### **State-of-the-Art of High-Power Gyro-Devices**

Update of Experimental Results 2023

Manfred Thumm

**M. Thumm**

State-of-the-Art of High-Power Gyro-Devices

 |

Update of Experimental Results 2023

Manfred Thumm

#### **State-of-the-Art of High-Power Gyro-Devices**

Update of Experimental Results 2023

**Karlsruhe Institute of Technology KIT SCIENTIFIC REPORTS 7765**

#### **State-of-the-Art of High-Power Gyro-Devices**

Update of Experimental Results 2023

by Manfred Thumm

Institut für Hochleistungsimpuls- und Mikrowellentechnik (IHM)

Report-Nr. KIT-SR 7765

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ISSN 1869-9669 DOI 10.5445/KSP/1000164947

### **Contents**


### **Abstract**

This report presents an update of the experimental achievements published in the review "State- of-the-Art of High-Power Gyro-Devices and Free Electron Masers", Journal of Infrared, Millimeter, and Terahertz Waves, **41**, No. 1, pp 1-140 (2020) related to the development of gyro-devices (Tables 2-34). Emphasis is on high-power gyrotron oscillators for long-pulse or continuous wave (CW) operation and pulsed gyrotrons for many other applications. In addition, this work gives a short update on the present development status of frequency step-tunable and multi-frequency gyrotrons, coaxial-cavity multi-megawatt gyrotrons, complex two-section stepped cavity gyrotrons, gyrotrons for technological and spectroscopy applications, relativistic gyrotrons, large orbit gyrotrons (LOGs), quasi-optical gyrotrons, fast- and slow-wave cyclotron autoresonance masers (CARMs), gyroklystron-, gyro-TWT- and gyrotwystron amplifiers, gyro-harmonic converters, gyro-BWOs 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 heating (ECH), 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 gyrotrons employing synthetic diamond output windows is 30 minutes (CPI and European KIT-SPC-THALES collaboration). The world record parameters of the European tube are: 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 electron energy recovery. PLLfrequency stabilization of such tubes has been demonstrated. A 1.5 MW version of this gyrotron is under development (IPP-KIT-THALES). The maximum output power of 1.5 MW in 4.0 s pulses at 45% efficiency was generated with the QST-CANON 110 GHz gyrotron. The first Japan 170 GHz ITER gyrotron prototype achieved 1 MW, 800 s at 55% efficiency and holds the energy world record of 2.88 GJ (0.8 MW, 60 min., 57 %). The Russian 170 GHz ITER gyrotron obtained 0.99 (1.2) MW with a pulse duration of 1000 (100) s and 57 (53) % efficiency. First frequency-injection-locked operation of a Russian 170 GHz-1 MW gyrotron has been demonstrated in short pulses using a PLL-frequency-stabilized 20 kW gyrotron master oscillator. The prototype tube of the KIT 2 MW, 170 GHz coaxial-cavity gyrotron (pulse duration 50 ms) achieved in 1 ms pulses the record power of 2.2 MW at 48% efficiency and 96% Gaussian mode purity. High-power CW 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%. 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%). The average powers produced by 94 GHz gyroklystrons, gyrotwystrons and gyro-TWTs are 10 kW, 5 kW and 2 kW, respectively.

### **Keywords**

Electron cyclotron maser, Gyrotron, Quasi-optical gyrotron, Gyroklystron-, Gyro-travelling-wave-, and Gyrotwystron amplifiers, Gyro-backward-wave oscillator, Cyclotron autoresonance maser, Dielectric vacuum windows

KIT – The Research University in the Helmholtz Association. Please send corrections, completions and supplements to manfred.thumm@kit.edu

## **Zusammenfassung**

Dieser Bericht bringt die im Review "State-of-the-Art of High-Power Gyro-Devices and Free Electron Masers", Journal of Infrared, Millimeter, and Terahertz Waves, **41**, No. 1, pp. 1-140 (2020) veröffentlichten experimentellen Ergebnisse zu Gyro-Röhren (Tabellen 2-34) auf den neuesten Stand. Der Schwerpunkt liegt dabei im Bereich der Entwicklung von Hochleistungs-Gyrotron-Oszillatoren für Langpuls- und Dauerstrichbetrieb (CW) sowie von gepulsten Gyrotrons für viele andere Anwendungen. Außerdem wird auch kurz über den neuesten Entwicklungsstand von stufenweise frequenzdurchstimmbaren Gyrotrons, Mehrfrequenz-Gyrotrons, Multi-MW-Gyrotrons mit koaxialem Resonator, Gyrotrons mit gestuftem, zweiteiligem Resonator, Gyrotrons für technologische und spektroskopische Anwendungen, relativistischen Gyrotrons, Large-Orbit-Gyrotrons (LOGs), quasi-optischen Gyrotrons, Zyklotron-Autoresonanz-Masern (CARMs) mit schneller oder langsamer Welle, Gyroklystron-, Gyro-TWT-, und Gyrotwystron-Verstärkern, Gyro-Harmonische-Konvertern, Gyro-Rückwärtswellen-Oszillatoren (BWOs) und von dielektrischen Vakuumfenstern für solche Hochleistungsmillimeterwellenquellen berichtet. Gyrotronoszillatoren (Gyromonotrons) werden vorwiegend als Hochleistungsmillimeterwellenquellen für Elektron-Zyklotron-Heizung (ECH), Elektron-Zyklotron-Stromtrieb (ECCD), Stabilitätskontrolle und Diagnostik von magnetisch eingeschlossenen Plasmen zur Erforschung der umweltfreundlichen Energiegewinnung durch kontrollierte Kernfusion eingesetzt. Die maximale Pulslänge von kommerziell erhältlichen 140 GHz, 1 Megawatt-Gyrotrons mit Austrittsfenstern aus künstlichem Diamant ist 30 min. (CPI und Europäische KIT-SPC-THALES Zusammenarbeitsgemeinschaft). Die Weltrekordparameter des europäischen 140 GHz-Megawatt-Gyrotrons sind: 0,92 MW Ausgangsleistung bei 30 min. Pulslänge, 97,5% Gaußsche Modenreinheit und 44% Wirkungsgrad mittels eines Kollektors mit einstufiger Gegenspannung (SDC) zur Energierückgewinnung. PLL-Frequenzstabilisierung solcher Röhren wurde gezeigt. Eine 1,5 MW Version dieses Gyrotrons ist in Entwicklung (IPP-KIT-THALES). Die maximale Ausgangsleistung von 1,5 MW bei 4,0 s Pulslänge und 45% Wirkungsgrad wurden mit dem QST-CANON 110 GHz Gyrotron erzeugt. Das erste japanische 170 GHz ITER-Prototyp-Gyrotron erreichte 1 MW, 800 s bei 55% Wirkungsgrad und hält den Energieweltrekord mit 2,88 GJ (0,8 MW, 60 min., 57 %). Das russische 170 GHz ITER-Gyrotron lieferte 0,99 (1,2) MW bei 1000 (100) s Pulslänge und 57 (53) % Wirkungsgrad. Erste Kurzpulsexperimente zum Frequenz-Injection-Locking eines russischen 170 GHz-1 MW Gyrotrons wurden mit Hilfe eines PLLfrequenzstabilisierten 20 kW Gyrotron-Master-Oszillators durchgeführt. Das KIT 2 MW, 170 GHz Prototyp-Gyrotron mit koaxialem Resonator (50 ms Pulslänge) erzielte 5 ms Pulsen die Rekordleistung von 2,2 MW bei 48% Wirkungsgrad und 96% Gaußscher Modenreinheit. CW-Gyrotrons finden jedoch auch in der Materialprozeßtechnik erfolgreich Verwendung. Dabei werden Röhren mit folgenden Parametern eingesetzt: f > 24 GHz, Pout =4-50 kW, CW, η > 30%. Gyrotrons mit gepulstem Magnet fürverschiedene Kurzpuls-Anwendungen arbeiten bei Frequenzen bis zu 670 GHz bei Pout = 210 kW und τ = 20 µs (η ≅ 4%), Pout = 5,3 kW bei 1 THz (η = 6,1%) und Pout = 0,5 kW bei 1,3 THz (η = 0,6%). Die höchsten von 94 GHz Gyroklystrons, Gyrotwystrons und Gyro-TWTs erzeugten mittleren Leistungen sind 10 kW, 5 kW und 2 kW.

KIT – Die Forschungsuniversität in der Helmholtz-Gemeinschaft. Bitte schicken Sie Korrekturen, Vervollständigungen und Ergänzungen an manfred.thumm@kit.edu

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# **1 Introduction**

The possible applications of gyrotron oscillators (gyromonotrons, or just gyrotrons) and other electron cyclotron maser (ECM) fast-wave devices (see Table 1) span a wide range of technologies [1-8]. 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: 8 GHz), electron cyclotron heating and non-inductive electron cyclotron current drive (ECH&CD: 14-170 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.

Most works on ECM devices have investigated the conventional gyrotron [9-31] in which the wavevector of the radiation in an open-ended, irregular cylindrical waveguide cavity is almost transverse to the direction of the applied magnetic field, generating transverse electric (TE) 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 0.1-1.2 MW at frequencies between 28 and 170 GHz have been used very successfully in thermonuclear fusion research for plasma ionization and start-up, ECH and local, current density profile control by non-inductive ECCD at system power levels up to 10 MW.

ECH has become a well-established heating method for both tokamaks [32-64] and stellarators [64-92]. 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 ≅ 5.5T) 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 new stellarator (W7-X) at the Max-Planck-Institute for Plasmaphysics in Greifswald, Germany, and the future tokamak experiment ITER (International Thermonuclear Experimental Reactor) in Cadarache, France, are between 18 and 40 MW at frequencies between 140 GHz and 170 GHz [23,26-31,39,57-61,65-86,92-116]. This suggests that mm-wave gyrotrons that generate output power of at least 1 MW, CW, per tube are required. Since efficient ECH 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 fundamental Gaussian beam mode (TEM00). Singlemode 77-170 GHz gyromonotrons with conventional, cylindrical cavity, capable of 1.5 MW per tube, CW [23-31], and 2 MW coaxial-cavity gyrotrons [97-112] 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 [26-31,39,98-121]. Frequency tuning has been shown to be possible in quasi-optical Fabry-Perot cavity gyrotrons [122,123] as well as in cylindrical and coaxial cavity gyrotrons by frequency tuning in steps (different operating cavity modes) [124-159].

This report updates the present status and future prospects of gyrotrons and RF vacuum windows for ECH&CD in fusion plasmas and for ECR plasma sources for generation of multi-charged ions, soft X-rays and UV radiation [160-187] (Tables 2-13), the development of very high frequency gyrotrons for active plasma diagnostics [188-244], 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) [245-362], remote detection of concealed radioactive materials [363-366], wireless communication [367] and medical applications [368-373] (Tables 14-18) and of quasi-optical gyrotrons (Table 22). Gyrotrons also are successfully utilized in materials processing (e.g. advanced ceramic and metal-powder-compound sintering, nano-particle production, surface hardening or dielectric coating of metals and alloys, semiconductor production, penetrating rocks) as well as in plasma chemistry [1-8,374-403]. 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 [404] and Gycom in Russia [382,393-396,405-410] are also employing permanent magnet systems. The state-of-theart in this area of gyrotrons for technological applications is summarized in Table 19.

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 [411-414]. 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 [415-428]. 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 25-27), gyrotron travelling wave tube amplifiers (Gyro-TWT) (Tables 28 and 29), gyrotwystron amplifiers(Tables 30-32), and broadband gyrotron backward wave oscillators (Gyro-BWO) (Tables 33 and 34).

The present report updates the experimental achievements (Tables 2 - 34) of gyro-devices reviewed in M. Thumm, State-of-the-Art of High-Power Gyro-Devices and Free Electron Masers, Journal of Infrared, Millimeter, and Terahertz Waves, 41, No. 1, pp. 1-140 (2020), and in KIT Scientific Report 7761 (2021). Former reviews were KfK Report 5235 (Oct 1993), FZKA Reports 5564 (Apr 1995),5728 (Mar 1996), 5877 (Feb 1997), 6060 (Feb 1998), 6224 (Jan 1999), 6418 (Feb 2000), 6588 (Mar 2001), 6708 (Feb 2002), 6815 (Feb 2003), 6957 (Feb 2004), 7097 (Feb 2005), 7198 (Feb 2006), 7289 (Feb 2007), 7392 (2008), 7467 (2009), and KIT Scientific Reports 7540 (2010), 7575 (2011), 7606 (2012), 7641 (2013), 7662 (2014), 7693 (2015), 7717 (2016), 7735 (2017) and 7750 (2018).

The list of references includes additional information about: principle and history of gyrotrons [437-452], effective cavity length [453], internal quasi-optical mode converters as transverse Gaussian beam or HE11 mode output couplers [454-469], electron beam space-charge neutralization [470,471], CARMs, other gyro-amplifiers and gyro-BWOs [472-486], magnicons [487-489], gyro-harmonic converters [490-492], and free electron masers (FEMs) [429-436,493-522].

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

### **2 Gyrotron Oscillators and Microwave Vacuum Windows for Plasma Heating**


SDC: Single-stage Depressed Collector 1) Communications & Power Industries, formerly VARIAN, 2) formerly VALVO, 3)Karlsruhe Institute of Technology, formerly FZK, 4) TED, formerly Thomson TE, 5) formerly TOSHIBA

Table 2: Performance parameters of gyrotron oscillators with frequencies between 5 and 95 GHz.


SDC: Single-stage Depressed Collector

1) Communications & Power Industries, formerly VARIAN, 2) formerly KfK, then FZK, 3) formerly JAERI, then JAEA, 4) formerly TOSHIBA 5) formerly Thomson TE, 6) formerly CRPP

Table 3: Present development status of high frequency gyrotron oscillators for ECH&CD and stability control in magnetic fusion devices (100 GHz ≤ f < 140 GHz, τ ≥ 0.1 ms).


SDC: Single-stage Depressed Collector 1) Comm. & Power Industries, formerly VARIAN, 2) formerly KfK, then FZK, 3) formerly VALVO, 4) formerly SPC, 5) formerly Thomson TE, 6) EGYC collaboration among SPC, Switzerland; KIT, Germany; HELLAS, Greece; CNR, Italy; ENEA Italy, 7) formerly JAERI, then JAEA, 8) formerly TOSHIBA

Table 4: Present development status of high frequency gyrotron oscillators for ECH&CD and stability control in magnetic fusion devices (f ≥ 140 GHz, τ ≥ 0.1 ms).


1) formerly KfK, then FZK, \* very similar cavity and tube design

2) EGYC is a collaboration among CRPP (now SPC), Switzerland; KIT, Germany; HELLAS, Greece; CNR, Italy; ENEA Italy

Table 5: Present experimental development status of short pulse (3 µs – 50 ms) coaxial cavity gyrotron oscillators.

Design studies on 4 MW, 170 GHz and 2 MW, 240 GHz coaxial-cavity gyrotrons for future fusion reactors were performed at KIT [997-1000]. 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 CVDdiamond windows.


SDC: Single-stage Depressed Collector; QOG: Quasi-Optical Gyrotron, EGYC: Cons. among SPC, Swisse; KIT, Germany; HELLAS, Greece; CNR, Italy; ENEA Italy 1) formerly VARIAN, 2) formerly KfK, then FZK, 4) formerly CRPP, 4) formerly Thomson TE, 5) formerly JAERI, then JAEA, 6) formerly TOSHIBA

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).


SDC: Single-stage Depressed Collector; 1) formerly KfK, then FZK, 2) formerly JAERI, then JAEA, 3) formerly TOSHIBA

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 singledisk output window.

IAP Nizhny Novgorod operated a 40 µs short-pulse gyrotron in 10 modes starting from TE12,4 at 133.9 GHz with 38 kW output power up to TE19,8 at 249.5 GHz with 183 kW and efficiencies from 10 to 27 % [1003].


SDC: Single-stage Depressed Collector; 1) formerly KfK, then FZK, 2) formerly CRPP, 3) EGYC collaboration among SPC, Switzerland; KIT, Germany; HELLAS, Greece; CNR, Italy; ENEA Italy, 4) formerly Thomson TE, 5) formerly JAERI, then JAEA, 6) formerly TOSHIBA

Table 8: Multi-frequency gyrotrons operating at different transmission maxima of a plane single-disk window.

The KIT 1 MW TE22,6-mode 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 [133]. 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%). The efficiencies are without SDC [1004].


SDC: Single-stage Depressed Collector;

1) formerly KfK, then FZK, 2) EGYC is a collaboration among CRPP (now SPC), Switzerland; KIT, Germany; HELLAS, Greece; CNR, Italy; ENEA Italy

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.

A specific feature of the coaxial gyrotron design is that it allows electron beam energy recovery and very fast frequency tuning via biasing the coaxial insert [987-990]. By biasing the inner rod of the KIT coaxialcavity gyrotron, such very fast (within ≈ 0.1 ms) frequency tuning was demonstrated at a power level of 1 MW. In particular, fast step frequency tuning between the 165.1 GHz nominal mode and its azimuthal neighbors at 162.8 GHz and 167.2 GHz (see Table 9) was obtained. In addition, operating in the nominal TE31,17-mode, continuous frequency pulling within 70 MHz bandwidth was achieved [940].


Note:\* and \*\* indicates that the power corresponds to that of a 1 MW (\*) and 0.8 MW (\*\*) HE11 mode, 1) formerly TOSHIBA

Table 10: Experimental parameters of high-power millimeter-wave vacuum windows [15,16,20,23-30,144-159, 450-452, 534-550,554,562,567,568,611-623,633-744,769-921,1004,1015-1074].


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. = polycrystalline, s.c. = single-crystalline) [95,118,1046,1052,1059,1061,1069-1073,1075-1079].


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) [1046].


Note that the power capability of options **,,** and is even 2 MW.

Table 13: Options for 1 MW, CW, 170 GHz gyrotron windows [93-98,118,1046].

First operation of a wideband short-pulse D-band megawatt gyrotron with elliptically brazed CVD-diamond Brewster window was published in [135-137]. A CVD-diamond Brewster window in corrugated HE11 waveguide with 32 mm inner diameter was tested at 110 GHz using 0.5 s pulses with powers up to 350 kW [1080-1082]. Broadband CVD-diamond Brewster windows are also developed for use in gyro-amplifiers [1083,1084].

### **3 Harmonic and Very High Frequency Gyrotron Oscillators**


1) Communications & Power Industries; formerly VARIAN \*) In collaboration with TOSHIBA, Ottawara

Table 14: Performance parameters of mm- and submillimeter-wave gyrotrons operating at the 2nd harmonic of the electron cyclotron frequency, with output power > 0.6 kW.


Table 15: Operation results of high harmonic gyrotrons with axis-encircling electron beam (LOG) and permanent magnet (Nd Fe B) at 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.

Operating at the fundamental, the 2nd harmonic or the 3rd harmonic of the electron cyclotron frequency, with one or two electron beams, 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) [205-362,1159-1169].

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 oscillated at the 3rd harmonic 285 GHz in the TE03-mode [1170-1174]. Simultaneous generation at the 2nd (37.5 GHz) and 4th (75 GHz) harmonic (140 W at 60 kV and 6A) was obtained by a self-excited gyromultiplier with single, sectioned cavity [1175,1176]. A high-harmonic sectioned TE35-mode gyrotron of IAP Nizhny Novgorod produced 0.5 kW at 740 GHz with 0.9% efficiency [1177-1180].


Table 17: Step tuning of MIT gyrotron oscillators (with large MIG [1140,1141]) operating at the fundamental electron cyclotron resonance frequency (pulse length 1.5 µs).


Table 18: Step tuning of MIT gyrotron oscillator (with small MIG [1140,1141]) operating at the fundamental electron cyclotron resonance frequency (pulse length 1.5 µs).

### **4 Gyrotrons for Technological Applications**


1) Communications & Power Industries, formerly VARIAN, 2) PM: permanent magnet

Table 19: Performance of present CW gyrotron oscillators for technological applications.

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 Table 33) [382,393,394]. This system has been installed at the University of Fukui, Japan.

# **5 Relativistic Gyrotrons**


r: rectangular waveguide

\*) operation from 28 to 49 GHz by magnetically tuning through a family of TEm2-modes, with the azimuthal index m ranging from 4 to 10.

Table 20: Present development status of relativistic gyrotron oscillators with MIGs or carbon fiber array cathode.


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

# **6 Quasi-Optical Gyrotrons**


SDC: Single-stage Depressed Collector

DEB: Dual Electron Beam (1 annular beam, 1 pencil beam), 1) Swiss Plasma Center, formerly CRPP, 2) formerly TOSHIBA

Table 22: Present development status of quasi-optical gyrotron oscillators.

### **7 Cyclotron Autoresonance Masers (CARMs)**


\* output mode

HERC Moscow, IAP Nizhny Novgorod, IHCE Tomsk, JINR Dubna



Table 24: State-of-the-art of slow-wave CARM experiments (short pulse).

### **8 Gyroklystrons, Gyro-TWTs, Gyrotwystrons, Gyro-BWOs and other Gyro-Devices**


#### **Weakly Relativistic Pulse Gyroklystrons**

Table 25: Weakly relativistic pulse gyroklystron experimental results.

#### **Weakly Relativistic CW Gyroklystrons**


QOGK: Quasi-Optical Gyro-Klystron;

SDC: Single-stage Depressed Collector

1) Communications & Power Industries, formerly VARIAN

Table 26: Weakly relativistic CW gyroklystron experimental results.

#### **Relativistic Pulse Gyroklystrons**


Table 27: Relativistic pulse gyroklystron experimental results.

#### **Weakly Relativistic Gyro-TWTs**


1) Communications & Power Industries, formerly VARIAN

Table 28: Present development status of weakly relativistic gyro-TWTs (short pulse and CW operation (IAP)).

#### **Relativistic Gyro-TWTs**


\*) This gyro-TWT operated near the "grazing intersection" in the dispersion diagram could also have been considered a CARM amplifier with frequency 4.4 times the relativistic cyclotron frequency.

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

#### **Weakly Relativistic Pulse Gyrotwystrons**


1) Communications & Power Industries, formerly VARIAN

Table 30: State-of-the-art of weakly relativistic gyrotwystron experiments (short pulse).

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


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

#### **Relativistic Pulse Gyrotwystrons**


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


#### **Weakly Relativistic Pulse Gyro-BWOs**

r = rectangular waveguide; c = circular waveguide, 1)formerly KfK, then FZK

Table 33: Experimental results on weakly relativistic pulse gyro-BWOs (short pulse and CW operation (IAP)).

#### **Relativistic Pulse Gyro-BWOs**


r = rectangular waveguide

Table 34: Experimental results on relativistic gyro-BWOs (short pulse: 0.01 – 1 µs).

### **Acknowledgments**

The author would like to thank K.A. Avramidis, Z.C. Ioannidis, G. Latsas, and J. Tigelis (NKU, Athens) M. Einat (Ariel University), J.J. Feng (BVERI, Beijing), L. Ives and M.E. Read (Calabazas Creek Research), L. Delpech and R. Magne (CEA, Cadarache), M. Blank, S.R. Cauffman, K. Felch, H. Jory and R. Schumacher (CPI, Palo Alto), W.A. Bongers (DIFFER, Nieuwegein), J.R. Brandon, T. Schaich, C.W.O. Thompson and C. Wort (Element 6, Charters), F. Albajar and P. Sánchez (F4E Barcelona), J. Anderson, J.L. Doane, R. Freeman, J. Lohr, C.P. Moeller and R.A. Olstad (General Atomics, San Diego), M.V. Agapova, V.I. Kurbatov, V.E. Myasnikov, V.B. Orlov, L.G. Popov, E.A. Solujanova, E.M. Tai and S.V. Usachev (GYCOM), V.L. Bratman, Yu. Bykov, G.G. Denisov, N.S. Ginzburg, M.Yu. Glyavin, V.A. Goldenberg, A.N. Kuftin, A. Litvak, V.N. Manuilov, S. Mishakin, V.V. Parshin, A. Peskov, M.I. Petelin, A.B. Pavelyev, R.M. Rozental, S.V. Samsonov, A.G. Savilov, E.V. Sokolov, V.E. Zapevalov and E.V. Zasypkin (IAP RAS, Nizhny Novgorod), J. Luo and Q. Xue (IECAS, Beijing), W. Kasparek, C. Lechte and B. Plaum (IGVP, Stuttgart), F. Leuterer, M. Münich, J. Stober, D. Wagner and H. Zohm (IPP Garching), H. Braune, V. Erckmann, H.P. Laqua, S. Marsen, T. Stange and F. Wilde (IPP, Greifswald), C. Darbos (ITER, Cadarache), S.-T. Han (KERI, Changwon), G. Aiello, E. Borie, G. Dammertz, B. Ell, L. Feuerstein, G. Gantenbein, S. Illy, J. Jelonnek, J. Jin, L. Krier, W. Leonhardt, G. Link, A. Marek, A. Meier, B. Piosczyk, S. Ruess, T. Rzesnicki, T. Scherer, M. Schmid, V.I. Shcherbinin, D. Strauss, M. Vöhringer and C. Wu (Karlsruhe Institute of Technology), K. Sakamoto (Kyoto Fusioneering), M.A. Shapiro and R.J. Temkin (MIT, Cambridge), H. Asano and T. Kikunaga (MITSUBISHI, Amagasaki), S. Kubo, M. Sato, T. Shimozuma and S. Takayama (NIFS, Toki), J.P. Calame, Y. Carmel, B. Danly, A. Fliflet, M. Garven, S.H. Gold and B. Levush (NRL, Washington D.C.), Y. Tsunawaki (Osaka Sangyo University), J. Neilson (SLAC), S. Alberti, J. Genoud, T. Goodman, and J.-P. Hogge (SPC, Lausanne), G.G. Sominski and O.I. Louksha (State Polytechnical University, St. Petersburg), W. He (Shenzhen University), C. Donaldson, A.D.R. Phelps and K. Ronald (Strathclyde University), E. Jerbi (Tel Aviv University), S. Kohler, A. Leggieri, F. Legrand, C. Lievin and R. Marchesin (THALES, Velizy), N.C. Luhmann, Jr. and D.B. McDermott (UC, Davis), L. Hongfu, Y. Liu and L. Shenggang (UESTC, Chengdu), K.R. Chu (National Taiwan University (NTU), Taipei), C.-Y. Tsai (National Tsing Hua University (NTHU, Hsinchu), S. Mitsudo, I. Ogawa, T. Saito and T. Tatematsu (University of Fukui), T.M. Antonsen, V.L. Granatstein, W. Lawson, G.S. Nusinovich and A.N. Vlasov (University of Maryland), R.M. Gilgenbach and Y.Y. Lau (University of Michigan), T. Imai, T. Kariya and R. Minami (University of Tsukuba), E. Choi (UNIST Ulsan), J. Hirshfield (Yale University), R. Ikeda, K. Kajiwara, H. Kobayashi, Y. Oda and K. Takahashi (QST, Naka). This work could not have been done without their help, stimulating suggestions and useful discussions.

The author also wishes to express his deep gratitude to Mrs. M. Huber for her help in doing the layout of this report.

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This report presents an update of the experimental achievements published in the review "State- of-the-Art of High-Power Gyro-Devices and Free Electron Masers", Journal of Infrared, Millimeter, and Terahertz Waves, 41, No. 1, pp 1-140 (2020) related to the development of gyro-devices. Emphasis is on high-power gyrotron oscillators for long-pulse or continuous wave (CW) operation and pulsed gyrotrons for many applications. In addition, this work gives a short update on the present development status of frequency step-tunable and multi-frequency gyrotrons, coaxial-cavity multi-megawatt gyrotrons, complex two-section stepped cavity gyrotrons, gyrotrons for technological and spectroscopy applications, relativistic gyrotrons, large orbit gyrotrons (LOGs), quasi-optical gyrotrons, fast- and slow-wave cyclotron autoresonance masers (CARMs), gyroklystron-, gyro-TWT- and gyrotwystron amplifiers, gyro-harmonic converters, gyro-BWOs 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 heating (ECH), 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 gyrotrons employing synthetic diamond output windows and single-stage depressed collectors is 30 minutes (CPI USA and European KIT-SPC-THALES collaboration). PLL-frequency stabilization of such tubes has been demonstrated. A 1.5 MW version of this gyrotron is under development (IPP-KIT-THALES). The first Japan 170 GHz ITER gyrotron prototype achieved 1 MW, 800 s at 55% efficiency and holds the energy world record of 2.88 GJ (0.8 MW, 60 min., 57 %). The Russian 170 GHz ITER gyrotron obtained 0.99 (1.2) MW with a pulse duration of 1000 (100) s and 57 (53) % efficiency. First frequency-injection-locked operation of a Russian 170 GHz-1 MW gyrotron has been demonstrated in short pulses using a PLL-frequency-stabilized 20 kW gyrotron master oscillator. The prototype tube of the KIT 2 MW, 170 GHz coaxial-cavity gyrotron (max. pulse duration 50 ms) achieved in 1 ms pulses the record power of 2.2 MW at 48% efficiency and 96% Gaussian mode purity.

**M. Thumm**

State-of-the-Art of High-Power Gyro-Devices

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Update of Experimental Results 2023