Model Of Secondary Electron Emission Research Articles

Study on the influence of magnetic field on the performance of a 5 kW hall thruster

Hall thruster is a kind of plasma optics device, which is used mainly in space propulsion. To study the influence of magnetic field on the performance of a 5 kW hall thruster, a two-dimensional PIC-MCC model was built. The Bohm diffusion was modeled by using a Brownian motion instead of the Bohm collision method and the near-wall conduction was modeled by a secondary electron emission model. When the mass flow rate is 5 mg/s, the thruster performance like thrust, efficiency and discharge current was simulated under a discharge voltage from 300 to 1,000 voltage. At first, the performance under constant magnetic field was simulated. The results showed that the magnetic field could not restrain the electrons as the discharge voltage increased. Later, the performance under varied magnetic field was simulated. The results showed that increasing the magnetic field strength with the increasing discharge voltage could restrain the electrons more efficiently, which proved that increasing the magnetic field strength is necessary for high specific impulse operation of hall thruster. At last, the performance measurement experiment of the thruster was carried out, and the experimental results verified the accuracy of the simulation results.

Numerical characterization of dual radio frequency micro-discharges

Micro-discharges have many excellent characteristics, such as generation of high-density and non-equilibrium plasmas at atmospheric pressure. In this paper, we used an implicit particle-in-cell/Monte Carlo collision method for three-dimensional velocities in a one-dimensional space combined with the secondary electron emission model to study the characteristics of micro-discharges driven by dual radio frequency (RF) power. The effect on plasma parameters was observed by varying the voltage of the RF power, the frequency, and the gas pressure of the discharge. Since the electrode spacing is very small in micro-discharges, the voltage change will affect the characteristics of micro-discharges. In addition, the plasma density increases with the frequency and the discharge mode changes at different frequencies. Finally, the influence of gas pressure on the characteristics of micro-discharges cannot be ignored. When the air pressure decreases, the ion flux reaching the electrodes is significantly increased, and the energy distribution of ions increases in the high-energy portion.

Secondary electron emission model for photo-emission from metals in the vacuum ultraviolet

Secondary electron emission model for photo-emission from metals in the vacuum ultraviolet

Performance Simulation of a 5 kW hall Thruster

Hall thruster is a kind of plasma optics device, which is used mainly in space propulsion. To simulate the discharge process of plasma and the performance of a 5 kW hall thruster, a two-dimensional PIC-MCC model in the R-Z plane is built. In the model, the anomalous diffusion of the electrons including Bohm diffusion and near-wall conduction is modeled. The Bohm diffusion is modeled by using a Brownian motion instead of the Bohm collision method and the near-wall conduction is modeled by a secondary electron emission model. In addition to the elastic, excitation, and ionization collisions between electrons and neutral atoms, the Coulomb collisions are included. The plasma discharge process including the transient oscillation and steady state oscillation is well reproduced. First, the influence of the discharge voltage and magnetic field on the steady state oscillation is simulated. The oscillation amplitude increases as the discharge voltage gets larger at first, and then decreases. While the oscillation amplitude decreases as the magnetic field gets stronger at first, and then increases. Later, the influence of the discharge voltage and mass flow rate on the performance of the thruster is simulated. When the mass flow rate is constant, the total efficiency initially increases with the discharge voltage, reaches the maximum at 600 V, and then declined. When the discharge voltage is constant, the total efficiency increases as the mass flow rate rises from 10 to 15 mg/s. Finally, a comparison between simulated and experimental performance reveals that the largest deviation is within 15%, thereby indirectly validating the accuracy of the model.

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.

Theory of Thermionic and Secondary Emission Properties of Palladium–Barium Cathodes of Microwave Electrovacuum Devices

A model of the surface structure of a palladium–barium cathode is proposed and the Dionne secondary electron emission model is used to calculate the dependence of the secondary electron emission coefficient of barium oxide crystallites forming on the surface of a cathode during its thermal activation on the temperature and oxygen vacancy concentration in the oxide. It is shown that the emission properties of a palladium–barium cathode significantly depend on both the emission properties of barium oxide crystallites and the cathode relative surface area occupied by them. The theoretical nomograms are built that relate the measured surface-averaged work function to the secondary electron emission coefficient of a palladium–barium cathode; the nomogram parameters are the temperature, oxygen vacancy concentration in barium oxide crystallites, and the relative cathode surface area occupied by them.

Analysis of Multipactor Effects by a Particle-in-Cell Algorithm Integrated With Secondary Electron Emission Model on Irregular Grids

We combine a novel finite-element-based electromagnetic particle-in-cell (EM-PIC) algorithm for the solution of Maxwell–Vlasov equations on irregular (unstructured) grids together with the Furman–Pivi probabilistic model governing the secondary electron emission process. The algorithm can be used for the analysis of resonant electron discharging phenomena (multipactor effects) in high-power radio frequency devices. In contrast to previous algorithms, the present EM-PIC algorithm yields a self-consistent time update of fields and particles on irregular grids with energy and charge conservation obtained from first principles. The use of unstructured grids enables local mesh refinement and simulation of complex geometries with minimal geometric defeaturing. We apply the algorithm to model multipactor effects on waveguides with flat or corrugated walls. We contrast the evolution of the electron population in various cases and investigate the respective saturation processes arising from self-field counterbalancing effects.

Feedback model of secondary electron emission in DC gas discharge plasmas

Feedback is said to exist in any amplifier when the fraction of output power in fed back as an input. Similarly, in gaseous discharge ions that incident on the cathode act as a natural feedback element to stabilize and self sustain the discharge. The present investigation is intended to emphasize the feedback nature of ions that emits secondary electrons (SEs) from the cathode surface in DC gas discharges. The average number of SEs emitted per incident ion and non ionic species (energetic neutrals, metastables and photons) which results from ion is defined as effective secondary electron emission coefficient (ESEEC,). In this study, we derive an analytic expression that corroborates the relation between and power influx by ion to the cathode based on the feedback theory of an amplifier. In addition, experimentally, we confirmed the typical positive feedback nature of SEE from the cathode in argon DC glow discharges. The experiment is done for three different cathode material of same dimension (tungsten (W), copper (Cu) and brass) under identical discharge conditions (pressure: 0.45 mbar, cathode bias: −600 V, discharge gab: 15 cm and operating gas: argon). Further, we found that the value of these cathode material controls the amount of feedback power given by ions. The difference in feedback leads different final output i.e the power carried by ion at cathode (). The experimentally obtained value of is 4.28 W, 6.87 W and 9.26 W respectively for W, Cu and brass. In addition, the present investigation reveals that the amount of feedback power in a DC gas discharges not only affect the fraction of power fed back to the cathode but also the entire characteristics of the discharge.

PIC/MCC simulation of breakdown dynamics inside high power microwave output window

High power microwave (HPM) has important applications in controlled thermonuclear fusion heating, microwave high-gradient accelerator, high-power radar, directed-energy weapon, super jammer, impact radar, etc. The window breakdown of HPM has been extensively studied, and some research progress in this respect has been made. However, the researches on the transition of window breakdown from multipactor discharge to rf plasma are still not enough in-depth. Especially, the influences of microwave frequency and microwave amplitude during breakdown need further studying. This paper focuses on the process of dielectric multipactor and background argon ionization during the discharge breakdown near the HPM dielectric window/vacuum interface. A one-dimensional-spatial-distribution-and-three-dimensional-velocity-distribution (1D3V) electrostatic model with using particle-in-cell simulation is adopted in present work. The model includes secondary electron emission, electrostatic field induced by the remaining positive charge on the dielectric window, the motion of charged particles under electrostatic and microwave field, and the collision process between electron and background gas, and the corresponding PIC/MCC code is also developed. We examine the effects of gas pressure, microwave frequency and microwave amplitude on discharge breakdown. It is found that there exists only electron multipactor process during the discharge breakdown on dielectric window in vacuum. At low pressures (10 mTorr, 500 mTorr) and slightly high pressure (10 Torr), electron multipactor and gas ionization are coexistent. However, at an extremely high pressure (760 Torr), the gas ionization dominates the breakdown process. At the same time, the position of plasma density peak moves away from the dielectric window as the gas pressure increases, which is the consequence of the competition between secondary electron multiplication on the dielectric window and gas ionization in the body region. It can be seen that the advantage of gas ionization gradually increases as the gas pressure increases. In addition, it is also observed that at 500 mTorr, the moment of gas ionization moves forward first and then backward with the increase of the microwave amplitudes or the microwave frequency, especially when the increment of frequency is numerically twice that of the amplitude, gas ionization occurs earliest. This phenomenon is explained by the secondary electron emission model. Meanwhile, the results show that the position of plasma density peak from gas ionization gradually approaches to the dielectric window as the microwave amplitude increases. However, with continually increasing the microwave frequency, the plasma density peak moves away from the dielectric window first and then approaches to the dielectric window.

Study of a high power hydrogen beam diagnostic based on secondary electron emission.

In high power neutral beams for fusion, beam uniformity is an important figure of merit. Knowing the transverse power profile is essential during the initial phases of beam source operation, such as those expected for the ITER heating neutral beam (HNB) test facility. To measure it a diagnostic technique is proposed, based on the collection of secondary electrons generated by beam-surface and beam-gas interactions, by an array of positively biased collectors placed behind the calorimeter tubes. This measurement showed in the IREK test stand good proportionality to the primary beam current. To investigate the diagnostic performances in different conditions, we developed a numerical model of secondary electron emission, induced by beam particle impact on the copper tubes, and reproducing the cascade of secondary emission caused by successive electron impacts. The model is first validated against IREK measurements. It is then applied to the HNB case, to assess the locality of the measurement, the proportionality to the beam current density, and the influence of beam plasma.

Effect of wall secondary electron distribution function on the characteristics of stable sheath near a dielectric wall

It is widely known that the energy distribution of secondary electrons induced by a single-energy electron beam presents typical bimodal configuration. However, the total velocity distribution of secondary electrons induced by a Maxwellian plasma electron group has not been revealed clearly, due to the lack of detailed theoretical calculation and calculation and experiment result. Therefore, researchers usually function satisfies single-energy distribution ( 0), half-Maxwellian distribution and so on, in order to study the characteristics of stable fluid sheath near a dielectric wall. For this reason, using the Monte Carlo method to simulate the wall secondary electron emission events based on a detailed probabilistic model of secondary electron emission induced by single-energy incident electron beam, we found that, when the incident electron follows an isotropic Maxwellian distribution, the total perpendicular-to-wall velocity distribution of the secondary electrons emitted from dielectric wall follows a three-temperature Maxwellian distribution. In the simulation, the incident angle of the plasma electrons and the emergence angle of the secondary electrons are considered, so the Monte Carlo method can discriminate whether the secondary electron velocity is perpendicular to or parallel to the wall surface. Then, a one-dimensional stable fluid sheath model is established under the wall boundary condition that the secondary electrons obey the three-temperature Maxwellian distribution; and some contrastive studies are made in order to reveal the effect of wall total secondary electron distribution functions such as single-energy distribution, half-Maxwellian distribution, and three-temperature Maxwellian distribution with the sheath characteristics. It is found that the total secondary electron distribution function can significantly influence the ion energy at the sheath interface, the wall surface potential, the potential and electron/ion-density distributions, and so on. Both the ion energy at sheath interface and the wall surface potential increase monotonously with the increase of wall total secondary electron emission coefficient. But the values of three-temperature Maxwellian distribution differ much from that of half-Maxwellian distribution and single-energy distribution. When the total secondary electron follows a three-temperature Maxwellian distribution, the critical space charge saturated sheath has no solution, indicating that with the increase of the wall total secondary electron emission coefficient, the sheath will directly transit from the classic sheath structure to the anti-sheath one. In the future work, a kinetic, static sheath model will be developed in order to study the characteristics of anti-sheath and space charge saturated sheath near a dielectric wall

Low-temperature electron microscopy of the surface of the superconductor Bi2Sr2CaCu2O8

Electron microscopy studies of the microstructure and composition of the surface of the high-temperature superconductor Bi2Sr2CaCu2O8 are performed. The secondary-electron emission intensity and its temperature dependence are estimated in local regions at temperatures lower than 300 K. The results are interpreted within the secondary-electron emission model.

Two dimensional particle-in-cell simulation of electron multipactor on high power microwave dielectric window surface

Based on the particle-in-cell (PIC) simulation method of the first principle, this paper studies the situation about formation and development of the electron process on the dielectric window surface in high power microwave (HPM) devices. We establish a simple two-dimensional model that the TEM wave irradiates the dielectric window surface vertically with the PIC simulation software VORPAL, employ the Vaughans secondary electron emission model, utilize Monte Carlo collision model to deal with the elastic collision, the excitation collision and the ionization collision between electron and background gas, and acquire the electron multipactor images on the dielectric window surface. The simulation results show that the electron number on the dielectric window surface reaches saturation in a period of time and its oscillation frequency is twice as much as that of the incident electric field. Changing the initial emissive number of the seed electrons, the amplitude of the incident electric field and the pressure of the background gas, we can obtain the variation of the electron number on the dielectric window surface in different conditions.

Three-dimensional simulation method of multipactor in microwave components for high-power space application

Based on the particle-in-cell technology and the secondary electron emission theory, a three-dimensional simulation method for multipactor is presented in this paper. By combining the finite difference time domain method and the particle tracing method, such an algorithm is self-consistent and accurate since the interaction between electromagnetic fields and particles is properly modeled. In the time domain aspect, the generation of multipactor can be easily visualized, which makes it possible to gain a deeper insight into the physical mechanism of this effect. In addition to the classic secondary electron emission model, the measured practical secondary electron yield is used, which increases the accuracy of the algorithm. In order to validate the method, the impedance transformer and ridge waveguide filter are studied. By analyzing the evolution of the secondaries obtained by our method, multipactor thresholds of these components are estimated, which show good agreement with the experimental results. Furthermore, the most sensitive positions where multipactor occurs are determined from the phase focusing phenomenon, which is very meaningful for multipactor analysis and design.

Suppression of secondary electron emission by micro-trapping structure surface

Suppression of secondary electron yield attracted much attention in areas such as accelerator and high power microwave components in recent years. To evaluate the suppression efficiencies of different surface topographies, the secondary electron yields (SEYs) of four kinds of micro-structured surfaces for trapping secondary electrons, i.e., triangular groove, rectangular groove, cuboid, cylindrical, are obtained by the phenomenological probabilistic model of secondary electron emission. The simulation results show that the SEYs of these structures are much dependent on the shape parameters such as aspect ratio or porosity. There are mainly three findings: 1) the SEY decreases with increasing aspect ratio and porosity; 2) the traps with cuboid or cylindrical shape are more efficient than triangular or rectangular traps for the SEY suppression; 3) the SEY dependence of micro-structured surface on incident angle is not as obvious as that of flat surface. Micro-trapping structure surfaces are fabricated by mechanical method, photolithography process and chemical etching respectively. The measured SEYs of these samples validate the theoretical results. All these results show that the proposed micro-structures as secondary electron traps have potential applications in SEY suppression in fields such as multipactor and electron-cloud effects.

Characteristics of wall materials secondary electron emission and sheath in Hall thruster

Hall thruster plasma discharge channels interact with wall forming sheath, and different secondary electron emission coefficients of wall material can considerably affect sheath characteristics. In this paper, we establish a two-dimensional physical model in wall sheath area to study the secondary electron emission characteristics of three different wall materials which are boron nitride (BN), silicon carbide (SiC) and aluminium oxide (Al2O3), and SiC material secondary electron emission model is improved. On the basis of improved secondary electron emission model, the relationship between the wall secondary electron emission coefficient and both electron temperature and magnetic field intensity is discussed by the particle in cell simulation method, and the different sheath properties of three materials (BN, SiC and Al2O3), are also investigated. The results show that fitting curve is consistent with the experimental results; at the same electron temperature, the values of secondary electron emission coefficient and electron density of three materials (BN, SiC and Al2O3) increase but the values of sheath electric field and sheath potential drop decrease in their sequence. The BN material has the appropriate secondary electron radiation coefficient, which makes the thruster work steadily under low current condition.

Investigation into anomalous total secondary electron yield for micro-porous Ag surface under oblique incidence conditions

We study the dependence of total secondary electron yield (SEY) of micro-porous silver surface on incident angle of primary electron. First, we produce regular and random micro-porous surfaces on silver plated aluminium samples by photolithography pattern process and direct chemical etching method, respectively. Second, we study the dependence of the SEY characteristics of these surfaces on the primary electron incident angle from 0° to 60°. The experimental results show that micro-porous surface with large porosity can suppress SEY effectively either for normal incidence or oblique incidence. The maximum SEY of the directly chemical etched sample can reach a 45% reduction relative to the original flat sample. We also show the anomalous experimental phenomenon that the SEY of directly chemical etched sample with large porosity does not increase with incident angle. Third, we explain the complicated SEY dependence on incident angle qualitatively by the secondary electron trapping effect in micro-pores as well as the Monte Carlo simulation based on phenomenological model of secondary electron emission. Finally, we suggest a third order polynomial formula to describe the SEY dependence on incident angle which will be useful in particle simulation. In total, we demonstrate that micro-porous surface with large porosity is effective in SEY suppression under both normal and oblique incidence and thus the proposed direct chemical etching method is prospective in related areas such as multipactor improvement.

Suppression of secondary electron yield by micro-porous array structure

We study secondary electron yield (SEY) suppression for metal materials using a roughened surface with a micro-porous array. First, we perform a Monte Carlo simulation of the electron trajectory in a single cylindrical well using a phenomenological model of secondary electron emission and the SEY suppression efficiency of a micro-porous array. The simulation results show that the SEY of a roughened surface is affected significantly by the aspect ratio of the micro-pores and the surface porosity of the metal plate. Then, to verify the simulation results, we produce a micro-porous array on metal plates using photolithography and measure their SEYs. We show that the micro-porous array structure can efficiently suppress the SEY of metal materials, and the measurements agree quantitatively with the corresponding simulation results. Finally, we derive an analytical formula to evaluate easily the SEY suppression efficiency of the Ag micro-porous array. In total, the micro-porous array proposed in this paper offers an alternative to SEY suppression in related areas such as multipactor effects in satellite payloads or electron cloud effects in accelerators.

Model of secondary electron emission at high incident electron energy for metal

The models of the true secondary electron emission and backscattered electron emission for metal are provided, based on the physical process of secondary electron emission from metal at high incident electron energy. The formula for the true secondary electron yield at high incident electron energy is derived with the Bethe equation known to provide accurate and simple analytical expressions for the stopping power and the probability that a secondary electron produced in a material reaches the surface and is emitted into the vacuum. The formula for the backscattered electron yield at high incident electron energy is derived with the absorbing rule for the incident electron in the material. All the above results lead to a new model for secondary electron emission from metal at high incident electron energy. The secondary electron yields of Au, Ag, Cu and Al derived with the new model are in good agreement with the results obtained from the scatters process simulation code-Casino.

A combined phenomenological model for secondary electron emission

The accuracy of the model for secondary electron yield (SEY) has a remarkable influence on the simulation result of multipactor threshold. A new combined phenomenological model for SEY was proposed based on the corrected Vaughan model and Furman model. It combines virtues of the latter two models by integrating corrected Vaughan model into Furman model for its calculation of yield of true secondary electron. The new model provides high flexibility and accuracy to fit experimental data of SEY as a function. For comparison, experimental data of silver and aluminum alloys were tested with the three models. It was found that the fitting accuracy has been improved by at least 10% under the circumstances of different incident angles of the original electron.