Eric J. Pencil, John E. Foster, Michael J. Patterson, Esther M. Diaz,

Jonathan L. Van Noord, and Heather K. McEwen

Glenn Research Center, Cleveland, Ohio

Ion Beam Characterization of a NEXT

Multi-Thruster Array Plume

NASA/TM—2006-214423

November 2006

AIAA–2006–5182
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Prepared for the

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Sacramento, California, July 9–12, 2006

Eric J. Pencil, John E. Foster, Michael J. Patterson, Esther M. Diaz,

Jonathan L. Van Noord, and Heather K. McEwen

Glenn Research Center, Cleveland, Ohio

Ion Beam Characterization of a NEXT

Multi-Thruster Array Plume

NASA/TM—2006-214423

November 2006

AIAA–2006–5182
Acknowledgments

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Level of Review: This material has been technically reviewed by technical management.
NASA/TM—2006-214423 1

Ion Beam Characterization of a 

NEXT Multi-Thruster Array Plume 

Eric J. Pencil, John E. Foster, Michael J. Patterson, Esther M. Diaz, 

Jonathan L. Van Noord, and Heather K. McEwen 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Three operational, engineering model, 7-kW ion thrusters and one instrumented, dormant thruster 

were installed in a cluster array in a large vacuum facility at NASA Glenn Research Center. A series of 

engineering demonstration tests were performed to evaluate the system performance impacts of operating 

various multiple-thruster configurations in an array. A suite of diagnostics was installed to investigate 

multiple-thruster operation impact on thruster performance and life, thermal interactions, and alternative 

system modes and architectures. The ion beam characterization included measuring ion current density 

profiles and ion energy distribution with Faraday probes and retarding potential analyzers, respectively. 

This report focuses on the ion beam characterization during single thruster operation, multiple thruster 

operation, various neutralizer configurations, and thruster gimbal articulation. Comparison of beam 

profiles collected during single and multiple thruster operation demonstrated the utility of superimposing 

single engine beam profiles to predict multi-thruster beam profiles. High energy ions were detected in the 

region 45° off the thruster axis, independent of thruster power, number of operating thrusters, and facility 

background pressure, which indicated that the most probable ion energy was not effected by multiplethruster operation. There were no significant changes to the beam profiles collected during alternate 

thruster-neutralizer configurations, therefore supporting the viability of alternative system configuration 

options. Articulation of one thruster shifted its beam profile, whereas the beam profile of a stationary 

thruster nearby did not change, indicating there were no beam interactions which was consistent with the 

behavior of a collisionless beam expansion. 

Nomenclature 

DCA Discharge Cathode Assembly 

DCIU Digital Control Interface Unit 

EM Engineering Model 

FP Full Power, corresponding to ~ 6860 W input power 

GRC Glenn Research Center 

IP Intermediate Power, corresponding to ~ 2780 W input power 

Ja Accelerator grid impingement current, mA 

Jb Beam current, A 

LP Low Power, corresponding to ~ 1120 W input power 

mN milli-Newton 

Mn Neutralizer Flow Rate, sccm Xe 

MTAT Multi-Thruster Array Test 

NCA Neutralizer Cathode Assembly 

NEXT NASA's Evolutionary Xenon Thruster 

PAT Performance Assessment Test 

Pin Input power into thruster, W 

PMS Propellant Management System 

PPU Power Processing Unit 
NASA/TM—2006-214423 2

Ptrue Facility pressure corrected for xenon, Torr 

RPA Retarding Potential Analyzer 

Va Acceleration voltage, V 

Vb Beam voltage, V 

Vg Coupling voltage (neutralizer common-to-facility ground), V 

I. Introduction 

The NASA Glenn Research Center is responsible for the development of NASA’s Evolutionary 

Xenon Thruster (NEXT) ion propulsion system. The objective of the NEXT project is to advance next 

generation ion propulsion technology readiness. The NEXT system consists of a high-performance, 7-kW 

ion thruster; a lightweight, high-efficiency, 7-kW power processor unit (PPU); a highly flexible advanced 

xenon propellant management system (PMS); a lightweight engine gimbal; and key elements of a digital 

control interface unit (DCIU) including software algorithms (refs. 1 to 4). This design approach was 

selected to provide future NASA science missions with the greatest value in mission performance benefit 

at a low total development cost. Technology validation and mission analysis efforts, conducted in Phase 

1, indicated the NEXT propulsion system can provide the capabilities to achieve aggressive outer 

planetary science missions and that further development was warranted in Phase 2 (ref. 5). 

A multi-thruster array test (MTAT) was beneficial to address thruster and gimbal-specific questions 

that drive the configuration of the ion propulsion system components as well as the configuration of the 

final multi-thruster system test to be executed at the completion of Phase 2. This MTAT utilized multiple 

engineering model (EM) NEXT ion thrusters as well as laboratory power consoles and laboratory 

propellant feed systems to operate multiple thrusters simultaneously. The engineering demonstration 

portion of MTAT focused on the characterization of performance and behavior of the individual thrusters 

and the array as affected by the simultaneous operation of multiple ion thrusters. The main focus of this 

characterization is a four-NEXT thruster array in a “3+1” configuration, where three thrusters are 

operational and one thruster is dormant (a spare). The results of the engineering demonstration are 

reported in Reference 6 and address the following objectives: 

• Document multi-thruster system performance and provide data to predict array lifetime; 

• Assess performance and lifetime implications of multi-thruster operations on individual thrusters 

as a function of operating mode, gimbal angle, and array configuration; 

• Assess thermal interactions; 

• Document alternative system modes and architectures, including various neutralizer 

configurations; and 

• Assess impact of multi-thruster operations on PPU and PMS subsystem performance. 

The MTAT physics effort focuses on the characterization of the plasma environment generated by the 

simultaneous operation of multiple ion thrusters. The interaction of this plasma environment with the 

spacecraft and the thrusters themselves plays an important role in the determination of spacecraft 

configuration, acceptable array operating condition, and array lifetime. The purpose of the MTAT physics 

effort is to address the following objectives: 

• Assess the effect of multi-thruster operation on ion thruster grid lifetime; 

• Assess how spacecraft structures are affected by the plasma particles and potential fields due to 

multi-thruster operation including gimbaling; 

• Assess how performance parameters such as neutralizer margin and plume divergence are 

affected by multi-thruster operation; and 

• Assess how the dormant thruster is affected by the ambient plasma environment produced by the 

multi-thruster array. 
NASA/TM—2006-214423 3

This paper documents the characterization of the ion beam produced by the MTAT using Faraday 

probes and a retarding potential analyzer. Specifically the ion beams are characterized during single- and 

multi-thruster operation to assess the degree of beam-beam plume interactions. The ion beams are 

characterized during various neutralizer configurations to investigate any unanticipated beam-neutralizer 

plume interactions during alternate neutralizer operation. Finally the ion beams are characterized during 

gimbal articulation to determine if significant beam-beam interactions exist. Significant changes to the 

thrust-generating ion beams during these test segments would indicate potential changes to the 

performance of the ion propulsion system due to operation in a multi-thruster array configuration. 

Companion papers document array local plasma (ref. 7), electron flowfield characteristics of the 

plume(ref. 8), and neutralizer coupling characteristics (ref. 9). 

II. Experimental Apparatus 

A. Array Configuration 

The thruster array consisted of four NEXT EM ion thrusters (EM1, EM2, EM4, and EM5) all 

manufactured at NASA GRC. The configuration of the three active thrusters is as identified in table 1 

with 'New' referring to pristine, previously untested hardware. The array's nominal configuration was a 

“3+1” geometry, whose spacing and thruster locations were chosen based on a spacecraft design activity 

conducted for a Titan orbiter mission using a NEXT ion propulsion system (ref. 10); the baseline thruster 

spacing was 0.64 m center-to-center both vertically and horizontally [0.91 m center-to-center diagonally]. 

The NEXT array in this nominal configuration is shown in figures 1 and 2. For these tests EM2 

functioned as the dormant (flight spare) thruster. Its optics were replaced with a collector gird and 

outfitted with plasma probes. Laboratory propellant feed systems and laboratory power consoles were 

used to operate the individual thrusters. Vacuum Facility 6 (VF6), a space simulation chamber at NASA 

GRC, was used for all array testing. VF6 was 7.6 m in diameter and 22.9 m long and was evacuated with 

12 cryogenic pumps. The array assembly was installed into the facility such that the thrusters fired along 

the long-axis of the facility. Additional information about the thruster array and supporting equipment can 

be found in reference 6. 

TABLE 1.—CONFIGURATION OF ACTIVE EM THRUSTERS 

Component EM1 EM4 EM5 

DCA New New New 

NCA New New New 

Ion Optics [Beam dia.] Pre-Operated [40 cm] Pre-Operated [36 cm] New [40 cm] 

Discharge Chamber Pre-Operated New New 

B. Planar Faraday Probes 

Circular, planar Faraday probes were implemented to characterize the ion density profile. The probes 

had a 1 cm2

 circular collection area made from molybdenum, and were surrounded by a grounded guard 

ring. Six probes were mounted on a vertical probe rake which was in turn mounted onto a two-axis 

motorized motion control system for measuring the ion beam current densities. Figure 3 shows a 

schematic of the path of the planar probes relative to the thruster array. Table 2 details the vertical 

locations of probes 1-6 on the probe rake relative to the array centerline. Probe 2 traversed the centerlines 

of EM1 and EM2, whereas Probe 6 traversed the centerlines of EM4 and EM5. The positioning system 

swept the probes in the radial (x') and axial (z') directions downstream of the engine ion optics, with 1.5 m 

travel in each axis. Data were collected during the outbound segment of the sweep or the + x' direction 

only. 
NASA/TM—2006-214423 4
NASA/TM—2006-214423 5

TABLE 2.—PLANAR FARADAY PROBE VERTICAL POSITIONS ALONG 

VERTICAL PROBE RAKE. POSITIVE DISTANCES DENOTE DISTANCE 

ABOVE ARRAY CENTERLINE. NEGATIVE DISTANCES DENOTE 

DISTANCE BELOW ARRAY CENTERLINE 

Probe Number Probe Location 

1 721 mm 

2 318 mm [aligned with centers of EM1 and EM2] 

3 157 mm 

4 0 mm [array centerline] 

5 –158 mm 

6 –317 mm [aligned with centers of EM4 and EM5] 

C. Off-Axis Retarding Potential Analyzer (RPA) 

The off-axis RPA, used to detect and determine the energy of the off-axis beam ions, was a 

commercially available, gridded Faraday cup. The entrance diameter was 5.0 mm while the physical 

diameter of the probe face was 28 mm. The probe had three grids: a grounded entry grid, an ion-retarding 

grid, and an electron-repelling grid biased to –27 V. The collector was biased to –32 V. The RPA data 

were collected by ramping the ion retarding grid up to 2000 V and recording the voltage and collected 

current as measured by an electrometer. In all cases, the probe current signal dropped to zero amperes 

near the beam voltage, thereby making scans to appreciably higher voltages unnecessary. As the ion 

repelling grid was biased more positive, it repelled ions of increasing energy. This provided a currentvoltage (I-V) characteristic from which the most probable ion energy and the ion energy distribution 

could be obtained. The off-axis RPA was located 1 m axially downstream and 1 m radially from the 

centerline of EM1 (45° from the thruster centerline). The off-axis RPA was vertically located at the same 

height as the centerline of EM1. 

D. Operating Conditions 

Table 3 lists the key thruster operating parameters of the thrusters during the test sequences discussed 

in this report. Reference 6 provides additional details about the experimental set up and instrumentation 

used to measure these parameters. 
NASA/TM—2006-214423 6

TABLE 3.—THRUSTER OPERATING PARAMETERS DURING VARIOUS TEST SEGMENTS. 

Thruster Condition Vb 

(V) 

Jb 

(A) 

Va 

(V) 

Ja 

(mA) 

Ptrue 

(torr) 

EM1@FP 1790 3.52 –211 12.6 1.2×10–6

EM4@FP 1788 3.52 –210 13.6 1.2×10–6

EM5@FP 1788 3.52 –210 13.7 1.1×10–6

EM1 during (EM1+EM4+EM5)@FP 1790 3.55 –210 22.2 3.0×10–6

EM4 during (EM1+EM4+EM5)@FP 1789 3.53 –210 23.5 3.0×10–6

EM5 during (EM1+EM4+EM5)@FP 1790 3.54 –210 22.9 3.0×10–6

EM1@IP 1169 2.00 –200 5.4 7.1×10–7

EM1@LP 671 1.21 –115 3.6 4.6×10–7

EM1 during (EM1+EM5)@LP 

 with 2 neutralizers 670 1.20 –115 3.6 8.2×10–7

EM5 during (EM1+EM5)@LP 

 with 2 neutralizers 670 1.20 –115 3.7 8.2×10–7

EM1 during (EM1+EM5)@LP 

 with 1 neutralizer 669 1.20 –115 3.7 7.5×10–7

EM5 during (EM1+EM5)@LP 

 with 1 neutralizer 669 1.20 –115 3.7 7.5×10–7

EM1 during (EM1+EM5)@FP 1791 3.54 –210 17.1 2.0×10–6

EM5 during (EM1+EM5)@FP 1789 3.52 –210 17.9 2.0×10–6

III. Results and Discussion 

A. Single Thruster Operation—Ion Current Density Profiles 

Figure 4 shows the beam profiles collected by Probe 2 across the thruster centerline for EM1 at full 

power operation (EM1@FP), corresponding to 6860 W into the thruster. The peak current density 

collected at z' = 250 mm was 2.3 mA/cm2

, which was near the center of the thruster, located at –320 mm. 

The left hand side of the beam profile for EM1 was truncated due to the length of travel limitations of the 

motion control tables. The peak current dropped exponentially and the beam widened as the axial distance 

increased as expected. The beam profile was approximately symmetric about the centerline. By assuming 

axisymmetry it is possible to integrate the radial beam profile and compare the results to the beam current. 

Integration of the beam profile from the thruster centerline toward the array centerline, yielded a current 

which was 5 to 12 percent higher than the measured beam current. The discrepancy was consistent with 

the effect of charge exchange ions and background plasma which would likely add to the measured 

current density profile. The beam divergence half angle was calculated to be 24 ± 1°, which is consistent 

with previous results obtained with this thruster design (ref. 11). 

Figure 5 shows the beam profiles collected by Probe 6 across the thruster centerline for EM4 at full 

power operation (EM4@FP). The peak current density collected at z' = 250 mm was 2.3 mA/cm2

, which 

was near the center of the thruster, located at –320 mm. Although a slight azimuthal asymmetry was 

noted, the beam profile was approximately symmetric about the thruster centerline, since the current 

density radial profiles matched within 4 percent when comparing the right-hand and left-hand sides. The 

peak current density of the beam profile decreased as the axial distance increased. The beam profile 

widened as the axial distance increased. Integration of the beam profile for EM4 matched the beam 

current to within 5 to 12 percent. The beam divergence half angle was determined to be 25 ± 1°. 
NASA/TM—2006-214423 7
NASA/TM—2006-214423 8

The beam profiles collected by Probe 6 across the thruster centerline for EM5 at full power operation 

(EM5@FP) are shown in figure 6. The beam profile was approximately symmetric about the thruster 

center, located at 320 mm. The right hand side of the beam profile for EM5 was truncated due to the size 

limitations of the motion control tables. Integration of the beam profile for EM5 matched the beam 

current to within 10 to 16 percent. The beam divergences half angle was determined to be 24 ± 1°. 

A comparison of the beam profiles for EM1, EM4 and EM5 collected at an axial distance of 250 mm 

is shown in figure 7. EM1 and EM4 profiles compare well despite the fact the EM4 optics were masked 

down to 36 cm. The result was expected given the current density in the outer 2 cm was negligible 

(ref. 12). The beam profile of EM5 was more peaked when compared to the beam profiles of either EM1 

or EM4. The difference in the EM5 beam profile as compared to the EM1 and EM4 beam profiles was 

attributed to small differences in the grid spacing measured during optics assembly. The smaller grid gap, 

associated with EM5 optics, would make the transparency higher along the thruster centerline resulting in 

higher current densities. It should be noted that although variations in the beam profiles existed from 

thruster to thruster, these differences were not significant enough to invalidate the MTAT test results. 

B. Single Thruster Operation—Off-axis Ion Energy Distribution 

Off-axis RPA data was collected for the various EM1 thruster power levels; Low Power (LP) or 

1120 W, Intermediate Power (IP) or 2780 W and Full Power (FP) or 6860 W. The current-voltage (I-V) 
NASA/TM—2006-214423 9

curves are shown in figure 8(a). The I-V curves were normalized by the peak current in each case. The 

theoretical RPA I-V curve for an ion beam would exhibit a step function behavior with discontinuities at 

the peak ion energies. The derivative of an RPA I-V curve at a given test condition provided the 

distribution of ion energies at that location, with the peaks located at the most probable ion energies. 

Given the RPA data was collected at discrete bias voltages with 20-V intervals, the slope for each interval 

was calculated and plotted against the average bias voltage in that interval. Given that the entrance grid 

was grounded, data was not corrected for the voltage-to-ground potential (Vg). Figure 8(b) shows the 

derivative (i.e., slope of the I-V curve) for each thruster power level. The most probable ion energy 

matched the beam voltage in all cases. The most probable ion energy was found to be 690 eV for LP, 

1190 eV for IP, and 1830 eV for FP, whereas the beam voltage was 671, 1169, and 1790 V for LP, IP and 

FP, respectively. The small discrepancy was attributed primarily to an uncertainty of ±10 V associated 

with data collection at discrete bias voltages. The minor peaks between 100 and 200 V were due to noise 

in the signal associated with low ion densities noted during low power operation. The results clearly 

showed that high-energy ions were present at 45° off the thruster axis at all operating conditions, which is 

important from a plume-spacecraft interaction perspective. 
NASA/TM—2006-214423 10
NASA/TM—2006-214423 11

C. Multiple Thruster Operation—Ion Current Density Profiles 

1. Ion Current Density Profiles 

Faraday probes were used to measure the ion density profiles during the simultaneous operation of 

multiple thrusters; EM1, EM4, and EM5. Both ion density profiles for both Probe 2 and Probe 6 are 

shown in figure 9. The ion current density measured by Probe 2 represented the beam profile of EM1 

measured across the thruster centerline. The ion current density measured by Probe 6 represented the 

beam profiles of EM4 and EM5 measured along the centerline of both thrusters. Note that the overlap of 

the beams of EM4 and EM5 occurred along the array centerline as early as 250 mm. The current density 

in the trough at the array centerline increased as axial distance to the thruster increased and was attributed 

to finite plume divergence. 

To gain insight into the effect of multiple thruster operation on the primary beam characteristics, an 

investigation of the effect of the number of active thrusters on the current density profiles was completed. 

The current density profiles for single engine operation (EM1@FP, EM4@FP, and EM5@FP) were 

compared to current density profiles for multiple engine operation, (EM1+EM4+EM5)@FP. Figure 10 

shows the comparison of the beam profiles collected during single engine and multiple engine operation 

at an axial distance of z' = 250 mm. The current density profiles collected by Probe 2 across the centerline 

of EM1 during single engine operation and multiple engine operation indicated there was only a minor 

difference between the two cases. Integration of the beam profiles in the two cases indicated the 

difference in beam currents was less than 0.2 A or 6 percent. The current density profiles collected by 

Probe 6 across the centerline of EM4 and EM5 during single engine operation and multiple engine 
NASA/TM—2006-214423 12

operation were comparable. The profiles agreed well near the thruster centerlines, where the beams did 

not overlap. Near the array centerline –75 mm < x' < 75 mm, the beam profile collected during multiple 

thruster operation indicated the beams had overlapped at this axial location. The beam profiles, in regions 

where beams intersected, did not match beam profiles collected during single engine operation, but could 

be matched by the superposition of the two individual beam profiles. In general overlay of single engine 

current density profiles onto multiple engine profiles indicated there was no significant difference in the 

beam profiles between single thruster operation and multiple thruster operation. 

Additional analysis was conducted to gain insight into the behavior of the beam profiles in the regions 

where the diverging beams overlapped. Since the ion expansion in this region of the plume was 

collisionless, the beam profiles collected during multiple engine operation should be predicted by the 

superposition of individual ion thruster beams. To determine if regions of beam overlap was due to simple 

superposition, the profiles in the region of overlap were added. Figure 11 shows a comparison of beam 

profiles measured during multiple engine operation at z' = 250 mm and beam profiles calculated by 

adding those measured during single engine operation at the same axial distance. The resulting 

summation agreed well the primary beam profile near the thruster centerline as well as the array 

centerline. The results indicated that multi-thruster beams could be predicted from single thruster beam 

profiles by superimposing the beam profiles collected during single thruster operation. 
NASA/TM—2006-214423 13

D. Multiple Thruster Operation - Off-axis Ion Energy Distribution 

The impact of the number of operating thrusters on the ion energy distribution was characterized by 

the off-axis RPA located at 45° from the centerline of EM1. The off-axis beam ions were characterized 

during single thruster operation (EM1@FP), double thruster operation (EM1+EM5)@FP, 

(EM1+EM4)@FP, and triple thruster operation (EM1+EM4+EM5)@FP for this study. The results are 

shown in figure 12(a) to (b). In figure 12(a), the collected current is plotted as a function of the probe bias 

voltage. The normalized I-V curves compare well in all cases. The derivatives of the I-V curves, which 

indicate the ion energy distribution, are shown in figure 12(b). The peak ion energy was the same in all 

cases; 1830 eV, which compared well with the beam voltage of 1790 V. The results indicated that there 

was no impact on the ion energy distribution due to the number of operating thrusters. Also the ion energy 

distribution was not significantly affected by the associated changes in background pressure due to the 

number of operating thrusters in this test configuration. 

E. Alternate Neutralizer Configurations 

Alternate neutralizer configurations were examined as part of this test program to evaluate these 

configurations as a system operational fault recovery mode, and as a performance-enhancing system 

architecture. Ion current density profiles were characterized for each neutralizer-thruster combination. In 

this report one alternate neutralizer configuration was selected as representative case for the general 

trends. The baseline configuration used for this investigation was the low power operation of both EM1 
NASA/TM—2006-214423 14
NASA/TM—2006-214423 15

and EM5 with both neutralizers operating at nominal conditions. Current density profiles were collected 

at three axial locations for nominal low power operation of EM1 and EM5 (with both neutralizers) and 

are shown in figure 13(a). Both the power and xenon flow rate to the neutralizer of EM5 were then turned 

off, leaving the neutralizer of EM1 as the sole operating neutralizer during the simultaneous operation of 

EM1 and EM5. The ion current density profiles were characterized at three axial locations for this 
NASA/TM—2006-214423 16

configuration and are shown in figure 13(b). A comparison of the beam profiles collected for both cases 

showed no change in the beam profile between the two neutralizer-thruster configurations, which 

indicated that EM1 neutralizer effectively neutralized the beam plasma for distances on the order of 1 m 

without degrading thrust performance of either engine. This result was promising in that it highlighted 

multiple-thruster, single-neutralizer operation as a system operation fault recovery mode and/or a possible 

performance-enhancing system architecture as discussed in reference 9. 

E. Gimbal Articulation 

The impact of gimbal operations on thruster ion beam was examined to identify possible sympathetic 

processes which may be related to thruster gimbal angle. The test involved the simultaneous operation of 

EM1 and EM5 at FP, while EM1 was articulated over a full range of motion by a gimbal actuator. The 

array was first configured into its nominal orientation (with gimbal angles of 0° on all thrusters). EM1 

was then actively articulated in the radial direction (relative to the array center) and then the tangential 

direction (perpendicular to the radial direction) while operating. The thruster orientations, shown 

graphically in figure 14, included; 12.4° radially in, 10.2° radially out, 11.3° tangentially up, and 11.1°

tangentially down. Ion beam profiles were collected at each orientation (see Reference 6 for additional 

information about test setup). 

Figure 15(a) shows current density profiles collected at an axial distance of 175 mm as EM1 was 

articulated radially. Figure 15(b) shows current density profiles collected at an axial distance of 175 mm 

as EM1 was articulated tangentially. The beam profile of EM5 did not change more than a few percent as 

EM1 was articulated in either the radial or tangential directions. The EM1 beam profiles shifted, as 

expected, as EM1 was articulated in both the radial and tangential directions. If the beam was perfectly 

symmetric and the probe was perfectly aligned, the peak current density of the beam profile would have 

decreased as the thruster was gimballed away from the 0° position. However, azimuthal asymmetry in the 

beam peak current density near the thruster centerline has been observed in other ion thruster tests. As 

EM1 was articulated away from the 0° position, a localized increase in the peak current density was 

observed. This trend was noted in two cases where EM1 was articulated 10.2° radially outward and 11.3°

tangentially upward. This may have indicated that there was a localized spot of higher current density 
NASA/TM—2006-214423 17
NASA/TM—2006-214423 18
NASA/TM—2006-214423 19

below the plane of the thruster centerline of EM1 or perhaps a small probe misalignment. Figure 15(c) 

shows current density profiles at an axial distance of 300 mm as EM1 was articulated radially. 

Figure 15(d) shows current density profiles collected at an axial distance of 300 mm as EM1 was 

articulated tangentially. Azimuthal asymmetry was again noted when EM1 was articulated 10.2° radially 

outward and 11.3° tangentially upward, although the resulting variations in peak current density were 

lessened as axial distance increased. The EM1 beam profiles shifted radially as EM1 was articulated in 

both the radial and tangential directions. Again the beam profile for EM5 did not change more than a few 

percent as EM1 was articulated. 

In general as the EM1 thruster was articulated its beam profile shifted radially as expected, whereas 

the beam profile of a stationary thruster (EM5) near the articulating thruster did not change. The results 

indicated that there were no beam-beam interactions occurring during the gimbal maneuvers, which was 

consistent with a collisionless beam expansion. 

IV. Conclusions 

Three operational NEXT ion thrusters were installed in a cluster array at NASA GRC. A series of 

engineering demonstration tests were performed to evaluate the system performance impacts of operating 

various thruster configurations in an array. A suite of diagnostics was installed to investigate array impact 

on thruster performance and life, thermal interactions, and alternative system modes and architectures. To 

ascertain impact on thrust-generating ions, the ion current density profile and ion energy distribution of 

the ion beam were measured by Faraday probes and an off-axis retarding potential analyzer. The ion 

beam was characterized during single thruster operation, multiple thruster operation, various neutralizer 

configurations, and single-thruster gimbal articulation. 

In general, overlay of single engine current density profiles onto multiple engine profiles indicated 

there was no significant difference in the beam profiles between single thruster operation and multiple 

thruster operation. The summation of beam profiles collected during single engine operation agreed well 

with measured multiple thruster profiles near the thruster centerline as well as at the array centerline. The 

results indicated that multi-thruster beams could be predicted from single thruster beam profiles by 

superimposing the beam profiles collected during single thruster operation. The results from the off-axis 

RPA clearly showed that high-energy ions are present at 45° off the thruster axis and that the peak ion 

energy did not vary due to the number of operating thrusters. Also, the peak ion energy was not effected 

by the associated changes in background pressure due to the number of operating thrusters in this test 

configuration. A comparison of the beam profiles collected for alternate neutralizer configurations 

showed no change in the beam profile between the two neutralizer-thruster configurations examined. In 

general, as one thruster was articulated its beam profile shifted as expected, whereas the beam profile of a 

stationary thrusters near the articulating thruster did not change. 

References 

1. Frandina, M., Carpenter, C. Soulas, G, Snyder, A. and Patterson, M., “Status of the NEXT Ion Engine 

Life Test,” AIAA–2005–4065, 41st Joint Propulsion Conference, Tucson, AZ, July 2005. 

2. Todd, P., Wiseman, S., Matinelli, R. and Pinero, L., “Status of the NEXT 7–kW Power Processing 

Unit,” AIAA–2005–3868, 41st Joint Propulsion Conference, Tucson, AZ, July 2005. 

3. Hoskins, W., Polaha, J., Hobson, D., Soulas, G. Patterson, M., Sovey, J. and Talerico, L., “Overview 

of the NEXT Ion Propulsion System Program at Aerojet,” AIAA–2005–3885, 41st Joint Propulsion 

Conference, Tucson, AZ, July 2005. 

4. Monheiser, J., Aadland, R. and Wilson, A., “Development of a Ground Based Digital Control 

Interface Unit (DCIU) for the NEXT Propulsion System,” AIAA–2004–4112, 40th Joint Propulsion 

Conference, Fort Lauderdale, FL, July 2004. 
NASA/TM—2006-214423 20

5. Benson, S., Patterson, M., Vaughn, D., Wilson, A. and Wong, B., “NASA's Evolutionary Xenon 

Thruster (NEXT) Phase 2 Development Status,” AIAA–2005–4070, 41st Joint Propulsion 

Conference, Tucson, AZ, July 2005. 

6. Patterson, M., Foster, J., McEwen, H., Pencil, E., Van Noord, J. and Herman, D., “NEXT MultiThruster Array Test-Engineering Demonstration,” AIAA–2006–5180, 42nd Joint Propulsion 

Conference, Sacramento, CA, July 2006. 

7. McEwen, H., Foster, J., Pencil, E., Patterson, M., Diaz, E. and Van Noord, J., “Characterization of 

Plasma Flux Incident on a Multi-Thruster Array,” AIAA–2006–5183, 42nd Joint Propulsion 

Conference, Sacramento, CA, July 2006. 

8. Foster, J., Patterson, M., Pencil, E., McEwen, H., Diaz, E. and Van Noord, J., “Plasma Characteristics 

Measured in the Plume of a NEXT Multi-Thruster Array,” AIAA–2006–5184, 42nd Joint Propulsion 

Conference, Sacramento, CA, July 2006. 

9. Foster, J., Pencil, E., McEwen, H. Patterson, M., Diaz, E. and Van Noord, J., “Neutralizer Plasma 

Coupling in a NEXT Multi-Thruster Array,” AIAA–2006–5184, 42nd Joint Propulsion Conference, 

Sacramento, CA, July 2006. 

10. Oberto, R., et al., “Advanced Projects Design Team, Titan Orbiter 2003–10,” Jet Propulsion 

Laboratory (JPL) Report ID #658, 2003. 

11. Kamhawi, H., Soulas, G., Patterson, M. and Frandina, M., “NEXT Ion Engine 2000 hour Wear Test 

Plume and Erosion Results,” AIAA–2004–3792, 40th Joint Propulsion Conference, Fort Lauderdale, 

FL, July 2004. 

12. Soulas, G., et al., “NEXT Ion Engine 2000 Hour Wear Test Results,” AIAA–2004–3971, 40th Joint 

Propulsion Conference, Fort Lauderdale, FL, July 2004. 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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November 2006

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E–15700

WBS 346620.04.05.03.01

26

Ion Beam Characterization of a NEXT Multi-Thruster Array Plume

Electric propulsion; Ion thrusters; Multi-thruster arrays

Unclassified -Unlimited

Subject Category: 20

Three operational, engineering model, 7-kW ion thrusters and one instrumented, dormant thruster were installed in a cluster array in a

large vacuum facility at NASA Glenn Research Center. A series of engineering demonstration tests were performed to evaluate the

system performance impacts of operating various multiple-thruster configurations in an array. A suite of diagnostics was installed to

investigate multiple-thruster operation impact on thruster performance and life, thermal interactions, and alternative system modes and

architectures. The ion beam characterization included measuring ion current density profiles and ion energy distribution with Faraday

probes and retarding potential analyzers, respectively. This report focuses on the ion beam characterization during single thruster

operation, multiple thruster operation, various neutralizer configurations, and thruster gimbal articulation. Comparison of beam profiles

collected during single and multiple thruster operation demonstrated the utility of superimposing single engine beam profiles to predict

multi-thruster beam profiles. High energy ions were detected in the region 45° off the thruster axis, independent of thruster power,

number of operating thrusters, and facility background pressure, which indicated that the most probable ion energy was not effected by

multiple-thruster operation. There were no significant changes to the beam profiles collected during alternate thruster-neutralizer

configurations, therefore supporting the viability of alternative system configuration options. Articulation of one thruster shifted its

beam profile, whereas the beam profile of a stationary thruster nearby did not change, indicating there were no beam interactions which

was consistent with the behavior of a collisionless beam expansion.

Eric J. Pencil, John E. Foster, Michael J. Patterson, Esther M. Diaz,

Jonathan L. Van Noord, and Heather K. McEwen

Prepared for the 42nd Joint Propulsion Conference and Exhibit cosponsored by the AIAA, ASME, SAE, and ASEE,

Sacramento, California, July 9–12, 2006. Responsible person, Eric J. Pencil, organization code RPP, 216–433–7463.