Large Orbit Gyrotron Research Articles

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.

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.

Large-Orbit Gyrotron Operation in the Terahertz Frequency Range

Coherent terahertz high-harmonic radiation has been obtained in a gyrotron with an axis-encircling electron beam. An electron-optical system with a cusp gun and a following drift section of adiabatic magnetic compression with an area factor of 3000 provides the formation of an 80-keV/0.7-A beam of gyrating electrons in a wide range of voltages and magnetic fields. Stable single-mode generation with a power of 0.3-1.8 kW in microsecond pulses is detected at four frequencies in the range 0.55-1.00 THz at resonant magnetic fields 10.5-14 T.

ELECTRON GUN FOR LARGE ORBIT GYROTRON (LOG) WITH DECREASED INFLUENCE OF CATHODE PLASMA ON ELECTRON BEAM PROPERTIES

High power Large Orbit Gyrotron (LOG) [1] is now under development at FIR FU. First version of this device was recently manufactured and then assembled with power supply ETIGO-IV [2]. Results of preliminary tests of electron-optic system are presented. The conditions when stableflat form of current pulse realized are discussed. Analytical estimations of cathode-anode distance to achieve small influence of cathode plasma during high voltage (HV) pulse are performed.

Measurement of the radiation frequency of a pulsed submillimeter-wave gyrotron using beat-note signals with the harmonics of a millimeter-wave frequency synthesizer

A method of frequency measurements of pulse signals in the millimeter/submillimeter wavelength region, including signals with a pulse duration shorter than 10 μs and a repetition rate of 0.1 Hz, is considered. This method is based on the analysis of amplitudes of beat notes between the signal under study and radiation of a millimeter-wave frequency synthesizer. Application of this method to frequency measurements and identification of modes of a pulsed relativistic gyrotron in a configuration of a large-orbit gyrotron operating at three lower harmonics of the cyclotron frequency in the range 100–400 GHz is demonstrated. This method allows radiation-frequency measurements with an accuracy corresponding to the frequency instability during an oscillation pulse. It is shown that the method developed is also applicable to investigations of the oscillation dynamics within a single pulse.

Design of a Powerful and Compact THZ Oscillator

According to simulations, a Large Orbit Gyrotron with a 30 keV/1 A electron beam and magnetic field of 5 T can produce CW radiation with output power of 102−103 W at the frequencies of 0.4–0.5 THz.

Electron-optical system for a large-orbit gyrotron

An electron-optical system generating a rectilinear or helical 250 keV/4 A/10 μs electron beam with a high compression factor is developed. For the former beam, a compression factor as high as 4400 and a current density of 25 kA/cm2 are achieved. In the process of forming the helical beam, the electrons rotating about the system’s axis (paraxial beam) acquire an initial velocity in a transverse magnetic field produced by a kicker. Their pitch factor is increased to a desired (operating) value in an adiabatically growing magnetic field. In tentative experiments with the helical beam in a large-orbit gyrotron, generation was obtained at the second cyclotron harmonic (223 GHz).

Design of Cavities for a Short Pulse Powerful Large Orbit Gyrotron

Designs of cavities for fundamental and high harmonic operation in Large Orbit Gyrotron (LOG) are discussed. The fundamental operation allows one to use beam currents in the range from 200 A to 300 A achieve output power of the order of 9–10 MW at frequency 143.6 GHz. Mode competition calculations show that stable oscillations in the TE1,4 mode using high beam currents are possible even with the pitch factor 1.3 which is significantly lower than the design value 1.55. For the second harmonic operation, the maximum current used for excitation of the TE2,4 mode is 60 A and the optimum magnetic field is 7.6 T. A cavity design for fourth harmonic operation using the TE4,4 mode is also presented.

Electron Optic System of Powerful Large Orbit Gyrotron with Pulse Magnetic Field

Short-pulse powerful Large Orbit Gyrotron with total electron energy about 400 kV and beam power in the cavity up to 100 MW is now under developing at FIR FU. Suitable for 200 ns pulse duration electron-optic system is analyzed. Results of numerical simulation for explosion emission cusp-type electron guns and magnetic field intensity about 8 T are presented. Sensitivity of the guns to small deviations from the nominal operating regime is investigated. Some versions of the gun with different accelerating potential as well as different beam current passing through the cavity (60-300A) are suggested. Current reduction simplifies the problems of mode competition and potential depression in the cavity, but at the same time decrease output power. To diminish current special diaphragms are suggested. Results of numerical simulation of collector corresponding to each version of the gun design including power density distributions along its surface are presented. It is shown that beam quality and collector regimes are suitable for LOG operation.

Development of high‐harmonic gyrotrons using a permanent magnet system

AbstractA large‐orbit gyrotron (LOG) operating at higher harmonics of the electron cyclotron frequency has been constructed by use of a permanent magnet system whose intensity is around 1 T. Operation tests have already been performed successfully up to the fourth harmonic. The operation efficiency is extremely high even at the third and fourth harmonics, because of highly efficient interaction between beam electrons and the high‐frequency electric field in the LOG. The test results of the LOG are summarized. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 149(4): 1–6, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20084

Reflections in Gyrotrons With Axial Output

Influence of reflections on operation of gyrotrons with axial output is studied both theoretically and experimentally. By way of example the Fukui large orbit gyrotron with a permanent magnet operating in third harmonic at frequency 89 GHz is considered. In the case of strong reflection (|R|=0.6), extreme sensitivity of output power on the reflection phase is found. A qualitative agreement between theory and experiment is observed.

Electron Gun for Powerful Large Orbit Gyrotron

In the article, results of numerical simulation of the gun with the cusp of magnetic field are presented. Short pulse version of the gun with explosion emission is investigated. Some preliminary analytical estimation of the beam and gun parameters are performed. Then, numerical optimization of the electrodes shape as well as magnetic field distribution is carried out. For preliminary separation of electrons and formation of the rectilinear beam, anode diaphragm is installed. After then, additional selection of electrons for decreasing the ripple is performed. For this purpose, channel walls are used for interception of some part of the electron beam. Reverse of the magnetic field in the diode part of the gun is formed. So, the formation of the rectilinear beam is combined with the region, where electrons obtain initial gyration energy. To prevent the disperse action of the own beam space charge forces, the system with big gradient of magnetic field (about 0.5-1 kGs/mm) is needed. According to results of the simulation, helical electron beam can be performed even at total compression ratio about 1000 and current density more than 50 kA/cm2. The designed electron gun provides acceptable performance for the large orbit gyrotron, such as operating current close to 300 A, the pitch-factor value about 1.5-1.7, deviation of the guiding centers from the axis (the ripple) λ/10 and λ/6 for operation on 3-rd and 5-th cyclotron harmonic correspondingly (wavelength λ=0.5 and 0.3 mm) and velocity spread within the range 10-15%.

Millimeter-Wave Relativistic Electron Devices

We briefly review advances and new ideas in the field of obtaining superpowerful coherent millimeter-wave radiation. Devices based on the stimulated bremsstrahlung and the Cherenkov radiation of curved and rectilinear relativistic electron beams, respectively, are considered, in particular, gyrodevices (including traditional gyrotrons, large-orbit gyrotrons, gyro-TWTs, and gyro-BWTs), free-electron masers (cyclotron-autoresonance masers and ubitrons), backward-wave oscillators (BWOs), traveling-wave oscillators (TWOs), and orotrons. Some of the above-mentioned devices are developed to the level ensuring their efficient use for a number of promising applications.

Development of High Harmonic Gyrotrons Using a Permanent Magnet System

A large orbit gyrotron (LOG) operating at higher harmonics of electron cyclotron frequency has been constructed by use of a permanent magnet system whose intensity is around 1 T. The operation test was already performed successfully up to the fourth harmonic. The operation efficiency was extremely high even at the third and the fourth harmonics, because of high efficient interaction between beam electron and high frequency electric field in LOG. The test results of the LOG are summarized.

Design of a Large Orbit Gyrotron with a Permanent Magnet System

The paper presents results of numerical analysis and outlines the computer-aided design of a novel high-harmonic gyrotron with a beam of electrons gyrating along axis-encircling trajectories. The electron beam is formed by a novel electron-optical system (EOS) based on an electron gun of diode type with thermionic cathode and gradual reversal of the magnetic field. The results of numerical simulations predict satisfactory performance of the EOS and appropriate beam quality parameters. The tube design allows one to install different cavities optimized for excitation of TE4,1 mode at the fourth harmonic of the cyclotron frequency or TE3,1 mode at the third one. The target parameters of the device are: frequency about 112 GHz; output power near 1 kW and efficiency of several percent.

Harnessing of the Axial Kinetic Electron Energy of a Large-Orbit Gyrotron by an Assistant Azimuthal Background Magnetic Field

A new method of providing a background magnetic field was proposed for a large-orbit gyrotron in a coaxial waveguide for an improvement in the device performance with respect to the gain and efficiency. The method, which consists in providing an assistant azimuthal magnetic field, over and above the conventional DC axial background field, was studied, using a kinetic theory. The method was compared with an earlier method of providing a DC radial background electrical field. The adjustment of the background fields and that of the DC kinetic energy distribution between the axial and transverse electron velocities of the beam provided a simultaneous increase in the gain and efficiency of the device. This may be attributed to the axial beam kinetic energy being harnessed by the method.

Design of a large orbit gyrotron with a permanent magnet system

In this paper, we present results of a conceptual design study of a prospective gyro-device with a permanent magnet system and an axis-encircling electron beam. Computer-aided electron-optical design of the tube has been performed by the new version of the software package GUN-MIG named GUN-MIG/CUSP. It is based on a self-consistent relativistic physical model and is developed as a problem-oriented tool for analysis of electron-optical systems with conventional magnetron injection guns (MIG) and electron guns with field reversal (cusp guns), forming axis-encircling beams. Results of numerical experiments predict satisfactory performance of the gun which is expected to form high-quality axis-encircling electron beams with small ripple and dispersion of the velocities. Parameters of the beams (current, radius, velocity ratio, etc.) are appropriate for weakly relativistic harmonic large orbit gyrotron (LOG). The cavity was optimized for excitation of TE 41 mode at the fourth harmonic of the cyclotron frequency. The results of this feasibility study will be used as a basis for development of a novel LOG with a permanent magnet system. The target parameters of such a prospective device are frequency of generated microwave radiation about 104 GHz, output power near 1 kW and efficiency of several percent.

Analysis of a large-orbit gyrotron in a coaxial waveguide under assistant background fields

The dispersion relation of a large-orbit gyrotron in a coaxial waveguide excited in the transverse-electric mode was developed starting from the beam-present wave equation. The relativistic Vlasov's equation was solved under tenuous beam approximations for the perturbed electron distribution function, hence the required current density to be used in the wave equation was found. The analysis was generalized, with respect to background fields, by considering a dc radial electric field and a dc azimuthal magnetic field, over and above conventional dc axial magnetic field. The dispersion relation was solved for the complex frequency for a given propagation constant and the imaginary part interpreted for the growth rate as well as the saturated efficiency of the device. Similarly, for a given frequency the dispersion relation was solved for the complex propagation constant and the imaginary part interpreted for the gain of the device, if configured as an amplifier. Two peaks were obtained in the gain-frequency response corresponding to two points of intersection between the beam-mode dispersion line and the waveguide-mode dispersion curve. The improvement in the growth rate, gain and saturated efficiency was predicted by the application of the additional background fields. A redistribution of beam kinetic energy between the axial and transverse electron velocities led to a remarkable enhanced saturated efficiency at the second peak. A detailed study of the variation of device performance with respect to the gain and saturated efficiency was presented, for a wide range of the dc background field and electron kinetic energy distribution parameters.

NUMERICAL ANALYSIS OF WEAKLY RELATIVISTIC LARGE ORBIT GYROTRON WITH PERMANENT MAGNET SYSTEM

High-harmonic gyro-devices with axis-encircling electron beams known as large orbit gyrotrons (LOG) represent an appealing alternative to the conventional gyrotrons. In this paper we investigate the feasibility of such device operating with low current and low energy electron beams formed by a novel electron gun with a permanent magnet system. The results from the numerical experiments indicate the possibility to excite TE41 mode at fourth harmonic of the cyclotron frequency. Simulations predict generation of microwave radiation with frequency 104 GHz and output power near 1 kW.

Computer simulation of axis-encircling beams generated by electron gun with permanent magnet system

Results from numerical simulations of a novel electron gun which forms axis-encircling electron beams with small velocity spread and ripple are presented and discussed. It utilizes magnetic system based on NdFeB permanent magnet. The parameters of generated beams are appropriate for a prospective compact, mildly relativistic large-orbit gyrotron (LOG) whose development is in progress at the Research Center for Development of Far-Infrared Region in Fukui University.