Vasco Guerra

Kinetic model of a low-pressure N/sub 2/-O/sub 2/ flowing glow discharge

A self-consistent kinetic model is developed to study dc flowing glow discharges in N/sub 2//O/sub 2/ mixtures. This model includes the calculation of electron energy distribution functions and electron rate coefficients coupled with detailed vibrational kinetics of N/sub 2/ molecules, chemical kinetics taking into account a large set of neutral, excited and charged species, interaction of N and O atoms at the discharge tube wall, and the thermal balance of the discharge. The results of this model agree reasonably well with the measurements of the electronic density, the gas temperature, the reduced electric field, the vibrational temperature of N/sub 2/ and the concentration of O, N atoms, NO molecules, N/sub 2/(C), N/sub 2//sup +/(B), and NO(/spl gamma/) excited states. The comparison was performed in a N/sub 2/-O/sub 2/ discharge at pressure p=2 Torr, for discharge currents I=15, 30, and 80 mA, a flow rate Q=100 sccm, and O/sub 2/ percentages ranging from 0 up to 100%,. >

Electron and heavy particle kinetics in a low-pressure nitrogen glow discharge

A detailed kinetic model is used to investigate the mechanisms for ionization, dissociation and atomic re-association in a low-pressure positive column. The approach is based on the self-consistent solutions to the electron Boltzmann equation coupled to a system of rate balance equations for the levels, the electronically excited states of and the and ions. The maintenance electric field is self-consistently determined from the continuity equations for electrons and ions. The model provides a satisfactory explanation of measurements conducted in these conditions, in the range p = 0.6 - 2.5 Torr and I = 10 - 100 mA, for the reduced electric field and the concentrations of N atoms and and states. The rate coefficients and are derived here for the two reactions leading to associative ionization by collisions between electronic metastables and , respectively. The dissociation due to the vibration - vibration (V - V) and vibration - translation (V - T) energy exchanges is shown to represent only a minor contribution for the total rate of dissociation, in opposition to previous studies, due to the effects of fast V - T exchanges associated with - N collisions. Finally, it is shown that the reaction does not constitute an effective depopulating mechanism of N atoms as most of the N atoms so created are reconverted to the N by collisions on the wall and quenching.

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.

Wave driven N2–Ar discharge. I. Self-consistent theoretical model

A theoretical model for a low pressure surface wave driven N2–Ar discharge accounting in a self-consistent way for electron and heavy particle kinetics, gas thermal balance, and wave electrodynamics is developed. The inhomogeneous wave power transfer along the discharge and the concentrations of active species as a function of the spatial position and mixture composition are investigated. N2+ are the dominant ions over a wide range of Ar percentages in the mixture due to the contributions of charge transfer processes between Ar+ and N2 and an effective associative ionization from N2(A 3Σu+). Dissociative recombination of N2+ with electrons provides an increase in the dissociation degree of nitrogen molecules at high Ar fractional concentration.

Modelling of a N2–O2 flowing afterglow for plasma sterilization

A kinetic model for a flowing microwave discharge in N2–O2 at ω/(2π)= 2450 and 915 MHz, in the pressure range p = 1–10 Torr, is constructed with the purpose of studying the conditions that maximize the concentrations of NO(B 2 � ) molecules and O( 3 P) atoms, which are known to play a central role in the sterilization processes. The former are responsible for the emission of UV photons associated with the NOβ bands. The NO(B) concentration is found to pass through a maximum, at approximately 1–3% of O2 added to the mixture, which is in good agreement with the measured maximum of UV emission intensity, and with the shortest time required for the inactivation of spores. For such an O2 percentage, the NO(B) also remains in the afterglow, with only a small reduction, up to a few ∼100 ms. Furthermore, the NO(B) concentration peaks with increasing pressure, with the corresponding maximum shifted to lower O2 percentages, in agreement with the observations of UV intensity. The concentration of O( 3 P) atoms is practically unchanged along the afterglow, at least up to times as high as 100 ms.

Theoretical insight into Ar–O2 surface-wave microwave discharges

A zero-dimensional kinetic model has been developed to investigate the coupled electron and heavy-particle kinetics in Ar–O2 surface-wave microwave discharges generated in long cylindrical tubes, such as those launched with a surfatron or a surfaguide. The model has been validated by comparing the calculated electron temperature and species densities with experimental data available in the literature for different discharge conditions. Systematic studies have been carried out for a surface-wave discharge generated with 2.45 GHz field frequency in a 1 cm diameter quartz tube in Ar–O2 mixture at 0.5–3 Torr pressures, which are typical conditions found in different applications. The calculations have been performed for the critical electron density for surface-wave propagation, ne = 3.74 × 1011 cm−3.It has been found that the sustaining electric field decreases with Ar percentage in the mixture, while the electron kinetic temperature exhibits a minimum at about 80%Ar. The charged and neutral species densities have been calculated for different mixture compositions, from pure O2 to pure Ar, and their creation and destruction processes have been identified. The O2 dissociation degree increases with Ar addition into O2 and dissociation degrees as high as 60% can be achieved. Furthermore, it has been demonstrated that the dissociation degree increases with the discharge tube radius, but decreases with the atomic surface recombination of O-atoms. The density of O− negative ions is very high in the plasma, the electronegativity of the discharge can be higher than 1, depending on the discharge conditions.

On the self-consistent modeling of a traveling wave sustained nitrogen discharge

We present a self-consistent formulation to study low-pressure traveling wave (azimuthally symmetric surface transverse magnetic mode) driven discharges in nitrogen. The theoretical model is based on a self-consistent treatment of the electron and heavy particle kinetics, wave electrodynamics, gas thermal balance, and plasma–wall interactions. The solution provides the axial variation (as a result of nonlinear wave power dissipation along the wave path) of all discharge quantities and properties of interest, such as the electron energy distribution function and its moments, population densities of all relevant excited and charged species [N2(X 1Σg+,ν),N2(A 3Σu+,a′ 1Σu−,B 3Πg,C 3Πu,a 1Πg,w 1Δu), N2+, N4+, e], gas temperature, degree of dissociation [N(4S)]/N, mean absorbed power per electron, and wave attenuation. A detailed analysis of the energy exchange channels among the degrees of freedom of the heavy particles is presented. Particular attention is paid to the axial variation of the gas and wall tempe...

Self-consistent kinetic model of the short-lived afterglow in flowing nitrogen

A detailed kinetic model for the flowing nitrogen microwave discharge and post-discharge is developed with the aim of gaining a deeper understanding into the processes responsible for the formation of the short-lived afterglow of nitrogen and for the enhancement of the concentration of N2(A 3 � + ) metastable, measured at approximately the same position in Sadeghi et al (2001 J. Phys. D: Appl. Phys. 34 1779). The present work shows that the peaks observed in the afterglow, for the density of molecules in radiative N2(B 3 � g) and N + (B 2 � + u ) and metastable N2(A 3 � + u ) states, can be explained as a result of a pumping-up phenomenon into the vibrational ladder produced by near-resonant V–V energy-exchange collisions, involving vibrationally excited molecules N2(X 1 � + g ,v ) in levels as high as v ∼ 35. The present predictions are shown to be in good agreement with the measured concentrations for N2(A 3 � + ) metastables and N( 4 S) atoms, and with the emission intensities of 1 + and 1 − system bands of N2.

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.

Active species downstream of an Ar-O2 surface-wave microwave discharge for biomedicine, surface treatment and nanostructuring

Self-consistent theoretical models have been developed in order to investigate the early and remote flowing afterglows of a surface-wave Ar?O2 microwave discharge generated at 2.45?GHz in a 0.5?cm diameter tube at pressures between 1 and 12?mbar. The early afterglow that occurs downstream of the discharge fills up the tube that connects the discharge region with the large-volume processing reactor, where the late afterglow develops. The models provide the time-dependent density profiles of different species along the afterglow and their 3D spatial distribution in the processing reactor. Systematic calculations are performed for all mixture compositions from pure Ar to pure O2 at different pressures.It is shown that the Ar+, and can survive up to 1?10?ms in the early afterglow depending on the mixture composition and pressure. In low O2 content mixtures the ion densities can increase in the early afterglow, depending on the operating conditions, as a result of Penning ionization involving the Ar(4s) states and forming Ar+, followed by charge transfer. In pure Ar the UV emitting resonant state atoms remain up to 0.1?ms in the afterglow, but with O2 addition their lifetime becomes considerably shorter. The oxygen species important for many applications, such as O(3P) atoms and O2(a) metastable molecules, survive up to 100?ms, thus are the main components of the late afterglow. It is shown that the O2 molecules are strongly dissociated in the discharge, dissociation being more efficient in high Ar content mixtures. However, the dissociation degree decreases to a few per cent in the early afterglow in about 10?ms. In the case of O2(a) molecules, yields above the threshold yield for the iodine laser operation are obtained at 12?mbar for afterglow times of up to 10?ms. In the large-volume reactor it has been found that at low pressure the density of O(3P) atoms decreases by about one order of magnitude towards the walls, while that of O2(a) changes about 20%, although with pressure the density decreases become more pronounced. Very similar density distributions are found at different mixture compositions for O(3P) atoms, while the quasi-homogeneous O2(a) distribution found in high Ar content mixtures progressively turns into a more inhomogeneous one with O2 addition.