JOURNAL OF PROPULSION AND POWER Vol. 34, No. 6, November–December 2018

Electrically Excited, Supersonic Flow Carbon-Monoxide with Air Species in Laser Mixture

M. Yurkovich,∗ Z. Eckert,∗ E. R. Jans,∗ A. P. Chernukho,† K. Frederickson,‡ J. W. Rich,§ and I. V. Adamovich¶ Ohio State University, Columbus, Ohio 43210 DOI: 10.2514/1.B37053 Performance parameters of an electric discharge excited, supersonic flow CO laser operated with significant amounts of air species in the laser mixture is studied experimentally and by kinetic modeling. The results demonstrate that adding nitrogen to the CO–He mixture increases the laser power significantly, by up to a factor of 3. Adding oxygen reduces CO vibrational excitation and results in a steep laser power reduction. Adding air to the laser mixture also produces higher output power, such that the positive effect of nitrogen outweighs the negative effect of oxygen. This result indicates that a chemical CO laser, not dependent on the electric discharge for excitation, may be capable of operating in a mixture of carbon vapor and air. Performance of a supersonic flow chemical laser excited by a reaction of carbon vapor and molecular oxygen is analyzed by kinetic modeling. Peak laser power is predicted at the C vapor mole fraction of 0.2%, when 15% of energy stored in CO vibrational energy mode (4.5% of the reaction enthalpy) is converted to laser power. At higher C vapor mole fractions, flow temperature rise caused by exothermic reactions and by CO vibrational relaxation results in rapid reduction of the predicted laser power.

I. Introduction Electric discharge excited CO emitting in midinfrared have RECENT experimental study [1] has shown that a chemical demonstrated high power and efficiency, up to 200 kW [7] and 50% reaction between carbon vapor and molecular oxygen, [8], with optimal performance achieved using electric discharges A operated at cryogenic temperatures. Laser is obtained on C3PO X3Σ− → COX1Σ O3P (1) rovibrational transitions of CO molecules initially populated by 2 g ;v electron impact in the discharge, by rapid vibration–vibration (V–V) energy exchange resulting in a highly non-Boltzmann population generates highly vibrationally excited CO molecules in the ground distribution among CO vibrational levels in low-temperature electronic state, up to at least v 14, with absolute population environments [9,10]. High power and efficiency are achieved due to inversion at v 4–7. Although formation of vibrationally excited the relatively slow vibration–translation (V–T) and vibration– CO in this reaction was detected previously in the afterglow of a rotation relaxation processes of CO. Chemical carbon-monoxide microwave discharge in nitrogen, with a small amount of cyanogen lasers, based on the reaction of carbon disulfide and molecular (a source of C atoms) and molecular oxygen added to the flow [2], the oxygen, present similar characteristics to the electric discharge results of Ref. [1] have been obtained without using nonequilibrium pumped lasers. The dominant CO product of CS2 O and CS O discharge excitation, with C atoms generated by vaporizing graphite reactions contains several vibrational quanta [11], with peak electrodes in a high-temperature arc discharge and oxygen added population in v 12 [12]. A major benefit of this system is the downstream [3]. These results are obtained in a collision-dominated formation of total among the lower vibrational environment (P 15–20 torr) at low rotational–translational temper- levels, resulting in higher gain on more rovibrational transitions, atures (T 400–450 K), conducive to producing laser action [4,5]. including R-branch lines. Carbon disulfide, however, is a highly toxic A high fraction of reaction enthalpy stored in the CO vibrational compound, making it a less desirable precursor compared to graphite mode at these conditions (at least 10–20%) [1] suggests that a or amorphous carbon. Thus, a novel chemical CO laser based on the chemical CO laser using carbon vapor and oxygen as reactants gas-phase reaction between atomic carbon vapor and molecular may operate at high efficiency. Generation of vibrationally excited oxygen would offer numerous advantages over the current chemical CO by reaction of Eq. (1) has been previously suggested to be the or electric discharge carbon-monoxide lasers. reason for output power increase in electric discharge excited CO One of the possible applications of the new chemical CO laser is lasers when small amounts of oxygen are added to the laser electrical power generation onboard a hypersonic vehicle. At these Downloaded by OHIO STATE UNIVERSITY LIBRARIES on January 3, 2019 | http://arc.aiaa.org DOI: 10.2514/1.B37053 mixture [6]. conditions, carbon vapor can be generated by surface ablation or by carbon powder evaporation in a hypersonic boundary layer and Presented as Paper 2017-1967 at the 55th AIAA Aerospace Sciences subsequently mixed with air to produce vibrationally excited CO in a Meeting (SciTech 2017), Grapevine, TX, 9–13 January 2017; received 14 with oxygen. Laser power would be generated in a January 2018; revision received 7 May 2018; accepted for publication 11 June supersonic flow laser cavity and converted to electrical power using 2018; published online 27 August 2018. Copyright © 2018 by M. Yurkovich, photovoltaic cells. Thus, the laser would have to operate in a gas Z. Eckert, E. R. Jans, K. Frederickson, J. W. Rich, and I. V. Adamovich. mixture with a significant fraction of air. Additionally, the flow Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. All requests for copying and permission to reprint should be temperature in the laser cavity may be fairly high, due to an oblique submitted to CCC at www.copyright.com; employ the ISSN 0748-4658 shock system decelerating the flow upstream of the cavity. CO laser (print) or 1533-3876 (online) to initiate your request. See also AIAA Rights operation at elevated temperatures has been demonstrated previously. and Permissions www.aiaa.org/randp. Optically pumped CO laser operating in a CO–Ar mixture, at *Graduate Research Associate, Department of Mechanical and Aerospace temperatures up to T 450 K, pressure of P 10 torr, and CO Engineering. Member AIAA. partial pressure of 0.2–0.3 torr, was demonstrated in Ref. [4]. †Visiting Scholar, Department of Mechanical and Aerospace Engineering. CS –O –He–Ar ‡ Chemical CO laser in a 2 2 mixture was demonstrated at Research Scientist, Department of Mechanical and Aerospace Engineering. temperatures up to T 500 200 K [5]. Member AIAA. § Adding nitrogen and oxygen to the laser mixture may significantly Professor Emeritus, Department of Mechanical and Aerospace Engineering. – Fellow AIAA. affect CO vibrational distribution function, both due to vibration ¶Professor, Department of Mechanical and Aerospace Engineering. vibration (V–V) energy transfer to N2 and O2 and due to vibration– Associate Fellow AIAA. translation (V–T) relaxation by O atoms. Previous measurements of

1494 YURKOVICH ET AL. 1495

vibrational populations of CO, N2,andO2 in optically pumped CO–N2, 1.6 mm thick, which prevents secondary electron emission from the CO–O2,andCO–air mixtures [13] produced quantitative insight into electrode surface and improves discharge stability. The electrodes are the effect of air species on the vibrational energy distribution of CO. In powered by a Dressler 5 kW, 13.56 MHz RF power supply with these experiments, low vibrational states of CO were excited by automatic impedance matching network. resonance absorption of CO pump laser radiation, followed by V–V Downstream of the discharge section, the flow enters an optical energy exchange among CO, N2,andO2 molecules. The results show access section 7.6 cm long, equipped with CaF2 windows flush- that presence of nitrogen has a weak effect on the CO vibrational mounted in the top and side walls. This section was in place only for distribution function, due to the fact that nitrogen has larger energy CO emission spectra measurements and was removed during spacing between vibrational levels, compared to that of CO, such that laser spectra and power measurements, to shorten the distance therateofV–VenergytransferfromCOtoN2 is lower compared to the between the discharge section and the supersonic nozzle. The rate of the reverse process. Because of this, most of the vibrational emission spectra are measured by a Varian 660-IR Fourier transform energy available in the mixture remains in the CO vibrational mode, infrared (FTIR) spectrometer. The 7-cm-long nozzle, with 0.32 × with relatively little vibrational excitation of N2 [13]. Adding oxygen to 10 cm throat cross sectional area, has an expansion ratio designed for the mixture, on the other hand, considerably reduces energy stored in an exit Mach number of M 3. Downstream of the nozzle exit, the the CO vibrational mode, such that O2 vibrational level populations flow enters the supersonic section with entrance height of 1 cm. The exceed those of CO [13]. This occurs because O2 has a smaller top and bottom walls of the supersonic channel diverge at 1.5 deg vibrational energy spacing compared to that of CO, such that vibrational angle each, to allow for boundary-layer growth. Several 0.5-in.-diam energy is preferentially transferred from CO to O2 by V–V exchange apertures in the side walls of the supersonic section, made of processes. These results demonstrate the negative impact that oxygen aluminum, provide optical access to the flow, as shown in Fig. 1. Two has on vibrational excitation of CO, which is critical for laser operation optical mounts with 1-in.-diam, 1-m-radius of curvature laser in mixtures containing considerable amounts of air. In addition to this, O mirrors, with 99.8% reflectivity between 4.5 and 5.75 μm (Rocky atoms generated in electric discharge excited or chemically reacting Mountain Instruments) are attached to the walls 3.2 cm downstream CO–air mixtures may further reduce vibrational nonequilibrium, due to of the nozzle exit, forming a transverse laser cavity, with laser power rapid vibrational relaxation of CO by O atoms and by CO2. coupled out on both sides of the cavity. During the experiments, laser The main objective of the present work is to determine how adding spectra and laser power are measured simultaneously. Laser power is significant amounts of nitrogen, oxygen, or air to the laser mixture measured using the beam on one side of the , by a Scientech affects CO laser power and spectrum. Previously, the effect of adding AC5000 power meter, and multiplied by 2. Laser spectrum is large amounts of oxygen and air on laser output power and efficiency measured using the beam on the other side of the resonator, by the was studied in an electric discharge excited CO laser sustained by a Varian FTIR spectrometer. Downstream of the supersonic section, pulsed electron beam [14], demonstrating high specific power in the flow enters a 36-cm-long diffuser with top and bottom walls CO–air mixtures for the first time. In the present work, we use an diverging at 3 deg, before exhausting into a 750 ft3 vacuum tank electric discharge excited, supersonic flow CO laser, discussed in pumped down by a 300 cubic feet per minute (CFM) vacuum pump. Sec. II. Supersonic expansion is used to reduce the temperature of the The steady-state supersonic flow run time is approximately 5 s. laser mixture and to study laser operation in a high-speed flow, such as would occur in a hypersonic flight. Comparison of the experimental results with predictions of a kinetic model, described in Sec. III, is used III. Kinetic Model to obtain insight into kinetics of energy transfer of vibrationally excited The kinetic model incorporates state-to-state vibrational kinetics, CO–N2–O2–He mixtures and to predict performance parameters of including vibration–vibration (V–V) energy transfer processes electrically excited and chemical CO lasers at the conditions where N O – – experimental data are not available, as discussed in Sec. IV. among CO, 2, and 2 and vibration translation (V T) relaxation of diatomic species by CO, N2, O2, He, and O atoms; chemical kinetics equations for species number densities in a reacting mixture; and quasi-one-dimensional compressible flow equations of vibrationally II. Experimental nonequilibrium, reacting flow [16]. The model also includes a set of The supersonic flow laser apparatus used in the present laser field equations governing the stimulated emission intensity on investigation, shown schematically in Fig. 1, is similar to the electric each lasing vibrational transition in transverse laser cavity [17]. discharge excited oxygen iodine laser developed in our previous In the present work, single-quantum rate coefficients for V–Vand work [15]. Briefly, gas mixture components (99.998% purity carbon V–T energy transfer processes in CO–N2–O2 mixtures have been monoxide, helium, oxygen, nitrogen, and dry air) are delivered from modified compared to Ref. [18], where they have been validated by high-pressure cylinders and premixed in a 1-in.-diam, 5-m-long comparing kinetic modeling predictions with spatially resolved delivery line, before flowing through a 10 × 2cmrectangular cross measurements of vibrational level populations of CO, N2, and O2 in section, 10-cm-long electric discharge section, which serves as a optical pumping experiments [13,18]. The rates have been modified

Downloaded by OHIO STATE UNIVERSITY LIBRARIES on January 3, 2019 | http://arc.aiaa.org DOI: 10.2514/1.B37053 plenum of a supersonic flow channel made of acrylic plastic taking into account measurements and semiclassical trajectory (see Fig. 1). Individual flows are activated by remotely controlled calculations of V–V rates for CO–CO [19,20], N2–N2 [21–23], solenoid valves. Two electrode blocks are flush-mounted in the top O2–O2 [24–27], CO–N2 [28,29], CO–O2 [30,31], and N2–O2 [32]. and bottom walls of the discharge section. Each electrode is a Although more recent calculations of N2–O2 V–V rates [33], using a 10 × 10 cm copper plate, covered by an alumina ceramic plate more accurate potential energy surface [34], are available, at the

Area Discharge Nozzle Nozzle reduction Diffuser section throat exit insert

Flow

Electrode block Optical access Optical access Resonator Optical access section (optional) window mirror window Fig. 1 Schematic of the supersonic flow CO laser apparatus. 1496 YURKOVICH ET AL.

present conditions the effect of V–Vexchange between N2 and O2 on discharge power, supersonic nozzle geometry, cavity gain length, predicted CO vibrational level populations and laser power is very cavity mirror reflectivities, and clear aperture mirror surface area. weak because high vibrational level populations of nitrogen and oxygen are extremely low (see Sec. IV). Dominant V–T processes included into the model are relaxation IV. Results and Discussion – N –He O –He by He and O atoms: CO He [35,36], 2 [37,38], 2 A. Operation and Kinetic Modeling of the Electric Discharge Excited [37,25], CO–O [39,40], N2–O [41,42], and O2–O [43–45]. Carbon-Monoxide Laser – N O Although V TrelaxationofCO, 2,and 2 by diatomic molecules Figure 2 plots the CO infrared overtone emission spectra is also included, based on semiclassical trajectory predictions measured downstream of the discharge section in two different [19,21,26], at the present conditions it is much slower compared to mixtures with the same amount of and helium, – V T relaxation by He and O atoms and has essentially no effect on 3.6 torr CO–97 torr He and 3.6 torr CO–97 torr He–13.8 torr N2,at – – the modeling predictions. Expressions for V TandVVrate the same RF discharge power of 2 kW and spectral resolution of coefficients as well as values of parameters used in the equations are 0.25 cm−1. These spectra, shown on the same scale, are used to infer given in the Appendix. CO vibrational level populations, as discussed in detail in Ref. [52]. Chemical reaction rate coefficients in Arrhenius form are taken CO fundamental emission spectra, taken at the same conditions, are from the combustion mechanism developed by Konnov [46] State- used for translational–rotational temperature inference. No CO2 specific rate coefficients of reaction of Eq. (1) are assumed to follow a emission has been detected in the spectra. Comparison of rotational Gaussian distribution over CO vibrational levels: temperature values inferred from emission spectra taken from optical access windows in the top and side walls of the optical access 2∕3 v − v2 k→ v; Tp exp − ⋅ k T (2) section showed that, at the present conditions, self-absorption of πσ σ f CO infrared emission is a minor effect that does not affect the P emission spectra. From Fig. 3, it can be seen that temperature 380–425 K where kfT k→ v; T is the overall forward reaction rate downstream of the discharge is in the range T .From coefficient; v 8 is chosen such that the fraction of reaction Fig. 2, it is evident that adding nitrogen to the mixture results in enthalpy going to CO vibrational mode; 30%, is the same as predicted considerable increase in emission intensity over a wide range of CO by the quantum chemistry calculations**; and σ 5 is chosen to overtone transitions. Kinetic processes causing this effect are produce the nascent vibrational distribution of CO product similar to discussed later. that of the reaction CS O → COvS [12]. In addition, 1∕3 of Figure 4 plots CO vibrational distribution functions, inferred the CO generated by reaction (1) is assumed to be produced in the from the spectra such as shown in Fig. 2, for the baseline CO–He ground vibrational level, v 0, which provides the best match to the mixture and in three mixtures with nitrogen, oxygen, or air added to experimental CO vibrational distribution in Ref. [1]. This assumption the baseline mixture, at the same RF discharge power of 2 kW. As is qualitatively consistent with the surface reaction between carbon expected for all operating conditions, CO “first level” vibrational clusters and molecular oxygen because solid phase carbon and temperature, defined as the slope of the vibrational distribution 0 atomic carbon vapor (C atoms) appear to be the dominant product of function at v , is significantly higher compared to the rotational graphite electrode ablation in the arc discharge [3]. CO produced in temperature (see the legend in Fig. 4). In addition, strong deviation other chemical reactions is assumed to be generated in equilibrium of CO vibrational populations from the Boltzmann distribution in – CO–N –He – – with the gas temperature, and rate coefficients of other chemical CO He, 2 ,andCO air He mixtures, due to anharmonic V–V pumping [9], is also evident. In CO–O2–He mixtures, this reactions that consume CO (e.g., CO O2 → CO2 O) are effect is not as pronounced. A “kink” at vibrational levels v assumed not to change CO vibrational populations. 10–11 Because direct measurements of the electric field in the positive is due to absorption spectrum by water vapor in atmospheric column of transverse RF discharges are challenging, the reduced air, which overlaps with CO first overtone emission spectra at ∼3500–4000 cm−1. It can be seen that adding nitrogen to the electric field (E/N) in the discharge is considered an adjustable – parameter, determined from matching the predicted CO vibrational CO He mixture significantly increases the CO vibrational level populations (see Fig. 4). On the contrary, adding oxygen to the distribution function (VDF) to the experimental VDF inferred from – CO infrared emission spectra measured downstream of the discharge CO He mixture dramatically reduces CO vibrational level populations (see Fig. 4), similar to the effect observed in optical section. This approach assumes that the discharge input energy pumping experiments [13]. Finally, adding air to the CO–He fractions going into vibrational excitation of CO, N2, and O2 by mixture results in a modest increase of CO vibrational level electron impact are controlled by the effective (RMS) reduced populations for v < 15 (see Fig. 4), such that the effects of adding N2 electric field value in the discharge. Rate coefficients of vibrational and O2 are nearly balanced. and electronic excitation of CO, N2, and O2 by electron impact as well as rate of O2 dissociation by electron impact are calculated by Downloaded by OHIO STATE UNIVERSITY LIBRARIES on January 3, 2019 | http://arc.aiaa.org DOI: 10.2514/1.B37053 solving Boltzmann equation for plasma electrons [47]. Electronically excited states of CO, N2, O2, and He decay radiatively or quenched collisionally, resulting in additional O2 dissociation and CO2 formation [48–50]. The laser cavity model is a simple Fabry–Perot model [17]. On each vibrational band, lasing is assumed to occur on a single vibrational–rotational transition with maximum gain, calculated for a Doppler-broadened spectral line. Einstein coefficients for CO spontaneous emission are calculated using the vibrational transition moments from Ref. [51]. To simulate lasing at steady-state conditions, the intracavity photon flux on each vibrational band is assumed to be constant throughout the cavity, chosen such that the laser power on that band matches the cavity-averaged power loss from the CO vibrational mode due to stimulated emission. Input parameters of the model are the gas mixture composition, RF

Fig. 2 CO overtone emission spectra downstream of the discharge in **Private communication with G. Shatz, Northwestern Univ., May 2012. 3.6 torr CO–97 torr He and 3.6 torr CO–97 torr He–13.8 torr N2 mixtures. YURKOVICH ET AL. 1497

Fig. 3 Post-discharge plenum flow temperature in CO–He (left) and mixtures with N2, O2, and air (right). He partial pressure 97 torr; CO partial pressure (right) 3.6 torr; discharge power 2 kW.

Figure 5 plots experimental CO vibrational distribution functions inferred from emission spectra such as plotted in Fig. 2, compared with the modeling predictions. The uncertainty of CO vibrational level populations is several percent, except for the two to three highest vibrational levels, for which the uncertainty is up to 30–50%. As discussed in Sec. III, in the modeling calculations, the reduced electric field value E∕N was adjusted to obtain the best match between the experimental and predicted CO VDFs. It can be seen that, in both the baseline CO–He mixture and in mixtures containing N2, O2, and air, experimental and predicted CO vibrational populations are fairly close to each other (see Fig. 5), indicating that the rates of V–V exchange for CO–CO and CO–N2, which control the CO VDF at these conditions, are represented fairly accurately. Note that varying E∕N to match CO vibrational populations at the V–V pumped plateau results in overprediction of higher vibrational level populations (at v>10–15), especially in CO–He and CO–N2–He mixtures, suggesting that the rates of CO–He V–T relaxation among the high vibrational levels, taken from Fig. 4 CO vibrational distributions downstream of the discharge Ref. [35], may be underpredicted. (baseline 3.6 torr CO–97 torr He mixture, and its mixtures with 13.8 torr From Figs. 4 and 5, it can be seen that adding nitrogen to the CO–He N2, 8.5 torr O2, and 21 torr air, discharge power 2 kW). mixture results in considerable increase of CO vibrational populations Downloaded by OHIO STATE UNIVERSITY LIBRARIES on January 3, 2019 | http://arc.aiaa.org DOI: 10.2514/1.B37053

Fig. 5 Experimental and predicted CO vibrational distributions: a) CO–He, b) CO–N2–He,c)CO–O2–He,d)CO–air–He. RF discharge power 2 kW. 1498 YURKOVICH ET AL.

at v>15. Kinetic modeling shows that this occurs due to N2 vibrational excitation by electron impact, with subsequent preferential V–V energy transfer from N2 to CO. Adding oxygen to the CO–He mixture, on the other hand, reduces CO vibrational level populations significantly. Analysis of modeling predictions shows that, at the present conditions, this is due to a combination of two effects: 1) V–T relaxation of CO by O atoms, generated by O2 electron impact dissociation and by quenching of electronically excited species, and 2) lower discharge energy fraction going to CO vibrational excitation by electron impact. O atom number density predicted in CO–air–He mixtures is lower compared to that in CO–O2–He mixtures, by approximately a factor of 2, due to lower average electron energy limited by additional vibrational excitation of N2. For example, O atom mole fraction predicted in a CO–air–He mixture with air partial pressure of 21 torr (O2 partial pressure 4.6 torr) is 1300 ppm, compared with 2700 ppm predicted in a CO–O2–He mixture at the same O2 partial pressure. This reduces the effect of CO V–TrelaxationbyO atoms in air-containing mixtures, while also enhancing CO vibrational Fig. 7 Predicted vibrational distributions of CO (solid lines), N2 (dashed lines), and O2 (dotted lines) at the conditions of Fig. 6. excitation via V–V energy transfer from N2 to CO. Therefore, in CO–air–He mixtures, the effects of V–Venergy transfer from N2 to CO and V–T relaxation of CO by O atoms are nearly balanced. Figure 6 plots predicted axial distributions of Mach number, static laser spectrum exhibits evidence of laser cascade, when lasing on pressure, static temperature, and flow velocity in a CO laser operating higher vibrational transitions overpopulates lower vibrational levels in a 3.6 torr CO–97 torr He–21 torr air mixture, at the RF discharge and triggers lasing on lower transitions (e.g., see Fig. 8). Lasing on 0 6–14 power of 2 kW. Figure 7 plots predicted vibrational distribution relatively low rotational transitions, J , is typical for low – functions of CO (solid lines), N2 (dashed lines), and O2 (dotted lines) translational rotational temperature in the laser cavity, predicted ≈ 110 K at these conditions. It can be seen that strongly nonequilibrium CO to be T . Adding nitrogen to the mixture results in lasing 2 → 1 15 → 14 vibrational distribution function is maintained through the nozzle on fundamental bands from v to v and throat and the laser cavity. At these conditions, vibrational significantly increases the number of rotational transitions on which temperatures of CO and N2 reach quasi-equilibrium via rapid V–V lasing occurs in each vibrational band (see Fig. 9). Adding oxygen results in the opposite trend, reducing both the number of vibrational exchange, with Tv (CO) being significantly higher compared to TvN2, due to the smaller CO vibrational quantum. This effect has bands and rotational transitions on which lasing occurs (see Fig. 10). been also detected in an optically pumped CO–N2 mixture [13]. On Finally, adding air produces a spectrum similar to the one observed in – the other hand, vibrational energy transfer from CO to O2 remains the baseline CO He mixture (see Fig. 11). In all cases other than CO–O –He very slow because CO–O2 V–V processes becomes near resonant 2 mixtures, the modeling predictions reproduce the range only for CO vibrational levels v ∼ 20, which have very low relative and qualitative trends of vibrational transitions on which lasing populations. occurs (except for the lowest vibrational transition in CO–He, Figures 8–11 compare experimental and predicted laser spectra in CO–N2–He, and CO–air–He), as well as the rotational quantum the baseline CO–He mixture and in mixtures with nitrogen, oxygen, numbers on which the laser power in each vibrational band peaks, and air, at the discharge power of 2 kW. It can be seen that, at the fairly well (see Figs. 8, 9, and 11). Note that, as discussed in Sec. III, baseline conditions, lasing occurs on fundamental vibrational bands the present model assumes that lasing occurs only on one vibrational– ranging from v 3 → 2 to v 14 → 13 (see Fig. 8), on several rotational transition in each band, on which gain reaches maximum. rotational transitions in each band (at the discharge power of 1 kW, Figure 12 plots laser output power versus CO partial pressure in a lasing on v 2 → 1 and v 15 → 14 bands is also detected). The CO–He mixture, at two different RF discharge powers, 1 and 2 kW. Downloaded by OHIO STATE UNIVERSITY LIBRARIES on January 3, 2019 | http://arc.aiaa.org DOI: 10.2514/1.B37053

Fig. 6 Predicted axial distributions of flow parameters in a supersonic flow CO laser operating in a 3.6 torr CO–97 torr He–21 torr air mixture, at RF discharge power of 2 kW. YURKOVICH ET AL. 1499

Fig. 8 Experimental and predicted laser spectra in a 3.5 torr CO–97 torr He mixture. Downloaded by OHIO STATE UNIVERSITY LIBRARIES on January 3, 2019 | http://arc.aiaa.org DOI: 10.2514/1.B37053

Fig. 9 Experimental and predicted laser spectra in a 3.5 torr CO–97 torr He–26.3 torr N2 mixture.

The results show that 1) laser power peaks at CO partial pressure of reduced the temperature in the laser cavity and resulted in a 3–5 torr (at 1 kW) and 3.5–4.0 torr (at 2 kW), and 2) laser power significant increase of gain and output power, at the same electric decreases as the RF discharge power is increased from 1 to 2 kW. This discharge power [53]. Thus, RF discharge power increase above is most likely due to higher translational–rotational temperature 1kWinCO–He mixtures is detrimental to the laser power. The at a higher discharge power, T 363 4Kat 1 kW versus T modeling predictions at the discharge power of 2 kW are in fair 393 6Kat 2 kW (both measured at 3.6 torr CO partial pressure), agreement with the data, overpredicting the laser power over the which reduces laser gain even if vibrational energy per CO molecule entire range of mixtures. This is expected because the kinetic model is increased. Indeed, precooling of the mixture in the plenum of an does not include additional losses in the cavity, other than photon flux electric-discharge-excited, supersonic-flow oxygen–iodine laser loss due to transmission through the laser mirrors. 1500 YURKOVICH ET AL.

Fig. 10 Experimental laser spectrum in a 3.5 torr CO–97 torr He–3.3 torr O2 mixture.

Fig. 11 Experimental and predicted laser spectra in a 3.5 torr CO–97 torr He–32 torr air mixture.

Downloaded by OHIO STATE UNIVERSITY LIBRARIES on January 3, 2019 | http://arc.aiaa.org DOI: 10.2514/1.B37053 Figure 13 illustrates the effect of adding N2, O2, and air to the baseline CO–He mixture at constant CO partial pressure of 3.6 torr and RF discharge power of 2 kW. The data show that adding nitrogen to the laser mixture results in a dramatic increase of laser power, from 8–10 W to approximately 30 W, at N2 partial pressures of 10–30 torr (see Fig. 13). The modeling predictions reproduce this trend quite well, although the laser power is again overpredicted by the model by about a factor of 2 (see Fig. 13). This trend is consistent with CO vibrational population increase in the presence of nitrogen (see Fig. 4), caused by vibrational energy transfer from N2 to CO. It is also affected by translational– rotational temperature reduction as N2 partial pressure is increased, from T 393 6Kin the baseline CO–He mixture to T 361 5Kin the mixture with 20.3 torr of nitrogen. The temperature reduction is caused by the increase in the fraction of discharge input energy going to vibrational excitation of nitrogen, thereby reducing energy fraction to excitation of electronic levels of CO, which contributes to temperature rise during their collisional quenching. Fig. 12 Experimental and predicted laser power in CO–He mixtures vs Adding oxygen to the baseline CO–He laser mixture dramatically CO partial pressure. He partial pressure 97 torr. reduces the laser power, by over an order of magnitude with less than YURKOVICH ET AL. 1501

CO–air flows, with significant mole fraction of air (up to at least up to 20–30%). Comparison of the experimental results and the modeling predictions also suggests that the model has predictive capability at the operating conditions that could not be achieved in the present work. Specifically, RF discharge operation in CO–air mixture, without helium, is challenging due to difficulty with maintaining diffuse discharge stability and impedance matching in the high- pressure plenum. However, exercising the kinetic model in a 3% CO– air mixture at plenum pressure of 100 torr and electric discharge power of 2 kW (with V–T relaxation of CO by O atoms turned off) predicts laser power of 18 W, suggesting that lasing in CO–air mixtures where vibrational nonequilibrium is sustained by other means than electric discharge excitation is feasible. To verify this, the kinetic model was exercised in airflow at the baseline plenum pressure of 100 torr, with the electric discharge turned off, and atomic carbon vapor injected and mixed with the flow 2.2 cm upstream of the nozzle throat. The flow channel and laser cavity Fig. 13 Experimental and predicted laser power vs N2, O2, and air partial geometry, as well as the mirror reflectivities, were the same as in the pressures. CO and He partial pressures 3.6 and 97 torr, respectively, RF modeling calculations discussed previously. At these conditions, discharge power 2 kW. carbon vapor reacts with molecular oxygen in the flow in reaction of Eq. (1), generating highly vibrationally excited CO [1] that rapidly forms a V–V pumped population distribution such as shown in Fig. 5. O This population distribution is maintained in the supersonic flow, such 7 torr of 2 added (see Fig. 13). This is consistent with CO vibrational as shown in Fig. 7, producing laser gain and generating laser power in population reduction in the presence of oxygen (see Fig. 4) and the laser cavity. Figure 14 plots laser power and flow temperature in the modest temperature rise as O2 partial pressure is increased, from 393 6K – 410 6K cavity predicted at these conditions, as a function of C vapor partial T in the baseline CO He mixture to T pressure (i.e., injection flow rate). At the partial pressure of ≈3 mtorr, in the mixture with 8.7 torr of oxygen. However, this trend is strongly sufficient gain is produced to overcome cavity mirror losses. Above 3 overpredicted by kinetic modeling predictions. Such significant mtorr, the laser power increases rapidly as 1) gain exceeds the loss difference is somewhat unexpected because CO vibrational threshold on additional CO vibrational transitions, and 2) highergain is distribution function at these conditions is predicted fairly accurately produced on these transitions due to higher CO number density. When (see Fig. 5c). Finally, adding air to the baseline mixture results in the C vapor partial pressure reaches ≈0.2 torr, the laser power peaks and modest laser power increase, by almost a factor of 2 (see Fig. 13), starts to decrease, due to flow temperature rise caused by energy release before it decreases gradually, such that the effect of nitrogen appears in the reaction of Eq. (1) and due to CO V–T relaxation by O atoms, to be balanced by that of oxygen. Although the modeling predictions also produced in the reaction. The predicted laser spectrum at peak (solid green line) reproduce this trend qualitatively, the laser power power of 16 W,shown in Fig. 15, is similar to the spectra measured and predicted by the model does not exhibit a well-pronounced peak, and predicted in electric discharge excited CO–N2–He and CO–air–He its reduction at high partial pressures is overpredicted. mixtures (see Figs. 9 and 11). At these conditions, 15% of energy The laser power dependence on air partial pressure was found to be stored in CO vibrational energy mode (4.5% of the reaction enthalpy) sensitive to state-specific CO V–T relaxation rates by O atoms. is converted to laser power. At low CO partial pressures (several mtorr), Although the vibrational relaxation time for this process has been the power is generated predominantly on low vibrational transitions, measured [39,40], the scaling of the rates with vibrational quantum v 3 → 2 to v 7 → 6, controlled by the nascent CO vibrational number is not known. A semiclassical theory of vibrational energy distribution. At high CO partial pressures (1–2 torr), lasing is predicted transfer in purely repulsive nonreacting collisions [54], used for V–T to shift to higher vibrational transitions, v 6 → 5 to v 11 → 10. rate scaling in the present work, predicts the rates to exceed the gas The modeling calculations show that peak laser power is controlled kinetic rate coefficient for CO (v ∼ 30), indicating that it is not primarily by the flow residence time between the injection point and applicable for CO–O collisions. Therefore, measurements and high- the nozzle throat. Further reduction of the injection/mixing region fidelity theoretical calculations of these rates are critical for the length would significantly reduce CO vibrational energy loss upstream predictive capability of the present model. The dashed green line in of the cavity and increase the laser power. Fig. 13 shows the modeling predictions in CO–air–He mixtures with V–T relaxation of CO by O atoms turned off. In this case, the agreement between the modeling predictions and the experimental Downloaded by OHIO STATE UNIVERSITY LIBRARIES on January 3, 2019 | http://arc.aiaa.org DOI: 10.2514/1.B37053 results is significantly better. This suggests that overpredicting V–T relaxation rates of highly vibrationally excited CO by O atoms may be one of the reasons for the discrepancy between the experimental data and the modeling predictions, in both CO–O2–He and CO–air– He mixtures. The highest laser output power at the conditions of present experiments is measured in a 3.3 torr CO, 97 torr He, 13.4 torr N2 mixture excited by a 2 kW RF discharge, producing 32.4 W, at an efficiency of 1.6%. Further increase of the discharge power results in a modest increase of the laser power, before it levels off. Optimizing laser power or efficiency is not the objective of the present work, which is focused mainly on demonstrating feasibility of supersonic flow CO laser operation in the presence of significant amounts of air species in the laser mixture and producing quantitative insight into the energy transfer kinetics in the laser mixture at these conditions.

B. Kinetic Modeling of a Chemical Carbon-Monoxide Laser The present results demonstrate that strong CO vibrational Fig. 14 Predicted output power and flow temperature in the laser cavity nonequilibrium and lasing can be obtained in vibrationally excited for a chemical CO laser operating in air at plenum pressure of 100 torr. 1502 YURKOVICH ET AL.

Fig. 15 Chemical CO laser spectrum predicted at the conditions of Fig. 14, when the output power reaches maximum.

V. Conclusions C vapor mole fraction of approximately 0.2% (partial pressure of In the present work, performance parameters of an electric- 0.2 torr), when 15% of energy stored in CO vibrational energy mode discharge-excited, supersonic-flow carbon-monoxide laser operated in (4.5% of the reaction enthalpy) is converted to laser power. At higher C a Mach 3 laser cavity with significant amounts of air species in the laser vapor mole fractions, flow temperature rise caused by the mixture have been studied experimentally and by kinetic modeling. exothermic reaction between carbon vapor and oxygen, as well as Laser spectra and output power have been measured and predicted in rapid CO vibrational relaxation, becomes detrimental, and the predicted laser power begins to decrease, such that laser action no CO–He, CO–He–N2, CO–He–O2,andCO–He–air mixtures. The longer occurs at C vapor mole fractions above ≈2%. Development experimental results and the modeling predictions demonstrate that of the chemical CO laser analyzed in the present work critically adding nitrogen to the baseline CO–He mixture results in significantly depends on increasing the yield of carbon vapor used to generate stronger vibrational excitation of CO, due to higher discharge energy vibrationally excited CO in the reaction with molecular oxygen, fraction going to CO and N2 vibrational excitation, and V–Venergy which is currently underway. The kinetic model developed in the transfer from N2 to CO. This effect also results in significantly higher present work may be used as a design tool for optimization of output laser power measured in CO–He–N2 mixtures (by up to a factor of 3 power and efficiency of this laser. compared to the baseline CO–He mixture) as well as lasing on a larger number of CO fundamental vibrational transitions. On the other hand, adding oxygen to the baseline CO–He mixture reduces CO vibrational Appendix: VV and VT Rate Parameterization excitation considerably, primarily due to the lower reduced electric Vibration–vibration exchange rate coefficient parameterization field in the discharge. It is also likely that CO vibrational levels are for diatom-diatom collisions, using species dependent coefficients in depopulated via V–T relaxation by O atoms, although the kinetic Table A1 [18]: model appears to overpredict this effect, due to significant uncertainty in scaling of the V–T rates with vibrational quantum number. Adding oxygen to the mixture also results in steep reduction of the laser power. − 1 → − 1 v;v−1 v;v−1 −ΔE∕2T Qij v; w v ;w Z Sw−1;w Lw−1;w e ; Again, this effect is overpredicted by the model, such that lasing is Δ θ 1 − 2δ − θ 1 − 2δ detected at O2 mole fractions up to 6%, whereas the model predicts that E i iv j jv ; adding only 1.5% of O2 to the baseline CO–He mixture would result in −1 v w −1 Sv;v S ⋅ T Fλv;v ; suppression of laser action. w−1;w 1 − δ v 1 − δ w w−1;w – i j Adding air to the baseline CO He mixture results in a higher 1 laser power, similar to adding nitrogen. The positive effect of λ 3 − −2λ∕3 −2λ∕3 F 2 e e ; adding nitrogen appears to outweigh the negative effect of adding v;v−1 2 w−1;w 2 Δ 2 oxygen. Laser action has been detected in the experiments and v;v−1 L g g exp − E – Lw−1;w 1;0 1;0 ; predicted by the kinetic model, for CO He mixtures with up to T g i g j bT 30% of air. In the experiments, further increase in the mole fraction Downloaded by OHIO STATE UNIVERSITY LIBRARIES on January 3, 2019 | http://arc.aiaa.org DOI: 10.2514/1.B37053 gv;v−1 2 a 1 2 va 2 − 2va 4 − 2v of air in the mixture was precluded by the limited range of ; 1;0 3 − 2 3 − operation of the RF discharge exciting the laser mixture in the g i a v a a v plenum. This result indicates that a chemical CO laser, which is not a 1∕δ ; dependent on the electric discharge for excitation of the laser i −10 1∕2 3 −1 mixture, may be capable of operating in a mixture of carbon vapor Z 3 × 10 T∕300 cm ⋅ s and air. Thus, the results of the present work show that, given a sufficiently strong CO vibrational excitation source, produced by electric discharge or by nonequilibrium chemical reaction at moderate stagnation temperatures (300–400 K), continuous-wave – lasing can be produced in CO air mixtures after supersonic Table A1 Parameters used in the V–V expansiontoMach3. rate expressions

Performance parameters of a supersonic flow chemical laser excited −1 −1 by a reaction of carbon vapor and molecular oxygen as well as Species S, K L,K c, K b,K operating in a mixture of carbon vapor and air at the same plenum CO–CO 1.64 ⋅ 10−6 1.614 0.456 40.85 −7 N2–N2 1.5 ⋅ 10 —— 0.36 —— conditions, flow channel geometry, laser cavity geometry, and mirror −6 O2–O2 2.0 ⋅ 10 —— 0.5 —— reflectivities have been studied using kinetic modeling. The modeling −8 CO–N2 7.0 ⋅ 10 0.06 0.185 87.73 predictions show that lasing can be achieved over a wide range of C −8 CO–O2 2.26 ⋅ 10 0.2 0.185 120 vapor mole fractions, starting from ≈30 ppm (partial pressure of −8 N2–O2 1.5 ⋅ 10 —— 0.25 —— ≈3mtorr). Peak laser power of 16 W is predicted at the YURKOVICH ET AL. 1503

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