James E. Pollard, Kevin D. Diamant, and Mark W. Crofton

The Aerospace Corporation, El Segundo, California 

Michael J. Patterson and George C. Soulas

Glenn Research Center, Cleveland, Ohio

Spatially-Resolved Beam Current and Charge-State 

Distributions for the NEXT Ion Engine

NASA/TM—2010-216790

September 2010

AIAA–2010–6779

https://ntrs.nasa.gov/search.jsp?R=20100033115 2018-05-17T12:02:55+00:00Z
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James E. Pollard, Kevin D. Diamant, and Mark W. Crofton

The Aerospace Corporation, El Segundo, California 

Michael J. Patterson and George C. Soulas

Glenn Research Center, Cleveland, Ohio

Spatially-Resolved Beam Current and Charge-State 

Distributions for the NEXT Ion Engine

NASA/TM—2010-216790

September 2010

AIAA–2010–6779

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

46th Joint Propulsion Conference and Exhibit

cosponsored by AIAA, ASME, SAE, and ASEE

Nashville, Tennessee, July 25–28, 2010
Acknowledgments

Kevin McCormick of NASA Glenn Research Center built the electrical and propellant interfaces to the test facility 

and installed the thruster and its power and propellant systems. This work was supported under 

The Aerospace Corporation’s Independent Research and Development Program.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://gltrs.grc.nasa.gov

Level of Review: This material has been technically reviewed by technical management. 

This report contains preliminary fi ndings, 

subject to revision as analysis proceeds.
NASA/TM—2010-216790 1

Spatially-Resolved Beam Current and Charge-State 

Distributions for the NEXT Ion Engine

James E. Pollard, Kevin D. Diamant, and Mark W. Crofton

The Aerospace Corporation

El Segundo, California 90245

Michael J. Patterson and George C. Soulas

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

Plume characterization tests with the 36-cm NEXT ion engine are being performed at The Aerospace 

Corporation using engineering-model and prototype-model thrusters. We have examined the beam current 

density and xenon charge-state distribution as functions of position on the accel grid. To measure the 

current density ratio j

++/j+

, a collimated E×B probe was rotated through the plume with the probe oriented 

normal to the accel electrode surface at a distance of 82 cm. The beam current density jb versus radial 

position was measured with a miniature planar probe at 3 cm from the accel. Combining the j

++/j+ and jb

data yielded the ratio of total Xe+2 current to total Xe+1 current (J

++/J+

) at forty operating points in the 

standard throttle table. The production of Xe+2 and Xe+3 was measured as a function of propellant 

utilization to support performance and lifetime predictions for an extended throttle table. The angular 

dependence of jb was measured at intermediate and far-field distances to assist with plume modeling and 

to evaluate the thrust loss due to beam divergence. Thrust correction factors were derived from the total 

doubles-to-singles current ratio and from the far-field divergence data.

Nomenclature

F thrust

Isp specific impulse

j

+ position-dependent current density of Xe+1

j

++ position-dependent current density of Xe+2

j

+++ position-dependent current density of Xe+3

J

+ total Xe+1 current

J

++ total Xe+2 current

J

+++ total Xe+3 current

jb position-dependent beam current density

Jb total beam current

Pin thruster input power (equals power processor output power)

PP planar probe

RPA retarding potential analyzer

TL throttle level

Vb beam power supply voltage

Vd discharge power supply voltage

Vg neutralizer potential relative to chamber ground

α thrust correction factor for doubly-charged ions

β thrust correction factor for beam divergence

γ combined thrust correction factor (= αβ)

δ divergence angle of jb vector relative to ion optics centerline
NASA/TM—2010-216790 2

ηe thruster electrical efficiency (beam power / thruster input power)

ηt overall thruster efficiency

ηu total propellant utilization efficiency

ηud propellant utilization efficiency for main discharge

θ probe angle relative to ion optics centerline, vertex at center of exit plane

ρ doubles current density ratio, j

++ / j+

σ triples current density ratio, j

+++ / ( j+ + j++)

ψ probe angle relative to ion optics centerline, vertex at turntable axis of rotation

ζ probe angle relative to ion optics centerline, vertex at accel grid edge

1.0 Introduction

NASA’s Evolutionary Xenon Thruster (NEXT) is engineered to be extremely flexible in terms of 

input power and specific impulse, while maintaining acceptable efficiency, and embodies a number of 

advances over previous ion engine systems (Refs. 1 to 6). NEXT as an integrated project is comprised of 

the development of an advanced xenon ion thruster, a power processor unit, a xenon feed system, a 

gimbal, and the control algorithms for system operation. Some of the operating characteristics of the 

thruster are being examined in a laboratory test program conducted at The Aerospace Corporation

(Ref. 7). The work is performed under the umbrella of a Space Act Agreement, with NASA participation 

in readying and operating the hardware being tested.

Mission application studies and long-duration life-testing of NEXT have focused on the standard 

throttle levels listed in Table 1. Potential enhancements in performance and service life relative to this 

baseline have been investigated analytically based on available laboratory data, showing that increased 

propellant utilization efficiency would improve the specific impulse and overall thruster efficiency

(Refs. 8 and 9). Higher propellant efficiency leads to a greater fraction of doubly-charged xenon in the 

main discharge, which causes (1) a small reduction in thrust relative to the performance if only Xe+1 is

present, and (2) an increase in the wear rate of the screen electrode and cathode keeper. Nevertheless,

higher propellant efficiency is predicted to increase NEXT service life by suppressing the wear rate of the 

accel electrode by charge-exchange ions. In this paper we revisit the laboratory measurements that 

support predictions of thruster performance and service life (Refs. 10 and 11). New test results include the

Xe+2 and Xe+3 current densities versus radial position on the accel electrode for the entire throttle table

and versus propellant utilization at selected operating points. We measured the beam current density

versus radial position very close to the accel to supplement the charge-state data and to allow a 

comparison of beamlet profiles with theoretical modeling. Finally, beam current density scans were 

performed at intermediate and far-field distances to provide input for plume effects modeling and to

evaluate the thrust correction factor associated with beam divergence. In a separate publication we present 

direct mechanical thrust measurements for NEXT and compare them with the corrected thrust derived 

from electrical parameters (Ref. 12).

TABLE 1.—BEAM CURRENT, BEAM SUPPLY VOLTAGE, DISCHARGE PROPELLANT EFFICIENCY

Jb

(A)

Vb

(V)

1800 1567 1396 1179 1021 936 850 679 650 400 300 275

3.52 TL40 TL39 TL38 TL37

3.10 TL36 TL35 TL34 TL33

2.70 TL32 TL31 TL30 TL29 TL28 ηud = 87% 93% 89%

2.35 TL27 TL26 TL25 TL24 TL23

2.00 TL22 TL21 TL20 TL19 TL18

1.60 TL17 TL16 TL15 TL14 TL13

1.20 TL12 TL11 TL10 TL09 TL08 TL07 TL06 TL05 TL04 TL03 TL02

1.00 TL01
NASA/TM—2010-216790 3

2.0 Test Setup

Measurements were performed in a 2.4-m diameter by 9.8-m long cryopumped vacuum chamber with 

the thruster oriented along the chamber centerline. Two re-entrant pumps were on the end-dome behind 

the thruster, two more re-entrant pumps were in the beam dump region, and four 1.3-m cryotubs were 

mounted on the cylindrical wall of the tank adjacent to the thruster. The base pressure with no gas load 

was less than 1×10–7 torr, with 90 percent of the residual gas being water vapor, and the remainder being 

air and hydrocarbons. The beam dump was lined with carbon composite sheets and flexible graphite to 

minimize the yield of sputtered material. Lab power supplies and a lab propellant feed system were used 

to operate the EM4 engineering model thruster and the PM1R prototype model (Ref. 6) thruster at the 

beam currents and voltages listed in Table 1. Also listed are the values for the discharge propellant 

efficiency ηud, as tested in this facility. Background pressure was measured by an ionization gauge located 

on the tank wall adjacent to the thruster. With an 1800-V, 3.52-A beam (TL40) the xenon pressure was

3.6×10–6 torr, applying the correction factor of 0.348 specified by the gauge manufacturer for xenon 

relative to nitrogen. The effective pumping speed for this condition was 2.1×105 liter/s.

A key objective of the test was to measure charge-state distribution versus position on the accel grid, 

but doing this directly with an intrusive probe might perturb the plume excessively. Instead, we used a 

spatially-resolving E×B probe at a downstream location, in combination with a miniature planar probe to 

measure beam current density versus radial position very close to the grid. Figure 1 shows the 

arrangement for scanning the probes at a constant distance from the spherical accel surface with the 

viewing direction normal to the surface. The probe arm was driven by a turntable located behind the 

thruster so that the axis of rotation coincided with the center of curvature of the grid. Four posts 

supporting the thruster were spaced far enough apart to allow the probe to scan across the full width of the 

accel in the thruster mid-plane. Prior to adopting this setup, we installed a different mechanical 

arrangement to confirm that the E×B signal was maximized in the grid-normal direction at all radial 

positions, as expected for the compensation design of the ion optics. The setup in Figure 1 differed from 

what was used in previous measurements of the NEXT charge-state distribution (Refs. 13 and 14), which 

were made on thruster centerline at 4.2 and 7.6 m downstream, or in a radial traverse at 1.5 m 

downstream with the probe in a fixed orientation parallel to the centerline. In earlier studies with a 30-cm 

ion engine and with the NSTAR thruster, the doubles-to-singles ratio was measured on centerline at 

15 cm from the accel (Ref. 15) or in the far-field using a fixed probe orientation (Ref. 16).

Figure 1.—Setup for plume angular scans. R is the ion optics radius of curvature.

ExB probe

NEXT

Turn2-axis table

stage

R 82 cm
NASA/TM—2010-216790 4

Figure 2.—Schematic of E×B charge-state analyzer with dimensions in mm.

With a gridded ion source the particle velocity scales as the square-root of the charge-to-mass ratio.

Figure 2 shows the dimensions of an E×B Wien filter for analyzing plume ions as a function of velocity to 

separate the charge states. For the case of orthogonal electric and magnetic fields, a charged particle with 

a velocity vector perpendicular to the fields is transmitted without deflection if the magnitude of its 

velocity equals E/B (Ref. 17). Given the narrow distribution of the kinetic-energy-to-charge ratio, and 

given that xenon was the only significant constituent of the beam, the charge-state resolution was 

determined mainly by the degree of collimation upstream of the fields and by the size of the ion collector 

aperture. The entrance collimator consisted of a conical skimmer machined from pyrolytic graphite 

followed by a stainless steel aperture, both having 1.6-mm diameter. A horseshoe-type sector magnet 

(0.186 T, 13-mm gap), originally part of a commercial mass spectrometer, was fitted with a pair of 

pyrolytic graphite electrode plates (7-mm gap) to establish the electric field. In order to have the desired 

flat-topped peaks when measuring the ion current versus velocity, the exit aperture diameter (10.3 mm) 

was larger than the diameter of the separated beamlets (ca. 6 mm). This insured that the peak-height ratios 

accurately represented the entire sample of ions entering the E×B probe. The collector cup was deep 

enough to prevent the escape of incoming ions and secondary electrons, and it was biased using a battery 

at −7 V relative to the grounded housing to suppress the electron background that leaked in from the 

thruster plume. Pump-out holes covered with fine mesh were provided in the entrance collimator and in 

the main housing to ensure that the dynamic pressure from neutralized beam ions (< 3×10–7 torr in the 

collimator) was much less than the static pressure due to xenon background gas. The electric field was 

swept using a Keithley 2410 SourceMeter connected to a voltage divider that established potentials of 

equal magnitude on the lower (+) electrode and the upper (−) electrode. The magnet poles were at ground 

potential. Collector current was recorded by a Keithley 6485 picoammeter as a function of ion velocity, 

probe angle, and probe axial position on centerline. Figure 3 shows the test facility configured with the 

thruster and E×B probe.

Angular scans were performed with the E×B entrance aperture at 82 cm from the spherical surface of 

the accel grid. Prior to pump-down the probe was aligned by temporarily detaching the collector housing 

and aiming a penlight back through the probe to produce a spot of light on the grid, while adjusting the 

turntable offset and probe azimuth to align the light with the grid edges and with the central grid hole. The 

penlight showed that the E×B viewing spot on the accel grid was about 8-mm diameter, which 

encompassed 13 to 14 accel holes and corresponded to an increment of 0.6° in the angular scans. We also 

used an alignment fixture attached to the ion optics that directed a laser beam downstream along the ion 

optics centerline. The location of the collimated E×B probe meant that only ions with velocity vectors 

normal to the grid surface were detected. Assuming the j

++/j+ ratio was uniform for all ions extracted 

through the plasma sheath of a given screen grid hole, the ratio measured downstream for that grid hole

was independent of probe angle relative to the grid surface, because the path of an ion through the grids 

Xe+2

Xe+3

Xe+1

E B Xe+1,2,3

67 145

3

1.6 Ø 1.6 Ø 10.3 Ø

149

entrance 

collimator

sector 

magnet with 

E-field 

plates

ion 

collector
NASA/TM—2010-216790 5

was independent of charge state. Therefore, the j

++/j+ ratio measured at 82 cm in grid-normal orientation 

was the same as what would be measured at the accel grid when averaged over the diameter of the 

viewing spot. This reasoning ignores the effect of charge-exchange collisions in the downstream region, 

for which we applied a correction based on axial scans with the E×B probe on thruster centerline (see 

Section 3.1).

Beam current density varied as a function of position on the grid, as did the fraction of ions having 

trajectories normal to the grid surface. The normal-current fraction was expected to decrease with

decreasing beamlet current, meaning that beamlets produced near the grid edge were more divergent than 

those produced near centerline. Therefore the current density measured by the E×B probe decreased faster 

toward the grid edge than the actual profile of jb at the accel. Because the probe measured j

++/j+ locally, 

we needed also to measure jb locally (i.e., close to the accel) for correctly determining the total current 

ratio J

++/J+

. Hence the E×B scans were supplemented by measurements with a miniature planar probe, 

again using the rotating arm to scan in a grid-normal orientation at constant distance from the accel (see 

Fig. 4). The collector was a stainless steel wire, viewed end-on, with a diameter of 0.4 mm. The wire was 

jacketed in a 0.8-mm OD alumina tube, within a 1.1-mm OD stainless steel tube that served as a guard 

ring, and finally within a 2.5-mm OD alumina tube that extended back to a connectorized housing.

Current versus applied voltage was measured with two Keithley 2410 SourceMeters, one for the collector 

and one for the guard. Data were recorded at 3 cm from the accel by stepping the miniature planar probe

Figure 3.—E×B probe on rotating arm at 82 cm from 

the accel grid of the PM1R.

Figure 4.—Miniature planar probe at 3 cm from accel 

grid of the PM1R.
NASA/TM—2010-216790 6

Figure 5.—Planar probe on rotating arm at 85 cm 

from the accel grid of the PM1R, and RPA on axial 

translator with turntable for adjusting the viewing 

direction.

through a set of 25 positions corresponding to the locations of the viewing spot for the E×B probe. For 

measurements of beamlet profiles at 0.5 to 2.0 cm from the accel, the probe was biased at −20 V and

moved in a continuous sweep while the SourceMeter stored data at 18.5 Hz until its internal buffer was 

filled (2500 readings). Although the nominal area of the collector was 1.3×10−3 cm2

, normalizing the 

scans to the known total beam current Jb implied that the effective area was 1.5 to 2.0×10−3 cm2 with the 

collector and guard ring biased at −20 V relative to ground. Probe I-V curves did not always show 

saturation of the ion current versus bias potential, so we relied on normalizing to Jb when attaching 

absolute units to the ion signal. However, only relative units for jb were needed to process the charge-state 

data. Modifying the probe design to narrow the dimensional tolerances and to extend the size of the guard 

ring might improve the ion saturation behavior.

Measurements of beam current density versus angle were made at 85 cm from the accel for 

comparison with E×B data taken at approximately the same distance. In this case jb was measured with a 

conventional guard-ring planar probe using the rotating arm to scan in a grid-normal orientation at a 

constant distance of 85 cm (see Fig. 5). The collector diameter was 1.27 cm, and the guard ring outer 

diameter was 2.54 cm. At each angle the I-V curve showed saturation of the ion current with collector and 

guard potentials of −20 V. To quantify the energetic ion flux at high divergence angles, a retarding 

potential analyzer (Ref. 18) was mounted on an axial translation stage at 51 cm radial distance from the 

thruster centerline (see Fig. 5). At the interface between the RPA and the axial stage there was a small 

turntable that adjusted the viewing angle of the probe as a function of axial position. Calibration of the 

RPA grid transmission as a function of beam energy was by comparison with a gridded flux probe having 

a known sensitivity (Ref. 18).

To evaluate the thrust loss due to beam divergence, we measured jb versus angle using the planar 

probe at 273 cm and the RPA at 281 cm from the accel in grid-normal orientation. For these far-field 

scans the thruster was pivoted by 12° away from chamber centerline to expand the angular range, and the 

probe axis of rotation coincided with the center of curvature of the ion optics, as in Figure 1. The two farfield probes were mounted together on the rotating arm and were displaced above and below the thruster 

mid-plane by +3 and −3 cm. For all other data reported in this paper the probes were confined to the 

thruster mid-plane.
NASA/TM—2010-216790 7

Figure 6.—E×B data at high utilization efficiency showing peaks for 

Xe+1 (40 km/s), Xe+2 (58 km/s), and Xe+3 (70 km/s). The duration of 

the sweep was 12 s.

Figure 7.—E×B data at low beam voltage showing peaks for Xe+1

(20 km/s) and Xe+2 (27 km/s).

3.0 Results

3.1 E×B Probe Performance

Figure 6 illustrates charge-state separation with the E×B probe for one of the highest levels of 

multiple ionization among the operating points that we examined. The beam voltage and current

corresponded to TL33 in Table 1, but the discharge utilization efficiency was increased to 96 percent, 

which gave a current density ratio for Xe+1, Xe+2, and Xe+3 of 100: 19: and 0.14 percent. Peak shapes were 

nearly flat-topped, as intended for this choice of aperture diameters, so that the full intensity could be

measured at the center of each peak. For TL04 to TL40 the shapes were similar to what is shown Figure

6, while for the lowest beam voltages (TL01 to TL03) a narrower shape was observed, probably because 

the beam inside the probe was displaced from the center of the exit aperture by stray fields. An example 

for TL01 at 275 V is shown in Figure 7. E×B voltage scans were performed at 25 angular positions from 

−15.1° to +15.1°, corresponding to radial distances on the accel of −18.5 cm to +18.5 cm in the thruster 

mid-plane, where a positive distance was to the right of the neutralizer if one were facing the ion optics.

The time needed to complete an angular scan was 6 min. Peak heights were taken to be the maximum 

value in the flat-topped region. Figure 8 shows an example of the radial dependence of the current 

TL33, PM1R, 96% utilization, +4.8 cm radial on accel 

0

20

40

60

80

100

120

20 30 40 50 60 70 80

Ion velocity (km/s)

Ion signal (nA)

x1

x100

TL01, PM1R, 88% utilization, +7.9 cm radial on accel 

0

2

4

6

8

12 17 22 27 32

Ion velocity (km/s)

Ion signal (nA)

x1

x10
NASA/TM—2010-216790 8

densities at high utilization efficiency. The departure from axisymmetry seen in E×B data for the PM1R

thruster was also evident in scans with planar probe at 3 cm from the accel (see Section 3.3).

Charge-exchange collisions between beam ions and neutral xenon caused the measured j

++/j+ to vary 

with distance from the accel grid. The relevant reactions were studied as a function of collision energy by 

Miller, et al (Ref. 19). Between 1800 and 600 eV (TL40 to TL04) the total cross sections are

Xe+1 + Xe → Xe + Xe+1, 43 to 50 Å2

Xe+2 + Xe → Xe + Xe+2

, 12 to 14 Å2

Xe+2 + Xe → Xe+1 + Xe+1

, 1 to 3 Å2

Therefore, Xe+1 at the beam energy was more likely to be lost by conversion to neutral Xe than was Xe+2.

The third reaction listed above produces a pair of singly-charged ions, one of which nominally retains the 

velocity of the reactant Xe+2 ion. However, this contribution to the j

++ signal from the E×B probe was 

minimal because (1) the reaction is less probable than the other charge-exchange processes, (2) the 

product ion carries one rather than two units of charge, and (3) the small cross section implies a non-zero 

momentum exchange between reactants which would deflect the product ion from the grid-normal 

direction.

Figure 9 shows the axial dependence of j

+

, j

++, and j++/j+ measured with the E×B probe at TL33 to 

illustrate the depletion of Xe+1 relative to Xe+2 with increasing distance. The ratio j

++/j+ at TL33 and other 

throttle levels approximated a linear function of distance over this range. If one knew the spatial 

dependence of xenon neutral density in the chamber, it would be possible to derive an analytic correction 

for the E×B results. Without neutral density data, an alternative is to characterize the axial dependence of 

j

++/j+ for all probe angles at every throttle level and to extrapolate back to the accel grid. Constraints of 

time and the mechanical setup limited us to measuring the axial dependence of j

++/j+ for a subset of 

throttle levels with the probe on thruster centerline. Axial scans were made as a function of beam current 

at 1179 V, namely TL09, 14, 19, 24, 29, 33, and 37. The data were extrapolated linearly to zero axial 

distance to determine the correction multiplier for j

++/j+ versus beam current, which was then applied to 

the E×B angular data at all beam voltages and probe angles. The multiplier, which was applied to j

+++

/j+

as well as j

++/j+

, ranged from 0.99 to 0.93 for beam currents between 1.2 and 3.5 A. Within the nominal 

fluctuation of thruster operating parameters we were not able to detect a variation in the correction 

multiplier as a function of beam voltage.

Figure 8.—Radial dependence of Xe+1 (blue), Xe+2 (x5, magenta), 

and Xe+3 (x500, yellow) signals at high utilization efficiency.

PM1R thruster, TL33, 96% utilization

0

20

40

60

80

100

120

140

-20 -10 0 10 20

Radial distance on accel grid (cm)

j+, 5*j++, 500*j+++ (arb.units)
NASA/TM—2010-216790 9

Figure 9.—Centerline current densities versus axial distance

measured with the E×B probe.

3.2 Radial Dependence of j

++/j+

For each of the 40 throttle levels in Table 1, E×B data were measured at 25 probe positions in the 

thruster mid-plane spanning the width of the perforated region of the accel grid. Figure 10 is a 

representation of this data set for the PM1R thruster, grouped into charts by beam current and corrected 

for the depletion of Xe+1 relative to Xe+2 with axial distance. It was anticipated that the maximum 

doubles-to-singles ratio would occur on thruster centerline, but this proved not to be the case. Instead, the 

centerline was typically a local minimum, with the maximum j

++/j+ occurring at radial distances of 5 to 

8 cm on either side of centerline. Presumably this would appear as an annular maximum if the entire accel 

grid were interrogated with the E×B probe. The radial dependence of j

++/j+ is an indicator for electron 

temperature upstream of the screen electrode, in response to the radial variation of propellant density and

of magnetic field. The data in Figure 10 were used to calculate the total current ratio J

++/J+ under the 

assumption that j

++/j+ in the mid-plane was representative of the entire beam (see Section 3.4).

 

 

 

TL33, centerline axial scan

80

100

120

140

160

0 20 40 60 80

Distance from accel grid (cm)

j+ and 10*j++ (nA)

TL33, centerline axial scan

y = 6.15E-05x + 6.88E-02

6.8%

7.0%

7.2%

7.4%

0 20 40 60 80

Distance from accel grid (cm)

Uncorrected j++/j+

 

 
NASA/TM—2010-216790 10

Figure 10.—Ratio of Xe+2 to Xe+1 current density j

++/j+ versus radial distance for the PM1R.

PM1R thruster, Jb=1.00 A and 1.20 A

0%

2%

4%

6%

8%

10%

12%

14%

-20 -10 0 10 20

Radial distance on accel grid (cm)

Corrected j++ / j+

TL01

TL02

TL03

TL04

TL05

TL06

TL07

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, Jb=1.20 A

0%

2%

4%

6%

8%

10%

12%

14%

-20 -10 0 10 20

Radial distance on accel grid (cm)

Corrected j++ / j+

TL08

TL09

TL10

TL11

TL12

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, Jb=1.60 A

0%

2%

4%

6%

8%

10%

12%

14%

-20 -10 0 10 20

Radial distance on accel grid (cm)

Corrected j++ / j+

TL13

TL14

TL15

TL16

TL17

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, Jb=2.00 A

0%

2%

4%

6%

8%

10%

12%

14%

-20 -10 0 10 20

Radial distance on accel grid (cm)

Corrected j++ / j+

TL18

TL19

TL20

TL21

TL22

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, Jb=2.35 A

0%

2%

4%

6%

8%

10%

12%

14%

-20 -10 0 10 20

Radial distance on accel grid (cm)

Corrected j++ / j+

TL23

TL24

TL25

TL26

TL27

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, Jb=2.70 A

0%

2%

4%

6%

8%

10%

12%

14%

-20 -10 0 10 20

Radial distance on accel grid (cm)

Corrected j++ / j+

TL28

TL29

TL30

TL31

TL32

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, Jb=3.10 A

0%

2%

4%

6%

8%

10%

12%

14%

-20 -10 0 10 20

Radial distance on accel grid (cm)

Corrected j++ / j+

TL33

TL34

TL35

TL36

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, Jb=3.52 A

0%

2%

4%

6%

8%

10%

12%

14%

-20 -10 0 10 20

Radial distance on accel grid (cm)

Corrected j++ / j+

TL37

TL38

TL39

TL40
NASA/TM—2010-216790 11

Figure 11.—Beam current density jb versus radial distance at 3 cm from the accel for the PM1R.

3.3 Radial Dependence of jb at 3 cm

For each of the 40 throttle levels in Table 1, miniature planar probe data were recorded at 25 positions 

in the thruster mid-plane at 3 cm from the accel grid. Figure 11 is a representation of this data set for the 

PM1R thruster, grouped into charts by beam voltage. Each curve is normalized to the known total beam 

current Jb with the collector area as a free parameter (see Section 2.0). A distance of 3 cm from the accel 

was far enough for the individual beamlets to merge, which was necessary when using jb data together 

with current densities measured by the E×B probe to derive the total current ratios for Xe+1, Xe+2, and 

Xe+3. Previously, the radial dependence of jb in the thruster mid-plane was measured in the EM3 LongDuration Test (Ref. 5) using linear traverses at 2 cm downstream from the geometric center of the accel 

grid. Our Figure 11 is in fair agreement with the EM3 data at 1800 V, with differences that are 

attributable to unit variation (PM1R versus EM3) and to the different methods for scanning the probes 

(constant 3 cm from accel versus linear traverse). The three units (EM3, EM4, PM1R) all showed an offcenterline bump on the otherwise smooth radial profile of jb at each TL. The origin of this feature is 

PM1R thruster, 3 cm from accel, Vb=275-936 V

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

-20 -10 0 10 20

Radial distance on accel grid (cm)

Beam current density (mA/cm2)

TL01

TL02

TL03

TL04

TL05

TL06

TL07

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, 3 cm from accel, Vb=1021 V

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

-20 -10 0 10 20

Radial distance on accel grid (cm)

Beam current density (mA/cm2)

TL08

TL13

TL18

TL23

TL28

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, 3 cm from accel, Vb=1179 V

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

-20 -10 0 10 20

Radial distance on accel grid (cm)

Beam current density (mA/cm2)

TL09

TL14

TL19

TL24

TL29

TL33

TL37

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, 3 cm from accel, Vb=1396 V

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

-20 -10 0 10 20

Radial distance on accel grid (cm)

Beam current density (mA/cm2)

TL10

TL15

TL20

TL25

TL30

TL34

TL38

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, 3 cm from accel, Vb=1567 V

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

-20 -10 0 10 20

Radial distance on accel grid (cm)

Beam current density (mA/cm2)

TL11

TL16

TL21

TL26

TL31

TL35

TL39

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, 3 cm from accel, Vb=1800 V

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

-20 -10 0 10 20

Radial distance on accel grid (cm)

Beam current density (mA/cm2)

TL12

TL17

TL22

TL27

TL32

TL36

TL40
NASA/TM—2010-216790 12

unknown. For the PM1R the location of the bump (−3 cm radial distance) coincided with a local 

maximum in the j

++/j+ distribution seen in Figure 10 for TL13 to TL40. We did not observe this feature in 

jb profiles at 85 cm from the accel (see Section 3.5). It would be of interest to measure the spatial 

dependence of jb over the entire accel grid to better understand the non-axisymmetric behavior. Beam 

flatness is defined as the ratio of average-to-peak beam current densities, where the average jb equals total 

beam current Jb divided by perforated area (1071 cm2

). For the PM1R mid-plane data in Figure 11, 

flatness was in the range of 0.69 to 0.81 and increased with total beam current in a roughly linear relation 

(= 0.646+0.047 Jb). There was no uniform trend in flatness versus beam voltage.

3.4 Multiply-Charged Ion Production Versus Throttle Level and Propellant Utilization

The results from the previous sections were used for calculating total current ratios, as explained in 

Appendix A. Figure 12 shows the discharge potential Vd and the doubles ratio J

++/J+ versus throttle level 

for the EM4 and PM1R thrusters. As noted in Table 1, the discharge propellant utilization for standard 

throttle conditions was 87 percent at TL01, 93 percent at TL02 to TL22, and 89 percent at TL23 to TL40. 

When the E×B data were recorded the discharge voltage was nearly the same for the two thrusters at 

TL23 to TL40, while at the lower TL’s it was 1 to 2 V higher for the PM1R than for the EM4. At other 

times the PM1R discharge voltage was 1 to 3 V less than is shown in Figure 12, while for the EM4 the 

variation between runs was only ±0.5 V. The cause of the PM1R variability has not been confirmed. 

Trends in the doubles ratio versus TL were nearly the same for the two thrusters, except for TL01 to TL09 

and TL13 to TL14 where J

++/J+ was higher for the PM1R than for the EM4, which was partly explained 

by the higher discharge voltage. Statistical error associated with probe signals on a given run was very 

small in the data of Figure 12, but the results at the lower TL’s were subject to uncertainty due to 

variation in Vd for the PM1R.

Figure 12.—Discharge voltage Vd and total current ratio J++/J+

versus throttle level.

 

24

25

26

27

28

29

0 5 10 15 20 25 30 35 40

Throttle level (TL)

Discharge potential (V)

EM4

PM1R

2

3

4

5

6

7

8

9

0 5 10 15 20 25 30 35 40

Throttle level (TL)

Total J++/J+ (%)

EM4

PM1R

 
NASA/TM—2010-216790 13

Figure 13.—Total current ratio J++/J+ versus propellant utilization

at 1179 V for a range of Jb.

Figures 13 and 14 show the total doubles ratio and the total triples ratio versus propellant utilization at 

Vb = 1179 V over a range of Jb, with separate graphs for the EM4 and PM1R thrusters. At the time of 

these measurements, Vd for the PM1R was typically 0.5 V less than Vd for the EM4, contrary to the 

pattern seen in Figure 12. Trending of J

++/J+ was very similar for the two thrusters (in the range of 2 to 

10 percent for utilization from 83 to 95 percent). At a given utilization the doubles ratio increased by 

about 2 percent as Jb increased from 1.2 to 3.5 A. The triples ratio at the higher TL’s varied from 0.005 to 

0.05 percent as utilization increased, with good agreement between EM4 and PM1R. For EM4 the triples 

ratio trending was about the same at all TL’s, while for PM1R the ratio increased less rapidly with 

utilization at the lower TL’s.

3.5 Angular Dependence of jb at Intermediate and Far-Field Distances

Planar probe angular scans were recorded in the thruster mid-plane at 85 cm from the accel to enable 

a comparison with the E×B signals measured at 82 cm. Measurements were extended to higher angles 

using RPA axial scans at 51-cm radial distance, as depicted on the graph in Figure 15. RPA scans were 

performed with grid-edge orientation, i.e. pointing toward the outermost apertures of the accel. This 

orientation maximized the signal over the range of the axial scan, demonstrating that the most divergent 

portion of the beam came from the outer apertures. As indicated in Figure 15, the exit plane was defined 

as the flat surface of the front mask, and the spherical accel surface extended to positive axial distances. 

Positive radial distances and positive angles were to the right of the neutralizer when facing the ion optics. 

Figure 16 shows jb at intermediate distances for the PM1R thruster, grouped into charts by beam voltage.

EM4 thruster, Vb=1179V

0

2

4

6

8

10

12

81 85 89 93 97

Discharge utilization (%)

Total J++/J+ (%)

1.20A

1.60A

2.00A

2.35A

2.70A

3.10A

3.52A

 

 

 

PM1R thruster, Vb=1179V

0

2

4

6

8

10

12

81 85 89 93 97

Discharge utilization (%)

Total J++/J+ (%)

1.20A

1.60A

2.00A

2.35A

2.70A

3.10A

3.52A
NASA/TM—2010-216790 14

Figure 14.—Total current ratio J+++/(J+ + J

++) versus propellant 

utilization at 1179 V for a range of Jb.

Figure 15.—Angle definitions and intermediate-distance 

probe locations in the thruster mid-plane. The planar 

probe used an angular scan in grid-normal orientation, and 

the RPA used an axial scan in grid-edge orientation.

EM4 thruster, Vb=1179V

0.00

0.01

0.02

0.03

0.04

0.05

81 85 89 93 97

Discharge utilization (%)

Total J+++/(J+ & J++) (%)

1.20A

1.60A

2.00A

2.35A

2.70A

3.10A

3.52A

 

 

 

PM1R thruster, Vb=1179V

0.00

0.01

0.02

0.03

0.04

0.05

81 85 89 93 97

Discharge utilization (%)

Total J+++/(J+ & J++) (%)

1.20A

1.60A

2.00A

2.35A

2.70A

3.10A

3.52A

-80

-60

-40

-20

0

20

40

60

-60 -40 -20 0 20 40 60 80 100

Axial distance (cm)

Radial distance (cm)

PP

RPA

ψ θ

ζ

turntable

probe

exit plane
NASA/TM—2010-216790 15

Here the angular and axial scans at each TL are plotted on the same graph as functions of the polar angle 

θ, noting that the distance from the probe to the center of the exit plane was not held constant. Planar 

probe results are plotted from θ = −48° to +31°, and RPA results are from θ = +33° to +96°. At many of 

the throttle levels the peak of the jb distribution was offset from thruster centerline by about −1° in a slight 

departure from axisymmetry. The planar probe signal at −20 V bias included components due to ions 

having less than the full beam energy and striking the probe at off-normal incidence. This flux arose 

mainly from charge-exchange collisions, as mentioned in Section 3.1. The RPA excluded the chargeexchange flux due to its repeller bias of +40 V. Hence the high-angle data points in Figure 16 show the 

fall-off of ions at the full beam energy, indicating that TL01, TL02, and TL03 produced fast ions with a 

higher divergence than was found at TL04 to TL40. The disparity was evident at angles greater than +45°, 

and for TL01 and TL02 the flux of fast ions was nearly constant from θ = +70° – +89°. Over most of the 

throttle table (TL04 to TL40) the current density of fast ions at θ = 75° – 89° was in the range of 10−6 to 

10−4 mA/cm2

. Analysis of the RPA signal versus repeller bias showed that the charge-exchange flux 

(< 2 eV kinetic energy) was in the range of 3×10–4 to 3×10–3 mA/cm2 depending on the total beam current 

(1.00 to 3.52 A), and was nearly independent of beam voltage and axial position.

We measured the beam current density versus angle in the far-field (281 cm from the accel) with the 

planar probe at −20 V collector bias and with the RPA at +30 V repeller bias. The planar probe current 

density was 10 to 20 percent higher than the RPA current density, partly because the planar probe 

collected ions from the charge-exchange plasma and because the RPA had a smaller field-of-view (ca. 

45° half-angle). Variation in the doubles-to-singles ratio versus TL and versus distance may also have 

played a role, because the planar probe signal included the electron current ejected by ion impact, for 

which the yield was higher for Xe+2 than for Xe+1. For these reasons the RPA was preferred over the 

planar probe when measuring jb in the far-field. To derive the thrust correction factor for beam divergence 

one needs to know the angle of the jb vector at each measurement location. Using the E×B probe we 

showed that the signal at 82 cm from the accel was maximized in the grid-normal orientation (see 

Section 2.0). Therefore the angle of the jb vector in RPA scans at 281 cm was assumed to be the same as 

the turntable angle ψ when in the range of −15°<ψ <+15°, corresponding to the perforated region of the 

accel grid. Outside of this range, we observed in RPA axial scans that the signal was maximized in gridedge orientation. Thus in the far-field scans the divergence angle δ was given by

δ = ψ if − 15° < ψ < +15°, otherwise δ = ζ, (1)

where ζ was the probe angle having its vertex at the grid edge (see Fig. 15). The rationale used here for 

determining δ helped to reduce the error due to the measuring jb at a finite distance from the 36-cm 

diameter source region. Figure 17 shows jb versus angle at 40 throttle levels for the PM1R, from which 

we derived thrust correction factors for beam divergence (see Section 3.7).
NASA/TM—2010-216790 16

Figure 16.—Beam current density jb versus polar angle θ for the PM1R at the locations in Figure 15. The curves show

planar probe data from –48° to +31° and RPA data from +33° to +96°.

PM1R thruster, Vb=275-936 V

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

-50 -25 0 25 50 75 100

Polar angle (deg)

Beam current density (mA/cm2)

TL01

TL02

TL03

TL04

TL05

TL06

TL07

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, Vb=1021 V

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

-50 -25 0 25 50 75 100

Polar angle (deg)

Beam current density (mA/cm2)

TL08

TL13

TL18

TL23

TL28

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, Vb=1179 V

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

-50 -25 0 25 50 75 100

Polar angle (deg)

Beam current density (mA/cm2)

TL09

TL14

TL19

TL24

TL29

TL33

TL37

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, Vb=1396 V

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

-50 -25 0 25 50 75 100

Polar angle (deg)

Beam current density (mA/cm2)

TL10

TL15

TL20

TL25

TL30

TL34

TL38

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, Vb=1567 V

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

-50 -25 0 25 50 75 100

Polar angle (deg)

Beam current density (mA/cm2)

TL11

TL16

TL21

TL26

TL31

TL35

TL39

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, Vb=1800 V

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

-50 -25 0 25 50 75 100

Polar angle (deg)

Beam current density (mA/cm2)

TL12

TL17

TL22

TL27

TL32

TL36

TL40
NASA/TM—2010-216790 17

Figure 17.—Beam current density jb versus probe angle ψ at 281 cm from the accel grid for the PM1R.

PM1R thruster, 281 cm from accel, Vb=275-936 V

0.000

0.001

0.010

0.100

-30 -25 -20 -15 -10 -5 0

Probe angle (deg)

Beam current density (mA/cm2)

TL01

TL02

TL03

TL04

TL05

TL06

TL07

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, 281 cm from accel, Vb=1021 V

0.001

0.010

0.100

1.000

-30 -25 -20 -15 -10 -5 0

Probe angle (deg)

Beam current density (mA/cm2)

TL08

TL13

TL18

TL23

TL28

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, 281 cm from accel, Vb=1179 V

0.001

0.010

0.100

1.000

-30 -25 -20 -15 -10 -5 0

Probe angle (deg)

Beam current density (mA/cm2)

TL09

TL14

TL19

TL24

TL29

TL33

TL37

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, 281 cm from accel, Vb=1396 V

0.001

0.010

0.100

1.000

-30 -25 -20 -15 -10 -5 0

Probe angle (deg)

Beam current density (mA/cm2)

TL10

TL15

TL20

TL25

TL30

TL34

TL38

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, 281 cm from accel, Vb=1567 V

0.001

0.010

0.100

1.000

-30 -25 -20 -15 -10 -5 0

Probe angle (deg)

Beam current density (mA/cm2)

TL11

TL16

TL21

TL26

TL31

TL35

TL39

 

 

 

 

 

 

 

 

 

 

 

 

PM1R thruster, 281 cm from accel, Vb=1800 V

0.001

0.010

0.100

1.000

-30 -25 -20 -15 -10 -5 0

Probe angle (deg)

Beam current density (mA/cm2)

TL12

TL17

TL22

TL27

TL32

TL36

TL40
NASA/TM—2010-216790 18

Figure 18.—Beam current density jb versus radial distance at

0.5, 1.0, and 2.0 cm from the accel grid. The 0.5 cm points 

are for the EM4. The 1.0 and 2.0 cm points are for the 

PM1R.

Figure 19.—Beam current density jb versus probe angle ψ for the EM4 thruster at TL40.

3.6 Radial and Axial Dependence of jb at 0.5-2.0 cm

To provide data for comparison with theoretical modeling of beamlet profiles we measured the 

current density at 0.5 to 2.0 cm from the accel grid in the thruster mid-plane. Figure 18 includes scans for 

the EM4 and PM1R thrusters. It shows radial profiles of jb for two individual beamlets with distance from 

the accel to the probe as a parameter, demonstrating that for TL05 the merger of beamlets was nearly 

complete at 2.0 cm from the accel. Figure 19 shows a sweep across the full width of the accel for the 

EM4, along with the smooth profiles of current density measured at 3 and 84 cm. Peaks from more than 

200 individual beamlets were detected at 0.5 cm, with intensities that followed the periodic variation in 

vertical offset between the aperture centers and the probe path. This periodicity occurred because of a 

design feature of the EM4 thruster that had the rows of apertures clocked by 10° relative to the mid-plane.

Figure 20 shows probe scans over a 2-cm wide region at 0.5 cm from the EM4 accel to illustrate the 

variation in beamlet divergence versus throttle level. To clarify this comparison, the ordinate of the 

graphs equals jb divided by its average value, namely jb (mA/cm2

)/Jb (mA)/1071 cm2. The trends were as 

follows:

• Among five operating points having Jb = 1.20 A (TL02, 05, 08, 10, 12), the minimum beamlet 

divergence was at Vb = 679 V (TL05), which corresponded to optimal focusing. At 679 V the 

beamlets appeared in high definition, while at 300 V (TL02) they were almost fully merged. With 

TL05, distance to accel grid as a parameter

0

1

2

3

4

5

3.4 3.5 3.6 3.7 3.8 3.9

Radial distance on accel grid (cm)

Beam current density (mA/cm2)

0.5cm

1.0cm

2.0cm

0

7

14

21

28

-16 -12 -8 -4 0 4 8 12 16

Probe angle (deg)

Beam current density (mA/cm2)

0.5 cm

3 cm

84 cm
NASA/TM—2010-216790 19

increasing beam voltage, namely 1021 to 1800 V (TL08, 10, and 12), there was an increase in 

divergence as the plasma sheath expanded upstream of the screen grid and the beamlets became 

over-focused. TL12 was known to produce over-focusing, especially on the outer apertures of the 

ion optics (Ref. 11). 

• At 2.35 A (TL23, 25, 27) and at 3.52 A (TL38 and 40) an increase in Vb at fixed Jb led to filling in 

of the jb profile in the region between apertures but had less of an effect on jb at the beamlet 

center. 1179 V (TL37) was known to produce a higher rate of accel aperture wall erosion than 

1800 V (TL40) (Refs. 5 and 11). This was, in part, due to under-focusing, which increased the

beamlet cross-section in the inter-grid region, leading to more charge-exchange ions created in 

regions where they could have a trajectory to the accel wall.

• At 1396 V (TL10, 25, and 38) and 1800 V (TL12, 27, and 40), an increase in Jb at fixed Vb

produced a reduction in beamlet divergence as the sheath contracted upstream of the screen grid.

Figure 20.—Scaled beam current density jb versus radial distance at 0.5 cm from the accel grid for the EM4.

EM4 thruster, 0.5 cm from accel, Jb=1.00A and 1.20A

0

1

2

3

4

5

6

7

0.0 0.5 1.0 1.5 2.0

Radial distance on accel grid (cm)

Scaled jb

TL01

TL02

TL05

 

 

 

 

 

 

 

 

 

 

 

 

EM4 thruster, 0.5 cm from accel, Jb=1.20 A

0

1

2

3

4

5

6

7

0.0 0.5 1.0 1.5 2.0

Radial distance on accel grid (cm)

Scaled jb

TL08

TL10

TL12

 

 

 

 

 

 

 

 

 

 

 

 

EM4 thruster, 0.5 cm from accel, Jb=2.35 A

0

1

2

3

4

5

6

7

0.0 0.5 1.0 1.5 2.0

Radial distance on accel grid (cm)

Scaled jb

TL23

TL25

TL27

 

 

 

 

 

 

 

 

 

 

 

 

EM4 thruster, 0.5 cm from accel, Jb=3.52 A

0

1

2

3

4

5

6

7

0.0 0.5 1.0 1.5 2.0

Radial distance on accel grid (cm)

Scaled jb

TL38

TL40

 

 

 

 

 

 

 

 

 

 

 

 

EM4 thruster, 0.5 cm from accel, Vb=1396 V

0

1

2

3

4

5

6

7

0.0 0.5 1.0 1.5 2.0

Radial distance on accel grid (cm)

Scaled jb

TL10

TL25

TL38

 

 

 

 

 

 

 

 

 

 

 

 

EM4 thruster, 0.5 cm from accel, Vb=1800 V

0

1

2

3

4

5

6

7

0.0 0.5 1.0 1.5 2.0

Radial distance on accel grid (cm)

Scaled jb

TL12

TL27

TL40
NASA/TM—2010-216790 20

3.7 Thrust Loss Correction Factors

The overall thruster efficiency ηt is proportional to the squares of the correction factors associated 

with doubly-charged ions (α) and with beam divergence (β):

( )

in

sp

in

b b g

t u e u P

F g I

P

J V V

2

0 2 2 2 = +

η = γ η η = α β η , (2)

++ +

++ +

+

+

α = J J

J J

1

1 2

2

, (3)

∫ ∫

− ° − °

β = δ ψ ψ ψ ψ

45

0

45

0

jb cos sin d jb sin d . (4)

Here β is the ratio of the axial component of beam current to the total beam current Jb. Equation (4) 

assumes axisymmetric flow at an average speed that is independent of angle, which is usually a good 

approximation for a gridded ion thruster. The integrand in the numerator is the product of the axial 

component of beam current density jb cosδ and the area weighting factor sinψ dψ. The scan range was 

from ψ = 0° to −29.5° in 0.5° steps, i.e., δ = 0° to −33.0°. For integrating Equation (4), the data were 

extrapolated to reduce the truncation error by assuming an exponential fall-off in jb versus ψ for angles 

between −29.5° and −45°. Our results are based on sampling a small portion of the far-field plume and 

may be affected by departures from axisymmetry that were not measured.

Figure 21 shows the correction factors versus throttle level for the PM1R, with error bars for the 

estimated systematic uncertainty. Random error in these measurements is small by comparison. Thrust 

loss due to divergence (ca. 2 to 4 percent) was greater than the loss due to doubly-charged ions (ca. 1 to 

2 percent). The factors α and β trended in opposite directions versus beam voltage and current, which 

moderated the dependency for the combined factor γ. At throttle levels with higher utilization (93 percent, 

TL02 to TL22) the combined factor stayed mostly in the range of 0.94 to 0.95, while at lower utilization 

(89 percent, TL23 to TL40) it was in the range on 0.95 to 0.96. Of the operating points in the standard 

throttle table, TL02 had the greatest thrust loss from divergence and TL05 had the smallest. At TL01 to 

TL03 the divergence loss could be more than what is shown in Figure 21, because of the fast ion flux at 

angles beyond the range of the far-field probe scans (see Fig. 16). Trending at a given utilization over 

most of the throttle table (TL05 to TL40) was as follows:

• The doubles correction factor α improved with increasing Vb at fixed Jb and utilization, because 

the discharge voltage decreased slightly as Vb increased, thereby suppressing Xe+2 production.

• The divergence correction factor β degraded with increasing Vb due to the increase in sheath 

curvature.

• The divergence correction factor β improved with increasing Jb due to the reduction in sheath 

curvature.

Trends in β versus TL in Figure 21 are not apparent in the profiles of jb versus radial distance at 3 cm 

from the accel (see Fig. 11). The latter were measured downstream of where the beamlets merged using a 

probe that collected current from multiple apertures on the accel grid, which allowed an assessment of 

beam flatness and axisymmetry but was not indicative of how β would vary as a function of Vb and Jb. 

Hence, the far-field divergence was a set mainly by beamlet focusing rather than by the distribution of 

beamlet current versus radial position.
NASA/TM—2010-216790 21

Figure 21.—PM1R thrust correction factors for doubly-charged 

ions α, beam divergence β, and their product γ, versus throttle 

level.

4.0 Conclusion

We have investigated the properties of the NEXT ion beam to better understand thruster performance, 

life-limiting factors, and plume interactions with the spacecraft. Measurements were performed with the 

EM4 and PM1R thrusters at 40 operating points that comprise the standard throttle table. They included 

the current densities of Xe+1, Xe+2, and Xe+3 as functions of radial position near the accel grid, and the 

beam current density jb as a function of angle and axial distance. Probe scans were confined to the thruster 

mid-plane. Departures from axisymmetry were evident in data recorded near the accel. An important 

feature of the setup was to scan the probes at a constant distance from the spherical grid surface with a 

viewing direction normal to the surface, which simplified the data reduction.

Plume centerline was generally a local minimum in the doubles-to-singles ratio j

++/j+

, with maxima

occurring at radial distances of 5 to 8 cm on either side of it. Presumably this would appear as an annular 

maximum in j

++/j+ if the entire accel grid were measured. The total current ratio J++/J+ was derived under 

the assumption that mid-plane data were representative of the entire beam, using j

++/j+ measured by a 

collimated E×B probe at 82 cm and jb measured at 3 cm from the accel grid. Axial scans of the E×B probe 

on centerline provided a correction for the depletion of Xe+1 relative to Xe+2 by charge-exchange 

collisions. Production of Xe+2 and Xe+3 was examined as a function of propellant utilization efficiency 

over the range of beam current Jb = 1.2 to 3.5 A at Vb = 1179 V to assess performance and lifetime 

implications for an extended throttle table.

The angular dependence of jb was measured at an intermediate distance range (50 to 85 cm from the 

accel) to provide data for plume modeling, and it was measured in the far-field (285 cm from the accel) to 

evaluate the thrust loss due to beam divergence. The flux of energetic ions at 89° from centerline was 100 

times higher at the two lowest throttle levels (TL01 and 02) than at the other TL’s. Beamlet profiles were 

measured at 0.5 to 2.0 cm from the accel for comparison with modeling, showing that the merger of 

beamlets was nearly complete at 2.0 cm. Beamlet focusing rather than beam flatness was the main factor 

in determining far-field plume divergence. Thrust loss correction factors were derived for doubly-charged 

ions and for plume divergence. At throttle levels with higher utilization (ηud = 93 percent) the combined 

correction factor stayed mostly in the range of 0.94 to 0.95, while at lower utilization (89 percent) it was 

in the range on 0.95 to 0.96.

PM1R thruster

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

0 5 10 15 20 25 30 35 40

Throttle level (TL)

Thrust correction factor

α

β

γ

NASA/TM—2010-216790 23

Appendix A.—Total Current Ratios

As noted in Section 2.0, the charge-state distribution measured with a collimated probe in the far-field 

is the same as what would be measured at the accel grid, assuming there is a correction for chargeexchange collisions with background gas (Section 3.1). In calculating the total current for each charge 

state one must apply a weighting factor proportional to the beam current density at the accel, which we 

measured using the miniature planar probe (Section 3.3). Neglecting the small contribution from triplycharged xenon, the beam current density is

+

++ + ++ ρ ≡ + ρ

ρ +

+ ρ = + = j

j j j j j j b b b , 1 1

1 , (A1)

For an assumed axisymmetric beam profile the ratio of total Xe+2 current to total Xe+1 current is

∑

∑

∫

∫

=

=

+ °

− °

+ °

++ + − °

ψ

+ρ

ψ

+ρ

ρ

≈

ψ ψ

+ ρ

ψ ψ

+ ρ

ρ

= 25

1

25

1

15

15

15

15

1

1

1

sin

1

1

sin

1

/

n

bn n

n

n

bn n

n

n

b

b

j

j

j d

j d

J J , (A2)

where ψ is the probe angle shown in Figure 15. A further assumption in Equation (A2) is that the 

direction of the measured jb is normal to the area element in the surface integrals. We measured ρn and jbn

at 25 angles ψn in the mid-plane of the thruster (Figs. 10 and 11). The integration was across the full grid 

diameter to average the current densities recorded on either side of centerline. In some cases we recorded 

the current density of triply-charged xenon j

+++ from which the total current ratio J+++

/(J

+

+J

++) was 

derived as follows:

+ ++

+++ + ++ +++

+

σ ≡ + σ

σ

+

+ σ = + + = j j

j j j j j j j b b b , 1 1

1 , (A3)

∑

∑

∫

∫

=

=

+ °

− °

+ °

− °

+ ++

+++

ψ

+σ

ψ

+σ

σ

≈

ψ ψ

+ σ

ψ ψ

+ σ

σ

= + 25

1

25

1

15

15

15

15

1

1

1

sin

1

1

sin

1

n

bn n

n

n

bn n

n

n

b

b

j

j

j d

j d

J J

J . (A4)
NASA/TM—2010-216790 24

References

1. M.J. Patterson and S.W. Benson, “NEXT Ion Propulsion System Development Status and 

Performance,” AIAA–2007–5199, 43rd Joint Propulsion Conference, 8–11 July 2007, Cincinnati, 

OH.

2. D.A. Herman, G.C. Soulas, and M.J. Patterson, “Performance Evaluation of the Prototype-Model 

NEXT Ion Thruster,” AIAA–2007–5212, 43rd Joint Propulsion Conference, 8–11 July 2007, 

Cincinnati, OH.

3. G.C. Soulas and M.J. Patterson, “NEXT Ion Thruster Performance Dispersion Analyses,” AIAA–

2007–5213, 43rd Joint Propulsion Conference, 8–11 July 2007, Cincinnati, OH.

4. D.A. Herman, G.C. Soulas, and M.J. Patterson, “Performance Characteristics of the NEXT LongDuration Test after 16,550 h and 337 kg of Xenon Processed,” AIAA–2008–4527, 44th Joint 

Propulsion Conference, 21–23 July 2008, Hartford, CT.

5. D.A. Herman, G.C. Soulas, and M.J. Patterson, “NEXT Long-Duration Test Plume and Wear 

Characteristics after 16,550 h of Operation and 337 kg of Xenon Processed,” AIAA–2008–4919, 44th 

Joint Propulsion Conference, 21–23 July 2008, Hartford, CT.

6. G.C. Soulas, M.J. Patterson, L. Pinero, D.A. Herman, and J.S. Snyder, “NEXT Single String 

Integration Test Results,” AIAA–2009–4816, 45th Joint Propulsion Conference, 2–5 Aug 2009, 

Denver, CO.

7. M.W. Crofton, et al., “Characterization of the NASA NEXT Thruster,” AIAA–2009–4815, 45th Joint 

Propulsion Conference, 2–5 Aug 2009, Denver, CO.

8. M.J. Patterson, “NEXT Study of Thruster Extended Performance (NEXT STEP),” Paper AIAA 2006-

4664, 42nd Joint Propulsion Conference, 9-12 July 2006, Sacramento, CA.

9. M.J. Patterson, “NEXT Study of Thruster Extended Performance II (NEXT STEP II),” AIAA–2008–

4808, 44th Joint Propulsion Conference, 21–23 July 2008, Hartford, CT.

10. J.L. Van Noord, “Lifetime Assessment of the NEXT Ion Engine,” AIAA–2007–5274, 43rd Joint 

Propulsion Conference, 8–11 July 2007, Cincinnati, OH.

11. J.L. Van Noord and D.A. Herman, “Application of the NEXT Ion Thruster Lifetime Assessment to 

Thruster Throttling,” AIAA–2008–4526, 44th Joint Propulsion Conference, 21–23 July 2008,

Hartford, CT.

12. K.D. Diamant, J.E. Pollard, M.W. Crofton, and M.J. Patterson, “Thrust Stand Characterization of the 

NASA NEXT Thruster,” AIAA–2010–6701, 46th Joint Propulsion Conference, 25–28 July 2010, 

Nashville, TN.

13. A. Snyder, H. Kamhawi, M. Patterson, and M. Britton, “Single-String Integration Test Measurements 

of the NEXT Ion Engine Plume,” AIAA–2004–3790, 40th Joint Propulsion Conference, 11–14 July 

2004, Ft. Lauderdale, FL.

14. T.A. Hickman, L.A. Arrington, M.M. Frandina, and G.C. Soulas, “Overview of Diagnostics for the 

NEXT Long Duration Test,” Paper AIAA–2005–4064, 41st Joint Propulsion Conference, 10–13 July 

2005, Tucson, AZ.

15. J.S. Sovey, “Improved ion containment using a ring-cusp ion thruster,” J. Spacecraft and Rockets, 

Vol. 21, No. 5, 1984, pp. 488–495.

16. V.K. Rawlin, J.S. Sovey, J.R. Anderson, and J.E. Polk, “NSTAR Flight Thruster Qualification 

Testing,” AIAA–98–3936, 34th Joint Propulsion Conference, 13-15 July 1998, Cleveland, OH.

17. A. Galejs and C.E. Kuyatt, “Focusing and Dispersing Properties of a Stigmatic Crossed-Field Energy 

Analyzer,” J. Vac. Sci. Technol., Vol. 15, No. 3, 1978, pp. 865–867.

18. J.E. Pollard and K.D. Diamant, “Hall Thruster Plume Shield Wake Structure,” AIAA–2003–5018, 

39th Joint Propulsion Conference, 20–23 July 2003, Huntsville, AL.

19. J.S. Miller, S.H. Pullins, D.J. Levandier, Y. Chiu, and R.A. Dressler, “Xenon Charge Exchange Cross 

Sections for Electrostatic Thruster Models,” J. Applied Physics, Vol. 91, No. 3, 2002, pp. 984–991.
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Spatially-Resolved Beam Current and Charge-State Distributions for the NEXT Ion Engine 

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14. ABSTRACT

Plume characterization tests with the 36-cm NEXT ion engine are being performed at The Aerospace Corporation using engineering-model 

and prototype-model thrusters. We have examined the beam current density and xenon charge-state distribution as functions of position on 

the accel grid. To measure the current density ratio j

++/j+, a collimated Eprobe was rotated through the plume with the probe oriented 

normal to the accel electrode surface at a distance of 82 cm. The beam current density j

b versus radial position was measured with a 

miniature planar probe at 3 cm from the accel. Combining the j

++/j+ and j

b data yielded the ratio of total Xe+2 current to total Xe+1 current 

(J++/J+) at forty operating points in the standard throttle table. The production of Xe+2 and Xe+3 was measured as a function of propellant 

utilization to support performance and lifetime predictions for an extended throttle table. The angular dependence of j

b was measured at 

intermediate and far-field distances to assist with plume modeling and to evaluate the thrust loss due to beam divergence. Thrust correction 

factors were derived from the total doubles-to-singles current ratio and from the far-field divergence data. 

15. SUBJECT TERMS

Ion thruster; Ion engine 

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