A model of fast heating of nitrogen–oxygen mixtures excited by a gas discharge in a broad range of reduced electric fields E/N is presented. It is shown that in air at E/N ⩽ 200 Td the main contribution to gas heating occurs due to dissociation reactions by electron impact of O2 molecules and due to processes of quenching of electronically excited N2(B  3Πg, C  3Πu, ) molecules by oxygen and excited O(1D) atoms by nitrogen. At E/N > 400 Td, dissociation reactions by electron impact of N2 molecules are dominant as well as the processes involving charged particles. The fraction of discharge energy converted to fast gas heating does not exceed 40%. An analysis of the experimental data on fast air heating in discharges at high reduced electric fields E/N is given. It was shown that, in a broad range of reduced electric fields, a fixed fraction of discharge power η E spent on the excitation of electronic degrees of freedom, ionization and dissociation of molecules is converted to fast heating of nitrogen–oxygen mixtures. In air, the value of η E is about 30 ± 3%. The value of η E diminishes with decreasing share of oxygen in a mixture. The significant role of heat release in the pooling reactions of molecules for fast gas heating in pure nitrogen and in nitrogen with small admixtures of oxygen is demonstrated. The simulation results agree with experimental data at E/N < 200 Td within the range of oxygen content δ = 0–20%.

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The results of a numerical study on kinetic processes initiated by a pulsed nanosecond discharge in air at high specific deposited energy, when the dissociation degree of oxygen molecules is high, are presented. The calculations of the temporal dynamics of the electron concentration, density of atomic oxygen, vibrational distribution function of nitrogen molecules, and gas temperature agree with the experimental data. It is shown that quenching of electronically excited states of nitrogen N 2 (B 3 Π g ), N 2 (С 3 Π u ), N 2 (a′ 1 Σ − u ) by oxygen molecules leads to the dissociation of O 2 . This conclusion is based on the comparison of calculated dynamics of atomic oxygen in air, excited by a pulsed nanosecond discharge, with experimental data.In air plasma at a high dissociation degree of oxygen molecules ([O]/[O 2 ] > 10%), relaxation of the electronic energy of atoms and molecules in reactions with O atoms becomes extremely important. Active production of NO molecules and fast gas heating in the discharge plasma due to the quenching of electronically excited N 2 (B 3 Π g , C 3 Π u , a′ 1 Σ − u ) molecules by oxygen atoms is notable. Owing to the high O atom density, electrons are effectively detached from negative ions in the discharge afterglow. As a result, the decay of plasma in the afterglow is determined by electron-ion recombination, and the electron density remains relatively high between the pulses.An increase in the vibrational temperature of nitrogen molecules at the periphery of the plasma channel at time delay t = 1-30 μs after the discharge is obtained. This is due to intense gas heating and, as a result, gas-dynamic expansion of a hot gas channel. Vibrationally excited N 2 (v) molecules produced near the discharge axis move from the axial region to the periphery. Consequently, at the periphery the vibrational temperature of nitrogen molecules is increased.

A review of experimental and theoretical investigations of the effect of atomic particles, and electronically and vibrationally excited molecules on the induction delay time and on the shift in the ignition temperature threshold of combustible mixtures is presented.The addition of oxygen and hydrogen atoms to combustible mixtures may cause a significant reduction in the ignition delay time. However, at relatively low initial temperatures, the non-equilibrium effect of the addition of atomic particles in ground electronic states is not pronounced. At the same time, the effect of excited O( 1 D) atoms on the oxidation and reforming of combustible mixtures is quite significant due to the high rates of reactions of O( 1 D) atoms with hydrogen and hydrocarbon molecules. In fuel-air mixtures, collisions with O( 1 D) atoms determine, under certain conditions, the dissociation of hydrocarbon molecules.Singlet oxygen molecules, O 2 (a 1 Δ g ), participate both in chain initiation and chain branching reactions, but the effect of O 2 (a 1 Δ g ) on the ignition processes is generally less important compared to oxygen atoms.The reactions of vibrationally excited molecules and the processes of VT-relaxation in combustible mixtures are discussed. The production of vibrationally excited N 2 (v) molecules in fuel-air mixtures at relatively low electric field is very important. However, at the moment, the effect of the reactions of N 2 (v) molecules on the oxidation and ignition of combustible mixtures is not completely clear, and requires further investigation.Therefore, with present knowledge, to reduce the ignition delay time and decrease the temperature threshold of combustive mixtures, the use of gas discharge systems with relatively high E/N values is recommended. In this case the reactions of electronically excitedmolecules, and atomic particles in ground and electronically excited states, are extremely important. The energy stored in electronically excited states of atoms and molecules is spent on the additional dissociation of oxygen and fuel molecules, on fast gas heating, and finally on triggering chain branching reactions.

Streamer-to-filament transition is a general feature of nanosecond discharges at elevated pressure. The transition is observed in different discharges by different groups: in the nanosecond surface dielectric barrier discharges (nSDBDs) in a single shot regime at high pressure (2-15 bar), in the point-to-point or point-to-plane open electrodes discharges at high repetitive frequency (so-called nanosecond repetitive pulsed discharges, NRPDs) at atmospherics pressure. The present paper contains experimental analysis of plasma properties in the filamentary nSDBD: the electrical current, the specific deposited energy, the electron density and the electron temperature were measured for a wide range of pressures and voltages. A model explaining plasma properties in filamentary nanosecond discharges and the role of excited species in streamer-to-filament transition is suggested and discussed.

The development of a nanosecond surface dielectric barrier discharge in air at pressures 1-6 bar is studied. At atmospheric pressure, the discharge develops as a set of streamers starting synchronously from the high-voltage electrode and propagating along the dielectric layer. Streamers cover the dielectric surface creating a 'quasi-uniform' plasma layer. At high pressures and high voltage amplitudes on the cathode, filamentation of the discharge is observed a few nanoseconds after the discharge starts. Parameters of the observed 'streamers-to-filaments' transition are measured; physics of transition is discussed on the basis of theoretical estimates and numerical modeling. Ionization-heating instability on the boundary of the cathode layer is suggested as a mechanism of filamentation.

The process of fast gas heating in air in the near afterglow of a pulsed nanosecond spatially uniform discharge has been investigated experimentally and numerically at moderate (3−9 mbar) pressures and high (200−400 Td) reduced electric fields. The temporal behavior of discharge current, deposited energy, electric field and temperature were measured. The role of processes with participation of excited and charged species was analyzed. It was shown that under the considered conditions the main energy release takes place in reactions of nitrogen and oxygen dissociation by electron impact and quenching of electronically excited nitrogen molecules, such as N 2 (A 3 Σ + u , B 3 Π g , C 3 Π u , a' 1 Σ − u) by oxygen and quenching of excited O(1 D) atoms by N 2. It was shown that about 24% of the discharge energy goes to fast gas heating during first tens of microseconds after the discharge.

O2(a 1Δg) production in a non-self-sustained discharge (ND) in pure oxygen and oxygen mixtures with inert gases (Ar and He) has been studied. A self-consistent model of ND in pure oxygen is developed, allowing us to simulate all the obtained experimental data. Agreement between the experimental and simulated results for pure oxygen over a wide range of reduced electric fields was reached only after taking into account the ion component of the discharge current. It is shown that the correct estimation of the energetic efficiency of O2(a 1Δg) excitation by discharge using the EEDF calculation is possible only with the correct description of the energy deposit into the plasma on the basis of an adequate discharge model. The testing of an O2(a 1Δg) excitation cross-section by direct electron impact, as well as a kinetic scheme of processes involving singlet oxygen, has been carried out by the comparison of experimental and simulated data. The tested model was then used for simulating O2(a 1Δg) production in ND in oxygen mixtures with inert gases. The study of O2(a 1Δg) production in Ar : O2 mixtures with small oxygen content has shown that the ND in these mixtures is spatially non-uniform, which essentially decreases the energetic efficiency of singlet oxygen generation. While simulating the singlet oxygen density dynamics, the process of three-body deactivation of O2(a 1Δg) by O(3P) atoms was for the first time taken into account. The maximal achievable concentration of singlet oxygen in ND can be limited by this quenching. On the basis of the results obtained and the model developed, the influence of hydrogen additives on singlet oxygen kinetics in argon–oxygen–hydrogen mixtures has been analysed. The simulation has shown that fast quenching of O2(a 1Δg) by atomic hydrogen is possible due to significant gas heating in the discharge that can significantly limit the yield of singlet oxygen in hydrogen-containing mixtures.