High-power Vacuum Electronic Devices Research Articles

Microstructure, mechanical properties and ultra-high temperature thermal stability of GdB6 ceramics fabricated by reactive spark plasma sintering

In this study, a highly dense GdB6 bulk ceramic material was fabricated using one-step in-situ reaction spark plasma sintering technology. The sample with a relative density of 96.7 % and a purity of 96.9 wt% was successfully obtained by sintering at 1800 °C for 30 min under a uniaxial pressure of 50 MPa. The Vickers hardness, elastic modulus and fracture toughness of the as-obtained GdB6 bulk ceramics were 18.9 GPa, 209 GPa and 1.6 MPa·m1/2, respectively. The thermal conductivity of GdB6 at room temperature was 28.68 W·m−1·K−1, 70 % of which was mainly attributed to phonon vibration, while the remaining 30 % to the electrical thermal conductivity. In addition, the as-obtained GdB6 ceramics showed no phase transformation after heat treatment at a relatively high temperature of 2100 °C for 20 min in Ar atmosphere. The highly covalent octahedral boron framework played a critical role in maintaining the phase stability of the GdB6 crystal lattice. This suggested that GdB6 ceramics could be used as potential ultra-high temperature ceramics for aerospace applications and high-power vacuum electronic devices operated at high temperatures.

Investigation of High-Power Ring-Bar Traveling Wave Tube With Metamaterial Absorber

A rectangular split resonant ring (RSRR) metamaterial absorber (MMA) was proposed to suppress backward wave oscillation (BWO) in a ring-bar traveling wave tube (RB-TWT). The unit structure parameters of the RSRR were optimized to maximize the absorption of backward waves. The amplification performance of the proposed slow wave structure (SWS) was also carried out by using particle-in-cell (PIC) simulations. With an input power of 0.125 W, the operating bandwidth of the RB-TWT loaded with RSRR-MMA was 8–13 GHz, which was higher than that loaded with centralized attenuators (CAs; 8–11.5 GHz). The output power exceeded 5 kW with a gain of 46 dB in the range of 9.1–12.7 GHz. Furthermore, its saturated output power exceeded 5 kW, the saturated gain was over 41 dB, and the saturated electronic efficiency was more than 14.5% over the entire working frequency range of 8–13 GHz. Therefore, the RB-TWT with RSRR-MMA achieved a wider bandwidth compared with that of the RB-TWT with CA, where exhibits the potential application of RSRR-MMA in high-power vacuum electronic devices.

Beam dynamic study of a Ka-band microwave undulator and its potential drive sources

Microwave undulators (MUs) have great potential to be an alternative solution to permanent magnet undulators in a free electron laser (FEL) when shorter undulator periods are required. In this paper, the factors that affect the choice of the high-power drive sources were studied via a Ka-band cavity-type MU with a corrugated waveguide proposed for the CompactLight X-ray FEL. They include the technology of the high-power vacuum electronic devices, the quality factor of the MU cavity that was demonstrated by prototyping a short section of the MU structure, and the beam dynamic study of the electrons’ trajectories inside the MU. It showed that at high beam energy, a high-power oscillator is feasible to be used as the drive source. At low beam energy, the maximum transverse drift distance becomes larger therefore an amplifier has to be used to minimize the drift distance of the electrons by controlling the injection phase.

The new application of boron-doped diamond film: Attenuator for high frequency and high power vacuum electronic devices

Boron-doped diamond film which was used as microwave attenuation material was prepared by DC arc jet chemical vapor deposition (CVD) and the boron-doped diamond attenuator for W-band traveling wave tube (TWT) was fabricated. The effects of boron concentration on microstructure, mechanical property and dielectric property were analyzed. Meanwhile, the performance of boron-doped diamond attenuators was evaluated. The results showed that the fracture strength of boron doped diamond was higher than 400 MPa, although it was slight decreased with the increasing of boron content in the diamond films. Very interestingly, the fracture strength of growth side was larger than that of nucleation side, the twinning band within the grains in the growth side play a critical role for this phenomenon. With the boron concentration increasing from 6.1 × 1018 cm−3 to 23.5 × 1018 cm−3, the average loss tangent of diamond films was increased from 0.05 to 0.32 in the range of 75–110 GHz, which indicated that the complex permittivity of diamond film could be controlled by tailoring boron content. Furthermore, the return loss and insertion loss of the boron-doped diamond attenuator was below −20 dB and −40 dB, respectively. These findings indicated that the boron-doped diamond displayed low reflection and good microwave attenuation. The boron-doped diamond attenuator was promising for using in vacuum electronic device with high performance.

Design Study on a Multiple-Tunnel Meander-Line Slow-Wave Structure for a High-Power V-Band Traveling-Wave Tube

Nowadays, there is a strong interest in miniature and high-power vacuum electron devices (VEDs) operating in millimeter and sub-THz bands. VED sources of coherent sub-THz radiation are required for many applications, especially for future advanced high-data-rate telecommunication systems. In this work, we present the results of design study of a meander-line slow-wave structure (SWS) suitable for multiple-sheet-beam operation to achieve a high level of output power. The results of cold-test electromagnetic simulation show a possibility of operation at beam voltage of 12 kV with high values of interaction impedance up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$12 \Omega $ </tex-math></inline-formula> . The 3-D particle-in-cell simulation of performance of the traveling-wave tube with 400-mA, 12-kV multiple sheet electron beam predicts up to 26-dB gain and up to 376-W output power.

High-Power Vacuum Electronic Devices from Microwave to THz Band: Way Forward

It is generally accepted that the 20th century was the age of electronics [...]

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.

Guest Editorial The Eighteenth Special Issue on High-Power Microwave and Millimeter-Wave Generation

The field of high-power microwave and millimeter-wave generation continues to be an active area of research and technological development to meet the demand of high peak, average, and prime power needs of government and industrial applications. High-power vacuum electronic devices, producing coherent electromagnetic radiation ranging from tens of centimeters to submillimeter wavelengths, continue to find application in advanced fusion and accelerator device applications, state-of-the-art radar and electronic countermeasures, high data-rate communications, and material processing. Advances in materials, plasmas, cathode technology, and electromagnetic structure design, as well as theory, modeling, and simulation, help to drive this technology to achieve unprecedented peak and average power, more compact size, enhanced waveform agility, greater bandwidth, and higher efficiency.

Effects of frequency-dependent surface impedance on the vacuum electronic terahertz sources

When the working frequency of vacuum electronic device reaches the terahertz frequency, the ohmic loss has a significant impact on the vacuum electronic device. To study the effect of the ohmic loss on the working characteristic of the vacuum electronic terahertz devices, this paper implements the frequency-dependent surface impedance boundary condition (SIBC) in the 3 dimensional particle in cell code UNIPIC-3D. Conformal mesh is adopted in the code to overcome the staircase error in traditional particle in cell method. By using the surface impedance boundary, we eliminate the need to study the field inside the lossy dielectric objects which require extremely small grid cells for numerical stability. In comparison with constant parameter SIBC, the dispersive SIBC is applicable over a very large frequency bandwidth and over a large range of conductivities. The correctness of the implementation is verified by simulating the lossy resonant cavity and circular waveguide, the simulated power loss is comparable with the theoretical predication. High power vacuum electronic devices of terahertz regime are attracting extensive interests due to their potential applications in science and technologies. The impulse-wave relativistic surface wave oscillator (SWO) and low-voltage continuous-wave planar grating backward wave oscillator (BWO) both made of copper are numerically studied by using UNIPIC-3D and dispersive surface impedance boundary condition. Numerical results show that the strongest field is very close to the slow wave structure where the beam-wave interaction occurs and that terahertz wave generates both in these two devices. The distributed wall loss has a considerable effect on the devices: the output power has a significant decrease and the startup time becomes longer, but the working frequencies of the two devices keep unchanged. To improve the efficiency of relativistic SWO, a resonant reflector is proposed between the diode and the slow wave structure. Numerical results show that the working frequency of the device with a resonant reflector keeps unchanged as the original one, but the output power increases to 60 MW from 41 MW of the original one when the ohmic loss is considered.

Photonic crystal-based compact high-power vacuum electronic devices

This paper considers how the finite dimensions of a photonic crystal placed inside a resonator or waveguide affect the law of electron beam instability. The dispersion equations describing e-beam instability in the finite photonic crystal placed inside the resonator or waveguide (a bounded photonic crystal) are obtained. Two cases are considered: the conventionally considered case, when diffraction is suppressed, and the case of direct and diffracted waves having almost equal amplitudes. The instability law is shown to be responsible for increase of increment of instability and decrease of length, at which instability develops, for the case when amplitude of diffracted wave is comparable with that of direct one, that happens in the vicinity of $\pi$-point of dispersion curve. Application of photonic crystals for development of THz sources at electron beam current densities available at modern accelerators is discussed.

Design and Experiment of 4 MW Ka Band Sheet Electron Beam TWT

High-power vacuum electron devices (VEDs) are attractive, because of their broad applications, such as advance radars, electromagnetic warfare, and plasma reactors. In this paper, a Ka band mega-watt sheet electron beam traveling wave tube (TWT) is reported. A two-staged rectangular waveguide grating slow wave structure (SWS) driven by a sheet electron beam (20 mm × 1 mm, 164.8 kV, 850 A, 20 ns) is used in this TWT. In order to suppress the self-excited oscillation, a rectangular BeO attenuator is inserted between the two-staged SWSs. The experimental results show that the TWT can produce more than 2 MW output power from 34.2 to 35.5 GHz. And the maximum output power is 4.24 MW at 35 GHz, corresponding to the gain of 25.9 dB. In the future, the TWT will be used to perform research on biomedical effects and imaging.

Development of a 100-W 200-GHz High Bandwidth mm-Wave Amplifier

A millimeter (mm) wave amplifier that combines both high-power output and a wide bandwidth would enable dramatic improvements in areas where high-power radio signals with significant information content are required. However, to date, the only available devices are either high-power vacuum electronic devices with limited bandwidths, or larger bandwidth semiconductor amplifiers with low-power output. Here, we show a new design of a vacuum electronic traveling-wave tube amplifier that has a measured output power of 107 W with a 3-dB bandwidth of 6 GHz centered on 200 GHz. The power-bandwidth product of this amplifier is 642 W-GHz. These results provide a demonstration of a high-power mm-wave source with significant bandwidth. This device is readily scalable to other frequency bands, particularly those in the mm- and sub-mm-wave frequency bands from 100 GHz to 1 THz.

A megawatt-level surface wave oscillator in Y-band with large oversized structure driven by annular relativistic electron beam

High power vacuum electronic devices of millimeter wave to terahertz regime are attracting extensive interests due to their potential applications in science and technologies. In this paper, the design and experimental results of a powerful compact oversized surface wave oscillator (SWO) in Y-band are presented. The cylindrical slow wave structure (SWS) with rectangular corrugations and large diameter about 6.8 times the radiation wavelength is proposed to support the surface wave interacting with annular relativistic electron beam. By choosing appropriate beam parameters, the beam-wave interaction takes place near the π-point of TM01 mode dispersion curve, giving high coupling impedance and temporal growth rate compared with higher TM0n modes. The fundamental mode operation of the device is verified by the particle-in-cell (PIC) simulation results, which also indicate its capability of tens of megawatts power output in the Y-band. Finally, a compact experimental setup is completed to validate our design. Measurement results show that a terahertz pulse with frequency in the range of 0.319–0.349 THz, duration of about 2 ns and radiation power of about 2.1 MW has been generated.

Investigation of Nanosized-Scandia-Doped Dispenser Cathodes With Machined Surfaces

Since the cathodes with machined surfaces are required for the practical applications in some high-power vacuum electron devices, the characteristics of nanosized-scandia-doped dispenser (SDD) cathodes with machined surfaces have been investigated, especially in dc applications. In order to reopen the surface pores, which may be smeared by machining procedure, the cathode surface was washed by Ethylene Diamine Tetraacetic Acid (EDTA) solution and prolonged time was taken for the cathode activation. The emission test results show that the SDD cathodes with machined surfaces exhibit better emission performances after treatment. The space-charge-limited dc emission current density of over 20 A/cm2 at 900 °Cb has been measured with temperature compensation for electron cooling after full activation. An on-going life test with a dc load of up to 40 A/cm2 at 970 °Cb has been completed $\sim 5000$ h of operation to date. Since the emission capability can be fully restored after exposing to air and reactivating for a short time, a preheating procedure is favorable for SDD cathode with machined surface in the practical applications. Surface analysis indicates that a recrystallization of tungsten grains occurs during the prolonged heating process, leading to further reopening of the surface pores. As a result, cathode surface is covered uniformly by active elements of Ba and Sc (as well as O) as on an unmachined surface, thereby maintaining the high emission property of the SDD cathode.

All-metal metamaterial slow-wave structure for high-power sources with high efficiency

In this paper, we have proposed a metamaterial (MTM) which is suitable for the compact high-power vacuum electron devices. For example, an S-band slow-wave structure (SWS) based on the all-metal MTMs has been studied by both simulation and experiment. The results show that this MTM SWS is very helpful to miniaturize the high-power vacuum electron devices and largely improve the output power and the electronic efficiency. The simulation model of an S-band MTM backward wave oscillator (BWO) is built, and the particle-in-cell simulated results are presented here: a 2.454 GHz signal is generated and its peak output power is 4.0 MW with a higher electronic efficiency of 31.5% relative to the conventional BWOs.

Investigation of intense sheet electron beam transport using the macroscopic cold-fluid model and the single-particle orbit theory

The focusing and the stable transport of an intense elliptic sheet electron beam in a uniform magnetic field are investigated thoroughly by using the macroscopic cold-fluid model and the single-particle orbit theory. The results indicate that the envelopes and the tilted angles of the sheet electron beam obtained by the two theories are consistent. The single-particle orbit theory is more accurate due to its treatment of the space-charge fields in a rectangular drift tube. The macroscopic cold-fluid model describes the collective transport process in order to provide detailed information about the beam dynamics, such as beam shape, density, and velocity profile. The tilt of the elliptic sheet beam in a uniform magnetic field is carefully studied and demonstrated. The results presented in this paper provide two complete theories for systemically discussing the transport of the sheet beam and are useful for understanding and guiding the practical engineering design of electron optics systems in high power vacuum electronic devices.

Thyristor-capacitor voltage regulator of high-voltage pulse generator

This paper presents a new thyristor-capacitor voltage regulator for a high-voltage pulse generator used in electrophysical installations based on different high-power vacuum electron devices. A thyristor-capacitor voltage regulator enables one to reduce the power intensity of storage capacitors that enter it. Expressions for calculating the parameters of components of the power supply are developed and results of simulation models of the power supply are reported.

Intense Sheet Electron Beam Transport in a Uniform Solenoidal Magnetic Field

In this paper, the transport of intense sheet electron beams in a uniform solenoidal magnetic field in high-power vacuum electronic devices is theoretically examined with the 3-D beam optics code MICHELLE. It is shown that a solenoidal magnetic field can be an effective transport mechanism for sheet electron beams, provided the beam tunnel is matched to the beam shape, and vice versa. The advantage of solenoidal magnetic field transport relative to periodic magnetic transport resides in the feasibility of transporting higher current density beams due to the higher average field strength achievable in practice and the lower susceptibility to field errors from mechanical misalignments. In addition, a solenoidally transported electron beam is not susceptible to voltage cutoff as in a periodic magnetic focusing system; hence, device efficiency is potentially higher.

High intensity, plasma-induced emission from large area ZnO nanorod array cathodes

High intensity electron emission cathodes based on a well-aligned ZnO nanorod array were fabricated. An investigation of the properties of the plasma and the electron beams produced by ZnO nanorod array cathodes was presented. Intense current electron beams were obtained from the cathodes. At an electric field of 7–8V∕μm and pulse duration of ∼100ns, the highest emission current density reached 76–91A∕cm2. The production mechanism of the electron beams was the plasma-induced emission. The morphology and structure of the ZnO nanorod after the application of the accelerating pulses were characterized. The plasma expanded at a velocity of about 10.7cm∕μs during the pulse interval. Whether the emission currents are high or low, the plasma on the cathode surface were always distributed uniformly. The ZnO nanorod array cathodes are expected to be applied to high power vacuum electronic devices as electron beam sources.

Novel cold cathode materials and applications

Field emission (FE) is based on the physical phenomenon of quantum tunneling, in which electrons are injected from the surface of materials into vacuum under the influence of an applied electric field. A variety of field emission cold cathode materials have been developed to date. In this review, we shall focus on several kinds of novel cold cathode materials that have been developed in the past decade. These include materials for microfabricated field-emitter arrays, diamond and related films, carbon nanotubes, other quasi one-dimensional nanomaterials and printable composite materials. In addition, cold cathode materials have a wide range of applications such as in flat panel displays, high-power vacuum electronic devices, microwave-generation devices, vacuum microelectronic devices and vacuum nanoelectronic devices. Applications are in consumer goods, military industries and also space technology. A comprehensive overview of the various applications is presented. Recently, recognizing the strong possibility that vacuum nanoelectronic devices using quasi one-dimensional nanomaterials, such as carbon nanotubes may emit electrons with driving voltages comparable to that of a solid-state device, there is a growing interest in novel applications of such devices. With such exciting opportunities, there is now a flurry of activities to explore applications far beyond those considered for the conventional hot cathodes that operate on thermionic emission. We shall discuss the details of a number of fascinating potential applications.