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(Color online) (a) Temporal and spatial evolution of plasma profiles in two-dimensional PIC simulations: initial uniform density (cyan squares), transient electron density n e under the initial preset accumulated negative charge (time 5 ns) (black diamonds),
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The time- and space-dependent optical emissions of nanosecond high-power microwave discharges near a dielectric-air interface have been observed by nanosecond-response four-framing intensified-charged-coupled device cameras. The experimental observations indicate that plasma developed more intensely at the dielectric-air interface than at the free-...
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... electromagnetic PIC simulation uses a finite-difference time domain method and a hybrid plasma- fluid model to compute interactions between the space charge and electromagnetic fields [21]. There is no initial surface charge or space-charge sheath, but only a uniform plasma den- sity n 0 = 10 7 cm −3 , indicated by the cyan curve in Fig. 5(a). The microwave E field of E rf0 = 3 MV/m is normal to the dielectric surface; the cases of the normal component much lower than the tangential E field are studied ...
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... the microwave field, can be treated as stationary during the nanosecond discharge. In contrast, electrons are periodically driven into the surface by the normal component of the microwave field, resulting in lower density close to the dielectric surface. The density difference between the electrons (purple curve) and ions (blue curve), shown in Fig. 5(a), in conjunction with the surface charge, forms a space-charge field normal to the dielectric ...
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... is some nonlinear positive feedback crucial to achieve the ultrafast discharge and the driven factors in positive feedback are the rf sheath field and the normal component of the microwave field. The intense rf sheath field accelerates electrons to a higher energy with a higher ionization rate as shown by the red curve in Fig. 5(a). The high collision rate results in the field energy becoming thermal energy for the electrons. Sampling the time with the maximum E rfz field, the temporal and spatial development of the sheath field profile is illustrated by the red, purple, and blue curves in Fig. ...
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... a higher energy with a higher ionization rate as shown by the red curve in Fig. 5(a). The high collision rate results in the field energy becoming thermal energy for the electrons. Sampling the time with the maximum E rfz field, the temporal and spatial development of the sheath field profile is illustrated by the red, purple, and blue curves in Fig. ...
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... microwave field, both of which drive electrons into the dielectric, charging it negatively until a balance is achieved between the electron temperature and the sheath drop. As positive feedback, the increased density difference between electrons and ions increases the amplitude of the sheath field [from the red to the purple to the blue curve in Fig. 5(b)] and the electrons exhibit locally increased temperature and hence higher ionization and inelastic collisions rates, including excitation of gas atoms to radiative and metastable states; consequently, both the ultraviolet illumination and the visible light emission become brighter, explaining the experimental observation in Fig. ...
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... repetitive pulses. It is found in the PIC simulation that the preset accumulated negative charge increases the sheath field, enhancing the elec- tron energy and ionization and accelerating sheath formation. Consequently, there is a peak of electron density close to the dielectric higher than that of the main plasma, as shown by the black curve in Fig. 5(a). Together with the higher energy in the sheath, the light emission is brighter near the dielectric surface, consistent with the experimental observation in Figs. 3(a) and ...
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... The residual sheath fields continue to drive local ionization and excitation, as the plasma decays on the ion mobility time scale, with the observed contraction of the visible light regions at about 1.6 × 10 6 m/s as the localized electron heating is confined to the near-sheath region. The sheath width collapses as shown by the black curve in Fig. 5(b), consistent with the observation in Fig. ...
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... microwave E field. When the normal component is much lower than the tangential part, i.e., E rfz E rfx,y , E rfz still plays an important role in accelerating the accumulation of negative surface charge and sheath development. For a small tilt angle of E rfz /E rfy = tan(5 • ) and E rfy = 3 MV/m, the total E z field shown as the cyan curve in Fig. 5(b) illustrates a space-charge field established above the dielectric surface. Consequently, the nonlinear space-charge field may exist at the dielectric-air interface for many HPM horn configurations during ultrafast HPM pulses and the mechanism is significantly different from the multipactor discharge [22][23][24][25][26] at the ...
Citations
... Therefore, the multipactor discharge is well described at the beginning of the discharge because the initial density chosen is low enough. This method is also used in the literature by Zhang et al. 43 and Chang et al. 44 Considering the important role of electrostatic interaction in the whole secondary electron multiplication process and the incident electromagnetic wave basically does not change in space, this paper intends to adopt the one-dimensional electrostatic model simulated by PIC to solve the space charge field. The basic equations are as follows: ...
In this work, we investigated the effects of an external magnetic field, a DC electrostatic field, and a normal rf electric field on the multipactor and plasma ionization breakdown process near a microwave window by performing kinetic particle-in-cell/Monte Carlo collision simulations, and the underlying mechanism is also given. The magnetic field, parallel to the surface and perpendicular to the tangential rf field, can effectively suppress the electron multipactor process by delaying the electron incidence on the dielectric window and push the plasma breakdown bulk away from the dielectric window. However, when the magnetic field is too strong, the mitigation effect is not significant, and may even enhance the multipactor process at the beginning of the plasma breakdown. The external DC electrostatic field, perpendicular to the surface, can inhibit electron multipactor when it points toward the surface. On the other hand, when the DC electric field direction is reversed, then the electron multipactor process is found to be promoted, and the gas ionization bulk is closer to the dielectric window. The external normal rf electric fields perpendicular to the surface with small amplitudes are found to be capable of promoting the multipactor process. With increasing the amplitude of normal rf electric field, the multipactor process can be suppressed to some degree at the initial stage of the plasma breakdown and the gas ionization bulk region is kept away from the dielectric window surface.
... Besides the electron avalanche induced under high vacuum conditions, multipactor discharges can also lead to a volume breakdown and the formation of an rf plasma. Initially, most studies were conducted via experimental measurements, focussing on the current, luminosity, soft X-ray emission, transmission and reflected microwave power, as well as spectroscopy of excited atoms that confirms the presence of desorbed gas from the surface [74,75]. Computational models, especially kinetic PIC simulations that can reveal the nonlinear multiphysics discharge dynamics, provided better understanding of the complicated plasma breakdown process in recent years [76][77][78][79]. ...
... The authors also experimentally confirmed that increasing the roughness or changing the profile of a dielectric surface, for example, grooving a flat surface to a corrugated shape, can increase the electric field strength, allowing for transmission of higher power [182]. In nanosecond time scale HPM breakdown, Chang et al. [75,183] investigated the ultrafast discharge in air and observed a thin layer of intense light emission above the dielectric/air interface after the HPM pulse, and it is explained by the formation of a space-charge microwave sheath superimposed on the components of the electromagnetic wave normal to surface, that enhances the local field amplitude and therefore the ionization and excitation. The photoelectrons produced by photons are found to profoundly promote discharge and fast propagation in the air. ...
Recent progress made in the prediction, characterisation, and mitigation of multipactor discharge is reviewed for single‐ and two‐surface geometries. First, an overview of basic concepts including secondary electron emission, electron kinetics under the force law, multipactor susceptibility, and saturation mechanisms is provided, followed by a discussion on multipactor mitigation strategies. These strategies are categorised into two broad areas – mitigation by engineered devices and engineered radio frequency (rf) fields. Each approach is useful in different applications. Recent advances in multipactor physics and engineering during the past decade, such as novel multipactor prediction methods, understanding space charge effects, schemes for controlling multipacting particle trajectories, frequency domain analysis, high frequency effects, and impact on rf signal quality are presented. In addition to vacuum electron multipaction, multipactor‐induced ionization breakdown is also reviewed, and the recent advances are summarised.
... Microwave devices operating with a high-power level and nearvacuum condition are prone to suffer potential risk of multipactor breakdown, 1 which is an electromagnetic phenomenon substantially driven by secondary electron avalanche and ubiquitously seen in microwave tubes, RF antennas and windows, particle accelerators, and space communication payloads. [2][3][4] Recent development of space communication techniques requires microwave systems to achieve a higher power capacity and further device miniaturization, which accordingly exacerbates the risk of multipactor. Because of the catastrophic hazard of multipactor, 5 it must be ensured that no multipactor occurs during practical device operation for engineering applications. ...
Multipactor occurrence essentially depends on the secondary emission property of the surface material, which is, thus, the requisite input for multipactor threshold prediction using the numerical and theoretical approaches. However, secondary emission yield (SEY) deviation in experimental measurements inevitably leads to uncertainty error in multipactor threshold prediction. Therefore, this paper presents a thorough quantitative analysis of multipactor threshold sensitivity to SEY including the effect of the device geometry, the multipactor mode, and the material type. Based on the statistical modeling, multipactor threshold voltages with respect to the SEY variation in critical SEY regions are calculated for both the parallel plates and coaxial lines with different multipactor orders and typical materials. Furthermore, the distribution of electron impact energy is also obtained to elucidate the underlying mechanism for the relevant sensitivity discrepancy. The result reveals that multipactor threshold is generally most sensitive to the energy region below the first crossover energy ( E 1 ), and this is changed to higher energies below the corresponding energy to the SEY maximum ( E m ) with a change in the device geometry, multipactor mode, or coating material. It is also found that the magnitude relation of the threshold sensitivity between different regions is radically determined with the distribution of electron impact energy, and the SEY variation close to E m merely affects the threshold result with a high multipactor order. This research provides useful reference for properly determining the threshold margin from the measurement error of SEY, thus promoting the performance optimization with multipactor prevention in the practical application of microwave devices.
... Microwave modes TE 10 and TE 11 are most often used in experiments. 6,24,25 In this study, the TE 10 microwave mode is used. A microwave electric field E rf = E peak cos (ωt) sin(πx/a + a/2) is applied in the y direction, where a is the size of the microwave window along the y-direction and E peak is the peak value of the microwave electric field (8 MV/m). ...
The high power microwave window breakdown characteristics of N2–SF6 mixtures are investigated with 3D particle-in-cell and Monte Carlo Collisions (PIC-MCC) simulation. The space and density distributions of electrons and ions are obtained. The results show that the threshold of breakdown increases with the ratio of SF6 when E/P is large. However, when E/P is small, the threshold of breakdown in 70% of SF6 and 30% of N2 is greater than that of pure SF6. This phenomenon is also observed in experiments. The theory analyses show that the energy loss of electrons is mainly caused by excitation collisions with N2 when the average energy of electrons Te is less than 6 eV, and is dominated by excitation and ionization collisions with SF6 when Te is greater than 6 eV. When E/P is small, the proportion of low energy electrons is large and Te increases with the ratio of SF6. Therefore, the effective ionization rate first decreases and then increases as the ratio of SF6 increases. Thus, the optimal ratio for improving the insulation properties is 60 ~ 80% SF6 when E/P is small. When E/P is large, the proportion of high energy electrons increases. Therefore, the effective ionization rate and density of electrons decrease as the ratio of SF6 increases. The maximum threshold of breakdown occurs when the ratio of SF6 is 100%.
... This is one of the main factors that limits high-power microwave (HPM) transmission and radiation [3,5]. The range of plasma observed in experiments [6,7] can reach several millimeters for nanosecond pulses, which is much larger than the scale of plasma obtained using the former particle-in-cell and Monte Carlo collision (PIC-MCC) simulation results [8][9][10]. A theoretical model for plasma propagation in vacuum window breakdown was proposed by Wang [11]. ...
... When only collision ionization is considered, as figure 2 shows, the thickness of the plasma extends to 0.3 mm. When the effect of photoionization is considered, the front of plasma in figure 3(a) can reach 3.96 mm, which is in agreement with past experiments [6,7]. At the beginning, the high excited states molecules and photons are produced near the dielectric surface. ...
... Therefore, the speed of plasma propagation decreases slightly, which equals to 0.94×10 6 m s −1 when t>1.25 ns. The light emission observed in experiment is concentrated in a 2 mm layer above the dielectric surface within 2 ns [7]. The speed of plasma propagation in experiment is 1×10 6 m s −1 . ...
The microwave window breakdown due to the plasma formation greatly limits the power handling capability of high-power microwave systems. However, the experimentally-observed fast plasma propagation cannot be explained using previous theory or simulation results. In this paper, the photoionization is considered to investigate the mechanism of microwave window breakdown at the air/dielectric interface by particle-in-cell simulation. The results show that photoelectrons produced by high-speed photons can profoundly promote discharge above the air/dielectric interface. Then a fast plasma formation and propagation occurs. The speed of plasma propagation can reach 1 × 10 ⁶ m s ⁻¹ , which agrees well with experiments. As a result, the transmitting power is attenuated more seriously than the case without the photoionization. Furthermore, the effects of size of microwave window, gas pressure, strength of microwave electric field and distribution of microwave electric field on the plasma propagation are investigated. The results show that the total number of electrons is nonlinearly increasing with the size of microwave window when a uniform microwave electric field is applied. The speed of the plasma propagation exponentially increases with the strength of microwave electric field. Therefore, the photoionization is an indispensable process in the microwave window breakdown with high-strength microwave electric field.
... A long and stable operational lifetime can also be promoted by controlling the strength of the electric field on the anode surface, enlarging the A-K gap, and avoiding window breakdown. The issues that affect the lifetime can be resolved to a certain extent by using innovative cathode materials 101,102 and an overmoded design, [103][104][105] treating the surfaces of high-frequency structures, 106 employing grooved windows, [107][108][109] and fitting advanced cooling systems. Generally speaking, high-impedance (>50 X) HPM sources tend to have longer lifetimes and to be more stable than low-impedance (20 X) HPM sources, which is most likely related to differences in large-area cathode plasma expansion and anode plasma formation. ...
Even after 50 years of development, narrowband high-power microwave (HPM) source technologies remain the focus of much research due to intense interest in innovative applications of HPMs in fields such as directed energy, space propulsion, and high-power radar. A few decades ago, the main aim of investigations in this field was to enhance the output power of a single HPM source to tens or hundreds of gigawatts, but this goal has proven difficult due to physical limitations. Therefore, recent research into HPM sources has focused on five main targets: phase locking and power combination, high power efficiency, compact sources with a low or no external magnetic field, high pulse energy, and high-power millimeter-wave generation. Progress made in these aspects of narrowband HPM sources over the last decade is analyzed and summarized in this paper. There is no single type of HPM source capable of excellent performance in all five aspects. Specifically, high pulse energy cannot be achieved together with high power efficiency. The physical difficulties of high power generation in the millimeter wave band are discussed. Semiconductor-based HPM sources and metamaterial (MTM) vacuum electron devices (VEDs) are also commented on here. Semiconductor devices have the advantage of smart frequency agility, but they have low power density and high cost. MTM VEDs have the potential to be high power efficiency HPM sources in the low frequency band. Moreover, problems relating to narrowband HPM source lifetime and stability, which are the important determinants of the real-world applicability of these sources, are also discussed.
... Gas breakdown by microwaves has been extensively studied [1][2][3][4][5][6]. One motivation of this research is attributed to the ever-growing power of the microwave radiation sources, in which gas breakdown is not desired [7]. Another motivation is due to the potential applications of microwave generated plasmas such as material processing [8]. ...
... The twenty five reactions between electrons and nitrogen molecules including momentum transfer, rotational excitation, vibrational excitation, electronic excitation, and ionization are considered to compute ν i , ν c , ν el , and ν inel in equations (4)- (7). The nitrogen reaction set can refer to table 1 of [26]. ...
... The comparison among prompt, delayed, and total excitation rates can be found in section 3. Therefore, the total excitation cross sections are adopted to calculate these rates ν exc in figure 1. The finite-difference time-domain (FDTD) method is employed to solve numerically equations (1)- (7). Let V r = N e υ r , V z =N e υ z , and e = U N e e e . ...
A two-dimensional model coupling Maxwell’s equations with plasma fluid equations is used to simulate nitrogen breakdown in a microwave field radiated from a circular waveguide. The balance equations of an excited molecule (ion) density, taking into account the production of excited states as well as quenching in radiation and collisional processes, are included in this model to predict the brightness of light produced in the breakdown. The electron energy distribution function (EEDF) obtained from the analytic expression is adopted to compute the rate coefficients in the model, and its effects on the light brightness are considered. The light brightness in the high electric field initially increases sharply over time. After the occurrence of the breakdown, the electron density saturates at a high level, but the local electric field collapses, leading to the decrease in the mean electron energy. In this case, the light brightness decreases in time since the production of excited molecules (ion) is less than its quenching. The plasma produced in nitrogen breakdown modifies the electric field distribution in space, and the light brightness in the enhanced electric field increases because of the enhancement of the electron density and mean electron energy. The difference in the evolutions of light brightness of different wavelengths is discussed. When the gas pressure increases, the critical electron density above which the incident wave is disturbed increases, and the corresponding light brightness for several wavelengths in the range of visible light first increases and then decreases, showing the similar trend as the experiment. The breakdown formation times predicted by the model also show the similar trend as the experiment.
... (a)~(d) 时序为 5 ns, 10 ns, 20 ns和 25 ns [98] Figure 8 Development and evolution of the HPM window surface breakdown. The times are (a) 5 ns, (b) 10 ns, (c) 20 ns, and (d) 25 ns images of visible light in pseudocolor[98] ...
... The electron energy is provided by the two electric field components of microwave to ionize gas in the volume breakdown process. However, in the window breakdown process, an extra electric field is generated by the normal electric field components, and it enhances the original electron energy provided by the microwave electric fields [10], [11]. According to the above-mentioned analysis, the model coupling the equations of electron number density 0093-3813 © 2018 IEEE. ...
In the near-field zone of an antenna or a microwave device, high-power microwave that may contain two orthogonal electric field components frequently causes air breakdown. The electric field power of microwave for heating electrons is deduced from the equation of electron momentum, and is introduced into a model coupling the equations of electron number density and electron energy to investigate air breakdown. Breakdown prediction obtained from the model considerably matches with that of the particle-in-cell Monte Carlo collision model. The amplitude of the mean electron energy changes periodically with the phase difference between the two electric field components with the same amplitude. At a high air pressure, breakdown formation time depends strongly on phase difference, but this dependence decreases as air pressure decreases. The dependence of air breakdown on the amplitude difference between the two electric field components is also unveiled under a fixed electric filed power. The approximate analytical solution of the mean electron energy is presented to analyze our numerical results. IEEE
... Breakdown at the vacuum-dielectric interface is triggered by multipactor and finally realized by plasma avalanche in the ambient desorbed or evaporated gas layer above dielectric. [4][5][6][7][8][9][10]16,17 Suppressing HPM vacuum multipactor and improving the breakdown threshold by periodically patterned profiles have been theoretically and experimentally studied. 14,15 The basic principle of multipactor suppression is to alternate the trajectories, flight times, and impact energies of electrons. ...
The three-dimensional periodic ripple profile with each unit of rotational symmetric surface is proposed to suppress multipactor for arbitrary electromagnetic mode with any polarization. The field distribution and multipactor electron
dynamics on the wavy surface are studied to illustrate the multipactor inhibition mechanism. High power microwave experiment was conducted to demonstrate the effect of wavy surface on significantly improving the window power capacity.
