Decel Grid Effects on Ion Thruster Grid Erosion

2122 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 5, OCTOBER 2008

Decel Grid Effects on Ion Thruster Grid ErosionRichard E. Wirz, John R. Anderson, Dan M. Goebel, Fellow, IEEE, and Ira Katz

Abstract—Jet Propulsion Laboratory (JPL) is currently assess-ing the applicability of the 25-cm Xenon Ion Propulsion System(XIPS) as part of an effort to infuse low-cost technically maturecommercial ion thruster systems into NASA deep space missions.Since these mission require extremely long thruster lifetimes toattain the required mission ΔV, this paper is focused on un-derstanding the dominate wear mechanisms that effect the lifeof the XIPS three-grid system. Analysis of the XIPS three-gridconfiguration with JPL’s CEX3D grid erosion model shows thatthe third “decel” grid effectively protects the accel grid frompits and grooves erosion that is commonly seen with two-gridion thruster grid systems. For a three-grid system, many of thecharge-exchange ions created downstream of the grid plane willimpact the decel grid at relatively low energies (∼25 V), instead ofimpacting the accel grid at high energies (∼200 V), thus reducingoverall erosion. JPL’s CEX3D accurately predicts the erosionpatterns for the accel grid, although it appears to overpredictthe pits and grooves erosion rates due, mainly, to uncertaintiesin incident energies and angles for sputtering ions and since itdoes not account for local redeposition of sputtered material. Sincethe model accurately simulates the erosion pattern but tends tooverpredict the erosion rates for both the two- and three-gridsets, this comparative analysis clearly shows how the decel gridsignificantly suppresses the erosion of the downstream surface ofthe accel grid as observed in experimental tests. The results alsoshow that the decel grid has a relatively small effect on barrelerosion (erosion of the aperture wall) and erosion of the upstreamsurface of the accel grid. Decreasing the accel grid voltage of theXIPS can reduce barrel (and total) erosion of the accel grid andshould be considered for high-ΔV missions.

Index Terms—Decel grid, grid erosion, ion thruster, Xenon IonPropulsion System (XIPS).

I. INTRODUCTION

NASA/JET Propulsion Laboratory (JPL) is considering theuse of commercial ion thruster technology for deep space

missions. L-3 Electron Technologies, Inc.’s 25-cm Xenon IonPropulsion System (L-3 ETI 25-cm XIPS) is an attractive candi-date due to its flight heritage, reliability, and performance. Sev-eral XIPS thrusters have thousands of hours of flight and groundtest data; however, to accommodate high-ΔV deep spacemissions, we must validate the thruster for tens of thousandsof hours of life. The Long-Duration Test (LDT), and then laterthe Extended Life Test (ELT), of the NASA Solar ElectricPropulsion Technology Application Readiness (NSTAR) ionthruster identified the erosion of the molybdenum accelerator

Manuscript received November 1, 2007; revised March 2, 2008 and April 25,2008. Current version published November 14, 2008. This work was carriedout by the Jet Propulsion Laboratory, California Institute of Technology, undercontract with the National Aeronautics and Space Administration.

The authors are with the Electric Propulsion at the Jet Propulsion Laboratory,Pasadena, CA 91109 USA (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPS.2008.2001041

Fig. 1. SEM images of the NSTAR accel grid after 30 352 h of operation.The accel grid experienced significant erosion on the downstream surface in a“pits & grooves” pattern. In the center region of the grids, the “pits” were wornthrough the grid around many of the apertures. Grid apertures were widened by∼25% while minimal erosion occurred on the upstream surface.

ion extraction grid (or simply “accel” grid) as a primary life-limiting mechanism for two-grid ion thruster life [1], [2].During the ELT, the accel grid apertures were eroded to asmuch as 25% beyond their original diameter; and, as shownin Fig. 1, pits were eroded entirely through the downstreamaccel grid face between many of the apertures near the thrusteraxis. The erosion on the downstream face is generally referredto as “pit & grooves” erosion while the erosion on the insideof aperture walls is referred to as “barrel” erosion. Both ofthese erosion types are important to minimize since they cancompromise the structural integrity of the grids, increase neutralpropellant loss, and increase the voltage required to preventelectron backstreaming.

The XIPS ion thruster uses a third molybdenum deceleratorgrid (“decel grid”), in addition to screen and accel molybdenumgrids. The decel grid is used on XIPS to minimize pits andgroove erosion of the accel grid, thus minimizing the amountof eroded molybdenum grid material that is ejected from thethruster. The primary disadvantage of a decel grid is the in-crease in complexity of the physical thruster assembly. To pre-dict the long-term life and performance of the XIPS thruster, wemust understand how the decel grid affects the erosion of the ac-cel grid. Reference [3] discusses that the decel grid reduces ioncurrent to the accel grid. Experimental tests by Brophy et al. [4]showed that the erosion rate for a 30-cm ion thruster founda greater than 100 times reduction in accel grid erosion whenusing a three-grid compared to a two-grid assembly.

More recently, 2-D and 3-D grid erosion models CEX2D andCEX3D were developed at JPL to help understand these erosionmechanisms [2], [5]. These models reveal that the erosion of theion extraction grids is due to charge-exchange (CEX) ions cre-ated between and downstream of the grids; this result was alsofound by grid erosion models created at other institutions [6].The culprit CEX ions are created when fast ions that areaccelerated through the grid apertures gain an electron fromslow neutral atoms, thus creating a slow ion and a neutral atom

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WIRZ et al.: DECEL GRID EFFECTS ON ION THRUSTER GRID EROSION 2123

that quickly travels downstream. A significant percentage ofthese slow ions (typically referred to as “CEX ions”) are bornin potential fields that accelerate them toward the accel gridat velocities sufficient to erode the grid surface. In previousefforts, the CEX2D and CEX3D models were compared againstLDT and ELT data for the NSTAR grid assembly and comparewell with the experimentally observed erosion behavior; how-ever, further code development is necessary to validate the exacterosion rates as discussed in [2] and [5]. For example, and mostimportantly to this effort, CEX3D accurately simulates the pitsand grooves erosion patterns observed on the accel grid fromthe LDT; however, the erosion rates from [2] overestimate theinitial wear rate by about 50%. This difference is due at least inpart to the uncertainties in sputter yield as a function of incidentenergy and angle, determination of local incident angles, andthe fact that the codes do not account for redeposition ofsputtered grid material locally at the erosion sites. Therefore,in this effort, we use the model results to perform a relativecomparison of erosion rates for a given geometry (using thebeginning-of-life (BOL) geometry) and not to determine long-term erosion values. This approach is valid for comparing theerosion of two- versus three-grid geometries since the uncer-tainties previously mentioned predominately affect the localsurface morphology over long durations; therefore the relativeaccuracy of the instantaneous erosion rates will not be affectedby differences between the two- and three-grid geometries.

A. Objective

In this paper, we examine the erosion characteristics of theXIPS accel grid in the presence of the decel grid and discusshow these characteristics will affect thruster life. To facilitatethis investigation, we first compare the erosion of the XIPSthree-grid accel grid with erosion of the NSTAR two-gridassembly and then compare the erosion of the XIPS accel gridwith and without the decel grid. We also investigate methodsfor reducing overall grid wear. The results presented herein arefor comparison of two- and three-grid systems and not intendedto provide absolute erosion rates.

II. ANALYSIS AND RESULTS

In this section, we compare the accel grid erosion of two-and three-grid geometries using JPL’s CEX3D grid erosionmodels. CEX3D is used since a 3-D domain is necessary toaccurately simulate the upstream erosion of the accel grid.All comparisons are for apertures near the thruster axis whereerosion is expected to be greatest for both the XIPS and NSTARthrusters due to the higher current density and intermediate neu-tral density near the axis [1], [7]. JPL’s ion thruster dischargemodel was used to determine upstream ion density, neutraldensity, and electron temperature conditions [8]. A detaileddescription of how CEX3D propagates these conditions intothe beamlet and computes erosion rates is given in [2]. Theaxial domain for all problems extends 5-cm downstream of theupstream surface of the screen grid to capture the erosion effectsdue to CEX ions created far downstream of the grid plane(note: Extending the axial domain to 10 cm produces a change

TABLE ITYPICAL THROTTLE POINTS FOR XIPS AND NSTAR

in the erosion rates predicted by the code of less than 0.5%since the majority of CEX that contribute to erosion are createdwithin 5 cm of the grid). The grid geometries and thrusterconditions used in the models are for BOL and include the hot-grip gap spacing observed during thruster testing [9]. Doubleions are not included in the analysis; however, equivalent beamcurrent densities are used. The propellant for all simulations isxenon, the propellant used by XIPS, NSTAR, and most noblegas electric thrusters. Throttle points used for model inputs areshown in Table I [1], [10].

A. Comparison of Accel Grid Erosion for XIPS and NSTARGrid Assemblies

In this section, we compare the BOL erosion rates of theaccel grid for the XIPS three-grid and NSTAR two-grid sys-tems. The XIPS and NSTAR grid systems have similar iontransparency. Ion transparency is a measure of the beamletcurrent extracted by a grid geometry for a given upstreamplasma density and grid voltage, which is analogous to theChild–Langmuir law for parallel electrodes. The screen andaccel grids for these thrusters have identical aperture diameters;however, the XIPS screen grid is thinner, and the screen-accelgrid gap is larger than the NSTAR thruster grid set. The thinnerXIPS screen grid tends to increase the ion transparency whilethe larger XIPS screen-accel spacing tends to decrease the iontransparency. Since these effects nearly offset, the performanceof the grid sets is comparable, although NSTAR has a slightlyhigher ion transparency. We compare the XIPS and NSTAR atthe TH15 maximum throttle condition for NSTAR operationas indicated in Table I. Since the diameter of the active gridarea of XIPS is ∼4 cm less than NSTAR, we used an upstreamion density for XIPS that is ∼18% greater than that usedfor NSTAR in these calculations to simulate equivalent thrustlevels. A XIPS thruster was successfully tested at the TH15throttle condition [10].

The results from Fig. 2 show that the erosion on thedownstream face is much greater for the two-grid geometry,while only a small amount of downstream erosion occurs atthe periphery of the accel grid aperture for the three-gridgeometry. The corresponding downstream erosion rates for asingle NSTAR and XIPS accel grid aperture are 0.826 and0.0173 mg/khr, respectively. To investigate the reason for thislarge difference in erosion rate between the two- and three-grid geometeries, we must determine where the CEX ions that


Fig. 2. Comparison of erosion of the downstream face of the accel grid for the NSTAR (two grid) and XIPS (three grid) geometries at TH15 as calculated byCEX3D. These data show that the decel grid of the three-grid geometry effectively shields the accel grid from erosion of the downstream face.

Fig. 3. Erosion rate (mg/khr/m) of downstream surface of accel grid due to CEX ions with respect to the position in the beamlet at which the CEX ions aregenerated; (above) XIPS three grid and (below) NSTAR two grid. The discharge sources are at the left-hand end of these figures. Axial domain used for CEX3Derosion calculations is approximately three times as long as shown in these images. This result shows that the decel grid effectively eliminates erosion of thedownstream surface of the accel grid due to CEX ions created downstream of the grid plane.

contribute to downstream erosion are generated in the beam. Todo this, we generated contour plots that show the magnitudes ofdownstream erosion caused by CEX ions with respect to the lo-cation from which they are generated. In Fig. 3, the downstreamerosion rate due to CEX ions created in the beam is plotted

as mg/khr/m, where “mg” is milligrams of molybdenum and“m” is meter of axial distance. These units avoid overweighingthe importance of downstream cells, since the cell sizes growlarger axially further downstream where CEX3D requires lessresolution. From the erosion rates in Fig. 3, we see that the decel


Fig. 4. Potential (φ) profiles for (above) XIPS three grid and (below) NSTAR two grid for throttle point TH15. Potentials for the two-grid geometry lead tohigh-energy impact (∼200 eV) of CEX ions on downstream accel grid surface, while the potentials created by the decel grid lead to relatively small impactenergies (∼25 V) and, hence, minimal erosion.

TABLE IICOMPARISON OF ACCEL GRID EROSION RATES (mg/khr) AT BOL

grid effectively eliminates almost all of the erosion due to CEXions created downstream of the grid plane.

The CEX ions generated in the beamlet originally havevery low velocity since they are made from the relativelyslow moving neutrals. The potentials in Fig. 4 show that theCEX ions generated in the downstream region of the beamletare influenced by the local electric field to move away fromthe beamlet axis. Therefore, for a two-grid geometry, a largefraction of the CEX ions created downstream are guided to the“pits & grooves region” between the apertures and acceleratedthrough a large electric field to energies (over 200 eV) that willcause relatively significant erosion on the upstream face of theaccel grid as shown in Figs. 1 and 2. For the three-grid geom-etry, most of these CEX ions are accelerated through smallerelectric fields due to the much higher potential of the decelgrid (0 V), resulting in relatively low impact energies (∼25 eV)incident to the decel grid. As discussed in [11], the sputter yieldfor molybdenum is three orders of magnitude smaller at theselower energies (in comparison to energies ≥ 200 eV) and istherefore insufficient to cause comparably noticeable erosion.This minimal erosion of the decel grid agrees with observationsfrom XIPS long-duration tests [5].

The erosion rates for the TH15 condition (as well as otherconditions, discussed in the next section) are summarized inTable II. From these values, we see that the barrel erosion issimilar for both geometries, although slightly lower for XIPS.The erosion of the upstream face of the accel grid is muchsmaller for the NSTAR grid. The lower upstream erosion forthe NSTAR geometry is due to the smaller spacing of the screenand accel grids, which provides a smaller region for barrel

erosion CEX ions, as discussed in [5]; also, the upstream iondensity used for XIPS is ∼18% higher.

B. Effect of Decel Grid on Accel Grid Erosion for the XIPSThree-Grid Configuration

In this section, we employ results from JPL’s CEX3D codeto understand how the decel grid affects accel grid erosionby comparing results for identical accel and screen grids withand without a decel grid, assuming XIPS grid geometries andpotentials. Fig. 5 shows the comparison of the “pits & grooves”and “bridge” erosion for the XIPS three-grid with a hypothet-ical XIPS two-grid configuration (assuming the decel grid isremoved) for the XIPS high-power throttle point. As discussedin the previous section, these results show that the decel grideffectively shields the downstream face of the accel grid fromerosion except for a small amount of erosion just around theperiphery of the accel grid aperture. Referring to Table II, thedecel grid reduces the downstream accel grid erosion over anorder of magnitude for both the high- and low-power operatingconditions. Referring to Figs. 6 and 7, we see that the electricfields are such that the majority of the CEX ions generateddownstream are guided away from the center of the beamlet andback toward the region between the apertures. As discussed inthe previous section, for the two-grid geometries the potentialfields result in high ion impact energy on the pits and groovesregion of the accel grid, while the three-grid geometries createa potential field that causes these same CEX ions to impact thedecel grid at much lower energies, resulting in significantly lesssputtering.


Fig. 5. Comparison of downstream erosion of the accel grid for XIPS two-grid and three-grid geometry at high power. The ordinate axis for the pits andgrooves erosion is an order of magnitude greater than the bridge erosion plot(see Fig. 2 for orientation of plots’ “Location” axes).

Referring to Table II, results for the XIPS low- and high-power conditions show that the barrel and upstream surfaceerosion rates are much less at low power. Reduced erosionfor the low-power condition for these surfaces is due in partto lower beamlet current as well as improved beam focusing.Improved beamlet focusing (also known as operating closer tothe optimal perveance fraction [5]) results in lower hole wallerosion since, upstream of the accel grid, only the CEX ionscreated near the edge of the beamlet contribute to barrel erosion,while the CEX ions created near the center of the beamlet tendto get accelerated into the beam [5]. The erosion results inTable II suggest that the improved focusing realized at the low-power condition also reduces the upstream erosion rate.

For the high-power condition, the barrel erosion for the two-grid geometry is about 33% greater than the three-grid geom-etry (0.750 and 0.562 mg/khr, respectively). This differenceappears largely due to the near-axis electric field just down-stream of the accel grid; for the two-grid geometry, the potential

field (Fig. 7) is such that a larger fraction of the downstreamCEX ions are generated in a potential field that will guide themback to the accel grid hole wall. Upstream erosion of the accelgrid is slightly higher for the two-grid geometry, 0.225 mg/khrcompared to 0.213 mg/khr for the three-grid geometry.

At the low-power point, we see (from Table II) that theaccel barrel erosion is ∼11% higher, and the upstream accelerosion is 27% higher for the three-grid configuration; however,the total erosion rate for the accel grid two-grid system isseveral times higher due to the much larger downstream erosiondiscussed earlier. The screen grid experiences minimal erosionfor both configurations due to low incident CEX ion energies,which is confirmed by experimental observations [1].

C. XIPS Accel Voltage Sensitivity

From the above results, we see that the barrel erosion ratefor the XIPS low-power condition (0.104 mg/khr) is noticeablyhigher than the rate for XIPS at TH15 (0.070 mg/khr). Sincethe two conditions use similar upstream plasma conditions, it isapparent that the lower voltage of the XIPS accel grid for thelow-power case (−300 V compared to −180 V for TH15) islikely the primary reason for the increased erosion rates, sinceit will result in increased impact energy of the CEX ions. The−300-V accel grid voltage used by XIPS is conservative sinceit is much lower than the electron backstreaming limit for theXIPS grid system [10]. Therefore, we can examine the changein erosion rate at higher accel voltages. To analyze the effect ofaccel voltage on barrel erosion rate for the XIPS thruster, weran the model for accel voltages −300, −240, and −180 V. Theresults from this analysis, shown in Fig. 8, suggest that higheraccel voltages will noticeably decrease barrel erosion for theXIPS accel grid. However, for the high-power case, an accelvoltage of −180 V increases the upstream erosion due to directimpingement caused by under focusing of the beam. From theseresults, we see that the optimal accel voltage from an erosionstandpoint is higher than the standard −300 V XIPS operatingcondition; however, one must consider electron backstreamingand changes in thruster geometry due to erosion to determinethe most desirable accel grid voltage for long-term missionperformance.

III. DISCUSSIONS AND CONCLUSION

Analysis of the XIPS grids with JPL’s CEX3D grid erosionmodel shows that, for BOL conditions, the decel grid signifi-cantly reduces the total erosion rate of the accel by effectivelyeliminating the pits and grooves erosion of the downstreamaccel surface commonly observed for two-grid systems. For alloperating conditions and geometries examined herein, the decelgrid reduced the downstream erosion of the accel grid by overan order of magnitude. Examining the results, it is apparent thatmost of the CEX ions created in the downstream region of thebeamlet are accelerated away from beamlet axis and towardthe pits and grooves region of the accel grid. For a two-gridgeometry, these CEX ions are accelerated through a potentialof at least 200 V, due to the low potential of the accel grid(−300 to −180 V), before impacting the upstream surface of


Fig. 6. Potential (φ) profiles for XIPS two- and three-grid geometries at high power.

Fig. 7. Potential (φ) profiles for XIPS two- and three-grid geometries at low power.

Fig. 8. Barrel erosion versus accel grid voltage for XIPS grid set. Lower accel grid voltage reduces barrel erosion but can increase erosion at high-power conditiondue to the onset of direct impingement on the upstream accel grid surface.

the accel grid at energies sufficient to cause the significant pitsand grooves erosion seen in many experiments. For a three-gridgeometry with the decel grid at only 0 V, the majority of thesepits and grooves ions see only a ∼25 V potential drop, thusattaining energies that lead to sputter yields that are three ordersof magnitude less than yields for the ≥ 200 V ions for the two-grid case. Therefore, the decel grid protects the accel grid frompits and grooves erosion while sustaining minimal erosion ofits own upstream surface. These observations are schematicallysummarized in Fig. 9 using approximate trajectories inferredfrom the information in Figs. 3, 4, 6, and 7 for CEX ions born at

relatively slow velocities. Per the recommendation of a reviewerof this paper, in future efforts, we will use the model to showsimulated CEX ion trajectories.

JPL’s CEX3D accurately predicts the erosion patterns forthe accel grid, although it appears to overpredict the pits andgrooves erosion rates due mainly to uncertainties in incidentenergies and angles for sputtering ions and lack of knowledgeof redeposition of sputtered material [2]. Since the model accu-rately simulates the erosion pattern but overpredicts the erosionrates for both the two- and three-grid sets, this comparativeanalysis is sufficient to show how the decel grid significantly


Fig. 9. CEX ions created in the downstream region of the beamlet attain significant impact energies in the two-grid configuration, leading to the pits and grooveserosion seen during NSTAR testing. In a three-grid geometry, the decel grid effectively protects the accel grid while sustaining minimal erosion due to muchsmaller impact energies. Trajectories shown are approximate.

suppresses the erosion of the downstream surface of the accelgrid as observed in experimental tests [10], [12].

Erosion rates for the aperture wall (barrel) were similar forthe cases examined; the most significant change was seen whenthe decel grid was removed from the XIPS thruster at highpower, resulting in a 33% increase in barrel erosion for thehypothetical two-grid XIPS geometry. As expected, decreasingthe accel grid voltage of the XIPS reduces barrel erosion ofthe accel grid. For BOL, the results show that an accel voltageas low as −180 V will decrease erosion of the accel grid forthe low-power case. From an erosion standpoint, an optimalvoltage for the XIPS high-power condition lies between −300and −180 V.

The significantly higher erosion for the XIPS high-powercase is due to the fact that the beamlet is far from the optimalperveance fraction. Essentially, at low power and TH15 thebeamlets are much better focused (closer to the optimalperveance fraction compared with the high-power case) anda larger fraction of CEX ions are created near the beamletaxis upstream of the grid plane; from the near-axis locationthe CEX ions are more likely to be accelerated into the beaminstead of into the grids where they can cause erosion [12]. Forthe high-power case, the beamlet occupies more of the apertureregion away from the beamlet axis; CEX ions created on theperiphery of these large beamlets are likely to cause erosion ofthe accel grid.

In future analyses, we will use additional experiments andcomputational modeling to determine the optimal accel gridvoltage for long term thruster life and performance to meetNASA mission needs. These efforts will include considerationof the electron backstreaming, double ion effects, detailederosion estimates for all grid surfaces, and the influence of thetemporal morphology of the grids on long-term erosion andelectron backstreaming limits. Future efforts will also includedetailed comparison with XIPS grid erosion once this informa-tion is available. We will also improve the codes by includingredeposition of the sputtered grid material and angular depen-dence of sputtering rates.

REFERENCES

[1] J. E. Polk, J. R. Anderson, and J. R. Brophy, “An overview of the resultsfrom an 8200 hour wear test of the NSTAR ion thruster,” presented atthe 35th Joint Propulsion Conf.Paper AIAA 99-2446, Los Angeles, CA,Jun. 1999, AIAA 99-2446.

[2] J. R. Anderson, I. Katz, and D. Goebel, “Numerical simulations oftwo-grid ion optics using a 3D code,” presented at the 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conf. Exhibit, Fort Lauderdale, FL,Jul. 2004, Paper AIAA 2004-3782.

[3] V. Rawlin and C. Hawkins, “Increased capabilities of the 30-cm diam-eter Hg Ion thruster,” in Proc. Conf. Adv. Technol. Future Space Syst.,Hampton, VA, May 1979.

[4] R. Brophy, L. Pless, and C. Garner, “Ion engine endurance testing at highbackground pressures,” presented at the AIAA/SAE/ASME/ASEE 28thJoint Propulsion Conf. Exhibit, Nashville, TN, Jul. 1992, Paper AIAA-1992-3205.


[5] J. R. Brophy, I. Katz, J. Polk, and J. R. Anderson, “Numerical simu-lations of ion thruster accelerator grid erosion,” presented at the 38thAIAA/ASME/SAE/ASEE Joint Propulsion Conf. Exhibit, Indianapolis,IN, Jul. 2002, Paper AIAA 2002-4261.

[6] J. Wang, J. Polk, J. Brophy, and I. Katz, “Three-dimensional particle sim-ulations of ion-optics plasma flow and grid erosion,” J. Propuls. Power,vol. 19, no. 6, pp. 1192–1199, 2003. 0748-4658.

[7] R. Wirz, “Discharge plasma processes of ring-cusp ion thrusters,”Ph.D. dissertation, Aeronautics, Caltech, Pasadena, CA, 2005. [Online].Available: http://etd.caltech.edu/etd/available/etd-05232005-162628/

[8] R. Wirz and I. Katz, “Plasma processes of DC ion thruster dischargechambers,” presented at the 41st AIAA/ASME/SAE/ASEE Joint Propul-sion Conf., Tucson, AZ, Jul. 2005, Paper AIAA-2005-3690.

[9] E. M. Diaz and G. C. Soulas, Grid Gap Measurement for an NSTAR IonThruster. IEPC-2005-244.

[10] W. G. Tighe et al., “Performance evaluation of the XIPS 25-cm thrusterfor application to NASA discovery missions,” presented at the 42ndAIAA/ASME/SAE/ASEE Joint Propulsion Conf. Exhibit, Sacramento,CA, Jul. 2006, Paper AIAA 2006-4666.

[11] R. P. Doerner, D. G. Whyte, and D. M. Goebel, “Sputtering yield measure-ments during low energy xenon plasma bombardment,” J. Appl. Phys.,vol. 93, no. 9, pp. 5816–5823, May 1, 2003.

[12] J. R. Beattie, J. N. Matossian, and R. R. Robson, “Status of xenon ionpropulsion technology,” J. Propuls. Power, vol. 6, no. 2, pp. 145–150,Mar./Apr. 1990.

Richard E. Wirz received the B.S. degree inaerospace engineering and the B.S. degree in oceanengineering from Virginia Polytechnic Institute andState University (Virginia Tech), Blacksburg, in1992 and 1993, respectively, and the M.S. and Ph.D.degrees in aeronautics and applied sciences fromCalifornia Institute of Technology (Caltech),Pasadena, in 2001 and 2005, respectively.

He is a Senior Engineer with the Electric Propul-sion Group, Jet Propulsion Laboratory, Pasadena,CA, specializing in modeling, experimental testing,

and mission integration of electric thrusters. He designed and developed theworld’s first noble gas miniature ion thruster and is a recognized expert inminiature and precision plasma thruster development, plasma modeling forelectric thrusters, and advanced propulsion concepts. He has three patentspending in this area and has authored over 30 publications. Previously, he wasthe Manager of Renewable Energy Technologies at Gibbs & Cox, Inc. andTechnical Lead at SeaSun Power Systems, Inc., both in Alexandria, VA, wherehe developed alternative energy technologies.

John R. Anderson, photograph and biography not available at the time of thepublication.

Dan M. Goebel (M’93–SM’96–F’99) received theB.S. degree in physics, the M.S. degree in electri-cal engineering, and the Ph.D. degree in appliedplasma physics from the University of California,Los Angeles, in 1977, 1978, and 1981, respectively.

He is a Senior Research Scientist with the JetPropulsion Laboratory, Pasadena, CA, where he isresponsible for the development of high-efficiencyion thrusters, advanced long-life components such ascathodes and grids, and thruster life model valida-tion for deep space missions. Previously, he was a

Research Scientist with Hughes Research Laboratories (HRL), Malibu, CA,and Principal Scientist with Boeing Electron Dynamic Devices, Inc., Torrance,CA, where he was the Supervisor of the Advanced Technology Group for mi-crowave tube development and the Lead Scientist of the Xenon Ion PropulsionSystem ion thruster program for commercial satellite station keeping. He isa recognized expert in advanced plasma and ion sources, microwave sources,high-voltage engineering and pulsed-power switches. He is the author of over100 technical papers, one book entitled Fundamentals of Electric Propulsion:Ion and Hall Thrusters to be published this year, and is the holder of 42 patents.

Dr. Goebel is the Chair of the American Institute of Aeronautics and Astro-nautics Electric Propulsion Technical Committee, Chair of the IEEE ElectronDevices Society (EDS) Technical Committee on Vacuum Devices, memberof the IEEE EDS Publications committee, and Life Member of the AmericanPhysical Society and Sigma Xi. In 2004, he received the William Dunbar HighVoltage Research Achievement Award.

Ira Katz received the B.S. degree in physical chemistry from Case Institute ofTechnology, Columbus, OH, in 1967, and the Ph.D. degree in chemical physicsfrom the University of Chicago, Chicago, IL, in 1971.

He is the Supervisor of the Electric Propulsion Group; Jet Propulsion Labora-tory (JPL), Pasadena, CA, which is responsible for electric propulsion systemsfor JPL’s space science missions. Previously, he was a Senior Vice Presidentof Maxwell Technologies’ S-Cubed Division, where he led investigationsin spacecraft–plasma interactions and electric propulsion generated plasmas.He is a recognized leader in computer models of ion thruster physics andspacecraft charging, and has authored over 70 peer-reviewed articles and almost100 conference publications.

Decel Grid Effects on Ion Thruster Grid Erosion

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