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Laser Induced Fluorescence Measurements on a Laboratory Hall Thruster (Postprint)

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6. AUTHOR(S) W.A. Hargus, Jr. and M.A. Cappelli (Stanford Univ.)

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Air Force Research Laboratory (AFMC) AFRL/PRSS 1 Ara Road. Edwards AFB CA 93524-7013

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13. SUPPLEMENTARY NOTES Presented at the 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Cleveland, OH, 13-15 July 1998. AIAA 1998-3645.

14. ABSTRACT In this paper, we describe the results of a study of laser induced fluorescence velocimetry of neutral xenon in the plume of a Hall type thruster operating at powers ranging from 250 to 725 W. Neutral velocities are seen to increase with thruster discharge voltage. There is no evidence for neutrals being accelerated in the near field plume. Velocities appear to remain constant past the cathode plane. In preparation for future ion velocimetry studies, the plume plasma potential profile is measured for a number of conditions. For a low power condition, the plasma potential profile is mapped through the ionization region into the interior of the thruster. For this condition, the electric field profile is calculated. We also find evidence of neutral xenon streaming toward the Hall thruster. These backstreaming neutrals make determination of neutral xenon velocities difficult. We believe the neutrals originate from the thruster plume wall impingement approximately 2 m from the thruster.

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Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std. 239.18

Laser Induced Fluorescence Measurementson a Laboratory Hall Thruster

W.A. Hargus, Jr.* and M.A. Cappelli**Mechanical Engineering Department

Thermosciences DivisionStanford University

Stanford, CA 94305

Abstract

In this paper, we describe the results of a study of laser induced fluorescence velocimetry of neutralxenon in the plume of a Hall type thruster operating at powers ranging from 250 to 725 W. Neutralvelocities are seen to increase with thruster discharge voltage. There is no evidence for neutrals beingaccelerated in the near field plume. Velocities appear to remain constant past the cathode plane. Inpreparation for future ion velocimetry studies, the plume plasma potential profile is measured for a numberof conditions. For a low power condition, the plasma potential profile is mapped through the ionizationregion into the interior of the thruster. For this condition, the electric field profile is calculated. We alsofind evidence of neutral xenon streaming toward the Hall thruster. These backstreaming neutrals makedetermination of neutral xenon velocities difficult. We believe the neutrals originate from the thrusterplume wall impingement approximately 2 m from the thruster.

Introduction

Due to their high specific impulse and highthrust efficiencies, Hall thrusters are now beingconsidered for use on commercial, research, andmilitary spacecraft. This technology provideseconomic advantages for a number of missionsthat can be translated into lower launch mass,longer time on station, or larger payloads [1].

There is a need for an increasedunderstanding of the complex phenomena thatgovern the operation of Hall thrusters. In order tomore fully understand the physics in thesedischarges, several laboratory model Hallthrusters have been constructed at StanfordUniversity. These thrusters serve primarily astest articles for plasma diagnostics. Thesediagnostics include laser induced fluorescence(LIF), probes of various types, and thrustmeasurements [2-6].

Laser based techniques have been developedto nonintrusively probe neutral and ionized xenon[6]. Such measurements in the plumes of Halland other types of ion thrusters provideinformation on the plasma discharge that isrequired to develop insight into the physical

processes occurring within the thrusters. Similaroptical diagnostic measurements have beenpreviously employed to examine plasmaproperties in other electric propulsion devices.For example, die hydrogen arcjet has beenextensively studied using lasers to measurevelocity, temperature, and electron numberdensity [7]. The high spatial resolution ofsingle-point laser-induced fluorescence isessential in probing nonuniform plasmaenvironments such as those in Hall thrusters andother electric propulsion devices.

Theory

Laser Induced Fluorescence

The interaction of a laser beam with aplasma may involve the optical excitation of anumber of atoms to a higher energy state. Theexcitation is more likely to occur if the laser istuned to the energy difference, hv,2, between anupper state and lower state. The interaction canbe investigated by either monitoring the resultingreduction in laser power following propagationthrough the plasma (absorption process), or bymonitoring the subsequent spontaneous emission

* Student Member AIAA** Member AIAA

as the resulting excited state relaxes to a lowerstate (laser-induced fluorescence process).Monitoring the fluorescence as the laser is tunedover the transition provides a measure of thefluorescence excitation line shape and is favoredfor the higher spatial resolution that it affords inthe determination of plasma parameters. Thespatial resolution for LIF is determined by theintersection of the probe laser beam with theoptical collection volume at the probe volume.

If the absorber has a velocity component, u,along the axis of the laser beam, it will absorbthe light at a frequency shifted from that ofstationary absorbers because of the Dopplereffect. The magnitude of this frequency shiftdepends on the velocity along the laser beam axisby

uc (1)

where c is the speed of light. The Doppler shiftof a species' fluorescence profile away from theline center vn of stationary absorbers is inproportion to the species velocity.

The measured fluorescence signal is givenby [8-10]

(2)

where r]d is the efficiency of the detectionsystem, ac accounts for factors involving thecollection system, and A is the Ensteincoefficient for spontaneous emission of therelevant transition.

For low laser intensities, rate equationanalysis indicates that the upper level populationN2 and therefore the fluorescence signal islinearly dependent on Iv at steady state, i.e.,

(3)

where Iv is the spectral irradiance at frequency vand B]2 is the Einstein stimulated absorptioncoefficient, v is the transition's spectral lineshape which accounts for the variation of theabsorption or laser excitation with frequency.The line shape is determined by the environmentof the absorbing atoms, so an accuratemeasurement of the line-shape function can leadto the determination of plasma parameters.

Several factors can affect the line shape andgive rise to a broadening and/or shift of thespectral line. In high temperature plasmas, themost significant of these is due to the Dopplereffect and gives rise to Doppler broadening.Doppler broadening reflects the fact that theabsorbing species is distributed in velocityaccording to the kinetic temperature, T^n. Whenthe absorbing species is characterized by aMaxwellian velocity distribution, Dopplerbroadening results in a Gaussian line shape.

Collisional interactions between theabsorbers and other species in the plasma giverise to spectral line shapes that are oftenLorentzian. This includes interactions withcharged particles (Stark broadening) anduncharged particles (van der Waals broadening).If both Doppler broadening and collisionbroadening are important and independent, theresulting line shape is a convolution of theGaussian and Lorentzian line shape into a Voigtline shape.

The measured fluorescence excitation lineshape is not necessarily equivalent to the spectralline shape. The line shape is an intrinsic propertyof the absorbers, whereas the fluorescenceexcitation line shape is the variation in thedetected fluorescence signal with frequency as thelaser is tuned across the absorption line feature.If the laser excitation significantly perturbs thepopulations in the coupled levels, it is said to besaturating the transition and the fluorescencesignal is then a nonlinear function of laserintensity. In cases where the laser intensity issignificantly below the saturation level and thelaser linewidth is small compared to the measuredlinewidth, the fluorescence excitation line shapereflects the spectral absorption line shape asgiven by Eqns. 2 and 3. When the laser intensityis sufficiently high to saturate the transition, thefluorescence excitation line shape is broader thanthe spectral line shape and the fluorescenceintensity is less than it would be if it were linearwith the laser intensity 7V. The saturationintensity, defined as that intensity Whichproduces a fluorescence signal half of what itwould be if the fluorescence was linear with 7V,depends inversely on the line strength of theparticular line. Stronger transitions have asmaller saturation intensity and thus a largersaturation effect for a given laser intensity.

Xenon Spectroscopy

The ground state of the xenon atom is noteasily accessible to LIF. With available laserwavelengths it is advantageous to probe moreaccessible excited states. The spectroscopy of the6s[3/2]2 -6p[3l2]z transition at 823.2 nmexamined in this study is discussed elsewhere[11]. The conventional Racah notation for theinert gases is used to denote the levelsinvestigated. Briefly, the seven isotopes eachhave a slight difference in their transitionenergies due to their differences in mass. The oddmass isotopes are further spin split due to nuclearmagnetic dipole and nuclear electric quadrapolemoments. The isotopic and nuclear-spin effectscontributing to the hyperfine structuredecomposes the 6si[3/2]£ -6p[3/2]2 transitioninto 21 lines.

As is the case with neutral xenon, theground state of the xenon ion is not easilyaccessible to LIF. The only ionic xenontransition for which the isotopic and hyperfinesplitting constants are known is the 605.1 nmtransition arising from the metastable 5d[3]j/2(4D7,2 ) state. In this case, the combinedisotopic and nuclear-spin effects cause the 605. 1nm transition to have 19 isotopic and hyperfinecomponents contributing to its line shape.Previous work has utilized nonresonantfluorescence at 529.2 nm collected from the

UP?61" state M-

For LIF measurements aimed at onlydetermining the velocity of the plasma flow, it isoften convenient to probe more accessibletransitions for which there is incompleteknowledge of the isotopic and nuclear spinsplitting constants. Manzellahas shown that theionic xenon transition at 834.7 nm,

, can be used to make velocitymeasurements in the plume of a Hall thruster[12-13]. In this case, the combined isotopic andnuclear-spin effects cause the 834.7 nm transitionto have 20 isotopic and hyperfine componentscontributing to its line shape. An additionalconvenient feature of this transition is a strongline emanating from the same upper state. The6.y[2 ]̂ -6/J3]s/£ transition at 541.9 nm allowsfor nonresonant fluorescence collection.

Local electric and magnetic fields can alsoaffect the spectrbscopy of a transition. Amagnetic field splits each fine or hyperfine lineinto a number of lines. An extremely strongelectric field can also cause a splitting of spectrallines. Furthermore, perturbations of an atom'senergy levels by nearby charged particles lead to abroadening and a shift of a spectral line. ThisStark shift has a linear dependence on the electronnumber density, ne, at the probe volume [14].

Plasma Potential Measurements

When an electrically isolated conductor isplaced in contact with a plasma, the conductorwill generally float at a potential different thanthat of the plasma potential [15]. This is due tothe formation of a sheath. The more mobileelectrons are depleted near the surface and anelectron retarding sheath causes the conductor tofloat at a potential lower than the plasmapotential.

One method to determine the plasmapotential is to heat a probe until a sufficientnumber of electrons are thermionically emittedfrom the probe surface to neutralize the sheath.Once the sheath is neutralized, the probe willfloat at the local plasma potential. An advantageof using an emissive probe instead of a sweptprobe is the issue of cleanliness. Oxide layers onswept probes are difficult to remove and distortthe current-voltage characteristic. When heated,emissive probes remove this oxide layer. Theyare thus cleaned in the course of their operation.

The relationship between the plasmapotential and the floating potential can bedetermined for a collisionless plasma byexamining the ion, Ft, and electron, Fe, fluxes.

F, = nsus (4)

(5)

where ns is the plasma density at the sheath edge,us is the drift velocity of the ions at the sheathedge, $f is the surface floating potential, p is theplasma potential, Te is the electron temperature,and ve is the electron mean thermal speed givenby

L.mi (6)

where k is Boltzmann's constant and m is theelectron mass. Several assumptions have beenmade in Eqns. 4 and 5. First, the ion flux isassumed to be constant. Second, the electronflux to the surface is retarded by a sheath. Thissheath is assumed to affect only the electronswhich have significantly lower masses than theions.

An estimate for the sheath edge ion velocityis obtained from the Bohm criterion [15]:

xenon flow rates of approximately 2.3 mg/sresult in chamber background pressures of in theregion of 10 Torr. This indicates that thefacility has a xenon gas pumping speed ofapproximately 2,000 1/s. Propellant flow to thethruster anode and cathode is controlled by twoUnit Instruments 1200 series mass flowcontrollers factory calibrated for xenon. Thepropellant used in this study was research grade(99.995%) xenon.

Hall Thnister

where M is the ion mass and UB is the Bohm driftvelocity. Strictly, Eqn. 7 only applies to acollisionless, field free, stationary plasma.However, it does provide a lower bound estimateof the speed of the ions entering the sheath [16].

In the steady state case of a floatingconductor in a plasma, the difference between theplasma and floating potentials can be determinedby equating the fluxes of Eqns. 4 and 5, andsubstituting Eqns. 6 and 7 for the electron andion speeds. This result is

(8)

With Eqn. 8, the plasma potential difference canbe calculated if the electron temperature isknown. Conversely, if the plasma and floatingpotentials are known, an acceptableapproximation to the electron temperature maybe determined. Finally, if the plasma potentialfield is sufficiently well known, it can benumerically differentiated to yield the electricfield, E.

(9)

Test Apparatus

Test Facility

The Stanford high vacuum test facilityconsists of a non-magnetic stainless steel vesselapproximately 1 m in diameter and 1.5 m inlength. The facility is pumped by two 50 cmdiffusion pumps backed by a 425 1/s mechanicalpump. Base pressure of the facility with no flowis approximately 106 Torr as measured by anionization gauge uncorrected for gas species.Chamber pressures during thruster testing at

The thruster used in this study has 4 outermagnetic windings consisting of 89 mm long,25 mm diameter, cores of commercially pure ironwrapped with 6 layers of 22 gauge insulatedcopper magnet wire. The inner core is also 25mm in diameter and 89 mm in height and has 12layers of magnet wire. A cutaway view of thethruster is shown in Fig 1. The depth of theelectrical insulator is 84 mm. The insulator isconstructed from 2 sections of cast 99.9%alumina tubing cut to length. These two piecesare cemented to a machinable alumina plateattached to the back plate of the thruster withnon-conducting fasteners (not shown). Thisthruster is modification of the thruster presentedin Ref. 5. The primary modification is to theouter magnetic cores which were increased indiameter to 25 mm from 9.5 mm. Thismodification increased the radial magnetic fieldstrength of the thruster significantly, but has notaffected the profile of the magnetic field, or itspeak location. More detailed information on themagnetic field characteristics can be found in Ref.5. One additional modification is a boron nitridelayer on the front surface of the thruster. Thislayer does not appear to affect the operatingcharacteristics of the thruster. However, theboron nitride layer prevents iron from the thrusterbody from sputtering and redepositing on to theinsulating channel.

The insulator supports the 90 mm outerdiameter stainless steel anode. Propellantdistribution through the anode is accomplishedby 32 evenly distributed holes each with adiameter of 0.5 mm. The anode is secured frombelow the insulator by three alumina bolts (notshown) which secure the anode and the insulatorto the thruster body. An isolated stainless steelfeed provides electrical and propellant feeds to thedischarge.

Fig. 1. Cross-section of modified Hall thruster. (A)central magnetic core, (B) outer magnetic cores, (C)anode, (D) insulated acceleration channel, and (E)magnetic windings.

The cathode required to neutralize the ionbeam and support the necessary electric field is anIon Tech, Inc. HCN-252 hollow cathode. It iscapable of supporting a maximum emissioncurrent of 5A. It is mounted to the rear of thethruster on a stainless steel bracket such that theplane of the cathode exit is 20 mm beyond thethruster exit plane.

The anode discharge is powered by aSorenson DCR300-6 laboratory power supplycapable of providing 300 V and 6 A. Thecathode heating element is powered by aPowerstat variable transformer supplying 8.5 Arequired to initially heat the cathode for startupand 4.0 A during thruster operation. The cathodekeeper uses a Sorenson SCR600-1.7 highvoltage power supply which provides 250 V ininitiate cathode startup and approximately 10 Vand 250 mA during thruster operation. Both theanode and cathode keeper power lines areconnected to the discharges through 4 Q powerresistors which act as electrical ballasts duringthe discharge start. The power required for themagnetic circuit solenoids is provided by acurrent controlled power supply producing 100-500mA of DC current.

The thruster is mounted on a two axistranslation system. In the vertical, the thrusterhas a range of travel of approximately 30 cm. Inthe axial, or horizontal, direction, the thruster isconstrained to a total travel of approximately 6cm. Both stages have resolutions on the order of10 }im, although the repeatability is considerablycourser.

Laser Induced Fluorescence

The experimental apparatus used for the laserinduced fluorescence measurements consists of atunable Coherent 899-21 single frequencytitanium sapphire laser. The laser is activelystabilized to provide line widths on the order of 1MHz with near zero frequency drift. Scan widthsof up to 20 GHz can be realized at centerwavelengths between 680 to 1060 nm. Thetitanium sapphire laser is pumped by Coherentsolid state Verdi pump laser. The pump laserprovides 5 W of single mode pump power at 532nm. The laser wavelength was monitored by aBurliegh Instruments WA-1000 scanningMichaelson interferometer wavemeter with aresolution of 0.01 cm4.

Figure 2 shows the experimental apparatusfor the LIF velocimetry measurements. Theprobe beam is directed into the Hall thrusterplume by a series of mirrors. The slightlydivergent beam (1.7 milliradians full angle) isfocused to a submillimeter beam waist at thethruster exit by a 50 mm diameter, 1.5 m focallength lens.

The collection optic consists of a 75 mmdiameter, 60 cm focal length, collimating lens.The collected light is then focused on to theentrance slit of a 0.5 m Ebert-Fastiemonochrometer with a 50 mm diameter, 30 cmfocal length, lens. An optical field stop is placedbetween the two lenses to match the F/# of theoptical train with that of the monochrometer.

The monochrometer is used as a narrow bandoptical filter so that only light from thetransition of interest is collected. With entranceand exit slits full open (425 /

Hall thruster studies [17]. Figure 3 shows aschematic of the probe electrical circuit.

MonochrometcrandPMT _ ThrusterTranslation System

Fig. 2. LJF velocimetry apparatus with laser systemand collection optics.

A portion of the probe beam is split fromthe main beam and passed through a xenon glowdischarge tube, used as an stationary absorptionreference. A silicon photodiode monitors thissignal. The use of a glow discharge tube for thispurpose is only possible for neutral xenon. Theglow discharge does not support a sufficientpopulation of excited state ions for detection.

The LIF signal is collected using a StanfordResearch Systems SRS-850 digital lockinamplifier. The probe bean is chopped by a SRS-540 optical chopper. The absorption signal fromthe stationary reference is collected using a SRS-530 lockin amplifier. Data from the absorptionsignal, laser power output, and the wavemeter arestored on the SRS-850 using 3 available analoginputs along with the LIF signal.

Typical tests consist of a 0.4 cm"1 scan ofthe probe laser frequency over a 10 minuteperiod. The beam is chopped at a frequency of 1.5kHz. Both lockin amplifiers use 3 s timeconstants. Data is sampled at 2 Hz, producingfour traces of approximately 1,200 points foreach velocity data point.

Plasma Potential Probe

The plasma potential probe is constructedfrom 150 //m diameter 2% thoriated tungstenwire. The filament is formed by winding thethoriated wire around a 0.9 mm mandrel for 8turns for a total length of approximately 1 cm.The filament is suspended between two 0.50 mmdiameter tantalum wires with an approximateseparation of 5 mm. The tantalum wires aresheathed in 1.25 mm diameter alumina tubes.The probe is similar to those used in previous

alumina sheathed/tantalum leads 10:1 Step DownTransformer

ProbeFilament

120 VACWall Current

Fig. 3. Plasma potential probe circuit schematic.

The probe is heated using a 10:1 step downtransformer powered by a 10 A Powerstat manualvariable transformer (Variac) which is connectedto wall current (120 V, 60 Hz). This transfersAC power to the probe while allowing the probecircuit to float at the plasma potential.

The probe filament is placed inside thevacuum chamber on a translation stage with a 15cm range of motion along the thruster's axialdirection. This allows the thruster to move inthe vertical direction and the probe to moveaxially into and out of the thruster plume. Theisolating transformer and the variable transformerare located outside the chamber. The voltage ofthe floating portion of the probe is monitoredrelative to ground by a hand held DC digitalvoltmeter (>100 kQ impedance). The probecurrent is monitored with a hand held ACammeter to ensure repeatable results.

The test procedure consists of placing theprobe in the required location. The floatingpotential is recorded, the probe current is raised to4 A, and then the heated probe potential isrecorded The procedure is then repeated.

Results and Analysis

Operating Conditions

The Hall thruster was operated at fourconditions. At each of these conditions, the peakmagnetic field was measured to be approximately175 G, the mass flow to the anode was 2 mg/s,mass flow to the cathode was 0.3 mg/s. Thefour conditions corresponded to dischargevoltages of 100, 160, 200, and 250 V. The

anode currents for these conditions were 2.1, 2.4,2.6, and 2.9 A, respectively. The total powerconsumed by the cathode and magnet circuit wasapproximately 30 W. It should be noted that thepower dissipated in the ballast resistors on theanode and cathode keeper lines (~10 W) are notincluded in these calculations.

Neutral Xenon Velocimetry

Figure 4 shows exit plane velocitiesmeasured at the center of the discharge channelfor a number of anode voltages. The samplevolume for all neutral data presented in this workis approximately 100/«n in diameter and 6 mmin length. This length was required for adequatesignal strength. Background emission wasapproximately a factor of 300 greater than theLIF signal. As seen in Fig. 4, the neutral exitvelocities increase with discharge voltage. Thisbehavior is likely due to two phenomena. First,as the discharge power increases, the anodeshould rise in temperature. If the neutral flow isindependent of the ionic flow, the neutrals wouldat best reach sonic velocities which wouldincrease with the square root of the anodetemperature. On the other hand, if the ions arecolliding with the neutrals in sufficient numbers,the neutrals may be accelerated by collisions withthe ions. If so, the velocity of the neutrals willdepend on the velocity of the ions which varieswith the square root of the discharge voltage.With thruster power primarily a function of thedischarge voltage, the ability to distinguishbetween these two processes is impossiblewithout temperature measurements of the anode.

Shown in Fig. 5 is a portion of a exit planeradial profile for a discharge voltage of 160 V.The profile is relatively constant up the edge ofthe discharge channel at 6 mm. When thevolume 3 mm beyond the edge of the accelerationchannel is examined. It reveals an interestingresult. The correlation between the stationarydischarge implies that the velocity isapproximately 0 mis. A closer examination ofthe LIF trace reveals that the peak may consist oftwo almost superimposed peaks. This wouldindicate that two distinct velocity classes ofneutrals exist. The fit of two such signal to thedata implies that the two velocities on the order230 and -150 m/s. These are indicated on Fig. 5by the round symbols.

700.

600•^500-

g400-

:>300-

§200-

100-

0 i100

I200

I300

Anode Voltage (V)Fig. 4. Channel centerline velocities at a number ofdischarge voltages.

500 ̂

|~400 J

p

1.0 -i

1 1 I I0.0 0.1 0.2 0.3 0.4

Scan Length (cm )Fig. 6B. Progression of LIF line shift evolutionwith distance from exit plane: possible bimodalvelocity behavior at 7.5 mm. Voltage discharge of200 V.

Scan Length (cm )Fig. 6C. Progression of LIF line shift evolutionwith distance from exit plane: negative velocity at15 mm. Voltage discharge of 200 V.

l.O-i

I 0.8-

•e 0.6-£• 0.4-V)fla 0.2-

0.0-J

D

r i i i i0.00 0.05 0.10 0.15 0.20 0.25

Scan Length (cm1)Fig. 6D. Extreme case of bimodal behavior seennear the exit plane for a discharge voltage of 250V.

Once this unexpected shift was found in theplume, one possible source could be dismissed.Near the thruster exit, the magnetic and electricfields would be the strongest. If either Zeeman,or Stark splitting of degenerate lines were tooccur, it would occur near the thruster. Thepoint in Fig. 5 at 9 mm from the channel centeris near to the inner magnetic core and the initialconcern was that Zeeman splitting wasappearing. With this same behavior, now

occurring away from the thruster, the conclusionreached is that there are two relatively distinctpopulations of neutrals with diametricallyopposed velocities.

Before proceeding further, the LIF lineshapes in Fig. 6 need some explanation. Theexcitation line shapes are distinctly more broadthan the absorption line shapes. This is due topartial saturation of the transition by the laserprobe beam. The laser system was capable ofproducing 200 mW at this wavelength. For thedata presented, neutral density filters were used toattenuate the laser output. The power deliveredto the probe volume was generally on the orderof 2 mW. Laser powers less than this did notproduce a recognizable signal that could becorrelated with the stationary reference absorptionline shape to determine a velocity.

~ 400-i•5 300-

1 200-D>

l100^3 o-caS-100-

-200-

-300

Thruster Exhaust

Reverse How

0 305 10 15 20 25Downstream Distance (mm)

Fig. 7. Axial near field plume velocities at adischarge voltage of 250 V.

The highest discharge voltage, 250 V,produced the highest neutral velocities. Thepopulation of the reversed flow was also larger asevidenced by the bimodal peak visible near theexit plane in Fig. 6D. The larger magnitude ofboth velocities shifted the two superimposedexcitation line shapes further apart. Estimates ofthe thruster exhausted neutral velocities in theplume are shown in Fig. 7 for a dischargevoltage of 250V. The data were calculated bysubtraction of the reversed flow contribution tothe LIF excitation line shape. This contributionwas modeled by use of an unsaturated line shape.There is much uncertainty in this procedure. Nocorrections were made for the partial saturation ofthe excitation line shape. The velocitiescalculated in this manner are presented along witha number of points near the exit plane whichexhibited little evidence of the reverse velocityneutrals. Also included in Fig. 7 are several ofthe calculated reverse velocities at this condition.

8

These appear to have a nearly constant velocityof approximately -200 mis. Although error barsare not shown in Fig. 7, the uncertainties in thevelocities are on the order to 25% near the exitplane and increase further in the plume as thereversed velocity neutrals begin to dominate theLIF signal.

The source of the reversed velocity neutralsis of considerable importance, that it may beeliminated. During the later portion of the testsperformed for this paper, it was discovered thatthe plume of the thruster was extending into aportion of the vacuum chamber over a diffusionpump. During this time, the laser window usedin mis study was coated with decomposeddiffusion pump oil. In addition, the pumpingspeed of the vacuum system was degraded. Aconical stainless steel baffle was placed before thediffusion pump to deflect the ion beam awayfrom the window. Later analysis of LIF datataken with the baffle in place indicates that thebaffle increased the amount of back flowsignificantly. It is important to note that theback flow was evident before the placement ofthe baffle, although to a lesser extent. It istherefore, almost certain that the xenon neutralback flow is due to reflected plume flow off thewall, and the baffle.

Plasma Potential Measurements

Plasma potential measurements wereperformed for two reasons. First, to explore thestructure of the electric field near the thruster todetermine whether the features seen in the LIFvelocimetry data were in fact due to Stark effects.The second and most important reason was todetermine the plasma potentials of the thrusternear plume flow field in preparation for ionvelocity measurements. Since the Hall thrusteris an electrostatic accelerator, knowledge of theplasma potential is important to understandingwhere the propellant acceleration occurs. Thecorrespondence between the ion velocitymeasurements and the plasma potentialmeasurements will provide an indication of theefficiency of the propellant acceleration process.

Figure 8 shows the plasma potentialmeasurements. The measurements were made inthe center of the acceleration channel and extendfrom approximately 50 mm into the plume, tothe thruster exit plane. Just inside the exit plane,the probe experienced severe heating. One of thealumina sheaths would usually becomeincandescent. At this point, the unheated probepotential would rise to near the plasma potential

as the probe was heated by the plasma. Theprobe was removed promptly from within thethruster when this occurred. Examination of theprobe after these incidents, revealed that thealumina had become semi-molten. For the 100V discharge voltage case, the probes did not heatand measurements were taken until the plasmapotential leveled.

120-i

0-,40-^

20-

0 I 1 1 1 1 1 I 1 1 1 1 1 1 1 1 1 1 1 1 1 i 1 1 1 1 1 1 1 I 1 1 1 1 1 1 1 i 1 1 I 1 1 1 i-30 -20 -10 0 10 20 30 40 50 60

Distance from Exit Plane (mm)Fig. 8. Plume plasma potential data at dischargechannel center.

The plasma potential is nearly constant inthe plume and rises significantly near the exitplane. For the 100 V anode potential case, it ispossible to numerically differential the plasmapotential data to produce the profile of the electricfield shown in Fig. 9. The electric field peaks atthe same location as the magnetic field, althoughthe electric field profile is narrower [5].Examining Fig. 8, the plasma potential for thiscase appears to level out near 85 V, 15V belowthe anode potential. This is consistent with theVon Engle theory which predicts that the anodefall of a glow discharge is somewhat greater thanionization potential of the gas in question(Xenon-12.2 eV) [18].

E

3.0-

2.5 -

2.0 -

•— 15-u_ i -o noI 1.0 H

Using floating and plasma potentials, anestimate of the electron temperature can becalculated by using Eqn. 8 as shown in Fig. 10.The electron temperatures quickly rise with theplasma potential. With only the exception of the100 V case, the electron temperatures rise abovethe xenon ionization potential near the exitplane, implying that ionization may be occurringin this region.

I jl I I Ij I I I I J I I I I I I I I I j l I I I ( I I I Ij I-30 -20 -10 0 10 20 30 40 50 60

Distance from Exit PlaneFig. 10. Electron temperatures calculated fromfloating and plasma potentials using Eqn. 8.

For the 250 V discharge voltage case, aradial profile of the plasma potential across theacceleration channel and approximately 13 mmdownstream of the exit plane is shown in Fig.11. The electron temperature follows the plasmapotential closely. Similar structure has beenseen further into the plume as well.

50-

>40-

330-

520-

10-

0.

I

a

I I I T I-30 -20 -10 0 10 20 30

Distance from Channel Center (mm)Fig. 11. Cross-section of plasma potential andelectron temperature across the acceleration channelat a distance of 13 mm from the exit plane at adischarge voltage of 250 V. Note that theacceleration channel is 12 mm wide.

The plasma potential data confirms thatwhile large electric fields do exist in the Hallthruster, they do not reside in the plume and assuch are not responsible for the distorted LIFexcitation line shapes. The plasma potential datahas given an indication of the energies of thexenon ions. Therefore, the ionic velocities canbe approximated and used as a starting point forLIF velocimetry.

Ionic Xenon Velocimetry

Measurements of ionic xenon velocities havenot been completed. The 824.7 nm transition isaccessible with the same laser and optical systemas the neutral 823.2 nm line. At this time, onlya small amount of work has been completed.For example, Fig. 12 shows a LIF excitationsignal of the 834.7 nm transition. These datawas taken at a discharge voltage of 85 V. Theline shape is shifted approximately 0.29 cm"1,corresponding to a velocity of approximately 7.2km/s. This transition is useful because of itsaccessibility, but in order to determine the ionictemperature, the ionic transition at 605.1 nm,where the isotopic and hyperfine splittingconstants are known, must be used.

_ I.O-iIn•E 0.8-1Z)

4 0.6-

5 0.4-•M

1 0.2 H\

0.0 0.2i

0.4I

0.80.6Scan Length (cm")

Fig. 12. LIF trace of ionic xenon transition at834.7 nm.

Conclusions and Future Work

From the data available, the neutral velocitydoes not appear to increase in the near fieldplume. The neutrals appear to exit at velocitiesbetween 300 and 600 m/s. The velocities appearto remain relatively constant, at least until thecathode plane at 20 mm. Reliable neutralvelocity measurements any further into theplume are not possible at this time. Themagnitude of the neutral velocity corresponds toprevious work, but it is now believed that thedeceleration of the neutrals evident in those

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measurements was caused unaccounted by reverseflowing neutrals [6].

The backstreaming neutrals are believed tobe a specular reflection of the recombined thrusterion beam. It should be possible to minimizethis effect by changing the orientation of thesurfaces upon which the plume impinges. Thisshould allow the unambiguous determination ofneutral velocities in the plume. The question ofwhat effect this back flow has on the thrusteroperation must also be answered before furthermeasurements can be performed.

The plasma potential measurements weresuccessful in measuring the plasma potentialcurves in the near plume. However, the inabilityof the probe to enter the thruster at higherdischarge voltages is an issue will be pursued.Future efforts will include a more robust probeable to survive entrance into the accelerationchannel at these conditions.

A possible solution to some of the facilityissues will be the new high vacuum facilityconstructed for our laboratory. It consists of astainless steel vacuum vessel, 1.3 m in diameter,and 3 m in length. It is pumped by two 1.2 mcryo-pumps with a combined rated speed onxenon of 67, 0001/s at 5x10"* Torr. This facilityis expected to be fully functional by October1998.

Acknowledgments

This work is supported by the Air ForceOffice of Scientific Research. W.A. Hargus, Jr.is supported under the Air Force Palace KnightProgram.

References

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