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Abstract A slow-wave structure improvement for enhancing the 2π-mode electronic efficiency is embodied in the validation of an extended interaction oscillator (EIO), which has an electronic efficiency of 6.52% at 0.22 THz from particle-in-cell (PIC) calculations. A 2π-mode bi-periodic slow-wave structure (BPSWS) with staggered long and short slots is utilized for optimizing the circuit performance. The proposed BPSWS has the capability of combining the respective advantages for both π and 2π-mode in terms of coupling performance and output performance, thus supporting a strongly-coupled 2π-mode with higher coupling capability. Compared with the typical mono-periodic SWS (MPSWS), the adopted strongly-coupled 2π-mode effectively improves the characteristic impedance M2R/Q by 103% to 66.79 Ω, the coupling coefficient by 66% to 0.497, and the normalized wave-amplitude by 22%. Accordingly, 503 W of average output power can be derived for the BPSWS-EIO with a 25.7 kV and 0.3 A sheet beam injected. Cold-test experiments were conducted, confirming that the 0.22 THz structure exhibits commendable fabrication precision and consistency and thus demonstrates the expected frequency response.

A slow-wave structure improvement for enhancing the 2π-mode electronic efficiency is embodied in the validation of an extended interaction oscillator (EIO), which has an electronic efficiency of 6.52% at 0.22 THz from particle-in-cell (PIC) calculations. A 2π-mode bi-periodic slow-wave structure (BPSWS) with staggered long and short slots is utilized for optimizing the circuit performance. The proposed BPSWS has the capability of combining the respective advantages for both π and 2π-mode in terms of coupling performance and output performance, thus supporting a strongly-coupled 2π-mode with higher coupling capability. Compared with the typical mono-periodic SWS (MPSWS), the adopted strongly-coupled 2π-mode effectively improves the characteristic impedance M2R/Q by 103% to 66.79 Ω, the coupling coefficient by 66% to 0.497, and the normalized wave-amplitude by 22%. Accordingly, 503 W of average output power can be derived for the BPSWS-EIO with a 25.7 kV and 0.3 A sheet beam injected. Cold-test experiments were conducted, confirming that the 0.22 THz structure exhibits commendable fabrication precision and consistency and thus demonstrates the expected frequency response. [ABSTRACT FROM AUTHOR]

In space applications, electronics must function while being strongly radiated. In this work, the radiation hardness property of the planar two-terminal nanoscale air channel device (NACD), which is a device of vacuum electronics, was demonstrated. Specifically, the total ionizing dose (TID) effect and single-event effects (SEEs) on the NACD were investigated. X-ray and pulse laser were utilized as the sources of TID radiation and SEE radiation, respectively. No significant degradation was observed after 1 Mrad-(Si) X-ray radiation, and no single-event transient phenomenon was detected at the pulse laser energy up to 5 nJ. Additionally, the physical mechanism of radiation hardness in the NACD was illustrated by theoretical analysis and finite element simulation. Taken together, the proposed work shows that the radiation hardness property of the NACD may have a prospect in deep space and nuclear energy applications. [ABSTRACT FROM AUTHOR]

In this paper, a scheme by optimizing electric field distribution with a two-output structure of the extended interaction oscillator (EIOs) is proposed to permit a significant increase in the efficiency at Ka-band. The distribution of the axial electric field and the synchronous conditions are optimized to make more fast electrons of the entire two-beam EIO circuit fall into the deceleration region and transfer energy to the electric field. Simulation results show that the efficiency is firstly improved from 28 % to 41 % by optimizing the external Q ( Q e ), the beam perveance ( P t ) and the last period. To further increase the efficiency, a peak output power of 254 kW and an efficiency of 50.8 % at 54 kV voltage and 4.6×2 A current is obtained by optimizing the all-period synchronous conditions. The idea of the optimization method provides an effective way to improve the efficiency of multibeam vacuum electronic devices at millimeter-wave and higher frequencies. [ABSTRACT FROM AUTHOR]

This article studies and summarizes the operating characteristics of a ${G}$ -band folded waveguide traveling wave tube (TWT) amplifier enhanced by the $\pi $ -mode extended interaction cavities. Compared with conventional ${G}$ -band TWTs, the proposed amplifier can at once obtain a higher gain in a shorter interaction circuit length and ensure a proper operating bandwidth. The key of the design is introducing the $\pi $ -mode multigap resonant cavity with alternating wide and narrow slots, which will improve the working performance of the whole device by shortening the length of the interaction circuit, enhancing its interaction effect, and improving its gain of unit length. In this design, the operation bandwidth is expanded by stagger tuning technique. In addition, the influence of cavity loss is examined so that the optimal performance is achieved. The PIC simulation results show that when the operating voltage is 21 kV and the operating current 80 mA, the maximum average output power of the designed TWT amplifier is 65.78 W at 217.4 GHz, which is corresponding to the maximum gain and efficiency of 37.38 dB and 3.9%, respectively. The gain per unit length is 12.64 dB/cm and the 3-dB bandwidth is 3.5 GHz. [ABSTRACT FROM AUTHOR]

Introduction/purpose: A fundamental theoretical explanation is given for the fact that in subnanosecond vacuum diodes there exists a group of electrons with kinetic energies much higher than the applied voltage (multiplied by the value of the elementary charge) qUmax. Methods: A mathematical method is used based on the numerical solution of the Vlasov-Poisson differential equations system for one-dimensional vacuum diodes of various designs. Results: It is shown in detail that the so-called "anomalous" electrons appear in the transient time domain characterizing the processes of establishing current flow in vacuum diodes. Conclusion: It has been convincingly shown that the presence of “anomalous” electrons is not associated with either the diode design or the presence of additional current carriers. In vacuum diodes with a subnanosecond leading edge of the voltage pulse, the excess of energy over qUmax can be over 20%.

A super klystron-like relativistic backward wave oscillator (RBWO) is presented. In the device, the distributed Cherenkov radiation generated in the slow wave structure (SWS) is absorbed by the electron bunch in the last corrugation and re-accelerated, creating high-energy electrons that overcome the space-charge repulsion force to further compress the bunch length and maximize the fundamental harmonic current, contributing to the extremely efficient microwave generation in the extraction cavity through concentrated transition radiation. A novel elliptical extraction cavity is used to improve the power capacity. A soft magnet is inserted to precisely control the beam path, which provides a higher beam kinetic energy to be extracted and a stronger electric field that interacts with the tighter bunch. In the experiment, as the diode voltage was 585 kV and diode current was 7.5 kA, a microwave pulse with a power of 2.9 ± 0.3 GW measured in the radiation field, a frequency of 8.23 GHz, and a pulse duration of 22 ns was obtained. The corresponding efficiency is up to 66 ± 8%, which is an experimental record for GW-class relativistic vacuum electronic devices. [ABSTRACT FROM AUTHOR]

chanism of compact electron bunching has been revealed by numerical modeling and described. The pattern of long-lived compact electron bunches continuously moving along the surface of the clinotron comb-type slow-wave structure holds owing to the continuous deposition of compact electron bunches on the comb surface. The compact bunches are formed in electron beam layers farthest from the comb surface but moving to it in consequence of the electron beam inclination under magnetic field action (the clinotron effect). The bunch size and its dependence on the problem parameters have been specified. The research results find use in analysis of operational regimes of conventional clinotrons. Clinotron designers are provided with relevant recommendations. Conclusion. A model of electron bunching in clinotron-type oscillators has been developed with regard to the process of deposition of electron beam layers on the comb-type slow-wave structure surface. It is the existence of a system of compact electron bunches continuously moving along the comb surface that makes possible efficient electron beam interaction with the high-frequency electromagnetic field of multiplied-mode oscillations. We justifiably claim that multipliers of the kind can be advanced to the short-wave end of the submillimeter band.

In this article, a scheme with a designed overmoded multigap circuit based on the TM13 mode and linearly distributed two electron beams is proposed for power enhancement of sub-THz extended interaction oscillators (EIOs). The analysis of the electromagnetic characteristics shows that the designed circuit can reproduce the resonant frequency, field distribution, and ohmic loss of the conventional multigap circuit with a single beam. When considering the same beam power injected into the designed and conventional circuit, the analysis of the beam loading shows the same effect on the circuit oscillation through the comparison in the electron conductance between two circuits. To examine the performance of the designed circuit, the EIOs are designed with two circuits at 0.22 THz. The analysis of the coupling coefficients between two circuits, combined with pic simulations, shows that the designed circuit produces the efficient interaction with two beams and, most importantly, two beam–wave interaction units can promote and achieve each other. The EIO with the designed circuit can deliver kW-level power at 0.22 THz. Accordingly, the proposed scheme has shown great potential in enhancing the power of sub-THz EIOs. [ABSTRACT FROM AUTHOR]

High power microwave (HPM) sources without guiding magnetic field have the advantages of compactness and miniaturization. However, the efficiency is generally limited due to the poor beam quality and low beam transmission rate. Here, we demonstrate a scheme that actually makes use of the large oscillation of electrons in the absence of a magnetic field. In a novel radial three-cavity transit-time oscillator (TTO) with two output ports, about half of the diode current is deposited on the walls of the first two cavities. On the one hand, the truncated electrons are with lower energy, which has delivered some of their kinetic energy to the standing-wave field in the first two cavities. Other electrons then gain energy from the standing-wave field and are accelerated to a much higher beam voltage than the diode voltage. On the other hand, after part electrons are truncated by the first two cavities, the remained electrons are characterized with smaller energy spread, higher current modulation coefficient, which results in a very efficient energy exchange between the beam current and the electromagnetic field in the third cavity with two extraction gaps. Particle-in-cell (PIC) simulation shows that when the diode voltage is 496 kV, beam current is 43.4 kA, the total output microwave powers are 11.4 GW, corresponding to an unprecedented beam-wave conversion efficiency of 53%. [ABSTRACT FROM AUTHOR]

This letter presents the design and experimental study of a $W$ -band traveling wave tube (TWT) with an overall efficiency of over 40% using multistage depressed collectors (MDCs). The folded waveguide (FWG) with a high coupling impedance and phase velocity tapering (PVT) circuit has been used to achieve the electronic efficiency of over 10%. Furthermore, a three-stage depressed collector with a high recovery efficiency was designed to enhance the overall efficiency in $W$ -band pulsed TWTs. A prototype tube was then fabricated and tested with a beam current of 193 mA and a beam voltage of 20.8 kV. The testing data shows that the overall efficiency of 40.6% was obtained at over 410 W output power within 5 GHz at 20% duty cycle, with the maximum efficiency of 47%. That is the highest efficiency test results of $W$ -band TWTs reported so far. [ABSTRACT FROM AUTHOR]

The backward wave oscillator (BWO) is one of the best choices for generating high-power radiation waves in the terahertz (THz) region. In order to improve the efficiency of beam–wave interaction and output power, a novel THz BWO scheme is proposed in this article, which uses double-slot embedded electron beams (DSEEB) to drive a grating loaded rectangular waveguide (GLRW) slow wave structure (SWS). The dispersion relation and the coupling impedance of the proposed SWS are derived by using an eigenfunction method (EFM) and solved with a numerical method. The results show that the upper limit frequency and the coupling impedances are remarkably improved, which are greater than those of traditional grating SWS. Based on this scheme, a BWO operating at the frequency of 0.5 THz is designed. The particle-in-cell simulations show that the output power reaches 8.86 W with an efficiency of 0.9%, which is 2.5 times the electronic efficiency of the conventional BWO. Thus, the proposed scheme provides a promising aim to develop the high-power THz source, which may be used in THz biomedical and material studies, and so on. [ABSTRACT FROM AUTHOR]

An effective overmoded circuit is proposed to support efficient interaction with two beams for the development of high power extended interaction oscillators (EIOs) toward sub-terahertz (THz) and higher frequency. The circuit design focuses on the effect of the frequency-determining parameter on mode characteristics of the $\mathrm{TM}_{{1} {n}}$ sequence. Frequency equivalence between $\mathrm{TM}_{{1} {n}}$ modes with different ${n}$ supports the mode selection for two-beam interaction. Accordingly, the TM13 mode is selected to duplicate the dispersion relation around the axial mode of $2\pi $ and ohmic loss of conventional TM11 mode used in conventional single-beam circuits. On this basis, the overmoded circuit is formed with two beams based on the TM13 mode. In order to verify its effectiveness, a 0.22-THz EIO is designed with the overmoded circuit and, most importantly, the effect of the uniformity of two beams on the EIO performance is simulated. When the current of one beam is assumed to be constant and that of another beam is increased, the power is increased linearly and then tends to be saturation. This shows good capability of the EIO in interacting with nonuniform two beams, which is critical for its stable operation. Simulation results have shown the achievement of the average power of 0.22-THz wave over 1.26 kW with double beams at 23 kV and each current of 0.2 A. [ABSTRACT FROM AUTHOR]

An improved topology of Extended interaction oscillators (EIO) is presented with enhanced efficiency at 0.35 THz. A bi-periodic ladder-type structure is used for optimizing the performance of the TM $_{{31}}- 2\pi $ mode operation, based on bi-periodic interaction gaps composed of staggered short and long slots. The bi-periodic interaction mechanism permits to operate the circuit with a standing-wave field between the $\pi $ and $2\pi $ modes, thus providing the potential to combine the advantages of both $\pi $ and $2\pi $ modes in terms of both coupling and output performance. The resulting transverse TM31 operating mechanism exhibits a good agreement with the bi-periodic structure in terms of coupling performance, with the short slot length positively correlated with the value of effective characteristic impedance ${M}^{\,{2}}{R}/{Q}$. The circuit demonstrates attractive coupling and output characteristics by optimization of ${M}^{\,{2}}{R}/{Q}$ and ${Q}_{e}$. A state-of-the-art value ${M}^{\,{2}}{R}/{Q}$ of 59.69 $\Omega $ and a moderate ${Q}_{e}$ of 2523.52 are achieved for a proposed eight-period ladder-type EIO. An RF power of 540 W is obtained with a 42.5 kV, 0.6 A sheet beam, with 2.1% electronic efficiency at 0.35 THz. [ABSTRACT FROM AUTHOR]

A complementary vacuum field emission device structure is proposed, and its operation is analyzed by multiphysics simulation. A freely moving double-clamped cantilever, which balances the electrostatic attraction force and elastic restoration force, is used as the source electrode while the drain electrode is fixed. The electron-emitting cathode is formed on the source for the n-type device and on the drain for the p-type. Thus, complementary current–voltage characteristics are obtained only with Fowler–Nordheim tunneling electron transport. The impact of various design parameters such as vacuum gap, cantilever beam thickness, gate width, and tip offset on the drain and gate leakage currents is investigated. Inverter logic operation is verified successfully using complementary devices. [ABSTRACT FROM AUTHOR]

In this work, exploitation of artificial intelligence algorithms for the design of forward-wave amplifier is proposed and discussed. The multilayer perceptron (MLP), a quintessential example of deep learning, is employed to get the cost function, i.e., the relationship between the geometrical parameter set and dispersion characterization from only a small proportion of the parameter sets. Due to the structural complexity, there did not exist a closed-form solution and the neural network shows its superiority compared to the time-consuming numerical calculation. Subsequently, the genetic algorithm (GA) is used to find the parameter sets best matching the goals that broadband beam–wave interaction at specific voltages and even specific phases or frequencies, from the huge 4-D parameter-set space. The intuitive goals can be easily accessible, as long as the goals are within the ability of the parameter-set space and converting the goals into the goal functions for GA with some attempts and adjustments. As a practical example, the recently proposed defect photonic crystal waveguide is utilized to demonstrate the design pattern. Through almost full-automatic design with little judgment and analysis, a response of 14-GHz 3-dB bandwidth and 25-dB gain at the 168.5-GHz central frequency has been realized, indicating the effectiveness of the design pattern. This design pattern will ease researchers from numerous simulation work and help to get deeper insight of the structure. [ABSTRACT FROM AUTHOR]

In vacuum electronics, the influence of the uniformity of electron beams on the performance of two- or multi-beam devices is a key scientific problem in the development of compact, high power millimeter-wave and terahertz (THz) sources. In this letter, we focus on this problem in extended interaction oscillators (EIOs) and propose a tractable extended resonant circuit that has no requirement for the uniformity of beams. The circuit design builds up a TM13 mode mechanism to support two nonuniform beams, based on maintaining the same electromagnetic characteristics of conventional resonant circuits used in single-beam EIOs. To verify this idea, we designed and simulated a 0.22-THz EIO with the proposed circuit. As the current of one beam is I1 and that of another beam is increased from 0 to 4I1, the EIO has shown the behavior of linearly increasing the output power and then becoming saturated through particle-in-cell (PIC) simulations. The ideal case is the currents of the two beams are the same at the same voltage to drive the EIO. This case can reduce the high current density required for start oscillation in single-beam EIOs and obtain high power. Simulation results show that the power of 0.22-THz wave over 1.32 kW is obtained using double beams at 22.2 kV and each current of 0.25 A. [ABSTRACT FROM AUTHOR]

A free-electron-driven multi-frequency terahertz (THz) radiation based on a super-grating structure is elucidated in this paper. The super-grating, i.e., periodically depth-modulated metallic grating, has a peculiar dispersion characteristic, similar to the energy bands in a crystal due to the Brillouin zone folding effect. The multi-frequency radiation is stimulated in several directions with the excitation of a free electron as the synchronization points are in the radiative region. The radiation frequency can be independently tuned by the groove depths of the super-grating. The number of frequencies is tailored by the modulated period. Additionally, the multi-frequency THz radiation exhibits a frequency-locked effect during the energy variation of the free electron. Moreover, the radiation field intensity shows a significant enhancement compared with that of a conventional Smith-Purcell radiation. The work is promising for developing efficient on-chip THz radiation sources and boosts advanced THz applications such as communications, multi-frequency imaging, and beam diagnostics, etc.

For extended interaction structure devices (EIDs), the determinants of field flatness are analyzed. It is found that the flatness of field distribution is determined by a cutoff condition existing in EIDs, which is based on the concepts of both resonant cavity and wave propagation. Such an analytical conclusion is verified by a novel scheme for the measurement of field distribution in an extended interaction oscillator (EIO) based on the perturbation technique. The sinusoidal-like axial field components with different flatness are obtained by utilizing the measured frequency shifts. Based on the cutoff condition, three field distribution patterns are established, including middle-concentrated, flat-distributed, and side-concentrated types. Particle-in-cell (PIC) simulations of a W-band EIO show that three field distribution patterns have the advantages of low surface-loss power, high power-capacity, and low oscillating-threshold, respectively. [ABSTRACT FROM AUTHOR]

We investigate by molecular dynamics simulations (Torfason et al., 2015; 2016) a beam of electrons released via field emission from a planar cathode surface of 1 μm2 with an inhomogeneous two-level work function, φlow and φhigh. A rectangular grid, where each cell can have one out of two values of the work function, is used as a model. The number of cells in the grid ranges from 6 × 6 to 96 × 96. We compare a periodic checkerboard arrangement with disordered distributions of patches. We perform multiple simulations and randomize the pattern each time. We study the beam behavior by calculating the position and velocity of each electron, r.m.s. emittance, and the brightness of the electron beam. The emittance increases while brightness decreases, with the mean distance between patches with φlow when they are in minority, and they switch the behavior versus the mean distance between patches with φhigh when these patches are in minority, respectively. The Coulomb interaction between all particles is fully included in our simulations. [ABSTRACT FROM AUTHOR]

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Introduction/purpose: A fundamental theoretical explanation is given for the fact that in subnanosecond vacuum diodes there exists a group of electrons with kinetic energies much higher than the applied voltage (multiplied by the value of the elementary charge) qUmax. Methods: A mathematical method is used based on the numerical solution of the Vlasov-Poisson differential equations system for one-dimensional vacuum diodes of various designs. Results: It is shown in detail that the so-called "anomalous" electrons appear in the transient time domain characterizing the processes of establishing current flow in vacuum diodes. Conclusion: It has been convincingly shown that the presence of “anomalous” electrons is not associated with either the diode design or the presence of additional current carriers. In vacuum diodes with a subnanosecond leading edge of the voltage pulse, the excess of energy over qUmax can be over 20%. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Traveling wave tubes are rapidly evolving to provide unprecedented power level in comparison to solid state devices in the millimeter waves region of the spectrum (80–300 GHz) thus enabling a wide range of applications. Wireless communications, imaging, plasma diagnostics, healthcare and many others will gain substantial features if high power at millimeter waves would be available from compact sources. The development of fabrication technologies is proving crucial for introducing new topologies and structures for millimeter wave vacuum electronic devices, compatible with the dimensions dictated by the short wavelength that poses substantial manufacturing challenges due to tight tolerances. This review paper will provide an overview of the principles, evolution and state of the art of one of the most widely utilized vacuum electronic device, the traveling wave tube (TWT). The wide band, high gain features of TWTs make those devices the most promising solutions for high power at millimeter waves and THz frequencies. [ABSTRACT FROM AUTHOR]

We simulate numerically the classical charge dynamics in a microscopic, planar, vacuum diode with a finite emitter area and a finite number of electrons in the gap. We assume that electrons are emitted under space-charge limited conditions with a fixed potential applied to the diode. The Coulomb interaction between all electrons is included using the method of molecular dynamics. We compare our results to the conventional 2-D Child–Langmuir and explain how it is limited in applicability for submicrometer diameter emitters. Finally, we offer some simple relations for understanding space-charge limited flow from very small emitters. [ABSTRACT FROM AUTHOR]

The phase stability of a 140-GHz, 1-kW pulsed gyro-amplifier system and the phase dependence on the cathode voltage were experimentally measured. To optimize the measurement precision, the amplifier was operated at 47 kV and 1 A, where the output power was ∼30 W. The phase was determined to be stable both pulse-to-pulse and during each pulse, so far as the cathode voltage and electron beam current are constant. The phase variation with voltage was measured and found to be 130 ± 30°/kV, in excellent agreement with simulations. The electron gun used in this device is non-adiabatic, resulting in a steep slope of the beam pitch factor with respect to cathode voltage. This was discovered to be the dominant factor in the phase dependence on voltage. The use of an adiabatic electron gun is predicted to yield a significantly smaller phase sensitivity to voltage, and thus a more phase-stable performance. To our knowledge, these are the first phase measurements reported for a gyro-amplifier operating at a frequency above W-band. [ABSTRACT FROM AUTHOR]

Influence of the density of mode spectrum in the gyrotron cavity on the possibility of stable single-mode operation has been investigated for high-order modes. Interaction of the operating mode with its two neighboring satellites, symmetrically spaced in frequency, is analyzed. Some threshold value of mode spectrum density dependent on the beam current is found. [ABSTRACT FROM AUTHOR]

Purpose. Preparation of brief scientific and technical review about sources, retrospective view, basic stages, achievements, problems, trends and prospects of development of world electronics for period of 20th-21st centuries. Methodology. Known scientific methods of collection, analysis and analytical treatment of the opened scientific and technical information of world level in area of a vacuum electronics, semiconductor electronics, vacuum microelectronics and nanoelectronics, and also optical electronics. Results. A brief analytical scientific and technical review is resulted about the primary and present states, achievements, trends and prospects of development of electronics in the developed countries of the world. From positions of approach of the systems advantages and lacks of semiconductor microelectronics are described as compared to a vacuum electronics. Considerable progress is marked in development of semiconductor element base (microtransistors, microprocessors, microcontrollers and other) for creation of different electronic devices and computing engineering. Information, touching the determining deposit of electronic companies of the «Silicon Valley» in the USA in providing of technological breach in area of modern microelectronics, production of computers and their microprocessors, software and creation of devices of mobile communication development, is resulted. The basic problems of space microelectronics are affected. The possible ways of further development are indicated in the world of electronics, including vacuum microelectronics and nanoelectronics, and also optical electronics. The special attention is turned for urgent development of high-efficiency protective facilities specialists against cyber attacks hackers on the computer systems. In a review an accent is done on the sharp necessity of acceptance the proper services of drastic measures for a fight with cyber terrorism. Originality. Systematization of the scientific and technical materials touching world history of development and creation of modern element base of a vacuum electronics and semiconductor microelectronics known from the sources opened in outer informative space is executed. Possible new perspective directions of development in the modern world of microelectronics are formulated. Practical value. Popularization and deepening for students, engineer and technical| specialists and research workers of front-rank scientific and technical knowledge in the topical area of development, creation and application in the modern technique of the different setting of high-computer-integrated electronic devices, extending their scientific range of interests and further development of scientific and technical progress in society.

In this article, a $3\pi $ band-edge dual frequency oscillator using a novel half-period staggered folded waveguide (FWG) is proposed. Simulation results from CST Particle Studio show that the half-period staggered FWG could form a stopband at $3\pi $ phase shift with a high coupling impedance, enabling the device operating at two frequencies. A prototype oscillator was then designed and fabricated. The experimental results showed that it was capable of operating at both 89.2 and 94.5 GHz with the maximum output power 148 and 65W, respectively. The testing results are also compared with the simulation. [ABSTRACT FROM AUTHOR]

The recirculating planar crossed-field amplifier (RPCFA) was designed, constructed, and tested at the University of Michigan. The RPCFA was driven by a number of RF sources ranging in frequency from 2.40 to 3.05 GHz and powers of 1 to 800 kW. Pulsed voltage was delivered to the cathode by the Michigan electron long beam accelerator with ceramic insulator (MELBA-C) which was configured to supply pulses of −300 kV, 1–10 kA, with 0.3–1.0- $\mu \text{s}$ pulse lengths. The RPCFA demonstrated zero-drive stability and a bandwidth of 15%. Amplification of microwave signals, at the design frequency of 3 GHz, below 150 kW, was observed with a mean gain of 7.87 dB and high variability, $\sigma = 2.74$ dB. Filtering this data set to only include shots with identical voltage and current profiles yielded a gain of 6.6 ± 1.6 dB. The mean gain increased to 8.71 dB and the variability decreased to $\sigma =0.63$ dB when the injected microwave power increased beyond 150 kW. Peak output powers of nearly 6 MW were achieved with RF breakdown limiting the maximum output power of the device. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations, with full Coulomb interaction and self-consistent field emission, are used to examine mutual space–charge interactions between beams originating from several emitter areas, in a planar infinite diode. The simulations allow observation of the trajectory of each individual electron through the diode gap. Results show that when the center-to-center spacing between emitters is greater than half of the gap spacing the emitters are essentially independent. For smaller spacing the mutual space–charge effect increases rapidly and should not be discounted. A simple qualitative explanation for this effect is given. [ABSTRACT FROM AUTHOR]

We review progress in kinetic plasma modeling by electromagnetic particle-in-cell (EM-PIC) algorithms on unstructured grids. These algorithms are implemented in modular CONPIC and BORPIC C++ codes that integrate a matrix-free explicit finite-element (FE) Maxwell solver based on a parallel sparse-approximate inverse (SPAI) algorithm and a first-principles charge-conserving scatter algorithm to transfer the charged particle information into dynamic variables on the grid. The Maxwell solver of the EM-PIC algorithm utilizes a mixed FE basis and discretizes the time-dependent coupled first-order Maxwell’s system explicitly. The explicit solver approximates the inverse FE system matrix (“mass” matrix) using hierarchical sparsity patterns based on the sparsity pattern of the original matrix. The resulting algorithm effectively accounts for multiscale plasma phenomena. We discuss the application of the developed EM-PIC algorithm to the analysis of laboratory plasmas, vacuum electronic devices for generation of high-power microwave signals, and RF electronics multipactor effects and apply the algorithm to the analysis of multipactor effects and its mitigation in coaxial cables. [ABSTRACT FROM AUTHOR]

This paper is devoted to the study of collisionless multicomponent plasma expansion in vacuum discharges. Based on the fundamental principles of physical kinetics formulated for vacuum discharge plasma, an answer is given to the following question: What is the main mechanism of cathode plasma transport from cathode to anode, which ensures non-thermal metallic positive ion movement? Theoretical modeling is provided based on the Vlasov–Poisson system of equations for a current flow in a planar vacuum discharge gap. It was shown that the non-thermal plasma expansion is of a purely electrodynamic nature, caused by the formation of a “potential hump” in the interelectrode space and its subsequent movement under certain conditions consistent with plasma electrodynamic transportation. The presented results reveal two cases of the described phenomenon: (1) the dynamics of single-component cathode plasma and (2) multicomponent plasma (consisting of multiple charged ions) expansion.

To simulate traveling-wave tubes (TWTs) in time domain and, more generally, the wave–particle interaction in vacuum devices, we developed the DIscrete MOdel with HAmiltonian approach (DIMOHA) model as an alternative to current particle-in-cell (PIC) and frequency approaches. Indeed, it is based on a longitudinal ${N}$ -body Hamiltonian approach satisfying Maxwell’s equations. Advantages of DIMOHA comprise: 1) it allows arbitrary waveform (not just field envelope), including continuous waveform (CW), multiple carriers, or digital modulations (shift keying); 2) the algorithm is much faster than PIC codes, thanks to a field discretization allowing a drastic degree-of-freedom reduction, along with a robust symplectic integrator; 3) it supports any periodic slow wave structure (SWS) design such as a helix or folded waveguides; 4) it reproduces harmonic generation, reflection, oscillation, and distortion phenomena; and 5) it handles nonlinear dynamics, including intermodulations, trapping, and chaos. DIMOHA accuracy is assessed by comparing it against measurements from a commercial ${Ku}$ -band tapered helix TWT and against simulations from a subterahertz folded-waveguide TWT with a staggered double-grating SWS. The algorithm is also tested for multiple carriers simulations with success. [ABSTRACT FROM AUTHOR]

This paper describes the design, fabrication, and initial testing of a Ka-band dielectric-loaded traveling-wave tube (TWT) with very high bandwidth. Applications for high-power mm-wave sources span radar (including imaging), biology, medicine, communications, national security, and other areas. Specifically, to achieve long-range, cm-scale imaging using synthetic aperture radar techniques, a radio-frequency (RF) source with an average power level of 1 kW and a bandwidth of 10 GHz will be required. We developed a novel Ka-band TWT architecture that approaches these requirements. To achieve a very wide bandwidth, we proposed to use a dielectric-lined waveguide slow-wave structure. A dielectric constant of larger than 13 is needed for the resonance with a 20-keV electron beam. In our design, we have used $\varepsilon = 20$ magnesium–calcium–titanate (MCT) ceramics. The two halves of the dielectric are shaped to ensure that the TM11-like mode has a flat electric field profile along the beam slot to accommodate transport of a 5-A sheet electron beam. The gain for the structure peaks at 33.25 GHz and is predicted to be 2.3 dB/cm with a total gain of 30 dB. The structure was fabricated and cold tested. Although the results of the cold test were inconclusive, we discuss possible reasons for discrepancies between simulations and measurements and propose simplifications to the tube’s geometry for future studies. [ABSTRACT FROM AUTHOR]

Thermionic electron emitter, a critical component in vacuum electronic devices, usually suffers from bulky size, difficulty to integrate, and slow temporal response. Here, to overcome these problems, we fabricate and test a new miniature electron emitter: on-chip thermionic electron emitter arrays based on horizontally aligned single-walled carbon nanotubes. The devices are fabricated and integrated on a Si chip with microfabrication technologies, providing compactness as well as fast temporal response. Emitter arrays with a packing density of up to $\sim \textsf {0.4}\times 10^{7}$ emitters/cm2 and an emission density of 1.73 mA/cm2 at a bias voltage of 2.64 V are realized. The turn-on/off response times of the devices are measured to be ~150/500 ns, which are 5 orders of magnitude faster than those of their bulky counterparts. The advantages of on-chip nature, high degree of integration, and fast temporal response make the thermionic emitter arrays promising in miniature vacuum electronic devices. [ABSTRACT FROM AUTHOR]

A stable high-order mode version of a ladder-type RF circuit (ladder cavity) is proposed to overcome the frequency limits of the commonly used fundamental mode counterpart for the development of millimeter-wave and higher frequency extended interaction oscillators (EIOs). The specific design for the stable operated circuit and its analysis based on the mode characteristic is carried out. The study of the transverse mode shows the potential of the TM31 mode in establishing more sufficient axial electric field than the fundamental TM11 version in a large size cavity. To enable the TM31 mode to overwhelm other possible competing modes in the circuit impedance, the field distribution of the TM31 mode is optimized in design through the analysis of transverse parameters. Key features that ensure stable operation are: 1) the large mode separation and nonoverlapping oscillation regions embodied in beam-loading conductance between related modes due to the specific design and 2) the large difference in the start-oscillation currents of the TM31 and TM11 modes based on the theoretical analysis. To demonstrate the circuit capability, a $W$ -band EIO with the proposed circuit has been designed. Simulations with a 3-D particle-in-cell code in CHIPIC predict a maximum output power over 10.6 kW around 93.1 GHz was obtained with a 20.5-kV and 8-A electron beam. The design provides a promising approach for making a stable high-order mode RF circuit applied in high-frequency EIOs. [ABSTRACT FROM AUTHOR]

Two samples of S-band BAC multiple-beam BT258 klystrons were developed, fabricated, and tested at DBT with the support of CERN. A maximum efficiency of 66% was obtained at an output level of 6.6 MW. The 60% efficiency for one of multiple beam klystrons was successively confirmed in dynamic tests at CERN. The BAC method of electron grouping is discussed and the output parameters of multiple-beam BAC-klystron are compared with the existing single-beam klystrons. The possibility of increasing the output power by the BAC method of electron grouping of S-band multiple-beam klystron at an output power level of 10 and 20 MW is discussed. [ABSTRACT FROM AUTHOR]

In this paper, we present the current status and both the ongoing investigation and the continuous improvements to the operational performance of a unique gyrotron, which is built using a 20 T superconducting magnet and holds a world record of 10-W terahertz wave generation at the highest frequency (1.08 THz) in the continuous wave (CW) operation. In addition, it has demonstrated high-purity single-mode generation on a sequence of modes that cover a wide range from subterahertz to terahertz frequencies at both the fundamental and second-harmonic resonances of the electron cyclotron frequency. As an illustration, the measurements of the observed radiation patterns of eight output modes radiated from a currently used resonant cavity with a linear up-taper are presented and compared with the corresponding patterns simulated by scattering matrix calculations. A new design of an optimized cavity with a nonlinear up-taper, which further improves the mode purity of the generated output radiation has been proposed and is being currently implemented as a replacement of the existing resonator. The overall operational performance and the output characteristics of this gyrotron (called FU CW III in accordance with the nomenclature adopted at FIR UF Center) make it a versatile and appropriate source of coherent CW radiation for many novel applications in the fields of high-power terahertz science and technologies. [ABSTRACT FROM AUTHOR]

Future nanoelectronics will require a high packing density of semiconductor devices so that electrons can be transported at speeds approaching their ultimate limit while operating at low power. Herein, we employ high-voltage pulses to create a nanoscale defect-void channel in an oxide to conduct an out-of-plane electron current from 2-D electron gas in a p-Si substrate to a metal electrode of metal–oxide–semiconductor structure. With this approach, we exploit the advantage of changes in the SiO2/Si structure after oxide breakdown and carrier transport properties to increase device performance. The electrical characteristics of the two-terminal device indicate ballistic electron transport in the nanochannel, providing potential energy-efficient and high-speed (in the picosecond regime) applications for computing systems and logic circuits with low-cost fabrication. Space-charge-limited current models offer physical insights into the formation of nanochannels containing a combination of free space and dielectric solid. Hot carrier injection in the nanochannel also provides an understanding of the defect physics via atomic luminescence and localized surface topography, as well as substantial insights via optical emission spectroscopy and atomic force microscopy into how defects rearrange themselves locally when their electronic state changes. [ABSTRACT FROM AUTHOR]

A W-band extended interaction oscillator (EIO) operating at two different types of $2\pi $ modes is presented. The modes and output characteristics were studied, and the circuit was optimized to ensure similar and efficient outputs of two standing-wave modes. For a cylindrical electron beam radius of 0.2 mm, voltage of 14 kV, and current of 0.5 A, particle-in-cell simulation predicts 616 W at 93.76 GHz for the $2\pi $ gap mode and 367 W at 94.22 GHz for the $2\pi $ cavity mode. In this paper, numerical simulations demonstrate the ability of the EIO circuit to be operated with two standing-wave modes in the millimeter-wave range. [ABSTRACT FROM AUTHOR]

Nuclear fusion energy is perhaps one of the most demanding challenges that the scientific community is facing. Unfortunately, the plasma is affected by microturbulence, which is still not fully understood, but which can degrade plasma confinement. The 0.346-THz backward-wave oscillator (BWO) is the enabling device for a high-k plasma collective scattering diagnostic that will provide unprecedented insight on turbulence thereby contributing to the realization of fully operational fusion reactors. This paper describes the final fabrication phase of the 0.346-THz BWO for the collective scattering diagnostic jointly performed in an international project, involving three leading institutions in vacuum electronics. The advancements in technology will open the route to new families of terahertz vacuum electron devices to enable new terahertz applications and provide industry with new advanced processes. [ABSTRACT FROM AUTHOR]

This paper presents experimental results that demonstrate the first known instance of harmonic frequency locking in a magnetron. The prototype crossed-field high-power microwave source, termed the multifrequency recirculating planar magnetron, consisted of two planar cavity arrays having design frequencies near 1 and 2 GHz, respectively, coupled by smooth-bore electron beam recirculation bends. The magnetron was driven by the MELBA-C modified Marx generator using −300-kV pulses supplying up to 3 kA for approximately 300 ns. Using a 0.17-T axial magnetic field, the harmonic frequency-locked state simultaneously produced microwave pulses of 32± 3 MW at 0.984± 0.001 GHz and 13±2 MW at 1.970±0.002 GHz. In addition, changes in the relative phase difference between the magnetron slow wave structures (SWSs) were correlated with changes in the axial magnetic field magnitude. This correlation, along with the results from experiments that tested each individual planar cavity array in isolation, suggested the locking was facilitated through the second harmonic content of the 1-GHz modulated electron beam recirculating from the 1-to 2-GHz SWS. Analysis of shots conducted near 0.17 T showed a ±17° shot-to-shot variation in the average relative phase difference between the SWSs, and the average variation in this phase difference during a locked shot was ±8°. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present a computationally efficient method for the calculation of impedance matrices of a large class of standing- and traveling-wave structures used in klystrons, traveling-wave tubes, and other vacuum electronic (VE) devices. By treating the gaps as “lumped ports,” this method avoids the time and disk spaces consuming steps employed in , which required postprocessing the electromagnetic fields from a finite-element (FE) simulation. By applying analytical transformations to the impedance matrix, we show that it may be cast in a form suitable for the large-signal 1-D and 2-D beam-wave interaction codes CHRISTINE-CC , and TESLA-Z. These transformations are formulated in a way to avoid numerical errors near structure resonance frequencies (singularity points). We also derive and apply formulas by which the impedance matrix of a structure may be constructed from the impedance matrices of its component parts, without additional FE simulations; by inverting the algebraic operations, we show that the impedance matrix of a structure from which a selected subsection has been removed may also be computed. Application of these formulas greatly facilitates the accurate calculation of the impedance matrices of large complex circuits that are difficult or impossible to model whole in a single FE simulation. [ABSTRACT FROM AUTHOR]