Recent research and development has been incredibly successful at advancing the capabilities for vacuum electronic device (VED) sources of powerful terahertz (THz) and near-THz coherent radiation, both CW or average and pulsed. Currently, the VED source portfolio covers over 12 orders of magnitude in power (mW-to-GW) and two orders of magnitude in frequency (from ; 10 THz). Further advances are still possible and anticipated. They will be enabled by improved understanding of fundamental beam-wave interactions, electromagnetic mode competition and mode control, along with research and development of new materials, fabrication methods, cathodes, electron beam alignment and focusing, magnet technologies, THz metrology and advanced, broadband output radiation coupling techniques.

Vacuum Electronic High Power Terahertz Sources

We demonstrate a high solubility limit of >9 mol% for MnTe alloying in SnTe. The electrical conductivity of SnTe decreases gradually while the Seebeck coefficient increases remarkably with increasing MnTe content, leading to enhanced power factors. The room-temperature Seebeck coefficients of Mn-doped SnTe are significantly higher than those predicted by theoretical Pisarenko plots for pure SnTe, indicating a modified band structure. The high-temperature Hall data of Sn1–xMnxTe show strong temperature dependence, suggestive of a two-valence-band conduction behavior. Moreover, the peak temperature of the Hall plot of Sn1–xMnxTe shifts toward lower temperature as MnTe content is increased, which is clear evidence of decreased energy separation (band convergence) between the two valence bands. The first-principles electronic structure calculations based on density functional theory also support this point. The higher doping fraction (>9%) of Mn in comparison with ∼3% for Cd and Hg in SnTe gives rise to a muc...

Valence Band Modification and High Thermoelectric Performance in SnTe Heavily Alloyed with MnTe

Resonant tunneling diodes (RTDs) have the potential for use as compact and coherent terahertz (THz) sources operating at room temperature. In this paper, sub-THz and THz oscillators with RTDs integrated on planar circuits are described. Fundamental oscillation up to 0.65 THz and harmonic oscillation up to 1.02 THz were obtained at room temperature in our recent study. Limiting factors for oscillation frequency and output power are theoretically analyzed including tunneling and transit-time effects and parasitic elements. Oscillation frequency and its dependence on RTD size are in good agreement with the measured results. Based on this result, it is shown that fundamental oscillation up to 2.3 THz and an output power of 60 µW at 1 THz are theoretically expected by improving the structures of the RTD and the antenna. Voltage-controlled oscillation, which is useful for the precise control of frequency, is observed in the RTD oscillators. Coherent power combining in an array configuration to achieve high output power as well as mutual injection locking between the array elements are also described.

https://iopscience.iop.org/article/10.1143/JJAP.47.4375/pdf

Resonant Tunneling Diodes for Sub-Terahertz and Terahertz Oscillators

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

Gyrodevice oscillators and amplifiers (or gyro-oscillators and gyro-amplifiers) are being utilized in a variety of applications where high power levels are required at millimeter-wave frequencies. Gyro-oscillators, developed primarily for magnetic fusion research applications, have achieved power levels near 1 MW for pulse durations in excess of 1 s at frequencies above 100 GHz. Continued work on these devices should enable them to achieve continuous-wave operation at multimegawatt power levels at frequencies in the 100-GHz to 200-GHz range, thereby meeting the requirements of planned magnetic fusion experiments. The development of gyro-oscillators for fusion experiments has led to the utilization of the devices in several industrial applications, such as ceramic sintering and metal joining. Activities in this area involve adapting the oscillators to the industrial environment where reliability, efficiency, and ease of operation are paramount. Gyro-amplifiers are being developed for applications requiring phase coherence and instantaneous bandwidth, such as in linear accelerators and millimeter-wave radar. Impressive results from X-band to W-band already suggest the promise of these devices. Potential new applications and novel gyrodevice design approaches continue to attract the attention of researchers around the world.

Characteristics and applications of fast-wave gyrodevices

The development of a high‐frequency, step‐tunable gyrotron operating at submillimeter wavelengths is described. The gyrotron design was optimized for operation at the second harmonic of the electron cyclotron frequency in the TE261 cavity mode, whose resonant frequency is 384 GHz. Experimental results show that second harmonic operation can occur without mode competition as long as the beam current is low (Ib ≲0.8 A), but as the current is increased, the fundamental TE231 cavity mode increases and eventually (Ib ≳1 A) suppresses the second harmonic. The competition between the two modes is studied in detail. The starting current for second harmonic operation is also studied experimentally and compared with calculated results. Other resonances have also been examined. With the present superconducting magnet, the maximum frequency achieved is 402 GHz (second harmonic operation in the TE551 cavity mode) at several kilowatts.

Development of a second cyclotron harmonic gyrotron operating at submillimeter wavelengths

The development of a high frequency electron spin resonance (ESR) spectrometer with a wide frequency range using a gyrotron as the radiation power source is described. GYROTRON FU-E, optimized for use in an ESR spectrometer in the millimeter wave range, was developed in Fukui University. In order to test the normal operation of the spectrometer, the ESR of two standard samples, single crystal and polycrystalline DPPH, has been measured, in the pulsed mode over the frequency range from 65 GHz to 135 GHz.

ESR spectrometer with a wide frequency range using a gyrotron as a radiation power source

Non-linear theory is used to examine the limits on the maximum power that might be obtained from a high harmonic gyrotron. In particular, special attention is paid to the situation where the power is limited by mode competition from a nearby fundamental resonance. We find good qualitative agreement between theory and some observations made on a high harmonic gyrotron at Fukui University.

Mode competition in a high harmonic gyrotron

GYROTRON FU IV, a frequency tunable gyrotron for spectroscopy in the submillimeter wavelength range using a 17 T superconducting magnet, has been designed and constructed. Simulations predict that the gyrotron will emit radiation at high frequencies up to 394 GHz (TE041 cavity mode) at the fundamental of the electron cyclotron frequency, up to 858 GHz (TE091 mode) at the second harmonic and up to 1301 GHz (TE8 11 1 mode) at the third harmonic. Preliminary experimental results have demonstrated operation from 158 to 443 GHz at the fundamental and from 336 to 838 GHz at the second harmonic. Output powers are several hundred watts and efficiencies several percent.

Development of a medium power, submillimeter wave gyrotron using a 17 T superconducting magnet

This letter describes the development of a submillimeter wave gyrotron operating at the second harmonic of the cyclotron frequency. The observed frequency is 383 GHz, which corresponds to the wavelength of 0.79 mm, and the output power is about 1 kW. We believe that the frequency is a world record for a stable, long pulse gyrotron operation at the second cyclotron harmonic, without a competition with the operation at the fundamental. Comparisons with the simulation results are represented.

Toshiaki Tatsukawa

Papers

Development of a second cyclotron harmonic gyrotron operating at submillimeter wavelengths

ESR spectrometer with a wide frequency range using a gyrotron as a radiation power source

Mode competition in a high harmonic gyrotron

Development of a medium power, submillimeter wave gyrotron using a 17 T superconducting magnet

Development of a second cyclotron harmonic gyrotron operating at 0.8 mm wavelength