Secondary Electron Avalanche Research Articles

Particle traps constructed on the insulator of conductive slip rings for surface flashover mitigation under particle adhesion

The generation of abrasive particles is an unavoidable consequence of sliding electrical contact wear in conductive slip rings (SRs). The adhesion of abrasive particles to the insulators may lead to a decrease in flashover voltage, posing a risk to satellite power transmission. In this paper, the effect of abrasive particles on flashover is first studied. Surface abrasive particles can distort the surface electric field of the dielectric, absorb scattered electrons, and then re-emit them, thereby accelerating the development and formation of secondary electron avalanches. Flashover test results indicate that surface abrasive particles lead to a significant reduction in flashover voltage. To mitigate the impact of particle adhesion on flashover, a method of constructing particle traps on the surface of insulators is proposed. The location and structural parameters of the particle trap are further optimized and determined. The flashover test results using planar dielectric samples and SR insulator samples both demonstrate that the optimized particle trap can significantly improve the flashover voltage. The dielectric maintains high electrical strength even when particles are trapped in particle traps. The physical details of the impact of particles on flashover and the effect of particle traps are analyzed utilizing an electron movement simulation, corroborating the experiment from a microscopic aspect.

Simulating the secondary electron avalanche of MCP by Geant4

Nowadays, Microchannel Plate (MCP), as a kind of electron multipliers based on the secondary electron emission, is widely used in many high-sensitive experiments, such as neutrino detection, which require the noise to be as low as possible, while the conventional straight-channel MCP will inevitably have ion feedback, resulting in the sequential after-pulses being the major source of noise. Normally, the problem can be effectively avoided by coupling two straight-channel MCPs in cascade and combining the channels into a “V” shape known as chevron MCPs, but this method is limited by the manufacturing techniques due to the unavoidable gap between the two pieces that will worsen the resolution and peak-to-valley ratio. However, the ion feedback can be inhibited significantly for MCPs with curved channels. Based on Geant4, we investigate how the geometrical parameters of curved-channel MCP influence the gain and get the optimum pore diameter for an MCP to reach the maximum gain with fixed thickness and applied voltage. Additionally, the track-by-track simulation reveals that the average acceleration distance of a secondary electron inside the curved-channel is approximately 20μm when the applied voltage, length-to-diameter ratio and pore diameter are 950 V, 50:1 and 20μm, respectively.

Research on ionization characteristics of atmospheric pressure pulse-modulated microwave He/air plasma jet

The atmospheric pressure pulsed microwave He plasma jet has the advantages of high electron density and abundant active particles, but its shrinking on the discharge electrode morphology limits its application range. In order to modulate a He plasma jet with a longer plume and study its ionization development characteristics, we constructed a dual-channel pulsed microwave coaxial discharge device. He and air were, respectively, injected into the inner and outer gas channels of the resonator to generate a double-layer atmospheric pressure microwave plasma jet with a longer plume. It is observed that the bifurcation of the stratified plasma jet will occur by changing the gas flow. The ionization development of plasma jet was observed by using enhanced charge-coupled device and microwave Rayleigh scattering apparatus measured the space-time evolution of plasma and observed the three times ionization enhancement process of plasma jet development. The spectral lines of the active products associated with Penning ionization were observed by using a fiber optic spectrometer. A fluid model was constructed to simulate and analyze that under the condition of sufficient He flow rate (He flow rate is above 0.6 slm), there will be sufficient and stable He mole fraction (64%) at the stratification of the plasma jet. The experimental and simulation results show that the jet profile of the microwave He plasma is related to the inlet structure of the discharger and He flow rate. Stratified intake structure can produce stratified He plasma jet, and the unique appearance of bifurcation of jet can be produced by changing the flow rate of He. In the bifurcation process of the plasma jet, the product of Penning ionization inhibits the development of the main branch of the plasma jet, and the secondary electron avalanche of the local electric field promotes the formation of the branch of the plasma jet and is accompanied by the enhancement of the second ionization. The ionization mechanism of microwave He plasma is the resonance excitation of local enhanced electric field, the advance of ionizing waves, and the interaction between the spatially distributed active particles.

Mechanism of runaway electron generation in nanosecond pulsed plate-plate discharge at atmospheric-pressure air

Classical discharge theory (Townsend theory and streamer theory) has limitations in explaining nanosecond pulsed gas discharge. In recent years, the research on nanosecond pulsed gas discharge theory based on the high-energy runaway electrons has attracted extensive attention. But so far, there have been few studies of the generation mechanism of runaway electrons in atmospheric-pressure-air nanosecond pulsed plate-to-plate discharge, which seriously hinders the application and development of nanosecond pulse discharge plasma. In this paper, a one-dimensional implicit particle-in-cell/Monte Carlo collision (PIC/MCC) model is developed to investigate the mechanism of runaway electron generation and breakdown in a 1 mm-long atmospheric-pressure-air gap between the plate electrode and plate electrode driven by a negative nanosecond pulse voltage with an amplitude of 20 kV. The results show that under the influence of space charge dynamic behavior, the electric field enhancement region appears between the plate electrode and plate electrode, so that electrons can satisfy the electron runaway criteria and behaves in the runaway mode. In addition, it is also observed that the pre-ionization effect of the runaway electrons in front of the discharge channel can cause the secondary electron avalanches. As the secondary electrons avalanche and the discharge channel continues to converge, the discharge is guided and accelerated, eventually leading to the breakdown of the air gap. This study further reveals the mechanism of nanosecond pulsed plate-plate discharge, expands the basic theory of nanosecond pulsed gas discharge, and opens up new opportunities for the application and development of nanosecond pulsed discharge plasma.

Exploring the Basic Physical Mechanisms of Cathode- and Anode-Initiated High-Voltage Surface Flashover

Surface flashover in vacuum imposes a substantial physical limit on modern, large-scale pulsed power. One of the ramifications is a minimum size requirement for new machines, which in itself becomes a hard barrier to the modernization and improvement of existing infrastructure. Pulsed power topologies require the physical mechanisms of both anode- and cathode-initiated flashover to be considered. Originally, the geometrical implications of field emission at the cathode triple junction (CTJ) motivated the usage of configurations that avoid electrons impinging on the insulating material. This will largely suppress the cathode-initiated flashover, which is best described by the secondary electron avalanche mechanism, gas desorption, and final breakdown in the desorbed gas. It depends on the cascade growth of a conducting plasma along the length of the insulator from the cathode. Mitigating the cathode-initiated flashover typically comes at the cost of a significant field enhancement at the anode triple junction (ATJ). In a typical implementation, the anode field may be three times higher than the cathode field for a given voltage, making the corresponding anode-initiated flashover much more common than otherwise. In the case of pulsed, anode-initiated flashover, experimental evidence suggests that charge is directly extracted from the insulator resulting in the insulator taking on a net positive charge advancing the anode potential. Along with accompanying gas desorption from the surface, the potential will then propagate from the anode toward the cathode until the effective length of the gap is sufficiently reduced to support flashover. The underlying physical mechanisms of cathode- and anode-directed flashover are discussed in light of previously gathered experimental data and recent experiments with pulsed, high-gradient, anode-initiated flashover.

Initiation and Growth of the Secondary Electron Avalanche along High-Gradient Insulators in Vacuum

High-gradient insulators (HGIs) are materials created by alternating layers of insulating and conducting materials, which can result in a higher holdoff voltage than a monolithic insulator of the same length. We model the HGIs periodic electric field and use both a particle-in-cell direct simulation Monte Carlo code and a test particle approach to parameterize the growth of the secondary electron emission avalanches (SEEAs). To determine the effect of various HGI design parameters on secondary electron trajectories and SEEA growth, the following factors are investigated: layer width, edge layer materials, secondary electron yield, cell coupling, and surface defects as an electron source. We propose an alternative mechanism for initiating surface flashover, which considers surface defects on the downstream side of insulating layers, which differs from the current theory of cell coupling between adjacent insulating layers. Our results indicate surface conditioning is necessary because surface defects are a significant electron source in many cases. Our results reinforce the theory that thin layer widths are better at minimizing the growth of a surface avalanche, even though they will be more susceptible to cell coupling. If an HGI will not be conditioned prior to use, then thin layers are essential because of the significant avalanche growth capable along thicker layers. Finally, a metal layer adjacent to the cathode and an insulating layer adjacent the anode seems the optimal configuration to minimize the probability of flashover.

Micro gap air discharge based on fractal theory

Micro-gap discharge is a form of gas discharge in which the discharge gap is on the order of sub-millimeters orless. To study the initial path of micro-gap discharge and the mechanism and law of particle density change during discharge, in this paper a micro-gap discharge experiment and discharge image acquisition device under atmospheric pressure is built and the COMSOL simulation software is used to simulate the electron density and space charge in the process of micro-gap air discharge. Furthermore, the MATLAB software is used to calculate the fractal dimension and probability development index of micro-gap discharge. The air discharge phenomena produced by applying positive DC voltage to needle tip at atmospheric pressure and room temperature with gap distance ranging from 50 μm to 150 μm are studied. It is found experimentally that there are twists and turns in the discharge channel, and the number of bifurcations in the discharge process with a short gap is less than that with a long gap. Observation of the micro-gap air discharge process with a gap of 100 μm under atmospheric pressure shows that the discharge process is divided into the following three processes: needle tip corona, corona breakdown streamer, and spark discharge channel. Based on the analyses of these experimental results, it can be concluded that the discharge mechanism follows Thomson's theory, supplemented by the streamer theory. The cathode secondary electron emission (including positive ions colliding with the cathode and photoelectron emission) and the space charge distortion electric field form a secondary electron avalanche to maintain the discharge together. The seed electrons formed by a small amount of space photoionization also form an electron avalanche under the action of the space charge distortion electric field. There are tortuous sections in the discharge channel, but the number of branches is small and the degree of tortuosity is low. Therefore, there are weak streamer forms. The discharge channel is tortuous and branched, but the number of bifurcations is relatively small, and the tortuousness is low. In addition, it is also found that a sheath is formed at the cathode, the distortion of electric field is 3–8 times that of original electric field, and the electron density reaches 2 × 10<sup>21</sup> m<sup>–3</sup> during discharge, obtained from the COMSOL simulation. Meanwhile, the fractal theory simulation is used to simulate the micro-gap discharge. In the process of research, the fractal dimension is found to be proportional to the voltage and the gap distance. When the probability development index <i>η</i> = 1.18–1.3, the fractal dimension of the simulated discharge process is closer to the experimental result. The findings in this paper lay the foundation for further exploring the discharge theory of sub-micro- and nano-scaled gaps.

Depositing chromium oxide film on alumina ceramics enhances the surface flashover performance in vacuum via PECVD

Surface flashover seriously restricts the electrical performance of insulating system in vacuum. Surface properties of the insulating materials are the key factors affecting the electrical property along the surface. In this paper, Atmospheric-Pressure Plasma Enhanced Chemical Vapor Deposition (AP-PECVD) is used to deposit chromium oxide film on alumina ceramics to improve its flashover voltage in vacuum. Chromium acetylacetonate and argon are used as the precursor and working gas, respectively. A homemade ultrasonic atomizer and gas distribution system are used to ensure the uniformity during the AP-PECVD. For comparison, deposition experiments are both carried out at atmospheric and sub-atmospheric pressures. The experimental results show that the surface flashover voltages in 10−4 Pa are increased by 20% and 26% after the PECVD at atmospheric and sub-atmospheric pressures, respectively. The physicochemical and electrical property indicates that more shallow traps are introduced into the surface of alumina ceramics. The initial potential of the deposited sample decreases by 36% after the AP-PEVCD. Therefore, the deposition of chromium oxide films on alumina ceramics by AP-PECVD can effectively inhibit the surface charge accumulation and improve the surface flashover voltage in vacuum by reducing the trap energy level and its density. Based on the SEEA (secondary electron avalanche) theory and the distribution of the surface traps, the effects of chromium oxide films deposited on alumina ceramics are summarized for the application of vacuum insulation. The feasibility of depositing chromium oxide films to improve the vacuum surface flashover voltage of alumina ceramics by PECVD is proved. The process of the PECVD at atmospheric pressure is flexible, which provides a promising technical support for enhancing surface flashover for other insulating materials.

Statistical and numerical analysis of secondary electron avalanches with ion-induced electron emission in air

Statistical and numerical analyses of secondary electron avalanches with ion-induced electron emission and multielectron initiation in air is carried out. Statistical analysis is based on the negative binomial distribution (NBD) and its mixtures and the mean number of electrons in avalanches is determined. The development of successive secondary electron avalanches for ion-induced electron emission from the cathode ($$ \gamma_{i} $$ process) in air is simulated by the fluid model. The statistics of secondary avalanches is described by a mixture of NBDs, and it is found that the mixing weights $$ a_{j} $$ are positively correlated with $$ \mu^{j} $$ for the contribution of secondary avalanche of $$ j $$th generation, where $$ \mu = \gamma_{i} [\exp (\alpha d) - 1] $$ is the avalanche regeneration factor. Gaussian continual approximations for the discrete negative binomial distribution are also applied to the experimental data and the mixing weights are determined. Our models can explain a deviation of the electron number distribution from exponential due to influence of ion feedback processes.

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.

Analysis of multiplication characteristics of coaxial waveguide loaded ceramic under external magnetic field

In recent years, a lot of research work has been done on secondary electron multiplication studies in parallel plate structures. A nonuniform field may not only affect the results quantitatively but also lead to a new phenomenon. This paper presents a Monte Carlo model to investigate the multipactor discharge in a coaxial waveguide structure loaded ceramic window, which is applied to a high power coupler. In order to suppress secondary multiplication and improve the transmitting power of the high-power microwave window, the application of an external magnetic field is theoretically analyzed and simulated. A Monte Carlo algorithm is used to track the secondary electron position and study the multipactor scenario on the surface of a ceramic window in the coaxial line, including 3-D trajectory program and 3-D particle distribution program. It is demonstrated that an appropriate DC magnetic field can generate a compensating trajectory and collision, which suppresses the secondary electron avalanche effectively. The optimal value of the external magnetic field for avoiding the multipactor phenomenon is also discussed.

Early stage of the discharge in ablative pulsed plasma thrusters

Currently the renewed interest in ablative pulsed plasma thrusters (APPTs) mostly results from the needs of satellites of the nano and micro sectors. However, the efficiency of APPTs significantly decreases when they are scaled down. A better understanding of the physical processes involved in plasma generation and maintenance in this type of thruster may help to minimize APPTs’ performance degradation, when they are miniaturized. One of the issues, which up to now seems to be only marginally clarified, is an initial phase of discharge between the APPT’s electrodes. In this paper a novel analytical model of a flashover development is proposed. In the model, movement of electrons between the PPT’s electrodes is described relying on the concept of a saturated secondary electron avalanche. Two analytical approaches are considered. In the simplest approach, secondary electrons are assumed to be monoenergetic, whereas in the second, the Maxwellian distribution function is assumed. Simple, analytical formulas for surface current density, electron density and the thickness of the electron layer are derived. The predicted values are in a good agreement with measurements and computer simulations of other authors. The work was inspired by experimental examination of a micro APPT fed with polymeric nonvolatile propellant. For completeness, the thruster, known as an LμPPT (liquid micro pulsed plasma thruster), is described and its performance is presented.

Characterization of flashover plasma across a large-scale ceramic vacuum interface initiated by explosive electron emission

Flashover plasma characteristics across a large-scale ceramic vacuum interface initiated by explosive electron emission (EEE) are investigated with the experimental and simulation methods. Driven by a negative high voltage pulse (-300 kV, 150 ns), flashover plasma luminescence processes were observed by a high-speed framing camera and the accompanied spectrum characteristics were measured by a spectrograph. Compared with flashover initiated by field electron emission (FEE), a faster light expansion velocity and higher electron temperature of flashover initiated by EEE were obtained which were 200 cm/μs and 4.57 eV, respectively. Radial and angular components characteristics of expansion velocity were analyzed as well and their maximum values were 200 cm/μs and 50 cm/μs. Furthermore, comparisons of flashover initiated by EEE and FEE were conducted by particle-in-cell methods and the results agreed with the experimental observations. From these results it can be concluded that due to higher primary electron energy and stronger secondary electron avalanche and gas ionization, flashover along the ceramic interface initiated by EEE has an easier and faster develop process compared with that initiated by FEE. This work can give a reference for evaluation and design of ceramic vacuum interfaces for high-current applications.

Upstream and downstream multipactor of dielectric window by electromagnetic PIC simulations

By using the electromagnetic Particle-In-Cell method, the multipactor of the dielectric window on the upstream side is observed directly even without presetting extra normal electric fields and compared to that on the downstream side. This paper shows: First, with the initial emission of electrons to provide extra normal electric fields, the secondary electron avalanche is much faster than that on the downstream side. Second, even without the initial emission of electron to provide extra normal electric fields, the secondary electron avalanche occurs on the upstream side, while it is nowhere to be found on the downstream side.

The suppression effect of external magnetic field on the high-power microwave window multipactor phenomenon

To suppress the surface multipactor phenomenon and improve the transmitting power of the high-power microwave window, the application of external magnetic fields is theoretically analyzed and simulated. A Monte Carlo algorithm is used to track the secondary electron trajectories and study the multipactor scenario on the surface of a cylinder window. It is confirmed that over-resonant magnetic fields (an external magnetic field whose magnitude is slightly greater than that of a resonant magnetic field) will generate a compensating trajectory and collision, which can suppress the secondary electron avalanche. The optimal value of this external magnetic field that will avoid the multipactor phenomenon on cylinder windows is discussed.

Calculation of the Effective Ionization Rate in Air by Considering Electron Detachment From Negative Ions

The effective Townsend ionization coefficient in dry air at different pressures and temperatures is calculated by considering ionization by direct electron impact, attachment of electrons to molecular oxygen, simplified negative ion kinetics, and detachment of electrons from negative ions in collisions with heavy neutrals. Effect of electron detachment from negative ions created by primary and secondary electron avalanches propagating in a uniform electric field in air at different pressures and temperatures and its contribution into the effective ionization rate are analyzed.

Dynamic Characteristics of SF6-N2-CO2 Gas Mixtures in DC Discharge Process

Dynamic characteristics of discharge particles are described within the framework of a two-dimensional photoionization-hydrodynamic numerical model for the discharge process of SF6-N2-CO2 gas mixtures at atmospheric pressure, under a uniform DC applied field. The finite difference—flux corrected transport (FD-FCT) algorithm is used in the numerical implementation for improving the accuracy and efficiency. Then the tempo-spatial distributions of the gap—space electric field and electron velocity are calculated from the microscopic mechanism, and the dynamic behaviors of charged particles are obtained in detail. Meanwhile, the tempo—spatial critical point of the avalanche-to-streamer in this model is discovered, and several microscopic parameters are also investigated. The results showed that the entire gap discharge process can be divided into two phases of avalanche and streamer according to Raether-Meek criterion; the electron density within the discharge channel is lower compared to that of positive and negative ions; space charge effect is a dominant factor for the distortion of spatial electric field, making the discharge channel expand toward both electrodes faster; photoionization provides seed electrons for a secondary electron avalanche, promoting the formation and development speed of the streamer.

Study of solid-conversion gaseous detector based on GEM for high energy X-ray industrial CT.

The general gaseous ionization detectors are not suitable for high energy X-ray industrial computed tomography (HEICT) because of their inherent limitations, especially low detective efficiency and large volume. The goal of this study was to investigate a new type of gaseous detector to solve these problems. The novel detector was made by a metal foil as X-ray convertor to improve the conversion efficiency, and the Gas Electron Multiplier (hereinafter "GEM") was used as electron amplifier to lessen its volume. The detective mechanism and signal formation of the detector was discussed in detail. The conversion efficiency was calculated by using EGSnrc Monte Carlo code, and the transport course of photon and secondary electron avalanche in the detector was simulated with the Maxwell and Garfield codes. The result indicated that this detector has higher conversion efficiency as well as less volume. Theoretically this kind of detector could be a perfect candidate for replacing the conventional detector in HEICT.

Effects of pressure and humidity on positive corona inception from thundercloud hydrometeors

This paper reports a study on the inception condition of positive corona discharges around thundercloud hydrometeors that are simulated as a spherical point electrode. The corona inception from the hydrometeor is investigated using a model developed by Naidis (2005), which suggests that the discharge becomes self-sustaining when the number of ionizing photons produced by all secondary electron avalanches is equal to that by a primary avalanche. For an isolated, charged hydrometeor in dry air of varying pressure, the results show that the onset of positive corona discharges from a large hydrometeor at high pressure requires a stronger avalanche multiplication than those from a small hydrometeor at low pressure. The onset voltage and surface electric field are obtained for thundercloud hydrometeors of different radii. For a hydrometeor of a radius of 1mm at atmospheric pressure, the onset surface field reaches about 2.75 times the conventional breakdown threshold field. In humid air (a gas mixture of dry air and water vapor), it is found that a stronger avalanche is required for the corona onset, and the effect of humidity is more pronounced for large hydrometeors. The absorption of the ionizing photons by water vapor is the main reason why a stronger avalanche is required for the corona inception in humid air. As corona discharges from an isolated hydrometeor remove the charge on it, this discharging mechanism sets an upper limit on hydrometeor charge that agrees reasonably with the reported observations of maximum precipitation charge. Finally, we discuss the corona inception condition from a charged hydrometeor in non-zero ambient field of thundercloud condition.

Model for calculating low-current moderate-pressure RF discharges with photon-driven secondary electron photoemission from the electrode surface

A model is developed for calculating a low-current moderate-pressure RF discharge with allowance for an electron cloud that is formed by electrons produced during the preceding periods of the RF field and fills the electrode sheath at regular intervals in accordance with the phase of the RF voltage applied to the electrodes. The cloud arises due to a phase shift of π/2 between the voltage across the sheath and that across the column of a low-current RF discharge plasma. The photon generation mechanism is as follows: as the cloud electrons fill the sheath, they acquire energy in superelastic collisions with metastables produced by the sheath electrons during the preceding periods of the RF field and then excite the metastable states to emitting levels. The discharge sheath forms due to the overlap of the secondary electron avalanches triggered by electron photoemission from the electrode surface. The parameters of the sheath in a low-current RF discharge are determined by the conditions under which the electron photoemission current in the sheath is self-sustaining, but the capacitive susceptance of the sheath is substantially higher than its active electrical conductance. The results of calculations are compared with the experimental data.