Jonathan L. Van Noord

Glenn Research Center, Cleveland, Ohio

Daniel A. Herman

ASRC Aerospace Corporation, Cleveland, Ohio

Application of the NEXT Ion Thruster 

Lifetime Assessment to Thruster Throttling

NASA/TM—2010-216920

November 2010

AIAA–2008–4526
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Jonathan L. Van Noord

Glenn Research Center, Cleveland, Ohio

Daniel A. Herman

ASRC Aerospace Corporation, Cleveland, Ohio

Application of the NEXT Ion Thruster 

Lifetime Assessment to Thruster Throttling

NASA/TM—2010-216920

November 2010

AIAA–2008–4526

National Aeronautics and

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NASA/TM—2010-216920 1

Application of the NEXT Ion Thruster 

Lifetime Assessment to Thruster Throttling

Jonathan L. Van Noord

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Daniel A. Herman

ASRC Aerospace Corporation

Cleveland, Ohio 44135

Abstract

Ion thrusters are low thrust, high specific impulse devices with typical operational lifetimes of 10,000 

to 30,000 hr over a range of throttling conditions. The NEXT ion thruster is the latest generation of ion 

thrusters under development. The NEXT ion thruster currently has a qualification level propellant 

throughput requirement of 450 kg of xenon, which corresponds to roughly 22,000 hr of operation at the 

highest input power throttling point. This paper will provide a brief review the previous life assessment 

predictions for various throttling conditions. A further assessment will be presented examining the 

anticipated accelerator grid hole wall erosion and related electron backstreaming limit. The continued 

assessment of the NEXT ion thruster indicates that the first failure mode across the throttling range is 

expected to be in excess of 36,000 hr of operation from charge exchange induced groove erosion. It is at 

this duration that the groove is predicted to penetrate the accelerator grid possibly resulting in structural 

failure. Based on these lifetime and mission assessments, a throttling approach is presented for the Long 

Duration Test to demonstrate NEXT thruster lifetime and validate modeling. 

Nomenclature

ds screen grid hole diameter

ELT Extended Life Test

EM Engineering Model

fp fraction of maximum perveance

Ib beam current

jb beam current density at the beamlet sheath

le effective grid gap

lg grid gap

LDT Long Duration Test

NEXT NASA’s Evolutionary Xenon Thruster

NSTAR NASA Solar Electric Propulsion Technology Application Readiness

P perveance

PFA probabilistic failure analysis

PMS propellant management system

PPU power processing unit

ta accelerator grid thickness

ts screen grid thickness

TRL technology readiness level
NASA/TM—2010-216920 2

Vb beam power supply voltage

VT total accelerating voltage

Introduction

NASA’s Evolutionary Xenon Thruster (NEXT) project encompasses the development of a high 

performance, nominal 7-kW, Electric Propulsion Thruster; a light weight, high efficiency power 

processing unit (PPU); a highly flexible advanced Propellant Management System (PMS); and a lightweight, low-cost gimbal (Refs. 1 and 2). The goal of the NEXT project is to develop the next generation 

ion propulsion technology to NASA Technology Readiness Level (TRL) 6 (Ref. 3). The NEXT project 

has built on knowledge gained from the current state-of-the-art NASA Solar Electric Propulsion 

Technology Application Readiness (NSTAR) thruster system to create a more efficient, higher specific 

impulse, and lower specific mass system (Ref. 2). The design approach will provide future NASA science 

missions with the greatest value in mission performance at a low total development cost.

The NEXT thruster is a 36 cm beam diameter, two-grid ion optics, xenon ion thruster with an input 

power of 0.5 to 6.9 kW, a maximum specific impulse of over 4100 sec, and a maximum thrust greater 

than 230 mN (Ref. 2). A throttle table containing several of the operating points for the NEXT thruster

along with corresponding thrust and specific impulse is given in Table 1 and Table 2. While a limited 

number of throttle points are indicated in the tables, NEXT is capable of operating at any additional point 

within the envelope shown. The arrows on the tables indicate typical throttling strategies used on mission 

analysis that favor either high thrust or high specific impulse use of the engines for the power available.

Several potential missions have been evaluated using the NEXT thruster (Ref. 4). These missions 

included a Saturn mission, Neptune mission, a near Earth asteroid return, comet rendezvous, Vesta-Ceres 

rendezvous, Titan direct lander, and a comet surface sample return. A 300 kg xenon throughput 

requirement, which corresponds to a 1.23×107 N-s total impulse, is derived from these studies with a 

corresponding 450 kg qualification level (Ref. 1). Typically these mission studies have followed a high 

thrust profile similar to that indicated in Table 1. It can be seen that this favors only a portion of the 

throttle table. This paper will provide a review of the anticipated lifetime of the NEXT thruster 

components and how the wear changes with throttling.

TABLE 1.—NEXT ION THRUSTER THROTTLE TABLE INCLUDING THRUST AND THRUSTER INPUT POWER

[Arrows indicate typical throttling profile followed to maintain maximum thrust for a given input power.]

Thrust /

Input Power 

Beam Voltage, V

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

Beam Current, A

3.52 mN

kW

236 

6.8

221

6.0

208

5.4

192

4.7

3.1 mN

kW

208

6.0

194

5.3

184

4.8

169

4.1

2.7 mN

kW

181

5.3

169

4.6

160

4.2

147

3.6

137

3.2

2.35 mN

kW

158

4.6

147

4.1

139

3.7

128

3.2

119

2.8

2 mN

kW

134

4.0

125

3.5

118

3.2

108

2.8

101

2.5

1.6 mN

kW

107

3.2

100

2.9

94

2.6

87

2.3

81

2.0

1.2 mN

kW

80

2.4

75

2.2

71

2.0

65

1.7

60

1.5

58

1.4

55

1.3

49

1.1

48

1.1

37

0.8

32

0.7

1 mN

kW

26

0.5
NASA/TM—2010-216920 3

TABLE 2.—NEXT ION THRUSTER THROTTLE TABLE INCLUDING SPECIFIC IMPULSE AND THRUSTER INPUT POWER

[Arrows indicate throttling profile followed to maintain maximum specific impulse for a given input power.]

Specific impulse/

input power 

Beam voltage, 

V

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

Beam current, A

3.52 s

kW

4190

6.8

3910

6.0

3690

5.4

3390

4.7

3.1 s

kW

4150

6.0

3870

5.3

3660

4.8

3360

4.1

2.7 s

kW

4150

5.3

3870

4.6

3660

4.2

3360

3.6

3130

3.2

2.35 s

kW

4100

4.6

3830

4.1

3620

3.7

3320

3.2

3090

2.8

2 s

kW

4310

4.0

4020

3.5

3800

3.2

3490

2.8

3250

2.5

1.6 s

kW

4190

3.2

3910

2.9

3690

2.6

3390

2.3

3160

2.0

1.2 s

kW

4000

2.4

3730

2.2

3530

2.0

3240

1.7

3010

1.5

2880

1.4

2750

1.3

2450

1.1

2400

1.1

1850

0.8

1590

0.7

1 s

kW

1400

0.5

Previous extended testing by the NSTAR program examined the wear mechanisms for ion thrusters 

(Refs. 5, 6, 7, and 8). Several of these wear mechanisms were minimized and mitigated during the 

NSTAR program, but a set of life-limiting mechanisms was established. Based on these related failure 

modes, an initial life assessment of the NEXT thruster was prepared during Phase I of the NEXT program 

(Ref. 9). To date several iterations of life models have been utilized based on the most recent data 

available for the NEXT thruster. These have included deterministic models for the ion optics (Refs. 10,

11, and 12). and the hollow cathodes (Refs. 9 and 13). Modeling of the thruster was also accomplished 

using a probabilistic failure analysis (PFA) process described in Reference 14. To date, PFA modeling 

uncertainties have led to unrealistic results that are not supported by test data. Until these uncertainties are 

resolved, PFA is of limited value. Further work is continuing to reduce these uncertainties, such as 

measurement of the grid gap during extended operation (Refs. 15 and 16).

Several tests of the NEXT thruster and its components to evaluate wear mechanisms have already been 

completed. The NEXT Engineering Model 1 (EM1) thruster was wear-tested for 2038 hr (43 kg of xenon 

throughput) to evaluate thruster wear (Ref. 17). Two wear mechanisms were revealed that could potentially 

lead to a premature failure. One was the excessive wear on the discharge cathode keeper orifice plate and 

the other was excessive wear on the accelerator grid holes beyond a radius of 15.3 cm. While loss of the 

keeper orifice plate does not result in thruster failure, the keeper electrode does protect the cathode and 

extend its life. The keeper material was changed to graphite for subsequent designs to achieve significant 

margin for life. The ion optics design was modified by removing some of the outer holes and making the 

effective beam diameter 36 cm instead of the previous 40 cm. The removal of holes was only in regions of 

low plasma density resulting in effectively similar discharge losses without the added optics wear. 

The NEXT Engineering Model 3 (EM3) is currently undergoing a long duration test (LDT) at the

NASA Glenn Research Center (Refs. 15 and 16). The intent of this test is to validate and qualify the 

NEXT propellant throughput capability to a qualification-level of 450 kg. The operation to date has been 

at a beam power supply voltage of 1800 V and a beam current of 3.52 A for the first 13,000 hr, then the 

thruster was throttled to a lower voltage of 1179 V at the same beam current. EM3 has processed 337 kg

of xenon to date and operated for 16,550 hr. As the data from this test continues to become available, it 

will be incorporated into the life assessment. 

Reference 18 provides a detailed NEXT life assessment for critical components including the 

accelerator grid, but lifetime limits due to electron backstreaming were not fully addressed. The intention 

of this paper is to examine the accelerator grid aperture hole wall wear that could result in the potentially 

life limiting reduction of electron backstreaming margin and review other life limiting mechanisms to 
NASA/TM—2010-216920 4

establish appropriate throttling conditions for the LDT. The NEXT lifetime assessment will continue to 

evolve with time based on the latest test data and modeling and this study provides an update the best 

understanding of the anticipated life of the thruster. A review of previous ion thruster endurance tests 

along with the causes of failure is given in Reference 19.

Throttling and Wear Mechanisms

This paper will not provide detail about the effect of throttling on each potential ion thruster wear 

mechanism as that can be found in a previously published paper (Ref. 18). The primary failure mechanism 

over the entire throttle table is predicted to be from accelerator grid groove penetration due to erosion from 

charge exchange ion possibly resulting in structural failure of the grid. Evaluation of accelerator grid barrel 

erosion and the related electron backstreaming limit will be expanded in this paper from previous 

assessments. A review will also be presented for the cathodes and how the mechanisms on each 

subcomponent vary with throttling, including at what throttle point are the expected to be the most severe.

Accelerator Grid

As previous mentioned, the primary failure to the thruster over its entire operational envelope defined in 

the throttle table is expected to be from accelerator grid groove penetration due to erosion from charge 

exchange ions impinging on the downstream face of the grid, possibly resulting in structural failure. Aside 

from the three lowest power throttle points operating at or below 780 W of input power, the harshest pit and 

groove wear occurs at the peak power throttle point corresponding to 3.52 A of beam current and 1800 V of 

beam voltage. The throughput and lifetime at this throttle point is predicted to be 770 kg and 36.2 khr 

respectively (Ref. 18). As can be seen from Figure 1, the lifetime and throughput generally increase as the 

operating power decrease. The three lowest power throttle points evaluated are predicted to have less

propellant throughput than the peak power point, but since they correspond to low beam currents and mass 

flow rates, the lifetime is expected to exceed the peak power point by 9000 hr. The three lowest power throttle 

points reduced propellant throughputs are due to the larger magnitudes of accelerator grid voltages required for 

operation. There is likely sufficient electron backstreaming margin such that the accelerator grid voltage can be 

adjusted and further increase the lifetime of the optics at the lowest three power points (Ref. 20). 

Figure 1.—Accelerator grid life based on structural failure from charge 

exchange pit and groove erosion (Ref. 18).

10

100

1000

10000

0 1000 2000 3000 4000 5000 6000 7000

Input Power, W

Propellant Throughput, kg

Lifetime, khrs
NASA/TM—2010-216920 5

Analysis was performed to assess the NEXT accelerator grid hole wall erosion and corresponding 

backstreaming limits. This wear mechanism leads to the primary life-limiting failure mode for the 

Dawn/NSTAR ion thrusters (Ref. 21). Previous modeling of erosion on the accelerator grid barrel from 

charge exchange ions showed that the normalized hole wall erosion rate per unit area is a strong function 

of the fraction of maximum perveance (Ref. 22). The fraction of maximum perveance is defined in 

Reference 22 as the perveance divided by the theoretical maximum perveance for xenon, which is 

4.77×10–9 A/V3/2. Perveance, P, is given as,

2

3

2

T

b e

V

j l P = ; ( ) 2

4

2 1 e g s ds l = l + t +

where jb is the current density between the grids, le is the effective grid gap, and VT is the total voltage or 

sum of the beam and accelerator voltage. Figure 2 shows the results from Reference 22 that indicate this 

correlation. Subsequent modeling has used the same computer code to predict the wear rates for the 

Dawn/NSTAR ion optics for various throttle points (Ref. 21). However, when modeling the aperture wall 

erosion of the Dawn/NSTAR ion optics, the normalized wear at the lowest throttle point, TH0, does not 

support this supposition. The ELT throttle points are given in Table 3. Figure 3 shows the predicted wall 

erosion rates from that reference versus the fraction of maximum perveance determined using the 

measured hot grid gaps (Ref. 23). The operation at NSTAR’s lowest power throttling point has a lower 

wear rate predicted than what would be expected based purely on its fraction of maximum perveance.

Modeling of the NEXT ion thruster optics using a different computer code also did not support the 

perveance—hole wall wear rate relationship (Ref. 24).

TABLE 3.—NSTAR ELT THROTTLE CONDITIONS

NSTAR 

throttle

number

Beam 

current, 

A

Beam 

voltage, 

V

Accelerator 

voltage, 

V

TH15 1.76 1100 –180

TH12 1.49 1100 –180

TH8 1.10 1100 –180

TH5 0.81 1100 –150

TH0 0.51 650 –150

Figure 2.—Normalized hole wall erosion rate as a function of 

perveance for given generic grid set (Ref. 22).

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 0.2 0.4 0.6 0.8

Fraction of Maximum Perveance, fp

Normalized Hole Wall Erosion Rate

lg/ds = 0.346

ts/ds=0.20

ta/ds = 0.42
NASA/TM—2010-216920 6

Figure 3.—Normalized hole wall erosion rate for NSTAR/Dawn

accelerator grids.

Figure 4.—NSTAR/Dawn normalized hole wall erosion rates predicted 

as a function of beamlet current density.

While it is not explicitly called out in Reference 22, it appears possible that the data to generate 

Figure 2 was at a fixed total voltage. If this is true, along with a fixed grid gap, it would indicate that the 

relationship is not defined by perveance, but with the beamlet sheath ion current density. The beamlet 

sheath current density is calculated in Reference 22 by taking the peak beam current density measured 

and dividing it by the screen grid ion transparency. The same approach was used in this assessment. For 

both the NSTAR and NEXT thrusters, the peak beam current densities and screen grid ion transparency 

are both based on measured data along with interpolations for conditions where measurements were not 

taken. Figure 4 shows the normalized wear predictions from Reference 21 for the NSTAR/Dawn thruster 

plotted versus the beamlet current density. The correlation between the beamlet sheath current density and 

the accelerator aperture wear rate appears strong. While total voltage is expected to have an effect, it 

appears less pronounced due to counteracting effects experienced in the beamlet plasma. For example, for 

higher voltages, the beamlet diameter, and hence ion flow volume, generally decreases under the 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Fraction of Maximum Perveance, fp

Normalized Hole Wall Erosion Rate

TH0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 20 40 60 80

Beamlet Current Density at Beamlet Sheath, A/m2

Normalized Hole Wall Erosion Rate
NASA/TM—2010-216920 7

conditions at the center aperture, yet the charge-exchange ion energy would likely increase. It should be 

noted that the exact relationship between the ion density and the wear rate would be expected to change 

for different ion optics geometries and gaps. 

While the NEXT grid geometry (lg/ds = 0.21, ts/ds = 0.2, ta/ds = 0.4) is slightly different that the 

NSTAR/Dawn geometry (lg/ds = 0.178, ts/ds = 0.2, ta/ds = 0.267), it is assumed that the NSTAR erosion 

data should provide insight into the anticipated hole wall wear of the NEXT optics. The values of lg are 

hot or operational grid gaps taken from Refs. 15 and 23. Other differences between the two grid sets

include the NEXT apertures having much less pronounced cusps and slightly larger accelerator grid 

aperture diameters at the grid center due to more tightly controlled manufacturing processes(Refs. 25

and 26).

Figure 5 shows the peak beamlet sheath current density over the entire range of the NEXT throttle 

table. The operating points from the NSTAR Extended Life Test (ELT) are also included for comparison.

It can be seen that peak beamlet current density for NSTAR/Dawn thruster corresponding to the worst 

case hole wall erosion is approximately 50 percent higher than the worst case for the NEXT optics. If the 

curve established for the NSTAR/Dawn optics (Figure 4) is used, the NEXT optics would experience a 

50 percent reduction in the aperture wear rate. By comparison, the worst case barrel erosion for NEXT 

could be likened to the NSTAR TH8 operating condition.

The beamlet sheath current density can be used to establish the worst case hole wall erosion rate for 

the NEXT optics. This will occur at a beam current of 3.52 A and beam voltage of 1179 V. Since there is 

only a marginal difference in beam current density, this operating point is only expected to produce a wall 

wear rate at most 5 percent larger than the NEXT full power operating condition. The maximum aperture 

wear rate and final diameter will have the biggest impact leading to the degradation of electron 

backstreaming margin.

The beam current density affects not only the rate, but also the final eroded diameter of the 

accelerator grid hole. Figure 6 shows the minimum aperture diameter of the NSTAR center hole during 

the ELT. It can be seen that while initially there is a substantial increase in the diameter, this slows down 

with time and eventually reaches what appears to be a steady state diameter. The diameter approaches 

between 120 to 125 percent of its starting diameter. The center aperture diameter wear profile with time is 

representative of a worst-case condition. The apertures which were observed during the ELT all appear to 

have an achieved a state where the minimum aperture diameter is no longer increasing. It has been 

previously shown that the mass loss is related to the beam current density (Ref. 27). Figure 7 shows the 

final aperture diameters measured from the NSTAR ELT normalized to the pretest accelerator center hole 

diameter. The beam current density measured at the TH15 throttle point is also included on that figure. It 

can be seen that there is virtually a one-to-one comparison between the final diameter size and the beam 

current density. The beam current density has not been corrected back to the grid plane and would likely 

improve the correlation further.

Since the NEXT optics are similar to the NSTAR optics, and the worst case center hole erosion is at a 

total voltage close to the NSTAR TH15 total voltage, Figure 7 provides a good means to determine the 

final aperture diameter for the NEXT thruster. The NEXT optics cusps are much less pronounced and the 

initial starting diameter of the accelerator center grid hole is larger than the NSTAR diameters. Based on 

Figure 7, the NEXT final accelerator center hole will only have a 2 percent increase from the beginning 

of life diameter based on a maximum beam current density of 4 mA/cm2

. It is expected that little cusp 

wear would occur at the highest beamlet sheath current density condition (Ib=3.52 A, Vb=1179 V) or at 

the peak power operation for NEXT (Ib=3.52 A, Vb=1800V) since both have similar beak beam current 

densities. This is supported by the negligible change in the NEXT EM3 thruster accelerator grid center 

hole minimum diameter after more than 16,000 hr of operation. Since minimal wear is anticipated on the 

cusp, it is predicted that there will be only a 10 V reduction on the electron backstreaming margin over 

the thruster’s lifetime.
NASA/TM—2010-216920 8

Figure 5.—Peak beamlet sheath current density for NEXT throttle table operating conditions

compared to NSTAR ELT operating points.

Figure 6.—NSTAR accelerator grid center aperture minimum diameter 

measured during the ELT.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 500 1000 1500 2000

Beam Voltage, V

Peak Beamlet Sheath Current Density, 

mA/cm2

3.52 A

3.10 A

2.70 A

2.35 A

2.00 A

1.60 A

1.20 A

1.00 A

NSTAR

TH15

 TH12

TH0

 TH8

TH5

1.00

1.05

1.10

1.15

1.20

1.25

1.30

0 5 10 15 20 25

Time, kh

Normalized NSTAR ELT Accelerator Grid 

Center Aperture Diameter 

TH15 TH8 TH15 TH0 TH15
NASA/TM—2010-216920 9

Figure 7.—Comparison of NSTAR TH15 beam current density and normalized 

post test accelerator hole diameters.

The minimally anticipated center aperture diameter wear is demonstrated in the data from the NEXT 

LDT. Figure 8 shows the center aperture minimum diameter that has been measured for the current 

duration of the test. No decrease in cusp diameter was experienced during the peak power condition. In 

fact, the cusp may have decreased slightly in diameter due to sputtered material being deposited. When 

operation was changed at 13,000 hr to the maximum beamlet current density (4.7 kW, Ib = 3.52 A, Vb = 

1179 V), the diameter increases slightly back to initial diameter. The extent of the wear appears to be only 

the removal of material that was sputtered onto the aperture. The measured wear data is supported by the 

measured electron backstreaming margin, as shown in Figure 9. The margin is seen to be extremely 

steady and to even increase slightly. This slight increase is consistent with the slight decrease in diameter 

of the accelerator grid hole. The effects of facility backsputtered carbon (<3.0 μm/khr) on accelerator grid 

life are beyond the scope of this paper.

Reference 21 indicates that the downstream chamfering also appeared to have a noticeable effect on 

the electron backstreaming limit effect late in the life of the NSTAR/Dawn optics (>12,000 hr). While the 

chamfering wasn’t measured during the NSTAR ELT, the electron backstreaming limit was measured

(Ref. 28). The approach in Reference 21 was to use the final accelerator chamfered dimensions along 

with the change in backstreaming limit to predict the chamfering during the ELT. After 12,000 hr of 

operation in the NSTAR ELT, the minimum diameter of the accelerator grid aperture appeared to reach a 

steady state, yet the backstreaming limit continued to change and appeared most affected by operation at 

the full power point, TH15. Operation at TH0 and TH5 appeared to have minimal effect on the change in 

the backstreaming limit and hence the chamfering erosion on the NSTAR optics. Since most of the NEXT 

operation has beamlet current densities that are appreciably lower than TH15 operation on NSTAR and 

also that the NEXT accelerator grid is 50 percent thicker, the chamfer wear would be expected to 

minimally affect the electron backstreaming limit. Chamfering has been seen on the NEXT optics, but 

this has essentially reached a steady state condition with no consequence on the electron backstreaming 

margin (Ref. 15).

0

1

2

3

4

5

6

7

0 5 10 15

Distance from Grid Center, cm

TH15 Beam Current Density, mA/cm2

0.95

1.00

1.05

1.10

1.15

1.20

1.25

1.30

Normalized Post Test Accelerator 

Hole Diameter

Beam Current Density

Accelerator Hole Diameter
NASA/TM—2010-216920 10

Figure 8.—NEXT thruster center aperture diameters measured during the NEXT LDT.

Figure 9.—NEXT thruster electron backstreaming margin as measured in the NEXT LDT.

While aperture wear was significant in the NSTAR thruster, current grid design, manufacturing, and 

operation in NEXT thruster have significantly reduced its impact. Operation over the current NEXT 

throttle table is not expected to cause much aperture wear affecting the electron backstreaming margin. 

This wear mechanism and failure mode has been reduced such that there will be significant life beyond 

the NEXT thrusters first anticipated failure mode due to groove erosion. The minimal wear experienced in 

the barrel of the NEXT accelerator grid also allows for an increase in beam current beyond the current 

throttle table while still maintaining ample life. 

The minimum perveance fraction can be used to evaluate the condition at which the highest degree of 

overfocusing will occur. This operating condition will likely result in the largest amount of grid material 

sputtered. This will occur in apertures with the lowest beamlet current densities (outer radius apertures) 

and highest total voltages. For the NEXT thruster, this is at a beam current of 1.2 A and a beam voltage of 

1800 V. While this phenomena is not expected to be a life limiter since the erosion stops once the grid 

material in the path of the beamlet is removed, it is expected to sputter the highest amount of material and 

contribute the most to grid recycling.

0.90

0.95

1.00

1.05

1.10

0 2 4 6 8 10 12 14 16 18

Elapsed Time, kh

Normalized Accelerator Grid.. 

Center Aperture Radius

Cusp: 6.9kW Cusp: 4.7kW

0

20

40

60

80

100

120

140

0 2 4 6 8 10 12 14 16 18

Total Elapsed Time, kh

E-Backstreaming Margin, V .

1.20 A, 679 V

1.20 A, 1179 V

1.20 A, 1800 V

2.00 A, 1179 V

2.70 A, 1179 V

2.00 A, 1800 V

3.52 A, 1179 V

2.70 A, 1800 V

3.52 A, 1800 V
NASA/TM—2010-216920 11

NEXT Cathode Lifetimes

As discussed more thoroughly in Reference 18, there are two major wearout mechanisms related to 

the discharge cathode. These are insert barium depletion and keeper wear. Figure 10 shows the previously

predicted maximum propellant throughputs for electron emitting insert lifetime and Figure 11 shows the 

previously predicted propellant throughputs for the discharge keeper wear. The depletion of barium from 

the insert is highly dependent on temperature and hence, the discharge current. While the lifetime due to 

barium depletion doesn’t change much for operation with a beam current of 3.52 A, the minimum lifetime 

of the insert is at a beam voltage of 1179 V. Operation at this throttle point has higher discharge losses 

and hence the highest discharge current. The lifetime of this insert is still predicted to be around 1000 kg 

of propellant throughput. Operation at Ib=3.52 A and Vb=1179 V is also expected to be the severest for the 

discharge cathode keeper. It is at this condition that ratio between the emission current and the discharge 

cathode flow rate is that highest and also has the highest discharge voltage. These contribute to the 

highest keeper wear rates. However, the thruster throughput is still expected to be several thousand 

kilograms of propellant before the keeper degrades enough to expose the heater. 

Figure 10.—Insert lifetime of the NEXT discharge cathode (Ref. 18).

Figure 11.—Lifetime of the NEXT discharge cathode keeper (Ref. 18).

1,000

10,000

1.00 1.50 2.00 2.50 3.00 3.50 4.00

Beam Current, A

Propellant Throughput, kg 

2000

2100

2200

2300

2400

2500

2600

2700

2800

2900

3000

1.00 1.50 2.00 2.50 3.00 3.50 4.00

Beam Current, A

Propellant Throughput, kg 
NASA/TM—2010-216920 12

Determination of LDT Throttling Conditions

The NEXT long duration test has been synergistic with the lifetime assessment of the NEXT ion 

thruster. The lifetime assessment has been used to establish what operating points are the most stressful 

on the life of the thruster and also establish recommended durations to provide sufficient data to 

demonstrate the wear rates that are present under the harshest conditions. Data is then received from the 

test and used along with other tests to further develop and validate analytical models. Some of the basic 

wear related measurements made during the NEXT LDT are grid aperture diameters, accelerator 

downstream groove erosion, cathode and keeper orifice diameters, and grid gap. Based on the lifetime 

assessment, four throttle points of interest were established for LDT operation. These throttle points 

bracketed the throttle table, as can be seen in Table 4, and also had the harshest conditions for 

corresponding wear mechanism. These throttle points include:

(1) Ib = 3.52 A, Vb = 1800 V

(a) Operation at this throttle point establishes wear rates of the first failure mode, namely 

structural failure of the accelerator grid. The accelerator grid is predicted to have groove 

penetration from downstream erosion at 36,000 hr of operation at this throttle point 

corresponding to 750 kg of propellant throughput (Ref. 18).This is thought to result in 

structural failure of the grid.

(2) Ib = 3.52 A, Vb = 1179 V.

(a) Operation at this throttle point corresponds to the worst case barrel erosion. While accelerator 

grid hole wall erosion is still anticipated to be minimal and failure due to electron 

backstreaming is expected to occur well beyond the penetration from groove erosion, 

operation at this throttle point will help establish the worst case rates. While this throttle point 

is not expected to have a dramatic effect on the discharge cathode, operation at this throttle 

point will be the most stressful for the discharge cathode insert and the discharge cathode 

keeper wear. The first failure mode at this throttle point is still predicted to be due to

accelerator grid groove erosion from charge exchange after 42,000 hr and corresponding to 

875 kg of propellant throughput (Ref. 18).

(3) Ib = 1.00 A, Vb = 275 V

(a) This throttle point corresponds to the lowest propellant throughput predicted before 

accelerator grid penetration. This is a result of the greatest negative accelerator voltage 

required. The propellant throughput predicted is 330 kg of xenon. However, while the 

throughput is lowest on the throttle table, it would correspond to 45,000 hr of operation

(Ref. 18).

(4) Ib = 1.20 A, Vb = 1800 V

(a) While this throttle point will produce the greatest overfocusing of beamlets on the outer 

apertures, it is not expected to be a life limiter. Operation here will demonstrate operation 

under the highest optics sputtering conditions. However, once the beamlet has removed the 

grid material from direct impingement, the wear is anticipated to cease. The first failure mode 

associated with operation at this throttle point is still predicted to be from groove erosion after 

200,000 hr of operation and over 1400 kg of propellant throughput (Ref. 18).
NASA/TM—2010-216920 13

TABLE 4.—NEXT THROTTLE TABLE WITH THROTTLE POINTS

CORRESPONDING TO A WORST CASE WEAR MECHANISMS

[Operational throttle points shaded.]

Beam

current, 

A

Beam voltage, 

V

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

3.52 Highest groove 

erosion rate

Highest barrel 

erosion rate, Shortest 

cathode/keeper 

lifetime

3.10

2.70

2.35

2.00

1.60

1.20 Highest outer 

aperture wear

Intermediate 

Point

1.00 Highest groove 

erosion based on 

throughput

In addition to the four throttle points that are associated with a wear mechanism, a fifth point is 

chosen that is intermediate (Ib = 1.2 A, Vb = 1021 V). This was chosen to provide fairly nominal operation 

for reference and to ensure there are no unknown and unexpected long-term effects. This throttle point 

represents a region of the throttle table that was used frequently during mission analysis. It is roughly in 

the middle of the high thrust profile. The durations chosen at each throttle point were a combination of 

ensuring sufficient operation to quantify the wear mechanism and also represent typical flight durations.

Two to four thousand hours of operation is expected to be the minimum necessary to establish trends 

from the various throttling conditions. These durations will be adjusted as the test continues to ensure that 

any wear mechanism is sufficiently quantified.

Conclusion

An evaluation of thruster wear mechanisms has been presented to determine the best operating 

conditions for the NEXT Long Duration Test. Several of the wear mechanisms were reviewed from 

previous work along with a further assessment of the accelerator aperture hole wall erosion and associated 

electron backstreaming margin. The primary life-limiting mechanism over the entire NEXT thruster

operating conditions is still expected to be from pit and groove erosion on the accelerator grid.

Penetration through the accelerator grid is expected to occur after 36,000 hr of operation of the NEXT 

thruster. While aperture wall wear was significant in the NSTAR thruster, current grid design, 

manufacturing, and operation in NEXT thruster have significantly reduced its impact. The minimal wear 

experienced in the barrel of the NEXT accelerator grid also allows for an increase in beam current beyond 

the current throttle table while still maintaining ample life. To validate the thruster life and aid in 

verifying its various wear rates, four throttle points of interest were determined. The first throttle point is

the peak power throttle point (Ib = 3.52 A, Vb = 1800 V) to evaluate the harshest wear rates from pit and 

groove erosion. The second throttle point includes the highest beamlet current density and discharge 

current (Ib = 3.52 A, Vb = 1179 V) to evaluate the accelerator hole wall erosion rate, the shortest discharge 

cathode insert life, and the highest discharge keeper wear. The third throttle point is at the lowest power

(Ib = 1.0 A, Vb = 275 V) to evaluate the highest pit and groove erosion based on propellant throughput.

The fourth throttle point (Ib = 1.2 A, Vb = 1800 V) will have the highest degree of beamlet overfocusing in 

the outer radius apertures and likely produce the highest degree of sputtered material. These four throttle 

points effectively span the NEXT operating conditions. Thruster lifetime estimates for these three throttle 

conditions range from 36,000 to 200,000 hr.
NASA/TM—2010-216920 14

References

1. Benson, S.W., Patterson, M.J., Vaughan, D.A., Wilson, A.C., and Wong, B.R., “NASA’s 

Evolutionary Thruster (NEXT) Phase 2 Developmental Status,” 41st Joint Propulsion Conference and 

Exhibit, AIAA–2005–4070, Tucson, AZ, Jul. 10–13, 2005.

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

Performance,” 43rd Joint Propulsion Conference and Exhibit, AIAA–2007–5199, Cincinnati, OH, 

Jul. 8–11, 2007.

3. Mankins, J.C., “Technology Readiness Levels,” White Paper, Apr. 6, 1995, URL: 

http://www.hq.nasa.gov/office/codeq/trl/trl.pdf [cited Jun. 5, 2007].

4. Oh, D., Benson, S., Witzberger, K., and Cupples, M., “Deep Space Mission Applications for NEXT: 

NASA’s Evolutionary Xenon Thruster,” 40th Joint Propulsion Conference and Exhibit, AIAA–2004–

3806, Fort Lauderdale, FL, Jul. 11–14, 2004.

5. Sengupta, A., et al., “An Overview of the Results from the 30,000 Hr Life Test of Deep Space 1 

Flight Spare Ion Engine,” 40th Joint Propulsion Conference and Exhibit, AIAA–2004–3608, Fort 

Lauderdale, FL, Jul. 11–14, 2004.

6. Polk, J.E., Anderson, J.R., Brophy, J.R., Rawlin, V.K., Patterson, M.J., Sovey, J., and Hamley, J., 

“An Overview of the Result from the 8200 Hour Wear Test of the NSTAR Ion Thruster, “ 35th Joint 

Propulsion Conference and Exhibit, AIAA–1999–2446, Los Angeles, CA, Jun. 20–24, 1999.

7. Patterson, M.J., Rawlin, V.K., Sovey, J.S., Kussmaul, M.J., and Parkes, J., “2.3 kW Ion Thruster 

Wear Test,” 31st Joint Propulsion Conference and Exhibit, AIAA–95–2516, San Diego, CA, 

Jul. 10–12, 1995.

8. Polk, J.E., et al., “A 1000-Hour Wear Test of the NASA NSTAR Ion Thruster, “ 32nd Joint 

Propulsion Conference and Exhibit, AIAA–1996–2717, Lake Buena Vista, FL, Jul. 1–3, 1996.

9. Patterson, M.J., et al., “NEXT: NASA’s Evolutionary Xenon Thruster Development Status,” 39th 

Joint Propulsion Conference and Exhibit, AIAA–2003–4862, Huntsville, AL, Jul. 20–23, 2003.

10. Emhoff, J.W. and Boyd, I.D., “Progress in NEXT Ion Optics Modeling,” 40th Joint Propulsion 

Conference and Exhibit, AIAA–2004–3786, Fort Lauderdale, FL, Jul. 11–14, 2004.

11. Farnell, C.C., Williams, J.D., and Wilbur, P J., “NEXT Ion Optics Simulation Via ffx,” 39th Joint 

Propulsion Conference and Exhibit, AIAA–2003–4869, Huntsville, AL, Jul. 20–23, 2003.

12. Emhoff, J.W. and Boyd, I.D., “Grid Erosion Modeling of the NEXT Ion Thruster Optics,” 39th Joint 

Propulsion Conference and Exhibit, AIAA–2003–4868, Huntsville, AL, Jul. 20–23, 2003.

13. Kovaleski, S.D., “Life Model of Hollow Cathode using a Barium Calcium Aluminate Impregnated 

Tungsten Emitter,” 27th International Electric Propulsion Conference, IEPC–01–276, Pasadena, CA, 

Oct. 14–19, 2001.

14. Polk, J.E., Moore, N.R., Newlin, L.E., Brophy, J.R., and Ebbeler, D.H., “Probabilistic Analysis of Ion 

Engine Accelerator Grid Life,” 23rd International Electric Propulsion Conference, IEPC–93–176, 

Seattle, WA, Sep. 13–16, 1993.

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

Characteristics after 17,200 h of Operation and 352 kg of Xenon Processed,” 44th ASEE Joint 

Propulsion Conference & Exhibit, AIAA–2008–4919, Hartford, CT, Jul. 20–23, 2008.

16. Herman, D.A., Patterson, M.J., and Soulas, G.S., “Performance Characteristics of the NEXT LongDuration Test after 17,200 h of Operation and 352 kg of Xenon Processed,” 44th Joint Propulsion 

Conference & Exhibit, AIAA–2008–4527, Hartford, CT, Jul. 20–23, 2008.

17. Soulas, G.C., Kamhawi, H., Patterson, M.J., Britton, M.A., and Frandina, M.M., “NEXT Ion Engine 

2000 Hour Wear Test Results,” 40th Joint Propulsion Conference and Exhibit, AIAA–2004–3791, 

Fort Lauderdale, Florida, July 11–14, 2004.

18. Van Noord, J.L., “Lifetime Assessment of the NEXT Ion Thruster,” 43rd Joint Propulsion 

Conference and Exhibit, AIAA–2007–5274, Cincinnati, Ohio, Jul. 8–11, 2007.
NASA/TM—2010-216920 15

19. Brophy, J.R., Polk, J.E., and Rawlin, V.K., “Ion Engine Service Life Validation by Analysis and 

Testing,” 32nd Joint Propulsion Conference and Exhibit, AIAA–1996–2715, Lake Buena Vista, FL, 

Jul. 1–3, 1996.

20. Soulas, G.C. and Patterson, M.J, “NEXT Ion Thruster Performance Dispersion Analysis,” 43rd Joint 

Propulsion Conference and Exhibit, AIAA–2007–5213, Cincinnati, OH, Jul. 8–11, 2007.

21. Brophy, J.R., “Propellant Throughput Capability of the Dawn Ion Thrusters,” 30th International 

Electric Propulsion Conference, IEPC–2007–279, Florence, Italy, Sep. 17–20, 2007.

22. Brophy, J.R., Katz, I., Polk, J.E., and Anderson, J.R., “Numerical Simulations of Ion Thruster 

Accelerator Grid Erosion,” 38th Joint Propulsion Conference and Exhibit, AIAA–2002–4261, 

Indianapolis, IN, Jul. 7–10, 2002.

23. Soulas, G.C. and Frandina, M.M., “Ion Engine Grid Gap Measurement,” 40th Joint Propulsion 

Conference and Exhibit, AIAA–2004–3961, Fort Lauderdale, FL, Jul. 11–14, 2004.

24. Emhoff, J.W., Boyd, I.D., Shepard, S.P., “Numerical Analysis of NEXT Ion Thruster Optics,” 28th 

International Electric Propulsion Conference, IEPC–03–110, Toulouse, France, Mar. 17–21, 2003.

25 Hoskins, W.A., et al., “Development of a Prototype Model ion Thruster for the NEXT System,” 40th 

Joint Propulsion Conference and Exhibit, AIAA–2004–4111, Fort Lauderdale, FL, Jul. 11–14, 2004.

26. Hoskins, W.A., et al., “Overview of the NEXT Ion Propulsion System Program at Aerojet,” 41st Joint 

Propulsion Conference and Exhibit, AIAA–2005–3885, Tucson, AZ, Jul. 10–13, 2005.

27. Polk, J.E., Duchemin, O.B., Ho, C., and Koel, B.E., “The Effect of Carbon Deposition on Accelerator 

Gird Wear Rates in Ion Engine Ground Testing,” 36th Joint Propulsion Conference and Exhibit, 

AIAA–2000–3662, Huntsville, AL, Jul. 16–19, 2000.

28. Sengupta, A., Brophy, J.R., and Goodfellow, K.D., “Status of the Extended Life Test of the Deep 

Space 1 Flight Spare Ion Engine After 30,352 Hours of Operation,” 39th Joint Propulsion Conference 

and Exhibit, AIAA–2003–4558, Huntsville, AL, Jul. 20–23, 2003.
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Application of the NEXT Ion Thruster Lifetime Assessment to Thruster Throttling 

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14. ABSTRACT

Ion thrusters are low thrust, high specific impulse devices with typical operational lifetimes of 10,000 to 30,000 hr over a range of throttling 

conditions. The NEXT ion thruster is the latest generation of ion thrusters under development. The NEXT ion thruster currently has a 

qualification level propellant throughput requirement of 450 kg of xenon, which corresponds to roughly 22,000 hr of operation at the highest 

input power throttling point. This paper will provide a brief review the previous life assessment predictions for various throttling conditions. 

A further assessment will be presented examining the anticipated accelerator grid hole wall erosion and related electron backstreaming limit. 

The continued assessment of the NEXT ion thruster indicates that the first failure mode across the throttling range is expected to be in 

excess of 36,000 hr of operation from charge exchange induced groove erosion. It is at this duration that the groove is predicted to penetrate 

the accelerator grid possibly resulting in structural failure. Based on these lifetime and mission assessments, a throttling approach is 

presented for the Long Duration Test to demonstrate NEXT thruster lifetime and validate modeling. 

15. SUBJECT TERMS

Ion thruster; Ion optics; Hollow cathodes; Electric propulsion; Electrostatic propulsion 

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