Vacuum Electronic Devices Research Articles

QUASIOPTICAL SOLID-STATE GENERATOR WITH OPEN QUASI-FREQUENCY SPHEREOSELETAL OSCILLATING SYSTEM

The results of research related to the development of an effective high-Q open oscillatory system of quasi-optical solid-state generators of millimeter and submillimeter wavelength ranges are presented. It is shown that for the stable operation of solid-state generators based on Gunn diodes and avalanche-transit diodes (ATDs) located inside an open resonator (OR), a significant rarefaction of the resonance frequency spectrum is required, which excludes jumps of the generation frequency. Taking this requirement into account, a spheroechelett open resonator (SE OR) is proposed as an open oscillatory system of solid-state generators, one of the mirrors of which is a reflective lattice of the “echelette” type. The paper presents the results of “cold” measurements of the spectrum of resonance frequencies of the SE OP, which indicate that the SE OP indeed has a significantly rarefied spectrum both in transverse and, which is very important, in longitudinal modes of oscillations. The conditions necessary for the excitation of high-quality stable oscillations with high energy efficiency in a solid-state generator based on an SE OR are discussed and implemented. The results of an experimental study of a quasi-optical generator based on an 8-mm ATD wavelength range are presented. The analysis of the obtained characteristics and their qualitative explanation on the basis of the proposed model of the processes occurring in the investigated generator are presented. It is shown that the SE OR can be used as a resonant system in the entire EHF range of not only solid-state, but also electronic-vacuum devices. Finally, a way is outlined in the direction of further rarefaction of the spectrum of the SE OR in order to create a really single-frequency open oscillatory system.

Vacuum Electronic Devices and Applications

This issue of Defence Science Journal (DSJ) contains a collection of nine manuscripts from National Symposium on Vacuum Electronic Devices and Applications (VEDA-2019). Vacuum Electronic Devices and Applications Society organises VEDA symposium/conference/workshop every year at different locations of India. VEDA has established itself as a leading platform for active and innovative research of microwave tubes. It provides a forum for knowledge sharing and exhibition of theoretical and technological development in the general areas of vacuum electronics devices and its system applications. It organises special session to enhance Research Institutes – Industries and Academia Interaction.

Potentials of Machine Learning in Vacuum Electronic Devices Demonstrated by the Design of a Magnetron Injection Gun

Great progress has been made on machine learning and its applications are expanding rapidly nowadays. Through the case study of optimizing a magnetron injection gun for gyrotron devices, the functions of machine learning were investigated by using two supervised learning algorithms, regression trees and artificial neural networks. They showed excellent performance in predicting the outputs, exploring the importance of the input parameters and the relationship with the output parameters. Machine learning can be a useful tool in the development of microwave vacuum electron devices.

A Carbon Nanotube-Based Hundred Watt-Level Ka-Band Backward Wave Oscillator

Carbon nanotube (CNT) cold-cathodes hold much promise in a variety of millimeter-wave and terahertz vacuum electronic radiation devices due to their inherent near instantaneous temporal turn-on and near-ideal ideal field electron emission performance. Here we report on the development of a CNT cold-cathode Ka -band backward-wave oscillator (BWO). Using a novel beam compression stage, theoretical studies, simulation results, and empirical findings collectively demonstrate that this device affords an unprecedentedly high output power of 230 W at a technologically important operating frequency of 33.65 GHz. The developed magnetic injection electron gun achieves a high emission current of 265.5 mA (emission current density of 188.3 mA/cm2) and a high focused beam current density of 18.5 A/cm2, which our studies suggest, is essential to the BWOs high output power.

Research on GaN-Based RF Devices: High-Frequency Gate Structure Design, Submicrometer-Length Gate Fabrication, Suppressed SCE, Low Parasitic Resistance, Minimized Current Collapse, and Lower Gate Leakage

As an important part of wireless communication systems, the technology of RF devices and circuits has been progressing rapidly. For high-frequency and high-power applications, vacuum electronic devices (e.g., traveling wave tubes) have been widely used, although the vacuum tube is large in size and hard to integrate.

Changes in the Editorial Board

After nine years (the maximum three consecutive terms) on the Editorial Board of IEEE Transactions on Electron Devices (TED), Prof. Farrokh Ayazi, GeorgiaTech, Atlanta, GA, USA (Solid-State Sensors and Actuators), Prof. Manfred Thumm, Karlsruhe Institute of Technology, Karlsruhe, Germany (Vacuum Electron Devices), and Dr. Wilman Tsai, TSMC, USA (MOS Devices and Technology) have stepped down from their Editor positions in the respective fields.

Study of a terahertz folded waveguide oscillator with sheet electron beam

High power vacuum electronic devices of the millimeter wave to terahertz regime are attracting extensive interest due to their potential applications in science and technologies. In this paper, a novel sheet beam (SB) folded waveguide (FWG) slow-wave structure (SWS) is applied and studied for enhanced output power of oscillators. Through the analysis of high-frequency characteristics of the SB FWG-SWS, the selection of an appropriate beam–wave interaction point on the dispersion curve and beam position is determined for an SB folded waveguide oscillator (FWO) operating near the 2π stopband point. The fundamental mode operation of the SB FWO was verified by the particle-in-cell simulation results, which also indicate that the SB FWO exhibits a better outstanding performance (84.6 W with a beam voltage and current of 25.1 kV and 200 mA, respectively) than the conventional circular beam FWO. The transmission stability of the SB was analyzed by observing the interception beam current in the FWO. The voltage and frequency tuning performance of the SB FWO were discussed in combination with clutter suppression. A preliminary experiment was conducted by fabricating a 220 GHz SB FWG-SWS and measuring the S parameter.

Zeroth radial modes of azimuthal surface waves in dense plasma-loaded, coaxial helix traveling-wave-tube-like waveguides

An analytical model of coaxial traveling-wave-tube-like waveguides with plasma filling has been justified and utilized to analyze the eigenmodes. Very often, introducing plasma into vacuum electronic devices leads to essential advantages as compared with evacuated tubes. The cylindrical structure under the present consideration consists of a central dielectric rod, placed inside a plasma coaxial layer with a metallic helix sheath on its outer interface, and a metal screen separated from the plasma by another dielectric layer. The dispersion properties of electromagnetic waves propagating across the external axial static magnetic field in such traveling-wave-tube-like waveguides are studied and summarized. The presence of a dense plasma coaxial layer makes the media nontransparent for waves in the electron cyclotron frequency range. However, surface type electromagnetic waves can propagate in this case. These waves are called azimuthal surface waves (ASWs). The helix sheath causes coupling of ordinarily and extraordinarily polarized ASWs. The zeroth radial ASW modes have been found to be most dangerous for parasitic wave excitation in dense plasma-loaded, coaxial traveling-wave-tube-like waveguides.

Over-Size Pill-Box Window for Sub-Terahertz Vacuum Electronic Devices

The pill-box window is one of the important components of microwave vacuum electronic devices (VEDs), and research into it is of great significance. As the operating frequency increases, the problems associated with the reduction in the structure size include the reduction of the brazing plane and the reduction in the tolerance of the pill-box window. These problems will cause traditional pill-box windows to be unsuitable in high-frequency bands, especially in terahertz and sub-terahertz regions. The most influential factor is the length of the circular waveguide in the box window. The welding plane of the over-size pill-box window is the annular bottom surface on both sides of the dielectric sheet, which is larger than the circular waveguide, and the operating frequency does not directly affect the area of the brazing surface. Choosing a suitable diameter for the dielectric sheet can effectively increase the tolerance to the length of the pill-box window circular waveguide. Therefore, an over-size pill-box window would be a practicable approach to improve the performance compared to the traditional pillow-box in high-frequency bands. This paper describes, in detail, the theoretical design, simulation optimization and experimental process of this improved pill-box window. An over-size pill-box window suitable for G band VEDs was successfully developed. The experimental result in the 215–225 GHz band is that the maximum transmission loss is −1 dB, and the overall transmission loss is close to −0.5 dB. The overall reflection is less than −11 dB.

Design Procedure for a Broadband TE11/HE11 Mode Converter for High-Power Radar Applications

The HE11 hybrid mode, propagating in an overmoded corrugated circular waveguide, is widely used for low loss transmission of high-power microwaves. Due to the inherent broadband frequency behaviour, this will be also essential for future broadband high-power radar applications, like space debris observation in low earth orbit (LEO). A promising amplifier concept for such radar sensors is a helical gyro-TWT. However, since the HE11 hybrid mode is not suitable for electron-beam-wave interaction in this kind of vacuum electron device, an additional mode converter is required. The present paper addresses the design procedure of a broadband high-power mode converter, designed for a helical gyro-TWT intended for future broadband high-power radar applications in the W-band. The interaction mode of the helical gyro-TWT under consideration can be easily transferred to the circular waveguide TE11 mode. Therefore, a TE11 ↦ HE11 mode converter is addressed here. The design procedure is based on a scattering matrix formalism and leads to a high HE11 mode content of ≥98.6% within the considered frequency range from 92 GHz to 100 GHz. Inside this frequency band, the mode content is even better and reaches ≈ 99.7% at ≈95 GHz. This allows broadband frequency operation of a helical gyro-TWT and is suitable for broadband high-power radar applications.

A Review of Microwave Vacuum Devices in China: Theory and Device Development Including High-Power Klystrons, Spaceborne TWTs, and Gyro-TWTs

Lately, R&D of microwave vacuum electron devices (MVEDs) has played a very important role in the technologies of broadcasting, communication, radar, remote imaging, electronic countermeasures, materials research, deep space exploration, and wave spectroscopy. In the 1970s, this widespread role could not have been predicted, as the development of vacuum electron devices faced a serious challenge from solid-state electron devices. However, in solid-state electron devices, the electron stream is a conduction (ohmic, collisional) current, whereas in vacuum electron devices, the current is a convection (ballistic, collisionless) current. So the vacuum is perfect for high-power devices needing high-power electron currents, especially because vacuum electron devices are intrinsically superior at handling high power or high power density, giving vacuum electronics research an edge in high power and high frequency [1]-[5].

Design of high-power wide-band G-band third harmonic amplifier

To meet the demand of high-power and wide-band signal sources for G-band vacuum electronic devices, the research on G-band third harmonic amplifier is carried out. The amplifier utilizes the third harmonic current in the nonlinear beam-wave interaction of E-band TWT, and realizes G-band electromagnetic wave amplification by cascading harmonic interaction section. The design scheme of high performance and practical G-band wide-band high-power source adopts folded waveguide slow wave structure with modified circular bends, and the G-band third harmonic amplifier is simulated and optimized by using the microwave tube simulator package (MTSS) software. The result shows that the device can obtain harmonic output power greater than 3.6 W in the range of 15 GHz, with conversion gain＞33.3 dB and electronic efficiency＞0.36%. Compared with other miniaturized terahertz radiation sources in this band, it has superior performance in terms of output power and bandwidth, and thus provides a design basis for the subsequent research of G-band third harmonic amplifier.

Ultralow Voltage GaN Vacuum Nanodiodes in Air.

The III-nitride semiconductors have many attractive properties for field-emission vacuum electronics, including high thermal and chemical stability, low electron affinity, and high breakdown fields. Here, we report top-down fabricated gallium nitride (GaN)-based nanoscale vacuum electron diodes operable in air, with record ultralow turn-on voltages down to ∼0.24 V and stable high field-emission currents, tested up to several microamps for single-emitter devices. We leverage a scalable, top-down GaN nanofabrication method leading to damage-free and smooth surfaces. Gap-dependent and pressure-dependent studies provide new insights into the design of future, integrated nanogap vacuum electron devices. The results show promise for a new class of high-performance and robust, on-chip, III-nitride-based vacuum nanoelectronics operable in air or reduced vacuum.

Enhanced interfacial reaction of silicon carbide fillers onto the metal substrate in carbon nanotube paste for reliable field electron emitters

Adhesion of carbon nanotube (CNT) onto a cathode substrate is very crucial for field electron emitters that are operating under high electric fields. As a supporting precursor of CNT field emitters, we adopted silicon carbide (SiC) nano-particle fillers with Ni particles and then enhanced interfacial reactions onto Kovar-alloy substrates through the optimized wet pulverization process of SiC aggregates for reliable field electron emitters. As-purchased SiC aggregates were efficiently pulverized from 20 to less than 1 micro-meter in a median value (D50). CNT pastes for field emitters were distinctively formulated by a mixing process of the pulverized SiC aggregates and pre-dispersed CNTs. X-ray photoelectron spectroscopy studies showed that the optimally pulverized SiC-CNT paste-emitter had a stronger Si 2p3/2 signal in the Ni2Si phase than the as-purchased one. The Si 2p3/2 signal would represent interfacial reaction of the SiC nano-particle onto Ni from the CNT paste and the Kovar substrate, forming the supporting layer for CNT emitters. The optimal paste-emitter even in a vacuum-sealed tube exhibited a highly reliable field emission current with a high current density of 100 mA cm−2 for over 50 h along with good reproducibility. The enhanced interfacial reaction of SiC filler onto the metal substrates could lead to highly reliable field electron emitters for vacuum electronic devices.

Nonlinear dynamics of radiation of high-current beams of charged particles in spatially periodic structures

Provided a review of physical processes underlying the functioning of vacuum electronic devices, namely, nonlinear processes of radiation of high-current beams of charged particles during their passage through spatially periodic structures (resonators) and their interaction with an electromagnetic field. Principles of functioning of travelling wave tubes, backward wave tubes, multi-wave Cherenkov generators, free electron lasers and free electron masers, volume free electron lasers are considered. These devices operate in a wide spectrum range from microwave to X-ray, have high efficiency, and allow obtaining high radiation powers in narrow spectral ranges. Increase the performance and reliability of electronic vacuum devices is based on improving the complex electromagnetic structures in resonators. The article demonstrates a variety of such devices with an obvious generality of physical principles used in them. Nonlinear chaotic dynamics of their functioning is considered. Particular attention is paid to consideration of principles of operation of volume free electron lasers as well as their difference from other devices.

Laser hybridizing with micro-milling for fabrication of high aspect ratio micro-groove on oxygen-free copper

High aspect ratio (HAR) micro-grooves are of great significance for vacuum electronic devices. However, the fabrication of HAR micro-groove on oxygen-free copper remains challenge since the high surface quality is hard to achieve. Aiming to improve the surface quality, a hybrid and successive method, obtained by combining nanosecond laser and micro-milling (NLMM), was presented. The nanosecond laser achieves the maximum workpiece removal rate, while the subsequent micro-milling provides the desired machining quality. Compared to one-way full slot micro-milling (FSMM), the presented manufacturing method (NLMM) could improve the surface quality, namely the top burr formation and the bottom and sidewall surface roughness. Furthermore, the NLMM displayed a lower tool wear rate compared to FSMM. Finally, a desired S-shaped groove with an aspect ratio of 2.5, a width of 0.2 mm, and the cycle number of 30 was successfully fabricated. The study offers a promising aspect for improving the HAR micro-groove fabrication quality.

Design of planar distributed three beam electron gun with narrow beam separation for W band staggered double vane TWT

A novel planar distributed three-beam electron gun with narrow beam separations is designed based on grids loaded sheet beam method. The dimensions of the three-beam gun in the y–O-z plane are determined using our basic theoretical design method developed for sheet beam gun. The results show that the profile of focusing electrode in the y–O-z plane is related to the beam width in the x-O-z plane. Then, the characteristics and parameters of three-beam array formation with their stability are analyzed thoroughly by adjustment of control grids in the x-O-z plane. Each of the beamlet obtained is with a small axial deviation of the two transverse waists. Based on the theoretical analysis and simulations, the planar three-beam electron gun is constructed with the beam voltage of 22 kV and the current of 3 times 0.15 A. The average radius of 0.08 mm at each beam waist is obtained with the compression factor of 4 for the 0.18 mm beam tunnel radius. The beam waist can be achieved at about 4.4 mm away from the cathode with the axis separation about 0.46 mm for each of beamlet. Thus, the design method can be generally used to construct such type of narrow beam separation and planar distributed multiple beam electron gun for the miniaturization and integrated vacuum electron devices in millimeter wave and terahertz band.

A Cascade Electron Source Based on Series Horizontal Tunneling Junctions

On-chip electron sources have wide potential applications in miniature vacuum electronic devices and emission efficiency is one of their performance benchmarks. A cascade electron source based on series metal-insulator-metal horizontal tunneling junctions is proposed, where free electrons are additively extracted from each tunneling junction. A cascade electron source with n horizontal tunneling junctions shows a theoretical emission efficiency of approximately {\eta}(n)=1-(1-{\eta}_0 )^n, with {\eta}_0 being the efficiency of a single tunneling junction. Experimentally, a cascade electron source with three Si-SiOx-Si tunneling junctions is demonstrated, achieving an emission efficiency as high as 47.6%. This work provides a new way of realizing highly efficient on-chip electron sources.

Effect of ion-beam surface treatment on photocathode emission

To meet the needs of high-frequency, miniaturized vacuum microwave devices, a new photocathode for microwave vacuum electronic device has been studied. Untreated oxygen-free copper, commonly used for photocathode substrate, exhibits relatively high photoemission characteristics. In this paper, we describe a specialized ion-beam bombardment procedure for textured copper surfaces, thereby improving the photoemission properties relative to untreated copper. The emission characteristics of antimony cesium photocathode on oxygen-free copper substrate before and after surface treatment are studied The photoemission and texture of post-treated oxygen-free copper surface are examined by scanning electron microscope. The results show that the treated surface has a particle-free, robust, uniformly highly-textured all-metal structure. This processing technology does not require to modify the copper machining and brazing, nor normal fabrication procedures of other photocathodes. In the experiment, the maximum photoemission current density of photocathode for the untreated substrate is 60.5 and that for the treated substrate is 146.0 mA/cm<sup>2</sup>, and their corresponding quantum efficiencies are calculated to be 2.67 × 10<sup>–3</sup> and 1.71 × 10<sup>–2</sup>, respectively. So, the quantum efficiency is enhanced by 6.41 times. The analysis indicates that the improvement of the quantum efficiency of the treated photocathode is mainly due to the enhancement of the light absorption rate. The results show that the photoemission is enhanced significantly after the substrate has been treated, and there is still much room for improvement.

MULTI-POINTED FIELD-EMISSION CATHODE AS A GENERATOR OF HIGHFREQUENCY OSCILLATIONS

Semiconductor field-emission cathodes have gained considerable popularity in modern radio electronics and electronic optics due to the high-power generation of the electron beam in the external electric field at temperatures close to the room ones. However, their wide application is restricted by the high dependence of the electron emission current on the value of the applied field and geometrical parameters of the cathode. This study aimed to examine the effect of resonance processes on amplifying the field emission of the multi-pointed semiconductor cathode. Modeling the behavior of resonant tunneling of electrons from semiconductors to vacuum was simulated by solving the one-dimensional Schrodinger’s equation, and the amplification due to resonant processes was estimated. The modeling results showed that as the electric field increases, the resonance conditions shift towards low energy levels. With the increase in the width of the barrier for the electron inside the solid body, the resonance conditions shift towards higher energies. It has been established that in onedimensional semiconductors with electrons of low conductivity width, the resonant energy coincides with the Fermi level. These cathode properties are optimal for amplifying the emission current and reducing failures of vacuum electronic devices based on semiconductive field cathodes. The proposed technique can be used to study the regularities of emission amplification due to resonant processes in multipoint semiconductor cathodes with multilayered structure and with metal tips.