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AIAA-96-3185

Comparisons of Hydrogen Atom Measurementsin an Arcjet Plume with DSMC Predictions

Ingrid J. Wysong, Jeffrey A. PobstHughes STXPropulsion DirectorateOL-AC Phillips LaboratoryEdwards AFB, CA

lain D. BoydSchool of Mechanical and Aerospace EngineeringCornell UniversityIthaca, NY

32nd AIAA/ASME/SAE/ASEEJoint Propulsion Conference

July 1-3, 1996/ Lake Buena Vista, FL

For permission to copy or republish, contact the American Institute of Aeronautics and Astronautics1801 Alexander Bell Drive, Suite 500, Reston, VA 22091

Comparisons of Hydrogen Atom Measurementsin an Arcjet Plume with DSMC Predictions

Ingrid J. Wysong* and Jeffrey A. Pobst**Hughes STX

Phillips Laboratory, Propulsion DirectorateEdwards AFB, CA

lain D. BoydSchool of Mechanical and Aerospace Engineering

Cornell UniversityIthaca, NY

Abstract

We present comparisons between hydrogen atom number density, velocity, and temperature measurements withDSMC predictions at nozzle exit for two power levels of a hydrogen arcjet thruster: 1.34 kW and 800 W.Comparisons between model and experiment are also presented in two downstream plume regions. Atomic densitypeaks at each power are not seen to vary substantially while velocity and temperature maximums decrease with lowerpower operation. Changes in profile shapes are noticed at the different powers and similar profile changes are alsoobserved in the model predictions. Differences in overall magnitude are observed at some locations betweenmeasured data and predicted results, but in general the model matches the measured data quite well.

Introduction becomes even greater in the plume expansion region,which is critical for determining spacecraft interaction

Arqjet thruster computational models have begun to and contamination issues. A DSMC model optimizedreach a level of maturity where new advanced thruster for arcjet prediction is currently under development atdesigns may soon be based upon model predictions. Cornell University.'Concurrently, there is interest in the development of anew arciet thruster class, designed for operation at Recently, measurements of atomic hydrogen numberpowers lower than 1000 watts. In order to design density, ground state temperature, and speciesthrusters of this type from model predictions, the velocity were taken in the plume of a one kilowattscaling of present thruster predictions into this lower class arcjet thruster using a two-photon laser inducedpower region must be explored, fluorescence (2PL1F) technique. Comparisons

between the density measurements and several arcjetAs thruster designs become physically smaller, computational models inferred that the models tendedchanges in the energy loss mechanisms of the to predict significantly lower atomic number densitiesnonequilibrium gas flow are expected. The direct than was measured with the 2PLIF technique.simulation Monte Carlo (DSMC) method, whilecomputationally intensive, provides a direct Additionally, molecular dissociation fraction at thesimulation of collision and plasma behavior and is center ofý the nozzle exit, as determined by atherefore able to model gas flows with high degrees combination of the 2PLIF work and molecularof nonequilibrium. The nonequilibrium of the gas density data from Raman spectroscopy,4 was found to

be higher than that predicted by the models perhapsPrincipal Scientist, Member AIAA indicating more frozen flow losses than previously

** Senior Scientist, Member AtAA thought.Associate Professor, Member A IAA

Copyright 0 1996 American Institute of Aeronautics and Frozen flow losses, already significant in I kWAstronautics. All rights reserved. No copyright, is asserted in arcjets may be more of a dominant factor in limitingthe United States under Title 17, U.S.C. Thc Government the attainable efficiency of lower power arcjethas a royalty free license to exercise all rights under the thrusters. Investigation into low power arejetcopyright claimed herein for Government purposes. All otherrights are reserved by copyright owner. This paper is operation and how it may differ from higher powerdeclared a work-of the U.S. Government and is not subject to operation is therefore warranted including acopyright protection in the United Stales. / 2005 15

Approved for public release; distribution unlimited.

comparison of how the models predict operation at focused by a 200 mm lens. For the work presentedthese lower powers. here, all data was taken down the axis 'of the arcjet

flow. Radial data from a 1.34 kW arcjet plume alongExperimental Technique and Analysis with a more detailed experimental setup description is

presented in previous work. 3

The experimental technique used to measure atomicdensities and temperatures has been described in The procedure for obtaining absolute number densityprevious papers. 23 Two-photon laser-induced values from the measured fluorescence intensity isfluorescence (2PLIF) at 205 nm is used to probe based .on a method reported by Meier et a]l.s andground state hydrogen atoms at various points in the Tserepi et al. 9 Prior to taking relative number densityarcjet plume. Through measured Doppler shifts, and lifetime data in the lit arcjet plume, the arciet isDoppler widths and calibrated intensities of the translated away from the collection volume with theresulting fluorescence, velocity, temperature and chamber open and a second calibration cell is placednumber density may be inferred. in the detection volume with the laser beam passing

through it. The LIF signal and lifetime is measuredSince any propellant gas is opaque for the VUV for the hydrogen atoms present in the cell. Atoms areradiation required for single-photon excitation of present due to the flow of hydrogen gas through aground state atoms, 2PLIF at correspondingly longer microwave discharge prior to entering the cell. Sincewavelengths is preferred. .The diagnostic technique, the laser beam, detection optics and::electronics havewhich has been. developed primarily for use in remained the same, the LIF from the arcjet and fromflames,''"7 uses two photons at 205 nm to promote the the cell will have the same proportionality to absoluteatoms from the n=l to the n=3 electronic state (the number density after correcting for any differences inLP3 transition). ^ Subsequent 3-2 fluorescence is quenching and window losses:observed (the Hoc transition), as shown in the energylevel diagram in Figure 1. A CN;=' .N . C

The apparatus is shown schematically in' Figure 2. A cThe laser is a pulsed dye laser pumped by a Nd:YAG S and' S are *the signal sizes (integrated over the

Awith a repetition rate of 10 Hz and a pulse width of 6 entire spectral width) from the arcjet and the cell, N•ns. The dye laser output at 615 nm is frequency nd NC

adN are the absolute number densities of atomictripled to achieve about 0.5 mJ per pulse at 205 nm.A mirror turns about 80% of the beam toward the hydrogen in the arcjet and the cell, and CQ is the

arcjet chamber through a variable attenuator placed in scaling factor for the difference in quenching. CQ isthe beam, the remaining 20% of beam energy is given by the ratio of fluorescence decay rate in thedirected toward a microwave-discharge source of arcjet to that in the calibration cell.atomic hydrogen.

The absolute number density of hydrogen atoms in

The laser beam and optics remain fixed, while the the discharge cell is obtained using a standardarcjet is mounted on a motion control x, y, z stage to chemical titration method. A schematic of thetranslate it for probing different regions of the plume, calibration cell used for this method is shown inDetection of the fluorescence takes place with a Figure 3. The cell is made out of glass tubing and isspectrally filtered and temporally gated approximately 30 cm (12 in.) long and 5 cm (2 in.) inphotomultiplier tube collecting light from the side of diameter. Important to the design of the cell is thethe chamber. A calibration cell with microwave long drift tube area with a Teflon tubing liner todischarge source outside th6 chamber provides a reduce gas/wall interactions. This drift tube allowswavelength reference during laser spectral tuning. complete mixing of the titration gas and the gas of

interest. In this case, the titration gas is NO2 and H is

For axial velocity measurements, the beam is sent the gas to be calibrated.directly down the axis of the arcjet flow (Path I) andis focused with a telescope lens configuration (not The hydrogen (with a helium carrier gas) enters theshown) outside the chamber with a focal length on the drift tube area near one end after passing through aorder of 2 m. For radial measurements. the microwave discharge. A dilute mixture of NO, inunfocussed beam can be sent to a turning prism inside helium is added to the flow through a long thin tubethe chamber located underneath the arcjet (Path 2), and enters the larger drift tube region at a locationdirected to pass vertically through the plume, and past the point where the hydrogen has entered. This

reduces the amount of NO, that might diffuse controller for the arcjet as well as the flow meters forupstream and enter the microwave discharge. the discharge cell were calibrated using a wet test

meter.Placing the end of the -tube somewhat closer to thecollection volume than the microwave discharge leads Figure 5 shows a sample spectral scan of theto a better response of signal to NO2 addition. This is hydrogen atom profile from the arcjet and from thebelieved to be due to reduced NO2 upstream diffusion discharge cell, taken with an axial laser beam. Eachwhich, if allowed to become significant, appears to profile is fit to a Gaussian shape using a Levenberg-influence the number of H atoms produced in the Marquardt least squares fit."t The wavelength shift ofmicrowave discharge. Placing the end of the tube the two centers yields the axial velocity, the width offive or six inches downstream of the'entrance of the each Gaussian yields the temperature, and the"atomic hydrogen appears to prevent this effect for the integrated fluorescence intensity is compared to thefollowing flow rates. calibration cell described above to determine a

niumber density value.The NO, reacts rapidly with hydrogen atoms whenthe two are completely mixed in the large drift tube A number of possible uncertainties enter into thisvolume. The'calibration involved flowing I slpm of a analysis. A focused pul sed laser beam creates a very2% hydrogen in helium mixture and then the addition high instantaneous energy density, which is needed toof 2% NO 2 in helium until the signal decreased, excite .the two-photon transition, but also may inducetypically at about .08 slpm of NOJ/He. A small other non-linear processes such as multiphotonvacuum pump brought the cell pressure down to 950 ionization (MPI), partial saturation of the transitionPa (7.2 Torr) during gas flow. (possible, even though the two-photon, absorption

cross section i's very small), and amplifiedThe LIF signal decreases until it is gone at the point spontaneous emission (ASE). Since the transition ofwhere the partial pressure of added NO2 is equal to interest is a two photon one, the unsaturated LIFthe partial pressure of hydrogen atoms in the cell. signal desired is proportional to laser power squared.Control of the amount of NO 2 added "allows the A laser power dependence study is performed todetermination Of the amount of hydrogen atoms verify that the laser power is attenuated enough topresent in the calibration cell. Figure 4 shows data remain in a region of fluorescence proportional tofrom one calibration run. The NO, concentration at laser power squared.the x intercept is equal to the concentration of atomichydrogen present as each hydrogen atom is reacted Another mechanism which can causeaway for each NO 2 molecule added in the fast misinterpretation of the fluorescence is Starkreaction: broadening. Based upon previous analysis, 3 the

maximum Stark effect in the center of the nozzle mayH+NO2 -- OH+NO cause us to overpredict temperatures by 7% and

densities by less than 3%.The large amount of buffer gas and low amount of Hsignificantly reduce possible secondary reactions, Additionally, a small amount of line broadening ismaking them negligible for the purposes of this caused by the linewidth of the dye laser beam which,experiment. Calibration took place just prior to each unlike that of continuous wavelength ring dye lasers,arcjet firing, is non-negligible compared with the atomic linewidth.

Studies have shown that the laser has a width of 0.28All thruster data were taken on a 1-kW-class arc.jet cm"1 or 0.012 A at 205 nm. The effect of the laserdesigned by NASA Lewis Research Center) 0 The width is taken into account using the followingC; 2cathode gap was set to 0.07". The arcjet operated at ielation:1either 134V, 10A (1.34 kW - a control conditionsimilar to previous results) or at 154V, 5.2A (800 W - (Av2 + 2Av, 2)1

(AV (AVD2 +2v2the lower power condition examined in this work)both on a hydrogen gas flow of 13.1 mg/s (8.74 slpm) where Av is the true Doppler width of the line (in

at aner oprain chme pressure ople wdt of 6h Pan (4(inr)cat an operating chamnber pressure of 6 Pa (45 reTort), c1), Av1 is the width of the laser, Av is the measured

A voltage-current curve of our arcjet, was shown in a transition width, and the factor of two is due to the

previous publication 2 and was verified prior to taking two-pholon probe method.

the results for this work. The hydrogen flow

3

Velocity is measured by observing the Doppler shift exponential and the lie time of each is compared.of the absorption line and determining the velocity This allows determination of the quenching effect andrelative to the incoming photons responsible for the what compensation is required to relate the plumeshift. fluorescence to that measured in the calibration cell.

V = -C Numerical Method

where v is the velocity, AX. is the wavelength shift, ?o The direct simulation Monte Carlo method (DSMC)is the zero velocity line center, and c is the speed of uses the motions and collisions of particles to performlight. Determining the line center of a profile with a direct simulation of nonequilibrium gas dynamics..the fitting process discussed earlier is relatively Each particle has coordinates in physical space, threesimple and quite accurate. The uncertainty from the velocity components, and internal energies. Adata analysis is less than the symbol size of the data computational grid is employed to group togethershown in velocity figures. particles that are likely to collide. Collision selection

is based on a probability model developed from basicLow signal to noise of the fluorescence (when low concepts in kinetic theory. The method is widely usedlaser power is used to prevent non-linear effects) for rarefied nonequilibrium conditions and findsmakes measurement of the Doppler width difficult, application in hypersonics, materials processing, andespecially at the edges of the plume, though Doppler micro-machine flows.shift measurements are less affected. Temperature isbased upon the square of the measured Doppler width The extension of the technique for application toand can be written in terms of width in the following arcjet flows is described in detail by Boyd.' Thismanner:13 required development of a number of new physical

models. Arcjets are characterized by highTtemperatures (as high as 30,000 K) which give rise to

0M" -strongly ionized flows (as much as 30% ionized). InM terms of intermolecular collisions, mechanisms must

be included in the numerical model to account for: (1)where 82, is the linewidth due to Doppler broadening, momentum transfer; (2) rotational excitation; (3)2, is the line center, M is the molar mass of the gas, vibrational excitation; (4) dissociation/recombination;and T is the translational temperature. The constant and (5) ionization/recombination. The relatively highin the equation is based upon the values of the speed level of ionization requires modeling of plasmaof light and the molecular gas constant. effects at some level.

Substantially greater uncertainty is associated with For a hydrogen arcjet, the four main species presenttemperature than with velocity due to the square are H2, H, H+, and e. In general, the most importantrelationship between temperature and width. The collision types that determine the arcjet flow field areuncertainty due to scatter can be seen in the H2-H2, H2-H, H-H, H2-e, and H-e. In the presenttemperature plots , which include data from several work, the momentum transfer cross-sections for thedifferent days. first three collision types are obtained from the work

of Vanderslice et al]. 4 in which collision integralsDensity measurements can be made by integrating the were tabulated for the temperature range of 1,000 tofluorescence for each transition and comparing the 15,000 K. The cross-sections for the H2-e collisionarea to the calibrated fluorescence taken previously, is determined using the experimental data ofAs described above, a calibration cell is placed in the Crompton and Sutton.15 The cross-section for the H-etest chamber at the same optical location as where the collision is determined using the simple analyticalarcjet is when it fires. Collisional quenching is expression given by Yos.16 The level of agreementpresent in the arcjet environment and variations in between the models used in the DSMC code and thequenching in the plume as well as differences original sources is very good with the exception ofbetween the arcjet and the calibration cell must be the H2-e collision. Other cross-sections are much lessaccounted for. This is accomplished through important and are assigned values such that the resultsmeasurements of fluorescence lifetime decay at each of hydrogen atoms are used for hydrogen ions. Forlocation where density measurements are taken in the example, the cross-sections for H2-H collisions areplume. Fluorescence decay is also measured in the also used for H2-H+ collisions. While this results indensity calibration cell. The traces are fitted to an some lack of accuracy, it is expected that the effect

4

will be small as these collisions are relatively 'the axis. Expansion is assumed to occur into a perfectinfrequent. vacuum.

Molecular hydrogen is an interesting diatomic The grids employed typically consist of 500 by 55molecule in that it has relatively large values for the cells. Due to the wide dynamic range in density forcharacteristic temperatures of rotation and vibration, these flows, the cells are adapted to the local meanThis results in long relaxation times for these modes. free path, and the time step in each cell is scaled byPreviously, a rate model for determining the the local mean time between collisions. Despite theprobability of translational-rotational energy transfer time step variation, the transient phase of thefor hydrogen at low temperatures was developed and simulation remains long. Typically, it requires at leastverified. 17 This same model is employed again here 15,000 iterations before steady state is :reached.

although it is recognized that it may be inaccurate at Macroscopic results are then obtained by samplingthe very high temperatures encountered in the arcjet. properties over a further 10,000 time steps. At steady

The probability of vibrational energy exchange state, there are- at least 300,000 particles present inemploys the familiar Landau-Teller relaxation time the simulation.with parameters obtained experimentally by Kiefer.'Once again it is recognized that this approach may be Results and Discussioninaccurate at the very high. temperatures found in thearcjet. There are only two chemical reactions Measurements are made.at 1.34 kW and at 800 Wattsimplemented in the present study. They are arcjet power. Both thruster configurations includedissociation of molecular hydrogen and ionization of mass flow of 13.1 mg/s hydrogen propellant andatomic hydrogen. The rate coefficients are obtained identical cathode spacings.from Refs. [19] drnd [20].

Figures 6-11 show nozzle exit atomic hydrogenA very simple scheme is employed to simulate properties at both 1.34 kW and at 800 W powers.electric field effects. The electrons are constrained to The figures are grouped such that density, velocity,move through the flow field with the aeerage ion and temperature profiles may be compared side byvelocity in each cell. This ensures charge neutrality, side with the higher power case on the left and thebut obviously omits the charge field effects. A second lower power case on the right.important plasma effect that is included in the DSMCsimulation is ohmic heating. This is an important Reduction of the power by 40% did not appear tosource of energy that results from the current significantly impact the number density of atoms atrepresented by the flux of charged species through the the center of nozzle exit. Both atomic hydrogenthruster. Using standard plasma physics theory, a density peaks appear to be around IX10' 6 cm-3 and aresource of energy is computed in each cell of the maximum at the center of the exit plane. The 800simulation due to ohmic heating. This energy is added watt profile in Figure 7, however, appears slightlyto the charged particles in each cell in each iteration more peaked than the 1.34 kW profile in Figure 6,of the calculation. The scheme is described in detail falling off an order of magnitude in density within 3.5in) Ref. [1]. mm of center, while the higher power case takes

about 4.5 mm to drop to lxl0'1 cm-3 .The collision and plasma models are implemented inan existing DSMC code that is designed for efficient Note that the model number density predictions bothperformance on v'ector supercomputers.21 The predict ld\kter values than what was measured, but likeDSMC computations are begun at a location the data do not show marked number density changesdownstream of the nozzle throat. The initial profiles with changes in power. The profile shape changeof flow properties are obtained from continuum observed in the experimental data between the twosimulations provided by the code of Butler.-2 2 The power cases is directly predicted by the model withinteraction of the gas with the wall of the nozzle is the more flat profile predicted in the high power caseassumed to be diffuse with ftll energy and the more peaked profile shown in the loweraccommodation to the wall temperature profile power case.obtained from the continuum simulation. Chargedspecies are neutralized upon contact with the wall. Comparing the velocity profiles in Figures 8 & 9, theThe computations extend through the nozzle exit 40% decrease in power for a constant flow rateplane to a distance of 30mm from the thruster along appears to have led 16 a 15% decrease in maximum

nozzle exit atomic hydrogen velocity and about a

5

25% to 30% decrease in velocity at the edges of the property (density, velocity, and temperature) and byvelocity distribution. The model prediction for both power (1.34 kW and 800 W). Both experimental datacases is quite close to the measured velocities and the DSMC prediction. (solid line) are shown inpredicting somewhat lower velocities in the higher each figure. Additionally, a DSMC prediction ispower case and quite closely matching the data given with the result normalized to the experimentalobserved in the 800 watt plume. For the 800 W case, data at the nozzle exit center (dashed line). Thisthe model predicts small increases in velocity near the provides a comparison of DSMC prediction in theouter edges of the nozzle walls. While this is not plume region assuming that the model and the datadirectly observed in the velocity data, the features more closely agreed at the center of the arcjet nozzlewould be well within the observed velocity scatter exit. The prediction of change at a location in theand could be present. plume relative to the nozzle exit conditions are thus

shown in addition to the absolute predictions that takeThe temperature distributions at nozzle exit are hard processes inside the arcjet into account.to accurately interpret using this technique due to thedifficulty in ascertaining accurate Doppler widths. Density distributions measured 10 mm downstream of

:Comparing the two power cases in Figures 10 & 11, a the nozzle exit are shown in Figures 12 & 13 andoverall decrease in average temperature can be seen appear to be slightly affected by the change in power.with decrease in power, though the scatter would The density maximum in the center of the distributionmake a quantitative measure of the decrease tenuous. drops from an average of about 2.4xl01 5 cm3 at 1.34

kW to an average of about 1.7xl0'5 cm 3 at 800 W,The DSMC model predicts a peak temperature of though the scatter and sample size make a conclusionsimilar magnitude to that experimentally observed, on power effect on peak density uncertain. At 5ramthough the predicted distribution appears to. drop in from center, the density does appear to drop as powertemperature with radius faster than the observed data is decreased from about 6x1014 crn"3 to 4x10 14 cm"3, adoes. The scatter in the experimental data is decrease of over 30%, similar to what was observedsignificant enough that it is not able to corroborate at nozzle exit.the predicted higher temperatures as -the radialposition approaches the nozzle walls. The model for The model density predictions at 10mm downstreamthe 800W case appears to underpredict the measured appear significantly lower than the measured values,temperature but the shape of the profile follows the but are extremely close in trend and shape. The ratedata as well as can be discerned in the scatter, of decrease from the center to the wings matches the

measured data almost exactly. The overall magnitudeThe high temperature environment of the arcjet difference already observed at nozzle exit has beennozzle exit often makes the use of diagnostic probes compensaied for in the dashed prediction and for the

or sensors difficult or inappropriate near or on the high power case the data and normalized predictionthruster. Because of this, many optical diagnostics match almost exactly. The 800 W prediction appearshave been conducied in the arcjet plume region in the to predict values lower than the data, but not by as

laboratory and ground- or space-based remote much as the normalization factor, which whenobservation of arcjet plumes are even being . applied, causes the normalized DSMC profile toconsidered for operational flight systems.22 predict higher densities than observed.

Understanding the behavior of the accessible arcjet The drop in peak atomic hydrogen velocity as powcrplume and how it relates to internal arcjet operation is is decreased is almost identical to the drop observedtherefore desirable. The DSMC computational at nozzle exit (from 13 km/s to under II km/s), butapproach is uniquely suited to modeling the plume the distribution is substantially wider at 10 mmenvironment from the arcjet nozzle. to the plume downstream (see Figures 14 & 15). The profile atregion and the following figures explore physical 1.34 kW appears to drop to about 8 km/s at 10 ammquantities at two locations in the plume: a from centerline and 6 km/s at 800 W. This is also adownstream radial distribution 10 mm from nozzle 25% decrease at the wings of the distribution similarexit and an axial distribution along the centerline to what was seen at the nozzle exit. The velocity

from the nozzle to 30mm downstream. profiles from the DSMC model, again show slightly

lower predicted velocities that almost exactly matchFigures 12-17 show radial distributions about the up with the data when normalized to the nozzle exitnozzle centerline 10mm downstream of the nozzle peak velocity. It would appear that for velocity, theexit. Again, the figures are grouped by physical

6

change between nozzle exit and 10 mm downstream vacuum chamber back pressure interaction with thepredicted by the model matches exactly the change expansion at these operating pressures. Both velocityobserved in the experimental data. profiles show essentially constant velocity consistent

with the centerline values previously seen at nozzleThe temperature profiles shown in Figures 16 & 17 exit and at 10 mm downstream. The model predictsare significantly different at 10 mm downstream than that at these pressures the velocities would remainwhat was observed at the nozzle exit in Figures 10 & constant as well, and though lower velocities areI1. The profiles of both powers are substantially predicted, normalization, as would be expected, leadsmore flat downstream and while noisy, both exhibit to very close match between model and experiment.average temperatures of around 700 K to 800 K at The atoms in the plume along the centerline appear toeither of the arcjet powers. The DSMC code, expand as an undisturbed beam of particles.however, predicts, strong variations in temperature atthe two different powers with a more peaked profile Decreases in temperature from nozzle exit into theat the higher power and a more flat profile at 800 W. plume are observed in Figures 22 & 23. In eachThe model prediction normalized to the nozzle exit figure, temperatures decrease with position from the'data does not appear to lead to better agreement observed nozzle exit values to somewhat constantbetween the predictions and the experimental data at downstream values of 700K to 800K. The scatter inthis plume location, the lower power case is again significantly less than

that observed in the higher power case. ModelExamination of the plume expansion along the predictions for temperature along the centerlinecenterline of the thruster axis takes place in Figures l8 exhibit similar decreases. Different from the densitythrough 23. Again, 1.34 kW operation is shown for case, the temperature predictions appear to predictdensity, velocity. and temperature in the even figures lower temperatures than what the measured valueson the left, and .800 W counterparts are shown in the indicate. Normalization of the model results lead theodd figures on the right, trends to match well for the first 10 mm and begin to

diverge as the data is compared farther out in theDensity for both powers is observed' to drop plume.exponentially (somewhat linear on a semi-log plot) Conclusionsthough the decrease is slightly faster in Figure 18 for1.34 kW than irn Figure 19 for 800W. Noticeably Two photon laser induced fluorescence has beendifferent is the. scatter observed at the different successfully used on various regions of an arcjetpowers. The density at the higher power was not as plume at both 1.34 kW and 800 W operation at arepeatable day to day' as was observed at 800 W constant flow rate. Atomic hydrogen ground stateoperation. Though the signal to noise ratio is much densities, velocities, and temperatures have beenbetter at the lower power arcjet operating condition, measured at nozzle exit, 10 mm downstream ofno explanation seems completely viable at this point nozzle exit, and along the centerline from nozzle exitto justify the differences in observed scatter. Note to 30 mm downstream.that at the chamber background pressure of 6 Pascals(45 mTorr), no discontinuities in density that would Comparisons between measured data profiles atindicate a shock formation are observed in the atomic different powers demonstrate that velocity. andspecies density. temperature magnitudes appear to be more affected

by arc jet power level than density peak values. ThisThe Monte Carlo model predictions for the centerline constant peak value of density implies a higherdensities indicate similar decreases in density with fraction of energy lost in frozen flow losses for thelower predicted magnitudes. When normalized with lower power case as a similar amount of energy isthe nozzle exit and plotted using ttie dashed line, the required to dissociate the molecules as in the highertrend for the 1.34 kW case matches tie data well, power case. Overall density profile shapes also areperhaps predicting slightly higher densities in the far observed to change as power is' varied becomingplume. The 800 W case also predicts lower densities more peaked in the center at lower powers.than measured, but the normalized slope appears to Centerline trends in these parameters do not appear todecrease in density less rapidly than the experimental significantly change with power.data indicates.

An advanced Direct Simulation Monte CarloFigures 20 & 21 indicate that no significant computational code has been optimized for use in theperturbations to vclocily are taking place due to arcjet plasma environment including collisional,

7

chemical, and plasma terms to account for the Laser-Excited Fluorescence." Combustion and Flame 71._

processes taking place in the high temperature 41-50 (1988).ofoesse ta rcg opaceration -the additio8. Meier, U., Kohse-Hoinghaus, K., and Just, Th.. "H and 0

environment atom detection for combustion applications: study, ofohmic heating in the vectorized plasma module marks quenching and laser photolysis effects," Chem. Phys. Lett.a significant step in the development of the code. 126, 567 (1986); U. Meier, K. Kohse-Hoinghaus. L. Schafer,

and C.-P. Klates, "Two-photon excited LIF determination

Comparisons between the model and expefimental of H-atom concentrations near a heated filament in a low-pressure H, environment." Appl. Opt. L..9, 4993 (I 990).measurements are generally quite good, Nvith profileprsueHeniom t"Ap.Ot.2 99(10)9. Tserepi, A. D., Dunlop. J. R., Preppernau, B. L., and Miller,trends in data and model matching for most cases. T. A., "Absolute H-atom Concentration Profiles inOverall magnitude differences, especially in nozzle Continuous and Pulsed RF Discharges," J. Appl. Phys. 72_.exit number density, have yet to be fully reconciled No. 7, 2638 (1992).

"and may be the subject of further investigation. 10. Curran F.M., and Haag, T.W., "An Extended Life andPerformance Test of a Low Power Arcjet," Paper AIAA-88-

Model predictions, throitgh normalization with 3106, 24th Joint Propulsion Conference, 1988, New York,respect to the nozzle exit data, appear to describe the New York.plurme expansion extremely well, leading to the 11. Press, W. H., Flannery, B. P., Teukolsky, S. A.. and

conclusion that the model appears to be mature with Vetterling, W. T., Numerical Recipes: The Art of Scientificsback to the nozzle Computing, Ist ed., Cambridge Univ. Press, Cambridge,

respect to relating plume quantities ba1986.exit. 12. Bamford, D.J., Jusinski, L.E., and Bischel, W.K., "Absolute

two-photon absorption and three-photon ionization crossAcknowledgments sections for atomic oxygen," Phys. Rev A 34. 185-198

(1986).13. Demtrdder, W., Laser Spectroscopy, Basic Concepts and

The experimental research is supported by the Air Instrumentation, 3rd Printing, Springer-Verlag, Berlin, 1988.Force Office of Scientific Research (AFOSR) and the 14. Vanderslice, J. T., Weissman, S., Mason, E. A., and Fallon,Phillips Laboratory. The modeling effort at Cornell R. J., "High-Temperature Transport Properties ofUniversity b.is supported byAFOSR grant F49620-94- Dissociating Hydrogen," Physics of Fluids, Vol. 5, 1962, pp.

t 155-164.1-0328. Dr. Mitat A. Birkan is the AFOSR technical 15. Crompton, R. W. and Sutton, D. J., "Experimentalmonitor for both efforts. Investigation of the Diffusion of Slow Electrons in Nitrogen

and, Hydrogen," Proceedings of the Royal Society, Vol.References A215, 1952, pp. 467-480.

16. Yos, J. M., "Transport Properties of Nitrogen, Hydrogen,

I. Boyd, .1. D., "Monte Carlo Simulation of Noneouilibrium Oxygen, and Air to 30,000K," AVCO Research andFlow In Low Power Hydrogen Arcjets," AIAA Paper 96- Advanced Development Technical Memorandum 63-7,

2022, 27"' Fluid Dynamics Conference, New Orleans, LA, 1963.June 1996. 17. Boyd, I. D., Beattie. D. R.. and Cappelli, M. A., "Numerical

2. Pobst, J. A., Wysong, 1. W., and Spores, R. A., "Laser and Experimental Investigations of Low-density SupersonicInduced Fluorescence of Ground State Hydrogen Atoms at Jets of Hydrogen," Joumrl of Fluid Mechanics, Vol. 280.

Nozzle Exit of an Arcjet Thruster," IEPC-95-28, 241h 1'994, pp. 41-67.

Intemational Electric Propulsion Conference, Moscow, 18. Kiefer, 1. H., Journal of Chemical Physics, Vol. 57, 1962, p.Russia, September 19-23, 1995. 1938.

3. Pobst. J. A.. Wysong, I. W., and Spores, R. A., "Laser 19. NIST Chemical Kinetics Database, Version 5.0, NIST,Induced Fluorescence of Ground Slate Hydrogen Atoms at Gaithersburg. MD.

Nozzle Exit of an Arcjet Thruster," AIAA-95-1973, 26'h 20. McCay, T. D. and Dexter, C. E., "Chemical Kinetic

Plasmadynamics and Lasers Conference. San Diego, CA, 19- Performance Losses for a Hydrogen Laser Thermal

22 June 1995. Thruster," Journal of Spacecraft and Rockeis, Vol. 24, 1987,4. Beattie, D. R., and Cappelli, M. A., "Raman Scattering pp. 372-376.

Meaqurements of Molecular Hydrogen in an Arcjet Thruster 21. Boyd, I. D., "\Vectorization of a Monte Carlo Method ForPlume." AIAA-95-1956. 26th AIAA Plasmadvyamics and Nonequilibrium Gas Dynamnics." lournal of ComputationalLasers Conference, San Diego, CA, June 19-22, 1995. Physics, Vol. 96, 1991. pp.

4 1 1-4 2 7 .

5. Goldsmith, J.E.M, Miller, J.A., Anderson, RIM., and 22. Butler, G. W., Boyd, 1. D.. and Cappelli, M. A., "Non-Williams, L.R., "Multiphoton - excited fluorescence Equilibrium Flow Phenomena in Low Power Hydrogen

measuretnents of absolute concentration profiles of atomic Arcjets," AIAA-95-2819, 31st Joint Propulsion Conference

hydrogen in tow-pressure flames," Proceedings of the 23rd and Exhihit, San Diego, CA, July 10-12, 1995.hymposiu intw-euemationa,) onrCombustions Combstin 223. J. Leduc, K.A. McFall, J.A. Pobst, D. Tilley, A. Sutton, L.Inmpostitute (1990r a). oJohnson, and D. Bromaghim, "Performance, Contamination,Institute, 182t (1990), Electro-magnetic, and Optical Flight Measurement

6. Preppernau, B.L, Dolson, D.A., Gottscho, R.A, .and Miller, Development for the Electric Propulsion Space Experiment,"T.A., "Tempotally resolved laser diagnostic measurements Paper AIAA-96-2727, 31' Joint Propulsion Conference,of atomic hydrogen concentrations in RF plasma Orlando, FL, July 1996.discharges," Plasma Chem. and Plasma Proc. 9, 157 (1989).

7. Bittner, J., Kohse-Hoinghaus, K., Meier, U., Kelm, S., andJust, T.H., "Determination of Absolute H AtomConcentralions in Low-Pressure Flames by Two-Photon

8

205 656

2

205

Figure 1. Energy level schematic (not to scale) of the hydrogen atom, showing the 2PLIF diagnostictechnique.

ArciatPath I

or

Lens Pashý2 rPPU Mirors Two e Nd:Yzg

telesentMe LaseSide Niew from PMT added fi. re

-for Path I

Motion TunableSystemt diode LaserContoa j Photo- [Dye

+ CDuabling Calibration Microwave|/

F i g u r e 2. 5 am pT ip lin g Cem nD isehar s

'Collection Lent Crysno Pellin-Broca

L,I ?risurPLenm

t arFilters

vLcnsr---. Digitizer

205nmCotnpster j

Figure. 2. Experimental apparatus.

Detection 12" long Gloss Tube with Tenlon Lionerfo ml

Pressue TapH/i-c mixtsure entens drill

tube after passing throughi microwave discharge

Bfrewster

2 205nnm Laser Beast

Figure 3. Calibration cell schematic for in-situ density calibration.

9

2.5 -

2

1.5

S *

0.5

0 2 4 6 8 10 12NO 2 Partial Pressure (mTorr)

Figure 4. Determination of atomic hydrogen density and emitted signal through titration of NO2into calibration cell.

) 0.8

S0.6

S0.4 Calibrati, Arcjet PlumeS~Cell

0.2

0205.140 205.145 205.150 205.1s5 205.160 205.165

Laser Wavelength (nm)

Figure 5. Sample 2PLIF spectrum of the hydrogen atom, showing the calibration signal from thedischarge cell, the signal from the arcjet plume, and least-squares Gaussian curve fits for each.

10

.00

, I • .''it 800' , IV ,

-6 -4 - 0 2 4 : 6z 2 80 2Nozzle Exit: 1.34 kW 38

DSMCdot e DSIMC, ,cI

. I l 14 . . . . . . . * , , , ,' .1x0 4

-6 -4 -2 0 2 4 6 -6 -4 -2 0 2 4 6

Radial Position (mm) Radial Position (mm)

Figures 6 & 7: H density measurements and predictions at arcjet nozzle exit for 1.34 kW and 800 W

14 14

12 1200• e So

10 10

- 8 ee 8

Nozzle Exit. 1.34 kW2 H data 2Hdt

-- DSM'C Model I

-6 -4 -2 0 2 4 -6 -4 -2 0 2 4

Radial Position (mam) Radial Position (ram)

SFigures 8 & 9: H velocity measurements and predictions at arcjet nozzle exit for 1..34 kW and 800 IV

20*0 2000

*0 S

S S

5000 1000

4I

Nozzle Exit: 1.34 kW N Ext:2 HWdata DSMC Model-DSMC Model

-6 -4 -2 0 2 4 6-6 -4 2 0 2 4Radial Position (mm) Radial Position (Mm)

Figures 10 & 11: H temperature measurements and predictions at arcjet nozzle exit for 1.34 kW and 800 W

2000 200

10 mm Downstream: 1.34 kW 10 mm Downstream: 800 W* H data if01 0 H data1xIO . H DSMC Model 1 DSMC Model

...... DSMC: Normalized to Nozzle Exit data ...... DSMC: Normalized to Nozzle Exit data

S S

4 . *. . .. .

i- . . . .

,,, z :" o / So "

-I0 -5 0 5 10 -10 0 5 10

Radial Position (mm) Radial Position (mm)

Figures 12 & 13:. H density measurements, predictions, and normalized predictions 10 mm from nozzle exit.

10 S10-S

140 .,5. 1x4."

$ O•

> >

4 -_4 "

10 mm Downstream: 1.34 kW 1 i ontem 0 NH • data Hdt

2 DSNIC' Model H2atSDSMC Model------. DSMC: Normalized to Nozzle Exit data ------ DSTNC: Normalized.to Nozzle Exit data

0 . . . . . . . . . . .I. . . ..I . . 0 . . . . I . . .. I . . " I-10 -5 0 5 10 -10 -5 0 5 10

Radial Position (mm) Radial Position (mm)

Figures 142& 15: H velocity measurements, predictions, and normalized predictions 10 mm from nozzle exit.

2000? 20)0010 mm Downstream: 1.34kW 10 mm Downstream: 800 WSH data * 1data

2DSMC Model - DSMC Model- DSMC: Normalized to Nozzle Exit data ------ DSMC: Normalized to Nozzle Exit data

1-500 1500

50 ....

0-

-10 -5 0 5 10 -10 -5 0 5 10Radial Position (mi) Radial Positio.n (mm)

Figures 16 & 17: H temperature measurements, predictions, and normalized predictions 10 mm from nozzle exit.

12

v

Axial Profile: 1.34 kW Axial Profile: 800 WOx10'0 0-- Hdata l16 Hdata

-- DSMC Model "DSMC Model"...... DSMC: Normalized to Nozzle Exit data ...... DSMC: Normalized to Nozzle Exit data

."? • . .

)0"• . ."1.. • ",..

*XOI S U *St x0o __to.o x0s I tC C 0.0 . .0 .,.

• S

lx1014 . . . . •1 4 . . . . . . . ,

0 5 10 15 20 25 30 0 5 10 15 20 25 30

Axial Position (mm) Axial Position (mm)

Figures 18 & 19: H density measurements, predictions, and normalized predictions along the axial centerline.

14 I4 14

"•"012 0 1 '" 2 • e!

10 10 U-- ----

6 6 6

4 4

Axial Profile: 1.34 kW Axial Profile: 800 Wi Hdata H data2 •DSMC Mlodel 2 • DSMC Model

-----. DSMC: Normalized to Nozzle Exit data .. DSMC: Normalized to Nozzle Exit data

10 15 2 25 3 . ... .. . .. I . . . . . . . . . . .0 5 10 15 20 25 30

Radial Position (mm) Axial Position (mm)

Figures 20 & 21: H velocity measurements, predictions, and normalized predictions along the axial centerline.

2500 - 2500Axial Profile: 1.34 kW Axial Profile: 800 W

a H data H Idataa DSMC Model

- DSMC Model2000 -.-.-.. DSMIC: Normalized to Nozzle Exit data 2000 ------ DSMC: Normalized to Nozzle Exit data

1000 C Z 10001,0,,00 100i

s00 '00

0 00 5 10 15 20 25 30

Axial Position (mm) Axial Position frmm)

Figures 22 & 23: H temperature measurements, predictions, and normalized predictions along the axial centerline.

13