Non-equilibrium Electron Energy Distribution Function Research Articles

Gas Discharge Properties of Inverse-Brush-Electrodes Driven by Alternating Current

The inverse-brush-electrodes (IBE) structure with a large area is designed to investigate the discharge mechanism driven by alternating current (ac). It is found that the IBE structure increases the number of secondary electrons and reduces the breakdown voltage by comparing the discharge processes of IBE and plate electrodes. Meanwhile, the breakdown mechanism of gas discharge driven by ac is analyzed in detail. Additionally, the designed IBE structure has a negative differential conductivity enhancement effect than that of plate electrodes when plasmas are generated in the heavy noble gas. Finally, the negative differential conductivity enhancement phenomenon of the IBE structure is analyzed by experiments and simulations. This effect originates from the non-equilibrium electron energy distribution function (EEDF), which is due to the more suprathermal electrons generated by the IBE structure.

Magneto-Oscillations of the Charge of a Field-Effect Transistor That Are due to a Microwave-Induced Nonequilibrium Electron Energy Distribution

The observation of microwave-induced changes in the charge of a field effect transistor with a channel formed by a bilayer electron system has confirmed that microwave radiation induces a nonequilibrium electron energy distribution function, which generates magneto-oscillations of the resistance of two-dimensional electron systems. The observed periodicity and beating of magneto-oscillations of the charge have been explained by the redistribution of electrons between the layers, which occurs because of the corresponding nonequilibrium occupation of electronic states.

Influence of space charge density on electron energy distribution function and on composition of atmospheric pressure He/O2/air plasmas

Atmospheric pressure non-equilibrium plasma represents an efficient source of reactive species for different kinds of applications. Rich chemistry of such plasmas develops on longer time scales and is difficult to handle by kinetic models so global models are often applied to study such processes. The Maxwell-Boltzmann (MB) distribution is often used in global modelling of these non-equilibrium systems, even for the calculation of electron rate coefficients. In order to test the sensitivity of plasma composition on the space charge density and consequently on the assumed electron energy distribution function (EEDF), the zero-dimensional global model is applied to the helium/oxygen mixture (0.5% of O2 with humid air impurities. To test the effect of the distribution function on global models we have included the data calculated based on the non-equilibrium EEDF for the processes where often exponential Arrhenius-like formulae as a function of the effective temperature are used. The initial calculation showed that the change in the form of distribution function mainly affects the processes with thresholds considerably higher than the mean electron energy while it does not change much the rates for the processes with the thresholds and peaks of the cross sections in the region of the mean energy. We have calculated variations of the EEDF with the charge density and the resulting changes in chemical kinetics.

Causes of plasma column contraction in surface-wave-driven discharges in argon at atmospheric pressure.

In this work we compute the main features of a surface-wave-driven plasma in argon at atmospheric pressure in view of a better understanding of the contraction phenomenon. We include the detailed chemical kinetics dynamics of Ar and solve the mass conservation equations of the relevant neutral excited and charged species. The gas temperature radial profile is calculated by means of the thermal diffusion equation. The electric field radial profile is calculated directly from the numerical solution of the Maxwell equations assuming the surface wave to be propagating in the TM_{00} mode. The problem is considered to be radially symmetrical, the axial variations are neglected, and the equations are solved in a self-consistent fashion. We probe the model results considering three scenarios: (i) the electron energy distribution function (EEDF) is calculated by means of the Boltzmann equation; (ii) the EEDF is considered to be Maxwellian; (iii) the dissociative recombination is excluded from the chemical kinetics dynamics, but the nonequilibrium EEDF is preserved. From this analysis, the dissociative recombination is shown to be the leading mechanism in the constriction of surface-wave plasmas. The results are compared with mass spectrometry measurements of the radial density profile of the ions Ar^{+} and Ar_{2}^{+}. An explanation is proposed for the trends seen by Thomson scattering diagnostics that shows a substantial increase of electron temperature towards the plasma borders where the electron density is small.

Vibrational kinetics of electronically excited states in H2 discharges

The evolution of atmospheric pressure hydrogen plasma under the action of repetitively ns electrical pulse has been investigated using a 0D state-to-state kinetic model that self-consistently couples the master equation of heavy particles and the Boltzmann equation for free electrons. The kinetic model includes, together with atomic hydrogen states and the vibrational kinetics of H2 ground state, vibrational levels of singlet states, accounting for the collisional quenching, having a relevant role because of the high pressure. The mechanisms of excitations, radiative decay and collisional quenching involving the excited H2 states and the corresponding cross sections, integrated over the non-equilibrium electron energy distribution function (EEDF) to obtain kinetic rates, are discussed in the light of the kinetic simulation results, i.e. the time evolution during the pulse of the plasma composition, of the EEDF and of the vibrational distributions of ground and singlet excited states.

Electron kinetics in capacitively coupled plasmas modulated by electron injection

The controlling effect of an electron injection on the electron energy distribution function (EEDF) and on the energetic electron flux, in a capacitive radio-frequency argon plasma, is studied using a one-dimensional particle-in-cell/Monte Carlo collisions model. The input power of the electron beam is as small as several tens of Watts with laboratory achievable emission currents and energies. With the electron injection, the electron temperature decreases but with a significant high energy tail. The electron density, electron temperature in the sheath, and electron heating rate increase with the increasing emission energy. This is attributed to the extra heating of the energetic electrons in the EEDF tail. The non-equilibrium EEDF is obtained for strong non-local distributions of the electric field, electron heating rate, excitation, and ionization rate, indicating the discharge has transited from a volume heating (α-mode dominated) into a sheath heating (γ-mode dominated) type. In addition, the electron injection not only modifies the self-bias voltage, but also enhances the electron flux that can reach the electrodes. Moreover, the relative population of energetic electrons significantly increases with the electron injection compared to that without the electron injection, relevant for modifying the gas and surface chemistry reactions.

Numerical Study of the H<sup>0 </sup>Atomic Density and the Balmer Line Intensity Profiles in a Hydrogen Negative Ion Source with the Effect of Non-Equilibrium Electron Energy Distribution Function

Numerical Study of the H<sup>0 </sup>Atomic Density and the Balmer Line Intensity Profiles in a Hydrogen Negative Ion Source with the Effect of Non-Equilibrium Electron Energy Distribution Function

Numerical study of atomic production rate in hydrogen negative ion sources with the effect of non-equilibrium electron energy distribution function

Spatial non-uniformity of the dissociative hydrogen atom (H0) production has been investigated in a large negative ion source (JAEA 10 A source) with the electron energy distribution function (EEDF) obtained by a Monte-Carlo simulation code for electron transport in 3D3V (three dimensional real and velocity) space. It has been shown that the H0 production rate becomes larger in the upper region (one side in the longitudinal direction) of the source chamber. This spatial non-uniformity of the H0 production profile is mainly explained by the non-equilibrium features of the EEDF in the upper region, i.e., the EEDF consists of thermal electron component with kinetic energy ε < 25 eV and fast electron component with energy ε > 25 eV in the upper region, while the EEDF mainly consists of only thermal electrons in the bottom region. These characteristics for the EEDF and the energy dependence of cross-sections for dissociation and dissociative ionization processes lead to the non-uniform profile of the H0 production. The above numerical results of the spatial H0 non-uniformity are validated and confirmed by comparisons with those by spectroscopic measurement. It has been clarified that the non-equilibrium (fast electron) component of the EEDF has a large contribution to the non-uniformity of the H0 production rate.

High power microwave breakdown in gas using the fluid model with non-equilibrium electron energy distribution function

The electron energy distribution function (EEDF) is usually assumed to be of the Maxwellian distribution in the fluid model in the simulation of high power microwave breakdown in gas. However, this assumption may lead to some large errors in the simulations. In this paper we compute the non-equilibrium EEDF via solving the Boltzmann equation directly, and incorporate it into the fluid model for argon breakdown. Numerical simulations show that the breakdown time obtained by the fluid model with the non-equilibrium EEDF accords well with the Particle-in-cell-Monte Carlo collision simulation result, while the Maxwellian EEDF has higher energy tail and results in faster breakdown time at low mean electron energy. Based on the non-equilibrium EEDF, the dependence of the breakdown threshold on the pressure predicted by the fluid model accord well with the argon breakdown experimental result.

EFFECTS OF MICROWAVE FREQUENCY ON ELECTRON ENERGY DISTRIBUTION FUNCTION AND AIR BREAKDOWN USING THE FLUID MODEL

The non-equilibrium electron energy distribution function (EEDF) obtained via solving the Boltzmann equation is introduced into the ∞uid model,and the efiects of the microwave frequency on the EEDF and air breakdown are investigated. Numerical simulations show that the breakdown threshold of the ∞uid model with the non- equilibrium EEDF agrees well with that of the reported experiments. The microwave frequency plays an important role on the shape of the non-equilibrium EEDF at low pressures. The breakdown time at the low pressures predicted by the Maxwellian EEDF is shorter than that from the non-equilibrium EEDF in low-frequency oscillating flelds, while matches the latter in high-frequency oscillating flelds.

Nitrogen dissociation in low-pressure microwave plasmas

This work reviews a number of important aspects of the kinetic processes occurring in microwave discharge plasmas in nitrogen containing mixtures. The fundamental problems which are addressed include: (1) the determination of the non-equilibrium electron energy distribution function and its coupling to the vibrational kinetics; (2) the processes determining the vibrational distribution function in N2, N2-H2, and N2-Ar plasmas; and (3) the influence of surface kinetic processes on the whole discharge physics. The mechanisms of dissociation and ionization in N2, N2-Ar and N2-H2 are discussed in detail. The important role of plasma-wall interactions is stressed. It is shown that in N2-Ar mixtures there is an increase in the dissociation degree of N2 molecules at high Ar fractional concentrations, which can be ascribed to the contribution of fast Ar++ N2→ N+2+ Ar exchanges followed by N+2 dissociative recombination. It is further demonstrated that surface electron-ion recombination processes involving NH+2and N+2 ions are the most important sources of N(4S) gas phase atoms in N2-H2 mixtures. As the amount of H2 in the discharge increases, the channel associated with HN+2 ions becomes dominant. At nearly constant electron density there is a smooth decrease in the dissociation degree of hydrogen in N2-H2 when the H2 percentage increases up to 50%, which is due both to electron impact dissociation and H2 dissociation upon quenching of nitrogen N2(A3∑+u) and N2(a' 1∑−u) metastables.

Negative electron mobility under condition of the resonant optical excitation in gas mixtures

The negative electron mobility (NEM) effect has been analyzed in gas mixtures consisting of heavy inert gas (Ar, Kr or Xe), molecular nitrogen and lithium vapor. The mixture parameters, which are optimal for NEM observation, have been determined by using an analytical technique and simulating the Boltzmann equation for the nonequilibrium electron energy distribution function. It has been shown that NEM may be realized under condition of ionization evolution generated by selective optical excitation of resonant transition of lithium atom Li(2S)–Li(2P).

Conductivity of a photoplasma of sodium vapor-inert gas mixtures

Parametric computations of electron energy distribution function (EEDF) and of electron mobility in photoplasma of sodium mixture with different inert gases are presented. The conditions of a weak external electric field are considered when electron heating is provided by super-elastic impacts (second kind collisions) with excited sodium atoms. It is demonstrated that nonequilibrium EEDF arising in heavy inert gases due to Ramsauer minimum on the cross section of elastic electron-atom collisions can considerably affect electron mobility. It results in complex nonmonotonous dependence of electron mobility from partial pressure of mixture components, plasma ionization degree, and from resonant-excited sodium atoms density. In Na+Kr, Na+Xe mixtures by low plasma ionization degree, the electron mobility can differ by more than two times from the one calculated with Maxwellian EEDF. In Na+Ar mixture, in certain conditions, the mobility may take on negative values. The critical plasma parameters (sodium excited atoms density, plasma ionization degree, electric field strength) determining this effect are revealed. The influence of nonequilibrium EEDF on electron mobility helps to explain abnormally low conductivity of photoplasma in Na+Ar mixture observed earlier in the experiment.

Determination of mean electron energy in an elevated-pressure plasma by the probe method

The possibility of determining mean electron energy from the slope of the dependence of probe electron current on probe potential under conditions of a nonequilibrium electron energy distribution function (EEDF) and electron drain to the probe is demonstrated. To accomplish this one must use the portion of the current-voltage curve (CVC) in the region of the null of the third derivative of the probe current. Given a drain parameter δ with an accuracy of ≅20%. For an EEDF close to equilibrium the maximum uncertainty of determination by this technique does not exceed 50%.