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

A novel rectangular-corrugated waveguide is proposed for submillimeter and terahertz vacuum devices. Two parallel corrugations that are enclosed in a rectangular waveguide create a beam channel that supports an interaction with a cylindrical electron beam. A notable advantage of the double-corrugated rectangular waveguide slow-wave structure (SWS) is the extension of well-established cylindrical beam technology to corrugated waveguide SWSs. The structure is also fully realizable with the most recent microfabrication techniques and is easily assembled. A detailed study to describe the electromagnetic behavior of the presented SWS is performed by 3-D electromagnetic simulation. A 650-GHz backward-wave oscillator and a 227-GHz traveling-wave tube are designed and simulated, by 3-D particle-in-cell code, to highlight the great potential of the double-corrugated rectangular waveguide for submillimeter frequency vacuum devices.

Double-Corrugated Rectangular Waveguide Slow-Wave Structure for Terahertz Vacuum Devices

The Child–Langmuir Law (CL), discovered a century ago, gives the maximum current that can be transported across a planar diode in the steady state. As a quintessential example of the impact of space charge shielding near a charged surface, it is central to the studies of high current diodes, such as high power microwave sources, vacuum microelectronics, electron and ion sources, and high current drivers used in high energy density physics experiments. CL remains a touchstone of fundamental sheath physics, including contemporary studies of nanoscale quantum diodes and nano gap based plasmonic devices. Its solid state analog is the Mott–Gurney law, governing the maximum charge injection in solids, such as organic materials and other dielectrics, which is important to energy devices, such as solar cells and light emitting diodes. This paper reviews the important advances in the physics of diodes since the discovery of CL, including virtual cathode formation and extension of CL to multiple dimensions, to the ...

100 years of the physics of diodes

This article gives a brief overview of the common vacuum electronic tube amplifiers used in high-power transmitters. Only three types of devices [travelling wave tubes (TWTs) including helix and coupled-cavity types, microwave power modules (MPMs), and klystrons] are covered, with emphasis on the recent advance in the millimeter-wave band.

Vacuum tube amplifiers

This article reviews the state-of-the-art millimeter-wave (mm-wave) power amplifiers (PAs), focusing on broadband design techniques. An overview of the main solid-state technologies is provided, including Si, gallium arsenide (GaAs), GaN, and other III–V materials, and both field-effect and bipolar transistors. The most popular broadband design techniques are introduced, before critically comparing through the most relevant design examples found in the scientific literature. Given the wide breadth of applications that are foreseen to exploit the mm-wave spectrum, this contribution will represent a valuable guide for designers who need a single reference before adventuring in the challenging task of the mm-wave PA design.

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A Review of Technologies and Design Techniques of Millimeter-Wave Power Amplifiers

Recent demonstration of low-voltage high-transconductance field emitter array operation holds promise for the successful development of reliable cold cathode vacuum electronics device technologies. This paper reports on the experimental studies of implementation of such field emitter arrays as the electron source for a moderate power traveling wave tube (TWT) operating in the C-band frequency regime. The cold cathode TWT has operated for over 150 h at duty factors up to 10%, beam currents up to 121 mA, and RF powers up to 100 W at 5 GHz. High cathode current densities of 15.4 A/cm2 were achieved concurrent with excellent beam control, resulting in 99.4% beam transmission under zero-RF-drive operating conditions and 97.3% transmission at maximum RF output power. The cathode is shown to operate with a 72% reduction in the operating voltage from the previous generation of emitters fabricated by SRI International, bringing the operating voltage for full current operation well below 100 V. Extensive device characterization and life testing has been performed, and interesting variation in cathode performance was observed during the high-duty high-current portion of the testing program. The results presented here represent the highest current, highest power, and highest duty factor ever reported for an RF vacuum device employing a field emission cold cathode electron source.

100 W Operation of a Cold Cathode TWT

The development of a compact radio frequency (RF) vacuum power amplifier for high-resolution airborne radar is described. The amplifier, a microwave power module (MPM), operates in the upper millimeter-wave frequency band of 231.5–235 GHz providing a peak output power of 32 W. Common with previous extremely high frequency MPM development at Electron Devices, the ${G}$ -band MPM consists of a periodic permanent magnet focused serpentine waveguide traveling wave tube (TWT) and a miniaturized 20-kV electronic power conditioner. Input drive to the MPM for saturation is around 10 mW. Low-loss chemical vapor deposition diamond WR-4.3 windows serve as the input and output ports of the TWT. Due to the high duty of the radar application, a four-stage TWT collector is employed for beam energy recovery. The MPM operates from a 270- $\text{V}_{\sf {dc}}$ power source. Two MPM configurations have been constructed: a single integrated unit for laboratory testing and a split-package configuration for integration in a standard electro-optical/infrared gimbal. The split-package flight test unit has a maximum prime power requirement of 176 W, corresponding to an overall amplifier efficiency of 9%. The radar sensor has been flown on a modified DC-3 test bed with high-resolution real-time video imagery obtained under cloud-obscured operating conditions.

A Compact Extremely High Frequency MPM Power Amplifier

A new field emitter geometry that utilizes a dielectric shield between the emitter tip and gate has been optimized to eliminate flashover in the cathode emitter cavities and improve cathode reliability. A high average power testbed has been designed and fabricated to test these cathodes at currents and current densities required for TWT operation in the X/Ku-Band frequency regime. Experimental tests in the water-cooled testbed demonstrated a record 100 hours of CW operation at 100 mA. A 200 mA X/KuBand TWT was designed to integrate these cathodes and to provide RF gain and power across the entire 6 - 18 GHz frequency band. Preliminary results taken at a cathode current of 50 mA show excellent focus of the emittance-dominated electron beam. RF results at 50 mA demonstrated positive gain over the entire frequency band with a maximum of 13.5 dB gain and 10 W output power at 10 GHz and low duty. Cathode testing as well as TWT design and preliminary operation will be described.

High average power field emitter cathode and testbed for X/Ku-band cold cathode TWT

Over the past decade, the Microwave Power Module (MPM) has undergone significant transformations, adapting to a wide variety of emerging radar and communications systems needs. Moreover, the recent success of unmanned aerial vehicles (UAVs) and mobile SATCOM terminals in various military theatres around the world has cultivated a renewed interest in MPM technology due to the size, weight, and efficiency advantages they offer when compared to traditional solid-state and travelling wave tube (TWT) amplifiers. This paper traces the evolution of the MPM from its beginning at L-3 Communications EDD to more modern applications as versatile building blocks in high data rate digital communications, synthetic aperture radars, and electronic warfare systems. The parallel evolution of MPM design tools and supporting technologies is also examined, both of which have been instrumental in the development of next-generation MPMs.

Technological evolution of the Microwave Power Module (MPM)

L-3 EDD has developed a pulsed mode WBand microwave power module (MPM) providing 100W RF power over 92 to 96 GHz. The MPM includes a serpentine waveguide PPM-focused TWT and a modular electronic power conditioner (EPC). A breadboard power supply was previously built and successfully tested with a W-Band TWT. The final power conditioner design is completing development for packaging with a TWT as an MPM.

Ramon Duggal

Papers

100 W Operation of a Cold Cathode TWT

A Compact Extremely High Frequency MPM Power Amplifier

High average power field emitter cathode and testbed for X/Ku-band cold cathode TWT

Technological evolution of the Microwave Power Module (MPM)

A 100 Watt W-Band MPM