Power Microwave Devices Research Articles

Electrical activity at the AlN/Si Interface: identifying the main origin of propagation losses in GaN-on-Si devices at microwave frequencies

AlN nucleation layers are the basement of GaN-on-Si structures grown for light-emitting diodes, high frequency telecommunication and power switching systems. In this context, our work aims to understand the origin of propagation losses in GaN-on-Si High Electron Mobility Transistors at microwaves frequencies, which are critical for efficient devices and circuits. AlN/Si structures are grown by Metalorganic Vapor Phase Epitaxy. Acceptor dopant in-diffusion (Al and Ga) into the Si substrate is studied by Secondary Ion Mass Spectroscopy and is mainly located in the first 200 nm beneath the interface. In this region, an acceptor concentration of a few 1018 cm-3 is estimated from Capacitance–Voltage (C–V) measurements while the volume hole concentration of several 1017 cm-3 is deduced from sheet resistance. Furthermore, the combination of scanning capacitance microscopy and scanning spreading resistance microscopy enables the 2D profiling of both the p-type conductive channel and the space charge region beneath the AlN/Si interface. We demonstrate that samples grown at lower temperature exhibit a p-doped conductive channel over a shallower depth which explains lower propagation losses in comparison with those synthesized at higher temperature. Our work highlights that this p-type channel can increase the propagation losses in the high-frequency devices but also that a memory effect associated with the previous sample growths with GaN can noticeably affect the physical properties in absence of proper reactor preparation. Hence, monitoring the acceptor dopant in-diffusion beneath the AlN/Si interface is crucial for achieving efficient GaN-on-Si microwave power devices.

Modes decomposition in particle-in-cell software CEMPIC**Project supported by the fund of the State Key Laboratory of Intense Pulsed Radiation Simulation and Effect (Grant No. SKLIPR1908).

The numerical method of modes analysis and decomposition of the output signal in 3D electromagnetic particle-in-cell simulation is presented. By the method, multiple modes can be resolved at one time using a set of diagnostic data, the amplitudes and the phases of the specified modes can all be given separately. Based on the method, the output signals of one X-band tri-bend mode converter used for one high power microwave device, with ionization process in the device due to the strong normal electric field, are analyzed and decomposed.

Multiple-GPU-Based Simulation of Ka-Band Helix Traveling Wave Tube

The finite-difference time-domain (FDTD) algorithm and the particle-in-cell (PIC) method-based simulation are classic approaches to design and optimize the helix traveling wave tubes (TWTs). In this article, a multiple-graphics processing unit (GPU)-based 3-D-FDTD-PIC parallel program is developed to complete the full-wave simulation of a Ka -band helix TWT in the time domain. The specific parallel simulation scheme is given. The code based on compute unified device architecture (CUDA) and message passing interface (MPI) can run on a scalable heterogeneous cluster consisting of multiple CPUs and GPUs, substantially improving the simulation speed and shortening the development period of TWTs. In this article, the specific parameters of the experimental TWT are given. The simulation results are found basically consistent with the measured values and theoretical analysis, verifying the correctness of this code. Moreover, this program can realistically restore the physical processes occurring in the tube in a short period of time, which means it can be applied as an efficient simulation tool for further research of TWTs or even other microwave power devices based on beam–wave interaction.

Structural, electrical transport and magnetic properties of Nd3+ substituted Mn–Cu nanoferrites

Abstract In present study, a series of spinel ferrite nanoparticles (NPs) with general formula Mn1-xCuxNd0.15Fe1.85O4 (x = 0.0, 0.1, 0.3 and 0.5) have been prepared by sol-gel route. The powder X-ray diffraction (XRD) and Raman spectroscopy revealed the formation of single phase of cubic spinel structure in doped samples. However, a formation of NdFeO3 secondary ortho-ferrite phase has been observed for higher content of x = 0.5. The average crystallite size calculated from Scherrer’s formula has been found in the range 6.5–16.5 nm. The lattice constant (a) estimated using XRD data has been found to increase from 8.4265 A to 8.4501 A on incorporation of Cu2+ ions. FTIR spectra of prepared NPs exhibit two prominent absorption peaks near 600 cm−1 and 400 cm−1 associated to 596 cm−1 and 496 cm−1 band. The dc resistivity measurements with a rise in temperature divulge the typical semiconducting behavior of the as-synthesized samples. The activation energies of the samples have been calculated from Arrhenius plot and found to be in between 0.29 eV and 0.59 eV. Furthermore, the frequency dependent dielectric permittivity, loss tangent and ac conductivity revealed the usual dielectric dispersion. The behavior of dielectric properties highlighted the Debye relaxation peak which is interpreted in the view of Koop’s theory in the light of the Maxwell-Wagner model. Real ( Z ’ ) and imaginary ( Z ’ ’ ) part of impedance confirm the relaxation mechanism associated to the grains and grain boundaries effect. Electron paramagnetic resonance (EPR) pattern shows a broad sharp resonance signal with Lange g-factor varies from 2.04 to 2.18, which is due to dominant spin-exchange interaction between Fe3+, Mn2+, and Cu2+ ions. P-E hysteresis loops found the ferroelectric nature of the ferrite samples. Typical soft ferrimagnetic behavior of Nd3+ substituted Mn–Cu ferrites have been observed at room temperature. The low losses, giant dielectric constant and high resistivity of prepared ferrite NPs can offer several applications like power transformers and microwave devices.

A high-efficiency relativistic magnetron with a novel all-cavity extraction structure

Relativistic magnetrons (RMs) are one of the most promising high power microwave devices. A high-efficiency relativistic magnetron (HERM) with a novel all-cavity extraction structure is presented and investigated theoretically and numerically in this paper. Compared with conventional RMs with an all-cavity extraction structure, the HERM has three improved features. First, a single stair is introduced into the HERM. Second, the angular width of the extraction structure is bigger than that of the anode slow-wave structure. Third, the extraction port is set behind the anode vanes in the axial direction instead of being set between two anode vanes. These improved features can enhance the extraction efficiency of the HERM. The typical simulation results are as follows: high-power microwaves of the transverse electromagnetic mode are generated with a peak power of 1.65 GW, a frequency of 1.61 GHz, and a power efficiency of 67.6%, when the voltage is 586 kV and the current is 4.19 kA. The simulation results verify the validity of the novel all-cavity extraction structure.

Epsilon-negative BaTiO3/Cu composites with high thermal conductivity and yet low electrical conductivity

Abstracts Epsilon-negative materials with high thermal conductivity and low electrical conductivity are of great importance for high power microwave devices. In this work, BaTiO3/Cu composites, as a class of epsilon-negative materials, are rationally designed to achieve a high thermal conductivity yet maintaining the electrical insulative character. Negative permittivity behavior induced by dielectric resonance and plasma oscillation is observed in these BaTiO3/Cu composites, which can be explained by the Lorentz and Drude model respectively. An outstanding absorption ability is achieved near the zero-cross point of the permittivity. Benefiting from the positive temperature coefficient of resistance and the weak temperature dependence of thermal conductivity in BaTiO3/Cu composites, sample containing 22.3 vol% of Cu content exhibits a thermal conductivity of up to 17.7 W/(m·k) and an electrical conductivity down to 0.0022 (Ω cm)−1 at 150 °C. Therefore, BaTiO3/Cu composite is a promising candidate for applications in electromagnetic attenuation and thermal management.

Design and experiment of X-band high-repetition rate high-power multi-beam relativistic klystron amplifier

The relativistic klystron amplifier (RKA) is a very important kind of high power microwave device, which has the advantages of high power, high efficiency, stable output phase and amplitude. The development of multi-injection RKA toward engineering and practical application needs to further improve operating frequency and output power of klystron amplifier, while the RKA of conventional circular waveguide drift tubes is restricted by the physical factors such as geometric size, space charge force and high-voltage breakdown. The multi-beam RKA based on the technology of multiple electron beams can work at low voltage and guiding magnetic field, and can also possess high electron beam current and diversion coefficient. The physical limitation of conventional structure RKA is overcome, and the working frequency and the output power are improved. In the experiment, the X-band GW level power of microwave is obtained. The multi-beam RKA needs to further improve its working frequency and working life. In the experiment, the power conversion efficiency of multi-beam RKA is about 35%, and most of the remaining electron energy will accumulate on the collection pole at the end. If the heat dissipation of the collector is not designed appropriately, the collection will be seriously ablated when working at high heavy frequency. Thus a large quantity of plasma and secondary electrons are generated, which affects the stability of the device. To solve the problem of electron reflux bombarding the output cavity after electron beam exchanging energy in the gap of output cavity, the reflux process of relativistic electron beam in the device is analyzed in this paper. On this basis, a coaxial extraction structure with four-gap extension interaction is designed to avoid electron reflux and reduce the gap electric field, thus improving the working life of the device. At the same time, in order to solve the problem that the conventional water cooling channel can affect the output microwave mode in a high-frequency over-mode device, a mode transformation structure of coaxial TEM mode-fan-shaped TE<sub>10</sub> mode-coaxial TEM mode-circular waveguide TM<sub>01</sub> mode is designed. The mode conversion efficiency is greater than 99.9%, and the influence of collecting polar water cooling channel on the output microwave mode is avoided. The stable operation of multi-beam RKA in the X-band with a repetition rate of 45 Hz is realized experimentally, while the output power is over 1 GW and the microwave pulse width is over 100 ns. At present, the multi-beam RKA runs about 10000 times in total, and the output microwave parameters do not decrease significantly.

A new design of divided solenoid with high homogeneity based on linear programming

A novel divided solenoid with high homogeneity is designed through linear programming (LP) for actual guiding magnet application in high power microwave devices. During the LP process, current density, magnetic field homogeneity, and total volume of coils are set as variable, constraint condition, and objective function, respectively. The divided solenoid is designed after calculating current density in the available zone where the coil current exists. After comparing the electrical parameters and magnetic field distribution of the conventional and the new solenoid, it is concluded that the inhomogeneity of the new design is reduced from 10% to 0.7%, the central magnetic field is equal, and other factors are in the same range. The divided solenoid magnet designed by LP is able to achieve high uniformity by improving the winding process and mechanical structure, which also has strong generalization.

Temporal single-surface multipactor dynamics under obliquely incident linearly polarized electric field

It is well-known that single-surface multipactor discharges can negatively affect electromagnetic wave transmission, especially in high power microwave devices and space-based communication systems. In this work, the temporal single-surface multipactor dynamics under an obliquely incident linearly polarized electric field are investigated by particle-in-cell simulations. The results reveal that multipactor discharges highly decay by adjusting the angle, θ, between the rf electric field and the dielectric window. For plane waves with θ = 0, the number of electrons as a function of time oscillates at twice the rf frequency. However, alternative oscillations decrease in magnitude at θ=0.05π and disappear at θ=0.15π. This is because the perpendicular component of the rf electric field alternatively reinforces or reduces the restoring field and increases or decreases the secondary electron emission in half of the rf period. In addition, the electrons are forced into a few branches in the phase space of velocity and position, i.e., the electrons have a few groups of velocities at a fixed position due to the largely increased electron flight time. Finally, multipactor suppression is investigated by a simple kinetic model, by which the susceptibility diagram is obtained. This diagram shows the upper and lower boundaries gradually approach each other upon increasing θ from 0 to 0.2π, indicating reduced parameter space, in which multipactor discharge can develop.

Surface roughness evolution induced low secondary electron yield in carbon coated Ag/Al substrates for space microwave devices

High power microwave devices have been suffering from secondary electron yield (SEY), which occurs at the surface and leads to increases in energy dissipate and noise level in microwave cavities. The suppression of SEY in microwave materials is significant for space applications. In this work, we report a surface roughness evolution mechanism induced by carbon coating on a widely-used microwave cavity material Ag/Al. The competition between the preferred growth and the island growth modes leads to the roughness enhancement in the 60 nm carbon coated Ag/Al substrates. By extracting the smooth factor Ks in Vaughan model from the angular dependent SEY measurement, we attribute the suppression of SEY to the intrinsic low SEY of the carbon film and the high surface roughness. Our work demonstrates a low-cost, easy-to-scale coating process to effectively prevent the formation of secondary electron avalanche in space borne microwave devices.

A high-efficiency dual-band relativistic Cerenkov oscillator based on dual electron beams

High efficiency and high frequency are key research topics for the development of high power microwave devices. There are, however, serious issues with mode competition in relativistic Cerenkov devices based on a single electron beam, which can lead to low efficiency in the beam-wave interaction process. In this paper, a high-efficiency relativistic Cerenkov oscillator based on a dual electron beam is proposed. The hollow slow wave structure is embedded in the inner conductor of the coaxial waveguide to obtain a double coaxial electromagnetic structure, which can suppress mode competition and improve the beam-wave interaction efficiency of the dual-band radiation. Moreover, each band of radiation is independent of the other, such that high frequency output radiation is generated. Using the particle-in-cell method, the physical processes involving the beam-wave interaction in the oscillator are studied with the aim of designing a high-efficiency double-electromagnetic structure. Simulation results indicated that the output power levels for the X-band and Ka-band were 1.50 GW and 710 MW, respectively, and the efficiencies were 34.7% and 30.7%, respectively, for a diode voltage of 550 kV and a guiding magnetic field of 0.85 T.

Suppression of single-surface multipactor discharges due to non-sinusoidal transverse electric field

It is of importance to suppress single-surface multipactor discharges in high power microwave devices. In this work, both particle-in-cell (PIC) and Monte-Carlo simulations demonstrate that multipactor discharges can be significantly suppressed by a temporal Gaussian-type transverse electric field waveform. Decreasing the half peak width of the Gaussian electric field can reduce the time-averaged positive charge density on the surface, corresponding to the strength of the multipactor, by an order of magnitude at fixed time-averaged input power. The underlying physical mechanism is revealed by examining the electron impact energy and angle distribution in detail, as well as the dynamic secondary electron yield (SEY) from PIC simulation. For the smaller half peak width and fixed average input power, more electrons striking the surface have energies either below the first crossover or higher than the second crossover of the SEY curve, giving rise to weaker secondary electrons emission and finally resulting in a weaker multipactor discharge. In addition, we give the analytical expressions of the frequency spectrum and phase shift needed to recover a Gaussian-type waveform, which is in excellent agreement with numerical calculations.

Transmission properties of microwave in rectangular waveguide through argon plasma**Project supported by the National Natural Science Foundation of China (Grant No. 61331002).

To study the impact of plasma generated by microwave breakdown on the propagation properties of microwave in high power microwave (HPM) devices, a three-dimensional (3-D) fluid model of argon plasma slab in rectangular waveguide is established and calculated by the finite-difference-time-domain (FDTD) method. A rectangular waveguide with a breakdown chamber filled with argon is set as the physics model, and HPM with frequency of 3–50 GHz propagates through this physics model. The time evolutions of the breakdown process are investigated, the reflection, transmission, and absorption coefficients of HPM are calculated, and the influences of some important parameters, including the thickness of the plasma slab and the microwave frequency on the propagation properties of the microwave are shown. Results of this work can offer theoretical instructions for suppressing the influence of breakdown to the performance of HPM devices, and for the use of microwave breakdown, such as the design of plasma limiter or absorber in HPM devices.

Polarization doping technology towards high performance GaN-based heterostructure devices

GaN-based III-V heterostructure devices are promising candidates in future power electronic, microwave and optoelectronic devices, owing to their superior material properties. The development of polarization doping technology in GaN-based material has attracted extensive interests in the recent years, because it allows the graded AlGaN layers to realize high conductivity n/p type bulk doping without introducing donor/acceptor dopants, which would significantly improve the performances of the GaN-based heterostructure devices. This lecture contains two parts: an overview of polarization doping technology and novel polarization-doped GaN-based electron devices. The review of polarization doping technology mainly focuses on its mechanism in realizing the n/p type doping in graded AlGaN layer without impurity dopants, and its unique carrier characteristics induced by polarization doping. The novel polarization-doped GaN-based power devices include the diodes and field effect transistors, which exhibit better performances than the ones with impurity-doped.

Feedback process study in over-sized backward wave oscillator

Internal field emission breakdown in the electro-dynamic structures of high-power microwave devices can seriously limit the devices’ output power and pulse duration. So an over-sized backward wave oscillator (BWO) is developed to increase the diameter of the electro-dynamic structure beyond the cut-off radius, and reduce these internal fields to levels, which are below critical breakdown levels. As a typical high power microwave (HPM) device, the oversized BWO is widely used and investigated. But some interaction phenomena between the beam and the microwave field in the device are not clearly understood because the beam-loaded effect is so obvious. And the physical process for the interaction is also considered to be complicated. Here as an oscillator, the feedback process is very important in the microwave device, which includes the oversized BWO. So the interaction process between the beam and the oversized BWO is explored from the feed back process instead of the field in the device. Then the physical mechanism for the feedback process in the oversized BWO is explored both in theoretical investigation and in particle-in-cell simulation. And the equivalent circuit is established for such a purpose. The mode control mechanism is explored based on the equivalent circuit. Finally an over-sized backward wave oscillator with rectangular profile corrugations is designed to produce TM<sub>01</sub> high power microwave radiation without mode-competition. An RF power of 7.9 GW at a frequency of 8.68 GHz is obtained in the particle in cell simulation driven by the beam with a beam voltage of 1 MW and a current of 20 kA, and the corresponding efficiency is 39.5%.

Study of ring-shaped cathodes for a high power microwave device without guiding magnetic field

The generation of annular relativistic electron beams (AREBs) with a good uniformity and longitudinal consistency needs intensive investigations, especially when no guiding magnetic field is supplied. Without a guiding magnetic field, ring-shaped cathodes with a width of several millimeters were utilized to generate AREB instead of knife-edged cathodes because of the space-charge force. Results of the fast framing optical photograph experiments and high power microwave (HPM) experiments show that the quarter-circular shielding electrode can improve the uniformity of the AREB obviously, and the carbon fiber cathode is validated to be an appropriate choice, which showed better emission characteristics and better performance in generating HPM than the graphite cathode and the polymer velvet cathode.

Coeffect of trapping behaviors on the performance of GaN-based devices * * Project supported by the National Natural Science Foundation of China (Nos. 61604137, 61674130).

Trap-induced current collapse has become one of the critical issues hindering the improvement of GaN-based microwave power devices. It is difficult to study the behavior of each trapping effect separately with the experimental measurement. Transient simulation is a useful technique for analyzing the mechanism of current collapse. In this paper, the coeffect of surface- and bulk-trapping behaviors on the performance of AlGaN/GaN HEMTs is investigated based on the two-dimensional (2D) transient simulation. In addition, the mechanism of trapping effects is analyzed from the aspect of device physics. Two simulation models with different types of traps are used for comparison, and the simulated results reproduced the experimental measured data. It is found that the final steady-state current decreases when both the surface and bulk traps are taken into account in the model. However, contrary to the expectation, the total current collapse is dramatically reduced (e.g. from 18% to 4% for the 90 nm gate-length device). The results suggest that the surface-related current collapse of GaN-based HEMTs may be mitigated in some degree due to the participation of bulk traps with short time constant. The work in this paper will be helpful for further optimization design of material and device structures.

Development and Investigation of a Field Emission Medium for Autocathodes of Mobile Power Microwave Devices

Silicon-diamond heterostructure based field emission media with silicon microtip arrays at the heterointerface were proposed and experimentally studied. The architecture of the heterostructures is optimized for the applications as an active medium for the field emission cathodes of mobile power microwave devices.

Review on high power microwave device with locked frequency and phase

There are many kinds of high power microwave devices.According to the phase and frequency characteristics of the output microwave,they can be divided into the phase and frequency locking high power microwave (HPM) devices and HPM oscillator.Among them,the frequency and phase of HPM devices with locked frequency and phase can be adjusted by the injecting microwave,which has achieved great progress of the HPM research.In this paper,the latest progress of HPM devices with locked frequency and phase which have been developed by the Institute of Applied Electronics,China Academy of Engineering Physics in recent years are reviewed,covering relativistic klystron amplifier (RKA) and relativistic backward-wave oscillator (RBWO) with injection-locked.Aiming at the problems encountered in the research of high power and long pulse RKA,in this paper we briefly introduce the key technologies in design and experiment,including the beam-wave interaction merits,the suppression of multi-frequency oscillation,pulse shortening, high frequency and high power operation,high gain,etc.The performances of RKA,such as power,phase stability and gain,have been improved remarkably.High-power output with stable frequency and phase has been realized by single-annular beam RKA in S-band,whose output power reaches more than 1 GW with a pulse width of 165 ns and phase fluctuation of 18 at a repetitive pulse of 25 Hz/1 s.The high gain RKA also achieves a similar output power and phase stability under the condition of injected microwave power of several kW.In X-band RKA,a structure of coaxial multi-beam has been used to break through the limitations of high frequency and high power capacity,which generates more than 1 GW output power with an input power of 30 kW,the beam-wave conversion efficiency is 34% and the phase fluctuation is 15 with a pulse width of 140 ns.On the basis of an in-depth understanding of RBWO technology, and using the advantages of high efficiency and compact structure,the RBWO research of injection modulated electron beam is proposed and carried out.More than 1 GW output microwave with locked phase is realized by 100 kW seed microwave.These results not only extend scientific and technological research of a large family of HPM devices,but also make it possible for HPM devices to be used in power synthesis,particle acceleration,high-performance radar,etc.

Multipactor susceptibility chart of coaxial transmission lines with stationary statistical modeling

Multipactor breakdown is a detrimental electromagnetic phenomenon caused by resonant secondary electron emissions synchronizing with field oscillation, which frequently takes place in powerful microwave devices and accelerating structures. Regarded as the principal failure mode of space microwave systems, multipactor may cause the performance to degenerate or even hardware operation to deteriorate catastrophically, thus multipactor becomes a major limitation in promoting the further development of space communication technology. Meanwhile, higher power capacity and volume integration accordingly lead to continuously growing multipactor hazard. In order to prevent multipactor from occurring, the accurate predictive technique to determine multipactor susceptibility has become a key issue for the mechanical design and performance optimization of microwave devices in the ground stage. Compared with the existing approaches to investigating the multipactor, statistical theories are able to conduct multipactor threshold calculation and mechanism analysis, with the stochastic nature of secondary emission fully considered from the probabilistic perspective. Currently, stationary statistical theory of multipactor has been developed for efficient multipactor threshold analysis of the parallel-plate geometry. However, it has not been further extended to the coaxial geometry which is commonly involved in radio frequency (RF) systems. For this reason, the stationary statistical modeling of the coaxial multipactor with all influencing factors considered is detailed in this paper. Due to the field nonuniformity and the secondary emission randomness, analytic equation of electron trajectories in the coaxial geometry is approximately derived by using the perturbation approach. Based on the implicit correlation between electron emission velocity and transit time, the joint probability density function is constructed for the calculation of the probability density distribution of electron transit time. Afterwards, a system of integral equations for depicting electron multiplication process in the coaxial geometry is formulated and solved with a novel and general iteration method. Finally, this stationary statistical theory is applied to the full multipactor susceptibility chart of coaxial transmission lines with typical coating materials in space engineering, such as silver, copper, alumina and alodine. A comparison shows that the calculation results are in reasonable agreement with the experimental measurements provided by the Europe Space Agent. What is more, there exists significant difference between multipactor susceptibility curves of the parallel-plate geometry and the coaxial geometry. This research is of great significance for optimizing the mechanism design and material selection of multipactor-free microwave devices.