Plasma Chemistry Model Research Articles

Kinetic study of plasma assisted oxidation of H2 for an undiluted rich mixture

Non-thermal plasma discharges are considered as an assisted method for supporting conventional thermal processes, because the discharge-generated radicals can be used to benefit them. In this regard, plasma assisted combustion has been investigated extensively, and plasma assisted fuel reforming has expanded its horizons to include plasma assisted chemical synthesis, in accord with a global megatrend—electrification. To scientifically advance this area, plasma-chemical kinetic studies must be conducted, and their mechanisms should consider both electron and gas temperatures to describe electron-induced and thermally-induced chemistry, respectively. Here we present a temperature-dependent plasma-chemical kinetic study for the plasma assisted oxidation of an undiluted rich H2/O2 mixture; it represents our continued effort to establish a foundational plasma assisted reaction mechanism for H2/O2 systems. A plasma-chemical kinetic model (KAUSTKin) and a modified plasma-chemical reaction mechanism were used for the zero-dimensional simulation; a temperature-controlled dielectric barrier discharge reactor was employed for the experiment. Non-linear oxidation behavior was observed in a range of 300–850 K for various discharge powers. To unravel the underlying mechanisms, analysis was broken down to a physical effect, described by the reduced electric field (E/N), and a chemical effect originating from the temperature dependence of the plasma assisted oxidation chemistry. We concluded that (i) the oxidation was initiated by the electron impact dissociation of H2, (ii) E/N governed the degree of H2 dissociation (and thus the degree of oxidation), (iii) the key intermediate was HO2, and (iv) a significant increase in the chain termination reaction, H + HO2 → H2 + O2, suppressed the oxidation (especially in the Negative Temperature Coefficient (NTC)-like regime of 600–725 K). The need to improve existing thermal reaction mechanisms for temperatures below 1000 K was emphasized, and a method for temperature-dependent plasma-chemical kinetic studies to provide a novel and complementary tool for this task was demonstrated.

Kinetic Study for Plasma Assisted Cracking of NH3: Approaches and Challenges.

Ammonia is considered as one of the promising hydrogen carriers toward a sustainable world. Plasma assisted decomposition of NH3 could provide cost- and energy-effective, low-temperature, on-demand (partial) cracking of NH3 into H2. Here, we presented a temperature-dependent plasma-chemical kinetic study to investigate the role of both electron-induced reactions and thermally induced reactions on the decomposition of NH3. We employed a plasma-chemical kinetic model (KAUSTKin), developed a plasma-chemical reaction mechanism for the numerical analysis, and introduced a temperature-controlled dielectric barrier discharge reactor for the experimental investigation using 1 mol % NH3 diluted in N2. As a result, we observed the plasma significantly lowered the cracking temperature and found that the plasma-chemical mechanism should be further improved to better predict the experiment. The commonly used rates for the key NH3 pyrolysis reaction (NH3 + M ↔ NH2 + H + M) significantly overpredicted the recombination rate at temperatures below 600 K. Furthermore, the other identified shortcomings in the available data are (i) thermal hydrazine chemistry, (ii) electron-scattering cross-section data of NxHy, (iii) electron-impact dissociation of N2, and (iv) dissociative quenching of excited states of N2. We believe that the present study will spark fundamental interest to address these shortcomings and contribute to technical advancements in plasma assisted NH3 cracking technology.

A reaction mechanism for plasma electrolysis of AgNO3 forming silver nanoclusters and nanoparticles

In plasma-driven solution electrolysis (PDSE), gas-phase plasma-produced species interact with an electrolytic solution to produce, for example, nanoparticles. An atmospheric pressure plasma jet (APPJ) directed onto a liquid solution containing a metallic salt will promote reduction of metallic ions in solution, generating metallic clusters that nucleate to form nanoparticles. In this article, results from a computational investigation are discussed of a PDSE process in which a radio-frequency APPJ sustained in helium impinges on a silver nitrate solution, resulting in growth of silver nanoparticles. A reaction mechanism was developed and implemented in a global plasma chemistry model to predict nanoparticle growth. To develop the reaction mechanism, density functional theory was used to generate probable silver growth pathways up to Ag9. Neutral clusters larger than Ag9 were classified as nanoparticles. Kinetic reaction rate coefficients for thermodynamically favorable growth pathways were estimated based on an existing, empirically determined base reaction mechanism for smaller Ag particle interactions. These rates were used in conjunction with diffusion-controlled reaction rate coefficients that were calculated for other Ag species. The role of anions in reduction of Agn ions in forming nanoparticles is also discussed. Oxygen containing impurities or admixtures to the helium, air entrainment into the APPJ, and dissociation of saturated water vapor above the solution can produce additional reactive oxygen species in solution, resulting in the production of anions and O2− in particular. For a given molarity, delivering a sufficient fluence of reducing species will produce similar nanoparticle densities and sizes for all applied power levels. Comparisons are made to alternate models for nanoparticle formation, including charged nanoparticles and use of direct current plasmas.

Laser-driven ionization mechanisms of aluminum for single particle aerosol mass spectrometry

Single particle aerosol mass spectrometry (SPAMS), an analytical technique for measuring the size and composition of individual micron-scale particles, is capable of analyzing atmospheric pollutants and bioaerosols much more efficiently and with more detail than conventional methods which require the collection of particles onto filters for analysis in the laboratory. Despite SPAMS’ demonstrated capabilities, the primary mechanisms of ionization are not fully understood, which creates challenges in optimizing and interpreting SPAMS signals. In this paper, we present a well-stirred reactor model for the reactions involved with the laser-induced vaporization and ionization of an individual particle. The SPAMS conditions modeled in this paper include a 248 nm laser which is pulsed for 8 ns to vaporize and ionize each particle in vacuum. The ionization of 1 μm, spherical Al particles was studied by approximating them with a 0-dimensional plasma chemistry model. The primary mechanism of absorption of the 248 nm photons was pressure-broadened direct photoexcitation to Al(y2D). Atoms in this highly excited state then undergo superelastic collisions with electrons, heating the electrons and populating the lower energy excited states. We found that the primary ionization mechanism is electron impact ionization of various excited state Al atoms, especially Al(y2D). Because the gas expands rapidly into vacuum, its temperature decreases rapidly. The rate of three-body recombination (e− + e− + Al+ → Al + e−) increases at low temperature, and most of the electrons and ions produced recombine within several μs of the laser pulse. The importance of the direct photoexcitation indicates that the relative peak heights of different elements in SPAMS mass spectra may be sensitive to the available photoexcitation transitions. The effects of laser intensity, particle diameter, and expansion dynamics are also discussed.

Energy partitioning in N2 microwave discharges: integrated Fokker–Planck approach to vibrational kinetics and comparison with experiments

This work investigates energy transfers between electrons, vibrational and translational degrees of freedom and their effect on dissociation mechanisms in a N2 microwave plasma in the pressure range between 50 and 400 mbar. A novel self-consistent 0D plasma chemistry model describing vibrational kinetics via the vibrational energy equation and the Fokker–Planck approach is developed. It is used to simulate conditions achieved experimentally, providing good agreement with measured values of vibrational and gas temperature and electron density. Above 100 mbar, energy efficiency of dissociation increases with power density, due to the significant contribution of collisions between vibrationally excited N2 and electronically excited molecules. Energy transfer to vibrations is maximum at low power density and low pressure due to reduced gas heating.

Consistent thermodynamics for plasma-assisted combustion

Excited-state species are a vital component of plasma chemistry modeling. Many previous works have studied the reactions of excited-state species. In almost all of these studies, the thermodynamic properties of the excited states are obtained via a modification of the formation enthalpy. It is often assumed that the error introduced by this procedure can be neglected. However, this is not true when vibrational states are included in the chemical mechanism. Specifically, we find that the error in adiabatic flame temperature can be as high as 100 K, and the error in NO concentration is as large as 50%. In this work, we will demonstrate how to consistently compute the thermodynamic data for these species. Key in computing thermodynamic data for excited-state species is the realization that an excited state is a subset of a full molecule description. The main challenge then becomes the consistently distributing the internal states over individual species.

Verification of Plasma Chemical Models of the Ionospheric D Region of the According to Radiophysical Data from Mikhnevo Geophysical Observatory

Verification of Plasma Chemical Models of the Ionospheric D Region of the According to Radiophysical Data from Mikhnevo Geophysical Observatory

Ignition thresholds and flame propagation of methane-air mixture: detailed kinetic study coupled with electrical measurements of the nanosecond repetitively pulsed plasma discharges

This study investigated the effects of nanopulsed plasma discharge on combustion enhancement. A disk-ring plasma actuator was developed to be applied to a rectangular cross-sectional burner. Experiments tests were performed with different applied voltages to estimate reduced electric field (E N), mean power and the energy input (E i) to be coupled with numerical analysis. The validation of the plasma chemistry model was carried out using ZDPlaskin numerical solver by comparing the experimental data of mole fraction of O atoms available in the literature. Plasma chemistry computations based on an extended kinetic mechanism of methane/air mixtures were performed by ZDPlasKin. Then, the results obtained by the ZDPlasKin in the form of kinetic effects (concentrations of O, OH, O3, CH, CH3, and H) and thermal effects (joule heating) were used in CHEMKIN to evaluate the combustion enhancement. Flame speed considerably increased with plasma discharge and maximum enhancement has been observed at about 20.9%. The flame temperature corresponding to the peak of heat release rate gets lower with the increase of the E i value. It has been observed that in a clean case, a lower flammability limit was retrieved at φ = 0.6, but in presence of plasma, it was reduced to φ = 0.5. The plasma actuation significantly reduced the time necessary to reach the ignition temperature with respect to autoignition (clean case). The maximum reduction of ignition delay timing was noticed at E i = 10.11 mJ cm−3 and E N = 200 Td, with a value between 10−8 and 10−7 s, significantly lower than the autoignition case is between 10−3 and 10−2 s. Adding to this, ignition analysis was also performed at low inlet temperatures. Results demonstrated that it was possible to reach ignition of CH4/air mixture at a lower inlet temperature than the one required for self-ignition (without plasma), meaning that ignition is even possible in cold flow conditions using plasma. Finally, thermal and kinetic effects of plasma discharge on the ignition enhancement of methane–air mixture have been predicted. It concluded the ignition improvement is mainly due to O, H, and OH species, while the thermal effect was negligible.

Kinetic study of plasma assisted oxidation of H2 for an undiluted lean mixture

For the past couple of decades, electrical discharges (or plasmas) have been widely investigated in pursuit of the advancement in combustion and fuel reforming. Particularly, nonthermal plasma has attracted researchers’ attention to improve ignition characteristics, promote flame stability, and reform hydrocarbons. Nevertheless, due to the nonthermal plasma's complex physicochemical nature, most of the experimental findings have not been fully explained yet. Recently, plasma-chemical kinetic studies have been initiated to address the important roles of plasma chemistry in hydrocarbon chemistry including combustion phenomena. However, we still have a long way to go to fully understand the underlying mechanisms and predict experimental outcomes. Here, we present a kinetic study of plasma-assisted low-temperature oxidation of H2 for an undiluted H2/O2 mixture. The aim of this study is to establish a foundation for low-temperature plasma assisted combustion as well as high-temperature plasma assisted reforming processes. We employed a plasma-chemical reaction mechanism and plasma-chemical kinetic modeling platform (KAUSTKin) and a temperature controlled dielectric barrier discharge reactor to study the plasma assisted oxidation of H2. As a result of systematically varying the gas temperature and discharge power, we found a non-linear oxidation behavior highlighting a Negative Temperature Coefficient (NTC)-like trend in a temperature range of 600–750 K. We investigated the effects of both the reduced electric field and the temperature on the plasma assisted oxidation chemistry. We found that (i) the oxidation is initiated by the electron impact dissociation of O2, which is governed by the reduced electric field and controls the oxidation degree, (ii) HO2 is the key intermediate for the full oxidation to H2O, and (iii) O3 and H2O2 production negatively affect the oxidation for temperatures below 400 K and over 600 K, respectively. We believe that these findings will further contribute to a better description and deeper understanding of the plasma chemistry with hydrocarbons as well as other H2 mixtures.

Hybrid numerical simulation of the generation and distribution characteristics of SF6 heavy particles under different DC PD energies

To investigate the decomposition law of SF6 under negative direct current partial discharge (PD) at different energies, a SF6 PD hybrid numerical model based on fluid dynamics and plasma chemical reaction models, in which 14 particle species and 24 chemical reactions are considered, is proposed. The effectiveness of the proposed model is validated with the current pulse waveform and the V–I discharge curve obtained by experiment. The influence of discharge energy on SF6 PD characteristic quantities and SF6 decomposition products is investigated with simulation and experiment. The results show that most of the discharge area of SF6 is neutral, and the cation clouds only exist in the ionosphere (4.79–5 mm). With the increase in applied voltage, the electric field intensity of the needle plate gap does not increase completely and even decreases in some areas. Moreover, different from the traditional opinion, the generation of SO2F2 under PD is mainly generated by the hydrolysis reaction of SOF4, which is formed by [SF5], [SF4], and [OH], [O]. The reaction path of [SF2] with O2 is not important. Thus, c(SO2F2)/c(SOF2 + SO2) can be used as the energy characteristic component ratio because of its ability to represent the low-fluorine sulfide ratio n([SF5])/n([SF4]).

Self-consistent modeling of microwave activated N2/CH4/H2 (and N2/H2) plasmas relevant to diamond chemical vapor deposition

The growth rate of diamond by chemical vapor deposition (CVD) from microwave (MW) plasma activated CH4/H2 gas mixtures can be significantly enhanced by adding trace quantities of N2 to the process gas mixture. Reasons for this increase remain unclear. The present article reports new, self-consistent two-dimensional modeling of MW activated N2/H2 and N2/CH4/H2 plasmas operating at pressures and powers relevant to contemporary diamond CVD, the results of which are compared and tensioned against available experimental data. The enhanced N/C/H plasma chemical modeling reveals the very limited reactivity of N2 under typical processing conditions and the dominance of N atoms among the dilute ‘soup’ of potentially reactive N-containing species incident on the growing diamond surface. Ways in which these various N-containing species may enhance growth rates are also discussed.

On the population density of the argon excited levels in a high power impulse magnetron sputtering discharge

Population densities of excited states of argon atoms in a high power impulse magnetron sputtering (HiPIMS) discharge are examined using a global discharge model and a collisional-radiative model. Here, the ionization region model (IRM) and the Orsay Boltzmann equation for electrons coupled with ionization and excited states kinetics (OBELIX) model are combined to obtain the population densities of the excited levels of the argon atom in a HiPIMS discharge. The IRM is a global plasma chemistry model based on particle and energy conservation of HiPIMS discharges. OBELIX is a collisional-radiative model where the electron energy distribution is calculated self-consistently from an isotropic Boltzmann equation. The collisional model constitutes 65 individual and effective excited levels of the argon atom. We demonstrate that the reduced population density of high-lying excited argon states scales with (p*)−6, where p* is the effective quantum number, indicating the presence of a multistep ladder-like excitation scheme, also called an excitation saturation. The reason for this is the dominance of electron impact processes in the population and de-population of high-lying argon states in combination with a negligible electron–ion recombination.

Analysis of the plasma transport in numerical simulations of helicon plasma thrusters

The accurate simulation of the plasma transport in helicon sources is a key aspect to improve the design of Helicon Plasma Thrusters (HPTs). Specifically, the 3D-VIRTUS code was proven to provide satisfactory estimations of the propulsive performance of realistic HPTs (difference between measures and numerical estimations of the thrust <30%). Nonetheless, further investigations are needed to deepen the influence that the plasma chemistry model, the formulation of the energy equation, and the definition of the diffusion coefficients have on the results of the simulation. First, a quantitative analysis has been conducted on a simplified configuration of HPT to study each phenomenon separately. Second, the generalized fluid model has been benchmarked against measures of plasma density performed on a helicon source. The radiative decay reactions affect the estimation of the performance (e.g., thrust) up to 40%. The quasi-isotherm formulation of the energy equation affects results (e.g., electron density) up to 30%. Accounting for anomalous transport or defining diffusion coefficients classically does not have a major effect on the simulation (e.g., thrust varies less than 20%). The generalized formulation of the fluid model provides estimations of the plasma density, which are within the uncertainty band of the measures (i.e., differences <20%).

(Invited) Microkinetic Models for Plasma-Catalytic NH3 Synthesis: Effects of Catalyst Material, Gas Temperature and Power Deposition on Rates and Yields

We developed microkinetic plasma catalysis models to investigate the plasma-assisted ammonia synthesis on transition metal catalysts in dielectric barrier discharge plasmas. These models aim at gaining insight in the reaction mechanisms and kinetics of plasma-activated species at the catalytic surface. We focus on the effects of vibrational excitations as well as plasma-generated radicals, on the turnover frequencies and equilibrium conversion.Based on the results of DFT calculations, we built scaling relations for the catalytic conversion of N2 and H2 towards NH3on transition metal catalysts. These scaling relations allow the generation of ‘volcano’ plots that show the logarithm of the turnover frequencies as function of a catalyst descriptor (in our case the binding energy of N). We compare the volcano plots for thermal catalysis with vibrationally-enhanced catalysis and radical-enhanced catalysis in order to investigate how the plasma can most efficiently aid in the catalytic conversion. Careful analysis of reaction flow diagrams shows which pathways are dominant in each of the regimes. We show that Eley-Rideal (ER) mechanisms can become very important in plasmas with high densities of radical species as opposed to thermal conditions where ER is usually considered negligible compared to Langmuir-Hinschelwood. We determined on which catalysts ER mechanisms are most important and how they impact the turnover frequency.This framework was introduced in a plasma chemistry model in ZDPlasKin to study conversions as function of catalyst material, gas temperature and specific energy input. Model results show the temporal behavior, with the specific role of microdischarge filaments and afterglows, of filamentary plasmas and how catalyst material, gas temperature and specific energy input can induce conversions beyond thermal equilibrium.

Model simulations of chemical effects of sprites in relation with observed HO<sub>2</sub> enhancements over sprite-producing thunderstorms

Abstract. Recently, measurements by the Superconducting Submillimeter-Wave Limb Emission Sounder (SMILES) satellite instrument have been presented which indicate an increase in mesospheric HO2 above sprite-producing thunderstorms. The aim of this paper is to compare these observations to model simulations of chemical sprite effects. A plasma chemistry model in combination with a vertical transport module was used to simulate the impact of a streamer discharge in the altitude range 70–80 km, corresponding to one of the observed sprite events. Additionally, a horizontal transport and dispersion model was used to simulate advection and expansion of the sprite air masses. The model simulations predict a production of hydrogen radicals mainly due to reactions of proton hydrates formed after the electrical discharge. The net effect is a conversion of water molecules into H+OH. This leads to increasing HO2 concentrations a few hours after the electric breakdown. Due to the modelled long-lasting increase in HO2 after a sprite discharge, an accumulation of HO2 produced by several sprites appears possible. However, the number of sprites needed to explain the observed HO2 enhancements is unrealistically large. At least for the lower measurement tangent heights, the production mechanism of HO2 predicted by the model might contribute to the observed enhancements.

Imaging and Modeling C2 Radical Emissions from Microwave Plasma-Activated Methane/Hydrogen Gas Mixtures: Contributions from Chemiluminescent Reactions and Investigations of Higher-Pressure Effects and Plasma Constriction.

Wavelength and spatially resolved imaging and 2D plasma chemical modeling methods have been used to study the emission from electronically excited C2 radicals in microwave-activated dilute methane/hydrogen gas mixtures under processing conditions relevant to the chemical vapor deposition (CVD) of diamond. Obvious differences in the spatial distributions of the much-studied C2(d3Πg-a3Πu) Swan band emission and the little-studied, higher-energy C2(C1Πg-A1Πu) emission are rationalized by invoking a chemiluminescent (CL) reactive source, most probably involving collisions between H atoms and C2H radicals, that acts in tandem with the widely recognized electron impact excitation source term. The CL source is relatively much more important for forming C2(d) state radicals and is deduced to account for >40% of C2(d) production in the hot plasma core under base operating conditions, which should encourage caution when estimating electron or gas temperatures from C2 Swan band emission measurements. Studies at higher pressures (p ≈ 400 Torr) offer new insights into the plasma constriction that hampers efforts to achieve higher diamond CVD rates by using higher processing pressures. Plasma constriction is proposed as being inevitable in regions where the local electron density (ne) exceeds some critical value (nec) and electron-electron collisions enhance the rates of H2 dissociation, H-atom excitation, and related associative ionization processes relative to those prevailing in the neighboring nonconstricted plasma region. The 2D modeling identifies a further challenge to high-p operation. The radial uniformities of the CH3 radical and H-atom concentrations above the growing diamond surface both decline with increasing p, which are likely to manifest as less spatially uniform diamond growth (in terms of both rate and quality).

Simulation of chemical composition nitrogen-argon-aluminum plasma

Plasma-chemical model of magnetron discharge is described. The model allows you to predict the densities of the main plasma components. As a result of numerical modeling, it was found that dissociation of N2 ranges from 10% to 30% with an increase in plasma power, the degree of dissociation increases. The addition of N2 to the plasma leads to a decrease in ion density due to energy consumption for non-ionizing molecular collisions. It can be assumed that aluminum nitride is deposited on the surface of a substrate with a high flow of metal ions in combination with a stream of neutral molecules N2 with a low adhesion coefficient and a relatively small flow of nitrogen ions.

Flowing Atmospheric Pressure Afterglow for Ambient Ionization: Reaction Pathways Revealed by Modeling.

We describe the plasma chemistry in a helium flowing atmospheric pressure afterglow (FAPA) used for analytical spectrometry, by means of a quasi-one-dimensional (1D) plasma chemical kinetics model. We study the effect of typical impurities present in the feed gas, as well as the afterglow in ambient humid air. The model provides the species density profiles in the discharge and afterglow regions and the chemical pathways. We demonstrate that H, N, and O atoms are formed in the discharge region, while the dominant reactive neutral species in the afterglow are O3 and NO. He* and He2* are responsible for Penning ionization of O2, N2, H2O, H2, and N, and especially O and H atoms. Besides, He2+ also contributes to ionization of N2, O2, H2O, and O through charge transfer reactions. From the pool of ions created in the discharge, NO+ and (H2O)3H+ are the dominant ions in the afterglow. Moreover, negatively charged clusters, such as NO3H2O- and NO2H2O-, are formed and their pathway is discussed as well. Our model predictions are in line with earlier observations in the literature about the important reagent ions and provide a comprehensive overview of the underlying pathways. The model explains in detail why helium provides a high analytical sensitivity because of high reagent ion formation by both Penning ionization and charge transfer. Such insights are very valuable for improving the analytical performance of this (and other) ambient desorption/ionization source(s).

Reactive collisions between electrons and BeT[formula omitted]: Complete set of thermal rate coefficients up to 5000 K

Rate coefficients for the dissociative recombination, vibrational excitation and vibrational de-excitation of the BeT+ ion for all vibrational levels of its ground electronic state (X1Σ+,vi+=0,…,27) are reported, including in the calculation the contribution of super-excited states of the BeT complex pertaining to three electronic symmetries - 2Π, 2Σ+, and 2Δ. These data are suitable for the kinetic modeling of beryllium and tritium containing plasma, as encountered in magnetic fusion devices with beryllium walls (JET, ITER). In the present study we restrict ourselves to incident electron energies from 10−3 up to 2.7eV, and to electron temperatures between 100 and 5000 K, respectively. Together with our earlier and closely related studies on the BeH+ and BeD+ systems, this present work completes the isotopic coverage for the beryllium monohydride ions. The vibrational energy (rather than the vibrational quantum state) is identified as a proper isotopic similarity parameter, e.g., for reduced but still isotopically correct plasma chemistry models.

The spatial distribution of hydrogen and oxygen atoms in a cold atmospheric pressure plasma jet

Cold atmospheric pressure plasma jets (CAPJs) are an emerging technology for the localised treatment of heat sensitive surfaces. Adding humidity to the CAPJ’s feed gas yields an effective production of highly reactive intermediate species, such as hydrogen atoms, oxygen atoms, and hydroxyl radicals, among others, which are key species for biomedical applications. This study focusses on the effluent of the CAPJ kINPen, which was operated with argon feed gas and a humidity admixture of 3000 ppm, while a gas curtain was used to limit the diffusion of ambient air into the effluent. The axial and radial density distribution of O and H atoms is measured by means of picosecond two-photon absorption laser induced fluorescence spectroscopy (ps-TALIF). A maximum O atom density of (3.8 ± 0.7) × 1015 cm−3 and a maximum H atom density of (3.5 ± 0.7) × 1015 cm−3 are found at the nozzle of the plasma jet. The experimental results are compared to a two-dimensional reacting flow model that is coupled with a local zero-dimensional plasma chemical model. With this model, the main H and O atom production mechanisms are determined to be the dissociation of H2O and O2 in the plasma zone of the plasma jet. The latter indicates, that a significant amount of oxygen (1%) was present inside the device. The reaction of OH with O atoms represents the main consumption pathway for O atoms and is at the same time a significant production pathway for H atoms. The main consumption of H atoms is through a three-body reaction including O2 to form HO2, which consumes more H and O atoms to form OH. It is pointed out, that most of the species are produced in the plasma zone, and that O and H atoms, OH and HO2 radicals, and O2 and H2O molecules are strongly connected.