-wave devices is shown to lead to a resonance in the plasma reduction factor R under typical gyrotron operation, resulting in a space charge related gain parameter Cs which modifies significantly the gain parameter C derived previously neglecting AC space charge effects. This result may modify the traditional approach of simulating gyrotron operation under large signal conditions

The lasing mechanism in fast-wave devices

This brief shows that ac space-charge effects lead to beam instabilities and to a lasing mechanism taking place in fast-wave devices.

Physics and classification of fast-wave devices†

Abstract This paper presents a new approach to the analysis and understanding of beam wave interaction taking place in fast-wave devices (beamlet theory). The theory takes fully relativistic effects into account, and, as it turns out, is applicable to slow-wave interactions as well. This is achieved by defining a generalized electromagnetic field whose polarization, phase velocity and origin need not be known to solve the relativistic equation of motion under small-signal conditions. As a result, two distinct but fundamental classes of interactions can be defined, referred to herein as synchronous and resonant. The characteristics of both classes of interactions are discussed in details and exemplified by the interactions taking place in gyrotrons and peniotrons. As a consequence, a systematic classification of commonly used high power microwave tubes is presented, and two new TM interactions occurring close fo waveguide cut-off are identified. The Pierce gain parameter is defined in terms of coupling and beam impedances, showing the gain of synchronous and resonant interactions to be respectively proportional to the cube and square root of the beam current.

Coupling impedances in synchronous fast-wave devices†

Abstract The coupling impedances for TE and TM gyrotron-type interactions occurring in rectangular and cylindrical waveguides are determined and found to be proportional to a functional impedance which decreases inversely proportional to' the fourth power of the propagation constant β, leading to a gain per unit length inversely proportional to the cube root of β, and proportional to the cube root of the current. Farther away from waveguide cut-off, the functional coupling impedance for TM-gyrotrons is found to be larger than the corresponding impedance for TE-gyrotrons, and decreases only inversely proportional to the square of β, leading to a gain per unit length which increases with frequency. However, under grazing-angle conditions, the coupling impedance for TM-gyrotrons vanishes, second order effects must be taken into account, and the gain per unit length becomes proportional to the square root of the current. Beam velocity spreads lead to an effective space charge parameter QC.