NASA /CR-2003-211817 AM-20024098 

Comparison of On-Orbit and Ground Based 

Hollow Cathode Operation 

Christian Carpenter 

QSS Group, Inc., Cleveland, Ohio 

. 

August 2003 
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NASA /CR-2003-211817 AIAA-2002-4098 

Comparison of On-Orbit and Ground Based 

Hollow Cathode Operation 

Christian Carpenter 

QSS Group, Inc., Cleveland, Ohio 

Prepared for the 

38th Joint Propulsion Conference and Exhibit 

cosponsored by AIAA, ASME, SAE, and ASEE 

Indianapolis, Indiana, July 7-10,2002 

Prepared under Contract NAS3-00145 

National Aeronautics and 

Space Administration 

a 

Glenn Research Center 

. 

August 2003 
Available from 

I NASA Center for Aerospace Information 

7121 Standard Drive 

, Mover, MD 21076 

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Available electronically at he:/ /~ltrs.prc.nasa.gov 
Comparison of On-Orbit and Ground Based Hollow Cathode Operation 

Christian Carpenter 

QSS Group, Inc. 

Cleveland, Ohio 44 135 

ABSTRACT 

The Plasma Contactor Units (PCUs) were developed at NASA Glenn Research Center (GRC) and Boeing for 

charge control on board the International Space Station (ISS). Since the first ignition of a PCU on 10/16/2000 

over 3,900 hours of operation have been demonstrated on a single unit. In order to guarantee that the PCUs’ 

hollow cathode assemblies (HCAs), which emit the electrons used for charge control, would satisfy the life 

requirement of 18,000 hours, a ground based hollow cathode life test program was initiated at GRC. The life 

test program aimed at 27,000 hours of operation on a single unit to demonstrate the industry standard 1.5 

times operational life requirement. As of this printing, over 18,000 hours of operation have been accumulated 

on a single hollow cathode. By comparing the data received from the on-orbit HCAs to the data obtained for 

the life test cathodes, a comparison may be drawn to determine if the on-orbit HCAs are operating normally, 

with a final goal of predicting lifetime. Based on the data taken thus far, it can be concluded that the on-orbit 

HCAs are operating within their design specifications. 

INTRODUCTION 

The International Space Station (ISS) high voltage solar 

arrays deliver output voltages of 140 V to 160 V. The 

electrical configuration of the ISS and the plasma 

current balance may cause the station to float at 

voltages predicted to be as much as 120 V below the 

ambient space plasma if no charge control device is 

used. If large negative floating potentials are present, 

the ISS could react with ambient space plasma. These 

reactions could include arcing and sputter erosion.”* 

The development of a plasma contactor for charge 

control aboard a space station began in 1992 with the 

United States’ Space Station Freedom plasma contactor 

project at the NASA Glenn Research Center (GRC). 

The information and research from this program was 

carried over into the design of the International Space 

Station (ISS) plasma contactors in 1994. Hollow 

cathodes were chosen for this application due to their 

efficient, variable, and rapid emission of high electron 

currents.’ 

The plasma contactor units (PCUs), shown in Figure 1, 

aboard the ISS use a hollow cathode assembly (HCA), 

shown in Figure 2, to generate electrons used for ISS 

charge control. To veri@ that the HCAs are capable of 

completing their designated mission, a life test was 

started at NASA Glenn Research Center (GRC) in 

1994.3 The goal of the test is to demonstrate 27,000 hrs 

of operation on a single HCA, which is 1.5 times the 

life requirement of the on-orbit HCAs. In addition to 

the life requirement, testing has been performed on the 

effects of ignition on hollow cathode performance. The 

data taken ftom this test should provide insight into the 

operation of the on-orbit HCAs and aid in problem 

diagnosis as well as life estimation. 

The first two plasma contactor units (PCUs) were 

delivered aboard the shuttle Discovery in October of 

2000 by the crew of STS-92. Two PCUs are used to 

ensure full redundancy, and they are co-located on the 

Z1 truss, shown in Figure 3, of the ISS in the area 

marked on Figure 4. The first ignition of a PCU aboard 

the ISS occurred on October 16,2000. 

PLASMA CONTACTOR REOUIREMENTS 

Three main requirements were instituted for the design 

of the ISS PCUs. First, each PCU must control the ISS 

structure floating potential at all points on the station to 

k40 V of the space plasma potential. The effect of v x B 

charging as the ISS moves through the Earth’s magnetic 

field tightens this requirement to 320 V.4*5 Next each 

HCA is restricted to emit a maximum electron current 

of 10 A at no more than 20 V. Finally, the PCU must be 

able to operate for no less than 18,000 hours, which is 

based on the amount of propellant in the PCU. 

In order to extend the operational life of the PCU an 

additional mode of operation was suggested, in which 

the PCU operates only during periods of active ISS 

charging. In order to ensure operation in this mode, the 

PCU was also required to be capable of 6000 ignitions 

with no less than 99% reliabilit~.~ 

NASNCR-2003-2 1 18 17 1 
GROUND BASED TEST OPERATION 

The ground based testing of flight type HCAs included 

testing four hollow cathodes. These cathodes are herein 

called life test cathodes and labeled HCA003, HCAOO6, 

HCAO10, and HCA013. The cathodes were tested in a 

configuration designed to simulate the conditions 

predicted to exist on the ISS.3 The test began July 7, 

1994 with HCA006. HCA006 was removed in 1996 for 

destructive analysis after 8000 hours of operation? 

Since the beginning of the ground based life test, 

18,873 hours have been demonstrated on HCAO13. 

HCA010 has operated for 15,876 hours and has 

demonstrated 4,424 ignitions. There have been no 

failures of the cathodes under test. HCA013 has 

successfully exceeded the lifetime requirement, and 

will be tested until the qualification requirement of 

27,000 hours is reached. The current status of the life 

test cathodes can be seen in Table I. The life test 

cathodes operate with 40 minutes of 3.0 A electron 

current to the anode. This mode of operation is herein 

called “Diode Mode”. This operation is followed by 50 

minutes of 2.5 A electron current to a close proximity 

bias electrode. This mode of operation is herein called 

‘Triode Mode”. The cyclic operation is designed to 

mimic the 90-minute orbit of the ISS, which consists of 

40 minutes of shade, at which time the arrays are not 

collecting current (simulated by diode mode), followed 

by 50 minutes of sunlight, during which time the solar 

arrays are collecting current (simulated by triode 

made). 

ON-ORBIT OPERATION 

The fist two PCUs (herein called flight cathodes and 

labeled PCUl and PCU2) are currently still in operation 

aboard the space station. Table 11 shows the current 

status of each flight cathode. Though originally 

designed for continuous operation, these units operate 

only during extra-vehicular activities (EVA), shuttle 

docking, or as deemed necessary. Typical discharge 

currents and anode voltages for these units are shown in 

Figures 5 and 6. Discharge current, which is the return 

current form the space plasma, ranges fkom 0.02 A to 

0.05 A with voltages ranging fkom 12.5 V to 15 V. 

Figure 5 shows that the discharge currents on orbit are 

substantially lower than the life test triode mode 

emission current of 2.5 A, this is due to the fact that all 

of the ISS solar arrays have not all been installed, and 

therefore the ISS is not collecting as much current as 

previously predicted. 

DATA ANALYSIS METHOD 

Due to the large amount of data taken during the life 

test and the ISS, a program was needed that could 

handle formatting, searching, and plotting millions of 

data points. A database program was chosen as the 

analysis platform, as databases can handle large 

amounts of records and can be integrated with other 

programs. 

Life test data are taken in increments of one to five 

minutes, which allows the entire test to be analyzed 

simultaneously. Currently the test data are split, based 

on the year of testing, but plans exist to create a 

database containing all life test data. Data are imported 

and formatted fkom comma-separated files generated by 

the test, after which the files are easily analyzed. Plots 

fkom this test are created by generating tables from the 

data by selecting records at regular intervals to create 

plots that do not exceed a maximum of 10,000 data 

points. 

ISS data are taken in onesecond increments. Thus, 

when downloaded from the ISS archive, the files are 

substantially larger than those of the life test. A typical 

ISS flight can create over 2 million data points. Due to 

the file size, the only practical way to analyze large 

amounts of ISS data is on a per flight basis. Data fiom 

each ISS flight are imported into a database from fixed 

width files. The imported table is then formatted into 

dates, times, or numbers. Mer formatting, data holes 

are filled by inserting the previous value of a parameter 

into the hole, creating a complete data set. This method 

was chosen because the ISS appears to create entries 

only when values change and complete data sets yield 

fewer errors when analyzed by the computer. Plots of 

this data are generated in the same method as life test 

plots. Cathode operating time is estimated by the 

database by testing for the “PCU on” condition and 

subtracting start hm end times of this condition. 

COMPARISON OF ON-ORBIT TO GROUND 

BASED OPERATION 

Ignition times and anode voltages have been shown to 

be a good indicator of cathode health. In order to 

diagnose the health of the flight cathodes, their 

performance was compared to the performance of life 

test cathodes. 

Ignition times for both life test and flight cathodes are 

shown in Table III. The listed ignition times begin with 

application of heater power. Ignition is completed when 

the anode voltage is less than the open circuit voltage. 

Life test cathode ignition times, plotted in Figure 7, 

show a span form 211 seconds to 400 seconds for the 

majority of the ignitions. HCAOlO has long ignition 

times during the first few ignitions, but this may be 

related experimentation with ignition procedures. 

Geometric changes, barium depletion, and 

NASAKR-2003-2 1 18 1 7 2 
contamination, as well as other factors, can all cause 

increases in ignition time. Evolution of barium 

tungstate from long duration operation appears to be the 

main cause of ignition time increases.’ For life test 

cathodes, there does appear to be a trend of increasing 

ignition time with the number of ignitions. 

The flight cathode ignition times span 210 seconds to 

353 seconds. There is a slight rising trend, seen in 

Figure 8, which is consistent with the life test plot in 

Figure 7. After ignition 10, both life test and flight 

cathode ignition times fall in the 210 s to 350 s range, 

and there were no incidents of abnormally long ignition 

times. 

As shown in Table 11, flight cathode PCUl has operated 

for 2742 hours. Based on this, a comparison between 

initial operation and operation at 3,000 hours was 

initiated. In order to make legible plots, only data for 

HCA006 and PCU2 are shown. Several long duration 

wear tests have shown that anode voltages begin to rise 

with operation time and oscillate after 500 to 1000 

hours of operation?-” Several mechanisms have been 

speculated as the cause of these changes such as: 

geometric changes in the cathode’ (due to deposition, 

erosion, or sputtering), pressure changes, and 

thermochemistrf‘12. To verify that no geometric 

changes are affecting the anode voltages of this test, the 

life test cathodes were inspected to verify their 

dimensions. The inspection consisted of using a pin 

gauge to manually inspect each cathode. Although the 

life test cathodes have accumulated thousands of hours 

of operation, no change was seen to the cathode orifice 

or keeper orifice as has been reported in similar tests.” 

As shown in Figure 9 and 10, data fkom the first 

operation of life test and flight cathodes show a 

distribution in anode voltage between 1 1.5 V and 17 V 

for life test cathodes and 12.2 V to 16.2 V for the flight 

cathodes. The peaks shown in Figure 9 correspond to 

cathode operation in diode mode, which simulates the 

ISS shade condition, while the valleys correspond to 

operation in triode mode, which simulates the sunlight 

condition. Peaks and valleys for the flight cathode 

anode voltages correspond to the position of the ISS. 

The data fiom first operation verify that the anode 

voltages of the flight cathodes are on the same order as 

the life test cathodes during the beginning of cathode 

life. 

. Figure 1 1 and 12 show anode voltage plots fbr life test 

HCA006 and flight cathode PCU2 operating at 

approximately 3,000 hours. Anode voltages range from 

11.5 V to 14 V for the life test cathode and 12.8 V to 

13.5 V for the flight cathode. Like the beginning of life 

data, the flight cathodes do not show peaks and valleys 

as seen in the life test data. The data from 3,000 

hours show that, while the patterns appear different, 

the anode voltage ranges are similar. Sinceboth 

of the flight cathodes and the life test cathodes are 

operating with similar maximum and minimum voltages, it should be possible to use data taken fkom 

the ground based life test to simulate operationof 

flight cathodes. 

CONCLUSIONS 

After comparing the ignition times and anode voltages 

of the ISS HCAs to the HCAs used in the ground based 

life test at GRC, it can be concluded that all cathodes 

are operating within design specifications and 

demonstrate similar anode voltages and ignition times. 

Anode voltages are in the same range at the beginning 

of life and after 3,000 hours. Ground tests of HCAs 

operating for periods of 18,000 hours are continuing to 

operate without incident. These results indicate that the 

ISS HCAs should continue to exhibit stable operation 

and ignite reliably. 

REFERENCES 

1. Patterson, M.J., et. al., “Plasma Contactor for 

Space Station Freedom,” AIAA Paper No. 93-2228, 

June 1993. 

2. Patterson, M.J., et. al., “Plasma Contactor 

Development for Space Station,” IEPC Paper No. 93- 

246, September 1993. 

Soulas, G.C., “Multiple Hollow Cathode Wear 

Testing for the Space Station,” AIAA Paper No. 94- 

3310, June 1994. 

4. Patterson, M.J., et. al., “Space Station Cathode 

Design, Performance, and Operating Specifications,” 

NASA TM-1998-206529, May, 1998. 

5. Kovaleski, S.D., and Patterson, M.J., 

“Correlation of Hollow Cathode Assembly Plasma 

Contactor Data from Ground Testing and In-Space 

operation on the International Space Station,” IEPC 

Paper No. 252, October, 2001. 

6. Soulas, G.C., and Sarver-Verhey, T.R, 

“International Space Station Cathode Life Test,” IEPC 

Paper No. 166, August, 1997. 

7. Verhey, T.R and MacRae, G.S., 

“Requirements for Long-Life Operation of Inert Gas 

Hollow Cathode - Preliminary Results,” AIAA Paper 

No. 2586, July 1990. 

8. Sarver-Verhey, T.R “28,000 Hour Xenon 

Hollow Cathode Life Test Results,” NASA CR 97- 

206231, November 1997. 

9. Zakany, J. and Pinero, L., “Space Station 

Cathode Ignition Test Status at 32,000 Cycles,” IEPC 

Paper No. 167, August, 1997. 

3. 

NASA/CR-2~3-211817 3 
10. Sarver-Verhey, T.R., “Continuing Life Test of 12. Kovaleski, S.D., “Life Model of Hollow 

a Xenon Hollow Cathode for a Space Station Plasma Cathodes Using a Barium Calcium Aluminate 

Contactor,” NASA CR 94- 19540 1, November, 1994. Impregnated Tungsten Emitter,” IEPC Paper No. 276, 

1 1. Brophy, J.R. and Garner, C.E., “Tests of High October, 2001. 

Current Hollow Cathodes for Ion Engines,” AIAA 

Paper No. 88-2913, July, 1988. 

HCA003 

HCA006 

Table I - Status of Life Test Cathodes 

Hours Ignitions 

12414.6 38 Under Test 

8029.8 Disassembled 

Unit I Accumulated I Accumulated I Status 

HCAO 1 0 

HCAO 1 3 

for Analysis 

15876.1 4424 Under Test 

18873.2 59 Under Test 

Table II - Status of Flight Cathodes 

Ignitions 

PCU2 3956 1 25 I 

4 
Ignition 

Number 

Ignition Times, sec 

PCUl I PCU2 IHCA003 I HCAOlO I HCAO13 

3 I 263 I 211 I 421 I 275 I 317 

4 I 215 I 210 I 421 I 398 I 250 

1 

2 

I 5 I 210 I 270 I 245 I 520 I 276 I 

210 211 855 

224 211 422 340 

6 I 220 I 211 I 260 [ 460 1 313 

7 

8 

9 

10 

11 

12 

243 241 421 517 273 

227 257 223 406 276 

305 251 256 3 54 264 

265 233 255 225 280 

242 247 274 258 273 

246 250 230 299 252 

13 1 256 I 347 I 303 I 290 I 268 

20 

21 

22 

23 

24 

25 

320 239 

277 267 

291 260 270 348 211 

318 252 253 348 240 

230 265 249 306 239 

226 256 246 295 263 

234 212 260 279 24 1 

252 270 281 295 276 

19 I 243 I 280 1 277 I 306 I 227 

30 I I 709 I 311 I 234 

1 i% 1 293 1 1 283 1 269 1 iiz 1 

266 3 02 292 

290 270 274 306 

29 224 26 1 358 248 

NASNCR-2003-211817 5 
Figure 1 - Plasma Contactor Unit 

. 

Figure 2 ~ Flight Hollow Cathode Assemblies 

NASNCR-2003-2 1 1 8 17 6 
. 

NASNCR-2003-2 

Figure 3 ~ Plasma Contactors on the Z 1 Truss 

- 

Figure 4 ~ Plasma Contactor Location on the ISS 

1817 7 
iSS313F 

Discharge Cwent - PCU 1 . Discharge Cwent - PCU 2 

Cwent 

005 7 I I 

Noise 

V iomm .. *... ................... 

........ *...*.. .... b 

2 002 4 

.. .~....................... ............................. 

............................. 0 01 : ............................. 

.................. 11 ,.* .....i.....*...l........ 

b..., ......... .....*.. b.... ......,.. ........ *...*..,................ ....................... I1 

Pcus Opcratulg 

Daie. dd&yyyy 

Figure 5 - PCU Discharge Current vs. Day for ISS Flight 3 13F 

lSS313F 

b Anode Voltam - FCU 1 . Anode Voltage - PCU 2 I 

I 

PCUs I Operaw 

1 

Voltage Offset 51 

Figure 6 - PCU Anode Voltage vs. Day for ISS Flight 313F 

NASNCR-2003-2 1 18 17 8 
-4-HCA003 -eHCAOlO -t HCA013 

900 

7M 

400 

300 

200 

0 

? 

A \ 

1 

S 10 1s 20 2s 30 

I@utmnNumber 

Figure 7 - Life Test Hollow Cathode Assembly Ignition Times 

I &PCUl +PCU2 1 

0 S 10 IS 20 

Ignition Number 

Figure 8 - Flight Hollow Cathode Assembly Ignition Times 

NASNCR-2003-2118 17 9 

2s 30 
17 

16 - 

15 - 

z 2 

: 

P 

- 14- 

13 - 

12 -- 

11 I 

Tnode Mode - c 

3 36 PM 448 PM 6WPM 712PM 824PM 936PM 10 48 PM l2WAM 2 24 PM 

Tune 

Figure 9 - HCA006 Anode Voltage During Initial Operation on 07/08/1994 

17 

16 

15 

> 

3 

2 14 

s 

P 

0 u 

4 

13 

12 

I1 

I 

I 

I- T 

p. 

2 24 PM 3 36 PM 448 PM 6 00 PM 712PM 824PM 9 36 PM 1048PM 1200AM 

Tune 

Figure 10 - PCU2 Anode Voltage During Initial Operation on 11/17/2000 

NASNCR-2003-2 1 1 8 17 10 
14 

13 

:- 

m 

.A 

3 

12 

11 

r 

I' \ 

c 

k 

1 .'dodeMode 

Diode Mode i 

14 

13 

e 3 

P 

- 

3 

d 

12 

1200AM 214AM 448AM 712AM 936AM l200PM 224PM 448PM 712PM 936PM l2mAM 

TJIW 

Figure 11 - HCA006 Anode Voltage After Approximately 3,000 Hours of Operation 

I1 

6 2, n 

12mAM 224AM 448AM 7:12AM 936AM l2aOPM 234PM 4:48PM 7:12PM 936PM 12aOAM 

Time 

Figure 12 - PCU2 Anode Voltage After Approximately 3,000 Hours of Operation 

NASA/CR-203-2 1 1 8 17 11 
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Comparison of On-Orbit and Ground Based Hollow Cathode Operation 

6. AUTHOR(S) 

Christian Carpenter 

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5. FUNDING NUMBERS 

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11. SUPPLEMENTARY NOTES 

Prepared for the 38th Joint Propulsion Conference and Exhibit cosponsored by AIAA, ASME, SAE, and ASEE, 

Indianapolis, Indiana, July 7-10,2002. Project Manager, Michael Patterson, Power and On-Board Propulsion Technology 

Division, NASA Glenn Research Center, organization code 5430,2 16977-748 1. 

14. SUBJECT TERMS 

Electron plasma; Hollow cathodes; Space plasmas; International Space Station 

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

The Plasma Contactor Units (PCUs) were developed at NASA Glenn Research Center (GRC) and Boeing for charge 

control on board the International Space Station 0.9s). Since the first ignition of a PCU on 10/16/2000 over 3,900 hours 

of operation have been demonstrated on a single unit. In order to guarantee that the PCUs' hollow cathode assemblies 

(HCAs), which emit the electrons used for charge control, would satisfy the life requirement of 18,000 hours, a ground 

based hollow cathode life test program was initiated at GRC. The life test program aimed at 27,000 hours of operation on 

a single unit to demonstrate the industry standard 1.5 times operational life requirement. As of this printing, over 18,000 

hours of operation have been accumulated on a single hollow cathode. By comparing the data received from the on-orbit 

HCAs to the data obtained for the life test cathodes, a comparison may be drawn to determine if the on-orbit HCAs are 

operating normally, with a final goal of predicting lifetime. Based on the data taken thus far, it can be concluded that the 

on-orbit HCAs are operating within their design specifications. 

13. ABSTRACT (Maximum 200 words) 

I I I 

NSN 7540-01 -280-5500 Standard Form 298 (Rev. 2-89) 

Pnscrlbed by ANSI SM. 239-18 

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