Large Orbit Gyrotron Research Articles

Third-Harmonic 1 THz Large-Orbit Gyrotron With an Improved Quasi-Regular Cavity

Third-Harmonic 1 THz Large-Orbit Gyrotron With an Improved Quasi-Regular Cavity

High-harmonic large-orbit terahertz gyrotrons for physical applications

An overview is given of works on two experimental installations that implement large-orbit gyrotrons (LOG) operating in the sub-terahertz frequency range at high harmonics of the electronic cyclotron frequency. At the installation of a pulsed LOG (80–100 keV/0.7–1.0 A), a radiation source with a frequency of 1 THz with a kilowatt output power level is being developed, which is planned to be used in plasma applications. A continuous subterahertz LOG (30 keV/0.7 A) is being created as a prototype of a universal multi-frequency source for spectroscopic applications. Complex electrodynamic systems are also described, designed to increase the selectivity and efficiency of excitation of high cyclotron harmonics in these devices, as well as to ensure the tuning of the generation frequency.

Spurious Fundamental-Harmonic Oscillations in the Horn Section of a High-Harmonic Gyrotron

In the experiments with a 30 keV/0.7 A sub-terahertz (THz) large-orbit gyrotron operating at the third cyclotron harmonic in the continuous-wave regime, it was found that the presence of reflections of the output signal from an external load leads to the excitation of parasitic oscillations at the fundamental cyclotron harmonic. Theoretical analysis shows that the excitation of parasitic oscillations occurs in the smooth-irregular output horn section of the microwave system, where the parasitic wave is close to the cyclotron resonance with the electron beam. Moreover, the fundamental-harmonic parasitic wave excited first in the output horn section can “penetrate” into the operating cavity, which can lead to suppression of the operating oscillation excited there at a high cyclotron harmonic.

High-Power Pulsed Terahertz-Wave Large-Orbit Gyrotron for a Promising Source of Extreme Ultraviolet Radiation

We study the possibility to make a gyrotron generating a power of several kilowatts at a radiation frequency of 1 THz, which will be used in promising plasma studies. In 2008, a pulsed largeorbit gyrotron (LOG) generating a power of 0.4 kW at the third cyclotron harmonic and an operating frequency of 1 THz was implemented successfully. We update this device to create a plasma source of extreme ultraviolet radiation. Earlier experiments showed that this will require a power that is almost an order of magnitude higher. Updating a large-orbit gyrotron will make it possible to increase the generation power up to 4 kW by raising the power of the electron beams and reducing the fraction of the ohmic loss in the cavity. Such a gyrotron is designed to operate with an electron beam with a current of 1.2 A and an accelerating voltage of 100 kV in a pulsed magnetic field of 14 T.

Universal Subterahertz Large-Orbit Gyrotron: Operation at the Second and Third Cyclotron Harmonics

Coherent subterahertz radiation in the continuous-wave regime was obtained in a large-orbit gyrotron (LOG) operated at a high harmonic of the cyclotron frequency. An electron–optical system containing a gun with a cusp followed by a section of adiabatic magnetic compression with a factor of 1000 ensures the formation of a 30 keV/0.7 A helical axis-encircling electron beam with acceptable velocity spread and pitch factor in a magnetic field of 5 T. Stable singlemode operation was obtained at frequencies of 0.394 and 0.267 THz with radiation powers of 0.37 and 0.9 kW at the third and second cyclotron harmonics, respectively. Tuning of the generation frequency in the gyrotron during its operation at the second cyclotron harmonic and complex regimes of competition of two waves excited at the second and third cyclotron harmonics are also studied.

Competition of Oscillations at Different Cyclotron Harmonics in the Subterahertz Large-Orbit Gyrotron

Recently, 30-keV/0.7-A continuous wave (CW) subterahertz large-orbit gyrotron was experimentally realized. In this experiment, two different modes at different (second and third) cyclotron harmonics were excited in the same cavity separately at slightly different operating magnetic fields. This work is dedicated to description and theoretical explanation of various regimes of competition of these two modes observed in the experiment.

State-of-the-Art of High-Power Gyro-Devices and Free Electron Masers

This paper presents a review of the experimental achievements related to the development of high-power gyrotron oscillators for long-pulse or CW operation and pulsed gyrotrons for many applications. In addition, this work gives a short overview on the present development status of frequency step-tunable and multi-frequency gyrotrons, coaxial-cavity multi-megawatt gyrotrons, gyrotrons for technological and spectroscopy applications, relativistic gyrotrons, large orbit gyrotrons (LOGs), quasi-optical gyrotrons, fast- and slow-wave cyclotron autoresonance masers (CARMs), gyroklystrons, gyro-TWT amplifiers, gyrotwystron amplifiers, gyro-BWOs, gyro-harmonic converters, gyro-peniotrons, magnicons, free electron masers (FEMs), and dielectric vacuum windows for such high-power mm-wave sources. Gyrotron oscillators (gyromonotrons) are mainly used as high-power millimeter wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control, and diagnostics of magnetically confined plasmas for clean generation of energy by controlled thermonuclear fusion. The maximum pulse length of commercially available 140 GHz, megawatt-class gyrotrons employing synthetic diamond output windows is 30 min (CPI and European KIT-SPC-THALES collaboration). The world record parameters of the European tube are as follows: 0.92 MW output power at 30-min pulse duration, 97.5% Gaussian mode purity, and 44% efficiency, employing a single-stage depressed collector (SDC) for energy recovery. A maximum output power of 1.5 MW in 4.0-s pulses at 45% efficiency was generated with the QST-TOSHIBA (now CANON) 110-GHz gyrotron. The Japan 170-GHz ITER gyrotron achieved 1 MW, 800 s at 55% efficiency and holds the energy world record of 2.88 GJ (0.8 MW, 60 min) and the efficiency record of 57% for tubes with an output power of more than 0.5 MW. The Russian 170-GHz ITER gyrotron obtained 0.99 (1.2) MW with a pulse duration of 1000 (100) s and 53% efficiency. The prototype tube of the European 2-MW, 170-GHz coaxial-cavity gyrotron achieved in short pulses the record power of 2.2 MW at 48% efficiency and 96% Gaussian mode purity. Gyrotrons with pulsed magnet for various short-pulse applications deliver Pout = 210 kW with τ = 20 μs at frequencies up to 670 GHz (η ≅ 20%), Pout = 5.3 kW at 1 THz (η = 6.1%), and Pout = 0.5 kW at 1.3 THz (η = 0.6%). Gyrotron oscillators have also been successfully used in materials processing. Such technological applications require tubes with the following parameters: f > 24 GHz, Pout = 4–50 kW, CW, η > 30%. The CW powers produced by gyroklystrons and FEMs are 10 kW (94 GHz) and 36 W (15 GHz), respectively. The IR FEL at the Thomas Jefferson National Accelerator Facility in the USA obtained a record average power of 14.2 kW at a wavelength of 1.6 μm. The THz FEL (NOVEL) at the Budker Institute of Nuclear Physics in Russia achieved a maximum average power of 0.5 kW at wavelengths 50–240 μm (6.00–1.25 THz).

High-Harmonic Gyrotrons with Axis-Encircling Electron Beams at IAP RAS

The Institute of Applied Physics of the Russian Academy of Sciences has for many years been developing sub-terahertz and terahertz large-orbit gyrotrons that permit selective oscillation at higher cyclotron harmonics than is possible in the conventional gyrotrons. Currently, experimental studies are conducted at two specialized facilities. A prototype universal sub-terahertz source for magnetic resonance spectroscopy is studied using a facility that generates long-pulse and continuous electron beams with particle energies of up to 30 KeV. Continuous selective oscillation at the second and third cyclotron harmonics with frequencies of 0.267 and 0.394 THz was obtained for a radiation power of 900 and 370 W, respectively. New resonators with periodic phase correctors have been developed to increase the efficiency of third-harmonic oscillation and obtain fourth-harmonic oscillation with frequencies of up to 0.65 THz. Using a facility with an electron energy of up to 80 KeV, we study the possibilities of increasing the pulse generation power at the third harmonic at frequencies close to 1 THz to employ in experiments on obtaining a gas discharge in a focused terahertz wave beam and generating high-power extreme ultraviolet radiation.

Axial modes in terahertz high-harmonic large-orbit gyrotrons

Terahertz (THz) gyrotrons can operate with a lower applied magnetic field in harmonic operation, but the weakened harmonic interactions in harmonic gyrotrons can introduce serious challenges when mode competition occurs. The use of an axis-encircling electron beam can greatly alleviate mode competition in a harmonic gyrotron. In this paper, we study axial modes for third-harmonic [Formula: see text]-mode large-orbit gyrotrons. Simulation results reveal that the minimum current for oscillation to begin in each axial mode in the gyrotron regime is associated with a specific range of applied magnetic field. To avoid mode competition, tapered applied magnetic fields and waveguide radii are employed to enhance the high-order axial modes and suppress the low-order axial modes. Furthermore, spurious transverse modes in a THz gyrotron are discussed below. A stable third-harmonic [Formula: see text]-mode large-orbit gyrotron at the third-order axial mode is predicted to yield peak output power of 6.5 kW at 768.1 GHz with an efficiency of 10% for a 75-kV, 0.85-A electron beam with an axial velocity spread of 3%.

Powerful continuous-wave sub-terahertz electron maser operating at the 3rd cyclotron harmonic

Coherent subterahertz radiation is achieved in an electron cyclotron maser (large-orbit gyrotron configuration) operating at a higher cyclotron harmonic in the continuous-wave generation regime. This auto-oscillator is based on the use of a gun with a magnetic field cusp and a section for adiabatic magnetic compression with a factor of 1000, forming a 30 keV/0.7 A axis-encircling electron beam in a magnetic field of 5 T. A stable single-mode generation is achieved in the case of operation at the third and second cyclotron harmonics at frequencies of 0.394 THz and 0.267 THz with radiation powers of 0.37 kW and 0.9 kW, respectively.

CW subterahertz gyrotron operating at high cyclotron harmonics

At the IAP RAS, a subteraherz continuous-wave large orbit gyrotron (LOG) operating directly at the second, third, and fourth cyclotron harmonics at frequencies of 0.26, 0.39, and 0.52 THz, is developed and created. The paper presents the results of experimental testing of LOG at the second and third harmonics carried out to date in the long-pulses regime (duration 0.1–60 s) with the transition to continuous- wave generation regime.

Influence of Perturbations in the Axial Symmetry on Formation of Helical Electron Beams in a System with the Reversed Magnetic Field

We perform three-dimensional numerical simulation of the electron-optical system (EOS) with the reversed magnetic field, which is designed for a large-orbit gyrotron (GBO) operated in a wide frequency range of 0.26–0.65 GHz, within the framework of a static model and analyze the influence of perturbations in the axial symmetry of electrodes and the magnetic field on the parameters of the axis-encircling electron beam. It is shown that the most significant perturbation of the parameters of the helical electron beam (HEB) is determined by the radial displacement of the cathode and the tilt of the cathode coil, which ensures the reversal of the magnetic field. Analytical estimations are presented, which allow determining the corresponding margins for the development of such systems.

Prospective THz Gyrotrons for High-Field Magneto-Resonance Spectroscopy

A high-harmonic Large Orbit Gyrotron and a low-voltage gyrotrino placed inside a spectrometer cryomagnet enable greatly simplify terahertz systems for magneto-resonance spectrometers. Large Orbit Gyrotrons provide a powerful third-harmonic generation at frequencies of 1 THz and 0.394 THz in pulsed and CW regimes, respectively, at significantly lower magnetic fields than conventional gyrotrons. According to simulations the gyrotrino with the voltage of 1.5 kV and frequency of 0.264 THz can generate a power of tens of watts; a possibility to operate at such a low voltage is demonstrated in the existing gyrotron with three-electrode magnetron-injection gun.

A Wideband Electron-Optical System of a Subterahertz Large-Orbit Gyrotron

Specific features of the electron-optical system, which forms the helical axis-encircling electron beam (HAEEB) in an electron gun with the moderate accelerating voltage $U_{o}$ (30 kV) and the cusp magnetic field, are considered. The optimal electrode geometry allowing one to obtain HAEEB without electrons reflected from the magnetic mirror for a pitch factor of up to 2 is found. The developed electron gun can be used in a set of pulsed and continuous-wave large-orbit gyrotrons employing high cyclotron harmonics (second, third, or fourth harmonics) in a wide frequency range of 0.26–0.52 THz (the magnetic field intensity B being 3 to 5 T). The frequency range can be extended even to 0.65 THz, if B is increased up to 6.3 T and $U_{o}$ , up to 45 kV, simultaneously.

Study of an Eighth-Harmonic Large-Orbit Gyrotron in the Terahertz Band

A 0.42-THz eighth-harmonic large-orbit gyrotron (LOG) operating with TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8,1,1</sub> mode has been designed and studied. The LOG design is characterized by the requirement of low external magnetic field and short cavity length to limit the ohmic loss. The cold cavity analysis is presented and followed by the hot cavity simulation with the 3-D particle-in-cell code. It is shown in the initial hot test that by proper choice of the electron beam source, the eighth harmonic of the electron cyclotron resonance can be selectively excited at 0.42 THz with an output power of 3.8 kW, which requires an external magnetic field as low as 2.8 T. Based on the model, the starting oscillation process of TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8,1,1</sub> mode at the eighth harmonic and the dynamic of beam-wave interaction are investigated. Upon verification of the design and modeling, a number of parameter variation studies are conducted to draw broader conclusions about the beam-wave interaction behavior in a high-harmonic LOG. The design procedure and the conclusion of this paper can be generalized and applied to other LOGs.

New Versions of Terahertz Radiation Sources for Dynamic Nuclear Polarization in Nuclear Magnetic Resonance Spectroscopy

Dynamic nuclear polarization in strong-field nuclear magnetic resonance (NMR) spectroscopy requires terahertz radiation with moderate power levels. Nowadays, conventional gyrotrons are used almost exclusively to generate such radiation. In this review paper, we consider alternative variants of electronic microwave oscillators which require much weaker magnetic fields for their operation, namely, large-orbit gyrotrons operated at high cyclotron-frequency harmonics and Cerenkov-type devices, such as a backward-wave oscillator and a klystron frequency multiplier with tubular electron beams. Additionally, we consider the possibility to use the magnetic field created directly by the solenoid of an NMR spectrometer for operation of both the gyrotron and the backward-wave oscillator. Location of the oscillator in the spectrometer magnet makes it superfluous to use an additional superconducting magnet creating a strong field, significantly reduces the length of the radiation transmission line, and, in the case of Cerenkov-type devices, allows one to increase considerably the output-signal power. According to our calculations, all the electronic devices considered are capable of ensuring the power required for dynamic nuclear polarization (10 W or more) at a frequency of 260 GHz, whereas the gyrotrons, including their versions proposed in this paper, remain a single option at higher frequencies.

Gyrotron Development for High Power THz Technologies at IAP RAS

The review summarizes recent experimental results in the field of THz gyrotrons developed for various applications. A CW gyrotron with the operation frequency of 0.26 THz has been successfully used for DNP spectroscopy A pulsed high-harmonic Large Orbit Gyrotron (LOG) with the frequency of 0.55 THz and kW level of output power has been used for THz breakdown and obtaining dense plasma in gases. A powerful pulsed 0.67 THz/200 kW gyrotron is under development for remote detection of ionization sources.

Linear theory of large-orbit gyrotron traveling wave amplifiers with misaligned electron beam

A linear theory of large-orbit gyrotron traveling wave amplifiers (gyro-TWAs), which can be applied to analyze the effect of electron beam misalignment, is developed by specializing the corresponding theory of small-orbit gyro-TWAs. The linear theory is validated by comparing with a nonlinear theory. Numerical results show that beam misalignment can reduce linear gain and amplification bandwidth of large-orbit gyro-TWAs and increase the starting length of large-orbit gyro-BWOs for modes in accordance with the mode-selective condition. In addition, beam misalignment can also break the limitation of mode-selective condition and make the instability problem more complex.

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 relatively low electron currents. Since diffraction Q factors of such cavities are very high, a large fraction of the radiated power is dissipated in Ohmic losses. A sectioned klystronlike cavity can be a way to combine a long electron-wave interaction region with a relatively low diffraction Q factor. In this paper, a design of a third-harmonic terahertz gyrotron is studied in detail and discussed. As compared to a regular cavity, the use of a sectioned microwave system provides an enhancement of the output rf power by several times along with the halving of the Ohmic losses.

Linear and nonlinear theories of a large-orbit gyrotron traveling wave amplifier

The linear and nonlinear theories of large-orbit gyrotron traveling wave amplifiers (gyro-TWAs) have been developed based on the corresponding theories of small-orbit gyro-TWAs. The linear theory is in good agreement with the nonlinear theory in the small signal region of large-orbit gyro-TWAs. The phenomenon that most electrons move toward the axis of interaction circuit during the beam-wave interaction is observed and its potential effect on the design of large-orbit coaxial gyro-TWAs is emphasized.