1 Introduction

Gyro-devices are microwave oscillators and amplifiers based on the Electron Cyclotron Maser (ECM) instability. The free energy for microwave generation is the rotational energy of a weakly relativistic helical electron beam in a longitudinal magnetic field. A net transfer of energy from the gyrating electrons to the electromagnetic (EM) field in the interaction circuit occurs as a result of mainly azimuthal phase bunching when the wave frequency is slightly larger than the relativistic electron cyclotron frequency or one of its harmonics. Many types of microwave sources are based on the ECM instability, such as the gyro-oscillator (gyrotron), gyro-klystron, gyro-twystron, gyro-travelling wave tube (gyro-TWT) or the gyro-backward-wave oscillator (gyro-BWO) [1–12].

The possible applications of gyrotrons and other gyro-type fast-wave devices span a wide range of technologies [11, 13–18]. The plasma physics community has already taken advantage of recent advances in producing high-power micro- and millimeter (mm) waves in the areas of RF plasma applications for magnetic confinement fusion studies, such as lower hybrid current drive (1-8 GHz), electron cyclotron resonance heating and current drive (28–170 GHz), plasma production for numerous different processes and plasma diagnostic measurements such as collective Thomson scattering or heat-pulse propagation experiments. Other applications which take advantage of the development of novel high-power mm-wave gyro-devices include deep space and specialized satellite communication, high resolution Doppler radar, radar ranging and imaging in atmospheric and planetary science, electron cyclotron resonance ion sources, sub-mm wave and THz spectroscopy, materials processing and plasma chemistry, as well as active denial systems and detection of concealed radioactive materials.

The history and the state-of-the-art of the various gyro-devices is summarized in [19], including more than 1,000 references on experimental results.

The present review paper is an introduction to this Special Issue and an update of [18] in the field of high-power gyrotrons. It reports on the present status of high-power gyrotrons for nuclear fusion plasma applications and active denial systems. In addition, it gives a short introduction of a new application of high-power pulsed sub-THz gyrotrons for detection of concealed radioactive materials by initiating a freely localized mm-wave break-down in air.

2 Gyrotrons for fusion plasma applications

At present, gyrotrons are mainly used as high-power mm-wave sources for EC-wave applications and for diagnostics of magnetically confined plasmas in controlled thermonuclear fusion research [20–22]. Long pulse (a few seconds) gyrotrons delivering output powers of up to 1.5 MW per unit, at frequencies between 28 and 170 GHz have been used very successfully for plasma ionization and start-up, electron cyclotron resonance heating (ECRH) and local current density profile control by noninductive electron cyclotron current drive (ECCD) in tokamaks [20, 21] and stellarators [22]. Gyrotron systems with total power of up to 4.5 MW have been installed [17, 18]. The 110 GHz multiple gyrotron system on the DIII-D tokamak at General Atomics (GA) in San Diego is described in this Special Issue in the contribution of Lohr et al. The maximum injected energy on a single plasma shot has been 16.6 MJ for six gyrotrons injecting a total of 3.4 MW for 5 seconds. Recent upgrades and extensions of the ASDEX-Upgrade tokamak multi-frequency ECRH system (105–140 GHz) at IPP Garching are presented in the paper of Wagner et al. In both ECRH systems corrugated HE11 waveguide transmission lines are in use to transmit the high-power mm-wave radiation from the gyrotrons to the plasma [23]. Shapiro and Temkin report on the design of a hyperbolic corrugated horn for conversion of the HE11 waveguide mode to the free space TEM00 Gaussian mode for optimum coupling to the plasma. The theoretical conversion efficiency is higher than 99.5%.

The confining magnetic fields in present day fusion devices are in the range of Bo = 1–3.6 Tesla. As fusion machines become larger and operate at higher magnetic field (Bo ≅ 5 T) and higher plasma densities in steady state, it is necessary to develop continuous wave (CW) gyrotrons that operate at both higher frequencies and higher mm-wave output powers. The requirements of the projected tokamak experiment ITER (International Thermonuclear Experimental Reactor) in Cadarache and of the future new stellarator W7-X at the Max-Planck-Institute for Plasma Physics (IPP) in Greifswald are between 10 and 40 MW at frequencies between 140 GHz and 170 GHz [17, 18]. This suggests that mm-wave gyrotrons generating output power of at least 1 MW, CW, per unit are required. Since efficient ECRH, ECCD and collective Thomson scattering need axisymmetric, narrow, pencil-like mm-wave beams with well defined polarization (linear or elliptical), single-mode gyrotron emission is necessary in order to generate a TEM00 Gaussian beam mode at the plasma torus antennas.

Modern high-power high-order volume mode (e.g. TE22,6, TE25,10, TE28,7, TE28,8 or TE31,8) gyrotrons for EC-wave fusion applications employ an internal quasi-optical (QO) mode converter with lateral mm-wave output in the linearly polarized fundamental Gaussian mode (Fig. 1) [18, 23]. The QO mode converter is a part of the internal electrodynamic system of the tube and separates the mm-waves from the spent electron beam, transforms the complicated high-order cavity mode to an easily transportable linearly polarized wave beam and allows to minimize harmful effects of possible reflections of mm-wave power to the gyrotron cavity. In addition, this design enables the simple integration of a single-stage depressed collector (SDC) for energy recovery. The QO mode converter consists of an irradiator (waveguide cut) and 2–3 mirrors (Fig. 1).

Fig. 1
figure 1

Schematic layout of a modern high-order volume mode gyrotron with quasi-optical mode converter and single-stage depressed collector [18].

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At present, chemical vapor deposition (CVD) diamond is the only solution for the realization of simple, water-edge-cooled single-disk 1 MW, CW gyrotron and torus windows for ECRH [24]. Current CVD capabilities allow samples of up to 120 mm diameter and 2.5 mm thickness (420 carat). The price is very high and there are only two suppliers: Element Six (former DeBeers Industrial Diamond Division, U.K.) and Diamond Materials, Germany. Synthetic diamond is attractive due to its good mechanical properties (bending strength: σB = 400 MPa), very small thermal expansion (α′ = 10−6/K), modest dielectric constant (εr′ = 5.67 ± 0.01), very low mm-wave attenuation (tan δ ≈ 2.10−5), very high thermal conductivity (k = 1,900 W/mK at room temperature) and nuclear radiation resistance up to <1021 neutrons/m2 and 0.8 Gy/s γ- or X-rays.

Thermal finite element computations show that the transmission capability is even above 2 MW, CW. Low and high power tests confirm the excellent material properties. Due to the temperature insensitivity of εr′, tan δ and α′, there is no change of the electrical thickness of the disk during long-pulse or CW operation.

2.1 State-of-the-art of fusion gyrotrons

Single-mode mm-wave gyrotron oscillators capable of high average power, 0.1–1 MW, in long pulse or CW operation, are currently under development in several scientific and industrial laboratories [10, 25–39]. Table 1 summarizes the performance parameters of modern gyrotrons for ECRH in the frequency range from 28 to 85 GHz and 95 GHz gyrotrons for active denial systems which also could be used for ECRH. The 75 GHz GYCOM gyrotron holds the worldrecord efficiency of 70% (with SDC) and Gaussian output mode purity of 98% [36]. Kariya et al. report in this Special Issue on the development of a 28 GHz 1 MW tube with TE8,3 mode cavity for the GAMMA10 tandem mirror at the University of Tsukuba and a 77 GHz, 1.2 MW gyrotron with TE18,6 mode cavity for the Large Helical Device (LHD) at Toki, Japan. The aims of this recent development is to get new record values of ion confining potential and electron temperature in GAMMA 10 and to upgrade the ECRH&CD system of LHD with an option to allow collective Thomson scattering diagnostics. High efficiency QO mode converters for such “low frequency” 28 GHz gyrotrons are a specific challenge. Minami et al. summarize their activities in the paper “High efficiency mode converter for low-frequency gyrotron”. The state-of-the-art at frequencies between 104 and 118 GHz is given in Table 2 and at frequencies from 140 to 170 GHz in Table 3. Figure 2 shows photographs of the megawatt-class 170 and 140 GHz gyrotrons for ITER and W7-X.

Table 1 Performance parameters of modern gyrotrons for ECRH (28–95 GHz) in plasmas for magnetic confinement fusion studies (τ ≥ 0.1 s) [19].
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Table 2 Present development status of modern high frequency gyrotrons for ECRH and stability control in magnetic fusion devices (104 GHz ≤ f < 118 GHz, τ ≥ 2 s) [19].
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Table 3 Present development status of modern high frequency gyrotrons for ECRH and stability control in magnetic fusion plasma devices (f ≥ 140 GHz, τ ≥ 1.0 s) [19].
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Fig. 2
figure 2

Megawatt-class cylindrical cavity gyrotrons for ITER (170 GHz) and W7-X (140 GHz).

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The maximum pulse length of commercially available 140 GHz, megawatt-class gyrotrons employing synthetic diamond output windows is 30 minutes (CPI and European KIT-CRPP-CEA-TED collaboration). The world record parameters for output power >0.9 MW of the European megawatt-class 140 GHz gyrotron are: 0.92 MW at 30 min. pulse duration, 97.5% Gaussian mode purity and 44% efficiency, employing a single-stage depressed collector (SDC) for energy recovery [26–28]. The contribution of Gantenbein et al. to this Special Issue reports about recent results of the series production of this tube. The maximum pulse length of the 110 GHz, 1 MW TOSHIBA-JAEA gyrotron for JT-60U is 31 s. The Japan 170 GHz ITER gyrotron holds the energy world record of 2.88 GJ (0.8 MW, 60 min.) and the efficiency record of 57% at 1 MW, 800 s for tubes with an output power of more than 0.5 MW [31, 32]. The very high efficiency has been obtained by operation in the so-called hard-excitation region which could be easily achieved by control of the electron beam velocity ratio with a triode-type magnetron injection gun (MIG) with a modulation anode. The paper of Kajiwara et al. in this Special Issue summarizes reliability tests on this gyrotron (0.8 MW/600 s with 20 min. interval). 72 shots out of 88 shots were successful (82% reliability). The Russian 170 GHz ITER gyrotron achieved 1 MW at 570 s pulse length and 0.8 MW with a pulse duration of 1000 s [36]. Recent results are described in the paper of Litvak et al. Denisov et al. report on recent developments in the field of millimeter wave multi-mode transmission line components in Russia.

The power modulation capabilities of the 140 GHz TED/KIT tube for use in plasma stabilization and diagnostic experiments have been experimentally investigated. In pulses of 1 s length, mm-wave power modulation higher than 80% has been obtained either by modulating the cathode voltage (50 kHz) or the depression voltage (1.5 kHz); in both cases the modulation frequency limitation was given by the corresponding HV power supply and not by the gyrotron itself. Detailed analysis of the collector loading with respect to the modulation scheme and the intrinsic gyrotron limitations for long-pulse operation with deep modulation are discussed in [40]. The peak power deposition on the collector of this gyrotron can be reduced by about a factor of 2 by combined vertical and transversal magnetic sweeping of the spent electron beam [41].

Experiments on 5 kHz (100 μs on/100 μs off) power modulation for 60 s of the 170 GHz, 1 MW TOSHIBA—JAEA gyrotron by beam current modulation via the modulation anode have been very successful [32].

2.2 Development of advanced fusion gyrotrons

In order to keep the number of the required gyrotrons and magnets as low as possible, to reduce the costs of the ITER 24 MW, 170 GHz ECRH&CD system and to allow four compact upper launchers for plasma stabilization, higher mm-wave power per tube (2 MW) is desirable. Conventional cylindrical waveguide gyrotron cavities are not suitable for this high frequency, high power regime because of high Ohmic wall losses and/or mode competition problems. However, in coaxial cavities the existence of the longitudinally corrugated and tapered inner conductor reduces the problems of mode competition and limiting current, thus allowing one to use even higher order modes with lower Ohmic attenuation than in cylindrical cavities. In addition, the inner rod enables a specific voltage depression scheme for energy recovery and ultra-fast frequency step tuning just by applying an appropriate voltage to this coaxial insert. CVD-diamond windows with a transmission capability of 2 MW, CW are feasible.

The availability of sources with fast frequency tunability (several GHz s−1, tuning in 1.5–2.5% steps for approximately ten different frequencies) would permit the use of a simple, fixed non-steerable mirror antenna for local ECCD experiments and plasma stabilization [21, 42]. Challenges in this development are the proper MIG as well as the broadband QO output coupler and CVD diamond window.

  1. A.

    Coaxial-Cavity Gyrotrons

A 2 MW, CW, 170 GHz coaxial-cavity gyrotron for electron cyclotron heating and current drive in ITER is under development in cooperation between European Research Institutions (EGYC) [43]. The design of critical components like electron gun, beam tunnel, cavity and QO mm-wave output system has been studied in a short-pulse coaxial gyrotron (pre-prototype) at KIT. At the magnetic field of B = 6.87 T a maximum mm-wave output power P out  = 2.2 MW has been obtained at U c  = 93 kV and I b  = 80 A with an efficiency of around 30% at 1 ms pulse duration in non-depressed collector operation [44]. If a CVD-diamond window would be installed instead of the fused silica window, the output power would be 2.3 MW with an efficiency of 31% due to lower mm-wave absorption in diamond. The Gaussian output mode purity is almost 96% [45].

The gyrotron development teams in Japan [32], Russia [36] and USA [39] are testing the power limits of conventional cylindrical-cavity CW gyrotrons. Table 4 summarizes the present state-of-the-art of advanced short-pulse 1.5–2 MW gyrotrons with TEM00 mode output and frequencies in the range of 110–170 GHz. In this Special Issue Tax et al. report experimental results at MIT on the 1.5 MW, 110 GHz gyrotron with a smooth mirror mode converter. The Gaussian beam content was (95.8 ± 0.5)% in both hot and cold tests. The paper of Litvak et al. summarizes recent experimental results on the Russian 170 GHz TE28,12 mode tube showing 1.5–2 MW power capability.

Table 4 State-of-the-Art of advanced 1.5–2 MW gyrotrons with TEM00-mode output [19].
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  1. B.

    Tunable Multi-Frequency Gyrotrons

In the case of local, non-inductive current drive for suppression of plasma instabilities in future tokamak fusion reactors the gyrotron is a good option, since ultra-broadband windows and specific gyrotron magnets allow stepwise frequency tuning in the seconds time-scale in the full D-band (110–170 GHz) [21]. Successful gyrotron experiments at KIT Karlsruhe employing a 140 GHz, TE22,6-mode cavity, a QO mode converter with dimple-type launcher, a broadband silicon nitride composite Brewster angle window and a SDC gave up to 1.6 MW output power (pulse duration: 1–5 ms) at efficiencies between 48 and 60% for the entire operating mode series in the frequency range from 114 GHz to 166 GHz (TEm,5 with m = 18 to 22, TEm,6 with m = 20 to 26 and TEm,7 with m = 22 to 26) [42]. Frequency tuning in steps of approximately 3.7 GHz was achieved by variation of the magnetic field strength in the cavity [46].

A new ECRH system is under development for the ASDEX-Upgrade tokamak at IPP Garching which is described by the contribution of Wagner et al. in this Special Issue. Four 1 MW gyrotrons with SDC will generate 4 MW power with a pulse duration of 10 s. GYCOM/IAP develops in collaboration with IPP Garching and KIT an industrial, frequency-tunable 1 MW gyrotron with almost 50% efficiency (SDC). A four-frequency tube (105, 117, 127 and 140 GHz) delivered in 10 s pulses 0.85 MW at 105 GHz and 0.95 MW at 140 GHz (two-frequency gyrotron) (see contribution of Litvak et al. and [36]). The single-disk CVD-diamond window has maximum transmission for these two frequencies. After the installation of a broadband CVD-diamond window, the GYCOM/IAP group will operate this gyrotron also at the two intermediate frequencies.

Preliminary operation of one of the 140 GHz, 1 MW gyrotrons of W7-X at IPP Greifswald as a two-frequency gyrotron delivered 0.41 MW in 10-s pulses at 103.8 GHz [28]. JAEA in Japan is testing a 137/170 GHz 1.4 MW two-frequency long-pulse gyrotron operating in the TE25,9 and TE31,11 modes, respectively [32].

Recently, the narrow-band fused silica output window of the pre-prototype 170 GHz coaxial-cavity gyrotron at KIT has been replaced by a broadband silicon-nitride Brewster window provided by NIFS, Japan, in order to be able to study the excitation of additional modes in the frequency range 130–170 GHz. Simulations have shown that the QO output coupler with the new launcher has a good conversion efficiency for a number of modes between 130 and 170 GHz [47]. In first experiments [44] an output power of 1.8 MW with 26% efficiency has been obtained in the TE28,16 mode at 141.3 GHz. In addition, in the next azimuthal neighbour mode TE29,16 at 143.3 GHz, an output power of 1.25 MW has been generated with an efficiency of 23%. Measurements of the intensity profiles of the mm-wave output beams have confirmed the numerical simulations resulting in a very high efficiency and good mode purity (up to 96%) of the QO output coupler also for these other frequencies and modes.

3 High-power terahertz gyrotrons

Russian gyrotrons for plasma diagnostics or spectroscopy applications deliver Pout = 40 kW with τ = 40 μs at frequencies up to 650 GHz (η ≥ 4%), Pout = 5.0 kW at 1 THz (η = 6.1%), and Pout = 0.5 kW at 1.3 THz (η = 0.6%). The status and new designs of THz gyrotrons are summarized in this Special Issue by Bratman et al.

Nusinovich et al. present the design data of a 200–300 kW, 670 GHz gyrotron for detection of concealed radioactive materials which would operate in a pulsed solenoid producing 27–28 T magnetic fields. The concept is based on the use of a high-power sub-THz gyrotron whose power, being focused into a small spot with dimensions in the order of a wavelength, exceeds the threshold level required for initiating a freely localized mm-wave breakdown in air. However, in the absence of radioactive materials, the ambient electron density is so small that there is a very small probability to find a free electron in this small volume to trigger the avalanche breakdown process. Therefore the fact that a breakdown is observed would indicate that there is hidden radioactive material in the vicinity of a focused wave beam. In addition, numerous issues in this specific application are discussed in the paper of Nusinovich et al., e.g. threshold conditions for initiating the breakdown, production of γ-rays by concealed radioactive materials and their role in producing low energy electrons outside a container, Gaussian wave beam focusing into a small spot by a limited-size antenna, random walk of energetic electrons which may result in generation of free electrons in a given volume during the sub-mm-wave pulse and comparison of diffusion time with the time required for competing processes, such as ionization and three-body attachment.

4 Conclusions

Industrial megawatt-class 140 GHz and 170 GHz CW gyrotrons for fusion plasma applications are available in EU, Japan, Russia and USA. The development of a 2 MW, 170 GHz, CW coaxial-cavity gyrotron for ITER is running in EU whereas 1.5 MW, 170 GHz, CW cylindrical-cavity gyrotrons are being developed in Japan and Russia. The development of 1–2 MW, step-tunable multi-frequency D-band gyrotrons for advanced ECCD and plasma stabilization is running in Japan, Russia and Germany.

High-power W-band gyrotrons for active denial systems and a pulsed sub-THz gyrotron (670 GHz) for detection of concealed radioactive materials by initiating a freely localized mm-wave break-down in air are under development in USA.