David Burnette

Picosecond CARS measurements of nitrogen vibrational loading and rotational/translational temperature in non-equilibrium discharges

Picosecond Coherent Anti-Stokes Raman Spectroscopy is used to study vibrational energy loading and relaxation kinetics in nitrogen and air nsec pulsed non-equilibrium plasmas, in both plane-to-plane and pin-to-pin geometries. In 10 kHz repetitively pulsed plane-to-plane plasmas, up to ~50% of coupled discharge power is found to load vibrations, in good agreement with a master equation kinetic model. In the pin-to-pin geometry, ~33% of total discharge energy in a single pulse in air at 100 torr is found to couple directly to nitrogen vibrations by electron impact, also in good agreement with model predictions. Post-discharge, the total quanta in vibrational levels v=0-9 is found to increase by a factor of ~2 in air and by a factor of ~4 in nitrogen, respectively, a result in direct contrast to modeling results which predict the total number of quanta to be essentially constant until ultimately decaying by V-T relaxation and mass diffusion. More detailed comparison between experiment and model show that the vibrational distribution function (VDF) predicted by the model during, and directly after, the discharge pulse is in good agreement with that determined experimentally. However, for time delays exceeding ~10 μsec, the experimental VDF shows populations of vibrational levels v≥2 greatly exceeding modeling predictions, which predicts their monotonic decay due to net downward V-V transfer and corresponding increase in v=1 population. This is at variance with the experimental results, which show an increase in the populations of levels v=2 and v=3, reaching a maximum at t~50-100 μsec after the discharge pulse, and relatively steady v=4-9 populations at t~10-100 μsec. It is concluded that a collisional process is feeding high vibrational levels at a rate which is comparable to the rate at which population of the high levels is lost due to net downward V-V energy transfer. A likely candidate for the source of additional vibrational quanta is quenching of metastable electronic states of nitrogen to highly excited vibrational levels of the ground electronic state.

Time-resolved temperature and O atom measurements in nanosecond pulse discharges in combustible mixtures

The paper presents results of time-resolved rotational temperature measurements, by pure rotational coherent anti-Stokes Raman spectroscopy and absolute O atom number density measurements, by two-photon absorption laser induced fluorescence. The experiments were conducted in nanosecond pulse discharges in H2–O2–Ar and C2H4–O2–Ar mixtures, initially at room temperature, operated at a high pulse repetition rate of 40 kHz, in a plane-to-plane double dielectric barrier geometry at a pressure of 40 Torr. Intensified charge-coupled device images show that O2–Ar and H2–O2–Ar plasmas remain diffuse and volume-filling during the entire burst. Images taken in C2H4–O2–Ar plasma demonstrate significant discharge filamentation and constriction along the center plane and in the corners of the test section. The experimental results demonstrate high accuracy of pure rotational psec CARS for thermometry measurements at low partial pressures of oxygen in nonequilibrium plasmas. The results are compared with kinetic modeling calculations, using two different H2–O2 chemistry and C2H4–O2 chemistry mechanisms. In H2–O2–Ar mixtures, the kinetic modeling predictions are in fairly good agreement with the data, predicting temperature rise and O atom accumulation in long discharge bursts, up to 450 pulses. The results show that adding hydrogen to the mixture results in an additional temperature rise, due to its partial oxidation by radicals generated in the plasma, essentially without chain branching. In C2H4–O2–Ar mixtures, the model consistently underpredicts both temperature and O atom number density. The most likely reason for the difference between the experimental data and model predictions is discharge filamentation developing when ethylene is added to the O2–Ar mixture, at fairly low temperatures.

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.

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.

Measurements of Vibrational Energy Loading in a Diffuse Plasma Filament

Picosecond CARS is employed to study vibrational loading in diffuse air and nitrogen plasma filaments while using a nanosecond discharge that couples a large fraction of energy into the vibrational mode. It was found that, as has been previously reported, the total energy coupled to the vibrational mode increases long after the pulse, reaching a peak more than 100μsec later at 100 torr. Spatially resolved Two Photon Absorption Laser Induced Fluorescence (TALIF) is used to measure absolute number densities of atomic nitrogen and oxygen within the plasma filament. The concentrations of these species remain approximately constant at long time scales, indicating that neither N atom recombination or N2(A 3 Σ) quenching into the lower vibrational levels of molecular nitrogen can account for the measured rise in vibrational quanta. Master equation modeling was applied, confirming that an electronic-vibrational coupling is needed to explain the data. The evidence presented here is consistent with the hypothesis that N2(A 3 Σ) is quenched into highly vibrationally excited levels of the ground state, after which vibrational population is re-distributed downward due V-V transfer.