The electron cyclotron maser (ECM) is based on a stimulated cyclotron emission process involving energetic electrons in gyrational motion. It constitutes a cornerstone of relativistic electronics, a discipline that has emerged from our understanding and utilization of relativistic effects for the generation of coherent radiation from free electrons. Over a span of four decades, the ECM has undergone a remarkably successful evolution from basic research to device implementation while continuously being enriched by new physical insights. By delivering unprecedented power levels, ECM-based devices have occupied a unique position in the millimeter and submillimeter regions of the electromagnetic spectrum, and find use in numerous applications such as fusion plasma heating, advanced radars, industrial processing, materials characterization, particle acceleration, and tracking of space objects. This article presents a comprehensive review of the fundamental principles of the ECM and their embodiment in practical devices.

/pdf/the-electron-cyclotron-maser-2tiz6d37qb.pdf

The electron cyclotron maser

This article reviews the state-of-the-art in high-power microwave source research. It begins with a discussion of the concepts involved in coherent microwave generation. The main varieties of microwave tubes are classified into three groups, according to the fundamental radiation mechanism involved: Cherenkov, transition, or bremsstrahlung radiation. This is followed by a brief discussion of some of the technical fundamentals of high-power microwave sources, including power supplies and electron guns. Finally, the history and recent developments of both high-peak power and high-average power sources are reviewed in the context of four main areas of application: (1) plasma resonance heating and current drive; (2) rf acceleration of charged particles; (3) radar and communications systems; and (4) high-peak power sources for weapons-effect simulation and exploratory development.

/pdf/review-of-high-power-microwave-source-research-4wfh5blzfg.pdf

Review of high-power microwave source research

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

/pdf/state-of-the-art-of-high-power-gyro-devices-and-free-5avfirj4z9.pdf

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

Physics and technology issues of importance to the high-gain gyrotron traveling wave amplifier (gyro-TWT) are investigated in theory and experiment. The gyro-TWT is known to be highly susceptible to spurious oscillations, especially in high gain operations. In the current study, oscillations of various origins are classified and characterized with detailed theoretical modeling. They are shown to be intricately connected to the interplay between the absolute/convective instabilities, circuit losses, and reflective feedback. Knowledge of these processes leads to the concept of an ultra high gain scheme which employs distributed wall losses for the suppression of spurious oscillations. A proof-of-principle Ka-band gyro-TWT experiment stable at zero drive has produced 93 kW saturated peak power at 26.5% efficiency and 70 dB gain, with a 3 dB saturated output power bandwidth of 3 GHz. The saturated gain is more than 30 dB beyond that previously achieved.

/pdf/theory-and-experiment-of-ultrahigh-gain-gyrotron-traveling-4s8khulzo7.pdf

Theory and experiment of ultrahigh-gain gyrotron traveling wave amplifier

Experimental results are presented on the first W-band gyrotron Traveling-Wave Tube (gyro-TWT) developed to exploit the 94 GHz atmospheric window for long-range, high-resolution radar applications. The gyro-TWT is designed to operate in the higher order TE01 mode and is driven by a 100 kV, 5 A electron beam with a pitch angle of v⊥/vz=1 and velocity spread of Δvz/vz=5%. Large-signal simulations predict 140 kW output power at 92 GHz with 28% efficiency, 50 dB saturated gain, and 5% bandwidth. The stability of the amplifier against spurious oscillations has been checked with linear codes. To suppress the potential gyro-BWO interactions involving the TE02, TE11, and TE21 modes, the interaction circuit with a cutoff frequency of 91 GHz has been loaded with loss so that the single-path, cold-circuit attenuation is 90 dB at 93 GHz. A coaxial input coupler with 3% bandwidth is employed with a predicted and measured coupling of 1 dB and 2 dB, respectively. The operating voltage is limited to below 75 kV because of oscillations encountered at higher voltages in this initial embodiment. Preliminary test at Vb=60 kV and Ib=3.7 A yielded 59 kW saturated output power at 92.2 GHz with 42 dB gain, 26.6% efficiency, and a 3 dB bandwidth of 1.2 GHz (1.3%).

Theory and experiment of a 94 GHz gyrotron traveling-wave amplifier

Absolute instabilities in the gyrotron traveling wave amplifier are investigated with a simulation approach which models electron and wave dynamics in the sever and distributed-loss section. Distributed wall losses are shown to be far more effective than the sever in stabilizing these instabilities. Physical interpretations are given and theoretical predictions are verified by a K„-band experiment which achieved 62 kW peak power with 12% bandwidth, 21% efficiency, and 33 dB saturated gain through the use of a mechanically adjustable magnetron injection gun.

Stabilization of absolute instabilities in the gyrotron traveling wave amplifier.

The magnetron injection gun (MIG) has been the predominant choice as the electron-beam generator in gyrodevices and it can be tuned by externally installed magnetic-field trim coils. The double-anode MIG, traditionally employed in most experiments, has the intermediate anode as an internal tuning mechanism. The single-anode design, on the other hand, is simpler to construct but lacks an internal beam tuning mechanism. The current paper describes a method to allow mechanical tuning of the MIG. Since the tip of the cathode is located in the region of the highest electric-field gradient, its extension and retraction relative to the rest of the cathode assembly provide an effective means for local electric-field profile modification and, therefore, a tuning mechanism for the electron-beam property and quality. It is demonstrated, in computer simulations and experimental tests, that the tuning mechanism adds a useful degree of freedom to the single-anode MIG for beam optimization. This method may also be applied to the double-anode design.

A mechanically tunable magnetron injection gun

A gyrotron traveling-wave amplifier (gyro-TWA) with a distributed-loss structure can achieve high gain over a broad bandwidth. In this study, a low-voltage, low-current subterahertz gyro-TWA with medium power (∼10 W) over a broad-frequency range (∼10 GHz) is designed. The excessive length of the copper section in this gyro-TWA leads to reflective oscillations and absolute instabilities. Reflective oscillations can be suppressed for a gyro-TWA with a high lossy-section resistivity and a short copper section. Absolute instabilities, conversely, can be suppressed for a gyro-TWA with a long lossy section and high lossy-section resistivity. The proposed gyro-TWA has a high lossy-section resistivity, a long lossy section, and a short copper section, which not only avoids mode competition but also achieves the desired characteristics of a high gain and broad-frequency range. For a 12 kV, 0.3 A electron beam, the frequency range of the 250 GHz 10 W level gyro-TWA with a distributed-loss structure, is ∼10 GHz when the input power is between 1 and 10 mW.

Y. S. Yeh

Papers

Stabilization of absolute instabilities in the gyrotron traveling wave amplifier.

A mechanically tunable magnetron injection gun

Broad-frequency range of a 250 GHz medium-power gyrotron traveling-wave amplifier with a distributed-loss structure