Secondary Electron Emission Coefficient Research Articles

Ammonia synthesis by plasma catalysis in an atmospheric RF helium plasma

The in-plasma-catalytic synthesis of ammonia from nitrogen and hydrogen admixed to a helium RF plasma is studied with infrared absorption spectroscopy, optical emission spectroscopy, and through chemical kinetics modeling. Sandblasted glass is used as support for iron, platinum, and copper catalysts up to a surface temperature of 150 ∘C . It is shown that the optimum ammonia production occurs at very small N2/(N2+H2) ratios with an increase of concentration with plasma power. The global kinetic modelling agrees well with the data for a variation of the N2+H2 admixture and the absorbed plasma power. The introduction of the catalyst enhances ammonia production by up to a factor of 2. Based on the comparison with the modelling, this is linked to a change in the electron kinetics due to the presence of the catalyst. It is postulated that introducing the catalyst increases the reduced electric field because it reduces the secondary electron emission coefficient. As a result, the dissociation of N2 is stimulated, thereby enhancing the NH3 formation. These experiments show that the impact of the catalyst on the plasma performance in noble gas-diluted RF plasmas can be more important than the impact of the plasma on any catalytic surface process.

Nonlinear change of ion-induced secondary electron emission in the κ-Al2O3 surface charging from first-principle modelling

Abstract Secondary electron emission (SEE) induced by the positive ion is an essential physical process to influence the dynamics of gas discharge which relies on the specific surface material. Surface charging has a significant impact on the material properties, thereby affecting the SEE in the plasma-surface interactions. However, it does not attract enough attention in the previous studies. In this paper, SEE dependent on the charged surface of specific materials is described with the computational method combining a density functional theory (DFT) model from the first-principle theory and the theory of Auger neutralization. The effect of κ-Al2O3 surface charge, as an example, on the ion-induced secondary electron emission coefficient (SEEC) is investigated by analysing the defect energy level and band structure on the charged surface. Simulation results indicate that, with the surface charge from negative to positive, the SEEC of a part of low ionization energy ions (such as Ei = 12.6 eV) increases first and then decreases, exhibiting a nonlinear changing trend. This is quite different from the monotonic decreasing tendency observed in the previous model which simplifies the electronic structure. This irregular increase of the SEEC can be attributed to the lower escaped probability of orbital energy. The results further illustrate that the excessive charge could cause the bottom of the conduction band close to the valence band, thus leading to the decrease of the orbital energy occupied by the excited electrons. The nonlinear change of SEEC demonstrates a more realistic situation of how the electronic structure of material surface influences the SEE process. This work provides an accurate method of calculating SEEC from specific materials, which is urgent in widespread physical scenarios sensitive to surface materials, such as increasingly growing practical applications concerning plasma-surface interactions.

Effective improvement of the micro-discharge threshold and environmental stability in microwave devices based on carbon/silver/titanium composite films

The micro-discharge effect limits the development of high-frequency microwave power components to miniaturization and integration. Therefore, it has become a focus of research to effectively suppress the micro-discharge effect, increase the micro-discharge threshold, and strengthen the environmental stability of high-frequency microwave devices. In this study, different elemental ratios of carbon (C)/silver (Ag)/titanium (Ti) composite films were obtained by multi-target magnetron co-sputtering technology and then systematically analyzed for micro-discharge effects. When the doping ratio of C/Ag/Ti was 3.538/1/0.013, the corresponding maximum secondary electron emission coefficient (δmax) decreased to 1.01 owing to the suppression of secondary electron emission by the increase in the content of sp2 hybrid bond. The micro-discharge threshold of Ku band waveguide filters coated with moderate C/Ag/Ti composite films showed an optimal performance of 10 000−12 500 W, which was increased by approximately 20 times. Moreover, the microstructure of the composite films exhibited higher density and flatness with a tiny increase in the titanium doping ratio, representing good environmental stability. Thus, the effective suppression of the secondary electron emission yield, significant improvement in the micro-discharge threshold, and enhancement of the environment stability of microwave components could be realized simultaneously by reasonably controlling the content of titanium in silver and titanium co-doped carbon-based composite films.

Effects of propellant species on the discharge characteristics of glow discharge hollow cathode

Hollow cathodes are the key sources of electrons for the operation of space electric thrusters. Traditionally, hollow cathodes utilize heating elements to raise the temperature of low work function materials to produce electrons. To accommodate the development of small satellites, researchers around the world have developed alternative neutralizer technologies such as radio frequency neutralizers, electron cyclotron resonance neutralizers, and coiled tungsten filaments cathode neutralizers to simplify the cathodes design and operation. In this work, a lower current glow discharge hollow cathode (GDHC) is developed, which can generate a discharge current of 10–100 mA. By testing with xenon, krypton, and argon propellants, it is found that GDHC has lower discharge voltage and power consumption with argon propellant. Simulation analysis reveals that during argon propellant discharge, the secondary electron emission coefficient on the cathode surface is larger, the electron density in the cathode sheath area is higher, and the high mobility of argon ions increases the conductivity within the cathode sheath. In addition, there are low and high extraction modes for GDHC electron extraction, depending on whether the keeper electrode absorbs or emits electrons.

Numerical Study of Ar/O2 Discharge Properties Influenced by Small Secondary Electron Emission in Dual-Frequency Atmospheric Pressure Discharge

A one-dimensional fluid model was employed to investigate the influence of the small secondary electron emission (SEE) coefficient on the characteristic properties of an Ar/O2 gas discharge at atmospheric pressure driven by a dual frequency source. The study includes basic physical quantities such as particle density, electron temperature, the electron heating mechanism, and energy loss. The research results illustrate that with an increase in the secondary electron emission coefficient, the electron density increases, while electron temperature and electric field decrease. The densities of various particles increase to different degrees, except for the metastable O2* molecule and the O atom. The density of the metastable O2* molecule and the density of the O atom are hardly affected by the SEE coefficient; however, the time required for both to reach steady state decreases. The time required for the electron density to reach steady state increases. Electron heating and energy loss increase to varying degrees when the SEE coefficient changes from 0.001 to 0.01.

Experimental validation of particle-in-cell/Monte Carlo collisions simulations in low-pressure neon capacitively coupled plasmas

Plasma simulations are powerful tools for understanding fundamental plasma science phenomena and for process optimisation in applications. To ensure their quantitative accuracy, they must be validated against experiments. In this work, such an experimental validation is performed for a one dimensional in space and three dimensional in velocity space particle-in-cell simulation complemented with the Monte Carlo treatment of collision processes of a capacitively coupled radio frequency plasma driven at 13.56 MHz and operated in neon gas. In a geometrically symmetric reactor the electron density in the discharge centre and the spatio-temporal distribution of the electron impact excitation rate from the ground into the Ne 2p1 level are measured by a microwave cutoff probe and phase resolved optical emission spectroscopy, respectively. The measurements are conducted for electrode gaps between 50 mm and 90 mm, neutral gas pressures between 20 mTorr and 50 mTorr, and peak-to-peak values of the driving voltage waveform between 250 V and 650 V. Simulations are performed under identical discharge conditions. In the simulations, various combinations of surface coefficients characterising the interactions of electrons and heavy particles with the anodised aluminium electrode surfaces are adopted. We find, that the simulations using a constant effective heavy particle induced secondary electron (SE) emission coefficient of 0.3 and a realistic electron–surface interaction model (which considers energy-dependent and material specific elastic and inelastic electron reflection, as well as the emission of true SEs from the surface) yield results which are in good quantitative agreement with the experimental data.

Dimensional analysis on microscale gas breakdown with electric field nonuniformity and positive space charge effects

A dimensional method was employed to evaluate the microscale gas breakdown characteristics at atmospheric pressure, resulting in a universal breakdown curve applicable to different types of gases (e.g., Ar, Xe, Ne, and N2). As the gap distance decreases, the breakdown mode transitions from ion-induced secondary electron emission to the field emission regime. In the field emission regime, the positive space charge effect becomes more significant. We discovered that incorporating the positive space charge effect in the field emission regime can be achieved by modifying the local electric field enhancement factor β. Consequently, we propose an effective electric field enhancement factor, βeff, which scales linearly with β, to accurately reproduce the breakdown curve while considering the positive space charge effect. This proposed approach significantly simplifies the numerical model. Additionally, we examined the effects of gas pressure, gap distance, cathode properties (e.g., work function and secondary electron emission coefficient), and electric field nonuniformity.

The electrical asymmetry effect in electronegative CF4 capacitive RF plasmas operated in the striation mode

The Electrical Asymmetry Effect (EAE) provides control of the mean ion energy at the electrodes of multi-frequency capacitively coupled radio frequency plasmas (CCP) by tuning the DC self-bias via adjusting the relative phase(s) between the consecutive driving harmonics. Depending on the electron power absorption mode, this phase control affects the ion flux in different ways. While it provides separate control of the mean ion energy and flux in the α-mode, limitations were found in the γ- and Drift-Ambipolar modes. In this work, based on experiments as well as kinetic simulations, the EAE is investigated in the striation-mode, which is present in electronegative CCPs driven by low frequencies. The discharge is operated in CF4 and is driven by two consecutive harmonics (4/8 MHz). The simulation results are validated against measurements of the DC self-bias and the spatio-temporally resolved dynamics of energetic electrons. To include heavy particle induced secondary electron emission realistically, a new computationally assisted diagnostic is developed to determine the corresponding secondary electron emission coefficient from a comparison of the DC self-bias obtained experimentally and from the simulations. Based on the validated simulation results, the EAE is found to provide separate control of the mean ion energy and flux in the striation mode, while the axial charged particle density profiles and the number of striations change as a function of the relative phase. This is understood based on an analysis of the ionization dynamics.

Study of TiO2 on the Voltage Holdoff Capacity of Cr/Mn-Doped Al2O3 Ceramic in Vacuum.

With the development of vacuum electronic devices toward high power, high frequency, and miniaturization, the voltage holdoff capacity of the insulation materials in devices has also been raised to a higher demand. Cr/Mn/Ti-doped Al2O3 ceramics were prepared, and the bulk density, micromorphology, phase composition, resistivity, secondary electron emission coefficient, and surface flashover threshold in the vacuum of the Al2O3 were characterized. The results show that the addition of TiO2 to the Al2O3 ceramic can promote the sintering of the ceramic. The Cr/Mn/Ti-doped Al2O3 ceramic with a homogeneous microstructure can be obtained by an appropriate amount of TiO2 addition. In the process of the heat treatment, the TiO2 in the ceramics was reduced to a certain degree, which had an impact on the microstructure of the Al2O3 ceramic. Adding a small amount of TiO2 can improve the voltage holdoff performance in the vacuum. The value of the surface flashover threshold in the vacuum of the Cr/Mn/Ti-doped Al2O3 ceramic containing 1 wt.% TiO2 reached a value of 33 kV, which is 32% higher than that of the basic Al2O3 ceramic. The preparation of Al2O3 ceramics with a high voltage holdoff capacity in a vacuum provides fundamental technical support for the development of vacuum electronic devices.

Investigation of conditions necessary for inception of positive corona in air based on differential formulation of photoionization

Sharp point electrodes generate significant electric field enhancements where electron impact ionization leads to the formation of electron avalanches that are seeded by photoionization. Photoionization of molecular oxygen due to extreme ultraviolet emissions from molecular nitrogen is a fundamental process in the inception of a positive corona in air. In a positive corona system, the avalanche of electrons in the bulk of the discharge volume is initiated by a specific distribution of photoionization far away from the region of maximum electron density near the electrode where these photons are emitted. Here, we present a new approach to finding the inception conditions for a positive corona, which is based on a differential formulation of the photoionization problem. The proposed iterative solution considers the same inception problem that has been solved in the existing literature by using either an integral approach to photoionization or a differential formulation of photoionization and considering the inception problem as a boundary-value eigenvalue problem. The results are validated by comparisons with previous integral formulations and time dynamic plasma fluid solutions in planar and spherical geometries. The results illustrate ideas advanced in Kaptzov (1950 Elektricheskiye Yavleniya v Gazakh i Vacuume p 610) providing a physically transparent connection between an effective secondary electron emission coefficient due to volume photoionization in a positive corona system and the secondary electron emission in conventional Townsend discharge theory. The results also demonstrate the significance of boundary conditions for accurate corona solutions that are based on a differential formulation of photoionization.

Spatio‐temporal distributions of accumulated charge density in atmospheric pressure dielectric barrier discharges

AbstractThe effect of the secondary electron emission coefficient on accumulated charge density distributions in atmospheric pressure dielectric barrier discharges have been simulated using two‐dimensional fluid model. Spatio‐temporal distributions of accumulated charge density on each side of the dielectric have been examined. The differences of conventional waveforms of accumulated charge density and the simulated results were clarified. And when the dielectrics were negatively accumulated, charges were uniformly accumulated, while positive charges were not. Randomness of streamers' occurrence, and self‐organized structures have been also examined by the accumulated charge distributions.

Spatio-temporal Distributions of Accumulated Charge Density in Atmospheric Pressure Dielectric Barrier Discharges

Effect of secondary electron emission coefficient on accumulated charge density distributions in atmospheric pressure dielectric barrier discharges have been simulated using two dimensional fluid model. Spatio-temporal distributions of accumulated charge density on each side of dielectric have been examined. The differences of conventional waveforms of accumulated charge density and the simulated results were clarified. And when the dielectrics were negatively accumulated, charges were uniformly accumulated, while positive charges were not. Randomness of streamers occurrence, and self-organized structures have been also examined by the accumulated charge distributions.

Realization of homogeneous dielectric barrier discharge in atmospheric pressure argon and the effect of beads on its characteristics

This paper describes the realization of a homogeneous dielectric barrier discharge (DBD) in argon at atmospheric pressure. The effect of the morphology of the dielectric surface (especially the dielectric surface covered by hollow ceramic beads (99% Al2O3) with different diameters) on discharge is investigated. With different dielectrics, the argon DBD presents two discharge modes: a filamentary mode and a homogeneous mode. Fast photography shows that the filamentary mode operates in a streamer discharge, and the homogeneous mode operates in a Townsend discharge regime. It is found that a homogeneous discharge can be generated within a certain voltage range. The voltage amplitude range decreases, and the breakdown voltage increases with the increase in the mean diameter of the ceramic beads. Waveforms of the total current and optical emission signal present stochastic pulses per half voltage cycle for the filamentary mode, whereas there is one single hump per half voltage cycle for the homogeneous mode. In the homogeneous mode, the intensity of the optical emission decreases with the mean diameter of the ceramic beads. The optical emission spectrum is mainly composed of atomic lines of argon and the second positive system of molecular nitrogen. It reveals that the electron density decreases with the increasing mean diameter of the ceramic beads. The vibrational temperature increases with the increasing mean diameter of the ceramic beads. It is believed that a large number of microdischarges are formed, and smaller ceramic beads have a larger activation surface area and more point discharge. Electrons liberated in the shallow well and electrons generated from microdischarges can increase the secondary electron emission coefficient of the cathode and provide initial electrons for discharge continuously. Therefore, the breakdown electric field is reduced, which contributes to easier generation of homogeneous discharge. This is confirmed by the simulation results.

Study on Secondary Electron Yield of Dielectric Materials

The secondary electron emission coefficient of dielectric materials will have an important impact on the performance of electronic devices and equipment. In order to understand the secondary electron emission coefficient of common dielectric materials, the collection electrode negative bias method is introduced in this study. First, the measurement method of secondary electron emission coefficient of dielectric materials is analyzed, and 20 common dielectric materials are introduced, the secondary electron emission coefficients of 20 kinds of common dielectric materials were measured and analyzed, which laid the foundation for the subsequent application of dielectric materials. The results show that among common dielectric materials, the secondary electron emission coefficients of alumina, silica and muscovite are relatively high, while those of polyimide, polycarbonate and polyester are relatively low; The surface roughness of the material will also have an important impact on the secondary electron emission coefficient. With the gradual increase of the surface roughness of the material, the undulation of the material surface will have a shielding effect and absorption effect on the secondary electrons, which will eventually lead to the gradual reduction of the secondary electron emission coefficient of the material. The research results can provide data support for the establishment of the database of secondary electron emission characteristics of common dielectric materials.

Расчетное исследование предельных характеристик электризации стенки при обтекании низкотемпературной плазмой

The purpose of this work lies in computational study of the floating potential distribution in the perturbed zone adjacent to the wall in the ionized flow of low-temperature plasma. As a result of electrophysical interaction, primary and secondary emission currents appear on the wall surface. Based on the probe theory, mathematical model was developed to determine the current values of the combustion products ionized flow and spatial potential on the surface of walls in the power plant flow path. Parametric calculation of the potential on the wall at its contact with the low-temperature plasma was carried out without and taking into account the secondary emission processes. The results obtained taking into account only the primary currents demonstrate alteration in the potential on the wall in the range of --(1.5--0.5) V at the plasma flow temperature of 1,000--3,500 K. Results are presented of the design research of the wall floating potential dependence with various secondary electron emission coefficients in comparison with the known experimental results. Metal walls were characterized by formation of a shielding layer, where only the primary currents appeared. It is shown that their floating potential changes the sign in dielectric walls, as coefficient of the secondary electron emission decreases. Parametric calculation of the current value near the wall was carried out showing that with an increase in pressure by 0.1--10 MPa, the current value was changing in the range of 0.005--0.025 A

Application of optical emission spectroscopy to predict the composition of films during reactive magnetron sputtering of Ti-Al composite targets

The processes of reactive magnetron sputtering of Ti-Al composite targets with different Al/Ti ratios have been studied. The dependences of the deposition rate, discharge voltage, elemental composition, and intensity of the control lines of optical plasma radiation on the oxygen concentration in the Ar/O2 gas mixture are established. It is shown that during reactive sputtering of Ti-Al composite targets, the discharge voltage is determined by the effective ion-induced secondary electron emission coefficient (SEEC) which depends on the area occupied by metals on the target, the degree of their oxidation, and the SEEC of these metals and their oxides. The rate of the deposition of TixAl1-xOy films in the metallic and transition sputtering modes increases proportional to the concentration of Al in the target, but the relative concentration of metals in the deposited films depends on the oxygen concentration in the Ar/O2 gas mixture and is determined by the reactivity of the target materials. It has been shown by optical emission spectroscopy (OES) that the ratio of the atomic concentrations of Al and Ti in deposited TixAl1-xOy films unambiguously depends on the ratio of the intensity of the control optical lines of aluminum AlI and titanium TiI in plasma. This makes it possible to use the OES method to predict the content of metals in a film during reactive magnetron sputtering of Ti-Al targets.

Влияние энергии электронного пучка на характеристики зарядки полимерных композитов с включением углеродных нанотрубок

The possibility of using polymer composite materials based on epoxy resins with the inclusion of carbon nanotubes as coatings with a low coefficient of secondary electron emission has been studied. Four types of samples were obtained: epoxy polymer, polymer composites with fillers (non-oriented and oriented carbon nanotubes, carbon soot). It is shown that the secondary electron emission yield depends on the structure of the introduced carbon filler, and polymer composites with oriented nanotubes exhibit antidynatron properties.

Laboratory Simulation of the Positron–Dust Interaction and its Implication for Interstellar Dark Clouds

We report the first laboratory experiment dealing with the interaction of a cosmic dust simulant with positrons emitted from a 22Na radioisotope. Measurements of a charge of micrometer SiO2 dust grains with an accuracy of one elementary charge e revealed +1 e steps due to positron annihilation inside the grain. The observed average rate of these charging events agrees well with prediction of a model based on the continuous slowing down approximation of energetic of positrons inside the grain. Less frequent charge steps larger than +1 e were attributed to emission of secondary electrons during positron slowing down. The determined coefficient of secondary electron emission is approximately inversely proportional to the grain radius. The experimental results led us to the formulation of a possible scenario of interstellar dark clouds charging.

Particle Accelerator and Vacuum Related Research at Universities

Particle accelerators and vacuum are inseparable. After a description of the situation surrounding the development of particle accelerators at universities, some related research areas are introduced, i.e., the J-PARC ACS linear accelerator, a liquid lithium target system for BNCT (Boron Neutron Capture therapy) and measurements of secondary electron emission coefficients of dielectrics for RF vacuum windows.

Current–voltage characteristics and mechanisms of electron emission from cold cathodes in a helium discharge

The abnormal discharge (AD) in pure helium and in helium containing molecular impurities was investigated. The obtained results were compared with the parameters of wide-aperture AD and different variants of ‘open’ discharge. It was shown that in all of the investigated types of discharges the current–voltage characteristics (CVCs) and, correspondingly, the emission properties of cold cathodes are determined mainly not by their material, but by the doping of cathodes with atoms of working gases and the purity of experimental conditions. With the impurity content less than 10−4% of the helium atom concentration, the CVCs begin to acquire S-shaped form, which is associated with a change in the electron emission mechanism. It was shown that the diversity of the CVCs is caused by uncertainty in the values of the secondary electron emission coefficients γ and the electron multiplication coefficient α in the cathode layer at reduced electric field strength E/N 103 Td. The reproducibility of CVCs and, correspondingly, emission properties of cathodes can be ensured by high purity of the working gas and by maintenance of the cathode doping only by the working gas atoms.