1

Experimental Performance Evaluations of Mo and ZrC-Coated Mo Field Emission Array Cathodes in Oxygen

Environments1

Colleen Marrese-Reading and Jay PolkJet Propulsion Laboratory (JPL), California Institute of Technology

M/S 125-109, 4800 Oak Grove Drive, Pasadena, CA 91106818-354-8179, colleen.m.marrese@jpl.nasa.gov

818-354-9275, james.e.polk@jpl.nasa.gov

Kevin L. JensenNaval Research Laboratory, Code 6841, ESTD

Washington, DC 20375-5347,202-767-3114, kevin.jensen@nrl.navy.mil

IEPC-01-278

This performance of Mo and ZrC-coated Mo field emission array cathodes in oxygen environmentsis discussed in this article. These results were obtained in the first year of a program to develop fieldemission cathodes to operate in low Earth orbit (LEO) environments for electrodynamic tether systems. Amodel is described that enables the comparison of the self-generated atomic oxygen fluxes in a molecularoxygen environment to the flux of atomic oxygen in LEO. The results of this model for experimentsperformed are presented. This report describes the experimental results obtained from evaluating theperformance of Mo and ZrC/Mo cathodes in oxygen environments. The results show that the performanceof ZrC-coated Mo field emission array cathodes is more stable than Mo field emission array cathodes, butthey suggest that neither material has acceptable stability in atomic oxygen environments.

1 Presented as Paper IEPC-01-278 at the 27th International Electric Propulsion Conference, Pasadena, CA, 15-19 October, 2001.

IntroductionField emission cathodes are under development to becompatible with the performance and lifetimerequirements of several space-based applications toreplace hollow and thermionic cathodesconventionally used to improve power, efficiency,mass and system complexity. Electrodynamic tethers,colloid thrusters, FEEP thrusters, micro ion enginesand Hall thrusters should significantly benefit fromfield emission cathodes compatible with their currentand lifetime requirements in the spacecraft andlaboratory environments. These applications willrequire field emission cathode materials with a highresistance to oxidation and sputtering by ionbombardment. The cathode architecture must providecontrol over the electron energy independent of theelectron extraction voltage to satisfy the lifetimerequirements and improve the space charge current

limits. [1] The cathode architecture should alsoprovide current limitations to protect them againstcatastrophic performance degradation from arcing anddebris impact to meet the demands on cathode lifetimein the space environment.

The current requirements for the tether and thrusterapplications range from 10 mA/cm2 to 200 mA/cm2 at<10 mW/mA for thousands of hours. The operatingenvironments can differ significantly and depend onthe laboratory and spacecraft environments. Duringground testing in simulated application environments,the cathodes must operate in oxygen partial pressuresgreater than 10-9 Torr, and even this environment canaffect the performance of these cathodes. Mostcontaminants can cause changes in the work functionof the tip material and/or the electric field distributionon the tips.

2

The field emission cathode current is exponentiallydependant on the work function and electric field onthe emitting tips. [2] Therefore the performance of thecathode is very sensitive to its operating environmentbecause it can cause changes in its work function andtip radius of curvature. Chemisorption orphysisorption of oxygen in these environments canincrease the cathode work function and decrease thesurface conductivity, causing temporary andpermanent performance degradation. Ambient ions orions generated between the tips and gate electrode cansputter the tips to increase the tip radii and decreasethe geometrically enhanced electric field at the tips.This environmental effect results in permanent cathodeperformance degradation.

The cathodes must be compatible with atomic oxygenenvironments for operation with electrodynamictethers in LEO. Some cathode performancedegradation in atomic oxygen may be acceptable if therequired current can be maintained by operating athigher voltages or turning on additional cathodes.However, the operating voltage will be limited so thatthe energy of the ions created between the tips andgate and downstream of the cathode will not exceedthe threshold for sputtering the cathode material. Theperformance degradation rates depend on the cathodematerial, operating voltage, cathode geometry,constituents of the environment and contaminationrate. Mo is the most common field emission arraycathode material with the most well developed andreliable fabrication processes. Therefore,wecharacterized its stability in oxygen even thoughpreviously reported results were not promising. [3,4]ZrC was identified as potentially more stable materialin oxygen because of promising results obtained atwith single crystal tips at Linfield Research Institute.[5,6]

Evaluating the performance of various cathodematerials in simulated LEO environments ischallenging because atomic oxygen is much morereactive than molecular oxygen and much moreexpensive to supply for contamination studies. Thesimplest cathode stability experiments have beenperformed by operating the cathode in molecularoxygen environments. Concerns about theapplicability of these results to understanding theperformance of the cathodes in a LEO environmentmotivated the development of the model presented inthis article. This model is used to estimate the flux of

atomic oxygen to the cathodes which is self-generatedby the interactions of the electron beams with themolecular oxygen environment.

This article presents the model developed tocharacterize the laboratory environment, a comparisonof the laboratory environment to the LEOenvironment, and the results from experimentalcathode performance evaluations in oxygenenvironments.

Laboratory Environment

The LEO environment is described in the Table 1.The most dangerous constituent of this environment iscurrently believed to be atomic oxygen (AO). AO isextremely reactive, capable of changing the workfunction and conductivity of the cathode surface if thematerial is not resistant to oxidation.

Table 1. Number densities of LEO constituent gases at300 km.

n (cm-3)O 109

O2 2x107

O+ 106

N2 108

He 106

H 104

To evaluate the compatibility of the cathode with thisenvironment, either the cathodes need to be tested inthe LEO environment or tested in an environment witha comparable flux of AO to the emitting area of thetips. AO sources in vacuum systems can simulate thefluxes and energies of AO which the cathodes wouldexperience in LEO. A unique aspect of testing FEAcathodes in molecular oxygen environments is thatthey self-generate a substantial population of AO andionized molecular oxygen by electron bombardment ofmolecular oxygen and collisions of ionized oxygenwith the field emission cathode surface. This uniqueaspect of these experiments is exploited to performrelatively low cost and quick cathode sensitivity tests.

The fluxes of ionized and atomic oxygen to theemitting areas of the tips were calculated using themodel included in Appendix A. The following threereactions take place in this environment to contributeto an effective flux of atomic oxygen to the cathode:

3

1) O2 + e -> O2+ + 2e

2) O2 + e -> 2O + e3) O2 + e -> O+ + O + 2e

Critical to this model is a statistical representation ofthe cathode emission distribution in an array of tips.The Jensen performance model uses the cathodegeometric and material characteristics withexperimentally acquired I-V data to determine thenumber of tips carrying the current load, the currentper tip, the effective tip radius of curvature, and thedistribution of tip radii in an array. [7,8] The fluxmodel provides first order estimates of the fluxes ofvarious oxygen species. It was assumed that oxygenatoms participate in only one reaction, therefore, oncemolecular oxygen is dissociated into 2O, those oxygenatoms could not later be ionized also. The majority ofthe oxygen bombarding the tips is ionized becausethese ions are accelerated to the emitting area of thetips by the local radial electric field. Withinapproximately 1 µm from the tips, all ions generatedbombard the emitting tip area while little of the neutralmolecular and atomic oxygen reach it. [9] Becauseionized oxygen molecules are bombarding the tipswith significant energies, they are highly reactive withthe surface. With molecular oxygen energiesexceeding a few eV, they will dissociate on impact andprovide highly reactive atomic oxygen to the surface.[10]

LEO and laboratory environments are described inTable 2. A cathode could be positioned in the ram,wake, or somewhere between these regions on aspacecraft. The cathode will not be positionedintentionally in the spacecraft ram because of thehostile environment with the highest probability ofdebris impact. This environment is described in Table2 as LEO RAM. The extremely low pressure in thewake of a spacecraft will prohibitively limit the space-charge current density. [11] The LEO environmentdescribed in columns 2 & 3 of Table 2 is the mostlikely environment for the FEA cathodes on aspacecraft in LEO. The fluxes of AO to the emittingarea of the tips was calculated with the neutral particlenumber densities, thermal energies, kinetic energies,and the emitting area of the tips.

We have used the model with experimental data toshow that operating FE cathodes in molecular oxygenenvironments in our laboratory produces AO fluxesthat are comparable to the flux of AO in LEO or in alaboratory facility with an AO source. The caveat is

that the cathode has to have decent performance with100 µA at ~50 V. If much higher voltages are requiredthe cathode could be sputter damaged during theexposures. The modeling results show that the self-generated flux of AO to the emitting area of the tips iscomparable to the flux of AO in LEO with the rightcombination of current per tip emitting and oxygenpressure. The biggest difference is the energy of theimpinging oxygen. This approach to simulating LEOis effective because the only environmental effect inLEO that will impact the cathode performance are thechanges that occur in the emitting area of the tip.These changes include work function, geometry, tipradius of curvature, and conductivity. The self-generated flux to the emitting area of the tip iscomparable to the AO flux in LEO because of theradial electric field focusing ions near the tips. Onlythe emitting tips self-generate this environment, butthis scenario is sufficient for our investigations.

Experiments have been conducted with the Lab Ia andIIa pressure and conditions, as described in Table 2.These experiments will be discussed later in thearticle. The cathode characteristics defined by the LabIa scenario describe the Mo cathode with the initialperformance shown in Figure 7. The cathodecharacteristics defined by the Lab IIa scenario describethe ZrC coated Mo cathode before oxygen exposures,with performance shown in Figure 16. Lab Ia andIIa environments have the same molecular oxygenpressures. In each environment, the cathodes areoperating at 50 V with the total current, Itot, the currentper emitting tip, Itip, number of tips emitting, Ntipsemitting,determined using Jensen’s model. The flux of atomicoxygen to the emitting tips (2ΦO2+ΦO) is alsodescribed in these columns as determined with themodel developed and described in Appendix A.Ntipsemitting identifies the number of tips which arecarrying the majority of the current in the arrays. Inthe Mo cathode test conditions described in the Lab Iaenvironment, only 120 of the 50,000 tips in the arrayare carrying the current. In 10-7 Torr of oxygen, theflux of atomic oxygen to those tips is 0.361 /s. Withthe effective tip radius identified for this cathode as2.28 nm, the flux of atomic oxygen in LEO is 1.5/s asshown in Table 2. Data in Lab Ib column show thatthe flux of atomic oxygen to the tips can be increasedto 3.61 /s by increasing the pressure from 10-7 to 10-6

Torr. This flux in the laboratory environment exceedsthe flux in LEO. The effect of oxygen ion energieson these results should also be determined.

4

The same result is shown in for the cathode describedin LEO II, Lab IIa and Lab IIb. This ZrC/Mo cathodehas significantly different operating characteristics andtip radius as identified with the Jensen model whileoperating at the same voltage and the same number oftotal tips in the array-50,000. In 10-7 Torr the atomicoxygen flux is 0.4 in the Lab Iia scenario. The flux toa tip with the same radius of curvature will be 5.5 /s inLEO. The flux of atomic oxygen in the laboratoryenvironment is linearly dependent on I/tip andpressure of molecular oxygen, as shown in Eqn 13.Therefore, increasing the pressure to 10-6 Torrincreases the flux of atomic oxygen by one order ofmagnitude so that the flux of atomic oxygen in thelaboratory environment is much closer than the flux inLEO. With slight increases in pressure and/or Itip, theflux in the laboratory will exceed the flux of atomicoxygen in LEO. These results could be presented moreeasily as flux densities, showing that the tip radius ofcurvature only slightly affects the results of the model;It influences the maximum radial distance from whichan ion created will impact the emitting area of thecathode tips.

The results of the atomic oxygen flux model show thatthe environment that is generated in the laboratory can

be identified and used to interpret results from theoxygen exposure experiments. The oxygen exposurescan be compared to the application environment toidentify how the cathodes responded to anenvironment which is more or less damaging than theLEO environment. This tool is useful at this stage inour program when many cathode materials are underinvestigation to identify the most appropriate material.The laboratory experiments in which AO is self-generated in the molecular oxygen environment canpresent a slightly more hostile environment than LEO.In this case the cathode is forced to perform in a worstcase scenario, but not extremely different than theLEO environment. Stable performance demonstratedin these experiments ensures stable performance in theLEO environment, which can be subsequentlydemonstrated in a vacuum system with a 5 eV atomicoxygen beam bombarding the cathode. Conversely,some experiments will generate a flux lower than theflux in LEO. A cathode demonstrating unacceptablestability in this environment will probably demonstrateeven worse performance in LEO. This result dependson the sensitivity of the sticking coefficient of oxygento the cathode material at energies up to 50 eV.

Table 2. A comparison of the LEO (300 km) and laboratory environments.LEORam

(300 km)

LEO I(300 km)

LEO II(300 km)

LabIa

LabIIa

LabIIb

LabIIb

nO (/cm3) 109 109 109

PO (Torr) 10-7 10-7 10-7

nO2 (/cm3) 2x107 2x107 2x107 109 109 1010 1010

PO2 (Torr) 2x10-9 2x10-9 2x10-9 10-7 10-7 10-6 10-6

nO+ (/cm3) 106 106 106

Te(eV) 0.1 0.1 0.1KE (eV) 5 - - <50 <50 <50 <50TI(eV) 0.1 0.1 0.1ΦΦΦΦO2 (#/s) 0.52 0.02 0.07 1.64 1.85 16.4 18.5ΦΦΦΦO (#/s) 36.2 1.5 5.42 0.33 0.37 3.3 3.72222ΦΦΦΦO2 + ΦΦΦΦO

(#/s)37.24 1.54 5.56 3.61 4.07 36.1 40.7

rtip (cm) 2.28x10-7 2.28x10-7 4.39x10-7 2.28x10-7 4.39x10-7 2.28x10-7 4.39x10-7

Itip (A) 9.2x10-8 2.8x10-8 9.2x10-8 2.8x10-8

Ntipsemitting 120 6000 120 6000Itot (A) 1.1x10-5 1.67x10-4 1.1x10-5 1.67x10-4

Vg 50 50 50 50∠ (°) 37 37 37 37 37 37 37

5

Cathode Experiments: Performance Sensitivity toOxygen Environments

Experimental ApparatusCathodesThe field emission array cathodes were fabricated atSRI Int. and Aptech. The cathode configuration isshown in Figure 1a. A single element of a fieldemission array (FEA) cathode is shown in Figure 1b.The Mo FEA cathode was developed at SRI Int. ZrCfilms were deposited on the FEA cathode at Aptech.[5] The thickness of the films is approximately 20Å.The height of the tips is ~1 µm and the gate apertureradius is 0.45 µm. The tip-to-tip spacing on thecathodes tested was 4 µm. The resistivity of the Siwafer was 2000 Ohm-cm. The arrays of tips consistedof 50,000 tips in a 0.78 mm2 circular pattern area.

a) b)Figure 1. a) SRI Int. Spindt-type field emission arraycathode package and b) a single tip in the array(Courtesy of SRI Int.).

Vacuum FacilityAn ultra high vacuum (UHV) facility is employed inthese experiments. The facility is pumped by aturbomolecular pump, a sublimation pump, and anionization pump to attain base pressures as low as7x10-11 Torr. Two UHV-compatible leak valves feedin oxygen and xenon for elevated pressure exposuretests. The cathode test flange is shown in Figure 2.The electrical schematic is shown in Figure 3.Cathode current through the base, gate and anode aremeasured with picoameters. A 1 MOhm resistor onthe gate electrode regulates gate voltage if the gatecurrent is excessive and helps to prevent arcingbetween the tip and gate electrode. A data acquisitionsystem is employed to record and store theexperimental data.

Experimental Results: Effect of the OxygenEnvironment on Cathode PerformanceMo and ZrC-coated Mo cathodes were tested in UHVand oxygen-rich environments at JPL to determine the

effect of oxygen on their performance. Specificobjectives included determining the emission stabilityof the cathodes while operating in oxygenenvironments and the upper limit on operatingvoltages to avoid sputtering the tips by oxygen ionbombardment. The results of experiments performedearlier at Linfield Research Institute showed thatsingle crystal ZrC tips were not easily damaged duringoperation in oxygen environments at voltages >1kV.[12] Those results motivated the investigationsdiscussed in this article. The data obtained from manyexperiments with Mo and ZrC/Mo cathodes in oxygenenvironments are included in Appendix B.

Figure 2. The cathode test flange in the facility at JPL.

-+ 15 V

A + -

A + -

Deflector

Anode

TO-5 Head

Gate

Cathode base

A 1 MΩ

100 kΩ

Figure 3. The cathode testing electrical schematic.

The experimental results show that the Mo andZrC/Mo FEA cathodes did not demonstrate stableemission in oxygen-rich environments. The cathodeswere operated in UHV environments until emissionstabilized before oxygen was introduced into thesystem. Typically oxygen was leaked into the vacuumsystem for exposures to 10-8 Torr for 1 hour and then

6

10-7 Torr for 1 hour. It was shown that the cathodecurrent decayed only when it was operating during theexposures. The cathode performance was not affectedby exposure to 10-5 Torr of oxygen during 1 hour whenit was not operating. The cathode I-V characteristicswere identical before and after the exposure. It isbelieved that the performance of the cathode is onlynoticeably affected by the oxygen environment whileoperating because the electron emission into thisenvironment after being accelerated through the gatepotential generates an much more reactiveenvironment with ionized and atomic oxygen as shownwith the model described in Appendix A.

During the exposures to oxygen while the cathode wasoperating, the current always decayed with anexponential behavior. When the oxygen was removedfrom the environment and it returned to ~10-9 Torr, theMo cathode performance recovered to the pre-exposure performance if the operating voltages andexposure doses of oxygen were low enough.Operating with gate electrode voltages ≤90 V at 10-7

Torr, the Mo FEA cathodes were not permanentlyaffected by these exposures as shown in Figure 8-Figure 12 in Appendix B. Table 3 in Appendix Bsummarizes the results from all of the experiments.Data is presented in the graphs and table in the orderthat it was acquired. The results suggest that nosputtering damage was done to the cathodes during theexperiments; only the work function was changed bythe oxygen adsorption. These results are verypromising in comparison to the operating voltagelimitations of ~36 V for the same cathodes in 10-5 Torrof xenon. [1]

Experiments performed on ZrC/Mo cathodes showedthat these cathodes were slightly more stable than theMo cathodes, the performance responses to increasedoxygen pressure was very repeatable and that totaldose of oxygen during the exposures had a moresignificant impact on the performance response toincreased oxygen pressure than operating voltage.These cathodes also demonstrated that they would notbe sputter-damaged by ion bombardment whileoperating at 50 V during a dose of 1.1x10-7 Torr-hourof oxygen, as shown in Figure 15. Figure 16 shows anI-V trace taken in UHV after experiment #100400.Repeatability in current degradation rate in the sameoxygen environment was demonstrated in two 50 Vexposures. The cathode recovered during a 50 Vexposure to 4.85x10-7 Torr-hours (Exp. #101100) as

shown in Figure 17; however, it did not recover within120 hours in UHV from an exposure to 7.64x10-7

Torr-hours (Exp. #101700), as shown in the samefigure. The cathode did recover later during a facilitybake-out. Figure 17 also shows that the performanceresponse to the oxygen exposure was totally repeatablefor the same dose and initial operating current. Figure17 shows the results from an exposure to an evengreater dose of oxygen, 10-6 Torr-hours (Exp.#103100). The cathode never demonstrated a fullrecovery from this exposure. Figure 17 shows thatpartial performance recovery will occur while thecathode is not operating in UHV. These resultssuggest that some of the oxygen adsorbed by thecathode will desorb in UHV. Physisorbed oxygen isexpected to quickly desorb in UHV. Performanceimprovements observed during operation in UHV afterexposures with cathodes operating can be attributed toelectron induced desorption also.

ZrC/Mo cathode 1096D was used in an oxygenexposure experiment to determine the cathode currentand work function when it was in equilibrium with arelatively high pressure environment and to determineif it was possible to maintain a constant current byregulating the voltage in this environment. Figure 19shows current and voltage data taken in UHV after thecathode demonstrated stable performance in the UHVenvironment at 10-9 Torr. Table 3 shows theexperimental conditions and response of the cathode toexposure to oxygen in this experiment, #032701, andin previous experiments. Data in Figure 20 show thestable cathode current observed in UHV before thecathode was exposed to elevated oxygen pressures.Figure 20 also shows the cathode current response tothe increased oxygen pressure. It was anticipated thatthe current would stabilize in this environment afterapproximately 1 hour. Instead, the current continuedto decay after more than 7 hours in this environment.Figure 20 can be misleading because it appears that 6hours into the exposure, the current had stabilized.Figure 21 shows some of the same data on a differentscale. With this scale it is obvious that the currentcontinues to decay after an exposure of 8.9x10-6 Torr-hours; oxygen continued to react with the cathodematerial changing its work function, conductivity orboth.

Figure 22 shows the ZrC/Mo cathode 1096D currentresponse to increased operating voltage in 10-6 Torr ofoxygen. Voltage increases were employed in an

7

attempt to demonstrate that the current densityrequired, ~10 mA/cm2, could be maintained in thisenvironment, albeit at higher operating voltages. Theinitial cathode operating voltage was 50 V to supply19.9 mA/cm2 with 156 µA from 0.78 mm2. The datain Figure 22 shows that, in this environment, thecathode operating voltage could not be increased tocompensate for the change in work function andconductivity and maintain a stable 19.9 mA/cm2. Thecathode operating voltage was increased up to 90 V inthis experiment. Each increase in gate voltage wasaccompanied by an initial increase in current andsubsequent current decay. The current decay rateseemed to increase with gate voltage. In subsequentcathode testing gate voltages up to 110 V in UHVwere employed to accelerate the recovery process, butthe cathode never fully recovered from the exposure; itcontinued to decay during operation, and demonstratedsignificantly reduced efficiency. The efficiencychange is characteristic of a sputter damaged cathode.

Figure 23 shows the normalized current degradation asa function of pressure and exposure time for thisexperiment and three experiments performedpreviously on this same cathode. Each of theexperiments was conducted with a gate voltage of 50V, however the initial current levels and oxygenpressures during the exposures were significantlydifferent as shown in Table 3. The experimentalresults were remarkably repeatable.

A second ZrC/Mo cathode, 1098F, was also tested inoxygen-rich environments. The performance of thiscathode was inferior to cathode 1096 D; however theexperimental results in response to the oxygenenvironment were similar. This cathode requiredmuch higher operating voltages for significant currentlevels with lower efficiency. Three experiments (Exp.#121100, 122100, 010501) were performed on thiscathode with the same exposure doses. The currentdecay in all three experiments differed by only 9%,demonstrating acceptable repeatability. These data areshown in Figure 24, Figure 26, and Figure 27. Thecurrent-voltage characteristics of this cathode areshown in Figure 25.

The performance responses of Mo and ZrC/Mocathodes are shown in Figure 28 and Figure 29. TheZrC/Mo cathodes behaved similarly to the oxygenenvironment demonstrating repeatability. The superiorstability of these cathodes could be responsible for this

repeatable performance. Mo and ZrC/Mo cathoderesponses to the oxygen environment clearly vary.The cathode performance should be characterizedfrom Jensen’s model and the I-V traces taken beforeeach exposure for comparison. Different performanceresponses may be attributable to these differences.

In all of the experiments conducted, the cathodecurrent never reached an equilibrium surface state inthe environments. In each of the experiments, theenvironment and cathode operating conditionsgenerated a flux of AO to the tips which is lower thanwould be expected in LEO, therefore, the performancedecay rates expected in LEO would be even higher.Future experiments will help to identify how theenergy of the impinging oxygen affects the adsorptionrate.

Conclusions

The results of this investigation include a betterunderstanding of the environment near the tips and theeffect of the environment on the performance of theMo and ZrC/Mo cathodes. The modeling resultsshowed that we can self-generate a flux of AO to thetips that is comparable to the flux of AO in LEO byscaling the emission current and oxygen pressure. Itwas shown that the cathodes are more sensitive to theoxygen environment than expected.

In the oxygen environment, both the work functionand conductivity seemed to be affected. Theimmediate physical adsorption of oxygen increases thework function to decrease the emission current. Theoxygen then diffuses its way into the bulk, eventuallyforming a oxide that decreases the conductivity of thecathode and degrades its performance further. Oncethis film is thick enough it can charge up and furtherdegrade the cathode performance. If the film is thinenough, the cathode can self-recover by electroninduced desorption of the oxygen in UHV. Webelieve that in some of our experiments, the oxide filmbecame too thick to self-remove by electron induceddesorption, therefore we observed some irreversibleperformance damage. In all experiments, the oxidefilm formed never passivated itself. The surface stateof the cathode continued to degrade after significantexposures in which the cathodes were exposed to alower flux of AO than is expected from the LEOenvironment.

8

The tolerable operating voltages demonstrated werevery promising. The Mo cathode performancerecovered from temporary performance degradation at50-90 V in oxygen. The ZrC/Mo cathodes weretolerant of the environment when operating at 50 V,but suffered from irreversible damage when operatingat 90 V. The upper limit has not yet been identifiedand may depend on environmental pressure, but isbetween 50 V and 90 V and possibly ~66 V.

Further experimentation is required to determinewhether the operating voltage and pressure togetherdefine operational limitations of the cathode. Theexperimental results showed that the Mo cathode couldbe operated up to 90 V for 1.1x10-7 Torr-hours ofoxygen without irreversible performance degradation.66 V seemed to be the operating voltage limitation at10-6 Torr. Larger dose experiments should also be

conducted at 50 V to demonstrate that this operatingvoltage is an upper limit which is independent ofoxygen dose.

Acknowledgements

The authors would also like to gratefully acknowledgeNASA MSFC (John Cole, Dr. Kai Hwang, RandyBagget, Les Johnson) and Tethers Unlimited Inc. (Dr.Rob Hoyt) for funding the development of fieldemission cathodes for space-based applications andDrs. Capp Spindt and Bill Mackie for their advice oncathode testing. The research described in this paperwas carried out at the Jet Propulsion Laboratory,California Institute of Technology, under a contractwith the National Aeronautics and SpaceAdministration.

9

APPENDIX A: Flux of Atomic Oxygen in Laboratory Environment

A. NomenclatureA1 = electron emitting area of tip (cm2)Atip = area of the tip emitting (cm2)dcc = center-to-center tip distance on array(cm)Ee= electron energy (eV)Itip = single tip current (A)Itot = total cathode current (A)Je= cathode electron current density (A/cm2)k = 1.38x10-23 J/KmO = 2.179x10-25 kgmO2 = 4.358x10-25 kgn = number density (/cm3)∠= half-cone emission angle (°)rbo = radial position of beamlet overlap (cm)rcea = radius of cathode emitting area (cm)rtip = radius of curvature of the tip (cm)rm = maximum radial distance from which an ion

formed will impact the emitting area of the tipsv = velocity (cm/s)Vg = gate electrode voltage (V)

V2 = volume element contributing to the generationof AO (cm3)

Q = collision cross-section (cm2)φ = flux hitting the emitting area of the tip (/s)Φ = flux of molecular oxygen (/cm2s)ψ = angle in volume integral (radians)ν = velocity (cm/s)Θ = angle between the atomic oxygen source

volume element and the tip surface normal(radians)

B. Reaction cross-sectionsThe reaction cross-sections for electron collisionswith molecular oxygen used in the model are shownin Figure 4. Only cross-sections for collisionscreating oxygen capable of bombarding the tips areshown and considered in the model. These datashow that the most probable collisions result in O2+and O+.

101

102

103

104

Rea

ctio

n C

ross

-sec

tion

(cm

/10-2

0 )

20018016014012010080604020

Electron Energy (eV)

Dissociation, O/O2 879Å (Sroka, 1968) Dissociation , O/O2 990Å (Sroka, 1968) Dissociation, O/O2 1304 Å (Ajello, 1971) Dissociation, O/O2 1356Å (Ajello, 1971) Dissociative Ionization O+/O2 (Englander-Golden & Rapp, 1965) Atomic Ionization, O+/O (Rothe, E.W., et al., 1962) Total Ionization, O2+/O2 (Rapp and Englander-Golden, 1965)

Figure 4. Reaction cross-sections for electron collisions with molecular oxygen.

10

C. Flux of oxygen to FEA cathode tipsC.1 Flux of Molecular OxygenC.1.1 Random

Eqn. 1 A rtip tip=∠2

3604 2π

Eqn. 2 ΦO O On v2 2 2

1

4= (#/cm2s)

Eqn. 3 vkT

mOO

2

2

8=

π(cm/s)

Eqn. 4 φ = ΦO tipA2

(/s)

C.1.2 Directed with 5 eVEqn. 5 ΦO O On v

2 2 2= (/cm2s)

Eqn. 6 veV

mOO

2

2

25

= (cm/s)

Eqn. 7 φO O tipA2 2= Φ (/s)

C.2 Flux of Ionized Molecular Oxygen

Eqn. 8 φO Otip

O O e

r

r

nI

eQ E r dr

tip

m

2 2 2 2= +∫ / ( ( ))

C.3 Flux of Atomic Oxygen

C.3.1 Near-tip contributionAtomic oxygen production rate

Eqn. 9dn

dtn n v Q n

J

eQO

O e e O O Oe

O O= =2 2 2 2/ /

Flux of atomic oxygen leaving dV2 and arriving at A1

Eqn. 10 φθ

πOOdn

dt

A

rdV= 1

2 24

cos

where A

r1

2 4

cosθπ

is the fraction of AO that reaches the

tip, A1.Eqn. 11

dA

rn

J

eQ E r dVO O

eO O eφ

θπ

= 12 4 2 2

cos( ( ))/

A rt122

3604=

∠π and J

I

ree=

∠4

360

22πIt is assumed here that Ie is not a function of Θ. Whilethis is not true, it will not affect our results since weare interested in the total flux to the emitting area ofthe tip only.Eqn. 12

φπ

θ θθ ψ

π

OO tip

tip O O e

r

rn I

er Q E r

rd d dr

t

= ( )( )∠

∫∫∫2

242

00

2

2/

maxcos sin

Eqn. 13

φO Otip

tipO O e

r

r

nI

er

Q E r

rdr

tip

bo

=∠∫2

222

24

sin ( ( ))/

C.3.2 Far-field Contribution (after the individualbeamlets overlap)

The initial beam radius is rcea which includes all50,000 beamlets in the array. It is assumed that thebeam is uniform in current density for this calculation.It is assumed that a single beamlet half-cone emissionangle is 37° and that the combination of all of thebeams expands at this angle also so that the currentdensity varies as a function of radial distance from thesource as

JI

r rtot

tot

cea

=+ ∠( )π tan

2

Then contribution to the flux of atomic oxygen to thetips from this far-field population isEqn. 14

φO Otot

tipO O e

cear

r

tip

bo

L

nI

er

Q E r

r rdr=

∠∠

+ ∠( )∫2

22

3602 2

2sin( ( ))

tan

/ ,

where rbo represents radial position of beamlet overlapand

Eqn. 15 rd

bocc=∠tan

.

In this calculation it is assumed that the energy of theelectrons does not exceed the gate electrode potential.In reality, the electrons will be accelerated between thegate and anode; however, the potential distribution inthat region has not yet been included in this model.

C.4 Flux of Ionized Atomic OxygenEqn. 16

φO Otip

O O e

r

r

tip

tip

m

nI

eQ E r dr= +∫2 2/ ( ( ))

D. Collision-induced Electron Beam AttenuationAs the electron beamlets participate in

ionization and dissociation reactions, the supply ofelectrons in the beam is depleted. This model did nottake into account electron beam depletion; therefore,significant depletion will introduce errors in theresults. Collision-induced electron beam depletionrate was determined to assess its significance on theresults of this model with the relationships

Eqn. 17dI

drn IQ E rO e= − ( )( )

2 which is evaluated

as

Eqn. 18I

In Q E r dr

tipO e

r

r

tip

L

= − ( )( )

∫exp

2.

The results from this analysis are shown in Figure 5.The depletion rate is shown for one type of reactiononly-total ionization by electron bombardmentforming O2+. This reaction is primarily responsiblefor depleting the electron beam as shown by thereaction cross-section data in Figure 4. The O2

pressure must be ~103 Torr before 0.25% of the beam

11

is depleted within 1 cm of the tip. The experimentsdiscussed in this study were conducted at ~107 Torr,therefore, an insignificant number of electrons arebeing depleted within 1 cm of the tip. Beyond this

distance, an insignificant number of ionized andatomic oxygen generated hit the emitting area of thetip.

1.000000

0.999995

0.999990

0.999985

0.999980

0.999975

I/Io

10-6 10-5 10-4 10-3 10-2 10-1 100

Radial Position from Tip (cm)

Vg= 100 VO2 + e-> 2e + O2+ nO2 = 109 cm-3

nO2 = 1010 cm-3

nO2 = 1011 cm-3

Figure 5. The variation in current with position downstream of the tip due to reaction collisions at variousenvironment pressures.

D. Modeling ResultsThe results of the model using the reaction

cross-sections shown in Figure 4 are shown in Figure6. The results in the figure show the fluxes to the tipsexpected in the LEO environment with the conditionsshown on the graph. Two scenarios were considered-the ambient environment and in the spacecraft ramwhere the oxygen will be impacting the spacecraft andcathodes with 5 eV. Figure 6 also shows the flux ofvarious oxygen species generated by a field emissioncathode operating in a molecular oxygen environment.The results show that the flux of oxygen to theemitting tips is primarily O2+ in a molecular oxygenenvironment with the tips emitting electrons withenergies that exceed oxygen ionization potentials. TheO+ population represents the second largest populationof oxygen bombarding the emitting tips. The high ionfluxes to the tips is attributed to the radial focusing ofthe trajectories by the electric field near the tips. Thefluxes of AO to the tips from the dissociation reactions

are orders of magnitude lower than the ion fluxes. TheO2+ ions bombard the tips with >12 eV of energy,therefore they dissociate on impact, providing twooxygen atoms for each O2+ impact. This source ofatomic oxygen is the most significant. To simulate theflux of atomic oxygen in LEO during the laboratoryexperiments, Figure 6 shows that the current per tipshould be increased by 10000x or the PO2 should beincreased by 10000x or some combination of thesechanges like a cathode emitting 100 nA/tip in 2x10-7

Torr of O2. These results show that it is possible toself-create an AO flux to the tips that is very similar tothe AO flux to spacecraft in LEO. To determine howthe self-generated flux of AO oxygen compares to theLEO environment, Jensen’s cathode performancemodel and experimental data are required. Many ofthe results of this modeling are presented in Table 2.The results can easily be scaled from the data given forchanges in number of tips emitting, pressure, tip radiusof curvature, or current per tip.

12

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102Fl

ux o

f O

xyge

n to

Sin

gle

Tip

Em

ittin

g A

rea

(#/s

)

1009590858075706560555045403530Vg (V)

Dissociation by Electron Bombardment atomic O 879Å atomic O 990Å atomic O 1304Å atomic O 1356Å

Dissociative Ionization by Electron Bombardment atomic O O+ Total Atomic O

Total Ionization by Electron Bombardment O2+

LEO Environment

Molecular Oxygen (2E7/cm3(1E-9 Torr)) Atomic Oxygen (1E9/cm3(1E-7 Torr)) Molecular Oxygen (2E7/cm3(1E-9 Torr), 5 eV) Atomic Oxygen (1E9/cm3(1E-7 Torr), 5 eV)

rtip= 4 nmItip = 1 nAItot = 50 uAnO2 = 2E7/cm3 (2 nT)nO = 10E9/cm3 (100 nT)

Figure 6. Fluxes of Ionized and neutral atomic and molecular oxygen to tips as generated in the laboratory andexpected in LEO.

APPENDIX B: Experimental Data

A compilation of experimental data is presented in Table 3. Data acquired from each experiment isincluded in the following figures. The data in each figure are identified by the experiment numbers. The detailsof the operating conditions of the experiments are given in Table 3.

13

Tab

le 3

. Sum

mar

y of

cat

hode

per

form

ance

res

pons

es to

exp

osur

e to

oxy

gen.

EX

P #

FE

AC

#V

G (

V)

VA (

V)

I I (µµµµ

A)

J I (mA

/cm

2 )P

O2

(Tor

r)t e

xp

(hr)

ΘΘΘΘ (

Tor

r-hr

)I F (u

A)

I F/I

I%∆∆∆∆ΙΙΙΙ

Rec

over

y ti

me

in U

HV

(hr

)

0913

00A

Mo

1100

C50

100

111.

4~1

0-81

111

~10-7

11.

26x1

0-74

0.36

641.

509

1300

BM

o 11

00C

6010

034

4.4

~10-8

130

0.88

12

30~1

0-71

1.08

x10-7

1909

1500

Mo

1100

C70

100

729.

2~1

0-81

660.

9266

~10-7

11.

35x1

0-730

0.45

589

0917

00M

o 11

00C

8013

017

021

.8~1

0-81

150

0.88

150

~10-7

11.

42x1

0-713

50.

921

209

1900

Mo

1100

C90

160

425

5.0

~10-8

139

00.

9239

0~1

0-71

1.23

x10-7

150

0.38

6529

0921

00M

o 11

00C

100

300

850

109.

0~1

0-81

700

0.83

700

~10-7

11.

36x1

0-715

00.

2182

>70

1004

00Z

rC/M

o10

96D

5013

018

824

.1~1

0-81

149

0.79

149

~10-7

11.

16x1

0-710

50.

744

1011

00Z

rC/M

o10

96D

5013

016

721

.4~1

0-74.

94.

85x1

0-715

.50.

0991

120

1017

00Z

rC/M

o10

96D

5013

017

021

.8~1

0-77.

77.

64x1

0-713

0.08

92no

ful

l rec

over

y in

120

hou

rs, b

utre

cove

ry d

urin

g ba

keou

t

1031

00Z

rC/M

o10

96D

5013

027

635

.4~1

0-7-

10-6

~3.7

1.00

x10-6

50.

0298

no f

ull s

elf-

reco

very

1211

00Z

rC/M

o10

89F

9020

088

11.2

~10-8

169

0.78

69~1

0-71

~1.1

x10-7

500.

7243

No

full

self

-rec

over

y

1221

00Z

rC/M

o10

89F

9016

068

8.7

~10-8

160

0.88

60~1

0-71

~1.1

x10-7

370.

6246

0105

01Z

rC/M

o10

89F

9016

048

6.1

~10-8

141

0.85

41~1

0-71

~1.1

x10-7

230.

5652

No

full

reco

very

. C

urre

nt s

tabi

lized

at 3

9.7

uA w

ithin

96

hour

s

0327

01Z

rC/M

o10

96D

5013

027

619

.910

-67.

68.

9x10

-60.

430.

0027

99.7

14

180x10-6

160

140

120

100

80

60

40

20

0

Current (A

)

928884807672686460Voltage (V)

-20.0

-19.6

-19.2

-18.8

-18.4

-18.0

-17.6L

n(I/

V2 )

15.0x10-312.51/V

Work function 4.4 eVTip radius 22.8 ÅNtipsemitting 120

Figure 7. I-V data taken with cathode Mo-1100C before Exp. #91300.

12x10-6

10

8

6

4

Cur

rent

(A

)

8.58.07.57.06.56.05.55.04.54.03.53.02.52.01.51.00.50.0

Time (hours)

10-8

10-7

Pres

sure

(T

orr)

Figure 8. Current and Pressure data from Exp. #91300A with cathode Mo-1100C.

35x10-6

30

25

20

15

Cur

rent

(A

)

2120191817161514131211109876543210

Time (hours)

10-8

10-7

Pres

sure

(T

orr)

Figure 9. Current and pressure data from Exp. #91300B with cathode Mo-1100C.

15

80x10-6

60

40Cur

rent

(A

)

7065605550454035302520151050Time (hours)

10-8

10-7

Pres

sure

(T

)

Figure 10. Current and pressure data from Exp. #91500 with cathode Mo-1100C.

200x10-6

160

120

Cur

rent

(A

)

36322824201612840Time (hours)

2

46

10-8

2

46

10-7

Pres

sure

(T

orr)

Figure 11. Current and pressure data from Exp. #91700 with cathode Mo-1100C.

400x10-6

350

300

250

200Cur

rent

(A

)

3432302826242220181614121086420Time (hours)

10-8

10-7

Pres

sure

(T

orr)

Figure 12. Current and pressure data from Exp. #91900 with cathode Mo-1100C.

16

500x10-6

450

400

350

300

250

200

150

100

50

0

Current (A

)

100908070605040Voltage (V)

-20.5

-20.0

-19.5

-19.0

-18.5

-18.0

-17.5

-17.0

-16.5L

n(I/

V2 )

20x10-318161412101/V

V (V) I (µΑ) 50.5 3.6 55.5 11.561.0 28.766.5 54.871.4 92.275.7 14181.3 23586.0 33990.2 434

Figure 13. Fowler-Nordheim plot and I-V data taken with cathode Mo-1100C before Exp. #91900.

800x10-6

600

400

200Cur

rent

(A

)

200180160140120100806040200

Time (hours)

10-9

10-8

10-7

Pres

sure

(T

orr)

1009896949290

Gate V

oltage (V)

gate voltage

pressure

current

Figure 14. Current and pressure data from Exp. #92100 with cathode Mo-1100C.

180x10-6

160

140

120

100

Cur

rent

(A

)

363432302826242220181614121086420Time (hours)

10-8

10-7

Pres

sure

(T

orr)

Figure 15. Current and pressure data from Exp. #100400 with cathode ZrC/Mo-1096D.

17

-20.5

-20.0

-19.5

-19.0

-18.5

-18.0

-17.5

-17.0

-16.5

-16.0

-15.5

Ln(

I/V

2 )

24x10-32220181/V

1.0x10-3

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

Current (A

)

605652484440Voltage (V)

V (V) Ia (µA) Ig (nA)39.1 0.26 5.040.7 4.6 7.946.1 48.2 33.251.8 195 12155.4 387 26759.6 791 68161.4 1071 1075

Work function 3.6 eVTip radius 43.9 ÅNtipsemitting 6000

Figure 16. Cathode ZrC/Mo-1096D characteristics before Exp. #101100 .

160x10-6

120

80

40

0

Cur

rent

(A

)

1401301201101009080706050403020100Time (Hours)

10-8

10-7

10-6

Pres

sure

(T

orr)

Current, Exp. #101100 Pressure , Exp. #101100 Current, Exp. #101700 Pressure, Exp. #101700

Exp. #101100

Exp. #101700

Figure 17. Current and pressure data from Exp. #101100 and 101700 with cathode ZrC/Mo-1096D.

400x10-6

300

200

100

0

Cur

rent

(A

)

302826242220181614121086420Time (hours)

10-8

10-7

10-6

Pres

sure

(T

orr)

175.0174.0173.0172.0

Cathode Off

Figure 18. Current and pressure data from Exp. #103100 with cathode ZrC/Mo-1096D.

18

-28

-26

-24

-22

-20

-18

-16

Ln(

I/V

2 )

50x10-34540353025201510

1/V

800x10-6

700

600

500

400

300

200

100

0

Current (A

)

6050403020100

Voltage (V)

V (V) Ia(µA) Ig(nA)32.2 0.19 0.1133.7 0.51 0.3136.0 1.29 0.7938.4 5.07 3.0740.5 11.3 6.9642.1 18.9 11.843.9 32.4 20.845.6 46.4 30.048.3 92.0 63.950.1 141 10252.1 191 15054.2 316 27356.4 377 36562.1 793 795

Figure 19. I-V characteristics of cathode 1096D before Exp.#032701 with an anode voltage of 100 V.

200x10-6

150

100

50

0

Cur

rent

(A

)

10.09.08.07.06.05.04.03.02.01.00.0Time (hours)

10-8

10-7

10-6

Pres

sure

(T

orr)

Figure 20. ZrC/Mo 1096D cathode current response to increase in oxygen pressure in Exp.# 032701.

1.75x10-6

1.50

1.25

1.00

0.75

0.50

Cur

rent

(A

)

10.810.610.410.210.09.89.69.49.29.08.88.68.48.28.0Time (hours)

789

10-6

2

Pres

sure

(T

orr)

Figure 21. Data from Figure 20 shown on a different scale.

19

220x10-6

200

180

160

140

120

100

80

60

40

20

0

Cur

rent

(A

)

11.3011.2011.1011.0010.9010.80

Time (hours)

90

80

70

60

50

Gate V

oltage (V)

1.6x10-6

1.20.80.40.0Pr

essu

re (

Tor

r) 200

150

100

50

Anode V

oltage (V)

Anode Voltage

Current

Pressure

Gate Voltage

Figure 22 .Cathode current response to operation at increased voltages in 10-6 Torr of oxygen in Exp. #032701.

1.00

0.90

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0.00

I/I o

10-10 10-9 10-8 10-7 10-6

Pt (Torr-hours)

#101100 #101700 #103100 #032701

Figure 23. Normalized current vs. pressure and time from Exp. #032701 and previous experiments #101100,101700, 103100.

20

160x10-6

140

120

100

80

Cur

rent

(A

)

5248444036322824201612840Time (hours)

10-9

10-8

10-7

Pres

sure

(T

orr)

Figure 24. Current and pressure from Exp. #121100 with cathode ZrC/Mo-1089F

-27

-26

-25

-24

-23

-22

-21

-20

-19

-18

-17

Ln(

I/V

2 )

18x10-3161412108

1/V

1.0x10-3

0.8

0.6

0.4

0.2

0.0

Current (A

)

1201008060

Voltage (V)

V(V) Ia(µA) Ig (nA)53.0 0.003 14.667.1 0.58 23.871.0 0.16 26.875.9 0.46 31.081.6 1.24 36.284.3 18.2 38.789.8 34.4 43.593.7 50.8 47.499.1 87.6 53.2104.0 139 59.3110.1 229 67.5115.3 335 74.7122.7 536 84.6129.0 760 94.2135.0 1005 106

Gate Current

Anode Current

Figure 25. I-V data taken after the Exp. #121100 with cathode ZrC/Mo-1089F and an anode voltage at260 V.

a)

100x10-6

75

50Cur

rent

(A

)

300280260240220200180160140120100806040200Time (hours)

10-8

10-7

Pres

sure

(T

orr)

21

b)

70x10-6

60

50

40

Cur

rent

(A

)

36.035.535.034.534.033.533.032.532.031.531.030.530.029.529.0Time (hours)

10-9

10-8

10-7

Pres

sure

(T

orr)

Figure 26a,b. Current and pressure from Exp. #122100 with cathode ZrC/Mo-1089F.

a)

50x10-6

40

30

20

Cur

rent

(A

)

1401301201101009080706050403020100Time (hours)

10-9

10-8

10-7

Pres

sure

(A

)

b)

50x10-6

40

30

20

Cur

rent

(A

)

76.075.575.074.574.073.573.072.572.071.571.070.570.0Time (hours)

10-9

10-8

10-7

Pres

sure

(A

)

Figure 27a,b. Current and pressure from Exp. #010501 with cathode ZrC/Mo-1089F.

22

0.01

2

3

456

0.1

2

3

456

1I/

I o

10-11 10-10 10-9 10-8 10-7 10-6

Pt (Torr-hours)

#100400 ZrC/Mo 1096D #101100 ZrC/Mo 1096D #101700 ZrC/Mo 1096D #103100 ZrC/Mo 1096D #121100 ZrC/Mo 1089F #122000 ZrC/Mo 1089F

Figure 28. Comparison of performance decay rates of both ZrC/Mo cathodes in oxygen.

2

3

4567

0.1

2

3

4567

1

I/I o

10-9 10-8 10-7 10-6

Pt (Torr-hours)

#91300A Mo 1100C #091300B Mo1100C #091500 Mo 1100C #091700 Mo 1100C #91900 Mo 1100C #092100 Mo 1100C #100400 ZrC/Mo 1096D #101100 ZrC/Mo 1096D #101700 ZrC/Mo 1096D #103100 ZrC/Mo 1096D

Figure 29. Comparison of performance decay rates of ZrC/Mo and Mo cathodes in oxygen.

References[1] Marrese, C. M., Polk, J. E., Jensen, K. L., Gallimore, A. D. , Spindt, C. , Fink, R. L., Tolt, Z. L., Palmer, W. D.,“An Investigation into the Compatibility of Field Emission Cathode and Electric Thruster Technologies: Theoreticaland Experimental Performance Evaluations and Requirements,” Micropropulsion for Small Spacecraft, AIAAProgess Series Vol. 187, edited by M. Micci and A. Ketsdever, AIAA , Reston, VA, 2000, Chapter 11.[2] Gomer, R., Field Emission and Field Ionization, Harvard University Press, Cambridge, Massachusetts, 1961.[3] I. Brodie and C. A. Spindt, “Vacuum microelectronics,” Adv. Electron. Electron Phys., vol.83, p.1, 1992.[4] B. R. Chalamala, R. M. Wallace, B. E. Gnade, “Effect of O2 on the electron emission characteristics of activemolybdenum field emission cathode arrays,” J. Vac. Sci. Technol. B 16(5), Sep/Oct 1998.[5] W. A. Mackie, T. Xie, M. R. Mathews, B. P. Routh, Jr. and P. R. Davis, “Field emission form ZrC films on Mofield emitters,” J. Vac. Sci. Technol. B 16(4), Jul/Aug 1998.[6] Marrese, C. M. "Compatibility of Field Emission Cathode and Electric Propulsion Technologies," Ph.D.dissertation, University of Michigan, 1999.

23

[7]Jensen, K. L., “An Analytical Model of an Emission-gated Twystrode Using a Field Emission Array,” J. Appl.Phys. 83(12), June 1998.[8] K. L. Jensen and C. M. Marrese, “Emission Statistics and the Characterization of Array Current,” InternationalVacuum Microelectronics Conference, Aug., 2001[9] I. Brodie, “Bombardment of Field-Emission Cathodes by Positive Ions Formed in the Interelectrode Region,”Int. J. Electronics 38(4), 1975.[10]C. T. Rettner, L. A. DeLouise, D. J Auerbach,. “Effect of Incidence Kinetic Energy and Surface Coverage onthe Dissociatve Chemisporption of Oxygen on W(100),” J. Chem. Phys. 85 (2) pp. 1131-1149. July 1986.[11] C. M. Marrese, C. Wang, K. D. Goodfellow, A. D. Gallimore, “Space-Charge Limited Emission from FieldEmission Array Cathodes for Electric Propulsion and Tether Applications,” Micropropulsion for Small Spacecraft,AIAA Progess Series Vol. 187, edited by M. Micci and A. Ketsdever, AIAA , Reston, VA, 2000, Chapter 18.[12] Marrese, C. M. "Compatibility of Field Emission Cathode and Electric Propulsion Technologies," Ph.D.dissertation, University of Michigan, 1999.