NASA/CR--1998-208677

International Space Station Cathode

Life Testing Status

Timothy R. Sarver-Verhey and George C. Soulas

Dynacs Engineering Company, Inc.

AIAA 98-3483

=

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October 1998
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NASA/CR--1998-208677 AIAA 98-3483

International Space Station Cathode

Life Testing Status

Timothy R. Sarver-Verhey and George C. Soulas

Dynacs Engineering Company, Inc.

Prepared for the

34th Joint Propulsion Conference and Exhibit

cosponsored by AIAA, ASME, SAE, and ASEE

Cleveland, Ohio, July 13-15, 1998

Prepared under Contract NAS3-98008

National Aeronautics and

Space Administration

Lewis Research Center

October 1998
Acknowledgments

The authors would like to acknowledge the efforts of M. Pastel, R.A. Jacko, G.E. Bowers, J.R. Miller, Jr.,

and J. Zakan34 without whom these tests could not have succeeded.

This report contains preliminary

findings, subject to revision as

analysis proceeds.

NASA Center for Aerospace Information

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Price Code: A03

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r

_zv_

m

International Space Station Cathode Life Testing Status

Timothy R. Sarver-Verhey and George C. Souias

: Dynacs Engineering Co., lnc.

' NASA Lewis Research Center Group

_' Brook ParL OH 44142

"_i r] , _ - -

To demonstrate adequate lifetime and performance capabilities of a hollow Cathode for use on the International Space Station (ISS) plasma contactor system, life tests of multiple hollow cathode assemblies

(HCAs) were initiated afoperating'iz0nditions simulating on-orbit operation. Three HCAs are presently

being tested. These HCAs are operated with a continuous 6 sccm xenon flow rate and 3 A anode curr_/nL Emission current requirements are simulated with a square waveform consisting of 50 minutes at

a 2.5 A emission current and 40 minutes with no emission current. As of July 1998, these HCAs have

accumulated between 11,700 and 14,200 hours. While there have been changes in operating behavior,

the three HCAs continue to operate stably within ISS specifications and are expected to demonstrate

the required lifetime.

Introduction tron emitter installed at the downstream end. A

sheathed heater is mounted on the exterior of the

A hollow cathode-based plasma contactor system cathode tube for conditioning and ignition. An intehas been baselined for use on the International gral anode shell is mounted around the cathode and

Space Station to reduce spacecraft electrical charg- electrically isolated from it.

ing) The plasma contactor system provides a low

impedance connection to space plasma with a hol- Test Configuration

low cathode assembly (HCA). The operational re- All of the HCAs are operated continuously with a

quirements of the HCA include an electron emission fixed current of 3 A between the cathode and anode.

capability of Up to 10 A and an operational lifetime For mission profile life testing, each HCA emits an

of 18,000 hours. Other component-level -tests, includ- additional current to an external anode plate

ing a hollow cathode, 2 cathode heater 3 life tests, mounted downstream. A bias voltage is applied to

and an accelerated cyclic ignition test, _ have been this plate to extract the simulated spacecraft charge

conducted as part of the effort to demonstrate corn- control current. A photograph of an HCA/bias anode

pliance with mission requirements, arrangement is shown in Figure 1. All HCA/bias anIn order to demonstrate the lifetime capability, ode configurations and their respective power supmultiple high-fidelity development model HCAs are plies are electrically isolated from each other, and

being life-tested to 27,000 hours at conditions simu- from vacuum facility ground.

lating on-orbit operation. To date, three HCAs, designated HCA-003, -010, and -013, have accumulated Power Supplies

12,400, l 1,700, and 14,200 hours, respectively. A Commercial power supplies are used to operate

fourth HCA had also been under test but this test the HCA heater and discharge and to extract the

was voluntarily terminated at 8,000 hours to conduct bias current. All power supplies are operated in curdestructive analyses prior to the HCA critical design rent controlled mode. The heater and anode power

review. The HCA condition was excellent and has supplies provide up to 8.5 A and 3.0 A, respectively.

been reported elsewhere. 5 This report will update an The bias power supply can provide up to 10 A, alearlier work _ by discussing the operating perform- though a current of 2.5 A is used for life testing. An

arlce of HCAs -003, -010, and -013 since hours breadboard ignitor supply _ provided a high-w_itage

10,900, 7,800, and 10,200, rcspectively, pulse 4 to ignite the HCA discharges.

.... Laboratory Gas Feed Systems

Apparatus One of the two gas feed systems used to deliver

xenon to the HCAs is shown in Figure 2. The xenon

Hollow Cathode Assembly flow rate is measured and controlled with a mass

A development model HCA design was used for flow controller. A mass flow meter is located upmission profile life testing. While there are minor stream of the flow controller to provide redundant

differences betwe.en the ffevelopme-nt model and flow measurement. Protocols developed for the

spaceflight designs, _'4 they are functionally identical plasma contactor program are used to mitigate xeand the test results are expected to be valid for the non gas contamination throughout the course of this

flight hardware. Briefly, the HCA consists of an test. Further details on the feed systems have been

orificed cathode tube with a low-work function elec- discussed previously. 7

NASA/CR-- 1998-208677
Vacuum Facility

The mission profile life testing is being performed

in a 1.0 m diameter x 1.5 m long stainless steel tank

evacuated with a 0.89 m diameter cryo-pump. This

eryo-pump has a pumping speed of approximately

i,300 L/s on xenon which maintains the facility at a

base pressure of 3.0 x I()-_ Pa (2.3 x 10-_ Tort) and a

pressure of 2.0 x 10.2 Pa (!.5 x 10`4 Torr) during HCA

operation.

Data Acquisition and Control System

Life testing is controlled and monitored by computer. Heater, anode, and bias currents, xenon flow

rate, and test sequencing are the control parameters

for the life test. Test chamber pressure, heater, anode, and bias voltages, and cathode temperature

data are automatically acquired and monitored to

terminate life testing in casc of an off-normal condition. Finally, AC components of anode and bias

voltages and currents are measured manually with

an oscilloscope.

Test Procedures and Operating Conditions

For all testing, cathode conditioning and ignition

procedures used in these tests are the same as those

for the spaceflight HCAs/ Whenever the HCA was

exposed to air, cathode conditioning procedures

were conducted to prepare the cathode electron

emitter for operation.

The HCA discharge is ignited with the following

procedure. A heater current of 8.5 A is applied. After

a fixed period, xenon gas flow is initiated and following a short flow stabilization period, the anode

open circuit voltage and ignition pulse are applied.

Heater power is maintained until an anode current is

detected. Immediately after ignition, the ignitor and

heater power supplies are turned off and the discharge between cathode and anode is allowed to

stabilize. The ignition procedure is described in further detail elsewhere. 4 A performance evaluation of

the HCA is typically conducted at the start of each

test segment during which anode voltages are measured as a function of xenon flow rate at a fixed anode current. Life testing is then initiated (or resumed).

The HCA operating conditions for all testing are

listed in Table I. Test conditions were defined by a

previous study which found that the International

Space Station wilt require on average an electron

current during approximately 50 minutes of the 90

minute orbital perk_t. _ Although the HCA is designed-to provide an electron emission current of up

to 10 A, the maximum sustained electron emission

current is anticipated to be approximately 2.5 A during this emission period, x During the remaining 40

minutes of the orbit, the idle period, no electron

emission current is required and the discharge is

maintained at an anode current of 3.0 A. This profilc

is repeated continuously.

Some deviations from these nominal test conditions are noted. HCA-010 was briefly operated in an

on-off mode to support a plasma eontactor system

trade study examining xenon usage. During operation

in this mode, the HCA discharge was ignited just

prior to each emission period and extinguished at the

start of each idle period. The on-off mode was initiated at hour 2,370 and was changed back to continuous-on mode after hour 6,445, after successfully

completing 4,369 cycles, as indicated in Table I.

Results and Discussion

To date, there have been 35 test interruptions.

These interruptions occurred as a result of facility or

test support equipment failures. None of the interruptions were due to the HCAs, and these shut-downs

were performed in a manner which did not compromise thc integrity of the HCAs. The behavior of the

anode and bias voltages at the end of idle and emission periods is used as the main indicator of HCA

health. The ignition times and performance evaluations are also used to assess the condition of each

HCA during life testing. The results of the HCAs

003,-010, and-013 collected since hours 10,900,

7,800, and 10,200, respectively will be presented

and discussed.

HCA Performance Evaluations

HCA performance was evaluated during the course

of life testing by measuring the anode voltage over a

range of xenon flow rates at a fixed current. Figure 3

shows the performance of HCA-003 at the start of

the test segments. As can be seen, the anode voltage behavior has remained approximately the same,

with some variation in voltage levels. The performance of HCAs -010 and -013 exhibited similar behavior, as shown in Figure 4. In this figure, the HCA

anode voltages were averaged from all profiles

measured to date. The voltages at 6 sccm have varied within a maximum bandwidth of 5.5 V over the

course of the life tests. This bandwidth is similar to

that observed in a 28,000 hour hollow cathode life

test. 2 The small differences in anode voltage between the three HCAs can be attributed to slight

variations in HCA geometry)

Life Test Performance

As of July 1998, HCAs -003, -010, and -013, have

accumuqated 12,400, 11,700, and i4.200 hours of

operation, respectively. Operation on all devices has

remained within the specifications of the ISS HCA.

These specifications include ignition within 30 minutes and operating anode voltages _< 40 VDC/ The

NASA/CR-- 1998-208677 2
i

I

i

anode voltage and ignition time behavior to date

will be examined in this section.

As mentioned previously, anode and bias voltages

are monitored to determine the condition of the

HCAs. Figures 5-7 show the anode and bias voltages

at the end of idle and emission periods for each

HCA. To date, none of the devices have exhibited

monotonic changes in anode or bias voltages over

the course of life testing which would indicate lifelimiting changes. There have been repeated instances of increases of several volts occurring over

periods of several thousand hours. However, similar

increases have occurred during these life tests as

well as during a hollow cathode life test'- after which

the voltages eventually returned to lower values.

HCA end-of-period w_ltages for all three devices

have varied within a maximum bandwidth of 6.9 V,

as shown in Table 2.

Another indicator of HCA condition is the time for

ignition to occur, as measured from the start of the

pre-heat. The ignition times for the three HCAs over

the course of the life test are shown in Figure 8. The

time required to ignite the HCAs has generally been

less than 6 minutes for all HCAs, which is characieristic for tl_is HCA design. _ Ignition times greater than

6 minutes occurred during the initial series of ignitions following HCA fabrication, which is typical.

Recently, two devices have required longer times to

ignite.

HCA-003 has required between 1i.8 and 23 minutes to ignite for ignitions 37, 38, and 42. Ignitions

39-41 and 43-45 all required 6.6 minutes or less. All

of these ignitions occurred between hours 10,860 and

12,415. The increased ignition times all occurred

following conditioning of the HCA whereas the

shorter ignition times occurred when the device was

restarted without conditioning (not required at these

times). This behavior suggests that the electron

emitter condition is responsible for the increased

ignition times, however, anode and b_as voltages

and their dynamic behavior has been relatively stable over the period of these ignitions.

HCA-010 ignition times were less than 6 minutes

up to ignition 31 of mnsslon profile life testing. These

include all of the 4,358 ignitions performed during

the operation in on-off mode when the device typically ignited within 3.7-4.0 minutes. However, the

ignition time exceed 6 minutes during ignitions 32,

33, and 37-39. The ignition time reached a maximum of 12 minutes during its last ignition. Unlike

HCA-003, the increased ignition times did not only

occur after conditioning, however the HCA had been

conditioned prior to ignitions 32, 33, and 38.

The AC components of anode and bias voltages

('noise') are checked periodically to verify that spot

mode operation is being maintained. _ While low

levels of noise are normal and have been present

throughout life testing, increased anode voltage

noise began to occur during idle periods after several

thousand hours of operation. This noise was similar

for all HCAs because it has been low-frequency (-I

Hz) and non-periodic. However, the magnitude and

time of appearance of the noise differs between the

three HCAs, with the noise ranging between 0.5 and

4.0 V peak-to-peak. Additionally, none of the devices has exhibited any monotonic increases in the

noise to date. The observed characteristics indicate

that the noise is not due to transition from spot to

plume noise.

While the cause of the increased ignition times

and anode voltage noise during the idle period is

unknown, it is suspected that they are likely due to

changes in the low work-function surface formation

process in the electron emitter. These changes would

be consistent with cathode aging, 2 in particular since

they have begun to occur after several thousand

hours of operation. Another factor that may contribute to these changes is the operating mode which

results in the HCA operating at two different emission levels. While the causes listed above are speculated to be occurring, a comprehensive explanation

for all of the changes will likely have to wait until

post-test destructive analyses of the HCAs can be

performed.

All of the HCAs continue to operate within ISS

specifications and have not exhibited any rapid or

monotonic changes which could indicate imminent

failure.

Concluding Remarks

Mission profile life testing of development model

HCAs is on-going. Three HCAs are presently being

life tested under conditions simulating projected onorbit conditions. As of July 1998, these devices have

accumulated between il,700 and 14,200 hours.

They=have demonstrated the.......longest lifetime of any

xenon hollow cathode operated at multiple emission

currents. HCA operation, as determined by anode

and bias voltage behavior, has been relatively stable. While there have been changes in HCA performance, operation has remained within the ISS

specification. At this time, none of the observed

changes are anticipated to prevent the demonstration

of the required lifetime.

References

Patterson, M.J., et al., "Space Station Cathode Design, Performance, and Operating Specifications,"

IEPC Paper 97-170, August 1997.

-' Sarver-Verhey, T.R., "28,000 Hour Xenon Hollow

Cathode Life Test Results," NASA/CR-97-20623[,

IEPC Paper 97-168, November 1997.

NASA/CR-- 1998-208677
Soulas,G.C.,"Statusof HollowCathode Heater

Development for the Space Station Plasma Contactor," AIAA Paper 94-3309, July 1994.

";Zakany, J.E. and Pinero, L., "Space Station Cathode Ignition Test Status at 32,000 Cycles," IEPC

Paper 97-167, August 1997.

5 Soulas, G.C. and Sarver-Verhcy, T.R., "International Space Station Cathode Life Testing,"

NASA/CR-97-206230, November, 1997.

Hamley, J.A., et al., "Development of a Power

Electronics Unit for the Space Station Plasma Contactor," IEPC Paper 93-052, September 1993.

v Soulas, G.C., 'qVlultiplc Hollow Cathode Wear Testing for the Space Station Plasma Contactor," AIAA

Paper94-3310, June 1994.

Personal Communication , Katz, I., S-Cubed Division of Maxwell Labs, San Diego, CA, July 1994.

HCA

Designation

Table 1 Life Test Operating Conditions

Life

Test

Hours

On-off

Cycles _

Anode

Current,

A

Bias

Current,

A

Emission

Period, min.

Idle

Period,

min.

HCA-003 0- n/a 3 2.5 50 40

12,415

HCA-0 ! 0 n/a 3 2.5 50

2.5

_ 0-2,370

2,370-

6,445

4,358 50

40

40

6,445- n/a 3 2,5 50 40

I1,667 ......

HCA-013 0- n/a 3 2.5 50 40

14,158

'1HCA discharge is extinguished during the idle periods.

Table 2 Life Test Voltage Bandwidths

HCA

Designation

Accumulated Idle Period

Voltage Bandwidths

Hours _ Anod% V Anode_ V

HCA-O03 ! 2,415 5.3 2.5

11,667 6.9

5.1

4.8

3.5

HCA-0t 0

HCA-013 14,158

Emission Period

Bias_

4.9

5.1

3.9

V

_' As of July 1998.

NASA/CR--1998-208677 4
Figure 1. Photograph of development model HCA with an external bias anode used for mission profile

life testing. The external bias anode is to the right in the photograph.

Figure 2. Xenon feed system used for mission profile and ignition testing.

NASA/CR--1998-208677 5
26 i

24

22 --

20

"_ 18 -

.<

Q

A'- ",:

,-, ....... -_

- ik •

12 _- _ .... =

M

10 : - i ........................... 0___ -- ra __

4 5 6 7 8 9 10

o Oh

Q 462h

A _6h

O 2847 h

• 3070 h

• 5168 h

• 5459 h

• 5706 h

0 6251 h

D 7427 h

A 9539 h

O 9768 h

• 10860h

• 10860h

• 12415 h

Xenon Flow, sccm

Figure 3. Performance evaluation profiles for HCA-003. Evaluations performed at beginning of test

segments following the accumulation of the indicated times.

26

24

22

20

_g

;_ 16

@

= 14

,<

12

.--4:1---HCA-013

10

4 5 6 7 8 9 10

Xenon Flow, sccm

Figure 4. Performance evaluation for the HCAs -010 and -013. Each trace represents the average of all

of the evaluation data sets for each device.

NAS MCR-- 1998-208677 6
26

18

18

t

, I I I I I _ ,

16

u

lO t

o

6

4

2

8 0

0 2,000 4,000 6,000 8,000 I0,000 12,000 14,000

Accumulated Operating Time, h

Figure 5. HCA-003 life test anode and bias voltages at the end of idle and emission periods.

26 18

• ..... Cycled On-Off ...... _.

24 .......................................................................................................................................................... :...... -: ....... 16

Bias (Emission Period) __

2[) - 12

_" 18 10

_ 4 6

_ 10 2

8 0

0 2,000 4,000 6 000 8,000 10.000 12,000 14,000 16,000

Accumulated Operating Time, h

Figure 6. HCA-010 life test anode and bias voltages at the end of idle and emission periods.

The indicated time is longer than the actual accumulated time because it includes the off periods

when this HCA was cycled on-off.

NASA/CR-- 1998-208677
26

24

18

"I

"F. i4

i

8

t

18

16 _

Bias (Emission Period) _.

- : - 10

Anode (Idle Period) 8 "_

L _ Anode (Emission Period)

4 <

I i I I i I • I , 1 x I , 1 • 0

2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000

Accumulated Operating Time, h

Figure 7. HCA-013 life test anode and bias voltages at the end of idle and emission periods.

24

L

22

20

18

16-

--O---HCA-003

--I]--HCA-010 ON-OFF Cycles Not Included)

----h-- HCA-013

i 14 -

12

I0

I

_ 8

4

0

0 10 20 30 40

Ignition Number

5O

Figure 8. Ignition time as a function of ignition number.

NASA/CR--1998-208677 8
REPORT DOCUMENTATION PAGE FormApproved

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1. AGENCY USE ONLY (Leave blank)

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4. TITLE AND SUBTITLE

2. REPORT DATE

October 1998

International Space Station Cathode Life Testing Status

6. AUTHOR(S)

Timothy R. Sarver-Verhey and George C. Soulas

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

Dynacs Engineering Company, Inc.

Brook Park, Ohio 44135

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

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

3. REPORT TYPE AND DATES COVERED

Final Contractor Report

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WU_322_)0-00-00

NAS3-98008

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REPORTNUMBER

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AGENCYREPORTNUMBER

NASA CR--1998-208677

AIAA 98-3483

11. SUPPLEMENTARY NOTES

Prepared for the 34th Joint Propulsion Conference and Exhibit cosponsored by AIAA, ASME, SAE, and ASEE,

Cleveland, Ohio, July 13-15, 1998. Project Manager, Sandra Hardy, Systems Engineering Division, NASA Lewis

Research Center, organization code 7830, (216) 433-2278.

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

13. ABSTRACT (Maximum 200 words)

To demonstrate adequate lifetime and performance capabilities of a hollow cathode for use on the International Space

Station (ISS) plasma contactor system, life tests of multiple hollow cathode assemblies (HCAs) were initiated at operating conditions simulating on-orbit operation. Three HCAs are presently being tested. These HCAs are operated with a

continuous 6 sccm xenon flow rate and 3 A anode current. Emission current requirements are simulated with a square

waveform consisting of 50 minutes at a 2.5 A emission current and 40 minutes with no emission current. As of July 1998,

these HCAs have accumulated between 11,700 and 14,200 hours. While there have been changes in operating behavior,

the three HCAs continue to operate stably within ISS specifications and are expected to demonstrate the required lifetime.

14. SUBJECT TERMS

Hollow cathode; Electron emission; Plasma contactor; Spacecraft charging

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