1 Introduction

The possible applications of gyrotron oscillators (gyromonotrons, or just gyrotrons) and other electron cyclotron maser (ECM) fast-wave devices (Table 1) span a wide range of technologies [1,2,3,4,5,6,7] . The plasma physics community has taken advantage of advances in producing high-power micro- and millimeter (mm) waves in the areas of radio frequency (RF) plasma applications for magnetic confinement fusion studies, such as lower hybrid current drive (LHCD 1–8 GHz), electron cyclotron resonance heating and non-inductive electron cyclotron current drive (ECRH&CD 28–170 GHz), plasma production for numerous different processes and plasma diagnostic measurements such as Collective Thomson Scattering (CTS) or heat-pulse propagation experiments. Other applications which await further development of novel high-power mm-wave sources include deep-space and specialized satellite communication, high-resolution Doppler radar, radar ranging and imaging in atmospheric and planetary science, remote detection of concealed radioactive materials, ECR sources of highly ionized ions, submillimeter-wave and THz spectroscopy, materials processing, and plasma chemistry.

Table 1 Overview of gyro-devices and comparison with corresponding conventional linear-beam (O-type) tubes

Most work on ECM devices has investigated the conventional gyrotron [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29] in which the wave vector of the radiation in an open-ended, irregular cylindrical waveguide cavity is almost transverse to the direction of the applied magnetic field, generating electromagnetic (EM) waves near the electron cyclotron frequency or at one of its harmonics. Long-pulse and continuous wave (CW) gyrotrons delivering output powers of 0.1–1.0 MW at frequencies between 28 and 170 GHz have been used very successfully in thermonuclear fusion research for plasma ionization and start-up, ECRH, and local current density profile control by ECCD at system power levels up to 10 MW.

ECRH has become a well-established heating method for both tokamaks [30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60] and stellarators [59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83]. The confining magnetic fields in present day fusion devices are in the range of Bo = 1–3.6 T. As fusion machines become larger and operate at higher magnetic field (Bo ≅ 5.5 T) and higher plasma densities in steady state, it is necessary to develop CW gyrotrons that operate at both higher frequencies and higher mm-wave output powers. The requirements of the future tokamak experiment ITER (International Thermonuclear Experimental Reactor) and of the new stellarator (W7-X) at the Max-Planck-Institut für Plasmaphysik in Greifswald are between 10 and 40 MW at frequencies between 140 GHz and 170 GHz [22, 25,26,27,28, 37, 50, 60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78, 84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99]. This suggests that mm-wave gyrotrons that generate output power of at least 1 MW, CW, per tube are required. Since efficient ECRH needs 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 fundamental Gaussian beam mode. Single-mode 110–170 GHz gyromonotrons with conventional cylindrical cavity, capable of 1.5 MW per tube, CW [22,23,24,25,26,27,28], and 2 MW coaxial-cavity gyrotrons [90,91,92,93,94,95,96,97,98,99,100] are currently under development. There has been continuous progress towards higher frequency and power but the main issues are still the long-pulse or CW cavity and collector operation. The availability of sources with fast frequency tunability would permit the use of a simple, non-steerable mirror antenna at the plasma torus for local current drive experiments [25,26,27,28, 37, 92,93,94,95,96,97,98,99,100,101,102,103,104]. Frequency tuning has been shown to be possible on quasi-optical Fabry-Perot cavity gyrotrons [105, 106] as well as on cylindrical cavity gyrotrons by frequency tuning in steps (different operating cavity modes) [107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142].

This review reports on the present status and future prospects of gyrotrons and RF vacuum windows for ECRH&CD in fusion plasmas and for ECR plasma sources for generation of multi-charged ions and soft X-rays [143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165] (Tables 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and 13), the development of very high-frequency gyrotrons for active plasma diagnostics [166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219], high-frequency sub-millimeter wave spectroscopy in various fields (e.g., dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) spectroscopy, molecular spectroscopy, hyperfine structure of the positronium) [220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262,263,264,265,266,267,268,269,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295,296,297,298,299,300,301,302,303,304,305], remote detection of concealed radioactive materials [306,307,308,309], wireless communication [310], and medical applications [311,312,313,314,315,316] (Tables 14, 15, 16, 17 and 18) and of quasi-optical gyrotrons (Table 22). Gyrotrons also are successfully utilized in materials processing (e.g., advanced ceramic sintering, surface hardening or dielectric coating of metals and alloys, semiconductor production, penetrating rocks) as well as in plasma chemistry [1,2,3,4,5,6,7, 317,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334,335,336,337]. The use of gyrotrons for such technological applications appears to be of interest if one can realize a relatively simple, low cost device, which is easy in service (such as a magnetron). Gyrotrons with low magnetic field (operated at the 2nd harmonic of the electron cyclotron frequency), low anode voltage, high efficiency and long lifetime are under development. Mitsubishi in Japan [338] and Gycom in Russia [324, 332,333,334,335, 339,340,341,342,343,344] are also employing permanent magnet systems. The state-of-the-art in this area of gyrotrons for technological applications is summarized in Table 19.

Table 2 Performance parameters of gyrotron oscillators with frequencies between 5 and 95 GHz
Table 3 Present development status of high-frequency gyrotron oscillators for ECRH and stability control in magnetic fusion devices (100 GHz ≤ f < 140 GHz, τ ≥ 0.1 ms)
Table 4 Present development status of high-frequency gyrotron oscillators for ECRH and stability control in magnetic fusion devices (f ≥ 140 GHz, τ ≥ 0.1 ms)
Table 5 Present experimental development status of short-pulse (3 μs–15 ms) coaxial cavity gyrotron oscillators
Table 6 Present development status of high-frequency gyrotron oscillators with conventional cylindrical or quasi-optical cavity and single-stage depressed collector (SDC) (τ ≥ 10 μs)
Table 7 Step-tunable 1-MW-class gyrotrons at KIT with Quartz, Silicon Nitride (Kyocera SN-287) or CVD-diamond Brewster window. The GYCOM 140 GHz TE22,10-mode tube was also operated in 50–150-ms pulses with a BN Brewster window (11 frequencies at 0.8 MW between 104 and 143 GHz). The QST and MIT gyrotrons used a plane single-disk output window
Table 8 Multi-frequency gyrotrons operating at different transmission maxima of a plane single-disk window
Table 9 Step-tunable 1-MW and 2-MW gyrotrons with coaxial cavity. IAP: Smooth inner rod and plane output window disk. KIT and EGYC: Tapered and longitudinally corrugated inner rod and broadband Silicon Nitride (Kyocera SN-287) Brewster window
Table 10 Experimental parameters of high-power millimeter-wave vacuum windows [15, 19, 22, 26,27,28, 127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142, 382,383,384, 458,459,460,461,462,463,464,465,466,467,468,469,470,471,472, 477, 484, 489, 490, 524,525,526,527,528,529,530,531,532, 542,543,544,545,546,547,548,549,550,551,552,553,554,555,556,557,558,559,560,561,562,563,564,565,566,567,568,569,570,571,572,573,574,575,576,577,578,579,580,581,582,583,584,585,586,587,588,589,590,591,592,593,594,595,596,597,598,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,614,615,616,617,618,619,620,621,622,623,624,625,626,627,628,629,630,631,632,633,634,635,636,637,638,639,640,641,642,643,644,645,646,647,648,649, 669,670,671,672,673,674,675,676,677,678,679,680,681,682,683,684,685,686,687,688,689,690,691,692,693,694,695,696,697,698,699,700,701,702,703,704,705,706,707,708,709,710,711,712,713,714,715,716,717,718,719,720,721,722,723,724,725,726,727,728,729,730,731,732,733,734,735,736,737,738,739,740,741,742,743,744,745,746,747,748,749,750,751,752,753,754,755,756,757,758,759,760,761,762,763,764,765,766,767,768,769,770,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787,788,789,790,791,792,793,794,795,796,797,798,799,800,801,802,803,804,805, 882, 883, 895,896,897,898,899,900,901,902,903,904,905,906,907,908,909,910,911,912,913,914,915,916,917,918,919,920,921,922,923,924,925,926,927,928,929,930,931,932,933,934,935,936,937,938,939,940,941,942,943,944]
Table 11 Thermophysical, mechanical and dielectrical parameters of window materials related to thermal load-failure resistance and power transmission capacity of edge-cooled windows at room temperature (p.c. = poly-crystalline, s.c. = single-crystalline) [86, 102, 919, 925, 932, 934, 942, 945,946,947,948,949,950]
Table 12 Thermophysical, mechanical and dielectrical parameters of window materials related to thermal load-failure resistance and power transmission capacity of edge-cooled windows at LN2-temperature—77 K (LNe-temperature—30 K) (p.c. = poly-crystalline, s.c. = single-crystalline) [919]
Table 13 Options for 1 MW, CW, 170 GHz gyrotron windows [84,85,86,87,88,89, 102, 919]
Table 14 Performance parameters of mm- and submillimeter-wave gyrotrons operating at the 2nd harmonic of the electron cyclotron frequency, with output power > 1 kW
Table 15 Operation results of high harmonic gyrotrons with axis-encircling electron beam (LOG) and permanent magnet (Nd Fe B) at the University of Fukui and pulsed magnet at IAP (THz gyrotron)
Table 16 Performance parameters of pulsed and CW millimeter- and submillimeter-wave gyrotron oscillators operating at the fundamental electron cyclotron resonance
Table 17 Step tuning of MIT gyrotron oscillators (with large MIG [993, 994]) operating at the fundamental electron cyclotron resonance frequency (pulse length 1.5 μs)
Table 18 Step tuning of MIT gyrotron oscillator (with small MIG [993, 994]) operating at the fundamental electron cyclotron resonance frequency (pulse length 1.5 μs)
Table 22 Present development status of quasi-optical gyrotron oscillators

The next generation of high-energy physics accelerators and the next frontier in understanding of elementary particles is based on supercolliders. For normal-conducting linear electron-positron colliders that would reach center-of-mass energies of > 1-TeV sources at 17 to 35 GHz with Pout = 300 MW, τ = 0.2 μs and characteristics that allow approximately 1000 pulses per second would be necessary as drivers [345,346,347]. These must be phase-coherent devices, which can be either amplifiers or phase-locked oscillators. Such generators are also required for super-range high-resolution radar and atmospheric sensing [348,349,350,351,352,353,354,355,356,357,358,359,360]. Therefore, this report also gives an overview of the present development status of relativistic gyrotrons (Tables 20 and 21), fast- and slow-wave cyclotron autoresonance masers (CARM) (Tables 23 and 24), gyro-klystrons (Tables 25, 26, and 27), gyrotron travelling wave tube (Gyro-TWT) amplifiers (Tables 28 and 29), gyrotwystrons (Tables 30, 31, and 32), peniotrons and gyropeniotrons (Tables 35 and 36) and magnicons (Table 37) for such purposes as well as of free electron masers (FEM) (Table 38) and broadband gyrotron backward wave oscillators (Gyro-BWO) (Tables 33 and 34) for use as drivers for FEM amplifiers.

2 Classification of Fast-Wave Microwave Sources

Fast-wave devices in which the phase velocity vph of the EM wave is higher than the speed of light c, generate or amplify coherent EM radiation by stimulated emission of bremsstrahlung from a beam of relativistic electrons. The electrons radiate because they undergo oscillations transverse to the direction of beam motion by the action of an external force (field). For such waves, the electric RF field is mainly transverse to the propagation direction.

The condition for coherent radiation is that the contributions of different electrons reinforce the originally emitted radiation in oscillators or the incident EM wave in amplifiers. This condition is satisfied if a bunching mechanism exists to create electron density variations of a size comparable to the wavelength of the imposed EM wave. To achieve such a mechanism, a resonance condition must be satisfied between the periodic motion of the electrons and the EM wave in the interaction region [18, 21, 27, 361]

$$ \omega -{k}_{\mathrm{z}}{v}_{\mathrm{z}}\cong \mathrm{s}\varOmega \kern0.75em ,\kern1em \mathrm{s}=1,2,\dots \kern1.75em \left({k}_{\mathrm{z}}{v}_{\mathrm{z}}=\mathrm{Doppler}\ \mathrm{term}\right), $$
(1)

where ω and kz are the wave angular frequency and characteristic axial wavenumber, respectively, vz is the translational electron drift velocity, Ω is an effective frequency, which is associated with macroscopic oscillatory motion of the electrons, and s is the harmonic number.

In ECMs, EM energy is radiated by relativistic electrons gyrating in an external longitudinal magnetic field. In this case, the effective frequency Ω corresponds to the relativistic electron cyclotron frequency:

$$ {\varOmega}_{\mathrm{c}}={\varOmega}_{\mathrm{c}\mathrm{o}}/\gamma \kern1em \mathrm{with}\kern1em {\varOmega}_{\mathrm{c}\mathrm{o}}={eB}_{\mathrm{o}}/{m}_{\mathrm{o}}\kern1em \mathrm{and}\kern1em \gamma ={\left[1-{\left(v/c\right)}^2\right]}^{-1/2}\approx 1+{eV}_{\mathrm{o}}/{m}_{\mathrm{o}}{c}^2=1+{eV}_{\mathrm{o}}/511 $$
(2)

where –e and mo are the charge and rest mass of an electron, γ is the relativistic Lorentz factor, Bo is the magnitude of the external magnetic field, and eVo is the energy of the accelerated electrons in keV. Here, Vo is the acceleration voltage. The nonrelativistic electron cyclotron frequency is given by the formula fco (GHz) = 28Bo(T). A group of relativistic electrons gyrating in a strong magnetic field will radiate coherently due to bunching caused by the relativistic mass dependence of their gyration frequency. Bunching is achieved because, as an electron loses energy, its relativistic mass decreases and it thus gyrates faster. The consequence is that a small amplitude wave’s electric field, while extracting energy from the particles, causes them to become bunched in the gyration phase and reinforces the existing wave electric field. The strength of the magnetic field determines the radiation frequency.

In the case of a spatially periodic magnetic or electric field (undulator/wiggler), the transverse oscillation frequency Ω = Ωb (bounce frequency) of the moving charges is proportional to the ratio of the electron beam velocity vz to the spatial period λw of the wiggler field. Thus,

$$ {\varOmega}_{\mathrm{b}}={k}_{\mathrm{w}}{v}_{\mathrm{z}}\kern0.75em ,\kern2em {k}_{\mathrm{w}}=2\uppi /{\lambda}_{\mathrm{w}} $$
(3)

The operating frequency of such devices, an example of which is the free electron maser (FEM) [362,363,364,365,366,367,368], is determined by the condition that an electron in its rest frame “observes” both the radiation and the periodic external force at the same frequency. If the electron beam is highly relativistic (vph ≅ vz ≅ c), the radiation will have a much shorter wavelength than the external force in the laboratory frame (λ ≅ λw/2γ2, so that ω ≅ 2γ2 Ωb). Therefore, FEMs are capable of generating EM waves of very short wavelength determined by the relativistic Doppler effect. Bunching of electrons in FEMs is due to the perturbation of the beam electrons by the ponderomotive potential well, which is caused by “beating” of the EM wave with the spatially periodic wiggler field. It is this bunching that enforces the coherence of the emitted radiation.

In the case of the ECMs and FEMs, unlike most conventional microwave sources and lasers, the radiation wavelength is not determined by the characteristic size of the interaction region. Such fast-wave devices require no slow-wave structure (e.g., periodically rippled walls or dielectric loading) and can instead use a simple hollow-pipe oversized waveguide as interaction circuit. These devices are capable of producing very high-power radiation at cm-, mm-, and sub-millimeter wavelengths since the use of large waveguide or cavity cross sections reduces Ohmic wall losses and breakdown restrictions, as well as permitting the passage of larger, higher-power electron beams. It also relaxes the constraint that the electron beam in a single cavity can only remain in a favorable RF phase for half of a RF period (as in klystrons and other devices employing transition radiation). In contrast with klystrons, the reference phase for the waves in fast-wave devices is the phase of the electron oscillations. Therefore, the departure from the synchronous condition, which is given by the transit angle θ = (ω-kzvz-sΩ)L/vz, where L is the interaction length, can now be of order 2π or less, even in cavities or waveguides that are many wavelengths long [369].

3 Dispersion Diagrams of Fast Cyclotron Mode Interaction

The origin of the ECMs traces back to the late 1950s, when three investigators began to examine theoretically the generation of microwaves by the ECM interaction [8, 9, 27]: Richard Twiss in Australia [370], Jürgen Schneider in the USA [371], and Andrei Gaponov in Russia [372]. A short note on the possibility to use the rotational energy of a helical electron beam for microwave generation was published by Hans Kleinwächter in 1950 [373]. In early experiments with devices of this type, there was some debate about the generation mechanism and the relative roles of fast-wave interactions mainly producing azimuthal electron bunching and slow-wave interactions mainly producing axial bunching [8, 9, 27, 374, 375]. The predominance of the fast-wave ECM resonance with its azimuthal bunching in producing microwaves was experimentally verified in the mid-1960s in the USA [376] (where the term “electron cyclotron maser” was apparently coined) and in Russia [377].

Many configurations can be used to produce coherent radiation based on the ECM instability. The departure point for designs based on a particular concept is the wave-particle interaction. Dispersion diagrams, also called ω-kz plots or Brillouin diagrams [378,379,380,381,382,383,384], show the region of cyclotron interaction (maximum gain of the instability) between an EM mode and a fast electron cyclotron mode (fundamental or harmonic) as an intersection of the waveguide mode dispersion curve (hyperbola):

$$ {\omega}^2={k_z}^2{c}^2+{k_{\perp}}^2{c}^2 $$
(4)

with the beam-wave resonance line (straight) given by eq. (1). In the case of a device with cylindrical resonator the perpendicular wavenumber is given by k = Xmn/Ro where Xmn is the nth root of the corresponding Bessel function (TMmn modes) or derivative (TEmn modes) and Ro is the waveguide radius. Phase velocity synchronism of the two waves is given in the intersection region. The interaction can result in a device that is either an oscillator or an amplifier. In the following subsections, the different ECM devices are classified according to their dispersion diagrams.

3.1 Gyrotron Oscillator and Gyroklystron Amplifier

Gyrotron oscillators were the first ECMs to undergo major development [27]. In autumn 1964 scientists at Institute of Applied Physics (IAP) in Nizhny Novgorod, Russia, operated the first gyrotron (TE101 mode in rectangular cavity, power: 6 W, CW) [18]. In 1966 the term “gyrotron” was coined by Arcady Goldenberg from IAP. Increases in device power were the result of Russian developments from the early 1970s in magnetron injection guns (MIGs), which produce electron beams with the necessary transverse energy (while minimizing the spread in transverse velocities) and in tapered, open-ended waveguide cavities that maximize the interaction efficiency by tailoring the electric field distribution in the resonator [8,9,10,11,12,13,14,15,16,17]. In 1967, Igor Antakov performed at IAP first gyroklystron amplifier experiments. As conventional klystrons, modern gyroklystrons consist of modulating input cavity, several bunching cavities and output cavity. Gyrotrons and gyroklystrons are devices which usually utilize only weakly relativistic electron beams (Vo < 100 kV, γ < 1.2) with high transverse momentum (pitch factor α = v/vz > 1) [381,382,383,384]. The wave vector of the radiation in the cavity is almost transverse to the direction of the external magnetic field (k⊥ > > kz, and the Doppler shift is small) resulting, according to eqs. (1) and (2), in radiation near the electron cyclotron frequency or one of its harmonics:

$$ \omega \cong \mathrm{s}{\varOmega}_{\mathrm{c}}\kern0.5em ,\kern1.5em s=1,2,\dots $$
(5)

In the case of cylindrical cavity tubes (see Figs. 1 and 2) the operating mode is close to cutoff (vph = ω/kz > > c) and the frequency mismatch ω - sΩc is small but positive in order to achieve correct phasing, i.e., keeping the electron bunches in the retarding phase [381,382,383,384]. The Doppler term kzvz is of the order of the gain width and is small compared with the radiation frequency. The dispersion diagrams of fundamental and harmonic gyrotrons are illustrated in Figs. 3 and 4, respectively. The velocity of light line is determined by ω = ckz. For given values of γ and Ro, a mode represented by its eigenvalue Xmn and oscillating at angular frequency ω is only excited over a narrow range of Bo. Quasi-optical gyrotrons employ a Fabry-Perot mirror resonator perpendicular to the electron beam, also providing k > > kz (Fig. 2). By variation of the magnetic field, a sequence of discrete modes can be excited. The frequency scaling is determined by the value of Bo/γ. Modern high-power high-order volume mode CW gyrotron oscillators for fusion plasma applications employ an internal quasi-optical (q.o.) mode converter with lateral microwave output [381,382,383,384,385,386,387,388,389,390,391,392,393,394,395,396,397] and a single-stage depressed collector (SDC) for energy recovery (Tables 2, 3, 4, 5, 6, 7, 8, 9, 10) (Fig. 5). Highly efficient, advanced q.o. mode converters utilize waveguide launchers with optimized wall perturbation: helical-type [385,386,387,388, 396, 397], mirror-type [389,390,391,392,393,394], or hybrid-type [395]. Cavity expansion due to ohmic wall heating (skin effect) and partial electron beam space charge neutralization reduce the operating frequency by a few hundred MHz [398, 399]. Cyclotron harmonic operation reduces the required magnetic field for a given frequency by the factor s. However, the measured efficiencies high-frequency gyrotrons operating at higher harmonics (s = 2 and 3) are lower than those operating at the fundamental frequency [8,9,10,11,12,13,14,15,16,17, 339,340,341,342,343,344,345,346,347, 378,379,380,381,382,383,384].

Fig. 1
figure 1

Schematic of VARIAN (CPI) CW gyrotron oscillator and scheme of irregular waveguide cavities of gyromonotron oscillator (left) and two-cavity gyroklystron amplifier (right)

Fig. 2
figure 2

Principle of a conventional gyrotron with cylindrical resonator and of a quasi-optical gyrotron with mirror resonator

Fig. 3
figure 3

Dispersion diagram of gyrotron oscillator (fundamental resonance)

Fig. 4
figure 4

Dispersion diagram of harmonic frequency gyrotron oscillator

Fig. 5
figure 5

Schematic layout of modern high-order volume mode gyrotron with quasi-optical mode converter and single-stage depressed collector

At low voltages, the number of electron orbits required for efficient bunching and deceleration of electrons can be large, which means that the resonant interaction has a narrow bandwidth, and that the RF field may have moderate amplitudes. In contrast with this, at high voltages, electrons should execute only about one cyclotron orbit. This requires correspondingly strong RF fields, possibly leading to RF breakdown, and greatly broadens the cyclotron resonance band, thus making possible an interaction with many parasitic modes.

3.2 Cyclotron Autoresonance Maser

In gyrotrons with highly relativistic beams (≥ 1 MeV), efficient interaction will lead to an average energy loss in the order of the initial electron energy. As a result, the change in the gyrofrequency is much larger than in the mildly relativistic case. It is therefore desirable to identify the condition under which such highly relativistic electron beams remain in synchronism with the RF field. A possibility for achieving synchronism is to utilize the interaction of electrons with EM waves propagating with a phase velocity close to the speed of light in the direction of the external magnetic field. In this case, the Doppler shift term kzvz is large, and the appropriate resonance condition is

$$ \omega \cong {k}_{\mathrm{z}}{v}_{\mathrm{z}}+\mathrm{s}{\varOmega}_{\mathrm{c}} $$
(6)

If vph ≅ c, the increase in cyclotron frequency due to extraction of beam energy (decrease of γ), nearly compensates the decrease in the Doppler-shift term. Therefore, if the resonance condition (6) is initially fulfilled, it will continue to be satisfied during the interaction. This phenomenon is called autoresonance, and the cyclotron maser devices operating in the relativistic Doppler-shifted regime are called cyclotron autoresonance masers (CARM) [18, 361]. Figure 6 shows how the Brillouin diagram of the fast cyclotron wave changes during the autoresonance interaction such that the working frequency ω remains constant even though both Ωc and vz are changing. The CARM interaction corresponds to the upper intersection and is based on the same instability mechanism as that of the gyrotron but operated far above cutoff.

Fig. 6
figure 6

Dispersion diagram of the cyclotron autoresonance maser (CARM)

The instability is convective, so feedback, e.g., by a Bragg resonator (see Fig. 7) [361] is required for a CARM oscillator and it is necessary to carefully discriminate against the other interactions corresponding to the lower frequency intersection in the dispersion diagram Fig. 6. The problem can be alleviated by employing the fundamental TE11 mode or the balanced HE11 hybrid mode (in circumferentially corrugated circular waveguide [101]) and properly choosing the system parameters to be within the stability limit. Compared to a gyrotron, there is a large Doppler frequency upshift of the output radiation (ω ≅ γ2Ωc) permitting a considerably reduced magnetic field Bo. Since the axial bunching mechanism can substantially offset the azimuthal bunching the total energy of the electron beam and not only the transverse component is available for RF conversion.

Fig. 7
figure 7

Schematic of the long-pulse MIT CARM oscillator experiment and scheme of a Bragg resonator (Adapted from: [400] K.D. Pendergast et al., Int. J. Electronics, 72, No. 5 and 6, 983-1004 (1992))

In contrast to the gyrotron, the CARM has an electron beam with low to moderate pitch factor (α < 0.7). The efficiency of CARMs is extremely sensitive to spread in the parallel beam velocity. The velocity spread Δvz/vz must be lower than 1% to achieve the full theoretically expected efficiency of 40% [361, 400].

It has been suggested that an ECM operating in the Cherenkov regime (vph < c) may be an attractive alternative high-power microwave source. This slow-wave CARM utilizes the coupling between the slow cyclotron wave of the electron beam and the slow EM waves of the circuit at the anomalous Doppler cyclotron resonance eq. (6) with s = -1 or any other negative integer. Such a slow-wave ECM can be driven by an electron beam with predominant axial velocity as in conventional Cherenkov devices. Experimental demonstrations were reported in [401,402,403,404], in which dielectric loaded and corrugated waveguide slow-wave structures were used. Since the transverse wavenumber of slow waves is imaginary, their fields are localized near the structure wall, and, therefore, the electron beam should also propagate close to the wall to couple to these EM waves.

3.3 Gyro-TWT and Gyrotwystron Amplifier

From the theoretical point of view, the gyro-TWT differs from the CARM only in regimes of operation. The gyro-TWT utilizes a moderately relativistic electron beam to interact with a fast waveguide mode near the grazing intersection of the frequency versus wavenumber plot (see Fig. 8) where the resonance line is tangent to the EM mode. This produces high gain and efficiency because the phase velocities of the two modes are nearly matched and the group velocity of the waveguide mode is nearly equal to vz. In the gyro-TWT regime (ω/kz > > c), the axial bunching mechanism is too weak to be of any significance. To benefit from autoresonance, the cutoff frequency should be reduced relative to the cyclotron frequency. The circuit employed in a gyro-TWT consists simply of an unloaded or loaded waveguide. Since no resonant structures are present, the gyro-TWT is potentially capable of a much larger bandwidth than a gyroklystron and thus can be used as a broadband amplifier in mm-wave radar and communication systems. Advanced devices employ tapered magnetic field and interaction circuit as well as two partially loaded stages in order to optimize the beam-wave interaction along the waveguide [405,406,407,408]. As in CARMs, it is necessary to carefully discriminate against the other interaction corresponding to the lower frequency intersection in the dispersion diagram Fig. 8 (see Section 3.4).

Fig. 8
figure 8

Dispersion diagram and scheme of interaction circuit of Gyro-TWT amplifier

The sensitivity to velocity spread can be strongly reduced by coupling between the second harmonic cyclotron mode of a gyrating electron beam and the radiation field in the region of near-infinite-phase velocity over a broad bandwidth by using a cylindrical waveguide with a helical corrugation on its inner surface (helical, coupled-modes gyro-TWT) [409,410,411].

The gyrotwystron [9], a hybrid tube, is derived from the gyroklystron by extending the length of the drift section and replacing the output cavity with a sligthtly tapered waveguide section like in a gyro-TWT. The output waveguide section is excited by the beam of electrons that are bunched because of modulation in the input and bunching cavities. The gyrotwystron configuration has a broader bandwidth than the gyroklystron and can mitigate the problem of microwave breakdown at high-power levels, since the microwave energy density in the output waveguide can be much smaller than in a gyroklystron output cavity. The inverted gyrotwystron is a device consisting of the input waveguide, drift section, and output cavity [412]. The traveling signal wave in the input waveguide may induce a high harmonic content in the electron current density. Then the prebunched electron beam can excite phase-locked oscillations in the cavity at a harmonic of the signal frequency.

3.4 Gyro-BWO

If the electron beam and/or magnetic field are adjusted so that the straight fast-wave beam line crosses the negative kz-branch of the waveguide mode hyperbola (see Fig. 9) then an absolute instability (internal feedback) with a “backward wave” occurs. In the gyro-BWO, the frequency of operation is now governed by the slope of the beam line, which is a function of vz, and thus of the beam acceleration voltage Vo. Consequently, just as in the case of slow-wave BWOs (e.g., Carcinotron), the frequency of oscillations can be continuously changed very fast over a broad range, using Vo in place of Bo. However, here, in contrast to conventional BWOs, also the phase velocity is negative (vph < 0). There is a Doppler down shift in frequency (Ωc/2 < ω < Ωc), so that very high magnetic fields are required for high-frequency operation.

Fig. 9
figure 9

Dispersion diagram and scheme of interaction circuit of Gyro-BWO

3.5 Overview on Gyro-Devices

Bunching of electrons in the gyrotron oscillator and in gyro-amplifiers has much in common with that in conventional linear electron beam devices, namely, monotron, klystron, TWT, twystron and BWO [9]. In both cases the primary energy modulation of electrons gives rise to bunching (azimuthal or longitudinal) which is inertial. The bunching continues even after the primary modulation field is switched off (in the drift sections of klystron-type and twystron-type devices). This analogy suggests the correspondence between conventional linear-beam (O-type) devices and various types of gyro-devices. Table 1 presents the schematic drawings of devices of both classes.

In Tables 15, 20, 21, 35, and 36, two other microwave source types similar to, but also fundamentally different in one way or another from, the ECMs will be briefly considered. The large orbit gyrotron (LOG) employs an axis-encircling electron beam in which the trajectory of each electron takes it around the axis of the cylindrical interaction region [378, 413]. For the operating modes TEmn a strong selection rule is valid: m = s in Eq. (5). Peniotron and gyro-peniotron are driven by an interaction that is phased quite differently from the ECM interaction; in practice, the peniotron and ECM mechanisms compete [379,380,381,382, 414].

4 Magnicons and Gyroharmonic Converters

The magnicon is a member of the class of scanning-beam amplifier tubes [16, 415, 416]. It is a magnetized device that uses a fast-wave output cavity. Therefore, it can also be grouped with gyro-devices in which electrons gyrating in an external magnetic field emit bremsstahlung radiation near the cyclotron resonance. In the earliest version of the magnicon, an electron beam was deflected in the unmagnetized input cavity, using a rotating TM110 mode and after an also unmagnetized drift space, the deflected beam is spun up to high transverse momentum by entry into a strong magnetic field at the entrance of the output cavity.

As a result of the phase-synchronous transverse deflection of the electron beam as a whole, the beam electrons entering the output cavity execute Larmor motion whose entry point and guiding center rotate in space around the cavity axis at the drive frequency. In the output cavity, the beam is used to drive a cyclotron-resonant fast-wave interaction with a synchronously rotating TM110 mode that extracts principally the transverse beam momentum. This interaction can be highly efficient, because the magnicon beam is fully bunched in space and in the gyro-phase, so that the phase bunching produced by the cyclotron maser instability is not required. With all the electrons decelerated identically, very high efficiencies can be achieved.

Later, higher perveance versions of the magnicon have been developed [416, 417], in which a fully magnetized electron beam is spun up to a high transverse momentum in a sequence of deflection cavities containing synchronously rotating TM110 modes, the first driven by an external RF source (Fig. 10). In addition, the output cavity can operate in the mth harmonic of the drive frequency by using TMm10 modes with m > 1, permitting extension of magnicon operation to higher operating frequencies. Again, the point of injection of the beam into the output cavity as well as the entry gyro-phase, rotate synchronously with a rotating RF mode of the output cavity. This makes possible much higher efficiencies than in most other gyro-devices. The key to the efficiency of these new magnicon designs is to spin the beam up to high transverse momentum (α > 1) without producing large spreads in energy and gyro-phase, so that the output cavity interaction will remain coherent over the entire ensemble of electrons, and not just synchronous in time. This requires great care in the design of the deflection cavities, in particular of the penultimate deflection cavity that produces more than half of the beam spin up. Since these spreads are generated by the fringing fields of the beam tunnel apertures in the deflection cavities and the output cavity, it also requires the use of a very small initial electron beam radius.

Fig. 10
figure 10

Schematic layout of the magnicon: 1—electron source; 2—vacuum valve; 3—drive cavity; 4—gain cavity; 5—penultimate cavity; 6—output cavity; 7—waveguide (× 2); 8—solenoid; 9—collector (Adapted from: [415] O.A. Nezhevenko, IEEE Trans. on Plasma Science, 22, No. 5, 756-772 (1994))

A summary of the development status of magnicons is given in Table 35.

A similar “scanning-beam” microwave device is the gyroharmonic converter in which dubbed “co-generation” arises from a near match in group and phase velocities between the input cavity TE11 mode at frequency ω and the TE72 mode at frequency 7ω in a cylindrical waveguide [418]. This match allows efficient power transfer into the 7th harmonic from a fundamental frequency wave that energizes an electron beam via cyclotron autoresonance acceleration (CARA). Theory indicates that high conversion efficiency can be obtained for a high-quality beam injected into CARA, and when mode competition can be controlled.

Generation of 0.5-MW power (3-μs pulse duration, 5 % efficiency) at 8.57 GHz (3rd harmonic of 2.856 GHz) in the TE31 mode has been observed in experiments using a 350-kV, 30-A electron beam [418,419,420].

5 Free Electron Masers

Free electron lasers (FELs) differ from the other high-power microwave sources considered in this report in that they have demonstrated output over a range of frequencies extending far beyond the microwave spectrum, well into the visible and ultraviolet range [361,362,363,364,365,366,367,368, 379, 380]. To achieve this spectral versatility, FELs exploit relativistic beam technology to upshift the electron “wiggle” frequency by an amount roughly proportional to γ2 (see Fig. 11 and Section 2). In this respect, perhaps a more descriptive name is that coined by R.M. Phillips [421]: UBITRON, for an “undulated beam interaction electron” tube. The magnetostatic wiggler is the most common, but not the sole means, for providing electron undulation. An electrostatic wiggler or the oscillatory field of a strong electromagnetic wave can also play this role. Devices with such electromagnetic wigglers are sometimes called scattrons [9, 18, 361]. The distinction between long-wavelength free electron maser (FEM) (λ ≥ 0.5 mm) and short-wavelength FELs is natural because higher current and lower energy beams are typically employed in this regime and space-charge effects are more important. In particular, the dominant interaction mechanism is often coherent Raman scattering. Also, while short-wavelength FELs excite optical modes, dispersion due to the beam dielectric effects and finite transverse dimensions in the drift tubes and cavities are important effects at longer wavelengths. A low power (3-W, 2-ms pulses) FEL operating at radio frequencies (FER) employing a 420 V, 0.2 A electron beam holds the world record for long wavelength (f = 266 MHz, λ = 1.1 m, λw = 0.04 m, Bw = 0.04 T) [422].

Fig. 11
figure 11

The basic FEM configuration. Electrons in an injected electron beam undulate in the periodic magnetic field of the wiggler (Adapted from: P. Sprangle et al., Nucl. Instrum. Meth. Phys. Res., A239, No. 1, 1 (1985))

The FEM appears to be potentially capable of fulfilling all the requirements for a frequency tunable high-power mm-wave source. Coverage of the entire frequency range of 130–260 GHz presents no severe problems, and even higher frequencies are quite feasible [4, 423,424,425,426,427,428,429,430,431,432,433,434]. Rapid tunability over more than ± 5% could be obtained by variation of the electron beam energy. The interaction occurs in an interaction circuit operating in low-order modes, which have very good coupling to a Gaussian beam output. The relatively low RF wall loading and the use of high electron beam energy (> 0.5 MeV) and a multi-stage depressed collector are compatible with a high unit power at efficiencies around 50% if the electron beam interception could be maintained at an acceptable level. A survey of FEM development status (experiments) is presented in Table 38. It is a great pity that the FOM-FEM project [423,424,425,426,427,428,429,430,431,432,433] was terminated in the autumn of 2001.

The highest CW power generated by a FEM is 36 W (X-band) [435] whereas the pulsed IR-FEL at the Thomas Jefferson National Accelerator Facility obtained a record average power of over 10 kW in the band from 1 to 14 μm (14.2 kW at 1.6 μm) and has the capability of more than 1 kW in the 250- to 1000-nm range. A recirculated electron beam power of up to 1 MW (Energy Recovering Linac) has been demonstrated resulting in an overall efficiency of approximately 2% [367, 368, 436,437,438,439,440,441]. The average output power in the THz regime is 100 W (train of sub-picosecond pulses).

The first stage of the Novosibirsk High Power Free Electron Laser (NovoFEL) had been commissioned in 2003. This 12-MeV THz-FEL generates coherent radiation, tunable in the range of 90–240 μm (3.33–1.25 THz), 60–117 μm, and 40–80 μm at the first, second, and third harmonics, respectively, with the corresponding maximum average output powers of 0.5 kW, 100 W, and 30 W. The maximum peak power is 0.5 MW (bunch duration: approx. 100 ps), the relative line width is 0.2–2% [442, 443].

The two-orbit energy recovery linac stage was assembled and commissioned in 2008. The first lasing of the two-stage THz-FEL (22 MeV) was achieved in 2009, providing radiation in the wavelength range 40–90 μm (bunch duration: 10–20 ps) at an average output power between 0.5 and 1 kW (2 MW peak power) with a maximum gain of 40%. The relative linewidth is 0.2–1%.

First lasing of the three-stage THz-FEL (42 MeV, 4 orbits), which is expected to deliver 1 kW average power at a pulse repetition rate of 3.75 MHz in the wavelength range of 5–10 μm, was obtained in 2015. The radiation wavelength was 9 μm at an average power of approximately 100 W [444,445,446,447].

The 200-ns LIU-3000 Induction LINAC (0.8 MeV, 200 A, 200 ns) FEM at JINR Dubna [448,449,450] has been operated as FEM multiplier (n = 1 (TE11) 24 GHz, 5 MW; n = 2 (TE21): 48 GHz, 1.5 MW; n = 3: 72 GHz, 0.1 MW) and as 2nd harmonic FEM oscillator.

A table, summarizing the parameters and the state-of-the-art of IR/THz FELs from around the world, is being continuously updated by J. M. Knopf (Helmholtz-Center Dresden-Rossendorf (HZDR), Germany): https://www.hzdr.de/db/Cms?pOid=56940.

6 Gyrotron Oscillators and Microwave Vacuum Windows for Plasma Heating

Tables 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and 13 present the current status of gyrotrons and RF vacuum windows for ECRH&CD in fusion plasmas.

Design studies on 4 MW, 170 GHz and 2 MW, 240 GHz coaxial-cavity gyrotrons for future fusion reactors were performed at KIT [877,878,879,880]. The 4-MW tube would operate in the TE52,31 -mode and its q.o. output coupler would generate two 2-MW fundamental Gaussian beams which leave the tube through two CVD-diamond windows.

The KIT 1 MW TE22,6 gyrotron operated at frequencies between 114 and 166 GHz has been investigated with respect to fast-frequency tunability in the frequency range from 132.6 to 147.4 GHz [116]. For that purpose, the gyrotron has been equipped with a special hybrid-magnet system consisting of superconducting (sc) magnets in the cryostat and additional normal-conducting (nc) copper magnets with a fast time constant at cavity and cathode. Special problems due to the magnetic coupling between the different magnets were investigated by calculation and experiment. Making use of these investigations different current regulation schemes for the nc magnets were implemented and tested experimentally. Finally, megawatt-class step-tuning operation between the five TEm,6 modes (m = 20 – 24) from TE20,6 to TE24,6 in time steps of 1 s has been achieved.

The Japan 1 MW ITER gyrotron was operated in a fast-tunable (3.5 s) sc magnet (JASTEC) at 170 GHz (TE31,8, 615 kW, 32%) and 167 GHz (TE30,8, 538 kW, 27%). These efficiencies where obtained without collector depression [883].

A specific feature of the coaxial gyrotron design is that it allows electron beam energy recovery and very fast frequency tuning by biasing the coaxial insert [869,870,871,872]. By biasing the inner rod of the KIT coaxial-cavity gyrotron, such very fast (within ≈ 0.1 ms) frequency tuning was demonstrated at a power level of 1 MW. In particular, step frequency tuning between the 165.1-GHz nominal mode and its azimuthal neighbors at 162.8 GHz and 167.2 GHz (see Table 10) was obtained. In addition, operating in the nominal TE31,17 mode, continuous frequency pulling within 70 MHz bandwidth was achieved [825].

In order to define the appropriate concepts for the development of 1 MW, CW mm-wave windows one has to compare the thermophysical, mechanical and dielectrical parameters of possible window materials related to the load-failure resistance R' and the power-transmission capacity PT at different temperatures [84,85,86,87,88,89, 102, 919]. The features of beryllia, boron nitride, silicon nitride (Kyocera SN-287), sapphire, Au-doped silicon, CVD diamond and silicon carbide at room temperature and of sapphire, Au-doped silicon and CVD diamond at cryo-temperatures are summarized in Tables 11 and 12, where

$$ \mathrm{R}\hbox{'}=k\cdot {\upsigma}_{\mathrm{B}}\cdot \left(1-\upnu \right)/E\cdot \upalpha $$
(7)
$$ {\mathrm{P}}_{\mathrm{T}}=\mathrm{R}\hbox{'}\uprho \cdot {\mathrm{c}}_{\mathrm{p}}\left(\left(1+\upvarepsilon \hbox{'}{}_{\mathrm{r}}\right)\ \mathrm{tan}\updelta \right). $$
(8)

For a 1 MW, CW mm-wave window the parameters R' and PT should exceed 250 and 100, respectively.

Comparison of R' and PT for the 4 materials BeO, BN, Si3N4 and sapphire shows that there is no chance to use these dielectrics for edge-cooled, single-disk CW windows at room temperatures. Experiments at CPI in the US and at NIFS and JAEA (now QST) in Japan confirmed that even a double disk FC75-face-cooled sapphire window has a CW-power limit of 0.3–0.4 MW. Nevertheless, these materials are widely used at lower frequencies and pulse operation.

At LN2-temperature 77 K (LNe-temperature 30 K) sapphire has a thermal conductivity of 900 (20000) W/mK and a loss tangent of 5.7·10−6 (2·10−6) leading to R' = 130 (2870) and PT = 71 (4460). The LN2-edge-cooled sapphire window of the 118 GHz TED gyrotron (0.5 MW, 210 s) [632,633,634,635,636,637,638,639,640,641,642] operates close to the allowable lower limits of R' and PT. However, the mechanical features and the required cooling auxillaries make such cryo-windows very complicated. Au-doped silicon at temperatures somewhat lower than 0 °C could avoid a thermal runaway and transmit 1 MW, CW; however, this material is too brittle and tends to mechanical cracking [907].

Using the available material parameters and employing various beam profiles, finite element computations revealed several options for 170 GHz, 1 MW, CW operation given in Table 13 [84,85,86,87,88,89, 102, 919]. The CVD diamond options 2 and 3 being water cooled are preferred for their simplicity, in particular for use as torus window.

A wide-band CVD-diamond Brewster window in corrugated HE11 waveguide with 32-mm inner diameter has been tested at 110 GHz using 0.5 s pulses with powers up to 350 kW [951,952,953].

7 Harmonic and Very High-Frequency Gyrotron Oscillators

Operating at the fundamental, the 2nd harmonic or the 3rd harmonic of the electron cyclotron frequency enables the gyrotron to act as a medium power (several 1–100 W) step tunable, mm- and sub-mm wave source in the frequency range from 38 GHz (fundamental) to 1.014 THz (TE4,12 mode, 2nd harmonic, 1 ms) (Tables 14, 15, 16, 17, and 18) [182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262,263,264,265,266,267,268,269,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295,296,297,298,299,300,301,302,303,304,305, 920,921,922,923,924,925,926,927,928,929,930,931,932,933,934,935,936,937,938,939,940,941,942,943,944,945,946,947,948,949,950,951,952,953,954,955,956,957,958,959,960,961,962,963,964,965,966,967,968,969,970,971,972,973,974,975,976,977,978,979,980,981,982,983,984,985,986,987,988,989,990,991,992,993,994,995,996,997,998,999,1000,1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1011,1012,1013,1014,1015,1016].

A 30 W two-cavity gyrotron with frequency multiplication achieved at IAP an efficiency of 0.43%. The first cavity operated in the TE01 mode near the fundamental cyclotron frequency at 95 GHz, the output cavity operated at the 3rd harmonic 285 GHz in the TE03 mode [1017,1018,1019,1020,1021]. Simultaneous generation at the 2nd (37.5 GHz) and 4th (75 GHz) harmonic (140 W at 60 kV and 6 A) was obtained by a self-excited gyromultiplier with single, sectioned cavity [1022, 1023]. A high-harmonic sectioned TE35 mode gyrotron of IAP Nizhny Novgorod produced 0.5 kW at 740 GHz with 0.9% efficiency [1024,1025,1026].

8 Gyrotrons for Technological Applications

The state-of-the-art of gyrotrons for technological applications is summarized in Table 19. IAP Nizhny Novgorod and GYCOM have developed a dual-frequency materials processing system employing a 15-kW, 28-GHz gyrotron and a 2.5-kW, 24.1-GHz tuneable gyro-BWO (see Tables 33 and 34) [324, 332, 333]. This system has been installed at the University of Fukui, Japan.

9 Relativistic Gyrotrons

Table 19 Performance of present CW gyrotron oscillators for technological applications
Table 20 Present development status of relativistic gyrotron oscillators

10 Quasi-Optical Gyrotrons

Table 21 Relativistic large orbit harmonic pulse gyrotrons with axis-encircling electron beam. The 21.6- to 74.9-GHz experiments at IAP used an explosive-emission cathode with kicker (τ = 10 ns) and the 115- to 469-GHz experiments employed a quasi-Pierce type thermionic gun with kicker (τ = 10 μs, 1 Hz)

11 Cyclotron Autoresonance Masers

Table 23 State-of-the-art of fast-wave CARM experiments (short pulse)
Table 24 State-of-the-art of slow-wave CARM experiments (short pulse)

12 Gyroklystrons, Gyro-TWT's, Gyrotwystrons, Gyro-BWOs and other Gyro-Devices

12.1 Weakly Relativistic Pulse Gyroklystrons

Table 25 Weakly relativistic pulse gyroklystron experimental results

12.2 Weakly Relativistic CW Gyroklystrons

Table 26 Weakly relativistic CW gyroklystron experimental results

12.3 Relativistic Pulse Gyroklystron

Table 27 Relativistic pulse gyroklystron experimental results

12.4 Weakly Relativistic Gyro-TWTs

Table 28 Present development status of weakly relativistic gyro-TWTs (short pulse)

12.5 Relativistic Gyro-TWTs

Table 29 Present development status of relativistic gyro-TWTs (short pulse)

12.6 Weakly Relativistic Pulse Gyrotwystrons

Table 30 State-of-the-art of weakly relativistic gyrotwystrons experiments (short pulse)

12.7 Weakly Relativistic Pulse Harmonic-Multiplying Inverted Gyrotwystrons/Gyro-TWT/Gyrotriotron

Table 31 State-of-the-art of weakly relativistic harmonic gyro-devices (short pulse)

12.8 Relativistic Pulse Gyrotwystrons

Table 32 State-of-the-art of relativistic gyrotwystron experiments (short pulse)

12.9 Weakly Relativistic Pulse Gyro-BWOs

Table 35 Experimental results of peniotrons

12.10 Relativistic Pulse Gyro-BWOs (pulse duration = 0.02–1 μs)

Table 36 Experimental results of gyropeniotrons

12.11 Peniotrons

Table 37 Experimental results of magnicons

12.12 Gyropeniotrons

Table 38 State-of -the-art of millimeter- and submillimeter-wave FEMs

12.13 Magnicons

Table 33 Experimental results on weakly relativistic pulse gyro-BWOs (short pulse)

13 Free Electron Masers

The design parameters of the FOM-FEM [423,424,425,426,427,428,429,430,431,432,433, 1385] are presented below. The project was terminated in The Netherlands in the autumn of 2001 and shipped to Israel.

13.1 Electron Beam Line (with Multi-stage Depressed Collector)

Electron beam current :

12 A

Body current :

< 20 mA

Gun voltage :

80 kV

Type of gun

triode gun, cathode operated in space-charge limited regime

Normalized beam emittance

6 p mm mrad (before interaction)

Electron beam energy

1.35–2.0 MeV (130–250-GHz operation)

Acceleration/deceleration

electrostatic

Focusing system

solenoids in period focusing arrays

Pulse length

2–100 ms

13.2 Undulator

Period

40 mm

Pole gap

25 mm

Number of periods

34

Peak field strength

Section 1

0.20 T, 20 cells

 

Section 2

0.16 T, 14 cells

Drift gap

35–60-mm length, adjustable

Focusing scheme

equal focusing in x- and y-direction

Matching scheme

1/2 cell 1/4 strength, 1/2 cell 3/4 strength

13.3 mm-Wave System

Primary waveguide

rectangular corrugated

Waveguide dimensions

15 × 20 mm2

Waveguide mode

HE11

Feedback and outcoupling

via optical beam multiplication in stepped waveguides

Feedback coefficient

adjustable 0–100 %

Output window

Brewster-angle boron-nitride window

13.4 mm-Wave Output Power

mm-wave frequency1)

130–260 GHz

On-line tunability2)

5% on ms time-scale

Output power

1 MW

Electronic efficiency

5%

System efficiency

> 50%

  1. 1)Slow frequency tuning by changing the electron beam energy from 1.35 to 2.0 MeV, and adjusting the height of the stepped waveguides (mechanical adjustment).
  2. 2)Frequency adjustable on ms-time scale, via a sweep of the electron beam energy. The bandwidth of the stepped waveguides is sufficient to sweep over 5%.

14 Comparison of Gyrotron and FEM for Nuclear Fusion

Table 39 lists a comparison of the main performance parameters and features of gyrotrons and FEMs for ECRH of plasmas in nuclear fusion research. The important advantage of the FEM is its fast and continuous frequency tunability and the possibility of very high peak power but the gyrotron is a much simpler device [4]. The cylindrical cavity gyrotron is the only mm-wave source which has an extensive on-the-field experience during fusion plasma heating experiments over a wide range of frequencies and power levels (8–170 GHz, 0.1–1.0 MW) [6, 22,23,24,25,26,27,28].

Table 34 Experimental results on relativistic gyro-BWOs (short pulse)
Table 39 Comparison of parameters and features of gyrotrons and FEMs for ECRH

15 New Trends in Gyrotron Development

Challenges in the development of future advanced gyrotrons are multi-frequency (multi-purpose) and stepwise frequency tunable tubes (see Tables 4, 5, 6, 7, 8, and 9) with frequencies higher than 200 GHz for ECRH&CD of plasmas in a DEMOnstration fusion reactor (DEMO) and sub-THz gyrotrons for Collective Thomson Scattering (CTS) plasma diagnostics and for very high magnetic field DNP-NMR spectroscopy (see Tables 14, 15, 16, 17, and 18). The unit power may be enlarged to 1.5–2 MW by empoying injection-locked and coaxial-cavity multi-megawatt gyrotrons (see Tables 5 and 6). Efficiency enhancement via multi-stage depressed collectors, frequency stability, fast oscillation recovery methods and reliability, availability, maintainability and inspectability (RAMI) are other important issues [1528].