Vacuum Electronic Devices Research Articles

Experiments of Sub-THz Wave Folded Waveguide Traveling-Wave Tube Amplifier

This study showed the possibility of using a sub-terahertz (THz) traveling-wave tube (TWT) via measuring the transmission characteristics and TWT performance of the circuit by applying X-ray LIGA, a micro-fabrication process, to the interaction circuit. The applied circuit type, an E-bend folded waveguide, is a simple structure most suitable for lithography. A total of three applied frequencies were used the W-band, G-band, and 850 GHz. Among the manufactured circuits, the W-band circuit was applied to the TWT, one of the vacuum electronics devices (VEDs). This was done to prove the manufacturing accuracy of the circuit by comparing the nonlinear characteristics of the circuit with the prediction results. Through such testing, the small signal gain was measured as 13 ± 2 dB under the conditions of 13.96-kV and 24.2-mA electron beam energy. The frequency bandwidth was extremely wide, about 9 GHz, and showed similar characteristics to the simulation predictions. The maximum output of the device was obtained up to 1 W or more at 87.12 GHz by slightly increasing the beam current. These characteristic achievements showed the suitability of the TWT for very small circuits fabricated using the X-ray LIGA process, further suggesting the applicability of other sub-THz bands.

The effects of Ar + N2 plasma power-based attachment of metal nanoparticles on the electron field emission properties of carbon nanotubes

An ideal vacuum-based electron emitter requires a low turn-on field and quick emission processes. Carbon nanotubes (CNTs) have great potential for emission applications, but have some limitations that affect their performance. Consequently, they can benefit from the alteration of their surface morphology upon decoration with plasma-based metal nanoparticles. In this work, we have attached iron nanoparticles on the CNTs surface via a radio frequency (rf) sputtering process using Ar + N2 gas with various plasma power in steps of 50 W. Field emission scanning electron microscopy (FESEM) micrographs have confirmed the attachment of the iron nanoparticles and etching of the nanotubes surface at high plasma power. The Raman spectra strongly confirm the formation of defects in the carbon sheet of the nanotubes. In the plasma treatment process, the increase in the Dband intensity enhanced the degree of disorder and may be beneficial for enhanced emission properties. The field electron emission properties are significantly enhanced after the Fe nanoparticles attached with plasma incorporated in terms of the reduction in Eto (1.64 from 2.02 V/μm) &Eth (1.98 from 2.64 V/μm), and increase in J (391.11–1179.24 μA/cm2) and β (1419–2900), respectively. The field emission parameters of the field emitters are modified due to incorporation of nitrogen species, attached Fe nanoparticles, and reduction in the nanotube density. The as-prepared field emitters also qualified the orthodox field emission hypothesis test by exhibiting fextr values in an acceptable range. Therefore, we can use the Ar + N2 plasma-based Fe nanoparticles decorated CNT field emitters in vacuum electronic device applications.

Physical Factors Governing the Shape of the Miram Curve Knee in Thermionic Emission

In a current density versus temperature (J-T) (Miram) curve in thermionic electron emission, experimental measurements demonstrate there is a smooth transition between the exponential region and the saturated emission regions, which is sometimes referred to as the "roll-off" or "Miram curve knee". The shape of the Miram curve knee is an important figure of merit for thermionic vacuum cathodes. Specifically, cathodes with a sharp Miram curve knee at low temperature with a flat saturated emission current are typically preferred. Our previous work on modeling nonuniform thermionic emission revealed that the space charge effect and patch field effect are key pieces of physics which impact the shape of the Miram curve knee. This work provides a more complete understanding of the physical factors connecting these physical effects and their relative impact on the shape of the knee, including the smoothness, the temperature, and the flatness of the saturated emission current density. For our analyses, we use a periodic, equal-width striped ("zebra crossing") work function distribution as a model system and illustrate how the space charge and patch field effects restrict the emission current density near the Miram curve knee. The results indicate there are three main physical parameters which significantly impact the shape of the Miram curve. Such physical knowledge directly connects the patch size, work function values, anode-cathode voltage, and anode-cathode gap distance to the shape of the Miram curve, providing new understanding and a guide to the design of thermionic cathodes used as electron sources in vacuum electronic devices (VEDs).

Analytical Solution for Space-Charge Waves in a Two-Stream Cylindrical Electron Beam

We present an analytical method to compute the wavenumbers and electric fields of the space-charge-wave eigenmodes supported by a two-stream electron beam, consisting of a solid inner cylindrical stream and a coaxial outer annular stream, both contained within a cylindrical metallic tunnel.We extend the analytical model developed by Ramo to the case of two streams. The method accounts for the interaction between the two streams with the presence of the beam-tunnel wall; it can be used to model the complex wavenumbers associated with the two-stream instability and the plasma frequency reduction effects in vacuum electronic amplifiers and other vacuum electronic devices.

Characterization of an X-Band Metamaterial Slow Wave Structure for HPM Applications

In this article, we have characterized an X-band complementary electric split-ring resonator (CeSRR) based metamaterial (MTM) slow-wave structure (SWS), suitable for high-efficiency vacuum electron devices (VEDs). The MTM SWS consists of an array of CeSRR and a circular waveguide that propagates below the cut-off frequency of an empty waveguide in the TM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{01}$</tex-math> </inline-formula> mode. The surface current distribution has been used to predict the resonance frequency of the CeSRR. An equivalent circuit approach has been used here to obtain its equivalent circuit capacitance and inductance per unit length, which is used further to analyze the dispersion characteristics and interaction impedance of the MTM SWS. The electromagnetic (EM)simulation result showed that the dispersion bandwidth ranges from 9.53 to 9.78 GHz, and the average interaction impedance is greater than 550 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega $</tex-math> </inline-formula> . Furthermore, we have designed, EM simulated, and experimentally characterized the proposed X-band MTM SWS. The experimental transmission coefficient ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{21})$</tex-math> </inline-formula> has been found approximately consistent with the EM simulated values.

Field emission cathodes made from knitted carbon nanotube fiber fabrics

Field electron emission cathodes were constructed from knitted fabrics comprised entirely of carbon nanotube (CNT) fibers. The fabrics consisted of a top layer array of ∼2 mm high looped structures and a bottom layer that was 1 mm thick with a flat underlying surface. Field emission (FE) experiments were performed on 25.4 mm diameter CNT fabric cathodes in both direct current (DC) and pulsed voltage (PV) modes, and the results were compared to those obtained from a CNT film cathode. The DC measurements were performed at a maximum voltage of 1.5 kV. The CNT fabric cathode emitted 20 mA, which was an 8× increase over the emission current from the CNT film cathode. The DC results were analyzed using the corrected form of the Fowler–Nordheim FE theory initially developed by Murphy and Good, which allows for the determination of the formal emission area and effective gap-field enhancement factor. The PV experiments resulted in Ampere level emission currents from both CNT fabric and CNT film cathodes. For a 25 kV, 500 ns voltage pulse, the CNT fabric cathode emitted 4 A, which was 2× more current than the CNT film cathode. Scanning electron microscopy imaging after PV testing revealed that the fibers remained intact after &amp;gt;5000 pulses. These results indicate that knitted CNT fabrics offer a promising approach for developing large area, conformable, robust FE cathodes for vacuum electronic devices.

Particle-In-Cell Simulations of High Efficiency 12-Vanes 2.45 GHz Continuous Wave Magnetron

As an essential vacuum electronic device for producing the microwave, the magnetron has various applications. This study developed a novel high-efficiency 12-vanes CW magnetron and anode resonance system that improved mode separation, expanded the working space of π-mode and made other modes more challenging to trigger, ultimately eliminating the possibility of mode jumping. A magnetron was simultaneously supplied with a particular quantity of anode voltage, and the cathode was generated by the electron, and high-frequency field interaction of a homogeneous magnetic field. The work efficiency of the 12-vanes CW magnetron was significantly enhanced. Given an anode voltage of 8000 V and a magnetic flux density of 3980 Gs as a consequence of particle simulation, the variation trend of a magnetron’s output power oscillation curve correlated with the development of hexagonal spokes. After a period of stable operation, the magnetron’s fundamental parameters were determined to be as follows: the primary frequency oscillation frequency was 2.466 GHz, the anode collision current was 1.08 A, the amplitude of sinusoidal oscillation was 125, the output power was 7812.5 W, and the corresponding power conversion efficiency was 90.42%. Changing the magnitude of the anode voltage or magnetic flux density resulted in a reduction in power conversion efficiency within a particular range; however, between 85% and 90% stability was maintained.

Secondary electron emission and vacuum electronics

Secondary electron emission serves as the foundation for a broad range of vacuum electronic devices and instrumentation, from particle detectors and multipliers to high-power amplifiers. While secondary yields of at least 3–4 are required in practical applications, the emitter stability can be compromised by surface dynamics during operation. As a result, the range of practical emitter materials is limited. The development of new emitter materials with high yield and robust operation would advance the state-of-the-art and enable new device concepts and applications. In this Perspective article, I first present an analysis of the secondary emission process, with an emphasis on the influence of material properties. From this analysis, ultra-wide bandgap (UWBG) semiconductors and oxides emerge as superior emitter candidates owing to exceptional surface and transport properties that enable a very high yield of low-energy electrons with narrow energy spread. Importantly, exciting advances are being made in the development of promising UWBG semiconductors such as diamond, cubic boron nitride (c-BN), and aluminum nitride (AlN), as well as UWBG oxides with improved conductivity and crystallinity. These advances are enabled by epitaxial growth techniques that provide control over the electronic properties critical to secondary electron emission, while advanced theoretical tools provide guidance to optimize these properties. Presently, H-terminated diamond offers the greatest opportunity because of its thermally stable negative electron affinity (NEA). In fact, an electron amplifier under development exploits the high yield from this NEA surface, while more robust NEA diamond surfaces are demonstrated with potential for high yields in a range of device applications. Although c-BN and AlN are less mature, they provide opportunities to design novel heterostructures that can enhance the yield further.

Generalized fractional Wentzel–Kramers–Brillouin approximation for electron tunnelling across rough metal interface

The conductive rough surfaces act as an integral part of several electron devices and systems. Electron tunnelling through the potential barrier imposed by the rough metal-vacuum interface is an important mechanism of charge transport in vacuum electron devices. Here, we analytically derive a generalized current–voltage relationship with a fractional image potential barrier that considers the reduced space-dimensionality encountered by the tunnelling electrons at a rough interface, in an effective manner. The traditional Schottky–Nordhiem equation based on the Schottky image potential barrier is shown to be a limiting case of our model for a perfectly flat surface. The fractional-dimension parameter used in this model accounts for the barrier reduction due to the geometrical roughness and it can be determined by fitting our model to a given current–voltage measurement. It is shown that the application of this model could reduce the error between measured current–voltage response and theoretical estimates based on the conventional model. This work provides an analytical framework for efficient design and engineering of quantum tunnelling in practical electron devices.

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.

Vertical Nanoscale Vacuum Channel Triodes Based on the Material System of Vacuum Electronics

Nanoscale vacuum channel triodes realize the vacuum-like transmission of electrons in the atmosphere because the transmission distance is less than the mean free path of electrons in air. This new hybrid device is the deep integration of vacuum electronics technology, micro-nano electronics technology, and optoelectronic technology. It has the advantages of both vacuum and solid-state devices and is considered to be the next generation of vacuum electronic devices. In this work, vertical nanoscale vacuum channel diodes and triodes with edge emission were fabricated using advanced micro-nano processing technology. The device materials were all based on the vacuum electronics material system. The field emission characteristics of the devices were investigated. The diode continued emitting at a bias voltage from 0 to 50 V without failure, and the current variation under different vacuum degrees was better than 2.1%. The field emission characteristics of the devices were evaluated over a wide pressure range of between 10-7 Pa and 105 Pa, and the results could explain the vacuum-like behavior of the devices when operating in air. The current variation of the triode is better than 6.1% at Vg = 8 V and Va = 10 V.

Mini-type low pressure helium detection from adsorption enhanced carbon nanotube field emission

The miniature low pressure helium detection technique is of great interest for some important industrial and scientific fields. A novel low pressure helium sensing method from carbon nanotube (CNT) field emission has been developed. The low field emission currents from multi-walled carbon nanotubes (MWNTs) with low crystallinities increase in helium pressure range of 10−7-10−3 Pa. The field emission analysis and first-principles simulation revealed that work functions of CNT emitters remain consistent with helium adsorptions. The simulation showed that the adsorption of He atom results in extra charge aggregation on carbon atoms at the defect, enhancing the field emission current. The helium sensing MWNT emitter normally doesn’t present good hydrogen, oxygen, and nitrogen sensing properties, benefiting the leak detection applications. The mini-type helium sensor in millimeter scale was demonstrated in various gas backgrounds with leak rates down to 10−8 Pa·l/s, very promising for helium detection and leak checking in wide types of in-situ measurement applications, including vacuum electronic devices, low power consumption instruments, and compact space facilities.

Micro Resistance Spot Welding Optimization for Ultra-Thin Mo-Re Sheet Used in TWT

<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$30~\mu $ </tex-math></inline-formula> m ultra-thin Mo-Re sheet had been joined by micro resistance spot welding (MRSW) and successfully applied to the low heating power cathode of traveling wave tube (TWT). The effects of different welding parameters, surface state and protection conditions on the joint’s appearance, micro-structure and lap shear strength (LSS) were analyzed in this paper. The results showed that a 63.5 N LSS of the MRSW joint was obtained under the following conditions: electrode force <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$F=9.0$ </tex-math></inline-formula> N, welding current <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I=1000\text{A}$ </tex-math></inline-formula> , ramping time <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$t_{1}=5$ </tex-math></inline-formula> ms, welding time <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$t_{2}=2$ </tex-math></inline-formula> ms and dip in the alcohol for protection. The fracture occurs in the heat affected zone (HAZ). Using metallurgical microscopy observed the nugget, metallurgical fusion could be found, the diameter of nugget was about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Phi 0.3$ </tex-math></inline-formula> mm, without gas pores. Scanning electron microscopy (SEM) was used to observe the fracture surface. It could be found some “whisker” shape diverged from the joint center to the surroundings in HAZ. The electrode tip was observed by energy dispersive spectroscopy (EDS) and optical profiler. It was found that the tip was uneven and adhered the base metal obviously after spark. Additional work about electrode sticking will be required. The thermal test data showed that the use of ultra-thin Mo-Re sheet can reduce a space TWT cathode heating power (CHP) by 0.57 W, which will effectively save satellite energy and improve the expected working life of vacuum electronic devices.

An On-Chip Microscale Vacuum Chamber with High Sealing Performance Using Graphene as Lateral Feedthrough.

On-chip microscale vacuum chambers with high sealing performance and electrical feedthroughs are highly desired for microscale vacuum electronic devices and other MEMS devices. In this paper, we report an on-chip microscale vacuum chamber which achieves a high sealing performance by using monolayer graphene as lateral electrical feedthrough. A vacuum chamber with the dimensions of π × 2 mm × 2 mm × 0.5 mm is fabricated by anodically bonding a glass chip with a through-hole between two Si chips in a vacuum, after monolayer graphene electrodes have been transferred to the surface of one of the Si chips. Benefiting from the atomic thickness of monolayer graphene, the leak rate of Si-glass bonding interface with a monolayer graphene feedthrough is measured at less than 2 × 10-11 Pa·m3/s. The monolayer graphene feedthrough exhibits a minor resistance increase from 22.5 Ω to 31 Ω after anodic bonding, showing good electrical conductance. The pressure of the vacuum chamber is estimated to be 185 Pa by measuring the breakdown voltage. Such a vacuum is found to maintain for more than 50 days without obvious degradation, implying a high sealing performance with a leak rate of less than 1.02 × 10-16 Pa·m3/s.

Nonlinear Dynamics of Radiation in Multiple-Beam Vacuum Electronic Devices

The article is devoted to overview of different types of vacuum electronic devices with two or more charged particle beams. There are travelling wave and backward wave tubes, free electron lasers and masers, volume free electron lasers. Two different cases take place in such situation: multiple-beam instability in such devices and multiple-stream instability. In the first case some charged particle beams moves in the system with different velocities. In the second one there are beams with almost equal velocities (streams). Two systems of equations for volume free electron laser with two electron beams are proposed. Some numerical results of VFEL numerical simulation are given and discussed.

Analysis of a Hollow Electron Beam Focusing and Transmission

For vacuum electronic devices (VEDs), the use of a hollow electron beam (HEB) increases the beam-wave interaction efficiency and permits a prominent decrease of the cutoff voltage in the focusing-electrode controlled electron gun. To obtain an HEB electron optics system (EOS) with stable propagation, lower ripple, high compression, and good laminarity, the issues related to the HEBs transmission characteristics in a uniform magnetic field are deeply investigated in this article. The studies on the Brillouin flow, which are different from the conclusion of the previous literature, are presented. Through combining the theoretical analysis and the simulation, the distinct motion ways of each layer in the HEB are clearly exhibited in both cases of the immersed flow and the partially shielding flow. This work can provide a meaningful reference for the design of a hollow beam EOS with high transmission and compression.

Miniature vacuum sensor based on gas adsorptions from carbon nanotube field emitters

A mini-type vacuum sensor based upon current enhancement from gas adsorption for multi-walled carbon nanotube (MWNT) field emitters is developed. The current increase rates in low emission state are proportional to gas pressures in nitrogen, hydrogen, and mixed gas environments. The field emission energy distribution (FEED) measurement demonstrated the reductions of electron tunneling energy barriers in H2 and N2 ambiences. The first-principles simulation indicates that the energy barrier reduction, which is related to the surface energy well from nitrogen adsorption, is attributed to the sensing effect for nitrogen. Both experiments and first-principles simulation affirmed that the high MWNT crystallinity benefits the sensing performance. The miniature sensor in millimeter dimensions based on MWNT field emitter with simple diode configuration can operate in a wide vacuum range from 10−7 to 10−2 Pa. The sensor was assembled inside the X-ray tube to measure the in-situ device vacuum, showing great potentials for vacuum electronic device applications.

Physics-Based Model for Nonuniform Thermionic Electron Emission from Polycrystalline Cathodes

A physics-based model that predicts the emitted current from thermionic cathodes is developed, which accurately spans from the temperature-limited (TL) to full-space-charge-limited (FSCL) regions. Experimental observations of thermionic electron emission demonstrate a smooth transition between TL and FSCL regions of the emitted-current-density-versus-temperature (J-T) (Miram) curve and the emitted-current-density-versus-voltage (J-V) curve. Knowledge of the temperature and shape of the TL-FSCL transition is important in evaluating the thermionic electron-emission performance of cathodes, including predicting the lifetime. However, there are no first-principles physics-based models that predict the smooth TL-FSCL transition region for real thermionic cathodes without applying a priori assumptions or empirical phenomenological equations that are physically difficult to justify. Previous work detailing the nonuniform thermionic emission found that the effects of three-dimensional space charge, patch fields (electrostatic potential nonuniformity on the cathode surface based on local work-function values), and Schottky barrier lowering can lead to a smooth TL-FSCL transition region from a model thermionic cathode surface with a checkerboard spatial distribution of work-function values. In this work, we construct a physics-based nonuniform emission model for commercial dispenser cathodes. This emission model is obtained by incorporating the cathode surface grain orientation via electron-backscatter diffraction and the facet-orientation-specific work-function values from density-functional-theory calculations. The model enables the construction of two-dimensional emitted-current-density maps of the cathode surface and corresponding J-T and J-V curves. The predicted emission curves show excellent agreement with experiment, not only in the TL and FSCL regions but, crucially, also in the TL-FSCL transition region. This model provides a method to predict the thermionic emission from the microstructure of a commercial cathode and improves the understanding of the relationship between thermionic emission and cathode microstructure, which is beneficial for the design of vacuum electronic devices.

Experimental Study on Positronium Detection under Millimeter Waves Generated from Plasma Wakefield Acceleration

Positronium (Ps) is an unstable system created by the temporary combination of electrons and negative electrons, and Ps generation technology under resonance conditions at millimeter waves is emerging as a new research topic. In general, Ps can be observed when an unstable separate state remains after electron and positron pair annihilation, as in positron emission tomography (PET). However, in this study, a plasma wakefield accelerator based on vacuum electronics devices (VEDs) was designed in the ponderomotive force generating electrons and positrons simultaneously using annular relativistic electron beams. It can induce Cherenkov radiation from beam–wave interaction by using dielectric materials. According to the size of dielectric materials, the frequency of oscillation is approximately 203 GHz at the range of millimeter waves. At this time, the output power is about 109 watts-levels. Meanwhile, modes of millimeter waves polarized by a three-stepped axicon lens are used to apply the photoconversion technology. Thus, it is possible to confirm light emission in the form of a light-converted Bessel beam.

Directly tuning the surface morphologies and electron pathway of graphite/diamond composite films for enhanced electron field emission

Cathode electron field emitters are widely used in high-power and high-frequency vacuum electron devices. The development of novel technique to fabricate the emitters with high electron field emission performance has stimulated extensive research. Here we present a unique one-step microwave plasma chemical vapor deposition process to fabricate the graphite/diamond composite films. The gaseous methane is replaced by liquid n-butylamine, which acts as the single reactive carbon source. The components and surface morphologies of the graphite/diamond composite films can be effectively tuned upon changing the deposition temperatures. The vertically oriented graphite/diamond composite film with a graphite-diamond-graphite nanostructure is obtained and shows superior electron field emission performance of low turn-on field (E0 = 4.0 V/µm) and high emission current density (J@8.8 V/µm = 13.0 mA/cm2). Our study reveal that the sharp edges, appropriate distribution density of the nanosheets, as well as good electron pathway from the bulk to the surface are the key ingredients for the electron field emission performance. This work on graphite/diamond composite films provides a new carbon source instead of methane to fabricate the emitters with high electron field emission performance, which may promote the industrial-scale application.