Secondary Emission Coefficient Research Articles

Effects of the ground-electrode temperature on electrical and optical characteristics of a coaxial dielectric barrier discharge in atmospheric pressure air

The applied power and electron energy are believed to play important roles in plasma applications, which can be significantly altered by controlling the ground-electrode temperature in a coaxial dielectric barrier discharge (DBD). The effects of the ground-electrode temperature on the electrical and optical characteristics of the DBD were investigated in atmospheric pressure air. The results show that the breakdown voltage decreases, and the number of discharge pulses, the transferred charges, and the applied power increase with the electrode temperature, while the peak current tends to decrease when the electrode temperature is below −35 °C and above 25 °C, respectively. When the electrode temperature remains constant, the breakdown voltage increases and the applied power decreases with the gap distance. The applied power increases with the applied voltage. The dielectric capacitance, the total capacitance, and the gap capacitance at the electrode temperature below −35 °C are generally higher than those at the electrode temperature above 25 °C. Moreover, they change in the same trend when the electrode temperature and the applied voltage increase. The dielectric and total capacitances increase while the gap capacitance decreases. The relative intensities of the spectral lines increase with the electrode temperature. The electron, vibrational, and rotational temperatures increase with the electrode temperature above 25 °C. However, the electron and vibrational temperatures increase and decrease with the electrode temperature below −35 °C. The secondary emission coefficients and electron energy distributions with the electrode temperature were discussed.

Assessing apparent secondary electron yield from current–voltage characteristics of abnormal DC glow discharges

The problem of assessing apparent secondary emission coefficient based on electrical properties of direct-current (DC) glow discharges using numerical modeling is revisited. An analysis of potential sources of uncertainties resulting from errors in experimental data as well as from model assumptions and approximations is presented. An estimation method based on a previously developed analytical model of a DC glow discharge is suggested. Application of the method is demonstrated in the example available in the literature of current–voltage characteristics of DC glow discharges in argon with copper electrodes. Values obtained for the considered data fall into two distinguishable groups, closely corresponding to those for clean and dirty copper cathode surfaces. The obtained preliminary results suggest the feasibility of estimating apparent secondary emission coefficients in DC discharges using numerical modeling.

Modelling of Nonthermal Dielectric Barrier Discharge Plasma at Atmospheric Pressure and Role of Produced Reactive Species in Surface Polymer Microbial Purification

A nonthermal atmospheric plasma reactor was used to sterilize polymer surfaces and satisfy safety constraints in a biological medium. A 1D fluid model was developed using COMSOL Multiphysics software® 5.4 with a helium-oxygen mixture at low temperature for the decontamination of bacteria on polymer surfaces. An analysis of the evolution of the homogeneous dielectric barrier discharge (DBD) was carried out through studying the dynamic behavior of the discharge parameters including the discharge current, the consumed power, the gas gap voltage, and transport charges. In addition, the electrical characteristics of a homogeneous DBD under different operating conditions were studied. The results shown that increasing voltage or frequency caused higher ionization levels and maximum increase of metastable species' density and expanded the sterilization area. On the other hand, it was possible to operate plasma discharges at a low voltage and a high density of plasma using higher values of the secondary emission coefficient or permittivity of the dielectric barrier materials. When the discharge gas pressure increased, the current discharges declined, which indicated a lower sterilization efficiency under high pressure. A short gap width and the admixture of oxygen were needed for sufficient bio-decontamination. Plasma-based pollutant degradation devices could therefore benefit from these results.

Saturation Effect of Secondary Emission Coefficient in MCP-based Multipliers

Saturation Effect of Secondary Emission Coefficient in MCP-based Multipliers

Vlasov simulation of the emissive plasma sheath with energy-dependent secondary emission coefficient and improved modeling for dielectric charging effects

A one-dimensional Vlasov–Poisson simulation code is employed to investigate the plasma sheath considering electron-induced secondary electron emission (SEE) and backscattering. The SEE coefficient is commonly treated as constant in a range of plasma simulations; here, an improved SEE model of a charged dielectric wall is constructed, which includes the wall charging effect on the SEE coefficient and the energy dependency of the SEE coefficient. Pertinent algorithms to implement the previously mentioned SEE model in plasma simulation are studied in detail. It is found that the SEE coefficient increases with the amount of negative wall charges, which in turn reduces the emissive sheath potential. With an energy-dependent SEE coefficient, the sheath potential is a nonlinear function of the plasma electron temperature, as opposed to the linear relation predicted by the classic emissive sheath theory. Simulation combining both wall-charging effect and SEE coefficient’ energy dependency suggests that the space-charged limited sheath is formed at high plasma electron temperature levels, where both sheath potential and surface charging saturate. Additionally, different algorithms to implement the backscattering in the kinetic simulation are tested and compared. Converting backscattered electrons to secondary electrons via an effective SEE coefficient barely affects the sheath properties. The simulation results are shown to be commensurate with the upgraded sheath theory predictions.

Analysis of secondary emission mechanism in electron avalanches propagating in cylindrical nanoruptures in liquid water

Recently, a bouncing-like mechanism for electron multiplication inside long nano-ruptures during the early stages of nanosecond discharge in liquid water has been proposed in (Bonaventura 2021 Plasma Sources Sci. Technol. 30 065023). This mechanism leads to the formation of electron avalanches within nano-ruptures caused by strong electrostrictive forces. The avalanche propagation is a self-sustaining process: the electrons emitted from the water surface to the cavity support the propagation of the avalanche and the avalanche itself is a source of the parent electrons impinging on the surface of the nano-rupture and causing secondary emission. We analyze the process of the electron secondary emission directly from the simulation results of the electron avalanche propagation. This allow us to perform an in situ study of the secondary emission and related physical processes. We present the results of an extensive parametric study performed using the state-of-the-art simulation toolkit Geant4-DNA for modeling electron-liquid water interactions. It is shown that the typical lifetime of an electron in an avalanche is about 0.1 to 0.2 picoseconds and that the electron experiences about 4 bounces before ending up in liquid water. In addition, it is shown that the secondary electrons are formed in a layer adjacent to the nano-rupture surface that is only a few nanometres thin. The secondary electron velocity distribution at the moment of the electron birth, the velocity space of electrons (re-)emitted from the water, and the velocity space of electrons at the moment of their impact to the cavity surface are analyzed in detail. Electron bouncing and secondary electron generation efficiency are quantified using the secondary emission coefficient, the secondary emission efficiency, and the effective energy consumed to produce new electrons.

Atmospheric pressure dual RF–LF frequency discharge: transition from α to α – γ -mode

This paper investigates the transition from α to α–γ-mode of a dual frequency (5 MHz/50 kHz) dielectric barrier discharge (DBD) at atmospheric pressure. The study is based on both experiments and modeling of a plane/plane DBD in a Penning mixture (Ar–NH3). The discharge is in the α-RF mode with three different voltage amplitudes (250, 300 and 350 V) and biased by a low-frequency (LF) voltage with an amplitude varying from 0 to 1300 V. At a given threshold of LF voltage amplitude (of about 400 V for a 2 mm gap and 133 ppm of NH3), a transition from α to α–γ-mode occurs. It is characterized by a drastic increase of both the argon and NH emissions. Increasing the NH3 concentration leads to a decrease of the LF voltage amplitude required to reach the α–γ-mode (experiment). The transition from α to α–γ-mode is initiated when the ionization in the sheath increases and the α–γ-mode is established when this ionization becomes higher than the self-sustainment criterion (1/γ). The transition from α to α–γ-mode results in an increase of the particle densities and a stabilization of the gas voltage independently of the LF voltage amplitude. Without secondary electron emission there is no transition. In the model, increasing the secondary emission coefficient from 0.05 to 0.15 leads to a decrease of the LF voltage amplitude required to switch from α to α–γ-mode from 700 to 550 V.

A computational study of positive streamers interacting with dielectrics

We use numerical simulations to study the dynamics of surface discharges, which are common in high-voltage engineering. We simulate positive streamer discharges that propagate towards a dielectric surface, attach to it, and then propagate over the surface. The simulations are performed in air with a two-dimensional plasma fluid model, in which a flat dielectric is placed between two plate electrodes. Electrostatic attraction is the main mechanism that causes streamers to grow towards the dielectric. Due to the net charge in the streamer head, the dielectric gets polarized, and the electric field between the streamer and the dielectric is increased. Compared to streamers in bulk gas, surface streamers have a smaller radius, a higher electric field, a higher electron density, and higher propagation velocity. A higher applied voltage leads to faster inception and faster propagation of the surface discharge. A higher dielectric permittivity leads to more rapid attachment of the streamer to the surface and a thinner surface streamer. Secondary emission coefficients are shown to play a modest role, which is due to relatively strong photoionization in air. In the simulations, a high electric field is present between the positive streamers and the dielectric surface. We show that the magnitude and decay of this field are affected by the positive ion mobility.

The Regimes for Sustaining a Hollow-Cathode Glow Discharge with a Hot Filament Inside the Cavity

The results of investigations of a low-pressure hollow-cathode glow discharge with a hot filament (thermionic cathode) inside the cavity are presented. The current-voltage characteristics (CVCs) of the discharge and the dependences of the current delivered to the thermionic and hollow cathodes on the discharge burning voltage are obtained at different gas pressures and filament currents. It is shown that the major fraction of the current is transferred in the thermionic cathode circuit. For interpretation of the CVCs, a model is accepted, which uses a generalized coefficient (rather than a conventionally utilized secondary-emission coefficient), which includes not only the cathode bombardment with ions but also the emission current induced by an external source. An estimation is made of the discharge parameters. The model is shown to be quite consistent with the experiment.

Plasma Simulation Benchmark for CCRF Discharge With Secondary Electron Emission

A comparative study of capacitively coupled radio frequency discharges was conducted using 2-D particle-in-cell simulation (PIC) and 2-D fluid plasma simulation. The selected scenario was a planar discharge with a 3-cm gap operated in helium, at a gas pressure of 400 Pa, with a driving voltage between 120 and 400 V at 3.2 MHz. The solvers were adapted to a 1-D problem by using periodic boundary conditions perpendicular to the gap, and the models included ion impact-induced secondary electron emission from the electrodes. A good agreement was obtained between the results of both solvers, with less than 23% deviation in the peak plasma density. For the studied range of secondary emission coefficients, $0.08 , it was found that a common transition point between “alpha” and “gamma” modes occurs at $V_{\mathbf {rf}} \sim 250$ V. Since the simulation results were shown to be less sensitive to the secondary emission coefficient below this transition voltage, these results can serve as a standard benchmark for plasma codes.

The dynamics of ionization waves formation in a transverse nanosecond plasma-beam discharge with a slotted cathode in argon

The article presents the results of an experimental study and numerical simulation of the spatio-temporal dynamics of ionisation processes in the gap between the electrodes and inside the cathode cavity during the formation of a nanosecond discharge in argon at pressures in the chamber from 1 to 10 Torr. The correspondence between the density distribution of charged particles and the optical patterns of the discharge is established. A numerical simulation of the formation of a limited discharge in argon under various external conditions is constructed that accounts for the influence of the charge deposited on the surface of the dielectric wall, the coefficient of secondary emission of electrons from the cathode surface and the effect of the space charge on the distribution of electric potential between the electrodes. The role of the surface charge deposited on the dielectric walls in the formation of the spatial structure of the discharge is determined. The results of numerical simulation are compared with experimental data. It is shown that the electron concentration in a discharge limited by dielectric walls is an order of magnitude higher than in an unlimited discharge under the same external conditions. The general patterns of formation and development of a limited discharge in argon are discussed and the main physical processes affecting the dynamics of development and the spatial structure of the discharge are established.

Influence of external impact on secondary emission characteristics of anti-multipactor nanocarbon

This work demonstrates that a thin nanocarbon film applied to a metal surface is capable of decreasing the coefficient of secondary emission down to the value of 1 ÷ 1.5. Thereby, eliminating the possibility of a multipactor discharge. In this paper we describe a method of making an anti-multipactor coating from a colloid solution of nanocarbon. The colloid solution of nanocarbon is made by creating a multi-spark high voltage pulse discharge in ethanol where argon is injected into the inter electrode space. Thermal, climate and tear-off tests have shown that emission properties of the surface are retained.

Cathode spot formation possibly explained by cathode electron emission from impact of excited state atoms

A theory is given for electron emission from electric arcs with non-thermionic cathodes, for example the iron cathodes used in arc welding, proposing that the principal process for electron emission is from the impact of excited atomic states at the cathode. For plasmas in contact with a metal cathode, a sheath of positive ions is formed over the cathode surface because electrons move away from the cathode. A sheath thickness of only 0.1 µm causes secondary electrons produced at the cathode surface to gain ~12 V. The electron transit through this thin sheath is collision-less. These 12 V electrons produce excited states of atoms to a distance of approximately ~10 µm from the cathode. Diffusion and direct transfer of these excited atoms back to the cathode in the almost collision free region next to the cathode is calculated to be in times of ~10−8 s. This time is less than the radiative lifetimes of almost all atomic transitions except for some resonance transitions where there would be an enhancement of lifetimes due to imprisonment of radiation. If these transfer times are less than the radiative lifetimes, secondary emission coefficients of electrons from the cathode are likely to be approximately 1, the measured coefficients for metastable atoms. The energies of these excited atoms are ~12 V or more, much larger than the usual work function of ~4.5 V required to eject electrons. If such secondary electrons, together with secondary electrons produced from the impact of positive ions and photons, replace or exceed the original number of electrons, amplification is possible in successive avalanches to cause electron densities to grow to supply the total arc current. The mechanism would then explain why observed cathode sheath voltages are independent of arc current for currents varying from 10 to 20 000 A for various low melting point electrodes and arc gases. Electron emission from cold cathodes, when temperatures are too low for thermionic emission, would also be explained. Solutions of the densities of electrons, positive ions and excited state atoms as a function of time and distance from the cathode are presented which give a physical explanation for the formation of cathode spots.

On the role of secondary electron emission in capacitively coupled radio‐frequency plasma sheath: A theoretical ground

AbstractWe propose a theoretical ground for emissive capacitively coupled radio‐frequency (rf) plasma sheath under low pressure. The rf sheath is assumed to be collisionless and oscillates with external source. A known sinusoidal voltage instead of current is taken as prerequisite to derive sheath dynamics. Kinetic studies are performed to determine mean wall potential as a function of secondary emission coefficient and applied voltage amplitude, with which the complete mean direct current sheath is resolved. Analytical analyses under homogeneous model and numerical analyses under inhomogeneous model are conducted to deduce real‐time sheath properties including space potential, sheath capacitance, and stochastic heating. Obtained results are validated by a continuum kinetic simulation without ionization. The influences of collisionality and ionization induced by secondary electrons are elucidated with a particle‐in‐cell simulation, which further formalizes proposed theories and inspires future works.

The Transition to Paschen\u2019s Law for Microscale Gas Breakdown at Subatmospheric Pressure

The decrease in electronic device size necessitates greater understanding of gas breakdown and electron emission at microscale to optimize performance. While traditional breakdown theory using Paschen’s law (PL), driven by Townsend avalanche, fails for gap distance d lesssim 15 μm, recent studies have derived analytic equations for breakdown voltage when field emission and Townsend avalanche drive breakdown. This study derives a new analytic equation that predicts breakdown voltage VB within 4% of the exact numerical results of a previously derived theory and new experimental results at subatmospheric pressure for gap distances from 1–25 μm. At atmospheric pressure, VB transitions to PL near the product of pressure and gap distance, pd, corresponding to the Paschen minimum; at lower pressures, the transition to PL occurs to the left of the minimum. We further show that the work function plays a major role in determining which side of the Paschen minimum VB transitions to PL as pressure approaches atmospheric pressure while field enhancement and the secondary emission coefficient play smaller roles. These results indicate that appropriate combinations of these parameters cause VB to transition to PL to the left of the Paschen minimum, which would yield an extended plateau similar to some microscale gas breakdown experimental observations.

Suppression of secondary emission of electrons from end plate in expander of open trap

The influence of secondary emission from the end plate in the expander divertor on the energy confinement in linear plasma devices was considered. A numerical model was developed to calculate a distribution of the electric potential along a magnetic field line in the expander divertor. The model assumes weak collisions and the absence of cross-field losses of both ions and electrons. The potential profiles in the case of both no and strong secondary emission are presented. It was shown that expansion of the magnetic field in the divertor can effectively suppress the influence of cold electrons on energy confinement even if the secondary emission coefficient is more than unity. The dependence of energy lost per one ion leaving the mirror on the secondary emission coefficient was studied. The influence of partial energy recuperation by cold electrons on the total energy balance was discussed. It has been shown that the energy loss per one ion is not sensitive to the recirculation of cold electrons if the secondary emission coefficient is not too large.

Электронно-эмиссионные свойства полупроводниковых субмикронных частиц

AbstractThe electron emission properties of submicron Si, GaAs, InSb, and InAs semiconductor particles and their multigrain structures have been investigated. The effect of the properties of nanoparticles on the field and secondary emissions has been established. A scanning electron microscopy-based method for measuring the secondary emission coefficient of semiconductors has been proposed. The effect of photoexcitation of the multigrain structure of submicron semiconductor particles on their secondary emission properties has been investigated by the vacuum triode method.

Systematic investigation of the effect of N2 admixture ratio on barrier discharge in helium

The influence of the N2 admixture ratio on a He–N2 uniform atmospheric pressure glow discharge (APGD) generated using an asymmetric 20 µs, 10 kHz bipolar applied voltage pulse is systematically investigated, using electrical and optical emission spectroscopic measurements as diagnostic tools. We focused our investigation on understanding the discharge characteristics of the plasma ignited after an off-voltage period of 80 µs duration. The measured breakdown voltages for various N2 admixture ratios of up to 3% are compared with a theoretically calculated breakdown voltage, taking into account the secondary electron emission coefficient of the dielectric surface. The results indicate that the value of the secondary emission coefficient should increase from 0.1 to 0.5 to realize the experimentally obtained breakdown voltage variation within a 3% N2 admixture ratio. Using experimentally obtained values of the breakdown voltage, discharge current densities for different N2 admixture ratios, and 66 reactions among electrons, helium and N2, the variations of the N2 (A), N2 (C) and (B) state densities with N2 admixture ratio are calculated. The N2 admixture ratio dependence of the calculated N2 (C) and (B) state densities are similar to the N2 admixture ratio dependences of the emission intensities of the 337 nm band of the N2 second positive system (SPS) and the 391.4 nm band of the first negative system, respectively. Experimental observation and numerical investigation indicates that below a 1% N2 admixture ratio, the He (23S) metastable state has a strong influence on the discharge characteristics, and above a 2% N2 admixture ratio, the N2 (A) metastable state influences the discharge behavior. The N2 (A) metastable state density is estimated experimentally using the emission intensity ratio of the 337 nm band of N2 SPS and the 247 nm band of a NO-γ system. The experimentally estimated and theoretically calculated value of N2 (A) metastable state density is of the same order of 1013 cm−3. The estimated N2 (A) metastable state density shows a rapid increase above a 2% N2 admixture ratio, which supports the idea that the secondary emission coefficient increases due to an increase in N2 (A) flux.

Electron Emission Properties of Submicron Semiconductor Particles

The electron emission properties of submicron Si, GaAs, InSb, and InAs semiconductor particles and their multigrain structures have been investigated. The effect of the properties of nanoparticles on the field and secondary emissions has been established. A scanning electron microscopy-based method for measuring the secondary emission coefficient of semiconductors has been proposed. The effect of photoexcitation of the multigrain structure of submicron semiconductor particles on their secondary emission properties has been investigated by the vacuum triode method.

Numerical simulation of negative point-plane corona discharge mechanism in SF6 gas

This paper presents the results of the numerical simulation of negative corona discharge in SF6 at 0.4 Mpa for a point-plane configuration. A 2D axisymmetric model considering three charged species has been investigated with an applied voltage equal to −25 kV. Three drift-diffusion equations coupled with Poisson’s equation were solved simultaneously to obtain the temporal and spatial properties of Trichel pulses. The distributions of charged species at different instants of time and the time-dependent total number of charged species have been determined. The characteristics of current pulses with voltages varying from −24 kV to −27 kV were compared and it has been found that the characteristics of the current magnitude and frequency are similar to those in air. The effects of the model coefficients—the ionization coefficient, the attachment coefficient, the mobility of charged species and the secondary emission coefficient—on the characteristics of current pulses have also been studied to investigate the sensitivities of the pulse parameters to these coefficients.