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This paper presents recent activities covering different plasma fields, from both theoretical and experimental point of views. An overview of the present interests of the scientific community is reported here. Starting from a brief description of the role of collisions in astrophysical plasmas, some fundamental aspects of gas discharges modelling, such as superelastic collisions and the basic concept of vibrational temperature, are discussed. Different plasma sources, as DBD and microwave discharges with their own specific applications, are reported. Edge plasmas in nuclear fusion reactors are investigated, focusing on the cooling mechanisms resulting from nitrogen puffing in the divertor region. Graphic abstract: [Figure not available: see fulltext.].

The time-dependent evolution of the energy transfer into gas heating in the afterglow of pulsed CO2 and CO2–N2 glow discharges produced in cylindrical tubes at low pressures (1–5 Torr) is theoretically investigated, by developing a self-consistent model that couples the time-dependent gas thermal balance equation with the vibrational kinetics. The modelling predictions are in good agreement with recently published experimental data on gas temperature, obtained via time-resolved in situ Fourier transform infrared spectroscopy. The cooling of the gas in the afterglow is found to be strongly dependent on the thermal conductivity and the wall temperature. It is verified that wall and volume deactivation of CO2 vibrationally excited species influences the gas heating along the afterglow, in different proportions depending on the pressure of the gas. The time-resolved contributions of each of these cooling and heating mechanisms are discussed in detail. The new results bring an additional validation of a set of mechanisms and rate coefficients for vibrationally-energy transfers previously proposed.

This work analyzes the temporal evolution of the electron kinetics in dry-air plasmas (80% N2: 20% O2), excited by electric-field pulses with typical rise-times of 10−9 and 10−6 s, applied to a stationary neutral gaseous background at pressures of 105, 133 Pa and temperature of 300 K. The study is based on the solution of the electron Boltzmann equation (EBE), adopting either (i) a time-dependent formulation that considers an intrinsic time evolution for the electron energy distribution function (EEDF), assuming the classical two-term expansion and a space-independent exponential temporal growth of the electron density; or (ii) a quasi-stationary approach, where the time-independent form of the EBE is solved for different values of the reduced electric-field over the duration of the pulse. The EBE was solved using the LisbOn KInetics Boltzmann solver (LoKI-B), whose original capabilities were extended to accept time-dependent non-oscillatory electric fields as input data. The role of electron–electron collisions, under specific conditions, is also reported and discussed. The simulations show that the quasi-stationary approach gives solutions similar to the time-dependent formulation for rise-times longer than the characteristic evolution time of the EEDF, i.e. 20 ns at 105 Pa and 20 μs at 133 Pa, meaning that a quasi-stationary description is possible in a high-collisionality situation and long rise-times (e.g. microsecond pulses at atmospheric pressure), failing for faster rise-times (e.g. nanosecond pulses for both pressures considered here).

A time-dependent kinetic model is developed to study the energy transfer to gas heating in N2 and N2–O2 plasmas. The model is based on the coupled solutions to the gas thermal balance equation and a system of rate balance equations for the most important neutral (including vibrational kinetics) and ionic heavy species produced in these plasmas. A complete set of gas heating mechanisms is taken into account, together with gas cooling by heat conduction to the wall. Calculations are performed for a cylindrical geometry and fractional concentrations of oxygen molecules [O2]/Ng (Ng being the gas number density) up to 80%. Modeling simulations are carried out for fixed values of the reduced electric field E/Ng and electron density ne with the purpose of focusing on the role of adding molecular oxygen O2 into the N2–O2 mixture. It is shown that the fractional discharge power and energy transferred to the translational mode (gas heating) increase with [O2]/Ng. The present work also shows that when the variation ...

LisbOn KInetics Boltzmann (LoKI-B) is an open-source simulation tool available at: https://github.com/IST-Lisbon/LoKI, The LisbOn KInetics Boltzmann (LoKI-B) is an open-source simulation tool (https://github.com/IST-Lisbon/LoKI) that solves a time and space independent form of the two-term electron Boltzmann equation, for non-magnetised non-equilibrium low-temperature plasmas excited by DC/HF electric fields from different gases or gas mixtures. LoKI-B was developed as a response to the need of having an electron Boltzmann solver easily addressing the simulation of the electron kinetics in any complex gas mixture (of atomic/molecular species), describing first and second-kind electron collisions with any target state (electronic, vibrational and rotational), characterized by any user-prescribed population. LoKI-B includes electron-electron collisions, it handles rotational collisions adopting either a discrete formulation or a more convenient continuous approximation, and it accounts for variations in the number of electrons due to non-conservative events by assuming growth models for the electron density. On input, LoKI-B defines the operating work conditions, the distribution of populations for the electronic, vibrational and rotational levels of the atomic/molecular gases considered, and the relevant sets of electron-scattering cross sections obtained from the open-access website LXCat (http://lxcat.net/). On output, it yields the isotropic and the anisotropic parts of the electron distribution function (the former usually termed the electron energy distribution function), the electron swarm parameters, and the electron power absorbed from the electric field and transferred to the different collisional channels. LoKI-B is developed with flexible and upgradable object-oriented programming under MATLAB (R), to benefit from its matrix-based architecture, adopting an ontology that privileges the separation between tool and data. This topical review presents LoKI-B and gives examples of results obtained for different model and real gases, verifying the tool against analytical solutions, benchmarking it against numerical calculation, This work was funded by Portuguese FCT-Fundacao para a Ciencia e a Tecnologia, under projects UID/FIS/50010/2013 and PTDC/FISPLA/1243/2014 (KIT-PLASMEBA).

This is the second of two papers presenting the study of vibrational energy exchanges in non-equilibrium CO2 plasmas in low-excitation conditions. The companion paper addresses a theoretical and experimental investigation of the time relaxation of ∼70 individual vibrational levels of ground-state CO molecules during the afterglow of a pulsed DC glow discharge, operating at pressures of a few Torr and discharge currents around 50 mA, where the rate coefficients for vibration-translation (V-T) and vibration-vibration (V-V) energy transfers among these levels are validated (Silva et al 2018 Plasma Sources Sci. Technol. 27 015019). Herein, the investigation is focused on the active discharge, by extending the model with the inclusion of electron impact processes for vibrational excitation and de-excitation (e-V). The time-dependent calculated densities of the different vibrational levels are compared with experimental data obtained from time-resolved in situ Fourier transform infrared spectroscopy. It is shown that the vibrational temperature of the asymmetric stretching mode is always larger than the vibrational temperatures of the bending and symmetric stretching modes along the discharge pulse - the latter two remaining very nearly the same and close to the gas temperature. The general good agreement between the model predictions and the experimental results validates the e-V rate coefficients used and provides assurance that the proposed kinetic scheme provides a solid basis to understand the vibrational energy exchanges occurring in CO2 plasmas.

A kinetic model describing the time evolution of ∼70 individual CO2(X1Σ+) vibrational levels during the afterglow of a pulsed DC glow discharge is developed in order to contribute to the understanding of vibrational energy transfer in CO2 plasmas. The results of the simulations are compared against in situ Fourier transform infrared spectroscopy data obtained in a pulsed DC glow discharge and its afterglow at pressures of a few Torr and discharge currents of around 50 mA. The very good agreement between the model predictions and the experimental results validates the kinetic scheme considered here and the corresponding vibration-vibration and vibration-translation rate coefficients. In this sense, it establishes a reaction mechanism for the vibrational kinetics of these CO2 energy levels and offers a firm basis to understand the vibrational relaxation in CO2 plasmas. It is shown that first-order perturbation theories, namely, the Schwartz-Slawsky-Herzfeld and Sharma-Brau methods, provide a good description of CO2 vibrations under low excitation regimes.

In this paper we present a self-consistent kinetic model to study the temporal variation of the gas temperature in the afterglow of a 440 Pa microwave nitrogen discharge operating at 433MHz in a 3.8 cm diameter tube. The initial conditions in the afterglow are determined by a kinetic model that solves the electron Boltzmann equation coupled to the gas thermal balance equation and a system of rate-balance equations for N-2(X-1 Sigma(+)(g), v) molecules, electronically excited states of N2, ground and excited states of atomic nitrogen and the main positive ions. Once the initial concentrations of the heavy species and gas temperature are known, their relaxation in the afterglow is obtained from the solutions to the corresponding time-dependent equations. Modelling predictions are found to be in good agreement with previously measured values for the concentrations of N(S-4) atoms and N2(A (3)Sigma(+)(u)) molecules, and the radially averaged gas temperatureTg along the afterglow of amicrowave discharge in N-2 under the same working conditions. It is shown that gas heating in the afterglow comes essentially fromthe energy transfer involving non-resonant vibration-vibration (V-V) collisions between vibrationally excited nitrogen molecules, as well as from energy exchanges in vibration-translation (V-T) on N-2-N collisions.

International audience; Herein, it is argued that Mars has nearly ideal conditions for CO 2 decomposition by non-equilibrium plasmas. It is shown that the pressure and temperature ranges in the ##IMG## [http://ej.iop.org/images/0963-0252/26/11/11LT01/psstaa8dccieqn1.gif] ∼ 96 % CO 2 Martian atmosphere favour the vibrational excitation and subsequent up-pumping of the asymmetric stretching mode, which is believed to be a key factor for an efficient plasma dissociation, at the expense of the excitation of the other modes. Therefore, gas discharges operating at atmospheric pressure on Mars are extremely strong candidates to produce O 2 efficiently from the locally available resources.

A self-consistent time-dependent kinetic model coupled to the gas thermal balance equation is presented for a N2-1%NO millisecond pulsed DC discharge at a pressure of 266 Pa (2 Torr) and a current of 35 mA. The model provides the temporal evolution of the most important heavy species of interest to this work such as N2(X1Σg +, v), NO(X2Π), N2(A3Σu +), N2(a′1Σu -), N(4S) and O(3P), simultaneously with the time-dependent variation of the gas temperature. Predicted results for NO number densities during the pulse are compared to experimental ones measured by time-resolved quantum cascade laser absorption spectroscopy (QCLAS). The agreement between experiment and modelling predictions is very reasonable, mainly until a pulse duration of 2 ms, revealing the temporal evolution of the most important creation and loss mechanisms of NO(X). Simulations show a slow gas heating during the first millisecond. Thereafter, gas heating is accelerated and levels off at a time ∼ 40 ms. These effects are explained and discussed in detail, together with the analysis of the fraction of the discharge power transferred to gas heating.

International audience; 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.

We present the results from a self-consistent kinetic model simulating the afterglow of a flowing microwave discharge in pure N2 in which CH4 is introduced in the post-discharge. The simulation is carried out for a discharge operating at 433 MHz, in a tube of 1.9 cm inner radius, at the pressure range 26.6–133 Pa. In the post-discharge the CH4 is primarily dissociated into CH3 and CH2 due to collisions with the metastables N 2 ( A 3 Σ u + ) and N 2 ( a ′ 1 Σ u - ) coming from the discharge, while HCN is obtained from collisions of CH3 and CH2 with N(4S) atoms. Our calculations show that numerous species existing in Titan’s atmosphere are then formed as a result of a complex interplay kinetics. The time-evolution of the species is investigated along the afterglow as well as the possibility of these species being precursors to the formation of organic aerosols.

The possibility of an afterglow plasma to be capable to produce N-containing hydrocarbon species of the type of those existing in Titan's atmosphere by varying the discharge plasma parameters and afterglow times is studied. The paper reports and discusses the results from a numerical simulation based on a self-consistent kinetic model for the afterglow of a flowing microwave discharge in pure N 2 , in which a 2% of CH 4 is introduced in the post-discharge. It is shown that concentrations of hydrocarbons and nitriles close to those in Titan's atmosphere can be obtained by varying either the gas pressure or the field frequency of the discharge, as well as by adjusting the instant at which the methane is introduced into the post-discharge. The simulations are carried out for discharges at 433, 915 and 2450 MHz, within the pressure range 26.6–133 Pa, and for the cases of methane addition at selected afterglow times in the interval 10 - 5 – 10 - 3 s . This system seems to present some advantages relatively to an experiment or a simulation conducted in a N 2 – CH 4 discharge to investigate Titan's atmosphere.

The flowing afterglow of a N 2 -O 2 microwave discharge intended to provide intense and spatially uniform UV emission for an efficient inactivation of bacterial spores is modelled with a 3-D hydrodynamic model leading to the spatial density distribution of the species in the reactor. The agreement of the calculated densities of the NO(A) and NO(B) UV emitting species with the corresponding measured emission intensities strongly supports the choice of the kinetic reactions retained in the ; model. In that respect, the specific contribution of N and O atoms to the spatial distribution of the NO(A) density ; (generating the NOy system) in the late afterglow is, for the first time, brought into relief.

A large volume post-discharge reactor placed downstream from a flowing N2–O2 microwave discharge is modelled using a three-dimensional hydrodynamic model. The density distributions of the most populated active species present in the reactor—O(3P), O2(a 1 Δg), , NO(X 2 Π), NO(A 2 Σ+), NO(B 2 Π), NO2(X), O3, and N(4S)—are calculated and the main source and loss processes for each species are identified for two discharge conditions: (i) p = 2 Torr, f = 2450 MHz, and (ii) p = 8 Torr, f = 915 MHz; in the case of a N2–2%O2 mixture composition and gas flow rate of 2 × 103 sccm. The modification of the species relative densities by changing the oxygen percentage in the initial gas mixture composition, in the 0.2%–5% range, are presented. The possible tuning of the species concentrations in the reactor by changing the size of the connecting afterglow tube between the active discharge and the large post-discharge reactor is investigated as well.

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.

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.

Etch rates of hexatriacontane (HTC) as high as ~10 mg s−1 m−2 in late O2 post-discharge are obtained at 333 K where no significant UV nor VUV irradiation occurs. Introducing N2 in the gas mixture helps control the ratio of O/O2 densities, which is shown to play a key role in the functionalization or etching of the HTC. The oxygen atoms are required for any further modification of the HTC because they initiate the formation of the radical chains by abstraction of one hydrogen. O(3P) atoms do not contribute directly to break the alkane chain close to room temperature but they can functionalise it. O2 is the important reactive species for the etching because of the role played by the peroxide groups on the scission of the hydrocarbon chains.

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.

The time-dependent evolution of the energy transfer to gas heating in a pure N2 discharge produced in a cylindrical tube at low pressures (1–10 Torr) is studied for different fixed values of the reduced electric field and electron density. We consider a model based on the self-consistent solutions to the time-dependent gas thermal balance equation coupled to the electron, vibrational, and chemical kinetic equations for the most important heavy species produced in N2 plasma discharges. The results of this model provide the temporal variation of the radially averaged value of the gas temperature, as well as the corresponding gas heating mechanisms. It is shown that the pooling reactions N2(A) + N2(A) → N2(B) + N2 and N2(A) + N2(A) → N2(C) + N2 are responsible for a smooth increase in the gas temperature before the first millisecond. For longer times, gas heating is found to be mainly caused by vibrational energy exchanges from non-resonant vibration–vibration (V–V) processes between N2 molecules and by vibration–translation (V–T) N2–N collisions. The heating rates of these different gas heating mechanisms and the gas temperature are calculated for a reduced electric field of 50 and 100 Td (1 Td = 10−17 Vcm2), an electron density of 1010 and 1011 cm−3, and a pressure of 1 and 10 Torr. The fractional power converted to gas heating from electronic and vibrational excitation is also calculated for these parameters, being respectively ~2% and in the range 10%–35%. The effect of having a contribution of non-resonant V–V processes to gas cooling within the time interval 0.1–1 ms is analysed. The role of the gas temperature on the temporal evolution of the vibrational distribution of N2(X, v) molecules is also discussed.

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.

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 Titan's 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.

International audience; A self-consistent model is developed to study the temporal variation of the gas temperature in millisecond single dc pulsed discharges and their afterglows in air-like mixtures (N2–20%O2) at low pressures. The model is based on the solutions to the time-dependent gas thermal balance equation, under the assumption of a parabolic gas temperature profile across the discharge tube, coupled to the electron, vibrational and chemical kinetics. Modelling results provide a satisfactory explanation for recently published time-resolved experimental data for the gas temperature in a 5 ms pulsed air plasma with a current of 150mA and the corresponding afterglow at a pressure of 133 Pa (1 Torr). It is shown that the main heating mechanisms during the first millisecond of the pulse come predominantly from O2 dissociation by electron impact through the pre-dissociative excited state O2(B 3− u ) and the quenching of nitrogen electronically excited states N2(A 3 u , B 3 g, a 1− u , a 1 g, w 1u) by O2, agreeing with other studies on fast gas heating in air plasmas. As the pulse duration increases, other gas heating sources become important, namely V–T N2–O energy exchanges, recombination of oxygen atoms at the wall, N2(A) quenching by O(3P) and reaction N(4S) NO(X) → N2(X, v ∼3) O, contributing altogether to an additional smooth increase in the gas temperature until the end of the pulse. In the first instants of the early afterglow, the gas temperature decreases very rapidly as a consequence of the minor role played by electronic collisions and due to a fast decay of N2 electronic states. For afterglow times up to 10 ms, the gas temperature continues to decrease, following the time-dependent kinetics of [N2(X,v)], [N(4S)], [O(3P)] and [NO(X)]. Sensitivity of the model to different input parameters such as thermal accommodation coefficient and probabilities for atomic recombination at the wall are reported.

International audience; Plasma-surface interactions are increasingly recognised as a key factor in explaining molecule production and conversion processes. In order to scrutinise such effects well-defined surface and gas phase conditions are required. Hence, the inner surface of a Pyrex tube was treated by a capacitively coupled RF plasma at low pressure. The post-plasma oxidation of gas mixtures containing 1 % NO into NO2 has been studied by means of quantum cascade laser absorption spectroscopy in the mid-infrared spectral range. The plasma pre-treatment experiments with oxygen containing precursors suggest O atom adsorption on the Pyrex tube with a surface density of 2.7 x 1014 cm–2. A simple kinetic model was developed and shows good agreement with the measured NO and NO2 gas phase concentrations. In the model a fraction of the surface is considered to be covered with chemisorption sites where atoms and molecules can be adsorbed, whereas they can be removed only by recombination (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Following a first study on a late afterglow in flowing pure nitrogen post discharge, we report new two-photon absorption laser-induced fluorescence (TALIF) measurements of the absolute ground-state atomic nitrogen density N(4S) and investigate the influence of methane introduced downstream from the discharge by varying the CH4 mixing ratio from 0% up to 50%. The N (4S) maximum density is about 2.2 × 1015 cm−3 in pure N2 for a residence time of 22 ms and does not change significantly for methane mixing ratio up to ∼15%, while above, a drastic decrease is observed. The influence of the residence time has been studied. A kinetic model has been developed to determine the elementary processes responsible for the evolution of the N (4S) density in N2/CH4 late afterglow. This model shows the same decrease as the experimental results even though absolute density values are always larger by about a factor of 3. In the late afterglow three-body recombination dominates the loss of N (4S) atoms whatever the CH4 mixing ratio. For high CH4 mixing ratio, the destruction process through collisions with CH3, H2CN and NH becomes important and is responsible for the observed decrease of the N (4S) density.

International audience; 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) N2(X, 5 ≤ v ≤ 14) → N2(B) N2(X, v = 0), while the relaxation of N2(C) is dominated by the pooling reaction N2(A) N2(A) → N2(C) N2(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.

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.

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(B2Π) 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 p=8 Torr, f=915 MHz and p=2 Torr, f=2450 MHz, and N2‐2%O2 mixture composition. Excluding the flow direction, sufficiently low temperatures ideal for sterilization have been found in most part of the reactor. The highest NO(B) and O(3P) concentrations at the reactor entrance are achieved at the highest pressure values here investigated. However, these larger densities rapidly decrease within a few centimeters below the values obtained at lower pressure. On the contrary, at low pressure the density distributions of NO(B) and O(3P) are quasi homogenous in most of the horizontal planes.

Titan, the biggest satellite of Saturn, have a dense atmosphere mainly composed of N2 and a few amount of CH4. High energy solar photons and electrons from the magnetosphere of Saturn generate a wide range of organic species from simple volatiles to organic solid particles. All around Titan, a dense and opaque brown aerosol layers prevents the observation of the soil. To get more information on Titan’s atmosphere, the Cassini‐Huygens space probes launched in 1997 and the Huygens module descend in the atmosphere of Titan on the 14th January 2005.Before the Cassini‐Huygens program, laboratory simulation approach was already initiated for the production of analogues of Titan’s aerosols, named “tholins”. Different types of plasmas have been used. Elementary analysis of “tholins” have also been done. From plasma modelling, the Electron Energy Distribution Function is calculated and compared to the solar energy spectrum. Some results on composition of tholins produced in RF plasmas are presented: morphology fro...

Titan, the largest satellite of Saturn, has been studied as an exo/astrobiological object for several years. Its dense atmosphere is made of nitrogen with a few percent of methane. Since Miller’s experiment, we know that reducing atmospheres are of great interest for prebiotic organic chemistry. Subjected to energetic particles bombardment (coming from solar radiations and Saturn’s magnetosphere), both hydrocarbons, nitriles (like HCN, a precursor of amino-acids) and organic aerosols are produced inside the atmosphere in notable amounts.

This paper uses experiments and modelling to study capacitively coupled radio-frequency (rf) discharges in pure nitrogen, at 13.56MHz frequency, 0.1–1 mbar pressures and 2–30W coupled powers. Experiments performed on two similar (not twin) setups, existing in the LATMOS and the GREMI laboratories, include electrical and optical emission spectroscopy (OES) measurements. Electrical measurements give the rf-applied and the direct-current-self-bias voltages, the effective power coupled to the plasma and the average electron density. OES diagnostics measure the intensities of radiative transitions with the nitrogen second-positive and first-negative systems, and with the 811.5 nm atomic line of argon (present as an actinometer). Simulations use a hybrid code that couples a two-dimensional time-dependent fluid module, describing the dynamics of the charged particles (electrons and positive ions N+2 and N+4 ), and a zero-dimensional kinetic module, describing the production and destruction of nitrogen (atomic and molecular) neutral species. The coupling between these modules adopts the local mean energy approximation to define space–time-dependent electron parameters for the fluid module and to work out space–time-averaged rates for the kinetic module. The model gives general good predictions for the self-bias voltage and for the intensities of radiative transitions (both average and spatially resolved), underestimating the electron density by a factor of 3–4., This work is supported by a PICS Cooperation Program, financed by the Portuguese Foundation for Science and Technology (FCT) and by the Centre National de la Recherche Scientifique (CNRS). The calculations were performed on SeARCH (Services & Advanced Computing with HTC/HPC) funded by FEDER through the COMPETE program and by the Portuguese FCT under contract CONC-REEQ/443/EEI/2005. Et Es-sebbar thanks the ANR programme (ANR-09-JCJC-0038 contract) for his Post-Doctoral grant.

Repetitively pulsed discharges in dry air (N2?20%O2) at low pressures (1?Torr) are studied by means of a time-dependent kinetic model. Simulations are carried out for both pulse and afterglow phases, accounting for the temporal evolution of the composition of the gas between each pulse. Modelling results are computed for a dc current of 40?mA, pulse durations between 200??s and 10?ms, and pulse repetition rates ranging from 20 up to 1000?Hz. This work shows that the vibrational distribution function of N2 molecules becomes more populated by increasing the pulse duration or the pulse repetition rate. A similar behaviour is also predicted for the final concentrations of NO(X), N(4S) and O(3P). It is shown that [NO(X)] increases with the duty cycle ratio independently of the pulse repetition rate, as observed experimentally under similar conditions. The influence of the pulse duration, repetition rate and number of pulses in the overall kinetics is analysed and discussed in detail.

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.

We present the modeling of a low pressure post-discharge system used for plasma sterilization, which consists of a flowing microwave discharge, an early-afterglow region developed downstream of the discharge in the same tube, and a large post-discharge reactor. The kinetic model valid for the discharge and early-afterglow region is based on the Boltzmann and rate balance equations, while in the post-discharge reactor a 3-D hydrodynamic model is used. The calculations are conducted in an N2-O2 mixture, which has seen to constitute an efficient system for sterilization purpose. The possibilities offered by modeling is presented through a discussion of the effects on relevant species densities and distributions caused by varying: (i) the initial gas mixture composition; (ii) the length of early-afterglow zone; (iii) the gas flow rate and gas pressure.

Part of this work was carried out under the support of the Portuguese Science and Technology Foundation FCT through a sabbatical fellowship ofCDP. The authorswould like to thank Dr Vasco Guerra for several fruitful discussions.; International audience; 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 80mA 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.