C. D. Pintassilgo

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.

Methane decomposition and active nitrogen in a N2-CH4 glow discharge at low pressures

Mass spectrometry and optical emission spectroscopy are used in a N2-xCH4 glow discharge with x = 0.5-2%, at low pressures (1-2 Torr) and small flow rates (6 sccm), in order to determine the CH4 and H2 absolute concentrations and the N2(B 3g) and N2(C 3u) relative concentrations. A kinetic model is developed based on the steady-state solutions to the homogeneous electron Boltzmann equation coupled to a system of rate balance equations for the most populated neutral and ionic species produced, either from active nitrogen and CH4 dissociation or as a result of reactions between radicals from N2 and CH4. It is observed that CH4 is very efficiently decomposed through a sequence of reactions in which at the end HCN and H2 appear as the most abundant products in the discharge. A brown deposition on the tube walls has been detected which is attributed to HCN, in agreement with other investigations of Titans atmosphere, since this species is poorly destroyed in volume. The accordance between theory and experiment is very satisfactory allowing an insight to be obtained into the basic elementary mechanisms in these discharges.

Spectroscopy study and modelling of an afterglow created by a low-pressure pulsed discharge in N2-CH4

Time-resolved emission spectroscopy is used to investigate the relaxation of N2(B 3Πg), N2(C 3Πu) and CN(B 2Σ) states in the time afterglow of a low-pressure N2-CH4 pulsed discharge, with time duration of 1 ms and in the range [CH4]/[N2] = 0-2%. The decays in the relative measured concentrations in the afterglow are interpreted by modelling the relaxation of a set of time-varying kinetic master equations for the various species produced in the discharge, with conditions at the beginning of the afterglow calculated from a time-dependent kinetic model for the pulsed discharge. It is observed that the N2(B 3Πg) state is populated in the afterglow mainly via the reaction N2(A 3Σu+) + N2(X 1Σg+, 5≤v≤14)→N2(B 3Πg) + N2(X 1Σg+, v = 0), since the pulse duration is large enough to populate the N2(X 1Σg+, v) levels at its end and, to a smaller extent, also by pooling of N2(A 3Σu+). The N2(C 3Πu) state is populated by pooling of N2(A 3Σu+) only, whereas the CN(B 2Σ) state is created through reactions involving either N2(A 3Σu+) states or N2(X 1Σg+, v) levels in collisions with CN(X 2Σ+) molecules. The agreement between measured and calculated concentrations of N2(B 3Πg) and N2(C 3Πu) states is very good in pure N2 and it may be considered satisfactory in the case of N2-CH4 mixtures, and for the CN(B 2Σ) state the agreement between theory and experiment is also reasonably good.

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.

Heavy species kinetics in low-pressure dc pulsed discharges in air

A time-dependent kinetic model is presented to study low-pressure (133 and 210?Pa) pulsed discharges in air for dc currents ranging from 20 to 80?mA with a pulse duration from 0.1 up to 1000?ms. The model provides the temporal evolution of the heavy species along the pulse within this range time, where the coupling between vibrational and chemical kinetics is taken into account. This work shows that the predicted values for NO(X) molecules and O(3P) atoms reproduce well previous measured data for these two species. A systematic analysis is carried out on the interpretation of experimental results. It is observed that the N2(X, v ? 13) + O ? NO(X) + N(4S) and the reverse process NO(X) + N(4S) ? N2 (X, v ~ 3) + O have practically the same rates for a pulse duration longer than 10?ms, each of them playing a dominant role in the populations of NO(X), N(4S) and, to a lesser extent, in O(3P) kinetics. Our simulations show that for shorter pulse durations, from 0.1 to 10?ms, NO(X) is produced mainly via the processes N2(A) + O ? NO(X) + N(2D) and N(2D) + O2 ? NO(X) + O, while the oxygen atoms are created mostly from electron impact dissociation of O2 molecules and by dissociative collisions with N2(A) and N2(B) molecules.

Modelling of a low-pressure N2-O2 discharge and post-discharge reactor for plasma sterilization

A model is used to study the afterglow of a flowing microwave discharge at ω/(2π) = 2450 MHz, p =667 Pa (5 Torr), in the mixture N 2 -xO 2 , with x = 0.7-7% of O 2 . This model considers a self-consistent kinetic description of the discharge and early-afterglow regions, followed by a 3D hydrodynamic analysis of the post-discharge chamber. The behaviour of NO(B) molecules and O( 3 P) atoms is discussed in detail, since these two species play an important role in the sterilization process, respectively, due to the UV emission associated with the NO β bands and due to erosion effects. The present work shows that a maximum in the UV emission intensity from NO β occurs in the range 0.7-2% of O 2 added to the mixture, which is in agreement with the survival curves of spores presented by Philip et al (2002 IEEE Trans. Plasma Sci. 30 1429). In general, the oxygen atoms concentration is more important as the added O 2 percentage increases. The interplay of N( 4 S), O( 3 P), NO(X), N 2 (X, v) and NO(B) species in the overall kinetics both in the discharge and in the early-afterglow region is discussed. Particular attention is devoted to the density of NO(B) and O( 3 P) in the sterilization vessel at different spatial planes and for various mixture compositions.

Modelling of a post-discharge reactor used for plasma sterilization

A three-dimensional hydrodynamic model is developed to simulate a post-discharge reactor placed downstream from a flowing microwave discharge in N2–O2 used for plasma sterilization. The temperature distribution and the density distributions of NO(B 2Π) molecules and O(3P) atoms, which are known to play a central role in the sterilization process, are obtained in the reactor in the case of discharges at 915 and 2450 MHz, pressure range 1–8 Torr and N2–xO2 mixture composition, with x = 0.2–2%. Excluding the flow direction, sufficiently low temperatures ideal for sterilization have been found in most parts of the reactor. The highest NO(B) and O(3P) concentrations at the reactor entrance are achieved at the highest pressure values investigated here. However, these larger densities rapidly decrease within a few centimetres below the values obtained at lower pressure. On the contrary, at low pressure the density distributions of NO(B) and O(3P) are quasi-homogeneous in most of the horizontal planes. At 8 Torr the densities increase orders of magnitude in the reactor as the gas flow increases from 1 × 103 to 4 × 103 sccm, while at 2 Torr this increase does not reach even one order of magnitude. In agreement with the experiment, the densities of NO(B) and O(3P) have been found to increase at 2 Torr as the O2 percentage increases in the discharge gas mixture, whereas at 8 Torr the density of NO(B) decreases with O2 percentage and the O(3P) density presents only minor changes.

NO kinetics in pulsed low-pressure plasmas studied by time-resolved quantum cascade laser absorption spectroscopy

Time-resolved quantum cascade laser absorption spectroscopy at 1897 cm−1 (5.27 µm) has been applied to study the NO(X) kinetics on the micro- and millisecond time scale in pulsed low-pressure N2/NO dc discharges. Experiments have been performed under flowing and static gas conditions to infer the gas temperature increase and the consequences for the NO line strength. A relatively small increase of ~20 K is observed during the early plasma phase of a few milliseconds. After some 10 ms gas temperatures up to 500 K can be deduced. The experimental data for the NO mixing ratio were compared with the results from a recently developed time-dependent model for pulsed N2–O2 plasmas which are well in accord. The early plasma pulse is determined by vibrational heating of N2 while the excitation of NO(X) by N2 metastables is almost completely balanced. Efficient NO depletion occurs after several milliseconds by N atom impact.

Modelling of an afterglow plasma in air produced by a pulsed discharge

A kinetic model is developed to study the afterglow plasma of a pulsed discharge in air. This model includes a detailed analysis of the temporal evolution of heavy species during the pulse, followed by their relaxation in the afterglow. The predicted results are compared with two experimental sets performed in the time afterglow of a pulsed discharge in N2–20%O2 at a pressure p = 133 Pa involving the measurements of (i) N2(B) and N2(C) fluorescences for a discharge current I = 40 mA and a pulse duration τ = 200 µs and 10 ms, together with (ii) the absolute concentration of NO(X) for I = 40 and 80 mA with τ varying from 1 to 4 ms. The results of the model agree reasonably well with the measurements of N2(B) and N2(C) decays. It is shown that under these experimental conditions, N2(B) is always populated mainly via the process N2(A) +N 2(X, 5 v 14) → N2(B) +N 2(X, v = 0), while the relaxation of N2(C) is dominated by the pooling reaction N2(A) +N 2(A) → N2(C) +N 2(X, v = 0). An almost constant concentration of NO(X) is experimentally observed until the remote afterglow, but the present model is only capable of predicting the same order of magnitude for afterglow times t 0.05 s. Several hypotheses are discussed and advanced in order to explain this discrepancy.

On the different regimes of gas heating in air plasmas

Simulations of the gas temperature in air (N2–20%O2) plasma discharges are presented for different values of the reduced electric field, E/N g, electron density n e, pressure and tube radius. This study is based on the solutions to the time-dependent gas thermal balance in a cylindrical geometry coupled to the electron, vibrational and chemical kinetics, for and 100 Td (1 Td = 10−17 V cm2), 109 ≤ n e ≤ 1011 cm−3, pressure in the range 1–20 Torr, and also considering different tube radius, 0.5, 1 and 1.5 cm. The competing role of different gas heating mechanisms is discussed in detail within the time range 0.01–100 ms. For times below 1 ms, gas heating occurs from O2 dissociation by electron impact through pre-dissociative excited states, e + O2 → e + → e + 2O(3P) and ... → e + O(3P) + O(1D), as well as through the quenching of N2 electronically excited states by O2. For longer times, simulation results show that gas heating comes from processes N(4S) + NO(X) → N2(X, v ~ 3) + O, N2(A) + O → NO(X) + N(2D), V–T N2–O collisions and the recombination of oxygen atoms at the wall. Depending on the given E/N g and n e values, each one of these processes can be an important gas-heating channel. The contribution of V–T N2–O exchanges to gas heating is important in the analysis of the gas temperature for different pressures and values of the tube radius. A global picture of these effects is given by the study of the fraction of the discharge power spent on gas heating, which is always ~15%. The values for the fractional power transferred to gas heating from vibrational and electronic excitation are also presented and discussed.