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).

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

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.

The Gyrotron at 50: Historical Overview

The present paper contains a brief overview of the studies of mode and wave interaction in gyrodevices. The main part of it is devoted to the discussion of physical effects caused by mode interaction in the free-running and phase-locked gyrotron oscillators. Also, the problem of the stability of large-signal operation of gyro-forward-wave amplifiers with respect to excitation of parasitic backward waves is discussed.

Review of the theory of mode interaction in gyrodevices

The problem of mode competition in cylindrical cavity gyrotrons is considered. The normalized variable equations are used to calculate the oscillation regions of possible operating modes in the energy-velocity-pitch-angle plane. The analysis is self-consistent and includes the effect of changing beam current, pitch angle, and energy during the startup phase. The time evolution of beam parameters during startup is computed for several types of startup methods and used to determine the oscillating cavity modes during startup. Depending on the type of startup chosen, the cavity can be made to oscillate in several modes or in a single chosen operating mode-even for high-order modes where many other possible operating modes exist. Some startup methods are seen to be less favorable than others, allowing for oscillation of unwanted modes and some methods are seen to be more sensitive to small beam/cavity misalignment. The accessibility to the high-efficiency hard-excitation region can also be determined and is seen to depend on the startup scenario. The startup analysis is linear whereas the stability and interaction efficiency computations are fully non-linear. The method is general and can be applied to any operating mode, with the mode competition analysis specifically useful for high-order modes where the spectrum is dense. The analysis of the accessibility to the hard-excitation region is applicable to high- and low-order operating modes. Both q=1 and q=2 longitudinal mode numbers are considered. >

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Mode competition and startup in cylindrical cavity gyrotrons using high-order operating modes

This report gives an update of the worldwide experimental achievements in the development of long-pulse gyrotron oscillators for fusion plasma heating, plasma diagnostics and materials processing. In addition, this work gives a short overview of the present development status of coaxial-cavity multi-megawatt gyrotrons, gyrotrons for technological and spectroscopy applications, relativistic gyrotrons, quasi-optical gyrotrons, cyclotron autoresonance masers, gyro-amplifiers, gyro-BWO's, free electron masers and of vacuum windows for such high-power mm-wave sources.

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State-of-the-Art of High Power Gyro-Devices and Free Electron Masers. Update 2011. (KIT Scientific Reports ; 7606)

The gyrotron is a powerful source of millimetre wave radiation. Fusion applications require more power per tube than is currently available This in turn means that the resonator must be highly overmoded, with a dense mode spectrum, which leads to mode competition. The influence of external parameters, such as the applied magnetic field, electron beam, and resonator geometry, on mode competition in tubes designed to operate at 150 and 140 GHz in the TE03 mode is investigated theoretically and experimentally. It can be shown that even when the mode spectrum is fairly dense, single mode operation of a gyrotron is possible. The influence of startup conditions on output power is investigated in the 150 GHz experiment. In some cases, modes with an axial index of 2 were excited.

Mode competition using TE03 gyrotron cavities

A scalar network analysis system suitable for the frequency range 110 - 170 GHz is presented. It offers a dynamic range up to 60 dB, thus having the capability to characterize the frequency response of millimeter wave networks with high insertion loss. The system consists of components, which are probably used at each microwave laboratory. The microwave source is a swept synthesizer. Its output signal is multiplied to the D-band frequency range. The paper describes the system's basic properties, as there are frequency stability and dynamic range, and how they are influenced by the components. Finally applications and results are presented.

Scalar MM-WAVB Network Analysis with High Dynamic Range

A vector network-analyzer is described, which is in use for waveguide component tests over the 130 - 150 GHz frequency range. The set up consists mainly of standard laboratory equipment, only a few components are custom designed for the 2 mm wavelength range. Measurement examples are given. The operation principle may briefly be described as follows: A test signal f1+f2 is generated by mixing a LO signal with fl ? 15 GHz and an auxiliary signal with f2 = 200 MHz. At the mixer output the LO signal and the lower sideband are suppressed. The upper sideband is multiplied by nine (n=9). Having passed the device under test, it is down converted by a harmonic mixer. The resulting IF-level and phase shift are measured at n·200 MHz = 1.8 GHz with the vector-voltmeter. The multiplied test signal and the local oscillator signal of the harmonic mixer are derived from the same signal source. Swept measurements are performed by tuning the LO frequency. The same principle can be used in a wide range of waveguide-bands.

T. Geist

Papers

Mode competition using TE03 gyrotron cavities

Scalar MM-WAVB Network Analysis with High Dynamic Range

Vector Network-Analyzer for 130 - 150 GHz