J Loureiro

Role played by the N2(A3Σu+) metastable in stationary N2 and N2-O2 discharges

The role played by the N2(A3Σu+) metastable on the overall kinetics of N2 and N2-O2 stationary discharges is illustrated by using a kinetic model based on the self-consistent solutions to the Boltzmann equation coupled to the rate balance equations for the vibrationally and electronically excited molecules, atoms and charged particles, in which the sustaining electric field is self-consistently determined. It is shown that together with the vibrational distribution of N2(X1Σg+,v) molecules, the metastable state N2(A3Σu+) plays a central role in the whole problem, since some important aspects of these discharges, such as ionization, gas phase chemistry and gas heating are associated with different processes involving the N2(A3Σu+) state.

ELECTRON AND HEAVY-PARTICLE KINETICS IN THE LOW PRESSURE OXYGEN POSITIVE COLUMN

A kinetic model for the low-pressure oxygen positive column is presented and discussed. The model is based on the electron Boltzmann equation and the rate balance equations for the dominant heavy-particle species, which are solved simultaneously in order to take into account the coupling between the electron and the heavy-particle kinetics. The effects of vibrationally excited molecules, dissociated atoms and metastable states on the electron kinetics are analysed in detail. The predicted populations of O2(X3 Sigma ), O2(a1 Delta ), O(3P), and O- are shown to agree satisfactorily with previously reported measurements. A combination of this kinetic model with the continuity and transport equations for the charged species e, O-, and O2+ is shown to provide characteristics for the maintenance field that agree reasonably well with experiment.

Self-consistent electron and heavy-particle kinetics in a low-pressure - glow discharge

A homogeneous kinetic model of a low-pressure - positive column is developed. The model is based on the self-consistent solutions to the electron Boltzmann equation coupled to the rate balance equations for the vibrationally excited molecules (X ) and (X ), the electronically excited states of , and NO(), and species. Further, this set is still solved together with the continuity equations for the electrons and the main positive ions (, , , , ) in order to determine the maintenance reduced electric field. This formulation allows us to determine the concentrations of the various neutral and ionic species, the electron density and the vibrational temperatures, and , as a function of the gas pressure, discharge current, gas temperature, tube radius and fractional composition. The calculated results are shown to be in satisfactory agreement with published experimental data. The complex interplay kinetics is analysed in detail and the effects of the poor knowledge of some collisional and surface data on the results are discussed.

Effects of electron-electron collisions on the characteristics of DC and microwave discharges in argon at low pressures

The authors present a systematic investigation of the effects caused by electron-electron collisions on the electron kinetics in Ar under the action of DC and microwave fields. The analysis is based on solutions to the homogeneous electron Boltzmann equation using the effective field approximation, i.e. assuming that the electron energy distribution function is time independent. The calculated electron transport parameters and ionization coefficient are used together with a generalized charged particle balance equation to derive the characteristics for the maintenance field and the mean absorbed power per electron for steady-state DC and microwave discharges in Ar. Results are provided for the cases of (i) single-step ionization; (ii) multi-step ionization, via the metastable and the resonance levels of Ar.

Dissociation rate and N (4S) atom concentrations in a N2 glow-discharge

Abstract The rate of dissociation in N 2 and the fractional densities of N ( 4 S) atoms have been self-consistently calculated for typical operating conditions of a low-pressure, moderate current, nitrogen discharge. The present model solves simultaneously the electron Boltzmann equation, a system of rate balance equations for the vibrational levels N 2 (X, ν) and the rate balance equation for the N ( 4 S) atoms. The Boltzmann equation includes both inelastic and superelastic e-V collisions, while the system for the vibrationallevels takes into account e-V, V-V and V-T exchanges as well as the processcs of dissociation, atom reassociation and vibrational deactivation on the walls of the container. The total dissociation rate includes both dissociation by electron impact and dissociation by the V-V and V-T energy exchange processes. The effects of wall deactivation on the vibrational distribution functions of N 2 (X, ν) molecules and on the rate of dissociation are analysed in detail. The various mechanisms fordestruction of N( 4 S) atoms are discussed and weighted. Predictions and measurements reported in the literature of the rate of dissociation and atom densities are shown to agree reasonably well.

A review of non-equilibrium dissociation rates and models for atmospheric entry studies

The appropriate simulation of vibrational exchange and dissociation processes in high-temperature (above 1000 K) plasmas mandates the application of more detailed state-to-state models, when compared with those based on first-order theories, utilized for the simulation of low-pressure plasmas. Such can be achieved through the application of approaches such as the forced harmonic oscillator model, or the quasi-classical trajectory model. This allows obtaining multiquantum state-resolved rates significantly more accurate at high temperatures, as compared with rates issued from first-order perturbation theories. This work reviews the more recent high-temperature datasets proposed by several groups, including our own. Such datasets have then been applied to the simulation of dissociation processes in high-temperature shock-heated nitrogen flows, and a comparison against traditional multi-temperature models has been carried out, for post-shock temperatures ranging from 20 000-100 000 K. Differences between the results predicted by the state-resolved and macroscopic models are significant, except in the high-temperature limit.

Non-equilibrium kinetics in N2 discharges and post-discharges: a full picture by modelling and impact on the applications

The main concerns associated with the establishment of a self-consistent model for N2 discharges and post-discharges at low pressures (typically p ~ 1 Torr), as well as in mixtures of this gas with O2 and CH4 are analysed and discussed. The focus is given on the coupling of the various kinetics involved: electrons, vibrational molecules N2 , dissociated atoms N(4S), ionic species, and various atomic and molecular electronic states. The impact of N2–O2 and N2–CH4 systems on the applications is briefly summarized by reviewing the essential kinetics. The difficulty in incorporating a self-consistent model for the surface kinetics is also discussed and a state-of-the-art approach for wall reactions is presented.

Electron kinetics in atomic and molecular plasmas

In the first part of this paper we briefly review some basic concepts of kinetic theory. The concept of the velocity distribution function is first introduced and its meaning is discussed. Then, the Boltzmann equation is presented on physical grounds and it is shown that the fluid equations are its moments. In the second part, the Boltzmann equation for free electrons in a low-temperature plasma is analysed. It is shown how this equation can approximately be solved for electrons under a HF field of frequency ω (including the particular case of a dc field, which corresponds to the limit ω = 0) by using a first-order double expansion in spherical harmonics in velocity space and a Fourier series in time. The electron transport parameters, particle balance and energy balance are analysed from a general point of view. Finally, an application to argon and nitrogen is given. The effects of changes in the field frequency on the electron energy distribution function, transport parameters and power balance are discussed. The importance of the coupling between the electron and the vibrational kinetics in N2 is emphasized.

Self-confinement of a fast pulsed electron beam generated in a double discharge

The construction of a double discharge pulsed electron beam generator and the study of the characteristics of the beam are presented in this paper. The electron beam generator consists of a fast filamentary discharge in superposition with an ordinary glow discharge in low-pressure gases. The filling gas is argon or helium at approximately 0.1 Torr pressure. The duration of the electron beam is shorter than 50 ns and the peak current intensity is of the order of amperes. The electron density is evaluated by making use of Stark broadening of the Hβ line and compared with the full computer simulation method. The pinch effect of the filamentary discharge is evaluated and its size compared with the diameter of the beam.

Dependence of volume-produced H− ions on the wall recombination probability of H atoms in a low pressure H2 positive column

A kinetic study of a low-pressure hydrogen positive column, at moderate currents, is carried out by solving the electron Boltzmann equation coupled to a system of rate balance equations for the populations of the vibrational levels H2(X 1Σg+,v), dissociated atoms H(1s), and negative ions H−. A systematic investigation of the effects of the wall recombination probability γH of H atoms on the enhancement of H− concentration is presented. It is shown that relative concentrations [H−]/ne larger than one can be obtained, at p=1–5 Torr and I=20–50 mA, as the probability for wall losses of H atoms increases beyond γH≃10−2. On the contrary, for lower values of γH, the concentrations of H atoms are high enough to produce a fast depopulating mechanism of the vibrational levels H2(X,v) by V–T exchanges in H2–H collisions, preventing H− formation by dissociative attachment from H2(X,v) molecules.