Ivan Shkurenkov

Two-stage energy thermalization mechanism in nanosecond pulse discharges in air and hydrogen–air mixtures

Time-resolved and spatially resolved temperature measurements, by pure rotational picosecond broadband coherent anti-Stokes Raman spectroscopy (CARS), and kinetic modeling calculations are used to study kinetics of energy thermalization in nanosecond pulse discharges in air and hydrogen?air mixtures. The diffuse filament, nanosecond pulse discharge (pulse duration ?100?ns) is sustained between two spherical electrodes and is operated at a low pulse repetition rate to enable temperature measurements over a wide range of time scales after the discharge pulse. The experimental results demonstrate high accuracy of pure rotational ps CARS for thermometry measurements in highly transient non-equilibrium plasmas. Rotational?translational temperatures are measured for time delays after the pulse ranging from tens of ns to tens of ms, spanning several orders of magnitude of time scales for energy thermalization in non-equilibrium plasmas. In addition, radial temperature distributions across the plasma filament are measured for several time delays after the discharge pulse. Kinetic modeling calculations using a state-specific master equation kinetic model of reacting hydrogen?air plasmas show good agreement with experimental data. The results demonstrate that energy thermalization and temperature rise in these plasmas occur in two clearly defined stages, (i) ?rapid? heating, caused by collisional quenching of excited electronic states of N2 molecules by O2, and (ii) ?slow? heating, caused primarily by N2 vibrational relaxation by O atoms (in air) and by chemical energy release during partial oxidation of hydrogen (in H2?air). The results have major implications for plasma assisted combustion and plasma flow control.

Femtosecond, two-photon-absorption, laser-induced-fluorescence (fs-TALIF) imaging of atomic hydrogen and oxygen in non-equilibrium plasmas

Femtosecond, two-photon-absorption laser-induced fluorescence (fs-TALIF) is employed to measure space- and time-resolved distributions of atomic hydrogen and oxygen in moderate-pressure, non-equilibrium, nanosecond-duration pulsed-discharge plasmas. Temporally and spatially resolved hydrogen and oxygen TALIF images are obtained over a range of low-temperature plasmas in mixtures of helium and argon at 100 Torr total pressure. The high-peak-intensity, low-average-energy fs pulses combined with the increased spectral bandwidth compared to traditional ns-duration laser pulses provide a large number of photon pairs that are responsible for the two-photon excitation, which results in an enhanced TALIF signal. Krypton and xenon TALIF are used for quantitative calibration of the hydrogen and oxygen concentrations, respectively, with similar excitation schemes being employed. This enables 2D collection of atomic-hydrogen and -oxygen TALIF signals with absolute number densities ranging from 2 × 1012 cm−3 to 6 × 1015 cm−3 and 1 × 1013 cm−3 to 3 × 1016 cm−3, respectively. These 2D images are the first application of TALIF imaging in moderate-pressure plasma discharges. 1D self-consistent modeling predictions show agreement with experimental results within the estimated experimental error of 25%. The present results can be used to further the development of higher fidelity kinetic models while quantifying plasma-source characteristics.

Electric field measurements in a dielectric barrier nanosecond pulse discharge with sub-nanosecond time resolution

The paper presents the results of time-resolved electric field measurements in a nanosecond discharge between two plane electrodes covered by dielectric plates, using picosecond four-wave mixing diagnostics. For absolute calibration, the IR signal was measured in hydrogen at a pressure of 440 Torr, for electrostatic electric field ranging from 0 to 8 kV cm−1. The calibration curve (i.e. the square root of IR signal intensity versus electric field) was shown to be linear. By measuring the intensities of the pump, Stokes, and IR signal beam for each laser shot during the time sweep across the high-voltage pulse, temporal evolution of the electric field in the nanosecond pulse discharge was determined with sub-nanosecond time resolution. The results are compared to kinetic modeling predictions, showing good agreement, including non-zero electric field offset before the main high voltage pulse, breakdown moment, and reduction of electric field in the plasma after breakdown. The difference between the experimental results and model predictions is likely due to non-1D structure of the discharge. Comparison with the kinetic modeling predictions shows that electric field in the nanosecond pulse discharge is controlled primarily by electron impact excitation and charge accumulation on the dielectric surfaces.

An Examination of Nitric Oxide Kinetics in a Plasma Afterglow with Significant Vibrational Loading

Laser-induced fluorescence measurements (LIF) of nitric oxide and two photon absorption laser-induced fluorescence (TALIF) of oxygen and nitrogen atoms are performed in a diffuse plasma filament with significant vibrational loading. The results are compared with kinetic modeling calculations. The experimental data shows that significant NO concentrations are achieved within a few microseconds after the pulse, after which the NO evolution is controlled by the reverse Zel’dovich reactions. The modeling calculation results show that the dominant formation channel is through collisions of oxygen atoms with multiple electronically-excited nitrogen states. Aside from acting as an energy reservoir, the large concentration of vibrationally-excited nitrogen in the ground electronic state seems to have no effect on NO formation. The present experimental results are accurately reproduced by incorporating all electronically excited N2 ∗ levels.

Time-resolved electron density and electron temperature measurements in nanosecond pulse discharges in helium

Thomson scattering is used to study temporal evolution of electron density and electron temperature in nanosecond pulse discharges in helium sustained in two different configurations, (i) diffuse filament discharge between two spherical electrodes, and (ii) surface discharge over plane quartz surface. In the diffuse filament discharge, the experimental results are compared with the predictions of a 2D plasma fluid model. Electron densities are put on an absolute scale using pure rotational Raman spectra in nitrogen, taken without the plasma, for calibration. In the diffuse filament discharge, electron density and electron temperature increase rapidly after breakdown, peaking at n e ≈ 3.5 1015 cm−3 and T e ≈ 4.0 eV. After the primary discharge pulse, both electron density and electron temperature decrease (to n e ~ 1014 cm−3 over ~1 µs and to T e ~ 0.5 eV over ~200 ns), with a brief transient rise produced by the secondary discharge pulse. At the present conditions, the dominant recombination mechanism is dissociative recombination of electrons with molecular ions, . In the afterglow, the electron temperature does not relax to gas temperature, due to superelastic collisions. Electron energy distribution functions (EEDFs) inferred from the Thomson scattering spectra are nearly Maxwellian, which is expected at high ionization fractions, when the shape of EEDF is controlled primarily by electron–electron collisions. The kinetic model predictions agree well with the temporal trends detected in the experiment, although peak electron temperature and electron density are overpredicted. Heavy species temperature predicted during the discharge and the early afterglow remains low and does not exceed T = 400 K, due to relatively slow quenching of metastable He* atoms in two-body and three-body processes. In the surface discharge, peak electron density and electron temperature are n e ≈ 3 1014 cm3 and T e ≈ 4.25 eV, attained after the surface ionization wave reaches the grounded electrode. The sensitivity of the present diagnostics is too low to measure electron density in the plasma during surface ionization wave propagation (estimated to be below n e ≈ 1013 cm−3). After peaking during the primary current pulse, the electron density decays due to dissociative recombination. Electron temperature decreases rapidly over ~150 ns after the primary current pulse rise, to T e ≈ 0.5 eV, followed by a much more gradual electron cooling between the primary and the secondary discharge pulses, due to superelastic collisions providing moderate electron heating in the afterglow.

Thomson Scattering Studies in He and He/H2 Nanosecond Pulse Nonequilibrium Plasmas

Thomson scattering and kinetic modeling are used to study time evolution of electron density and electron temperature in a nanosecond pulse, diffuse filament electric discharge sustained between two spherical electrodes and operated at a low pulse repetition rate. The experiments have been done for three representative cases: (1) Helium, P=200 torr, discharge pulse energy 17 mJ/pulse; (2) Helium, P=100 torr, discharge pulse energy 0.6 mJ/pulse; and (3) 1% hydrogen in helium, P=100 torr, discharge pulse energy 0.8 mJ/pulse. In Case 1, peak electron number density and peak electron temperature are ne ≈ 3.5·10 15 cm -3 and Te ≈ 4 eV, respectively. The kinetic model predictions agree well with the temporal trends detected in the experiment (rapid initial rise of electron temperature and electron density during the discharge pulse and gradual decay in the afterglow), although peak electron temperature and electron density values during the pulse are somewhat overpredicted. Similar temporal trends, although at lower peak ne and Te, are observed at lower discharge pulse energies. The electron number density decay in a 1% H2 – He mixture is found to be much faster, by approximately a factor of 4, compared to helium at the same pressure and nearly the same discharge pulse energy. This is likely due to more rapid dissociative recombination of electrons in collisions with H2 + ions compared to dissociation recombination of electrons with He2 + ions. At these lower pulse energies, model predictions reproduce temporal trends fairly well, but overpredict peak electron density as well as the rate of electron cooling. D ow nl oa de d by I go r A da m ov ic h on F eb ru ar y 2, 2 01 4 | h ttp :// ar c. ai aa .o rg | D O I: 1 0. 25 14 /6 .2 01 413 58 52nd Aerospace Sciences Meeting 13-17 January 2014, National Harbor, Maryland AIAA 2014-1358 Copyright

Kinetics of NO Formation and Decay in Nanosecond Pulse Discharges in Air-Fuel Mixtures

Time-resolved, absolute NO and N atom number densities are measured by NO Laser Induced Fluorescence (LIF) and N Two-photon Absorption LIF in a diffuse plasma filament, nanosecond pulse discharge in dry air, hydrogen-air, and ethylene-air mixtures at 40 Torr, over a wide range of equivalence ratios. The results are compared with kinetic modeling calculations incorporating pulsed discharge dynamics, kinetics of vibrationally and electronically excited states of nitrogen, plasma chemical reactions, and radial transport. The results show that in air afterglow, NO decay occurs primarily by the reaction with N atoms, NO + N → N2 + O. In the presence of hydrogen, this reaction is mitigated by reaction of N atoms with OH, N + OH → NO + H, resulting in significant reduction of N atom number density in the afterglow, additional NO production, and considerably higher NO number densities. In fuel-lean ethylene-air mixtures, a similar trend (i.e. N atom concentration reduction and NO number density increase) is observed, although [NO] increase on ms time scale is not as pronounced as in H2-air mixtures. In nearstoichiometric and fuel-lean ethylene-air mixtures, when N atom number density was below detection limit, NO concentration was measured to be lower than in air plasma. The results show the need for further kinetic modeling to provide quantitative insight into NO kinetics in hydrocarbon-air plasmas.

Kinetics of NO formation and decay in nanosecond pulse discharges in Air, H2-Air, and C2H4-Air mixtures

Time-resolved, absolute NO and N atom number densities are measured by NO Laser Induced Fluorescence (LIF) and N Two-Photon Absorption LIF in a diffuse plasma filament, nanosecond pulse discharge in dry air, hydrogen-air, and ethylene-air mixtures at 40 Torr, over a wide range of equivalence ratios. The results are compared with kinetic modeling calculations incorporating pulsed discharge dynamics, kinetics of vibrationally and electronically excited states of nitrogen, plasma chemical reactions, and radial transport. The results show that in air afterglow, NO decay occurs primarily by the reaction with N atoms, NO + N → N2 + O. In the presence of hydrogen, this reaction is mitigated by reaction of N atoms with OH, N + OH → NO + H, resulting in significant reduction of N atom number density in the afterglow, additional NO production, and considerably higher NO number densities. In fuel-lean ethylene-air mixtures, a similar trend (i.e. N atom concentration reduction and NO number density increase) is observed, although [NO] increase on ms time scale is not as pronounced as in H2-air mixtures. In near-stoichiometric and fuel-lean ethylene-air mixtures, when N atom number density was below detection limit, NO concentration was measured to be lower than in air plasma. These results suggest that NO kinetics in hydrocarbon-air plasmas is more complex compared to air and hydrogen-air plasmas, additional NO reaction pathways may well be possible, and their analysis requires further kinetic modeling calculations.

Time-resolved electron temperature and electron density measurements in a nanosecond pulse filament discharge in H2-He and O2-He mixtures

Time evolution of electron density and electron temperature in a nanosecond pulse, diffuse filament electric discharge in H2–He and O2–He mixtures at a pressure of 100 Torr is studied by Thomson/pure rotational Raman scattering and kinetic modeling. The discharge is sustained between two spherical electrodes separated by a 1 cm gap and powered by high voltage pulses ~150 ns duration. Discharge energy coupled to the plasma filament 2–3 mm in diameter is 4–5 mJ/pulse, with specific energy loading of up to ~0.3 eV/molecule. At all experimental conditions, a rapid initial rise of electron temperature and electron density during the discharge pulse is observed, followed by the decay in the afterglow, over ~100 ns–1 µs. Electron density in the afterglow decays more rapidly as H2 or O2 fraction in the mixture is increased. In He/H2 mixtures, this is likely due to more rapid recombination of electrons in collisions with and ions, compared to recombination with ions. In O2/He mixtures, electron density decay in the afterglow is affected by recombination with and ions, while the effect of three-body attachment is relatively minor. Peak electron number densities and electron temperatures are n e = (1.7–3.1) 1014 cm−3 and T e = 2.9–5.5 eV, depending on gas mixture composition. Electron temperature in the afterglow decays to approximately T e ≈ 0.3 eV, considerably higher compared to the gas temperature of T = 300–380 K, inferred from O2 pure rotational Raman scattering spectra, due to superelastic collisions. The experimental results in helium and O2–He mixtures are compared with kinetic modeling predictions, showing good agreement.