Microwave Breakdown Research Articles

Theoretical and Experimental Study on Transient Electrical Behavior of Coaxial Bandpass Filter With Low-Pressure Microwave Breakdown

Theoretical and Experimental Study on Transient Electrical Behavior of Coaxial Bandpass Filter With Low-Pressure Microwave Breakdown

Effect of desorbed gas on microwave breakdown on vacuum side of dielectric window

The gas desorbed from the dielectric surface has a great influence on the characteristics of microwave breakdown on the vacuum side of the dielectric window. In this paper, the dielectric surface breakdown is described by using the electromagnetic particle-in-cell-Monte Carlo collision (PIC-MCC) model. The process of desorption of gas and its influence on the breakdown characteristics are studied. The simulation results show that, due to the accumulation of desorbed gas, the pressure near the dielectric surface increases in time, and the breakdown mechanism transitions from secondary electron multipactor to collision ionization. More and more electrons generated by collision ionization drift to the dielectric surface, so that the amplitude of self-organized normal electric field increases in time and sometimes points to the dielectric surface. Nevertheless, the number of secondary electrons emitted in each microwave cycle is approximately equal to the number of primary electrons. In the early and middle stages of breakdown, the attenuation of the microwave electric field near the dielectric surface is very small. However, the collision ionization causes a sharp increase in the number density of electrons, and the microwave electric field decays rapidly in the later stage of breakdown. Compared with the electromagnetic PIC-MCC simulation results, the mean energy and number of electrons obtained by the electrostatic PIC-MCC model are overestimated in the later stage of breakdown because it does not take into account the attenuation of microwave electric field. The pressure of the desorbed gas predicted by the electromagnetic PIC-MCC model is close to the measured value, when the number of gas atoms desorbed by an incident electron is taken as 0.4.

A combined theoretical, computational, and experimental investigation to evaluate performance of suitable thermocouple for microwave processing

Accurate temperature measurement during any microwave processing of material is a contentious issue. While thermocouples are routinely utilized in conventional heat furnaces, their presence in microwave cavities may cause complications such as localized distortion of electromagnetic field, heat dissipation, thermal instabilities, microwave breakdown, and significant inaccuracies in temperature measurement. Therefore, in the present investigation thermocouple effects are discussed by providing the results of temperature estimation by various thermocouples for microwave cladding systems. The present study theoretically and experimentally explored the impact of multiple thermocouples on temperature measurements during microwave heating. COMSOL simulation models were developed to explore microwave and thermocouple interaction during microwave heating. Simulation models were used to assess the effects of the microwave power intensity, thermocouple orientation, and geometry on temperature measurement. The investigation outcomes can be utilized as a guide for selecting appropriate thermocouples in the applications of microwave cladding and other microwave-based manufacturing processes.

Three-dimensional modeling of microwave discharges in a waveguide-based plasma source with experimental comparison.

This paper proposes a transient three-dimensional model to simulate microwave-induced discharges in a waveguide-based plasma source under intermediate pressures. A plane-symmetric simplification method is applied to simplify half of the microwave plasma source in the calculation domain, dramatically reducing the demand for computational resources and calculation time. Meanwhile, the numerical simulations remain in three dimensions without dimensionality reduction, which allows us to directly calculate the efficiency of power coupling from the incident microwave to the plasma. Besides, the computation decrease improves the convergence performance of the mathematical model, making it possible to model the entire discharge process from 1×10^{-9} to 1×10^{4}s. This period covers the instantaneous microwave breakdown to the formation of a stable plasma column near steady state. The results have revealed the electromagnetic waveguide structure change of the microwave plasma source during the discharge process. Several microwave power-coupling efficiencies of the waveguide-based plasma source with different thicknesses and permittivities of the glass tube are calculated and compared with the experimentally measured data. Furthermore, the effects of the glass tube on the electromagnetic modes of the traveling microwave propagating along the plasma column and the discharge properties are also investigated. The numerically obtained results generally agree with the theoretical analysis and the experimental data in our previous studies, demonstrating the validity of the proposed mathematical model and the plane-symmetric simplification method.

Particle-in-cell simulations of the ionization process in microwave argon microplasmas

The importance of microwave device reliability and performance for microscale devices motivates a more fundamental understanding of breakdown mechanisms in this regime. Microwave breakdown theories predict breakdown when electron production balances electron loss. Electron production depends strongly on the ionization rate νi; however, previous studies either used the measured νi in macroscale gaps or the empirical formula for DC voltage, inaccurately predicting νi in microscale gaps. Alternatively, this work characterizes νi in microwave microplasmas by using particle-in-cell simulations. We calculated νi in argon gas at atmospheric pressure for 2–10 μm gaps under AC fields ranging from 1 to 1000 GHz. The behavior of νi may be separated into two regimes by defining a critical frequency fcr that depends on the amplitude of the applied voltage, gap distance, and pressure. For frequency f<fcr, the electrodes collect the electrons during each cycle and the electron number oscillates with the electric field, causing νi/f to roughly scale with the reduced effective field Eeff/p. For f>fcr, the phase-space plots indicate that the electrons are confined inside the gap, causing the electron number to grow exponentially and vi/p to become a function of Eeff/p. These results elucidate the ionization mechanism for AC fields at microscale gap distances and may be incorporated into field emission-driven microwave breakdown theories to improve their predictive capability.

Efficient Dynamic Mesh Refinement Technique for Simulation of HPM Breakdown-Induced Plasma Pattern Formation

Numerical simulation of the complex plasma dynamics associated with high power, high-frequency microwave breakdown at high pressures, leading to the formation of filamentary plasma structures such as self-organized plasma arrays, is a computationally challenging problem. The widely used 2-D electromagnetic (EM)–plasma fluid model, which accurately captures the experimental observations, requires a runtime of several days to months to simulate standard problems due to stringent numerical requirements in terms of cell size and time step. This article presents a self-aware mesh refinement (MR) algorithm that uses a coarse mesh and a fine mesh that dynamically expand based on the plasma profile topology to resolve the sharp gradients in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E$ </tex-math></inline-formula> -fields and plasma density in the breakdown region. The dynamic MR (DMR) technique is explained in detail, and its performance has been evaluated using a standard benchmark microwave breakdown problem. We observe a speedup of 8 (of the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$O(r^{3})$ </tex-math></inline-formula> , when the refinement factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$r$ </tex-math></inline-formula> ) is 2) compared with a traditional single uniform fine-mesh-based simulation. The technique is scalable and performs better when the problem size increases. We also present a comprehensive spatio-temporal visual analysis to explain the complex physics of high-power microwave (HPM) breakdown, leading to self-organized plasma filaments as an application of the DMR technique.

Heterogeneous CPU-GPU Accelerated Subgridding in the FDTD Modelling of Microwave Breakdown

Microwave breakdown is crucial to the transmission of high-power microwave (HPM) devices, where a growing number of studies have analyzed the complex interactions between electromagnetic waves and the evolving plasma from theoretical and analytical perspectives. In this paper, we propose a finite-difference time-domain (FDTD) scheme to numerically solve Maxwell’s equation, coupled with a fluid plasma equation for simulating the plasma formation during HPM air breakdown. A subgridding method is adopted to obtain accurate results with lower computational resources. Moreover, the three-dimensional subgridding Maxwell–plasma algorithm is efficiently accelerated by utilizing heterogeneous computing technique based on graphics processing units (GPUs) and multiple central processing units (CPUs), which can be applied as an efficient method for the investigation of the HPM air breakdown phenomena.

An Electromagnetic-Plasma Fluid Model Simulation of Waveguide Plasma Limiter Filled With Different Easily Ionized Inert Gas

To protect electronic systems against high-power microwave (HPM), a plasma waveguide limiter is presented. In response to the above phenomenon, a self-consistent 3-D multiphysics electromagnetic-plasma fluid model coupling full-wave Maxwell’s equations with plasma fluid equations is established to describe the operating mechanism of a 3-D simplified sandwich structure plasma limiter filled with four easily ionized inert gases. The plasma limiter system is analyzed with the spectral-element time-domain (SETD) method. The choice of filling easily ionizable inert gas, influence of the number of layers, and pressure on the plasma limiter are discussed. Numerical results demonstrate that the gas with low critical breakdown field strength is more suitable for protecting HPMs. In the case of 20 torr, the order of microwave breakdown is xenon, argon, neon, and helium, and the two-layer plasma limiter (plasma–slab–plasma) has better protective characteristics than a one-layer plasma limiter (slab–plasma–slab) with the same length of gas chamber. Our research can provide theoretical guidance for designers and give the complete physical physics process.

Microwave breakdown of sub-atmospheric argon gas in the vicinity of a microgap

The microwave electric field at breakdown for argon gas in a microgap (generally, gap size &amp;lt;1 mm) over a wide range of pressures and gap sizes was found to exhibit two behavioral pressure regimes. A high microwave electric field was established in the gap of a re-entrant mode resonator. By slowly raising the microwave electric field level, a breakdown level is reached where impact ionization induces low pressure argon gas to breakdown preferentially outside of the gap. At high pressure, breakdown is found to occur preferentially inside the gap, but only after the pressure is high enough for impact ionization to produce breakdown there. Impact ionization has a lower threshold electric field than the boundary-controlled processes, such as secondary electron emission, that would eventually produce breakdown inside the microgap if impact ionization did not occur first. Thus, the lower pressure regime is characterized by breakdown outside of the gap, while in the high-pressure regime, breakdown occurs inside the gap, in both cases by impact ionization. The transition pressure PT between these two pressure regimes depends on the gap size dgap, roughly as PT ∝ dgap−2. Consistent with the model developed here, this corresponds to the full-amplitude of electron oscillation in the gap field being commensurate with the gap size.

Plasma-based microwave power limitation in a printed transmission line: a self-consistent model compared with experimental data

Plasma-based microwave power limitation in a suspended microstrip transmission line integrating a micro hollow cathode discharge (MHCD) in its centre is experimentally and numerically studied. Transient and steady state microwave power measurements exhibit a limitation threshold of 28 dBm and time responses of 25 microseconds. Intensified charge-coupled device imaging shows that microwave breakdown occurs at the top of the MHCD. The plasma then extends towards the microwave source within the suspended microstrip transmission line. Besides, a self-consistent model is proposed to simulate the non-linear interaction between microwave and plasma. It gives numerical results in agreement with the measurements, and show that the plasma expansion during the transient response is related to a shift between the ionization source term and the electron density maximum. The propagation speed, under the tested conditions, depends mainly on the stepwise ionization from the excited states.

Effects of gas adsorbed on solid surface during gas breakdown in electron cyclotron resonance discharges

The microwave breakdown power (P wb) in an ECR plasma source was not merely determined by pressure (gas flow rate), but found to vary with the time interval between two successive breakdowns. The measured P wb dropped rapidly from a high value at a short time interval to a low level at a long time interval. The obtained dependence of P wb on pressure (gas flow rate) exhibited distinct features: the normal monotonicity and abnormal non-monotonicity at the short and long time intervals, respectively. The effective zone in the antenna’s surface, bombarded by hot electrons heated in the ECR layer, was validated by (1) masking the antenna with a film having a variable radius; (2) calculating the distribution of the vertical component of the microwave electric field with respect to the static magnetic field; and (3) imaging glows of transient breakdown discharges with a fast camera. The reduction in P wb was mainly attributed to the enhanced emission of δ-electrons from the gas-adsorbed antenna under the bombardment of energetic electrons coming from the ECR layer. The correlation between the dynamic gas coverage and the emission coefficient of δ-electrons was established to understand the abnormal ECR breakdown features.

Experimental Investigation of Material and Geometry Effects on Microwave Breakdown of Evanescent-Mode Cavity Resonators

This article presents an experimental investigation of microwave breakdown in evanescent-mode cavity resonators with different surface materials and post geometries, aiming at unveiling the role of secondary electron emission (SEE), attachment, and diffusion on microwave breakdown in inhomogeneous electric fields. The breakdown power of six cavities resonating at 2.6 GHz was measured at pressures ranging from 1 to 100 mbar in ambient air. The cavity with lower secondary emission yield surface material has a higher breakdown power on the left side of the breakdown curves, indicating the importance of SEE in microwave breakdown, especially at low pressures. Meanwhile, except for the cavity with hemisphere post, it was found that the quality factor has little effect on the breakdown power since electric field distributions inside these cavities are not significantly linked to post geometry. Two distinct regimes are observed in the breakdown curves with a transition pressure of 10 mbar. Namely, the breakdown curves present an emission-free branch above the transition, where microwave breakdown occurs in the gap, and a diffusion branch below the transition, where the microwave plasma expands out of the gap. We performed a theoretical calculation and found that the transition occurs when the attachment length equals the post radius.

Numerical simulation of multi-carrier microwave breakdown in air-SF6 mixtures

In this paper, the multi-carrier microwave breakdown in air-SF6 mixtures is analyzed with the spectral-element time-domain method. In this process, a three-dimensional multi-physics model coupling Maxwell's equations with electron fluid equations is established. The tail-erosion and frequency shift phenomenon caused by multi-carrier microwave breakdown in different proportions of air-SF6 mixtures can be observed. Numerical results demonstrate that multi-carriers will make the wireless communication system sensitive to microwave breakdown. On the other hand, the different proportions of air-SF6 mixtures can obviously improve the breakdown threshold to inhibit the multi-carrier microwave breakdown, which is favorable to the propagation of multi-carrier microwaves in air. Meanwhile, increasing the pressure can suppress the frequency shift phenomenon, and the proportion of different SF6 in the mixed gas has little effect on the frequency shift of the transmitted wave. Our research provides theoretical guidance for comprehensively exploring the characteristics and physical mechanism of multi-carrier microwave breakdown in air-SF6 mixtures and is helpful for protection of microwave devices and plasma application.

Microwave breakdown in an overmoded relativistic backward wave oscillator operating at low magnetic field

Microwave breakdown has always been a huge challenge to the development of high-power microwave (HPM) sources. Some unique breakdown phenomena in a novel and powerful overmoded relativistic backward wave oscillator (RBWO) operating at low magnetic field are demonstrated. Three different breakdown mechanisms are utilized to explain these phenomena through detailed electromagnetic field calculation and particle-in-cell demonstration and effective methods are applied or suggested to mitigate the breakdown. The breakdown in the slow wave structure (SWS) mainly results from bombardment by the main electron beam under the intense radial electric field. Increasing the span between the main electron beam and the SWS or applying a coaxial extraction structure operating at coaxial TM01 mode might decrease the radial electric field and lessen the bombardment. The breakdown in the internal reflector originates with the field-induced emission in the inner ring under the intense axial electric field of the TM01 and TM02 mode and the subsequent electron-triggered emission in the outer ring. Removing the central part or constructing a complex reflector surface can suppress the emission. The breakdown in the slot retained for the Rogowski coil results from low pressure gas discharge initiated by microwave leakage from the RBWO into the diode region. Pasting microwave absorbing material into the coaxial diode region helps to obtain normal beam current waveforms measured by the Rogowski coil.

Propagation of microwave breakdown in argon induced by a 28 GHz gyrotron beam

An atmospheric argon discharge plasma was induced by a high-power microwave beam using a 28 GHz gyrotron and investigated at pressures of 40 kPa–100 kPa and Gaussian peak intensities of 0.115 GW/m2 (0.204 MV/cm) and 0.168 GW/m2 (0.246 MV/cm). According to high-speed imaging results, the propagation velocity of the discharge front increased with the backpressure to maintain a range of 600 m/s–1000 m/s. The propagation velocity was 25 times larger in argon than in air. Applying the collisional-radiative (CR) model to the spectroscopic results showed that the electron density increases with the pressure from 5.0×1021 m−3 at 40 kPa to 5.0×1022 m−3 at 100 kPa and lies on the critical density curve. The electron temperature decreases as the background pressure increases from 2 eV to 0.5 eV, and the gas temperature was 300–400 K. According to CR analysis, the population densities of 3dn and 2sn excitation levels in the fast velocity condition are much lower than that in any other conditions. The results revealed that the energy transfer from electrons to ionized particles is more remarkable in the high background pressure and the fast velocity condition.

Monte Carlo simulation of microwave air breakdown in parallel plates considering electron-surface interaction

In this paper, a Monte Carlo simulation method was proposed to investigate the initial cascade process of low-pressure microwave air breakdown. Calculations were performed by tracing electrons only, with the assumption that space charge was negligible. Two main electron generation mechanisms, electron-impact ionization and surface secondary electron emission (SEE), were introduced to simulate the initial breakdown phase more realistically. Detailed investigations were performed in a parallel plate waveguide operating at 9.4 GHz with pressures ranging from 0.2 Torr to 100 Torr. The results confirm that microwave breakdown is controlled by impact ionization at high pressures. However, as the gas pressure decreases, secondary electron emissions prevail against electron-neutral collisions, which leads to the sensitivity of the breakdown field on SEE. The threshold breakdown electric field obtained by the Monte Carlo method has been compared with experimental results, showing good agreement in the case where an appropriate secondary electron emission model is given. The developed Monte Carlo method provides an inexpensive way to determine the breakdown field and can be further applied to other microwave components of interest.

Transient Analysis of High-Power Microwave Air Breakdown under External DC Magnetic Field

This article concentrates on the high-power microwave (HPM) breakdown process and behaviors in air-filled rectangular waveguide under the external dc magnetic field. To solve the persuasive issue, a three-dimensional (3-D) multiphysics model coupling full-wave Maxwell’s equations with plasma fluid equations is established and solved by the spectral-element time-domain (SETD) method. In our simulation, the Galerkin’s method is employed for the space discretization and the central difference scheme is used for the temporal discretization during the SETD process. The numerical results reveal that the external dc magnetic field applied in breakdown volume can significantly enhance the breakdown threshold to suppress the HPM breakdown, which is beneficial for the propagation of HPM pulse in air-filled waveguide. Our research provides theoretical guidance for suppressing the impact of HPM breakdown for HPM devices protection.

Space and time evolution of high-power microwave breakdown on the atmosphere side of the dielectric surface

The high-power microwave breakdown on the atmosphere side of the dielectric surface leads to the formation of the thin plasma layer. A two-dimensional model coupling Maxwell’s equations with quasi-neutral plasma fluid equations is used to study the breakdown evolution. We concentrate on the breakdown caused by the incident electric field parallel to the dielectric surface. The results show that the electric field enhancement at the tips of plasmoid in the direction parallel to the dielectric surface accelerates the propagation of the plasma front, while the propagation speed in the perpendicular direction decreases in time since the upstream plasma collapses the local electric field. The difference between the two speeds is responsible for the formation of the thin plasma layer. The effect of air pressure on the breakdown evolution is discussed. The breakdown delay time from simulations as a function of pressure shows the same trend as the experiment.

Influence of secondary electron emission characteristic of dielectric materials on microwave breakdown

For a microwave device filled with dielectrics, the secondary electron (SE) emission has a very important influence on the mechanism of microwave breakdown including low pressure discharge and multipactor. In this work, the SE yields (SEYs) and the SE energy spectra of seven kinds of dielectric materials are first measured and then used to examine their effects. In the positive charging process under electron irradiation, the surface potential of the dielectric layer trends to be steady with the SEY being one. Based on the measurement data, the steady surface potential is calculated under the charging stability condition. The steady surface potential is bigger for a bigger SEY. For a given SEY, the steady surface potential is found to be proportional to the peak energy &lt;i&gt;E&lt;/i&gt;&lt;sub&gt;peak&lt;/sub&gt; of the SE energy spectrum. Furthermore, the effect of steady surface potential on low pressure discharge and multipactor are respectively studied for a parallel plate system filled with a dielectric layer. A static electric field related to the positive charging is introduced. The electron diffusion model in low pressure discharge process is modified by considering the static electric field. The electrons drift in a fixed direction under the action of static electric field, and the electron diffusion length decreases. Consequently, the effective electrons for low discharge decreases and the threshold microwave power increases. Therefore, a dielectric material with higher SEY and bigger &lt;i&gt;E&lt;/i&gt;&lt;sub&gt;peak&lt;/sub&gt; is helpful in suspending the inhibition of low pressure discharge. Furthermore, the effect of steady electric field on multipactor is also explored. Two effects related to dielectric material and metal are analyzed in detail. The SE emission from dielectric material is held back by the steady electric field and some low energy electrons return back to the dielectric materials. The effective SEY thus decreases. On the other hand, the electric field reduces the landing electron energy on the metal, and the corresponding SEY also decreases. The electron oscillation condition with considering both microwave field and stead electric field is derived and the threshold values for microwave power of multipactor are calculated. The susceptibility curves corresponding to different materials are plotted. Our result may be used to choose the filling dielectric materials for a microwave device.

Transient Electric Field Shaping With the Linear Combination of Configuration Field Method for Enhanced Spatial Control of Microwave Plasmas

The demonstration of enhanced spatial control of nanosecond microwave plasmas generated by the time reversal plasma source is presented in this paper. This new microwave plasma source relies on the spatio-temporal control of the electric field inside an all-metal plasma reactor by modifying the waveform of a high power microwave signal. More specifically, it originally used the spatio-temporal focusing capabilities of the time reversal method to focus a high electric field in a small location. However, a parasitic microwave breakdown can still occur at sharp corners or wedges inside the cavity due to the local enhancement of the residual electric field during time reversal focusing. Thus, it is proposed to use the linear combination of configuration field method to improve field control inside the reactor. Its transient electric field shaping capabilities turn out to be a good candidate for the development of a low pressure microwave “plasma brush”.