Kraig Frederickson

Mitigation of electron attachment to oxygen in high pressure air plasmas by vibrational excitation

A series of time resolved microwave attenuation measurements are performed of the electron number density of an electron beam generated, CO laser excited nonequilibrium O2∕N2 plasma. Resonant absorption of infrared radiation from the CO laser produces the nonequilibrium state, in which the heavy species vibrational modes are disproportionately excited, compared to the rotational and translational modes (Tvib≈2000–3000K vs TR∕T≈300K). It is shown that this results in an increase in the plasma free electron lifetime by two orders of magnitude compared to the unexcited cold gas, an effect which is ascribed to complete mitigation of rapid three-body electron attachment to molecular oxygen. A series of heavy species filtered pure rotational Raman scattering measurements are also presented, which exhibit minimal temperature change (+50K), indicating that the observed lifetime increase cannot be due to heavy-species thermal effects. Finally, computational modeling results infer an increase in the rate of O2− det...

Spatially and Temporally Resolved Atomic Oxygen Measurements in Short Pulse Discharges by Two Photon Laser Induced Fluorescence

Two Photon Laser Induced Fluorescence (TALIF) is used to measure absolute oxygen atom concentrations as a function of time in O2/He, O2/N2, and methane/air plasmas produced with a 20 nanosecond duration, 20 kV pulsed discharge. While the pulser is capable of repetition rates as high as 50 kHz, the data reported here was purposefully obtained at much lower repetition rate, 10 Hz, in order to limit the number of pulses experienced by the flowing, room temperature gas sample during its resonance time within the plasma to one, or at most two. While not directly measured, the low repetition rate insures negligible temperature rise due to direct plasma heating. Relative atomic oxygen concentration data are put on an absolute scale by means of a calibration procedure in which the observed signal level is compared to that of reference TALIF spectra of atomic xenon, obtained under known conditions of pressure and temperature, and identical optical and spectroscopic conditions. Calibrated TALIF spectra show that a single discharge pulse creates initial atomic oxygen concentrations in the range (2.0 – 3.0) x 10 14 cm -3 for air, 10% O2 in helium and 10% O2 in nitrogen mixtures. Peak atomic oxygen concentration is a factor of approximately two lower in fuel lean (φ=0.5) methane/air mixtures. In pure helium buffer, the initially formed atomic oxygen decays monotonically, with decay time consistent with formation of ozone and oxygen. In all nitrogen containing mixtures, atomic oxygen concentrations are found to initially increase, for time scales on the order of 10-100 microseconds, due presumably to additional O2 dissociation caused by collisions with electronically excited nitrogen. The long time scale decay in O2/N2 mixtures occurs on a time scale, which is similar to that of O2/He, whereas in the methane/air mixture, the decay rate is greater by a factor of approximately five.

MITIGATION OF OXYGEN ATTACHMENT IN HIGH PRESSURE AIR PLASMAS BY VIBRATIONAL EXCITATION

We present a series of measurements of the temporal evolution of electron density in high pressure, room temperature pulsed e-beam generated molecular plasmas. We show that vibrational excitation, to Tvib of order 2,000 – 3000 K, results in nearly complete mitigation of direct O2 attachment, which is the principal free electron loss process under equilibrium conditions. Spatially and temporally resolved temperature measurements, performed using spectrally filtered pure rotational Raman scattering, indicate heavy species rotational/translational temperature is only slightly increased as a result of vibrational excitation, to approximately 350 K. Kinetic modeling of two limiting cases, accelerated detachment and inhibited attachment, suggests that two non-equilibrium mechanisms may be playing a role simultaneously: i), detachment enhancement by collision of O2 - (or other negative) ions with vibrationally excited neutrals, and ii), attachment inhibition due to electron heating by superelastic collisions with vibrationally excited neutrals.

Dual-Pump CARS Measurements in a Vibrationally Nonequilibrium Supersonic Mixing Layer

The paper presents effect of vibrational relaxation between a vibrationally excited supersonic flow and jet stream of a rapid vibration-translation (V T ) relaxer. For the discharge excited supersonic flow of nitrogen up to TV =2000 K, mixing of CO2 downstream of the expansion corner results in the decrease of expansion angle by up to ∆θ = 8◦, as a consequence of heat release during the vibrational relaxation and the resultant pressure rise by up to 55%. Single / dual pump Coherent Anti-Stokes Raman Scattering measurements demonstrated the decrease of the vibrational temperature across the mixing layer, which is accompanied with the increase of local CO2 mixture ratio. A two-dimensional vibrational temperature distribution coincides with the spatial profile of the modified shear layer measured in the schlieren visualization. The region of the vibrational relaxation is also consistent with the layer of the V T relaxer penetration into the supersonic flow downstream of the trailing shocks, with the subsequent flow residence on the corresponding time scale of the V T relaxation.

Progress in Development of a Chemical CO Laser Driven by a Chemical Reaction between Carbon Vapor and Oxygen

A chemical flow reactor is used to study the vibrational population distribution of CO produced by a reaction between carbon vapor generated in an arc discharge and molecular oxygen. The results demonstrate formation of highly vibrationally excited CO, up to vibrational level v=14, at low temperatures, T=400-450 K, with population inversions at v=4-7, in a collision-dominated environment, 15-20 Torr. The average vibrational energy per CO molecule formed by the reaction is 0.6-1.2 eV/molecule, which corresponds to 10-20% of the reaction enthalpy. The results show feasibility of development of a new CO chemical laser using carbon vapor and oxygen as reactants. A supersonic flow CO laser excited by a transverse RF discharge in the plenum is used to determine the effect of adding air species to the laser mixture. Carbon monoxide infrared emission spectra are used to measure CO vibrational level populations and temperature in subsonic CO-He, CO-He-N2, CO-He-O2, and CO-He-air flows excited by the discharge. Laser power and spectra generated in the transverse resonator in the M=3 supersonic flow are measured for each mixture. Nitrogen addition to the baseline CO-He mixture increases energy stored in the CO vibrational mode, resulting in a significant increase in laser power. Addition of oxygen had the opposite effect, reducing both CO vibrational populations and laser power. Adding air resulted in a modest increase of CO vibrational distribution, as well as an increase in laser power, although not as significant as when nitrogen was added to the flow. The results demonstrate feasibility of operating a supersonic flow CO laser in mixtures with significant amounts of air. 1 Graduate Research Assistant 2 Research Scientist 3 Professor Emeritus, Fellow AIAA 4 Professor, Fellow AIAA D ow nl oa de d by I go r A da m ov ic h on J an ua ry 1 4, 2 01 7 | h ttp :// ar c. ai aa .o rg | D O I: 1 0. 25 14 /6 .2 01 719 67 55th AIAA Aerospace Sciences Meeting 9 13 January 2017, Grapevine, Texas AIAA 2017-1967 Copyright

Development of a Chemical Carbon Monoxide Laser

The initial development of a novel chemical carbon monoxide laser driven by the exothermicity of the chemical production of carbon monoxide via the reaction between gasphase atomic carbon and molecular oxygen is presented. A flowing chemical reactor has been constructed for the investigation of this chemical reaction, where sublimation of amorphous carbon is achieved within an electrically-driven arc, and injection of rf-discharge-activated oxygen results in the formation of carbon monoxide. Detection and quantification of the chemical product is performed via ex situ absorption spectroscopy. The production rate of carbon monoxide is determined to be ~4.3e 18 molecules/sec.

Femtosecond TALIF Imaging of Atomic Hydrogen in Pulsed, Non-Equilibrium Plasmas

In recent decades significant interest has been paid to investigate the applications of non-equilibrium plasma discharges to a variety of practical combustion related applications. However in order to develop a more fundamental understanding of such phenomena, additional insight into important kinetic processes is of great importance. A two-photon absorption laser-induced fluorescence (TALIF) technique is developed utilizing wide-bandwidth, short-time duration, femtosecond (fs) laser pulses. Due to its increased bandwidth and short pulse duration, fs-TALIF has reduced impact from photo-dissociation and increased signal compared to traditional nanosecond TALIF schemes. This fs-TALIF technique is used to image key atomic species within nanosecond pulsed, non-equilibrium discharges at moderate pressures. These two dimensional results provides both spatial and temporal information that can be used in more predictive plasma kinetics models. 1 Research Engineer, Spectral Energies, LLC., 5100 Springfield St, Suite 301, Dayton, OH 45431, AIAA Member 2 Senior Research Scientist, Spectral Energies, LLC., 5100 Springfield St, Suite 301, Dayton, OH 45431, AIAA Associate Fellow 3 Senior Research Scientist & CEO, Spectral Energies, LLC., 5100 Springfield St, Suite 301, Dayton, OH 45431, AIAA Associate Fellow 4 Research Associate, The Ohio State University, Columbus, OH 43210, AIAA Member 5 Professor, The Ohio State University, Columbus, OH 43210, AIAA Associate Fellow 6 Principal Research Chemist, Air Force Research Laboratory, WPAFB, OH 45433, AIAA Associate Fellow

Scaling Up Generation of Vibrationally Excited CO in a Chemical Reaction between Carbon Vapor and Oxygen

Carbon products generated in a DC arc discharge with graphite electrodes sustained in argon buffer are characterized using time-resolved mass spectra measured by a Residual Gas Analyzer (RGA). The results show that atomic carbon vapor is one of the dominant vapor-phase components produced in the discharge. The yield of C atoms is much higher compared to that of C2 molecules. C3 signal exhibits essentially no dependence on the arc current and may be due to carbon particulates accumulated on the walls of the discharge cell from the previous runs. The yield of heavier carbon species (up to C17) is insignificant. However, the atomic carbon vapor yield is approximately two orders of magnitude lower than the net rate of carbon electrode consumption, indicating that carbon may be ablated from the electrodes as solid particulates. Carbon atoms produced in the arc discharge are used to generate highly vibrationally excited CO by a chemical reaction with molecular oxygen, added to the flow downstream of the discharge cell. Based on previous CO laser experiments, it is estimated that the rate of C atom production in the arc discharge needs to be scaled up by approximately three orders of magnitude, to achieve optical gain sufficient for laser power generation. To increase the high-temperature plasma volume up to several cm3, inductively coupled RF discharge is used. Fourier Transform Infrared (FTIR) emission spectroscopy was used to measure temperature in the Inductively Coupled Plasma (ICP) sustained in argon flow seeded with 2.5% CO. Temperatures up to T=2600 K were observed at a discharge power of 1 kW in the ICP. Micron-size carbon particles added to the argon flow using a custom-designed particle seeder are vaporized in the ICP discharge cell, and the products are injected into the main argon flow. Oxygen is injected into the main flow downstream of the ICP discharge cell. Vibrationally excited CO is formed by a chemical reaction between carbon vapor and oxygen in the main flow, at a relatively low temperature of T=400 K. CO vibrational levels up to v=6 are detected from FTIR emission spectra. Further experiments quantifying the yield of vibrationally excited CO are underway. 1 Graduate Research Assistant 2 Research Scientist 3 Undergraduate Research Assistant 4 Professor Emeritus, Fellow AIAA 5 Professor, Associate Fellow AIAA 1 D ow nl oa de d by I go r A da m ov ic h on J an ua ry 1 1, 2 01 8 | h ttp :// ar c. ai aa .o rg | D O I: 1 0. 25 14 /6 .2 01 801 79 2018 AIAA Aerospace Sciences Meeting 8–12 January 2018, Kissimmee, Florida 10.2514/6.2018-0179 Copyright

OH Radical Measurements in Hydrogen-Air Mixtures at the Conditions of Strong Vibrational Nonequilibrium

This work presents time-resolved, absolute measurements of OH number density, nitrogen vibrational temperature, and translational-rotational temperature in nitrogen, air, and lean hydrogen-air mixtures excited in a diffuse filament nanosecond pulse discharge, at a pressure of 100 Torr and high specific energy loading. The main objective of these measurements is to study a possible effect of nitrogen vibrational excitation on low-temperature kinetics of HO2 and OH radicals. N2 vibrational temperature and gas temperature in the discharge and the afterglow are measured by ns broadband Coherent Anti-Stokes Scattering (CARS). Hydroxyl radical number density is measured by Laser Induced Fluorescence (LIF) calibrated by Rayleigh scattering. The results show that the discharge generates strong vibrational nonequilibrium in nitrogen and air for delay times after the discharge pulse of up to ~ 1 ms, with peak vibrational temperature of Tv ≈ 2700 K at T ≈ 550 K (in nitrogen) and Tv ≈ 1900 K at T ≈ 650 K (in air). Nitrogen vibrational temperature in air peaks ≈ 200 μs after the discharge pulse, before decreasing due to vibrational-translational relaxation by O atoms (on the time scale of a few hundred μs) and diffusion (on ms time scale). OH number density increases gradually after the discharge pulse, peaking at t ~ 100-300 μs and decaying on a longer time scale, until t ~ 1 ms. Both OH rise time and decay time decrease as H2 fraction in the mixture is increased from 1% to 5%. OH number density in a 1% H2-air mixture peaks at approximately the same time as vibrational temperature in air. Since vibrational temperature and gas temperature in air and in a 1% H2-air mixture are expected to be close, this suggests that OH kinetics may be affected by N2 vibrational excitation. Additional CARS measurements in H2-air mixtures are necessary to verify this conjecture and obtain further insight. 1 Graduate Research Assistant 2 Research Scientist 3 Professor, Associate Fellow AIAA D ow nl oa de d by I go r A da m ov ic h on J an ua ry 1 4, 2 01 7 | h ttp :// ar c. ai aa .o rg | D O I: 1 0. 25 14 /6 .2 01 715 84 55th AIAA Aerospace Sciences Meeting 9 13 January 2017, Grapevine, Texas AIAA 2017-1584 Copyright

Experimental and Kinetic Modeling Studies of Novel Carbon Monoxide Gas Lasers 1

A chemical flow reactor has been used to study the vibrational population distribution of carbon monoxide produced by a reaction between vapor-phase carbon generated in an arc discharge and oxygen, to determine feasibility of extracting the chemical energy released from this reaction by laser radiation. Additionally, a supersonic flow, electric discharge excited CO laser has been developed and characterized over a range of operating conditions. The same supersonic laser apparatus can be adapted to produce population inversion via oxidation of vapor-phase carbon, generating vibrationally excited CO. Resultant laser power and spectra are compared with the predictions of a kinetic model of a supersonic flow CO laser.