NASA/TM--2001-211276 IEPC-01-081

Improving the Total Impulse Capability of

the NSTAR Ion Thruster With

Thick-Accelerator-Grid Ion Optics

George C. Soulas

Glenn Research Center, Cleveland, Ohio

1

0

December 2001

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NASA/TM--2001-211276 IEPC-01-081

Improving the Total Impulse Capability of

the NSTAR Ion Thruster With

Thick-Accelerator-Grid Ion Optics

George C. Soulas

Glenn Research Center, Cleveland, Ohio

Prepared for the

27th International Electric Propulsion Conference

cosponsored by the AFRL, CNES, ERPS, GRC, JRL, MSFC, and NASA

Pasadena, California, October 14-19, 2001

National Aeronautics and

Space Administration

Glenn Research Center

December 2001
This report contains preliminary

findings, subject to revision as

analysis proceeds.

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Available fi'om

National Technical Information Service

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Available electronically at http://gltrs.grc.nasa.gov/GLTRS
Improving the Total Impulse Capability of the NSTAR Ion Thruster

With Thick-Accelerator-Grid Ion Optics

George C. Soulas

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Phone: 216-977-7419

Email: Geo_llge,.(_.,.S._?.t_.!aA.@.g!q, nasa, o_ov

The results of performance tests with thick-accelerator-grid (TAG) ion optics are presented.

TAG ion optics utilize a 50% thicker accelerator grid to double ion optics' service life.

NSTAR ion optics were also tested to provide a baseline performance for comparison.

Impingement-limited total voltages for the TAG ion optics were only 0 to 15 V higher than

those of the NSTAR ion optics. Electron backstreaming limits for the TAG ion optics were

3 to 9 V higher than those for the NSTAR optics due to the increased accelerator grid

thickness for the TAG ion optics. Screen grid ion transparencies for the TAG ion optics

were only about 2% lower than those for the NSTAR optics, reflecting the lower physical

screen grid open area fraction of the TAG ion optics. Accelerator currents for the TAG ion

optics were 19 to 43% greater than those for the NSTAR ion optics due, in part, to a sudden

increase in accelerator current during TAG ion optics' performance tests for unknown

reasons and to the lower-than-nominal accelerator aperture diameters. Beam divergence

half-angles that enclosed 95% of the total beam current and beam divergence thrust

correction factors for the TAG ion optics were within 2° and 1%, respectively, of those for

the NSTAR ion optics.

Introduction

The success of the NSTAR (i.e., NASA Solar Electric

Propulsion Technology Applications Readiness

Program) 30 cm ion thruster system on the Deep Space

1 mission has demonstrated the viability of ion

• • 12 propulsion for deep space rmsslons. ' As a result, ion

propulsion is a candidate for several deep space

missions, such as the Neptune Orbiter, Titan Explorer,

Mars Sample Return, Europa Lander, and others.

However, most of these missions require increasing

the NSTAR thruster's propellant throughput and peak

input power capabilities beyond the demonstrated

88 kg of xenon at 2.3 kW and approximately 140 kg at

various power levels. 35

The total impulse capability of the NSTAR ion

thruster is limited, in part, by sputter erosion of the ion

optics. Brophy, Polk, and Rawlin identified five

credible failure modes related to sputter erosion of the

NSTAR ion optics that must be addressed to increase

optics longevity. 6 These failure modes included:

1. accelerator grid structural failure from chargeexchange ion sputter erosion of the downstream

accelerator surface;

2. electron backstreaming due to enlargement of the

accelerator grid apertures from charge-exchange

ion sputter erosion;

3. a grid electrical short that cannot be eliminated by

the grid clearing circuit;

4. screen grid sputter erosion by discharge chamber

ions; and

5. accelerator grid structural failure due to direct ion

impingement from defocused beamlets as a result

of foreign material on a screen grid aperture.

The foreign material of failure mode 5 is generally

presumed to originate either from the discharge

chamber walls or cathode assembly as sputterdeposited material that had spalled or debris from a

foreign source. 6 As a result, the discharge chamber

flake containment design of the source of debris, and

not the ion optics, controls failure mode 5, Although

the downstream side of the NSTAR screen grid is

NASA/TM--2001-211276 I
sputter-depositedwithacceleratorgridmaterialduring

operation,spallingof thismaterialwouldbeexpected

to contributeto failuremode3 (i.e.,a grid electrical

short)andnot failuremode5. Failuremode5 will,

therefore,notbeconsideredhere.

TestresultsfromRef.[3]alsoshowedthatscreengrid

sputtererosion(i.e., failure mode4) wasminimal,

likely dueto thederatedoperatingconditionsof the

NSTARthruster.Althoughscreengrid erosionmust

beaddressedinanyionoptics'servicelife assessment,

lifetimeswellbeyondthosedemonstratedin Refs.[3]

to [5] areexpectedas long as theselow discharge

voltagesandplasmadensitiesaremaintainedandthe

ratio of doubly-to-singlycharged ions is not

significantlyincreased.

Long durationtests in the NSTAR program3-5 and

others 6'v have demonstrated, however, that sputter

erosion of the downstream surface and aperture wails

of the accelerator grid (i.e., failure modes 1 and 2)

must be addressed to increase ion optics' longevity.

While accelerator aperture erosion would be expected

to aggravate the issue of an unclearable electrical short

(i.e., failure mode 3), grid shorting from accelerator

grid erosion products was not found to be an issue

during any long duration test of the NSTAR program•

A development effort to extend ion optics' service life

was initiated at the NASA Glenn Research Center

(GRC). 8 Several methods of extending ion optics'

service life are presently being pursued. One method

involves changing the ion optics' material to one with

a lower volumetric sputter erosion rate than

molybdenum. Grid material technologies presently

investigated include carbon-based materials, titanium,

and ion-implanted materials, s-t1

Another method for increasing ion optics service life

involves changing the ion optics' design and not the

material. This prevents the loss of the large database

already available with molybdenum ion optics and

eliminates all fabrication development costs since the

fabrication technique is unchanged, The ion optics'

design change was to merely thicken the accelerator

grid. Service life is enhanced primarily because there

is more material available for sputter erosion.

Additional benefits to thickening the accelerator grid

include increased electron backstreaming limits and

reduced neutral loss rates.

A service life assessment was conducted with thicker

accelerator grids and compared to that of the NSTAR

accelerator grid. 9 Service life analyses included, in

part, sputter erosion of the downstream surface and

aperture walls of the accelerator grid, shown in Fig. 1,

and sputter erosion of the upstream surface of the

screen grid. Failure criteria included accelerator grid

structural failure from complete bridge erosion

through the grid thickness, the onset of electron

backstreaming from accelerator aperture enlargement,

and screen grid structural failure from erosion through

50% of the _id thickness. Failure due to an

unclearable grid electrical short was not assessed. The

accelerator grid thickness for the thick-accelerator-grid

(TAG) ion optics was increased 50% over the NSTAR

accelerator grid thickness. This thickness was selected

because the grid manufacturer had experience etching

this thickness and there was prior performance test

experience with this accelerator grid thickness. I-'l

These analyses were conservative in that the

accelerator voltage setting did not exploit the higher

initial electron backstreaming limits of the TAG ion

optics• Accelerator currents were, however, assumed

to be the same as those of NSTAR ion optics. Results

showed that TAG ion optics could more than double

the NSTAR thruster total impulse capability at thruster

input powers of 2.3 kW (i.e., the peak NSTAR input

power), 3.0 kW, and 4.6 kW. The dominant failure

mode was the onset of electron backstreaming from

accelerator apertul'e enlargement at the grid center.

The following paper reports on the results of

performance tests with TAG ion Optics. These

performance tests were conducted over a thruster input

power range of 0.5 to 3.0 kW and are compared to

those of NSTAR ion optics using the same thruster.

Because _pri0r tests with =_ew titanium ....and

molybdenum ion optics had exhibited rapid initial

changes in ion optics performance parameters, termed

• • • 4 I0.11

mn optics burn-m, the TAG ion optics were

initially operated until ion optics performance

parameters achieved steady state values. Afterwards,

ion optics' performance was determined. Ion optics

performance parameters that included impingementlimited perveances, electron backstreaming limits,

screen grid ion transparencies, accelerator currents,

near-field beam current density profiles, beam

divergence half-angles, and thrust correction factors

due to beam divergence were determined throughout

the thruster input power range.

NASA/TM--2001-211276 2
Test Hardware and Operating Procedures

TAG Ion Optics

All nominal dimensions for the TAG ion optics were

identical to the NSTAR design except for the

accelerator grid thickness. The thickness for the TAG

ion optics was increased 50% over the NSTAR

thruster accelerator grid thickness. The nominal grid

cold gap was set to the NSTAR dimension since prior

testing had shown that this accelerator grid thickness

had little effect on perveance. L_ Other electrode

nominal dimensions were identical to those of the

NSTAR thruster so that the NSTAR thruster ion optics

mounting assembly design would require minimal

changes, t .15

Two TAG ion optics sets were successfully

manufactured. One TAG ion optics set was assembled

for the tests reported here with hardware utilizing the

NSTAR thruster design, t4 Screen and accelerator grid

aperture diameter variations were within -1%/-5%

and +0%/-22%, respectively, of the nominal NSTAR

thruster design throughout the active area. t4't5 Screen

and accelerator grid aperture diameters were smallest

at the active area perimeter. The resulting screen and

accelerator grid open area fractions at the active area

perimeter were estimated to be about 9% and 30%

lower-than-nominal, respectively. Grid cold gap

variations throughout the active area were within

+0%/-8% of the nominal NSTAR design. The centerto-center alignment of the screen and accelerator

apertures was measured in the radial direction and

found to be misaligned by as much as 10% of the

nominal accelerator aperture diameter for the outer

perimeter apertures. This misalignment was due to the

increased thickness of the accelerator grid and was

anticipated to cause the ion beam to be slightly more

divergent.

NSTAR Ion Optics

NSTAR ion optics fabricated at NASA Glenn were

also tested to provide a baseline performance for

comparison. Screen and accelerator grid aperture

diameter variations were within +0%/-3% and __.4%,

respectively, of the nominal NSTAR thruster design

throughout the active area. t4't5 Accelerator grid

aperture diameters were larger-than-nominal at the

active area center while screen grid aperture diameters

were lower-than-nominal at the active area perimeter.

The resulting screen grid open area in the active area

perimeter fraction was estimated to be only 3 to 4%

lower-than-nominal. Grid cold gap variations

throughout the active area were within _+8% of the

nominal NSTAR thruster design.

Ion Thruster

Both ion optics sets were mounted onto a 30 cm ion

thruster. This thruster serves as a test bed for 30 cm

thruster development at NASA. 8 The mechanical

designs of the thruster discharge chamber and ion

optics were nearly identical to those of the NSTAR

thruster, described in detail in Refs. [14] and [i5]. The

thruster was fitted with thermocouples for thermal

tests and the exterior was modified so that a second

neutralizer could be installed.

Power Console and Gas Feed System

A power console similar to that described in Ref. [16]

powered the thruster. This power console was

modified to allow the thruster to be throttled up to

5 kW. A high purity gas feed system was used to

provide xenon to the discharge cathode, discharge

chamber, and neutralizer through separate mass flow

controllers.

Diagnostics

During thruster operation, voltages and currents were

measured with digital multimeters and xenon flows

with mass flow meters. These measured parameters

were used to set thruster operating conditions, as well

as to determine thruster performance.

The thruster was connected to an electrically floating

power supply circuit used to determine the screen grid

transparency to discharge chamber ions. The circuit

electrically tied the screen grid to the discharge

cathode during normal operation, but biased the grid

negative relative to discharge cathode potential to

repel electrons and measure the collected ion

current, tO,l7

Beam current density profiles were measured with a

probe mounted onto a two-axis probe motion system.

9

The probe was a planar geometry with a 1.0 cmcircular current-collecting area) 7 The probe was

biased negative with respect to beam plasma potential

to repel electrons and was grounded through a resistor

that acted as a shunt to measure collected currents.

The positioning system swept the probe in the radial

and axial directions through the vertical center of the

thruster ion optics. The positioning system had a

NASA/TM--2001-211276 3
1.25m maximumtravelin eachaxis,whichenabled

near-fieldradialbeamcurrentdensitymeasurements at

different axial locations,as measuredfrom the

geometricenterof theion optics.Thecurrentdensity

measurementswere then used to determinebeam

currentdensityprofiles,beamdivergencehalf-angles,

andthrustcorrectionfactors._°

Vacuum Facility

Testing was conducted in Vacuum Facility II at

NASA Glenn. This 2.2 m diameter by 7.9 m long

facility is evacuated with seven cryogenic pumps and a

turbomolecular pump. The total measured facility

pumping speed was greater than 100,000 l/s with

xenon. The facility base pressure was typically less

than 2.6×10 -5 Pa and background pressures were as

high as 5.2×10 4 Pa at the peak thruster input power of

3.0 kW.

Ion Optics Performance Parameters

Ion optics performance parameters were determined

throughout testing. These parameters included

impingement-limited total voltages, electron

backstreaming limits, screen grid ion transparencies,

accelerator grid currents, beam divergence, and beam

divergence thrust losses.

Impingement-limited total voltages were determined

from plots of accelerator current as a function of total

voltage where the slope was -0.02 mAN. Total

voltage is the sum of the absolute values of the beam

and accelerator power supply voltages. Perveance

margins were defined as the difference between the

total voltage during normal operation and the

impingement-limited total voltage. Uncertainties in

impingement-limited total voltage determinations

(and, therefore, perveance margins) were within

_+t0V.

The electron backstreaming limit was determined by

lowering the magnitude of the accelerator grid voltage

until the indicated beam current increased by 0.1 mA

due to backstreaming electrons. Uncertainties in

electron backstreaming limit measurements were

estimated to be within _1 V.

Screen grid ion transparencies were determined by

biasing the screen grid 20 V below discharge cathode

potential to repel electrons and to measure the

collected ion current. The method used to determine

screen grid ion transparency from these measurements

is discussed in detail in Ref. [10]. Uncertainties in

screen grid ion transparency measurements were

estimated to be within __.0.002.

Radial beam current density profiles were used to

determine beam divergence, and to provide peak

current densities for comparisons of electron

backstreaming limits/° Regarding beam current

density measurements, no attempt was made to repel

charge-exchange ions from the probe or to account for

secondary electron emission due to ion bombardment.

Integration of the radial beam current density profiles

(assuming azimuthal symmetry) yielded beam currents

that were higher than the measured beam current by as

much as 15%. It is anticipated that these errors were

caused by a combination of effects, which included the

large probe surface area, charge-exchange ions in the

beam, secondary electron emission from both

singly- and doubly-charged ions, and a slightly

asymmetric beam.

Radial beam current density profiles were taken at five

axial locations to determine beam divergence halfangles and thrust losses due to beam divergence. The

methods used to determine divergence half-angles and

beam divergence thrust losses are discussed in detail in

Ref. [10]. Uncertainties in the beam divergence halfangles and thrust losses due to beam divergence

cannot presently be assessed due to the unknown

sources of error in the beam current density

measure ments.

Operating Procedures ......

TAG and NSTAR ion optics were tested on the 30 cm

ion thruster at the thruster input power leveI s and

corresponding operating parameters listed in Table 1.

These power levels included NSTAR operating points

that encompassed the full 0.5 to 2.3 kW throttling

range, 18 and an operating point with a higher beam

voltage for operation up to 3.0 kW. During thruster

operation, main and discharge cathode flows were

maintained at fixed values while the discharge current

was adjusted to maintain a constant beam current.

During each test, the ion optics were initially operated

for an extended duration to allow ion optics

performance parameters to achieve steady state values.

The thruster was operated primarily at TH15-

(see Table 1) while monitoring changes in

impingement-limited total voltages, electron

backstreaming limits, screen grid ion transparencies,

NASA/TMm2001-211276 4
andradialbeamcurrentdensityprofiles.Thethruster

wasnot operatedcontinuously,but wasintentionally

interruptedon severaloccasions.When theseion

opticsperformanceparametershad achievedsteady

statevalues,the thrusterwas step-rampedthrough

operatingpointslistedin Table 1 to characterize ion

optics' performance. At each operating point, ion

optics performance parameters such as impingementlimited total voltages, electron backstreaming limits,

screen grid ion transparencies, and beam current

density profiles, as well as other thruster performance

parameters, were determined. Upon completion of

each ion optics' performance data sets, the ion optics

were re-characterized at TH15 to confirm that ion

optics' performance had not changed.

Results and Discussions

Ion Optics Burn-in

The results of ion optics burn-in at THI5 for both ion

optics' designs are plotted in Fig. 2. Performance

parameters were plotted as a function of propellant

throughput and not accumulated operation because the

ion optics were operated at several different power

levels during this bum-in period. Monitored ion optics

performance parameters included perveance margin,

electron backstreaming limit, screen grid ion

transparency, and beam current density profiles.

TAG Ion Optics

Initial performance tests with TAG ion optics revealed

that ion optics performance parameters were indeed

changing rapidly with accumulated operation. The

thruster was, therefore, operated until these

performance parameters achieved steady state values.

The thruster was operated at THI5 throughout most of

the bum-in period. The thruster was not operated

continuously, but was restarted 16 times throughout

the bum-in period, and was vented to atmosphere at a

propellant throughput of about 240 gm.

All monitored the ion optics performance parameters

of Fig. 2 achieved steady state values after processing

about 570 gm of xenon. Perveance margins and screen

grid ion transparencies increased by about 80 V and

2%, respectively, while the electron backstreaming

limit decreased by about 9 V. During this burn-in

period, the peak beam current density 49 mm

downstream of the optics changed less than about

0.2 mA/cm 2. This change is only about 3% of the total

peak beam current density, and is, therefore,

considered too small to appreciably affect these

performance parameters.

A post-test examination of the TAG ion optics

(discussed in a later section) revealed that the

accelerator grid aperture diameters had increased by

about 4% throughout the perforated diameter and the

cold grid gap had also decreased by about 8% at the

geometric center of the active area. Using changes in

the cold grid gap as an indicator of hot grid gap

behavior, it is speculated that the increases in

accelerator aperture diameters and grid gap caused the

increases in perveance margins and the decreases in

electron backstreaming limits because both

performance parameters are directly affected by these

geometric changes. L__9Geometric changes at the ion

optics' center are speculated to have contributed

significantly to these performance changes because the

beam current density is peaked there and, therefore,

beamlet currents are largest there. It is further

speculated that the cold grid gap decrease at the ion

optics' geometric center may have been responsible

for the slight increase in screen grid ion transparency.

This is because a smaller grid gap is known to reduce

discharge losses by improving screen grid ion

transparency.12 TAG ion optics' performance

characterization results presented in the remainder of

the paper were measured following ion optics burn-in

unless otherwise noted.

NSTAR Ion Optics

The results for NSTAR ion optics shown in Fig. 2, and

discussed in Ref. [11], showed no significant changes

in any of the monitored performance parameters. If a

bum-in had occurred for this ion optics set, it would

have to have occurred prior to a propellant throughput

of 230 gin. Ion optics' performance characterizations

were initiated at 570 gm of propellant throughput.

Following performance characterizations, Fig. 2 shows

that although perveance margins and electron

backstreaming limit remained unchanged, the screen

grid ion transparency had decreased by 0.8% for

unknown reasons.

Burn-in has been noted with molybdenum ion optics

during an NSTAR thruster long duration test. 4 Rapid

decreases in perveance margins, screen grid ion

transparencies, and electron backstreaming limit

magnitudes were noted within the first 150 hours, or

1.3 kg of xenon, of life testing. These changes are

consistent with an increasing grid hot gap.

NASA/TM--2001-211276 5
Impingement-LimitedTotal Voltage

Beam current as a function of impingement-limited

total voltage is pIotted in Fig. 3 for the TAG and

NSTAR ion optics. Perveance margins for the TAG

ion optics are listed in Table 2 with results from

NSTAR ion optics for comparison.

As Fig. 3 and Table 2 demonstrate, impingementlimited total voltages for the TAG ion optics were only

0 to 15 V higher than those of the NSTAR ion optics.

This close agreement is not surprising, however, since

prior studies had shown that the thicker accelerator

grid of the TAG ion optics design has relatively little

effect on the impingement-limited total voltage) z2°

Although the accelerator aperture diameters varied

significantly for the TAG ion optics, they were similar

to the NSTAR ion optics at the active area center,

where beam current densities and, therefore, beam

currents per hole were highest.

Electron Backstreaming Limit

Electron backstreaming limit voltages for the TAG ion

optics are listed in Table 2 with results with NSTAR

ion optics for comparison. Since the electron

backstreaming limit is also a function of the peak

beam current density, t° the electron backstreaming

limit is plotted as a function of the peak beam current

density in Fig. 4 for a more appropriate comparison.

The data for each grid set are presented at separate

beam voltages because the electron backstreaming

limit is also a function of beam voltage) 9

Table 9 shows that electron backstreaming limits for

the TAG ion optics were 3 to 9 V higher than those of

the NSTAR optics. Fig. 4 also shows that the electron

backstreaming limit as a function of peak beam current

density for the TAG ion optics was about 5 to 11 V

higher than that of the NSTAR ion optics at a beam

power supply voltage of 1100 V. The higher electron

backstreaming limits for TAG ion optics were not only

expected due to the increased accelerator grid

thickness, 19 but had also been demonstrated for this

accelerator grid thickness using a mercury ion

thruster, t3

Screen Grid Ion Transparency

Table 3 lists screen grid ion transparencies for TAG ion

optics with results from NSTAR ion optics for

comparison. Screen grid ion transparencies for the TAG

ion optics were only about 2% lower than those for the

NSTAR optics, reflecting the lower physical screen grid

open area fraction of the TAG optics from smaller

screen grid aperture diameters. It is noteworthy that

although screen grid ion transparencies for the TAG ion

optics were lower than those for the NSTAR ion optics,

dischmge losses were still 0 to 2% lower than those for

the NSTAR ion optics, as shown in Table 3. It is

speculated that this was largely due to reduced neutral

loss rates for the TAG ion optics due to the lower

accelerator grid open area fraction, especially at the

active area outer perimeter.

Accelerator Current

Table 3 lists accelerator currents for TAG ion optics

with results from NSTAR ion optics for comparison.

Accelerator currents for the TAG ion optics were 19 to

43% greater than those for the NSTAR ion optics. This

was due, in part, to a sudden increase in accelerator

current during TAG ion optics' performance tests. This

is demonstrated in Fig. 5 where accelerator current in

plotted as a function of propellant throughput during

burn-in and performance testing at TH15. Here, the

accelerator current increased from 7.5 to 8.7 mA to just

over 10 mA with no change in facility pressure (also

shown in Fig. 5). The cause of this sudden change is

presently unknown, however, past NSTAR thruster life

tests have exhibited initial accelerator current increases

followed by a decrease to steady state values within the

first 16 kg of xenon throughput.4 ..........................

Prior to this sudden change, however, accelerator

currents were still higher thanthose for the NSTAR

ion optics (shown in Fig. 5 prior to performance

characterizations). It is speculated that these

differences were largely due to the lower-than-nominal

accelerator aperture diameters.

A one-to-one comparison of accelerator currents for

TAG ion optics versus NSTAR ion optics is critical for

properly assessing the total impulse improvement of

TAG ion optics. This is because the accelerator grid

erosion rate is directly proportional to the

impingement current. 9 TAG ion optics service life

assessments to date have assumed accelerator currents

equal to those of the NSTAR ion optics. The varying

accelerator currents of this TAG ion optics set have

made one-to-one comparisons difficult. Proper

comparisons of accelerator currents may require long

duration testing since prior tests have demonstrated

that up to 1500 hours of wear testing may be required

for NSTAR ion o_tics accelerator currents to reach

steady state valuesJ '4

NASA/TM--2001-211276 6
Beam Current Density Profiles, Beam Divergence,

and Thrust Loss

Peak beam current densities were determined from

radial beam current density profiles taken 48 to 49 mm

downstream of the grid center. Sample radial beam

current density distributions at THe l9 and TH0 are

shown in Fig. 6 for both ion optics sets. Radial beam

current density distributions at several axial locations

are shown in Fig. 7 for TAG ion optics. Table 3 lists

peak beam current densities for the TAG ion optics

with results from NSTAR ion optics for comparison.

As Table 3 shows, peak beam current densities for the

TAG ion optics were up to 5% higher than those for

NSTAR ion optics. It is speculated that the larger peak

beam current densities for the TAG ion optics was due

to the smaller screen grid aperture diameters at the

perimeter of the active area. This reduced the physical

open area fraction in this region, requiring a higher

discharge plasma density to provide the required beam

current. This caused more ion current to be extracted

through the center of the active area.

Figures 6 and 7 also show that beam current density

profiles were slightly non-axisymmetric near the

active area center for both ion optics' designs. This

slight asymmetry occurred at all power levels tested

and is an artifact of the thruster discharge chamber

plasma. _o

Beam divergence angles that enclosed 95% of the total

beam current are listed in Table 4 for TAG ion optics

with results from NSTAR ion optics for comparison.

Divergence half-angle data for the TAG ion optics

were within 2° of those for the NSTAR ion optics.

Sample plots of percentage of total beam current that

enclosed a given divergence half-angle are shown in

Fig. 8 for TAG and NSTAR ion optics. Beam

divergence half-angles were similar for both ion

optics' designs.

Table 4 lists these thrust correction factors due to

beam divergence for TAG ion optics with results from

NSTAR ion optics for comparison. All values for

titanium ion optics were within 1% of those with

molybdenum ion optics. This close agreement

demonstrates that thrust losses from beam divergence

are similar for TAG and NSTAR ion optics.

Post-test TAG Ion Optics Examination

Following performance tests, the TAG ion optics were

removed and re-inspected. Measured post-test

dimensions included accelerator grid aperture

diameters and the cold grid gap. Accelerator aperture

diameters increased by about 4% throughout the active

area, likely due to sputter erosion, The increase in

aperture diameter is initially rapid because the aperture

wall cusp height can be sputter eroded quickly. The

aperture wall cusps are an artifact of the grid

fabrication technique. The cold grid gap decreased by

about 8% at the geometric center of the ion optics,

however, this change was within the accuracy of the

measurement.

Pre- and post-test photomicrographs of screen and

accelerator apertures were compared. Notches in the

accelerator aperture wall cusps were found at and near

the apertures of the active area outer perimeter.

Pre- and post-test examples are shown in Fig. 9. These

notches occurred in apertures as far as 17 mm from the

outer-radius and occurred throughout the

circumference. The notches are speculated to have

been primarily due to the 22% smaller-than-nominal

accelerator aperture diameters in this region.

Notching likely occurred during perveance

measurements, where reducing the total voltage

between the grids defocused ion beamlets, which then

impinged directly upon the accelerator aperture wall

cusps. The notched appearance is likely due to noncircular beamlets, which are caused by interactions

with adjacent beamlets. Beamlet distortion

(i.e., non-circularity) would be expected to increase

towards the smaller beamlet currents of the outerperimeter apertures because beamlet distortion

increases with decreasing normalized perveance per

hole. 2_ Notches only appeared on the portions of the

aperture wall cusps that were radially opposite to the

geometric center of the grid due to the slight

misalignment of the screen and accelerator apertures.

Conclusions

The results of performance tests with TAG ion optics

were presented. TAG ion optics utilize a 50% thicker

accelerator grid to double ion optics' service life.

NSTAR ion optics were also tested to provide a

baseline performance for comparison. Both sets were

initially operated until ion optics performance

parameters achieved steady state values. Afterwards,

performance characterizations were conducted. All

tests were conducted on the same 30 cm ion thruster

NASA/TM--2001-211276 7
thatwasnearlyidenticalin mechanicaldesignto that

of theNSTARthruster.

All performance parameters for the TAG ion optics

achieved steady state values after processing 570 gm

of propellant. During this ion optics burn-in, perveance

margins and screen grid transparencies increased while

electron backstreaming limits decreased. Accelerator

aperture diameter increases and cold grid gap

decreases were speculated to have caused these

changes.

Impingement-limited total voltages for the TAG ion

optics were only 0 to 15 V higher than those of the

NSTAR ion optics. This close agreement is not

surprising, however, since prior studies had shown that

the thicker accelerator grid of the TAG ion optics

design has relatively little effect on the impingementlimited total voltage,

Electron backstreaming limits for the TAG ion optics

were 3 to 9 V higher than those for the NSTAR optics.

The higher electron backstreaming limits for the TAG

ion optics were expected due to the increased

accelerator grid thickness.

Screen grid ion transparencies for the TAG ion optics

were only about 2% lower than those for the NSTAR

optics, reflecting the lower physical screen grid open

area fraction of the TAG ion optics from smaller

screen grid aperture diameters.

Accelerator currents for the TAG ion optics were 19 to

43% greater than those for the NSTAR ion optics. This

was due, in part, to a sudden increase in accelerator

current during TAG ion optics' performance tests. The

cause of this sudden change is presently unknown,

however, past NSTAR thruster life tests have

exhibited initial accelerator current increases followed

by a decrease to steady state values. Prior to this

sudden change, however, accelerator currents were

still higher than those for the NSTAR ion optics. It is

speculated that these differences were largely due to

the lower-than-nominal accelerator aperture diameters.

Beam divergence half-angles that enclosed 95% of the

total beam current for the TAG ion optics were within

2° of those for the NSTAR ion optics. All beam

divergence thrust correction f_ctors for the TAG ion

optics were within 1% of those with NSTAR ion optics.

A post-test examination of the TAG ion optics

revealed that accelerator aperture diameters had

increased by about 4% throughout the active area and

the cold grid gap had decreased by about 8% at the

geometric center of the ion optics. Photomicrographs

of TAG accelerator grid apertures showed notching of

the accelerator aperture walI cusps located at and near

the outer-radius apertures. It is speculated that this

notching was created during perveance measurements

and was the result of the smaller-than-nominal

aperture diameters in this region and the slight undercompensation of the aperture patterns.

References

[1] Rawlin, V.K., et al. "An Ion Propulsion System

for NASA's Deep Space Missions," AIAA Paper

99-4612, September 1999.

[2] Polk, J.E., et al., "In-Flight Performance of the

NSTAR Ion Propulsion System on the Deep

Space One Mission," IEEE Aerospace

Conference Paper 8.0304, March 2000.

[3] Polk, J.E., et al., "An Overview of the Results

from an 8200 Hour Wear Test of the NSTAR Ion

Thruster," AIAA Paper 99-2446, June 1999.

[4] Anderson, J.R., et al., "Performance

Characteristics of the NSTAR Ion Thruster

During an On-going Long Duration Ground

Test," IEEE Aerospace Conference Paper 8.0303,

March 2000.

[5] Personal Communication, Anderson, J.R., Jet

Propulsion Laboratory, Pasadena, CA, May 2001.

[6] Brophy, J.R., Polk, J.E., and Rawlin, V.K., "Ion

Engine Service Life Validation by Analysis and

Testing," AIAA Paper 96-2715, July 1996.

[7] Rawlin, V.K., "Erosion Characteristics of TwoGrid Ion Accelerating Systems," IEPC Paper 93-

175, September 1993.

[8] Patterson, M.J., et al., "Ion Propulsion

Development Activities at NASA Glenn

Research Center," AIAA Paper 2000-3810, July

2000.

[9] Soulas, G.C., "Grid Technology Analyses for

CNSR," Presentation at NASA Glenn Research

Center, January 2000.

[10] Soulas, G.C., Foster, J.E., and Patterson, M.J.

"Performance of Titanium Optics on a NASA 30

cm Ion Thruster," AIAA Paper 00-3814, July

2000.

NASA/TM--2001-211276 8
[ 11 ] Soulas, G.C., "Performance Evaluation of

Titanium Ion Optics for the NASA 30 cm Ion

Thruster," IEPC Paper 01-92, October 2001.

[12] Rawlin, V.K., "Performance of 30-cm Ion

Thrusters with Dished Accelerator Grids," AIAA

Paper 73-1053, October-November 1973.

[13] Rawlin, V.K., "Studies of Dished Accelerator

Grids for 30-cm Ion Thrusters," AIAA Paper 73-

1086, October-November 1973.

[14] Christensen, J.A., et al., "Design and Fabrication

of a Flight Model 2.3 kW Ion Thruster for the

Deep Space 1 Mission," AIAA Paper 98-3327,

July 1998.

[15] Sovey, J.S., et al., "Development of an Ion

Thruster and Power Processor for New

Millennium's Deep Space 1 Mission," AIAA

Paper 97-2778, December 1997.

[16] Pinero, L.R., Patterson, M.J., and Satterwhite,

V.E., "Power Console Development for NASA's

Electric Propulsion Outreach Program," IEPC

Paper 93-250, September 1993.

[17] Foster, J.E., Soulas, G.C., and Patterson, M.J.,

"Plume and Discharge Plasma Measurements of a

5 kW NSTAR-Derivative Ion Thruster," AIAA

Paper 2000-3812, July 2000.

[18] Rawlin, V.K., et al., "NSTAR Flight Thruster

Qualification Testing," AIAA Paper 98-3936,

July 1998.

[ 19] Kaufman, H.R., "Technology of Electron

Bombardment Ion Thrusters," Advances in

Electronics and Electron Physics, vol. 36,

Academic Press, Inc., New York, 1974, pp. 265-

373.

[20] Rovang, D.C. and Wilbur, P.J., "Ion Extraction

Capabilities of Two-Grid Accelerator Systems,"

IEPC Paper 84-86, July 1984.

[21] Hayakawa, Y. and Kitamura, S., "Ion Beamlet

Divergence Characteristics of Two-Grid

Multiple-Hole Ion Accelerator Systems," AIAA

Paper 97-33195, July 1997.

Table I. Nominal thruster operating parameters.

Power Input Beam Beam Accelerator Neutralizer Main Discharge Neutralizer

Level P°wer'a Current'b V°ltage'b Voltage, V Keeper Flow, Cathode Flow,

Designation kW A V Current, A sccm Flow, sccm sccm

TH0 c 0.5 0.51 650 - 150 2.0 5.98 2.47 2.40

TH4 c 1.0 0.71 1100 -150 2.0 8.30 2.47 2.40

TH8 _ 1.4 1.10 1100 -180 1.5 14.4 2.47 2.40

TH10 _ 1.7 1.30 1100 - 180 1.5 17.2 2.56 2.49

THI 2_ 1.8 1.49 1100 - 180 1.5 18.5 2.72 2.65

TH 15_ 2.3 1.76 1100 - 180 1.5 23.4 3.70 3.60

THe 19 3.0 1.76 1500 -250 1.5 23.1 3.54 3.60

"Nominal values.

bPower supply current or vohage,

¢Nominal NSTAR operating condition,

Table 2. Perveance margins and electron backstreaming limits

for TAG and NSTAR ion optics.

Power Level

Designation

TH0 a

TH4 a

TH8 a

THI0 a

TH 12_

THI5 a

THeI9

Perveance Margin, V

TAG

175

535

450

405

350

285

755

NSTAR

175 -65

540 -121

455 -132

405 -136

365 -140

295 -147

760 -194

Electron Backstreaming Limit, V

TAG NSTAR

-69

-126

-138

-143

-148

-156

-197

'Nominal NSTAR operating condition.

NASA/TM--2001-211276 9
Table 3. Peak beam current densities, screen grid ion transparencies, discharge losses,

and accelerator currents for TAG and NSTAR ion optics.

Power Level

Designation

TH0 b

TH4 b

TH8 b

THI0 b

THI2 b

TH 15b

THe 19

Peak Beam Current

Densities, a mA/cmTAG NSTAR

2.4 2.4

3.3 3.2

4.5 4.3

5.0 4.8

5.5 5.3

6.1 5.8

6.3 6.0

Screen Grid Ion

Transparencies

TAG NSTAR

0.820 0.836

0.868 0.882

0.872 0.877

0.864 0.870

0.856 0.863

0.839 0.849

0.892 0.895

Discharge

Losses, b W/A

TAG NSTAR

Accelerator

Currents, mA

TAG NSTAR

250 250

240 240

200 200

190 191

185 187

184 188

176 178

2.0 1.4

2.2 1.8

5.0 3.5

6.3 4.5

7.8 5.5

10.1 7.2

8.1 6.8

"Peak beam current densities at 48-49 man downstream of the geometric center of the ion optics; peak beam current densities occurred at

approximately the radial center of the grid active area.

_'ominal NSTAR operating condition.

Table 4. Divergence half-angles that enclosed 95% of total beam current and

beam diver[[ence thrust correction factors for TAG and NSTAR ion optics.

Divergence Half-angle at 95% Thrust Correction Factor for

Power Level

Designation

TH0 b

TH4 b

TH8 b

TH 10b

THI2 b

THI5 b

THe 19

of Beam Current, degrees

TAG a NSTAR

28 29

29

29 29

28

27

25 26

31 29

Beam Divergence

TAG _ NSTAR

0.97 0.97

0.97

0.97 0.97

0.97

0.98

0.98 0.98

0.97 0.97

"Determined with data taken prior to ion optics burn-in.

bNomlnal NSTAR operating condition.

\

New Grid

Aperture

Enlargement

Eroded Grid

Fig. 1. Typical accelerator grid erosion sites.

NASA/TM--2001-211276 10
>

o_

¢.)

qO

310

290

270

250

230

210

190

[ O NSTAR Ion Optics

-- "I" ill ITAG Ion Optics ....

ii a" I t

........... 1 1

I

Performance

Characterizations

0 1_ 2_ 3_ 4_ 5_ 6_ 7_ 8_ 9_

Propellant Throughput, gm

a. Perveance margin as a function of accumulated propellant throughput.

-137

> -140

-143

"_ -146

-149

_ -152

-155

-158

/

O NSTAR Ion Optics

/ IITAG Ion Optics

..... i];

r

I

Performance

Characterizations

I

1

I

J

0 1_ 2_ 3_ 4_ 5_ 6_ 7_ 8_ 900

Propellant Throughput, gm

b. Electron backstreaming limit as a function of accumulated propellant throughput.

Fig. 2. Burn-in performance parameters at THI5 for TAG and NSTAR ion optics.

NASA/TM--2001-211276 11
0.855

0.850

'O

0.845

_ 0.840

0.835

"_ 0.830

_, 0.825

r/?,

0.820

0.815

!- __JL[_ IITAG Ion OP tics 1 i ii " I

0 1_ 2_ 3_ 4_ 5_ 6_ 7_ 8_ 9_

Propellant Throughput, gm

c. Screen grid ion transparency as a function of accumulated propellant throughput.

Fig. 2. Burn-in performance parameters at TH15 for TAG and NSTAR ion optics. (Concluded)

0.60

0.40

z

. | .I' ._ [ O NSTAR Ion Optics

__ ?TAGI_ onOptics

i

I

t

600 650 700 750 800 850 900

Impingement-Lirr_ted Total Voltage, V

950 1000

Fig. 3. Beam current as a function of impingement-limited total voltage

for TAG and NSTAR ion optics.

NASA/TM--2001-211276 12
4-a

,.d

7

o

0_

-6O

-80

-IOO

.... O TAG Ion Optics at a 650 VBeam Voltage

• NSTAR Ion Optics at a 650 VBeam Voltage

UITAGIon Optics at a 1100 VBeam Voltage

• NSTAR Ion Optics at a 1100 V Beam "voltage

A TAGIon Optics at a 1500 VBeam Voltage

• NSTAR Ion Optics at a 1500 VBeam Vo]tage

_l , p

i

3.5 4 4.5 5 5.5 6

-120 ......

-140

-160

-180 L

2 2.5 3

-2OO

Peak Beam Current Density, mA/cm 2

Fig. 4. Electron backstreaming limit as a function of peak beam current density for TAG and

NSTAR ion optics. Peak beam current densities were measured 48 mm downstream

of the ion optics' center. Beam voltages here are power supply voltages.

10.5 1.0E-03

10.0

"_ 9.5

g 9.0

8.5

u 8.0

<

7.5

7.0

O0 O0 I

• i

• I

[ I

• Current

o Pressure

I

0 0 OlO 0

i

0 100 200 300 400

do ob o• o

e

500 6OO

l.I

° I

0]

I

Characterizations

700 800 900

Propellant Throughput, gm

Fig. 5. Accelerator current and corrected tank pressure as a function of

propellant throughput for TAG ion optics at THI5.

1.0E-04

O

¢"a

8'

-.]

NASA/'FM--2001-211276 13
el

O

7

6J

5

4

3

2

1

0

___ [ _ TAG Ion Optics at THe19

, NSrARIonOptic...,THe19

_-

[ /" _'_'_,_, ] ......... TAG Ion Optics at TH0

_t i - _ -! .... NSTAR Ion Optics at TH0

i RE

-180 -120 -60 0 60 120 180

Radius, nma

Fig. 6. Radial beam current density profiles for TAG and NSTAR ion optics at THeI9 and TH0.

Beam current densities were measured 48-49 mm downstream of the ion optics' center.

Beam current densities for TAG ion optics were measured prior to ion optics burn-in.

L)

O

7

6

5

4

3

2

t

0

Axi mm:

k

-300 -200 - 100 0 100 200 300

Radius, mm

Fig. 7. Radial beam current density profiles at several axial locations for TAG ion optics at THel9.

Beam current densities for TAG ion optics were measured prior to ion optics burn-in.

NASA/TM_2001-211276 14
100

9O

80

7o

_ 60

m 50

4o

0

_ 30

20

10

0 5 10 15 20 25 30 35 40

Divergence Half-Angle, degrees

Fig. 8. Percentage of total beam current enclosed in a given divergence half-angle for TAG

and NSTAR ion optics at a THel9. Beam current densities for TAG ion optics

were measured prior to ion optics burn-in.

NASA/TM--2001-211276 15
Grid

Center

T

a. Pre-test photomicrograph.

Grid

Center

T

b. Post-test photomicrograph.

Fig. 9. Pre- and post-test photomicrographs of TAG ion optics' screen and

accelerator apertures at the outer-radius of the active area.

NASA/TM--2001-211276 16

REPORT DOCUMENTATION PAGE FormApproved

OMB No. 0704-0188

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

December 2001 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Improving the Total Impulse Capability of the NSTAR Ion

Thruster With Thick-Accelerator-Grid Ion Optics

[6. AUTHOR(S)

George C. Soulas

7, PERFORMING ORGANIZATION NAME(B) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

9. SPONSORING,q_ONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

WU-755-B4-04-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-13075

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--2001-211276

IEPC-01-081

11. SUPPLEMENTARY NOTES

Prepared for the 27th International Electric Propulsion Conference cosponsored by the AFRL, CNES, ERPS, GRC, JRL,

MSFC, and NASA, Pasadena, California, October 14-19, 2001. Responsible person, George C. Soulas, organization code

5430, 216-977-7419.

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Unclassified - Unlimited

Subject Category: 20 Distribution: Nonstandard

Available electronically at htto:l/_ltrs._,rc.nasa._ov/GLTRS

This publication is available from the NASA Center for AeroSpace Information, 301_621_0390.

13. ABSTRACT (Maximum 200 words)

The results of performance tests with thick-accelerator-grid (TAG) ion optics are presented. TAG ion optics utilize a

50 percent thicker accelerator grid to double ion optics' service life. NSTAR ion optics were also tested to provide a

baseline performance for comparison. Impingement-limited total voltages for the TAG ion optics were only 0 to 15 V

higher than those of the NSTAR ion optics. Electron backstreaming limits for the TAG ion optics were 3 to 9 V higher

than those for the NSTAR optics due to the increased accelerator grid thickness for the TAG ion optics. Screen grid ion

transparencies for the TAG ion optics were only about 2 percent lower than those for the NSTAR optics, reflecting the

lower physical screen grid open area fraction of the TAG ion optics, Accelerator currents for the TAG ion optics were

19 to 43 percent greater than those for the NSTAR ion optics due, in part, to a sudden increase in accelerator current

during TAG ion optics' performance tests for unknown reasons and to the lower-than-nominal accelerator aperture

diameters. Beam divergence half-angles that enclosed 95 percent of the total beam current and beam divergence thrust

correction factors for the TAG ion optics were within 2 ° and t percent, respectively, of those for the NSTAR ion

optics.

14. SUBJECT TERMS

Ion thruster; Ion optics; Grids

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION

OF REPORT OF THIS PAGE

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NSN 7540-01-280-5500

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OF ABSTRACT

Unclassified

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