NASA/TM--2001-211220

Performance Evaluation

Optics for the NASA 30

George C. Soulas

Glenn Research Center, Cleveland, Ohio

IEPC-01-092

of Titanium Ion

cm Ion Thruster

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

https://ntrs.nasa.gov/search.jsp?R=20020009015 2018-05-15T10:44:51+00:00Z
This report contains preliminary

findings, subject to revision as

analysis proceeds.

NASA Center for Aerospace Information

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Performance Evaluation of Titanium Ion Optics for

the NASA 30 cm Ion Thruster

George C. Soulas

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Phone: 216-977-7419

Email: _ c_,_2_:_y=C=_ !2_!!_____Lg!__:._t_a_____, g?_

The results of performance tests with titanium ion optics were presented and compared to

those of molybdenum ion optics. Both titanium and molybdenum ion optics were initially

operated until ion optics performance parameters achieved steady state values. Afterwards,

performance characterizations were conducted. This permitted proper performance

comparisons of titanium and molybdenum ion optics. Ion optics' performance was

characterized over a broad thruster input power range of 0.5 to 3.0 kW. All performance

parameters for titanium ion optics of achieved steady state values after processing 1200 gm

of propellant. Molybdenum ion optics exhibited no burn-in. Impingement-limited total

voltages for titanium ion optics were up to 55 V greater than those for molybdenum ion

optics. Comparisons of electron backstreaming limits as a function of peak beam current

density for molybdenum and titanium ion optics demonstrated that titanium ion optics

operated with a higher electron backstreaming limit than molybdenum ion optics for a

given peak beam current density. Screen grid ion transparencies for titanium ion optics

were as much as 3.8% lower than those for molybdenum ion optics. Beam divergence halfangles that enclosed 95% of the total beam current for titanium ion optics were within 1 to

3 ° of those for molybdenum ion optics. All beam divergence thrust correction factors for

titanium ion optics were within 1% of those with molybdenum ion optics.

Introduction

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 propulsion for deep

• • 12

space masslons.' Ion propulsion is, therefore, a

candidate for several deep space missions, such as the

Neptune Orbiter, Titan Explorer, Mars Sample Return,

Europa Lander, and others• However, ion propulsion

system mass and volume could be significantly

reduced for many of these missions by increasing the

NSTAR thruster's propellant throughput and peak

input power capabilities beyond that already

demonstrated by past and ongoing wear tests. 3"4

Propulsion system mass and volume reductions occur

because fewer thrusters and, therefore, fewer

accompanying power processors and propellant feed

system components, would be required•

Increasing propellant throughput and thruster power is

limited, in part, by charge-exchange sputter erosion of

the accelerator grid. 5'6 Significant charge-exchange

accelerator grid sputter erosion can lead to electron

backstreaming due to accelerator aperture

enlargement, grid structural failure due to pit and

groove erosion of the downstream surface, and an

unclearable grid short by a flake from sputter-eroded

accelerator grid material. 6 Utilizing an ion optics

material with a lower volumetric sputter erosion rate

addresses all of the aforementioned failure

mechanisms to extend propellant throughput and

increase thruster power density.

A development effort was initiated at the NASA Glenn

Research Center (GRC) to identify a material with a

lower accelerator grid volumetric sputter erosion rate

than molybdenum (i.e., the present NSTAR thruster

NASA/TM--2001-211220 1
grid material).SuchanactivitycouldutilizeNSTAR

thrustergriddesignandfabricationtechniquesto keep

development costs low. 7"8 Titanium was found to offer

a 45% reduction in volumetric erosion rates and could

be fabricated using the same fabrication techniques as

molybdenum ion optics. Accelerator grid life was

expected to improve by a factor of 1.9x.

Several titanium grid sets were successfully fabricated

and performance tested with NSTAR 30 cm

engineering model ion thrusters. Titanium ion optics

were successfully operated over an extended power

range of 0.5 to 4.6 kW. Ion optics performance

parameters that included impingement-limited total

voltages, electron backstreaming limits, screen grid

ion transparencies, and beam divergence were

compared to those of molybdenum ion optics.

However, these performance parameters were found to

be changing rapidly with accumulated operating time. 8

This initial rapid change in ion optics' performance,

referred to as an ion optics burn-in in this report, was

speculated to have been due to small initial changes in

either accelerator grid aperture diameters from sputter

erosion or grid hot gap from stress-relieving of the

domed material. This burn-in phenomenon has been

noted with molybdenum ion optics during an NSTAR

long duration test. 4

Because performance parameters from these prior

titanium ion optics tests had not yet achieved steady

state values, a proper one-to-one performance

comparison with molybdenum ion optics could not be

completed. An investigation was, therefore, initiated to

burn-in titanium ion optics by operating them until ion

optics performance parameters achieved steady state

values. Afterwards, ion optics' performance was

determined. The same was done for a molybdenum ion

optics set, and the performance of these two materials

was compared. Ion optics' performance was

characterized over a broad thruster input power range

of 0.5 to 3.0 kW. This paper reports on the results of

these tests.

Test Hardware and Operating Procedures

Titanium Ion Optics

A photograph of 30 cm titanium grids is shown in

Fig. 1. The titanium ion optics set utilized for this

investigation was set B of Ref. [8]. Screen and

accelerator grid aperture diameter variations were

within +0%/-9% and _+9%, respectively, of the

0.10 nominal NSTAR design throughout the active area.

In general, accelerator grid aperture diameters were

lower-than-nominal at the active area center while

screen grid aperture diameters were lower-thannominal at the active area perimeter. The resulting

screen grid open area in the active area perimeter

fraction was estimated to be 10 to 13% lower-thannominal. Grid cold gap variations throughout the

active area were within +4%/-0% of the nominal

NSTAR design, which was better than the variations

for the molybdenum ion optics used for these tests.

The screen grid thickness was also 7% thicker than the

nominal NSTAR design. Although this grid set had

bonded together due to temperature differences

between the grids during thruster start-up on a prior

test, the grids were subsequently separated, and

aperture alignment and cold grid gap changes were

found to be negligible. 8 Thruster start-up procedures

were modified to preclude this from occurring and will

be discussed in a later section.

Molybdenum Ion Optics

Molybdenum ion optics fabricated at NASA Glenn

were also tested to provide a baseline performance for

comparison. These ion optics were the same as those

used in Ref. [8]. Screen and accelerator grid aperture

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

respectively, of the nominal NSTAR design

throughout the active area. 9'H)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°7oof the

nominal NSTAR design.

Ion Thruster

Both ion optics sets were mounted onto a 30 cm ion

thruster, shown in Fig. 2, which was the same thruster

as that reported in Ref. [8]. This thruster serves as a

test bed for 30 cm thruster development at NASA. _

The mechanical designs of the thruster discharge

chamber and ion optics are nearly identical to those of

the NSTAR thruster, described in detail in Refs. [9]

and [10]. The thruster was fitted with thermocouples

for thermal tests and the exterior was modified so that

a second neutralizer could be installed.

NASA/TM--2001-211220 2
PowerConsoleandGasFeedSystem

A powerconsolesimilarto thatdescribedinRef.[12]

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.

Beam current density profiles were measured with a

probe mounted onto a two-axis probe motion system.

The probe was a planar geometry with a 1.0 cm 2

circular current-collecting area. j3 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

1.25 m maximum travel in each axis, which enabled

near-field radial beam current density measurements at

different axial locations, as measured from the

geometric center of the ion optics. The current density

measurements were then used to determine beam

current density profiles, beam divergence half-angles,

and thrust correction factors. 8

Vacuum Facility

Testing was conducted in Vacuum Facility i1 at

NASA Glenn. This 2.2 m diameter x 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 1/s with

xenon. The facility base pressure was typically less

than 2.6×10 -5 Pa (2x10 -7 Torr) and background

pressures were as high as 5,2×10 -4 Pa (3.9×10 -_ Torr)

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 _id 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 mA/V. 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 (i.e., the settings

defined in Table 1) and the impingement-limited total

voltage. Uncertainties in impingement-limited total

voltage determinations (and therefore, perveance

margins) were within _10 V.

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 -*! 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. [8]. 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 thrust loss, and to

provide peak current densities for comparisons of

electron backstreaming limits, which will be discussed

in a later section. Regarding beam current density

measurements, no attempt was made to repel chargeexchange 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

NASA/TM--2001-211220 3
causedbyacombinationof effects, which included the

large probe surface area, measurement of chargeexchange 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

thrust losses are discussed in detail in Ref. [8].

Uncertainties in the beam divergence half-angles and

beam divergence thrust losses due to beam divergence

cannot presently be assessed due to the unknown

sources of error in the beam current density

measurements,

Operating Procedures

Molybdenum and titanium ion optics were tested on

the 30 cm ion thruster at the thruster input power

levels 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, 14as well as operating points

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 the TH15

operating point (see Table 1) while monitoring

changes in impingement-limited total voltages,

electron backstreaming limits, screen grid ion

transparencies, and radial beam current density

profiles. The thruster was not operated continuously,

but was intentionally interrupted on several occasions.

When these ion optics performance parameters had

achieved steady state values, the thruster was stepramped through operating points listed in Table I to

characterize ion optics' performance. At each

operating point, ion optics performance parameters

such as impingement-limited 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' pelformance had not changed.

Prior titanium ion optics tests had revealed that 30 cm

titanium ion optics can make contact and bond

together if beam extraction is initiated immediately

following discharge ignition, s This was speculated to

have been due to the initial uneven temperatures of the

grids during thruster startup from room temperature.

This issue was overcome in this investigation by

merely allowing the discharge chamber to heat the ion

optics for a minimum of 20 minutes prior to initiating

beam extraction. This duration is conservative,

however, since temporal electron backstreaming

measurements from room temperature to THI5 have

indicated that beam extraction may have been initiated

as soon as 10 minutes following ignition. 7

Results and Discussions

Ion Optics Burn-in

The results of ion optics burn-in at TH 15 for both grid

materials are plotted in Fig. 3. 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 these

burn-in periods. Monitored ion optics performance

parameters included perveance margin, electron

backstreaming limit, screen grid ion transparency, and

beam current density profiles.

Titanium 1on Optics

For the titanium ion optics, the performance

parameters of Fig. 3 achieved steady state values at

different propellant throughputs. Perveance margins

achieved steady state at about 750 gm, then electron

backstreaming limits at about 1030 gin, and finally

screen grid ion transparencies at about 1200 gm.

Perveance margins and screen grid ion transparencies

generally increased during the first 400 gm of

propellant throughput, and decreased thereafter until

steady state values were reached. Electron

backstreaming limits generally increased after the first

400 gm of propellant throughput.

Radial beam current density profiles at various

propellant throughputs are plotted in Fig. 4 after

400 gm of propellant throughput. The profiles show

that although the peak beam current density did not

change significantly, the beam current densities

NASA/TM--2001-211220 4
increasedatradiiof-100 mmto-20 mm and 20 mm to

100 ram. Specifically, positive radial locations showed

a steady increase in beam current density until 1205 gm

of propellant throughput, while negative radial locations

increased only until 705 gm of propellant throughput.

These changes, however, were small.

Although the post-test cold grid gap was measured to

be 0% to +8% of the pre-test gap at the outer-radius,

mid-radius, and center of the grid active area, this

change was also within the accuracy of the

measurement. Post-test accelerator aperture diameters

showed little or no change at the active area outer- and

mid-radii, but increased by 2 to 5% within 5 to 10 mm

of the active area center.

Trends for each performance parameter changed at

about 400 gm of propellant throughput. This may be

related to the grids making contact and bonding

together at 193 gm of propellant throughput, however,

this cannot be confirmed. 8 The increasing perveance

margin prior to 400 gm of propellant throughput may

have been due to the increased accelerator aperture

diameters at the active area center, where beam current

per hole is highest. The changes after 400 gm in all

monitored performance parameters are consistent with

an increasing grid hot gap, especially at the geometric

center of the grid active area. An increasing hot grid

gap decreases perveance margins and increases in

electron backstreaming limit magnitudes. _5 A larger

hot grid gap is also known to increase discharge losses

by reducing screen grid ion transparency. _6 The

changes in the cold grid gap, however, were too small

to confirm this change. Furthermore, it is presently

unclear how this speculated hot gap increase may have

contributed to the changes in the radial beam current

density profiles. Regardless, all ion optics performance

parameters achieved steady state values by 1200 gm of

propellant throughput. Titanium ion optics'

performance characterization results presented in the

remainder of this paper were measured following this

point. As shown in Fig. 3, the aforementioned ion

optics performance parameters did not change

following performance characterizations.

Molybdenum Ion Optics

The results for molybdenum ion optics shown in Fig. 3

showed no significant changes in any of these

performance parameters. If a burn-in had occurred for

this ion optics set, it would have to have occurred prior

to a propellant throughput of 230 gm. Ion optics'

performance characterizations were initiated at 570 gm

of propellant throughput. Following performance

characterizations, Fig. 3 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.

Impingement-Limited Total Voltage

Beam current as a function of impingement-limited

total voltage is plotted in Fig. 5 for titanium and

molybdenum ion optics. Perveance margins for

titanium ion optics are listed in Table 2 with results

from molybdenum ion optics for comparison.

As Fig. 5 and Table 2 demonstrate, impingementlimited total voltages for titanium ion optics were

about 45 to 55 V greater than those of molybdenum

ion optics at the highest beam current. This disparity

decreased to 10 to 15 V for the lowest beam current.

The difference between the two grid materials was due

to the larger beam current densities at the active area

center for the titanium ion optics (presented in a later

section). This is demonstrated in Fig. 6, where the

peak beam current density is plotted as a function of

impingement-limited total voltage for titanium and

molybdenum ion optics. The larger beam current

densities at the active area center of the titanium ion

optics were the result of higher beam currents per hole

in this region, which caused higher impingementlimited voltages. Regardless, the agreement in

perveance margin shown in Table 2 is considered

sufficient because there is adequate margin at all

operating points in Table 1. Furthermore,

impingement-limited total voltages are known to

decrease with thruster operation. 3

Electron Baekstreaming Limit

Electron backstreaming limit voltages for the titanium

ion optics are listed in Table 2 with results with

molybdenum ion optics for comparison. Since the

electron backstreaming limit is also a function of the

peak beam current density, 8 the electron

backstreaming limit is plotted as a function of the peak

NASA/TM--2001-211220 5
beamcurrentdensity(listedin Table3)in Fig.6for a

moreappropriatecomparison.Thedatafor eachgrid

setarepresentedatseparatebeamvoltagesbecausethe

electronbackstreaminglimit isalsoafunctionof beam

voltage._s

Table2 showsthattheelectronbackstreaminglimits

of titaniumionopticswereI to 7V higherthanthose

of molybdenumion optics. However,peak beam

currentdensities,listedin Table3, werealso0.1 to

0.5 mA/cm:higherthanthosefor molybdenumion

optics. Fig. 7, which correlatesboth parameters,

demonstratesthattitaniumion opticsoperatedwitha

higherelectronbackstreaminglimit thanmolybdenum

ionopticsfor agivenpeakbeamcurrentdensity.This

disparity,thoughsmall,isspeculatedto havebeendue,

inpart,to thelarger-than-nominal acceleratoraperture

diametersfor themolybdenumion opticsandlowerthan-nominalacceleratoraperturediametersfor the

titanium ion opticsat the activeareacenter.The

electronbackstreaminglimit decreaseswithincreasing

acceleratoraperturediameter.iS

ScreenGrid Ion Transparency

Table 3 lists screen grid ion transparencies for titanium

ion optics with results from molybdenum ion optics

for comparison. Screen grid ion transparencies for

titanium ion optics were as much as 3.8% lower than

those for molybdenum ion optics. This reflected the

smaller physical open area fraction of the titanium ion

optics' screen grid at the perimeter of the active area

and slightly thicker titanium screen grid.

Accelerator Current

Table 3 lists accelerator currents for titanium ion

optics with results from molybdenum ion optics for

comparison. Accelerator currents for titanium ion

optics were within 4% of those for molybdenum ion

optics.

Beam Current Density Profiles, Beam Divergence,

and Thrust Loss

Sample radial beam current density distributions for

both grid materials are shown in Fig. 8, taken 48 rnm

downstream of the grid center. Radial beam current

density distributions at several axial locations are

shown in Fig. 9 for titanium ion optics. Peak beam

current densities were determined from radial beam

current density profiles taken 48 mm downstream of

the grid center. Table 3 lists peak beam current

densities for titanium ion optics with results from

molybdenum ion optics for comparison. As Table 3

shows, peak beam current densities for titanium ion

optics were 4 to 8% higher than those for molybdenum

ion optics. It is speculated that the larger peak beam

current densities for titanium ion optics was due to the

smaller screen _id 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. This higher discharge

plasma density was reflected in the 2 to 5% higher

discharge losses for the titanium ion optics, as shown

in Table 3. It is noteworthy that the larger peak beam

current densities for titanium ion optics likely

contributed to the lower screen grid ion transparencies

and higher impingement-limited total voltage, as

discussed in earlier sections.

Figures 8 and 9 also show that beam current density

profiles were slightly non-axisymmetric near the

active area center for both ion optics materials. This

slight asymmetry occurred at all power levels tested

and is an artifact of the thruster discharge chamber

plasma. 8

Beam divergence angles that enclosed 95% of the total

beam current are listed in Table 4 for titanium ion

optics with results from molybdenum ion optics for

comparison. Divergence half-angle data for the

titanium ion optics were within 1 to 3° of those for the

molybdenum ion optics. Sample plots of percentage of

total beam current that enclosed a given divergence

half-angle are shown in Fig. 10 for titanium and

molybdenum ion optics. Beam divergence half-angles

were almost identical for both ion optics materials.

Table 4 lists these thrust correction factors due to

beam divergence for titanium ion optics with results

from molybdenum 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 very similar for titanium and molybdenum ion

optics.

NASA/TM---2001-211220 6
Conclusions

The results of performance tests with titanium and

molybdenum ion optics were presented. Both titanium

and molybdenum ion optics were initially operated

until ion optics performance parameters achieved

steady state values. Afterwards, performance

characterizations were conducted. This permitted

proper performance comparisons of titanium and

molybdenum ion optics over a broad thruster input

power range of 0.5 to 3.0 kW.

All performance parameters for titanium ion optics

achieved steady state values after processing 1200 gm

of propellant. Trends for each performance parameter

changed at about 400 gm of propellant throughput.

Changes in performance parameters after 400 gm are

consistent with an increasing grid hot gap, especially

at the geometric center of the grid active area.

Unfortunately, changes in the cold grid gap were too

small to confirm this change. Molybdenum ion optics

exhibited no burn-in.

Impingement-limited total voltages for titanium ion

optics were about 45 to 55 V greater than those of

molybdenum ion optics at the highest beam currents,

This disparity decreased to 10 V for the lower beam

currents. The difference between the two grid

materials was due to the larger beam current densities

at the active area center for the titanium ion optics.

This agreement is considered sufficient because there

was adequate perveance margin at all operating points.

Comparisons of electron backstreaming limits as a

function of peak beam current density for

molybdenum and titanium ion optics demonstrated

that titanium ion optics operated with a higher electron

backstreaming limit than molybdenum ion optics for a

given peak beam current density. This disparity is

speculated to have been due, in part, to the larger-thannominal accelerator aperture diameters for the

molybdenum ion optics and lower-than-nominal

accelerator aperture diameters for the titanium ion

optics at the active area center.

Screen grid ion transparencies for titanium ion optics

were up to 3.8% lower than those for molybdenum ion

optics, reflecting the smaller physical open area

fraction of the titanium ion optics' screen grid at the

perimeter of the active area and slightly thicker

titanium screen grid.

Beam divergence half-angles that enclosed 95% of the

total beam current for titanium ion optics were within

l to 3° of those for molybdenum ion optics. All beam

divergence thrust correction factors for the titanium

ion optics were within 1% of those for the

molybdenum ion optics.

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

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

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Rawlin, V.K., "Titanium Optics for Ion

Thrusters," IEPC Paper 99-149, October 1999.

[8] Soulas, G.C., "Performance of Titanium Optics

on a NASA 30 cm Ion Thruster," AIAA Paper

2000-3814, July 2000,

[9] 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.

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

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Millennium's Deep Space 1 Mission," AIAA

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

Development Activities at NASA Glenn

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2000.

NASA/TM--2001-211220 7
[12] 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.

[13] 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.

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

Qualification Testing," AIAA Paper 98-3936,

July 1998.

[ 15]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.

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

Thrusters with Dished Accelerator Grids,"

AIAA Paper 73-1053, October-November 1973.

Power Input

Level Power, _

Designation kW

Table 1. Nominal thruster operating parameters.

Beam Beam Neutralizer Main

Current b Voltage,b Accelerator Keeper Flow,

A V Voltage, V Current. A sccm

Discharge Neutralizer

Cathode Flow.

Flow, sccm sccm

TH0 _ 0.5

TH4 _ 1.0

TH8 ¢ 1.4

THI0' 1.7

TH t 2_ 1.8

THI5 _ 2.3

THe4 0.6

THe8 0.8

THel 1 1.1

THeI4 1.3

THeI6 2.5

THeI9 3.0

0.51 650 -150 2.0 5.98 2.47 2.40

0.71 1100 -150 2.0 8.30 2.47 2.40

1.10 1100 - 180 1.5 14.4 2.47 2.40

1.30 i 100 -180 1.5 17.2 2.56 2.49

1.49 1100 -i 80 1.5 18.5 2.72 2.65

1.76 1100 -180 1.5 23.4 3.70 3.60

0.56 1500 -250 2.0 6.50 2.47 2.40

0.85 1500 -250 2.0 10.6 2.47 2.40

1.06 1500 -250 2.0 14.0 2.47 2.40

1.27 1500 -250 1.5 17.0 2.56 2.46

1.44 1500 -250 1.5 19.2 2.81 2.72

1.76 1500 -250 1.5 23.1 3.54 3.60

"Nominal values,

bPower supply current or voltage,

_Nominal NSTAR operating condition.

Table 2. Perveance margins and electron backstreaming limits

for titanium and molybdenum ion optics.

Power Level

Designation

TH0 a

TH4 _

TH8"

TH 10a

THI2 _

THI5 _

THe4

THe8

THe 11

THeI4

THe 16

THeI9

Perveance Margin, V

Ti

165

515

425

370

325

240

1090

970

910

850

715

Electron Backstreaming Limit, V

Mo Ti

175 -70

540 -124

455 -137

405 -141

365 -144

295 -149

1105 -142

1000 -167

940 -180

885 -184

845

760 -192

Mo

-69

-126

-138

-143

-148

-156

-145

-171

-181

-183

-189

-197

_Nommal NSTAR operating condition.

NASA/TM--2001-211220 8
Table 3. Peak beam current densities, screen grid ion transparencies, discharge losses,

and accelerator currents for titanium and molybdenum ion optics.

Peak Beam Current Screen Grid Ion Discharge

Power Level

Designation

TH0 b

TH4 _

TH8 _

THI0 b

TH12 b

TH15 b

THe4

THe8

THe 11

THe 14

THeI6

THeI9

Densities, a mA/cm-"

Ti Mo

2.5 2.4

3.4 3.2

4.6 4.3

5.2 4.8

5.6 5.3

6.2 5.8

2.8 2.6

3.8 3.7

4.5 4.3

5.1 4.8

5.3

6.3 6.0

Transparencies

Ti Mo

Losses, W/A

Ti Mo

Accelerator

Currents, mA

Ti Mo

1.4 1.4

1.8 1.8

3.5 3.5

4.6 4.5

5.7 5.5

7.5 7.2

1.5 1.5

2.6 2.5

3.5 3.5

4.2 4.4

5.1

6.6 6.8

0.827 0.836

0.872 0.882

O.864 O.877

0.854 0.870

0.843 0.863

0.818 0.849

0.904 0.912

0.906 0.909

0.904 0.913

0.898 0.908

- 0.904

0.875 0.895

250 250

250 240

200 200

197 191

194 187

197 188

233 225

200 200

190 190

190 180

179

185 178

"Peak beam current densities at 48 nnn downstream of the geometric center of the ion optics: peak beam current

approximately the radial center of the grid active area.

_Nommal NSTAR operating condition.

densities occurred at

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

and beam divergence thrust correction factors for

Power Level

Designation

TH0 a

TH4 a

TH8 a

TH 10a

TH12 a

TH15 a

THe4

THe8

THe 11

THeI4

THel6

THeI9 29

_Nonfinal NSTAR operating condition.

titanium and molybdenum ion optics.

Divergence Half-angle at 95% Thrust Correction Factor

of Beam Current, degrees Beam Divergence

Ti Mo Ti Mo

28

28

27

25

32

29

29

29

28

27

26

29

29

29

29

29

29

for

0.97 0.97

- 0.97

0.97 0.97

0.98 0.97

- 0.98

0.98 0.98

0.96 0.96

- 0.96

- 0.97

0.97

0.97

0.97 0.97

NASA/TM--2001-211220 9
Fig. 1. Photograph of titanium grids with and without the ion optics mounting ring.

Fig. 2. Titanium ion optics installed onto a NASA 30 cm ion thruster.

NASA/TM--2001-211220 10
>

_z

_D

e.o

>

t",_

310

290

270

250

230

210

_ Performanc

Characterizations

__ [OMo ybdenum Ion Optics [ I ;

I ]ITitanium Ion Optics l i :

l__f

190 .... ! ..... 4 _i .....

i !

i!

17o i J r _ r

0 200 400 600 800 1000 1200 1400

Propellant Throughput, gm

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

>

_O

e=

-145 1

-149 i

-153

-157 l

-161

0

O Molybdenum Ion Optics

IITitanium Ion Optics

i

i

i ̧ q

i

Performance

Characterizations

i

i

200 400 600 800 1000 1200 1400

Propellant Throughput. gm

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

Fig. 3. Burn-in performance parameters at TH15 for titanium and molybdenum ion optics.

NASA/TM--2001-2 i 1220 11
0,855 ..............

T'T_ Perforn_ance

5

_ 0.825 _ ; Molybdenum Ion Optics ...... ! - t_ _ I. .... \ --:

[. Titanium ion Optics !' _, _ _ _11_t_t__

0.815 i I T

0 200 400 600 800 1000 1200 1400

Propellant Throughput, gm

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

Fig. 3. Burn-in performance parameters at TH15 for titanium and molybdenum ion optics. (Concluded)

-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180

Radius, mm

Fig. 4. Radial beam current density profiles for titanium ion optics during ion optics burn-in

at various propellant throughputs. Profiles were measured 48 mm axially downstream

of the grid center. Gray arrows indicate changes with accumulated propellant throughput.

NASA/TM--2001-211220 12
<

E

r_

1.8

1.6

1.4

1.2

0.8

0.6

0.4

+

0 Molybdenum Ion Optics

[] Titanium Ion Optics

fl30 I:_1

/

O _ [] /

O r

OO O I:F

jJ . .....

J

zO _ 43

I

600 650 700 750 800 850 900 950 1000 1050

Impingement-Limited Total Voltage, V

Fig. 5. Beam current as a function of impingement-limited total voltage

for titanium and molybdenum ion optics.

c_

E

e_

7.0

[

i

6.0 t

5.0

4.0

3.0

2.0

!

O Molybdenum Ion Optics

[] Titanium Ion Optics

I

!

th

!I i o

I Jo

i

6b° i i ,

O

iO

00

[]

0

I .... i o•

l l [] []

o° ............. i

1

r

600 650 700 750 800 850 900 950 1000 1050

Impingement-Limited Total Voltage, V

Fig. 6. Peak beam current density as a function of impingement-limited total voltage

for titanium and molybdenum ion optics.

NASA/TM--2001-211220 13
:>

.,d

e- °_

..d

e-,

eer_

e-.

krd

-6O

-8O

-10O

-120

-140

-160

- ! 80

-20O

0 F

] OMolybdenum Optics at a 650 V Beam Voltage

i • + _ - Optics at a 650 V Beam Voltage

|

OTitanium

, [

r'lMolvbdenum Optics at a 1100 V Beam Voltage

i

5, .... ITitanium Optics at a 1100 V Beam Voltage

AMolybdenum Optics at a 1500 V Beam Voltage

&Titanium Optics at a 1500 V Beam Voltage

i

j i ,

2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

Peak Beam Current Density, mA/cmFig. 7. Electron backstreaming limit as a function of peak beam current density for titanium and

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

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

[

7 = t Titanium Optics at THe 19

[' Molybdenum Optics at THe 19

6 .... "..........Titanium Optics at TH0

"'E Molybdenum Optics at TH0

_ 5_ r/'_ ..... _ .....

"_. 4 __'_

o ,

- i 80 - 120 -60 0 60 120 180

Radius, nun

Fig. 8. Radial beam current density profiles for both titanium and molybdenum ion optics at THel9

and TH0. Beam current densities were measured 48 mm downstream of the ion optics' center.

NASA/TM--20O 1-211220 14
e_

E

_o

_D

L)

r0

-300

\

!

i

!

Axial Location, ram: '

m48

-- 198

273

.......................... 348

498 .... 1

i

-2_ -1_ 0 1_ 2_ 3_

Radius, mm

Fig. 9. Radial beam current density profiles at several axial locations

for titanium ion optics at THel9.

v.

L)

E

o

o

100

90

80

70

60

50

40

30

20

10

0÷

0

,,¢,J

4

I

I

5 10 15 20 25 30 35

Divergence Half-Angle, degrees

Fig. 10. Percentage of total beam current enclosed in a given divergence half-angle

for both titanium and molybdenum ion optics at a THel9.

4O

NASAfI'M--2001-211220 15
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

Performance Evaluation of Titanium Ion Optics for the NASA 30 cm Ion Thruster

6. AUTHOR(S)

George C. Soulas

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

5. FUNDING NUMBERS

WU-755-B4-04--O0

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-13073

10. SPONSORING/MONWORING

AGENCY REPORT NUMBER

NASA TM--2001-211220

IEPC--O 1---092

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--977-7410.

12a, DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category: 20 Distribution: Nonstandard

Available electronically at httt_://eltrs.mxz.nasa.f, ov/GLTRS

This publication is available from the NASA Center for AeroSpace Information. 301--621-0390.

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

The results of performance tests with titanium ion optics were presented and compared to those of molybdenum ion optics.

Both titanium and molybdenum ion optics were initially operated until ion optics performance parameters achieved steady

state values. Afterwards, performance characterizations were conducted. This permitted proper performance comparisons

of titanium and molybdenum ion optics. Ion optics' performance was characterized over a broad thruster input power range

of 0.5 to 3.0 kW. All performance parameters for titanium ion optics of achieved steady state values after processing

1200 gm of propellant. Molybdenum ion optics exhibited no burn-in. Impingement-limited total voltages for titanium ion

optics were up to 55 V greater than those for molybdenum ion optics. Comparisons of electron backstreaming limits as a

function of peak beam current density for molybdenum and titanium ion optics demonstrated that titanium ion optics

operated with a higher electron backstreaming limit than molybdenum ion optics for a given peak beam current density.

Screen grid ion transparencies for titanium ion optics were as much as 3.8 percent lower than those for molybdenum ion

optics. Beam divergence half-angles that enclosed 95 percent of the total beam current for titanium ion optics were within

1 to 3 ° of those for molybdenum ion optics. All beam divergence thrust correction factors for titanium ion optics were

within 1 percent of those with molybdenum ion optics.

14. SUBJECT TERMS

Ion thruster; Ion optics; Grids

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

18. SECURITY CLASSIFICATION

OF THIS PAGE

Unclassified

NSN 7540-01-280-5500

! 19. SECURITYCLASSIFICATION

OF ABSTRACT

Unclassified

15. NUMBER OF PAGES

21

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20. LIMITATION OF ABSTRACT

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