Aaron Montello

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.

Vibrational and rotational CARS measurements of nitrogen in afterglow of streamer discharge in atmospheric pressure fuel/air mixtures

The use of nonequilibrium plasma generated by nanosecond discharges to ignite fuel/air mixtures, known as transient plasma ignition (TPI), has been shown to effectively reduce ignition delay and improve engine performance relative to spark ignition for combustion engines. While this method is potentially useful for many engine applications, at present the underlying physics are poorly understood. This work uses coherent anti-Stokes Raman spectroscopy (CARS) to measure the rotational and vibrational excitation of nitrogen molecules in the discharge afterglow in a variety of fuel/air mixtures outside the limits of combustion in order to elucidate the thermal behaviour of TPI. The time evolution of relative populations of vibrationally excited states of nitrogen in the electronic ground state are reported for each gas mixture; it is shown that generation of these vibrationally excited states is inefficient during the discharge in air but that generation occurs at a high rate roughly 5??s following the discharge; with the addition of fuels vibrationally excited states are observed during the discharge but an increase in population is still seen at 5??s. Possible mechanisms for this behaviour are discussed. In addition, rotational temperature increases of at least 500?K are reported for all gas mixtures. The effect of this temperature increase on ignition, reaction rates, and thermal energy pathways are discussed.

Picosecond USED-CARS for Simultaneous Rotational/Translational and Vibrational Temperature Measurement of Nitrogen in a Nonequilibrium Mach 5 Flow

Picosecond Unstable-resonator Spatially Enhanced Detection Coherent Anti-Stokes Raman Scattering (USED-CARS) is used for measurement of nitrogen Q-branch (ΔJ = 0) spectra in the subsonic plenum and supersonic flow of a highly nonequilibrium Mach 5 wind tunnel. Spectra are processed to infer simultaneous rotational / translational (Trot) and 1 st level vibrational (Tvib) temperatures in the 200 – 370 Torr plenum. Operation of the nominally high reduced electric field (E/npeak ~ 500 Td), nsec pulsed discharge alone results in fairly significant vibrational loading, Tvib ~ 720 K / Trot ~ 380 K; addition of an orthogonal low E/n (~10 Td) DC sustainer discharge produces substantial vibrational loading, Tvib ~ 2000 K / Trot ~ 450 K. Effects of injection of CO2, NO, and H2 downstream of the pulser-sustainer discharge are examined, which result in vibrational relaxation accompanied by simultaneous gas heating, Tvib ~ 800-1000 K / Trot ~ 600 K. CARS measurements within very low density flows in the Mach 5 expansion nozzle are also performed, with Tvib measured in both the supersonic free-stream and downstream of a bow shock created by a 5 mm diameter cylindrical test object in the Mach 5 flow. Measurements within 300 μm of the cylinder leading edge show that for pure N2, or N2 with 0.25 Torr CO2 injection, no vibrational relaxation is observed behind the bow shock.

Picosecond CARS Measurements of Vibrational Distribution Functions in a Nonequilibrium Mach 5 Flow

The design and implementation of a new picosecond Coherent Anti-Stokes Raman Scattering (CARS) spectroscopy instrument for measurement of nitrogen Vibrational Distribution Function (VDF) in a highly nonequilibrium Mach 5 flow is described. First level vibrational temperatures of the order of 2000 K are achieved in the 300 Torr nonself-sustained plasma wind tunnel plenum, generated by a high E/n (300 Td) nanosecond pulsed discharge, which provides ionization, in combination with an orthogonal low E/n (~10-30 Td) DC sustainer discharge, which efficiently loads the nitrogen vibrational mode. It is also shown that operation with the nanosecond pulsed plasma alone results in significant vibrational energy loading, with Tv of the order of 1100 K. Downstream injection of CO2, NO, and H2 results in vibrational relaxation, demonstrating the ability to further tailor the vibrational energy content of the flow.

Energy Coupling and Heat Release in Air and Ethylene-Air Nanosecond Pulse Discharge Plasmas

A new analytic model of energy coupling to repetitive nanosecond pulsed discharge plasmas has been developed and shown to agree well with previous estimates from single pulse nanosecond discharges in air. The new, quasi-one dimensional model provides accurate expressions for electric field, electron density, and coupled pulse energy, including effects of dielectric surface charge accumulation, and corresponding sheath development. Primary conclusions which result directly from this new model are: (i) the pulse energy coupled to the plasma during an individual nanosecond discharge pulse is controlled primarily by the capacitance of the dielectric layers and by the breakdown voltage, and (ii) the pulse energy coupled to the plasma during a burst of nanosecond pulses decreases as a function of the pulse number in the burst, rather than remaining constant. This occurs primarily because of plasma temperature rise, and resultant reduction of breakdown voltage, with the result that the coupled pulse energy varies approximately proportionally to the number density. The model has been validated by a series of experimental temperature measurements in air and ethylene-air discharges, where the nanosecond discharge was operated in repetitive burst mode. Experimentally determined temperature, by pure rotational Coherent Anti-Stokes Raman Spectroscopy (CARS) and N2 emission spectroscopy, as a function of number of pulses in a burst, was found to agree well with predictions of the model. It is found that heating rate in fuel-air plasmas is much faster compared to air plasmas, primarily due to energy release in exothermic reactions of fuel with O atoms generated by the plasma. The initial heating rate in fuel-air plasmas is controlled by the rate of radical (primarily O atoms) generation and is nearly independent of the equivalence ratio. At long burst durations, heating rate in lean fuel air-mixtures is significantly reduced when all fuel is oxidized.

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.