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

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.

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The electron cyclotron maser

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).

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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

Systems for THz transmission using new types of THz waveguides with low loss, negligible group velocity dispersion and structural simplicity are described herein. The THz system incorporates the use of a waveguide with two or more substantially parallel conductive elements which may enable many new THz sensing applications. It is now possible to direct the THz pulse inside of containers or around corners, where line-of-sight optics are not practical. Moreover, the systems allow use of either radially polarized or linearly polarized THz antennas. The disclosed systems are compatible with existing terahertz generation and detection techniques.

Method and system for transmitting terahertz pulses

The design and experimental study of a 35-GHz gyrotron-traveling-wave tube (gyro-TWT) amplifier operating in the circular TE/sub 01/ mode at the fundamental cyclotron harmonic are presented. The interaction circuit in this experiment consisted of a new type of ceramic loading that provided the required loss for stable operation. A saturated peak power of 137 kW was measured at 34.1 GHz, corresponding to a saturated gain of 47.0 dB and an efficiency of 17%, with a -3-dB bandwidth of 1.11 GHz (3.3%). Peak output powers in the range of 102.1 to 148.6 kW with -3-dB bandwidths of 1.26 and 0.94 GHz, respectively, were measured by varying the operating parameters. The gyro-TWT was found to be zero-drive stable at these operating points, demonstrating that ceramic loading is a highly effective means of suppressing spurious oscillations in gyro-TWTs. This type of ceramic loading has the added advantage of being compatible with high average power operation.

A gyrotron-traveling-wave tube amplifier experiment with a ceramic loaded interaction region

We present the first vacuum electronic traveling wave amplifier to incorporate an interaction circuit fabricated by ultraviolet (UV) photolithography and electroforming, demonstrating over 60 W of output power at 214.5 GHz from a 12.1 kV, 118 mA electron beam. The tube also achieved an instantaneous bandwidth of ${\sim}{\rm 15}~{\rm GHz}$ in $G$ -band in the small signal regime. The all-copper circuit was fabricated in two layers using a UV-transparent polymer monofilament embedded in the photoresist to form the beam tunnel prior to electroforming. Effects arising from fabrication errors and target tolerances are discussed. This microfabrication technique and demonstration paves the way for a new era of vacuum electron devices that could extend into the 1–2 THz range with advances in high-current-density electron guns.

Demonstration of a High Power, Wideband 220-GHz Traveling Wave Amplifier Fabricated by UV-LIGA

Current amplifier research at the Naval Research Laboratory Vacuum Electronics Branch emphasizes techniques to extend the bandwidth and average power capability of gyro devices for millimeter wave radar applications. This paper will discuss the implementation of a wideband high-gain gyro-traveling wave tube amplifier design, with a measured peak output power of 78 kW, gain /spl sim/60 dB, and a 3-dB bandwidth of 4.2 GHz (12%) at 52 kW in K/sub a/-band. The 3-dB saturated bandwidth at 70 kW is 6 GHz (17%), which is also the instantaneous bandwidth with appropriately tailored input power (e.g., gain equalizer). The amplifier operates in the TE/sub 11/ mode and for stabilization employs a high-average power compatible diffractive loading technique.

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A TE/sub 11/ K/sub a/-band gyro-TWT amplifier with high-average power compatible distributed loss

Details of the amplifier physics design are given in [7], but a summary of the major design issues is given here. Given the accelerating voltage and desired frequency range, gain, and output power, the various tradeoffs for the interaction region can be explored. The lengths of the loaded and unloaded sections of the interaction circuit are selected to satisfy stability constraints, overall amplifier gain, and efficiency requirements. The loss section needs to be sufficiently long and with sufficient loss for the desired gain and zero-drive stability, but not too long or velocity spread will compromise performance. It must especially load the spurious modes. In our case, the loss design must also be high-average power capable. A short unloaded section is added for nonlinear gain and high power. Its length must be less than the critical length for start oscillation in the given magnetic field. A too long section will also compromise bandwidth due to beam velocity spread. The appropriate magnetic field must be chosen for near grazing to slightly above grazing with the waveguide mode for the given waveguide size and beam voltage. The magnetic field is tapered where possible in regions of uniform diameter waveguide in order to ensure no oscillations. For this design, the field is tapered at the input coupler and at the output at the unloaded section and uptaper. A nonlinear uptaper is used to couple RF from the interaction circuit to the outside world with miminal reflection of the operating mode and minimal conversion to other modes. For each set of beam and field values, the RF input power is chosen for flattest response. The implementation of these design considerations is described below. The wall geometry, magnetic field profile, and calculated power for this amplifier design are shown in Fig. 1 for the operational parameters: kV, A, mm,

http://ieeexplore.ieee.org/iel5/27/29266/01321255.pdf

-Band Gyro-TWT Amplifier With High-Average Power Compatible Distributed Loss

The experimental demonstration of a four cavity W-band (93 GHz) gyroklystron amplifier is reported. The gyroklystron has produced 67 kW peak output power and 28% efficiency in the TE011 mode using a 55 kV, 4.3 A electron beam. The full width at half maximum instantaneous bandwidth is greater than 460 MHz, a significant increase over the bandwidth demonstrated in previous W-band gyroklystron amplifier experiments. The amplifier is unconditionally stable at this operating point. Experimental results are in good agreement with theoretical predictions.

Dean E. Pershing

Papers

A gyrotron-traveling-wave tube amplifier experiment with a ceramic loaded interaction region

Demonstration of a High Power, Wideband 220-GHz Traveling Wave Amplifier Fabricated by UV-LIGA

A TE/sub 11/ K/sub a/-band gyro-TWT amplifier with high-average power compatible distributed loss

-Band Gyro-TWT Amplifier With High-Average Power Compatible Distributed Loss

Experimental demonstration of a w-band gyroklystron amplifier