Rotational Temperature measurements by pure

51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 07 - 10 January 2013, Grapevine (Dallas/Ft. Worth Region), Texas

AIAA 2013-0431

Rotational Temperature measurements by O2 CARS in a repetitively pulsed low temperature, low pressure nonequilibrium plasma S. Lanier1, S. Bowman1, I.V. Adamovich1, and W.R. Lempert2 Michael A. Chaszeyka Nonequilibrium Thermodynamic Laboratories Department of Mechanical and Aerospace Engineering The Ohio State University, Columbus, OH 43210

Downloaded by Igor Adamovich on January 9, 2013 | http://arc.aiaa.org | DOI: 10.2514/6.2013-431

Picosecond pure rotational CARS thermometry is applied to the study of plasma assisted chemical oxidation in O2/Ar/H2 mixtures, excited with a burst of ~70 nsec duration pulses at 30 kHz repetition rate, at a total pressure of 40 Torr. Experimental precision of approximately +/- 14 K, defined as the 95% confidence interval, is demonstrated, although the inferred temperatures showed a systematic error of ~- 25 K. Results in 20 percent O2/Ar mixtures, obtained as function of number of pulses in the 40 kHz burst, show an increase in temperature which agrees well with predictions of a plasma chemical oxidation model, indicating that energy coupling to the plasma, and effective E/N, are well predicted. Temperatures inferred upon addition of H2 fuel showed no discernible increase in temperature, relative to the corresponding O2/Ar cases, a result which is unexpected and requires further study.

______________________ 1 2

PhD Candidate, Chemistry, W090 Scott Laboratory, Student Member, AIAA . Professor, Mech. & Aero. Engineering, E438 Scott Laboratory, Associate Fellow, AIAA 1 American Institute of Aeronautics and Astronautics

Copyright © 2013 by Suzanne Lanier. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.


I. Introduction Plasma assisted combustion has recently attracted intense interest due to the potential to enhance and control combustion processes. For example, Stariskovii [1] demonstrated a significant decrease in ignition temperature, by ~600K, and simultaneously observed an increase by a factor two in the flame’s blow-off velocity using high voltage nanosecond discharge in pressures as low as 0.3 atm. Similarly both Adamovich et. al [2], and Starikovskaya et. al. [3] have determined that ignition delay time can be reduced by at least an order of magnitude by utilizing similar discharges under similar conditions. In addition, Marcum et al [4] demonstrated an increase in flame speed by a factor of two or more in comparable electric discharges, and Hemawan et al [5] reported an extension of lean flammability limits ~20% or more in low energy microwave plasmas. Stariskovii [1], and others, have noted that low-pressure, low temperature diffuse nonequilibrium plasmas generate large initial radical pools and super-equilibrium populations of metastable electronic states such as N2(A3∑) and O2(a1∆). Such species can participate in reactions that either lead to ignition or generate species in fast reactions leading to ignition. These processes are typically highly temperature dependent and as such experimental determination of temperature constitutes a critical measurement for elucidating fundamental phenomena and for validation of plasma chemical kinetic mechanisms. This paper reports our initial progress towards the development of molecular oxygen-based picosecond pure Rotational Coherent Anti-Stokes Raman Spectroscopy (RCARS) for application to low (~10 Torr) non-equilibrium plasma thermometry. The work presented here is specifically motivated by a desire to perform fundamental plasma chemical fuel oxidation measurements in which argon is used in place of the more standard nitrogen as a diluent gas. The choice of argon is deliberate, primarily for three reasons. First, and most fundamentally, both ground and excited state N2 can participate in a variety of reactions producing nitrogen containing species such as NO or NO2, thus removing N2 from the system simplifies the kinetics and allows the focus to be on the primary chemical oxidation processes. Second, argon has a thermal diffusivity which is similar to that of nitrogen, and as such the heat transfer properties of the mixtures being studied more accurately mimic those of air. Finally, due to the rather high energy of argon’s ground electronic state, ~11.2-eV, a larger fraction of discharge energy will be deposited into oxygen, producing the desired dissociation and excited electronic states in argon plasmas relative to nitrogen. Coherent Anti-Stokes Raman Spectroscopy (CARS) is a well-established diagnostic method that has been used extensively in combustion environments [6]. Its application as a plasma diagnostic has also been reported by several groups. As one example, Devyatov et al. [7], reported CARS measurements of vibrational levels v=0-4 in a 200 nsec duration pulsed discharge in nitrogen at 60 torr. The results suggested that collisional processes involving excited electronic states of N2 result in increased vibrational level populations in the ground electronic state for time scales of several hundreds of microseconds after decay of the plasma itself. Recent picosecond vibrational CARS measurements of Montello et al. [8] have confirmed this result. Montello et al. has also reported measurements of rotational/translational temperature and vibrational distribution function in a Mach 5 non-equilibrium wind tunnel [9]. As a final example, Messina et al. [10] used CARS to demonstrate significant vibrational loading in an atmospheric pressure, nanosecond pulse, point-to-point discharge. Similar to the work of Devyatov and Montello, Messina reported an increase in N2 vibrational temperature in air plasmas at time delays exceeding approximately 1 μsec after discharge initiation, reaching a maximum at ~50 μsec delay. This work focuses on picosecond pure rotational CARS where O 2 is the target species rather than the more common N2. The selection of pure rotational CARS was based on two primary characteristics of the plasmas under study. First, our anticipated temperatures were quite low, of order 300 – 600 K. Pure rotational CARS has the advantage of readily providing well resolved rotational structure, resulting, at least potentially, to high accuracy and precision low temperatures [11]. Second, our studies are performed at relatively low pressure, 40 Torr for the work to be presented here, with the partial pressure of O2 even lower (8 Torr or less). It was anticipated, therefore, that it would be crucial to utilize sufficiently high laser beam intensities where Stark broadening could lead to significant systematic error if the more common vibrational Q-branch CARS were to be used [12]. However, due to the greatly increased spacing being adjacent rotational transitions in pure rotational spectra, it was anticipated that the effect of Stark broadening on temperature inference would be negligibly small. 2 American Institute of Aeronautics and Astronautics

This paper presents our progress to date on the application of picosecond pure rotational CARS to studies of plasma chemical oxidation at low pressure. It is demonstrated that experimental precision of approximately +/- 10 K can be readily achieved in O2/Ar/H2 mixtures with partial pressure of O2 as low as ~6 Torr. Experimentally determined temperature rise upon addition of bursts of up to 450 nanosecond duration pulses in O2/argon mixtures is found to agree well with plasmas-chemical modeling predictions, although inferred temperature rise upon addition of hydrogen fuel is lower than predicted. There exist some systematic deviations in the observed CARS spectra, in comparison with spectral modeling, which is a result still under investigation.

II. Experimental


A. Plasma The experiments conducted in this study were performed in the plane-to-plane dielectric barrier discharge cell, identical to that of our previous pure rotational CARS studies [13], shown schematically in Fig. 1. The rectangular quartz channel has dimensions 220 mm x 22 mm x 10 mm with 1.75 mm thick walls. Two copper plate electrodes, 14 mm x 65 mm, with rounded edges for field uniformity are attached to the outside of the walls. The discharge was created using an in-house high-voltage nanosecond pulse generator described in more detail in [14]. The pulse generator uses a magnetic pulse compression method to produce pulses with voltage of 10-30kV, pulse duration of 50-100 ns, and pulse repetition rate of up to 50 kHz, with alternating pulse polarity. In the experiments presented here, the pulser is operated in a repetitive “burst” mode at 10-Hz where individual pulses within the burst have a peak applied voltage of ~20kV, pulsed duration (FWHM) of ~70 ns, and pulse repetition rate of 40 kHz. Figure 6 (Section III) shows typical pulse voltage and current waveforms, which will be discussed in more detail below. CARS data is obtained 35 microseconds after the end of a burst that is between 15 and 451 pulses in mixtures consisting of 20% O2/Ar as well as in mixtures of 20% O2/Ar with added H2 at equivalence ratios of ϕ=1.0, 0.5, and 0.1. Flow rates of each individual component were controlled by mass flow controllers at a total pressure of 40 torr and a flow velocity of ~0.5 m/s, which is sufficiently fast to ensure that the flowing gas experiences only a single burst, and sufficiently slow that the probed gas experiences the full set of pulses within the burst. Discharge uniformity is confirmed by ICCD imaging, which is described in section III.

Figure 1: Test cell schematic. Copper electrodes attached to the outside top and bottom of the rectangular quartz channel. B. CARS CARS is a non-linear optical diagnostic involving in which three incident photons mix to produce a fourth photon via the induced third order polarization [15]. As illustrated in the energy level diagram of figure 2 the incident pump/Stokes beam pair creates a coherent polarization in the medium which oscillates at the difference frequency. Raman scattering of the probe beam off of this coherent polarization (sometime termed a “grating”) 3 American Institute of Aeronautics and Astronautics


creates a coherent fourth beam which is anti-Stokes shifted in frequency such that ωcars=ωpump – ωstokes + ωprobe. For this work, the pump and Stokes beams both derived from a broad band dye laser, the details of which will be summarized below. The second harmonic of Nd:YAG, at 532 nm, provided both the probe beam and pumped the dye laser. To reiterate, the resulting anti-Stokes beam is slightly upshifted in frequency from the probe.

Figure 2: Energy diagram describing CARS process. The CARS process is subject to what is known as “phase matching”, a requirement arising from the requirement to conserve photon angular momentum. This can be achieved from a variety of geometries, the most common of which is linear phase matching, which is experimentally simple to implement, but results in poor spatial resolution along the beam propagation direction [6]. On the other hand, BOXCARS [6] has the advantage of much higher spatial resolution, typical of order hundreds of microns or less. In this work, a planar boxcar geometry was employed [16], as illustrated in figure 6. To further aid in signal discrimination and noise reduction from probe beam scatter which has a nearly identical frequency to the anti-Stokes beam, the pump and Stokes are orthogonally, but linearly, polarized relative to one another. This method was first developed by Vestin et al [16], and later utilized by Zuzeek et al [13]. The result is an anti-Stokes beam that has linear polarization orthogonal to the probe beam and results in a factor of greater than 1000 reduction of the stray light from the pump beam when a polarizer is inserted in the CARS beam path (See Fig. 4) . As described by Vestin et al [16], this polarization scheme results in a reduction of the CARS resonant and non-resonant signals by a factor of 9/16 and 1/9, respectively. The pump and Stokes beams are generated with the custom modeless dye laser, shown in figure 3, patterned after that of Roy et al, [17], and used in our previous work [9]. The dye utilized is pyrromethene 597 in ethanol at a concentration of 0.157g/L for the oscillator and pre-amplifier and 0.016g/L for the final amplifier. The conversion efficiency is ~13% and the bandwidth of the output is ~300 cm-1 centered at ~576 nm.

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Figure 3: Modeless Dye Laser schematic. Note the reflecting prism is on a translation stage to serve as a delay line for the CARS probe beam. Figure 4 shows a schematic of the complete CARS diagnostic system. Briefly, the pump/Stokes and 532 nm probe beam, with energy of ~6 mJ/pulse, are arranged in the linear BOXCARS geometry on a 500 mm focal length plano-convex lens. After re-collimation the CARS beam is brought incident to a 0.75 m Andor Shamrock spectrometer with 1800 line/mm grating. A back illuminated – electron multiplying CCD (Andor Newton) is used for detection. As will be discussed in section IV, it was found that the electron multiplying feature of this CCD camera results in significant improvement in the sensitivity of the CARS system. Timing and synchronization was performed with a pair of digital delay generators such that spectra were obtained 35 microseconds after termination of the final pulse in the burst sequence.

Figure 4: RCARS schematic 5 American Institute of Aeronautics and Astronautics


III.

Plasma Chemistry and Nanosecond Discharge Models

To obtain insight into kinetics of plasma/chemical fuel oxidation, we use a kinetic model developed in our previous work [18-22]. Briefly, the model incorporates nonequilibrium air plasma chemistry [23], expanded to include hydrocarbons and hydrogen dissociation processes in the plasma [19,24,25], as well as electron impact excitation of argon and metastable argon quenching reactions [A1,A2]. The dominant radical species generation processes in H2/CxHy/N2/O2/Ar nonequilibrium plasmas are listed in Table 1. The plasma chemistry model is coupled with “conventional” hydrogen-oxygen chemistry mechanism developed by Popov [24]. Note that this mechanism has been developed and validated for relatively high-temperatures, significantly higher than found in the present experiments, and may well be inaccurate at the present conditions. Assessing applicability of these mechanisms, used as a starting point for development of a nonequilibrium plasma assisted combustion mechanism, is one of the objectives of the present work. The list of air plasma chemistry processes and hydrogen-oxygen chemical reactions incorporated in the present model is given in Ref. [19] and Refs. [24,25], respectively. The species concentration equations are coupled with the two-term expansion Boltzmann equation for plasma electrons. The model incorporates the energy equation for the temperature on the discharge centerline [22] as well as quasi-one-dimensional flow equations, with heat transfer to the walls (dominant energy loss mechanism at the present conditions) included. The model is validated using previous measurements of O-atom concentration (TALIF) [18,19], temperature (rotational CARS) [13,26], OH concentration (LIF) and ignition time [20-22] in nsec discharges in air, and mixture of air and CH4, C2H4, and H2. Key parameters controlling plasma chemistry in the nsec pulse discharge include the reduced electric field, E/N, and coupled pulse energy. Energy coupled to the plasma using two different nsec pulse generators, including the FID pulser used in the present work, was measured in air in a low-temperature cell outside of the furnace, over a wide range of pressures [27]. It has been shown that at P=10-100 torr, coupled pulse energy is nearly independent on pulse repetition rate (at ν=1-40 kHz), remains nearly constant during the pulse burst (up to ~100 pulses), and is proportional to discharge pressure, i.e. energy coupled per molecule remains constant. These experimental results are in good agreement with the analytic model of energy coupling in a nsec pulse discharge [28], which incorporates key effects of pulsed breakdown and sheath development on the nsec time scale. The model predicts pulse energy coupled to the plasma vs. pulse voltage waveform, discharge geometry, pressure, and temperature. In the present work it was determined that coupled pulse energy could not be measured accurately since it represents a small difference between the energy forwarded to and reflected from the load, and as such is very sensitive to stray phase shift between the current and voltage. For this reason, predictions for coupled pulse energy, used in the kinetic modeling, were taken from predictions of the pulsed discharge model [28] rather than experimental i-V traces. As described in more detail in the next section, the applied electric field was approximated as a Gaussian pulse with peak amplitude and rise time equal to that of the experimental voltage waveform. From this the discharge model was used to predict the total coupled pulse energy and an effective E/N value for which this energy was coupled. This resulted in an effective E/N value of 60 Td and coupled pulse energy of 0.12 meV/molecule (0.2 mJ/pulse) for Ar/O2/fuel mixtures at 40 Torr.

IV.

Results and Discussion

A. Plasma Characterization Figure 5 shows a representative sample of ICCD images obtained in a 20% O2/Ar mixture at 40 Torr total pressure. Note that individual images shown were obtained from different bursts and are not representative of discharge development within a single burst. Note also that these images are faint relative to N2 plasmas which exhibit intense emission from the N2 first and second positive band. Nevertheless, other than some weak structure observed in the first pulse it can be seen that the plasma remains homogeneous and diffuse throughout the course of the burst sequence, a necessary condition for comparison of experimental data to the 1-D model described in the previous section. 6 American Institute of Aeronautics and Astronautics


Figure 5: Plasma emission captured with a gated ICCD camera at a variety of pulsed within a 40 kHz burst. 20% O2/Ar, P = 40 Torr, 2 microsecond camera gate. Current and voltage waveforms were also obtained, an example of which, for the 20% O 2 in argon mixture, is shown in Fig. 6. While in principle, coupled pulse energy can be determined from integration of the I(t)V(t) product, in practice, as discussed in section III, the result for these data were very dependent upon phase shift between the current and voltage traces which is difficult to accurately determine. For this reason the nanosecond discharge model [28] was used to obtain a prediction for coupled pulse energy and effective E/N.

Figure 6: Current and voltage waveforms. 20% O 2/Ar, P = 40 Torr, pulse 100 in a 40 kHz burst. Figure 7 summarizes the results for the 20% O2 in argon mixture at 40 Torr. The left side of Fig. 7 shows both the experimental and theoretical applied voltage waveforms and electric field within the plasma. As would be expected, significant shielding is predicted subsequent to discharge breakdown. The right side of Fig. 7 shows the predicted temporal evolution of the reduced electric field within the plasma, the coupled plus stored (in the dielectric) pulse energy, and the coupled pulse energy. It can be seen that the final coupled energy (~0.2 mJ) is a small fraction of the peak coupled plus stored energy (1.3 mJ), indicating that the majority of the energy which initially charges the capacitor formed by the electrodes and dielectric is ultimately reflected back to the pulser. Furthermore, the majority of the energy coupling occurs post breakdown, when the reduced electric field is relatively low (~60 Td). As indicated in the figure, the coupled pulse energy of 0.12 meV per molecule is used for the kinetic modeling which is described below. Again, the value used by the model of effective reduced electric field for this coupled energy is 60 Td. The same values were also used for O2/Ar/H2 mixtures.



Figure 7: Nanosecond pulse discharge model predictions for the temporal evolution of applied and plasma fields (left), and E/N, coupled + stored, and coupled pulse energy (right). Mixture is 20% O2/argon at P = 40 Torr. Net coupled pulse energy is 0.20 mJ corresponding to 0.12 meV/molecule. B. CARS Results 1. Measurement Precision and Accuracy Figure 8 shows a typical experimental CARS spectrum obtained in an Ar-O2-H2 mixture at an equivalence ratio of 0.5, and burst size of 31. The total pressure is 40 Torr, and the spectrum has been averaged over 200 laser shots. Also shown is the best fit theoretical spectrum, obtained from CARSFIT. While the agreement is reasonably good, there are obvious systematic differences. First, despite attempts to perform background subtraction, by obtaining a spectrum under identical conditions with the pump beam blocked, it is clear that some residual probe beam scatter is contaminating the spectrum. It should be noted that the combination of scattering from the test cell windows and the lost partial pressure of oxygen (~6 Torr for this case) greatly exacerbates this issue. Future work will aim to improve this by including additional spatial filtering of the CARS signal. In addition, it can be seen that the J = 7 transition is slightly overestimated and the J = 9, 13, 17, and 23 are underestimated, particularly J = 13. It was found that this general trend, in particular the significantly increased intensity of the J = 13 transition, was quite reproducible and appeared in all of the data, including that taken in O 2/Ar at room temperature (no pulser operation) and a variety of pressures. While not shown, a similar systematic error in the intensity of the rotational envelop was also observed in Stokes spectra. Many potential sources of this systematic error were examined, including dye laser spectra profile, or more specifically the intensity envelope of the non-resonant background signal, imaging/focusing issues onto the CCD detector, and, as discussed by Roy et al [29], potential J-dependent dephasing times which can lead to non-Boltzmann intensity distribution in psec CARS spectra. To eliminate this as a possibility, the temporal arrival of the pump, Stokes, and probe beams was examined closely and arranged, via translation stages (See Fig. 4) to be as close to coincident as possible. In addition, while not studied in as much detail, initial pure N 2 spectra did not appear to be distorted in this manner. The effect of this systematic error, as will be shown in more detail below, is a systematic offset in temperature by ~-25 K. Future work will focus on delineating the origin of this systematic uncertainty.



Figure 8: Experimental CARS spectrum for 20% O2 /Ar /H2 mixture. φ = 0.5, P = 40 Torr. As stated in section II, the electron multiplying feature of the CCD camera was found to significantly improve the S/N of the raw spectra. To illustrate this, figure 9 shows a collage of four spectra obtained in 20% O2/Ar at 44 Torr (8.8 Torr partial pressure of O2) under ambient room conditions (No discharge). In each case the spectra were averaged for 10 laser shots (1 sec). The EM gain settings, from top to bottom, are 0 (disabled), 50, 100, and 200, where the maximum setting is 255. . The improvement in signal to noise ratio is clear, increasing from essentially zero, with gain disabled, to ~15 with gain set at 200. An assessment of the experimental precision has also been performed. Specifically, between five and nine spectra were obtained for each of the following seven conditions. All spectra were averaged for 200 laser shots. (a) Five spectra of 20% O2 in Argon at ~40 torr with no pulser. This comprises the “room temperature” set (b) Nine spectra of 20% O2 in Argon at ~40 torr with a burst size of 51 (c) Nine spectra of 20% O2 in Argon at ~40 torr with a burst size of 201 (d) Nine spectra of 20% O2 in Argon at ~40 torr with a burst size of 351 (e) Nine spectra of 20% O2 in Argon and H2 where ϕ=0.1 at ~40 torr with burst size of 201 (f) Six spectra of 20% O2 in Argon and H2 where ϕ=0.5 at ~40 torr with burst size of 201 (three points were thrown out as CARSFIT could not fit due to large baseline) (g) Nine spectra of 20% O2 in Argon and H2 where ϕ=1.0 at ~40 torr with burst size of 201 The spread of these data points are shown in figure 10 which plots each condition as a “cluster” of data points, as labeled. The left most cluster corresponds to condition (a), which is the “baseline” 20% O 2/Ar mixture. The next three correspond to the 20% O2/Ar mixture in which the number of pulses in the burst is increased from between 51 and 201. A small, but definitely discernible temperature increase, total ~ 60 K, is observed, as well as a clear offset of ~-25 K for the room temperature “baseline” data set. The rightmost three clusters correspond to cases (e) – (g), in which H2 has been added to the mixture and the number of pulses in the burst is held constant, at 201. No discernible increase in temperature is observed between these three cases, or case (c), which is the same number of pulses but without fuel. As will be shown below, this result is at variance with plasma chemical model predictions. The table below shows the standard deviations for each of the seven data sets, which range between 5.0 K and 6.7 K with an average value of 5.5 K. Applying standard statistics of small data sets, assuming a value of 8 (the average) for the number of data points in each set, results in a 95% confidence interval of ~+/- 14K 9 American Institute of Aeronautics and Astronautics

20% O2 in Argon at ~40 torr with no pulser 20% O2 in Argon at ~40 torr with a burst size of 51 20% O2 in Argon at ~40 torr with a burst size of 201 20% O2 in Argon at ~40 torr with a burst size of 351 20% O2 in Argon and H2 where ϕ=0.1 at ~40 torr with burst size of 201 20% O2 in Argon and H2 where ϕ=0.5 at ~40 torr with burst size of 201 20% O2 in Argon and H2 where ϕ=1.0 at ~40 torr with burst size of 201 Average Standard Deviation

σ 5.9 4.9 5.0 5.1 6.7 5.3 5.5 5.5


Table 1: Statistical analysis of CARS temperature precision from seven data sets.



Figure 9: O2 RCARS spectrum at O2 partial pressure of 8.8 torr O2. 10 laser shot average EM gain at 0 (disabled), 50, 100, and 200, from top to bottom. 11 American Institute of Aeronautics and Astronautics


Figure 10: 20% O2 in Ar at ~40torr. Seven data sets, each corresponding to a different mixture and/or burst size as described in the text. Figure 11 summarizes the above data by plotting inferred temperature as a function of number of pulses in the burst for all mixtures. “One” sigma error bars are included.

Figure 11: Summary of data from figure 10, plotted as inferred temperature as a function of number of pulses in the burst. 2. Comparison With Modeling Predictions Comparison of the experimental CARS temperatures with predictions from the plasma chemical kinetic model described in section III are given in figure 12. It can be seen that the predicted temperature rise for the O 2/Ar mixture, ΔTmodel ~ 80 K, agrees quite well with the observed value ΔT CARS ~ 75 K, although as stated previously, there is a clear systematic offset of ~ - 25 K in the experimental data. However, the model clearly predicts that addition of H2 should result in a discernible, and within the precision of the CARS data, temperature rise of ~25 K for burst sizes exceeding ~200 K. It is stressed that this temperature data, particularly for the fuels, is rather preliminary and further measurements, planned for the immediate future, are necessary to explore this further. 12 American Institute of Aeronautics and Astronautics


Figure 8: Comparison of experimental and predicted temperatures for O 2/Ar and O2/Ar /H2 mixtures as a function of number of pulses per burst. P = 40 Torr.

V. Summary and Conclusions Picosecond pure rotational CARS thermometry has been applied to the study of plasma assisted chemical oxidation in O2/Ar/H2 mixtures, excited with a burst of ~70 nsec duration pulses at 30 kHz repetition rate, at a total pressure of 40 Torr. Experimental precision of approximately +/- 14 K, defined as the 95% confidence interval, has been demonstrated, although the inferred temperatures showed a systematic error of ~- 25 K. Results in 20 percent O2/Ar mixtures, obtained as function of number of pulses in the 40 kHz burst, show an increase in temperature, which agrees well with predictions of a plasma chemical oxidation model. This indicates that energy coupling to the plasma, and effective E/N, are well-predicted. Temperatures inferred upon addition of H 2 fuel showed no discernible increase in temperature, relative to the corresponding O 2/Ar cases, a result which is unexpected and requires further study. Acknowledgements The authors wish to acknowledge support from the U.S. Air Force Office of Scientific Research MURI program in Plasma Assisted Combustion, Chiping Li technical monitor, and the National Science Foundation, Steven Gitomer technical monitor. References [1] A. Yu. Starikovskii. Proceedings of the Combustion Institute 30, 2405-2417 (2005) [2] I.V. Adamovich, I Choi, N. Jiang, J. H. Kim, S. Keshav, W. R. Lempert, E. Mintusov, M. Nishihara, M. Samimy, and M. Uddi. Plasma Sources Science and Technology. 18 (2009) 034018 (13pp) [3] S.M. Starikovskaya, N. L. Aleksandrov, I. N. Kosarev, S. V. Kindysheva, and A. Yu. Starikovskii. High Energy Chemistry. 43(3) (2009) 213-218 [4] S.D. Marcum, and B.N. Ganguly. Combustion and Flame. 143 (2005) 27-36 [5] S.D. Hemawan, Indrek S. Wichman, Tong hun Lee, Timothy A. Grotjohn, and Jes Asmussen. Review of Scientific Instruments. 80 (2009) 053507 (9pp)


[6] Alan C. Eckbreth. “BOXCARS: Crossed-beam-phase-matched CARS generation in gases.” Appl. Phys. Lett. (1978) Vol. 32, 421-423 [7] A.A. Devyatov, S.A. Dolenko, A.T. Rakhimov, T.V. Rakhimova, N.N. Roĭ, and N.V Suetin. “Investigation of kinetic processes in molecular nitrogen by the CARS method.” Sov. Phys: JETP (1986) 63(2) [8] A. Montello, Z. Yin, D. Burnette, I.V. Adamovich, and W.R Lempert, “Picosecond CARS Measurements of Nitrogen Vibrational Loading and Rotational/Translational Temperature in Nonequilibrium Discharges,” AIAA2012-3180, 43rd AIAA Plasmadynamics & Lasers Conference, 25-28 June 2012, New Orleans, LA [9] Montello, M. Nishihara, J.W. Rich, I.V. Adamovich, and W.R Lempert,” Picosecond CARS Measurements of Nitrogen Rotational/Translational and Vibrational Temperature in a Nonequilibrium Mach 5 Flow,” Accepted for publication in Experiments in Fluids, Dec, 2012.


[10] D. Messina, B. Attal-Trétout, F. Grisch. “Study of a non-equilibrium pulsed nanosecond discharge at atmospheric pressure using coherent anti-stokes Raman Scattering.” Proceedings of the Combustion Institute. (2007), vol. 33, 825-832 [11] L. Martinsson, P.-E. Bengtsson,and M. Aldén. “Oxygen concentration and temperature measurements in N 2-O2 mixtures using rotational coherent anti-stokes Raman spectroscopy.” App. Phys. B. (1996) Vol. 62, 29-37 [12] Gaetano Magnotti, Andrew D. Cutler, G.C. Herring, Sarah A. Tedder, and Paul M. Danehy. “Saturation and Starks effects in dual-pump CARS of N2, O2, and H2.” Journal of Raman Spectroscopy. (2012) Vol. 43, Issue 5, 611-620 [13] Y. Zuzeek, I. Choi, M. Uddi, I.V. Adamovich, and W.R. Lempert, “Pure Rotational CARS Thermometry Studies of Low Temperature Oxidation Kinetics in Air and Ethene-Air Nanosecond Pulse Discharge Plasmas”, Journal of Physics D: Applied Physics, vol. 43, 2010, p. 124001. [14] Takashima, K., I.V. Adamovich, Z. Xiong, M.J. Kushner, S. Starikovskaia, U. Czarnetzki, and D. Luggenhölscher. “Experimental and Modeling Analysis of Fast Ionization Wave Discharge Propagation in a Rectangular Geometry” 2011. Physics of Plasmas. 18 083505. [15] Syvlie A. J. Druet and Jean-Pierre E. Taran.”CARS spectroscopy.” Prog. Quant. Electr., (1981), Vol.7, 1-72 [16] Fredrik Vestin, Mikael Afzelius, and Per-Erik Bengtsson. “Development of rotational CARS for combustion diagnostics using a polarization approach.” Proceedings of the Combustion Institute. (2007) Vol. 31, 833-840 [17] S. Roy, T.R. Meyer, and J.R. Gord. Optics Leters 30, 3222-3224 (2005). [18] M. Uddi, N. Jiang, E. Mintusov, I.V. Adamovich, and W.R. Lempert, “Atomic Oxygen Measurements in Air and Air/Fuel Nanosecond Pulse Discharges by Two Photon Laser Induced Fluorescence”, Proceedings of the Combustion Institute, vol. 32, 2009, pp. 929-936 [19] M. Uddi, N. Jiang, I. V. Adamovich, and W. R. Lempert, “Nitric Oxide Density Measurements in Air and Air/Fuel Nanosecond Pulse Discharges by Laser Induced Fluorescence”, Journal of Physics D: Applied Physics, vol. 42, 2009, p. 075205 [20] I. Choi, Z. Yin, I.V. Adamovich, and W.R. Lempert, “Hydroxyl Radical Kinetics in Repetitively Pulsed Hydrogen-Air Nanosecond Plasmas”, IEEE Transactions on Plasma Science, vol. 39, 2011, pp. 3288-3299 [21] Z. Yin, I.V. Adamovich, and W.R. Lempert, “OH Radical and Temperature Measurements During Ignition of H2-Air Mixtures Excited by a Repetitively Pulsed Nanosecond Discharge”, accepted for publication in Proceedings of the Combustion Institute, 2012 [22] Z. Yin, K. Takashima, and I.V. Adamovich, "Ignition Time Measurements in Repetitive Nanosecond Pulse Hydrogen-Air Plasmas at Elevated Initial Temperatures", IEEE Transactions on Plasma Science, vol. 39, 2011, pp. 3269-3282 [23] I.A. Kossyi, A. Yu. Kostinsky, A. A. Matveyev, and V.P. Silakov, “Kinetic Scheme of the Nonequilibrium Discharge in Nitrogen-Oxygen Mixtures, Plasma Sources Science andTechnology, vol. 1, 1992, pp. 207-220 14 American Institute of Aeronautics and Astronautics

[24] N.A. Popov, “Effect of a Pulsed High-Current Discharge on Hydrogen–Air Mixtures”, Plasma Physics Reports, 2008, vol. 34, No. 5, pp. 376–391 [25] A. Konnov, “Remaining uncertainties in the kinetic mechanism of hydrogen combustion”, Combustion and Flame, vol. 152, 2008, pp. 507–528 [26] Y. Zuzeek, S. Bowman, I. Choi, I.V. Adamovich, and W.R. Lempert, “Pure Rotational CARS Studies of Thermal Energy Release and Ignition in Nanosecond Repetitively Pulsed Hydrogen-Air Plasmas”, Proceedings of the Combustion Institute, vol. 33, Issue 2, 2011, pp. 3225-3232 [27] Takashima, K., Yin, Z., and Adamovich, I.V., “Measurements and Kinetic Modeling Analysis of Energy Coupling in Nanosecond Pulse Dielectric Barrier Discharge, accepted for publication in Plasma Physics Science and Technology, 2012


[28] I.V. Adamovich, M. Nishihara, I. Choi, M. Uddi, and W.R. Lempert, “Energy Coupling to the Plasma in Repetitive Nanosecond Pulse Discharges”, Physics of Plasmas, vol. 16, 2009, p. 113505 [29] Joseph D. Miller, Sukesh Roy, Mikhail N. Slipchenko, James R. Gord, and Terrence R. Meyer. Optics Express. (2011) Vol. 19, No.16, 15627-15640 A1. A. Bogaerts, “Effects of oxygen addition to argon glow discharges: A hybrid Monte Carlo-fluid modeling investigation”, Spectrochimica Acta Part B 64 (2009) 1266–1279 A2. J.E. Velazco, J.H. Kolts, and D.W. Setser, “Rate constants and quenching mechanisms for the metastable states of argon, krypton, and xenon”, J. Chem. Phys. 69, 4357 (1978)
