(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

At AM A01-31413

AIAA 2001-2958

FIBER OPTIC SENSORS FOR THE STUDY OFSPACECRAFT-THRUSTER INTERACTIONS:ION SPUTTERING

Andrew D. KetsdeverAir Force Research LaboratoryPropulsion DirectorateEdwards AFB, CA 93524

Brian M. EcclesUniversity of Southern CaliforniaDepartment of Aerospace and Mechanical EngineeringLos Angeles, CA 90089

Mohamed Abid, Greg Netherwood, and Colleen FitzpatrickRice Systems, Inc.Irvine, CA 92614

35th AIAA Thermophysics ConferenceJune 11-14, 2001 / Anaheim, CA

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(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

Fiber Optic Sensors for the Study of Spacecraft-ThrusterInteractions: Ion Sputtering

Andrew D. Ketsdever1"Air Force Research Laboratory

Propulsion DirectorateEdwards AFB, CA 93524

Brian M. Eccles*University of Southern California

Department of Aerospace and Mechanical EngineeringLos Angeles, CA 90089-1191

Mohamed Abid*, Greg Netherwood**, and Colleen Fitzpatrick§

Rice Systems, Inc.Irvine,CA 92614

ABSTRACT

Fiber optic contamination sensors offer increased flexibility over single point data obtained by quartzcrystal microbalances for the investigation of spacecraft contamination events. A proof-of-principledemonstration of the fiber optic sensor's sensitivity to material sputtering from energetic thruster plumeions is given. For this case, the cladding of the fiber is etched in an aqueous HF solution to simulate thesputtering from energetic ions. The intensity of a 1304 nm diode laser is measured as a function of time orcladding thickness as the cladding is being etched. The experimental data is combined with an estimate ofthe expected sputter rate from Hall thruster plumes in order to predict the sensitivity of the optical fiber inspacecraft applications. The sputter rate was estimated based on a sputter yields derived from numericalmodels and previous experimental data and the details of the thruster plume.

INTRODUCTION

The interaction between spacecraft and thrustereffluents has received considerable attention inrecent years. Adequate methods arenecessary for mission designers to predict,monitor and abate detrimental contamination ofspacecraft surfaces. In previous missions,thruster interaction concerns have focused onself-contamination from non-direct and highangle (measured from the thruster centerline)plume impingement. However, the advent ofdistributed networks of co-orbiting micro- and

f Senior Research Engineer, AIAA SeniorMember, e-mail: andrew.ketsdever@edwards.af.mil* Undergraduate Student, AIAA Student Member* Principle Scientist, AIAA Member, e-mail:abid@usc.edu** Member of Technical Staff, AIAA Members Senior Scientist, AIAA Senior Member

This material is declared a work of the U.S. Government andis not subject to copyright protection in the United States.

nanosatellites has brought about a need toaddress direct plume impingement, also termedcross-contamination. Shortly after separationfrom a launch vehicle, the microsatellites of aparticular cluster will be in close proximity toone another. In order to form and maintain thecluster, these microsatellites will be required toperform propulsive maneuvers relative to eachother, which could result in significant cross-contamination. Therefore, the future ofspacecraft-thruster interactions involves bothdirect impingement (cross-contamination) andhigh angle plume impingement (self-contamination) of thruster effluents on satellitesystems.

The growing popularity of micro- andnanosatellite missions has also brought about aneed to design contamination monitors whichaddress the limitations on microsatellite power,mass and volume. In the past, quartz crystalmicrobalances (QCMs) have been used to look at

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

spacecraft contamination and thrusterinteractions where the major interaction is theadsorption of molecular species on criticalsurfaces.12 QCMs have several limitations whenassessing the contamination potential ofpropulsion systems. The most severe limitationis that the QCM can only provide interactiondata at a single point. Since the plume of atypical thruster can vary by several orders ofmagnitude in very small distances, a large arrayof QCMs would be required to adequately assessthe contamination potential of a propulsionsystem.

This study focuses on the proof-of-principledemonstration of a low mass, low power fiberoptic contamination sensor (FOCS) as analternative to QCMs. Although this work isprimarily concerned with assessing the FOCS forhighly energetic ion interactions that inducematerial sputtering, the sensor may also beappropriate as an adsorption monitor formolecular contaminants. Figure 1 shows how anFOCS system might be used to assess theinteraction characteristics of a thruster across anentire solar cell array providing a 2-D map of theinteraction.

Evanescent wave guides similar to the FOCShave been investigated for use as chemicalsensors. Carter, et al.3 used this technique tosense hydrazine propellants at a level below 10ppb. In this case, the fiber cladding was replacedwith a material that reacted with hydrazine. Inmost cases, the sensors are used to detect thepresence of adsorbed or reacted species. Incontrast, the FOCS measures the depletion oflight traveling through the fiber as the claddingmaterial is removed (sputtered) by energeticplume ions. The FOCS principle of operation isdetailed in the following sections.

THEORY

Principles of Fiber Optics

The difference in the index of refraction betweenthe core and cladding is responsible forpropagating the light signal through the opticalfiber. Total internal reflection (TIR) occurs in afiber when the light ray traveling through ahigher index of refraction material (core)encounters an interface with a lower index ofrefraction material (cladding). Although the lighttraveling in the fiber can be thought of as

undergoing TIR at the core-cladding boundary, afraction of the light actually penetrates into thecladding before being reflected back into thecore.4 As a consequence, a portion of theelectromagnetic wave is transmitted through thecladding called the evanescent wave. Thedistribution of the light intensity in the claddingis given by

(1)

where I0 is the intensity of the light travelingthough the fiber, x is the radial distance into thefiber measured from the core-cladding interface,and 1/p is the characteristic penetration length ofthe light into the cladding material. The p termis given by

(2)

where X is the wavelength of the lighttransmitted through the fiber and ncore and nclad arethe indices of refraction for the core and claddingmaterial, respectively.

As Eq. (1) indicates, the intensity of theevanescent wave decreases exponentially withincreased distance from the core-claddinginterface. The distance at which the evanescentwave intensity drops by 1/e of I0 is thecharacteristic length of penetration or P"1 asshown in Fig. 2a. For a concentric core fiber, theintensity profile is symmetric (roughly Gaussian)with the fiber axis as shown in Fig. 2b. Lightpropagates differently in an eccentric core fiberas shown in Fig. 2c. If the core is sufficientlyeccentric, the tail of the evanescent wave extendsbeyond the cladding into the surroundingenvironment. This condition is representative oflight propagation through a sputtered (or etched)cladding optical fiber where the claddingthickness on one side of the core has beensignificantly reduced.

Principles of FQCS Operation

The interaction between the evanescent waveand the surrounding environment forms the basisof the FOCS. For this reason, the sensitivity ofthe FOCS would be dependent upon the fractionof light that penetrates into the surroundingmedium, which is dependent on the claddingthickness. The FOCS can operate in two modesas a spacecraft- thruster interaction monitor,

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

either as a molecular contaminant adsorptionmonitor or as a sputtering monitor. In thepractical application of FOCS to study thrustereffluent deposition, parameters such as therefractive index of the contaminant and thecontamination layer thickness are important. Fora relatively thin cladding thickness (i.e. on theorder of fT1), the contamination layer will causea change in the transmitted light intensity, asshown in Fig. 3, due to a change in the index ofrefraction. If the index of refraction of thecontamination layer is greater than that of thecladding, then the light intensity will decreaseand vice versa. If the wavelength of light ischosen to match an absorption peak of a givencontaminant, the signal would decrease due tolight being absorbed in the contaminant layer.3

In studying interactions caused by sputtering ofexposed materials to energetic ions, theenvironment surrounding the FOCS system willnot change (i.e. vacuum); however, the claddingthickness would decrease due to the sputteringprocess. As this process continues, more of theevanescent tail would be exposed to the sensorenvironment causing a decrease in thetransmitted signal strength as shown in Fig. 4.The rate of this decrease would be proportionalto the sputter (or etch) rate and couldconceivably be calibrated to allow for theestimation of ion flux at the sensor location.

ESTIMATED SPUTTERING RATES FORION THRUSTERS

Sputtering Yields: The TRIM Code

The Transport of Ions in Matter (TRIM) code isa computational model which utilizes a MonteCarlo numerical approach with a fully quantummechanical treatment of ion-atom collisions.5Although TRIM can calculate all kineticphenomena associated with the loss of ionenergy, material sputtering is of interest in thisstudy. The level of material sputtering is trackedin terms of a sputter yield or sputtered atoms perincident ion. The sputter yield is calculated as afunction of incident ion energy and incidentangle for xenon ions (singly charged) impactinga 3 pirn thick SiO2 surface. The silicon dioxidesurface is representative of the cladding materialused in commercially available optical fibers.To insure adequate statistics, calculations wereperformed using 105 incident ions. The bindingenergy in the silicon dioxide molecule used in

the TRIM calculations is 2.0 eV and 4.7 eV foroxygen and silicon, respectively. In this study,TRIM is used instead of analytical formulationsfor the sputter yield found in the literature.6

Figure 5 shows individual sputter yields forsilicon and oxygen from an SiO2 surface as afunction of incident xenon ion energy andnormal incidence. As expected, the lowerbinding energy and atomic mass of the oxygenatom produces a larger sputter yield than silicon.Figure 6 shows the total sputter yield for silicondioxide as a function of incident energy forvarious angles of incidence measured from thesurface normal. Figure 7 shows the total sputteryield as a function of incident angle for variousion energies. The sputter yield decreases at highangles from the surface normal (i.e. nearlyparallel to the surface) for a given incidentenergy due to the decrease in the magnitude ofthe normal momentum and penetration depth ofthe incident ion. The data in Fig. 7 is fit with asixth order polynomial, which is consistent withthe findings of other studies.7 Figures 6 and 4suggest that a maximum sputter yield occurs atapproximately 75° independent of the incidentenergy. Figure 7 also shows that the sputteryield is above 1.0 for most incident energiesabove 0.3 keV at angles greater than 50°.

Properties of Ion Thruster Plumes

In order to estimate the sputter rate for xenon ionthruster effluents impinging on fiber cladding,the thruster plume must be characterized. Thereare typically three components of an ion thrusterplume: the plume core which is composed ofenergetic ions, the neutral plume which iscomposed on un-ionized propellant, and thecharge exchange component. The chargeexchange8 and the neutral components of theplume are neglected in this study since onlyenergetic ions above the sputter threshold willcontribute to sputtering.

Hall Thrusters

The plume ion flux of two different Hallthrusters is shown in Fig. 8 as a function of anglefrom the thruster centerline. The data is taken 40mm downstream of a D55 Hall thruster9 and 1.0m downstream of an SPT-100.10 The ion energydistribution for an SPT-100 is centered atapproximately 270 eV for singly charged ions asshown in Fig. 9.11 The associated energy level ofdoubly and triply charged ions is expected to be

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

much larger; however, the abundance of theseions is relatively small. As indicated in Fig. 8,the ion flux is a maximum on the thrustercenterline and drops off gradually with plumeexpansion angle. Although the plume densitydecreases as a function of radial position, thesputter yield for a material normal to the thrusterexit plane increases up to about 75° from thecenterline as Fig. 6 indicates.

Ion Engines

In general, the divergence of the plume of agridded ion engine is less than the plumedivergence from a Hall thruster. Figure 10shows the calculated ion flux at the exit plane ofthe NSTAR ion engine.12 In general, the ionflux from the NSTAR engine is less than that ofa typical 1 kW-class Hall thruster throughoutmuch of the plume. However, the typical plumeenergy is on the order of 1 keV suggesting thatthe sputter yield for a material impacted by theplume would be higher than for the Hall thruster(Fig. 6).

Estimated Ion Sputter Rate

In order to determine whether the FOCS systemwill make an attractive sputter sensor for ionelectric thrusters, the sputter rate of the claddingmaterial under impingement by the plume mustbe estimated. The sputter rate for a materialimpacted by an energetic ion beam is given by

dxdt (3)

where <£ (x,cj>) is the ion flux from the thruster atsome axial and radial location downstream of theexit plane [ions/cm2 sec], y (E^O) is the averagesputter yield as a function of ion impact energyand angle, and p is the number density of thesputtered material [atoms/cm3]. The sputteryield at a given point in the plume averaged overthe ion energy distribution at that point can befound by

(4)

where f(E) is the ion energy distribution functionat a given axial and radial position in the plume.

The integral in the numerator of Eq. (4) isestimated by combining the centerline ion energydata in Fig. 9 with TRIM calculated sputteryields for normal incidence in Fig. 6. From thisanalysis, the average sputter yield for the SPT-100 Hall thruster is approximately 0.162. FromEq. (3), the sputter rate for the SPT-100 for aFOCS with a pure silicon dioxide claddingwould be about 8.4 A/sec assuming the sensor ison the plume centerline 1 m downstream of thethruster exit plane (using the centerline flux inFig. 8).

Some data exists for the sputter rate of silicondioxide impacted by energetic xenon ions.Thomas13 gives a maximum sputter rate of 40A/min for 1 keV xenon ions impacting SiO2;however, an ion flux rate is not quoted.Randolph, et al.10 give a sputter rate ofapproximately 100 A/min for a quartz sampleplaced 1 m downstream of an SFT-100 close tothe thruster centerline. The relatively largedifference between these experimental resultsand the theoretical result obtained in this workmay be due to several factors including thenature of the material used, the cleanliness of theexposed surface, the discharge characteristics ofthe thruster, and the background pressure in thefacility. For the sputter yield derived by TRIM,the surface is assumed to be composed of a clean(on an atomic level), pure silicon dioxide layerimpacted by a monoenergetic, directed xenon ionbeam. This is obviously a poor representation ofa real thruster plume impinging on anengineering surface; however, it allows order ofmagnitude comparison with experimental etchrates detailed in the following sections. It will beassumed that the sputter rate for the sensorcladding impacted (normal to the surface, 1 mdownstream of the thruster exit plane) byenergetic ions in the plume of a Hall thruster willbe between 0.01 and 0.05 /^m/min.

EXPERIMENTAL SET UP: SIMULATEDION SPUTTERING

In order to simulate the effects of xenon ionsputtering on optical fiber, the fiber wasimmersed in an acid solution. Hydrofluoric acid(HF) was selected for this purpose, due to itshigh level of reactivity with silicon dioxide andother glasses. (NOTE: HF is an extremely

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

dangerous acid, and it is recommended that allsafety precautions be used.) The primary goal ofthis experiment was to observe the decrease intransmitted light level as a function of the opticalfiber cladding thickness as the cladding is etchedby the HF acid solution. Corning® SMF-28™single-mode glass fiber was used exclusively inthis experiment, which consists of an 8.2 |umdiameter core of SiO2 doped with GermaniumDioxide (GeO2), a 125 [urn outer diametercladding of essentially pure SiO2, a 245 p,m outerdiameter dual acrylate Corning CPC™ protectivecoating, and a final, 900 (tim outer diameterjacket. For light with a wavelength of 1550 nm,the indices of refraction of the core and claddingare 1.4505 and 1.4447, respectively.14 In theinitial preparation of the fiber, the 900 (nm outerjacket was removed using a hot plate, and the245 |iim acrylate coatings were removed in anacetone bath. These layers were removed from asegment of the fiber approximately 5 cm inlength, leaving the silicon dioxide claddingexposed for acid etching. The light signal wasprovided by a 1.6 mW, fiber coupled diode laser,which was determined to have an operatingwavelength of 1304 nm. A Gallium-Arsenide(GaAs) infrared detector was connected at theother end of the fiber to monitor output signalfrom the diode laser. All fibers had factory-installed FC style connectors on both ends,allowing for easy and repeatable couplings toboth the laser and the detector. The experimentalset up is shown in Fig. 11.

Two solutions of aqueous acid were used inetching, one of 49% HF and another of 20% HF(by weight). Hydrofluoric acid is commerciallyavailable in the 49% concentration, and thissolution has been shown to have an etch rate onsilicon oxides that varies between 1.8 and 2.3/^m/min.15 The fiber was supported in thesolution by a Teflon mount allowing it to becompletely surrounded by the acid solution. Inthis configuration, the acid was able to etch thecircumference of the fiber evenly from alldirections. This is physically different fromwhat would occur if the fiber were beingsputtered by energetic ions, which would impactonly on the surface facing the incident ion beam.The fibers were etched until no signal wasreceived at the detector.

RESULTS

Figures 12 and 13 shows the laser light intensityat X = 1304 nm as a function of time (claddingthickness) for two different SMF-28 fibers in a49% HF solution. The etch rate for thisconcentration of HF is estimated to be 1.6/^m/min which is similar to published results forsilica.15 In Fig. 12, the intensity remainsrelatively constant until the fiber is etched to adiameter of approximately 21.3 /mi. Based on acore diameter of 8.2 /mi, this corresponds to aremaining cladding thickness of 6.6 pirn or 5K.From Eq. (2), the characteristic length ofpenetration of the laser light into the claddingmaterial is approximately 3.5 //m or 2.7X. Thisrepresents the location at which the intensity ofthe evanescent wave is reduced by a factor of1/e; thus, the transmitted signal is not expectedto degrade until the cladding thicknessapproaches 1/p. The noise seen in the traces ofFigs. 12 and 13 is most likely due to theextremely fast etch rate, which is known to etchin a manner that leaves a rough surface. Notethat in Fig. 13, the fiber actually broke before theetch was completed, suggesting that ways toimprove the sensor's survivability will berequired. One alternative would be usemultimode fiber, which has a thicker corediameter (~ 60 /<m) that may allow for a morerobust sensor.

Figure 14 shows the laser intensity transmittedthrough the fiber as a function of time for an etchin a 20% HF solution. The etch rate is estimatedto be 0.16 /*m/min which suggests that the etchrate is not linear with HF concentration insolution. Again, the light intensity through thefiber remained relatively constant until the fibercladding was reduced to a thickness of about 6.6/mi. Although the etch rate is dependent onsolution concentration and can vary from fiber tofiber, the first indication of change in lightintensity occurs for the same cladding thicknessvery near 5X. The etch rate in 20% HF is anorder of magnitude lower than for the 49%solution; however, the noise level is still ratherhigh. This suggests that the nature of the HFetch of the cladding material (i.e. surfaceroughness) may be the major cause of the noise.

The repeatability of the intensity versus claddingthickness will be critical for sensor calibration.Figure 15 shows the results of four differentfibers etched in a 20% HF solution. Although

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

some of the traces exhibit a large amount ofnoise, the repeatability of the data is quite good.This suggests that a fiber sensor used toinvestigate sputter removal of cladding materialcan be adequately calibrated.

DISCUSSION: SENSOR UTILITY

The estimated range of sputter rates of the sensorcladding impacted along the surface normal byxenon ions 1 m downstream of a Hall thruster isbetween 0.01 and 0.05 ^m/min. This isapproximately an order of magnitude lower thanthe etch rate of the 20% HF solution. Therefore,the degradation in signal shown in Fig. 14 wouldoccur over approximately 15.5 hours at a sputterrate of 0.01 /^m/min. Since the sensors will mostlikely be attached to a spacecraft at high anglesrelative to the thrust vector, the sputter rate isexpected to vary over several orders ofmagnitude. However, the trend towards lowersputter rates at high angles due to decreased ionenergy and flux in the plume is somewhatcounteracted by the increase in sputter yield forshallow angles of incidence.

As an example, consider an FOCS placed 1 mfrom the exit plane of an SPT-100 Hall thrusteron a solar cell array at an angle of 45° from thethrust vector as shown in Fig. 16. Taking thelowest sputter rate for this geometry of 0.1 A/secor 6.0 x 10"4 ^m/min from Randolph, et al.,10 itwould require 258.3 hours to produce an effectsimilar to that seen in Fig. 14 (i.e. maximumfiber throughput decreasing to zero). This is areasonable fraction (-I/IO^) of the averagelifetime of an SPT-100 [ref. 6] implying that theFOCS sensitivity is adequate for missionapplicability. This analysis requires that thefiber cladding be reduced to a thickness of about5X prior to installation on the spacecraft.

The most important feature of the FOCS systemis the ability to gain spacecraft-thrusterinteraction data over a large fraction of thespacecraft geometry as alluded to in Fig. 1. In amission mode, several fibers can be placed onthe solar cell array with varying claddingthickness. This would allow a sputter analysis atdifferent operational times over the thrusterlifetime.

Commercially manufactured optical fibers havecladding thicknesses much greater than (3"1

where the evanescent wave intensity decreases tozero within the cladding. Therefore, it isnecessary to remove a significant portion of thecladding material before it can be useful inmeasuring the sputter potential of a thrustersystem. This requires that the remaining sensorbe extremely thin for single-mode fiber such asthat used in this study (core diameter = 8.2 /mi).Obviously this can severely weaken the fibercausing breakage. Using multimode fiber with athicker core diameter (~ 60 /<m) may help thesurvivability of the device while maintaining theFOCS sensitivity.

CONCLUSIONS

The FOCS principle of operation has beenverified as the cladding thickness decreased foran etch process in aqueous HF solution. Theexperimental results obtained in this studysimulate the sputtering of cladding material byenergetic ions in the plume of ion electricthrusters. The repeatability of the HF etchresults indicate that the sensor can be calibratedfor transmitted light intensity as a function ofcladding thickness. This information can then beused to estimate the sputter rate of an ion thrusterplume. Estimates for an SPT-100 Hall thrusterindicate that the sputter rate (0.01 to 0.05^m/min) will be approximately an order ofmagnitude lower than the etch rate (0.16/mi/min) observed for a 20% aqueous HFsolution (by weight). The slower etch rate andthe expected smoother "etch" from the ion plumemay reduce the amount of noise evident in thedata.

Initial estimates indicate that the transmittedlight intensity will go from maximum signal tozero signal in approximately 15.5 hours if thesensor is placed 1 m downstream of an SPT-100normal to the thruster centerline. This "effect"time is increased to about 258 hours for thesensor placed 1 m downstream at an angle of 45°from the thruster centerline. This is on the orderof II 10th of the design lifetime of a typical Hallthruster.

o

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

ACKNOWLEDGEMENTS

This work was funded by the Air Force Office ofScientific Research and the Air Force ResearchLaboratory. The authors wish to thank Dr.Ingrid Wysong (AFRL) for her support of thisproject. The authors also wish to thank Dr.Stephen Vargo (SiWave, Inc.) and Dr. ThomasMathew (USC) for their help with HF etchingprocedures and safety. The TRIM code wasmade available by IBM Research. The assistanceof Mr. Luke Uribarri (USC) and Mr. Josh Cripps(USC) is also greatly appreciated.

REFERENCES

1. Wood, B., Hall, D., Lesho, J., Uy, O., Boies,M., Silver, D., Benson, R., Dyer, J., Green, D.,Galica, G., "On-orbit Midcourse SpaceExperiment (MSX) satellite environment flightexperiments," AIAA paper 99-0252, January1999.

2. Spanjers, G., Schilling, J., Engelman, S.,Bromaghim, D., Johnson, L., "Preliminaryanalysis of contamination measurements fromthe ESEX 26 kW ammonia arcjet flightexperiment," AIAA paper 1999-2709, June1999.

3. Carter, M., Smith, J., Mowry, D., Patel, J.,"Reversible evanescent wave sensors forhydrazine," Proceedings of the InternationalSociety for Photo-Optical InstrumentationEngineers, Vol. 3540, 123-133, 1998.

4. Hecht, J., Understanding fiber optics. ThirdEdition, Prentice Hall, Upper Saddle River, NJ,pp. 64, 1999.

5. Ziegler, J., Biersack, J., Littmark, U., Thestopping and range of ions in solids, PergamonPress, New York, NY, 1985.

6. Tribble, A., Searing, R., Scheps, R.,Underwood, D., "Analysis of spacecraft-thrusterinteractions for the SPT-70 and SPT-100stationary plasma thrusters," AIAA paper 94-0330, January 1994.

7. Smith, R., Jakas, M., Ashworth, D., Oven, B.,Bowyer, M., Chakarov, I., Webb, R., Atomic andion collisions in solids and at surfaces: theory,simulation and applications, edited by R. Smith,

Cambridge University Press, Cambridge, UK,pp. 293,1997.

8. Ketsdever, A., "Design Considerations forCryogenic Pumping Arrays in Spacecraft-Thruster Interaction Facilities," Journal ofSpacecraft and Rockets, May-June, 2001 (to bepublished).

9. Boyd, L, "Computation of the plume of theD55 Hall thruster," AIAA paper 98-3798, July1998.

10. Randolph, T., Pencil, E., Manzella, D., "Far-field plume contamination and sputtering of thestationary plasma thruster," AIAA paper 94-2855, June 1994.

11. King, L., Gallimore, A., "Ion energydiagnostics in the plume of an SPT-100 fromthrust axis to backflow region," AIAA paper 98-3641, July 1998.

12. Wang, J., Brophy, J., Brinza, D., "3-Dsimulations of NSTAR ion thruster plasmaenvironment," AIAA paper 96-3202, July 1996.

13. Thomas, S., "Electron-irradiation effect inthe Auger analysis of SiO2," Journal of AppliedPhysics, Vol. 45, No. 1, pp. 161-166, January1974.

14. Corning, Inc. "Corning SMF-28 opticalfiber: product information," PI1036, April 2001.

15. Williams, K., Muller, R., "Etch rates formicromachining processing," Journal ofMicroelectromechanical Systems, Vol. 5, No. 4,pp. 256-269, December 1996.

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(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

Detector

Solar panel

Source

Source

OpticalFibers

Detector

Figure 1: Possible FOCS system configuration on a spacecraft solar cell array.

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

CORE

CLADDING

(A)

Oadding

4 - 2 0 2 4 6

Core

(B)

Siirrounding: Environment

•Evanescent Wave

Etdied Gadding

4 .2 0 2

Core

(Q

Figure 2: A.) Evanescent wave penetration into cladding material, B.) Light transmission through aconcentric fiber, C.) Light transmission with an etched or sputtered cladding.

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

Absorb"tion

k Contaninan"t Layer

CLADDING

(A)

Con~taninan"b Layer

CLADDING

(B)

Figure 3: Adsorbed molecule contamination on cladding material with a thickness of a few A,. A.)Absorption of light by contamination layer, B.) Contamination index of refraction different from claddingmaterial.

10

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

CLADDING

(A)

Xe

CLADDING

(B)Xe

CLADDING

(C)

Figure 4: Energetic ion sputtering of optical fiber cladding material. Transmitted light signal inverselyproportional to the cladding thickness. A.) Total internal reflection with relatively thick cladding, B.)Reduced cladding thickness due to ion sputtering, C.) Further reduction in cladding thickness.

11

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

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CD

P.00

0.01 -

0.001 -

0.00010.5 1

Ion Energy (keV)

1.5

Figure 5: Silicon and oxygen sputter yield as a function of incident xenon ion energy. Ions impact withnormal incidence on a silicon dioxide surface.

10

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O

0.1

0.01

—O~• B" — -Q

i- -o-

0° (Normal incidence)—^ -45°- B- - 85°

0.2 0.4 0.6

Ion Energy (keV)

0.€

Figure 6: Total sputter yield for silicon dioxide as a function of xenon ion energy for several impact angles(surface normal at 0°).

12

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

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Figure 7: Total sputter yield for silicon dioxide as a function of incident angle for various xenon ion impactenergies.

101

.oxEo

101

101

10 13

O D55 - 0.04mo SPT100-1.0m

-100 -50 0

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Figure 8: Hall thruster ion flux as a function of plume expansion angle. D55 data taken 0.04m downstreamof the thruster exit plane and SPT-100 data taken 1.0m downstream, [refs. 6,7]

13

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

X<§

U

100 200 300 400 500 600 700

Ion Energy / q (eV, wrt plasma)

Figure 9: Ion current as a function of ion energy at 1.0 m downstream on the SPT-100 thrust axis. [ref. 11]

o

§

§

10'

10'

10'

10'

10 13

-100 -50 50 100

Angle (degrees)

Figure 10: NSTAR ion engine flux as a function of angle at the thruster exit plane, [ref. 12]

14

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

FIBEROPTICCABLE

FIBER STRIPPEDTO GLADDING

DETECTOR'ACID BATH

Figure 11: Experimental set up.

1.4

1.2

£gO

P-,

0.8

0.6

0.4

0.2

-0.2

METER

0 500 1000 1500 2000

Time (sec)(A)

Figure 12: A.) Transmitted laser intensity as a function of time during 49% aqueous HF etch of cladding.

15

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

££

O

-0.21700 1750 1800 1850 1900 1950 2000 2050 2100

Time (sec)(B)

Figure 12: B.) Detailed view of signal loss starting with a cladding thickness of- 5 A..

££<D

O

1.4

1.2

0.8

0.6

0.4

0.2

-0.2500 1000 1500 2000 2500 3000

Time (sec)Figure 13: Transmitted laser intensity as a function of time during 49% aqueous HF etch of cladding.

16

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

IOH

fc

%

1.4

1.2

1

0.8

0.6

0.4

0.2 -

0 -

-0.214000 16000 18000 20000 22000

Time (sec)

Figure 14: Transmitted laser intensity as a function of time during 20% aqueous HF etch of cladding.

OPH

"O1)N

O

1000 2000 3000 4000

Adjusted Time (sec)

Figure 15: Normalized laser power transmitted through fiber as a function of time during 20% aqueous HFetch of cladding. Time adjusted to account for slightly different etch rate from fiber to fiber.

17

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

SPACECRAFT

SDLAR CELL ARRAY

FDCS SENSOR 45deg, OFF-AXISAND. In DOWNSTREAM DF THRUSTER_________L_

THRUSTER

OPTICAL FIBER GRID WITH FOGS CLADDINGSTRIPPED IN LOCATIONS' DF INTEREST

Figure 16: Example mission geometry.