Hani Kamhawi, John T. Yim, Michael J. Patterson, and Penni J. Dalton

Glenn Research Center, Cleveland, Ohio

Operational Status of the International Space Station 

Plasma Contactor Hollow Cathode Assemblies From

July 2011 to May 2013

NASA/TM—2014-216579

April 2014

AIAA–2013–4114
NASA STI Program . . . in Profi le

Since its founding, NASA has been dedicated to the 

advancement of aeronautics and space science. The 

NASA Scientifi c and Technical Information (STI) 

program plays a key part in helping NASA maintain 

this important role.

The NASA STI Program operates under the auspices 

of the Agency Chief Information Offi cer. It collects, 

organizes, provides for archiving, and disseminates 

NASA’s STI. The NASA STI program provides access 

to the NASA Aeronautics and Space Database and 

its public interface, the NASA Technical Reports 

Server, thus providing one of the largest collections 

of aeronautical and space science STI in the world. 

Results are published in both non-NASA channels 

and by NASA in the NASA STI Report Series, which 

includes the following report types:

• TECHNICAL PUBLICATION. Reports of 

completed research or a major signifi cant phase 

of research that present the results of NASA 

programs and include extensive data or theoretical 

analysis. Includes compilations of signifi cant 

scientifi c and technical data and information 

deemed to be of continuing reference value. 

NASA counterpart of peer-reviewed formal 

professional papers but has less stringent 

limitations on manuscript length and extent of 

graphic presentations.

• TECHNICAL MEMORANDUM. Scientifi c 

and technical fi ndings that are preliminary or 

of specialized interest, e.g., quick release 

reports, working papers, and bibliographies that 

contain minimal annotation. Does not contain 

extensive analysis.

• CONTRACTOR REPORT. Scientifi c and 

technical fi ndings by NASA-sponsored 

contractors and grantees.

• CONFERENCE PUBLICATION. Collected 

papers from scientifi c and technical 

conferences, symposia, seminars, or other 

meetings sponsored or cosponsored by NASA.

• SPECIAL PUBLICATION. Scientifi c, 

technical, or historical information from 

NASA programs, projects, and missions, often 

concerned with subjects having substantial 

public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientifi c and 

technical material pertinent to NASA’s mission.

Specialized services also include creating custom 

thesauri, building customized databases, organizing 

and publishing research results.

For more information about the NASA STI 

program, see the following:

• Access the NASA STI program home page at 

http://www.sti.nasa.gov

• E-mail your question to help@sti.nasa.gov

• Fax your question to the NASA STI 

Information Desk at 443–757–5803

• Phone the NASA STI Information Desk at

 443–757–5802

• Write to:

 STI Information Desk

NASA Center for AeroSpace Information

 7115 Standard Drive

 Hanover, MD 21076–1320
Hani Kamhawi, John T. Yim, Michael J. Patterson, and Penni J. Dalton

Glenn Research Center, Cleveland, Ohio

Operational Status of the International Space Station 

Plasma Contactor Hollow Cathode Assemblies From

July 2011 to May 2013

NASA/TM—2014-216579

April 2014

AIAA–2013–4114

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

49th Joint Propulsion Conference and Exhibit

cosponsored by AIAA, ASME, SAE, and ASEE

San Jose, California, July 14–17, 2013
Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2014-216579 1 

Operational Status of the International Space Station Plasma 

Contactor Hollow Cathode Assemblies From 

July 2011 to May 2013 

Hani Kamhawi, John T. Yim, Michael J. Patterson, and Penni J. Dalton 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The International Space Station has onboard two Aerojet Rocketdyne developed plasma contactor 

units that perform the function of charge control. The plasma contactor units contain NASA Glenn 

Research Center developed hollow cathode assemblies. NASA Glenn Research Center monitors the onorbit operation of the flight hollow cathode assemblies. As of May 31, 2013, HCA.001-F has been ignited 

and operated 123 times and has accumulated 8072 hours of operation, whereas, HCA.003-F has been 

ignited and operated 112 times and has accumulated 9664 hours of operation. Monitored hollow cathode 

ignition times and anode voltage magnitudes indicate that they continue to operate nominally. 

Introduction 

The International Space Station (ISS) power system is designed with high voltage solar arrays 

operating at output voltages of typically 140 to 160 V (Ref. 1). The power system is configured with a 

“negative ground” that electrically ties the habitat modules, structure, and radiators to the negative tap of 

the solar arrays. As a result of the electrical configuration and the plasma current balance, early analyses 

indicated that the habitat modules, structure, and radiators may float to voltages as high as 120 V with 

respect to the ambient space plasma (Ref. 1). Furthermore, given the large negative floating potentials, 

there exist the probability for deleterious interactions of the ISS and the space plasma. In 1992, a decision 

was made to baseline a plasma contactor on the ISS, from a variety of solutions, as a means to mitigate 

these interactions (Ref. 2). A plasma contactor establishes a low impedance plasma bridge to control the 

spacecraft potential relative to the local space plasma. Figure 1 illustrates the current path enabled through 

a plasma contactor. The plasma contactor unit (PCU) consists of a hollow cathode assembly (HCA), 

power electronics unit (PEU), and gas feed system (GFS). The entire unit is contained within an avionics 

box referenced as the orbit replaceable unit (ORU) (Ref. 1). The HCA within each PCU generates the 

electrons required for ISS charge control, and is electrically tied to the negative single point ground of the 

ISS main truss segment. The HCA consists of a low-work function insert for electron emission, a body 

tube with a heater, and an electrical isolator. During HCA operation, a continuous xenon flow of 6.0 to 

7.5 sccm is maintained, a 3.0 A current to the anode is emitted, and the HCA emits to the space plasma 

for space charge control. 

NASA Glenn Research Center (GRC) developed and delivered the flight HCAs, shown in Figure 2, 

for integration into the PCUs. The NASA GRC HCA development program included manufacture, test, 

and installation of engineering, qualification, and flight model HCAs according to the program 

requirements (Refs. 3 and 4). This included long duration testing of the devices to meet the 18,000 

operational hours requirement. A 28,000+ hours operation test (Ref. 5) and a 19,000+ hours operation test 

(Ref. 6) were among the tests performed in support of this goal. Table 1 presents the final status of the life 

tested hollow cathode assemblies where all life tests were terminated voluntarily. In addition, a high 

fidelity development model HCA attained 32,000 successful ignition cycles, demonstrating the ISS 

requirement of 6,000 ignition cycles with at least 99 percent reliability (Ref. 7). HCA heater testing was 

also conducted to determine the cyclic life capability of the heater, which was found to be around 
NASA/TM—2014-216579 2 

12,000 cycles (Ref. 8). Finally, prior to installing the HCAs in their PCUs, the flight HCAs underwent 

multi-step performance acceptance tests at NASA GRC (Refs. 1, 3, 4, 6, 7, and 8). The HCAs were 

subsequently integrated into the PCU ORUs at the flight contractor site where the PCU ORUs underwent 

acceptance-level environmental testing. Afterwards, the PCU ORUs were shipped to NASA GRC where 

they underwent further testing to verify the operability of the HCA and the PCU to assess the plasma 

clamp voltage. These tests established that the HCA and PCU were fully functional and capable of 

emitting the required electron current within the 20 V potential requirement. The current emitted by the 

HCAs to the space plasma, and which is collected by the ISS solar arrays, is referred to as the clamping 

current. The PCU ORUs were subsequently shipped to NASA Kennedy Space Center (KSC) for 

integration to the ISS Z1 structure and launch. 

Figure 2.—Flight hollow cathode assemblies. 

TABLE 1.—SUMMARY OF ACCUMULATED HOURS 

AND NUMBER OF IGNITIONS FOR LIFE-TESTED 

HOLLOW CATHODES 

Unit Accumulated 

hours 

Accumulated 

ignitions 

HCA-003a 12,415 38 

HCA-006a 8,030 1 

HCA-010a 15,876 4424 

HCA-013a 18,873 59 a

Test terminated voluntarily.

+

- PEU 

Anode 

Supply

+

-

HCA

Solar

Arrays

ISS Structure

Space + Plasma

- PEU 

Anode 

Supply

+

-

HCA

Solar

Arrays

ISS Structure

Space 

Plasma

Figure 1.—ISS simplified plasma contactor unit 

electrical schematic. 
NASA/TM—2014-216579 3 

Figure 3.—Location of plasma contactor on the Z1 truss. 

Figure 4.—Photograph of PCU flight model 3 inside VF5 at NASA GRC. 

There were two initial flight models, PCU FM01 and PCU FM02, which incorporated the HCAs 

designated HCA.001-F and HCA.003-F, respectively. These PCUs, shown in Figure 3, were developed 

by the Rocketdyne Division, part of the Boeing Company at that time. They were delivered to the ISS 

aboard the Space Shuttle Discovery in October of 2000. These two PCUs are co-located on the ISS Z1 

truss as shown in Figure 3. 

For a third flight model, PCU FM03, NASA GRC personnel installed HCA.006-F into the PCU at 

Aerojet Rocketdyne’s Canoga Park facility in April 2008. A series of tests were performed at NASA 

GRC vacuum facility 5 (VF5) on PCU FM03, shown in Figure 4, in the summer of 2008. The tests 

included fifteen ignition cycle tests, a clamp voltage test, and a 24 hr test to confirm HCA stable 

operation (Ref. 9). PCU FM03 was launched on STS-129, on November 16, 2009, on the External 

Logistics Carrier 1 (ELC1). The unit’s latch valves were closed prior to launch, at L-85 hr. On 

November 18, 2009 the unit was moved from the Space Shuttle cargo bay to the ISS as is shown in 

Figure 5. Currently, ELC1 is stored on the P3 (port) truss of the ISS. PCU FM03 will not be activated 

until 2028 when PCU FM02 is expected to run out of xenon expellant. 
NASA/TM—2014-216579 4 

Figure 5.—Relocation of ELC1 from Shuttle 

cargo bay to ISS. 

On-Orbit Operation of PCU FM01 and PCU FM02 

The first two flight HCA units installed aboard the ISS, HCA.001-F in PCU FM01 and HCA.003-F in 

PCU FM02, are currently in operation. On October 16, 2000, the first ignition of a PCU aboard the ISS 

took place. The PCUs now typically operate during extra-vehicular activities (EVAs), shuttle docking, or 

as deemed necessary. During spacewalks, the two PCUs are operated simultaneously for redundancy.

The on-orbit status and health of the HCAs are assessed by monitoring the ignition time and anode 

voltage. The ignition time is defined as the time between the initiation of the cathode warm-up procedure 

(i.e., current flow to heater) to the detection of anode current (current between the cathode and keeper 

plate) indicating plasma formation between the cathode and anode. Increases in the ignition time and/or 

anode voltage may indicate hollow cathode performance degradation. This degradation can occur due to 

changes in the emitter surface chemistry, depletion of the emitter impregnate material, heater degradation, 

and physical changes to the cathode/keeper orifice plate configurations (Refs. 5 to 7). The data from 

operating the HCAs in the time period from July 1, 2011 to May 31, 2013 will be presented in the next 

two sections. HCAs operational status results from ground-based tests and aboard the ISS prior to July 1, 

2011 have been presented earlier (Refs. 9 to 15). 

Prior to the Space Shuttle fleet retirement in July of 2011, the PCUs were operated for extended 

durations (100 to 200 hr) when the Space Shuttle was docked to the ISS and there were plans for multiple 

spacewalks during the mission. However, starting with the last Space Shuttle mission (Atlantis STS-135), 

the PCUs were only operated for durations of 10 to 15 hr each during spacewalks. Table 2 in Appendix 1 

lists the spacewalks that took place since July 1, 2011. 

Hollow Cathode Assembly HCA.001-F on PCU FM01 

Operational details of HCA.001-F are presented in Table 3. During the time period from July 1, 2011 

to May 31, 2013, HCA.001-F was ignited 8 times (segments 116-123) and operated for a total duration of 

106.8 hr. For the flight HCAs, a “segment” is defined as the period of time between HCA ignition and 

until it is commanded off and the emission current reaches zero. At the end of segment 123, the total 

accumulated operating time was 8072 hr. HCA.001-F ignition times for segments 116-123 were between 

3.6 and 4.7 min as presented in Figure 6 and Table 2. These ignition times are consistent with previous 

segments ignition times. Figures 7, 9, and 11 present the anode voltage profiles during segments 116, 120, 

and 123, respectively. Figures 8, 10, and 12 present the clamping current profiles during segments 116, 
NASA/TM—2014-216579 5 

120, and 123, respectively. The profiles shown in Figures 7 to 12 are representative of typical HCA.001-F 

operation (Refs. 9 to 15). Results in Table 3 show that the anode voltage range is approximately 13.4 to 

15.7 V; the higher voltage occurs after HCA ignition (typical after ignition) and drops from that point to 

the nominal operating value of 13 to 14 V (Ref. 6). Table 3 shows that the largest operating clamping 

current is 0.176 A; clamping current readings below 0.018A are considered noise. 

TABLE 3.—HCA.001-F OPERATION DETAILS FOR PERIOD FROM JULY 1, 2011 TO MAY 31, 2013 

Ignition/operating 

segment number 

Operation start date Duration,

hr 

Ignition 

time, 

min 

Accumulated 

operating hours,

hr 

Anode 

voltage 

range, 

V 

Clamping 

current 

range, 

A 

116 July 12, 2011 16 3.8 7981.2 13.7 to 14.8 0 to 0.129 

117 August 3, 2011 13.1 4.7 7994.3 13.6 to 15.5 0 to 0.080 

118 February 16, 2012 13.2 4.0 8007.5 13.7 to 15.6 0 to 0.076 

119 August 20, 2012 12.8 4.1 8020.3 13.5 to 15.7 0 to 0.176 

120 August 30, 2012 11.8 3.8 8032.1 13.4 to 14.7 0 to 0.117 

121 September 5, 2012 11.8 3.8 8043.9 13.4 to 14.6 0 to 0.135 

122 November 1, 2012 13.9 3.6 8057.8 13.5 to 15.6 0 to 0.064 

123 April 19, 2013 14.2 3.8 8072.0 13.4 to 15.7 0 to 0.111 

Figure 6.—Ignition times for all ISS flight data segments. 

 

0

2

4

6

8

10

12

0 20 40 60 80 100 120

Ignition Time, min

Segment #

HCA.001-F Ground

HCA.001-F

HCA.003-F Ground

HCA.003-F

July1,2011

July1,2011
NASA/TM—2014-216579 6 

Figure 7.—HCA.001-F anode voltage profile 

during segment 116. 

Figure 8.—HCA.001-F clamping current profile 

during segment 116. 

Figure 9.—HCA.001-F anode voltage profile 

during segment 120. 

Figure 10.—HCA.001-F clamping current profile 

during segment 120. 

Figure 11.—HCA.001-F anode voltage profile 

during segment 123. 

Figure 12.—HCA.001-F clamping current profile 

during segment 123. 
NASA/TM—2014-216579 7 

Hollow Cathode Assembly HCA.003-F on PCU FM02 

Operational details of HCA.003-F are presented in Table 4. During the time period from July 1, 2011 

to May 31, 2013, HCA.003-F was ignited 8 times (segments 105-112) and operated for a total duration of 

108.9 hr. At the end of segment 112, the total accumulated operating time was 9664.4 hr. HCA.003-F 

ignition times for segments 105-112 were between 3.6 and 4.5 min. These ignition times are consistent 

with previous segments ignition times. Figures 13, 15, and 17 present the anode voltage profiles during 

segments 105, 109, and 112, respectively. Figures 14, 16, and 18 present the clamping current profiles 

during segments 105, 109, and 112, respectively. The profiles shown in Figures 13 to 18 are 

representative of typical HCA.003-F operation (Refs. 9 to 15). Results in Table 4 show that the anode 

voltage range is approximately 13.1 to 16.4 V; the higher voltage occurs after HCA ignition (typical 

behavior) and drops from that point to the nominal operating value of 12 to 13 V (Ref. 6). Table 4 shows 

that the largest operating clamping current was 0.205 A. For the clamping current, readings below 0.018A 

are considered noise. 

Figure 13.—HCA.003-F anode voltage profile during 

segment 105.

Figure 14.—HCA.003-F clamping current profile 

during segment 105.

 

Figure 15.—HCA.003-F anode voltage profile during 

segment 109. 

Figure 16.—HCA.003-F clamping current profile during 

segment 109.
NASA/TM—2014-216579 8 

Figure 17.—HCA.001-F anode voltage profile during 

segment 112. 

Figure 18.—HCA.001-F clamping current profile during 

segment 112. 

TABLE 4.—HCA.003-F OPERATION DETAILS FOR PERIOD FROM JULY 1, 2011 TO MAY 31, 2013 

Ignition/Operating 

segment number 

Operation start date Duration,

hr 

Ignition 

time, 

min 

Accumulated 

operating hours,

hr 

Anode voltage 

range, 

V 

Clamping 

current range,

A 

105 July 12, 2011 16.2 4.0 9571.7 13.8 to 14.9 0 to 0.130 

106 August 3, 2011 13.4 3.9 9585.1 13.3 to 14.6 0 to 0.115 

107 February 16, 2012 13.0 3.8 9598.1 13.3 to 14.6 0 to 0.082 

108 August 20, 2012 12.3 3.6 9610.4 13.3 to 16.4 0 to 0.205 

109 August 30, 2012 12.8 3.7 9623.2 13.4 to 14.7 0 to 0.170 

110 September 5, 2012 12.1 3.8 9635.3 13.6 to 15.1 0 to 0.176 

111 November 1, 2012 14.1 4.2 9649.4 13.5 to 15.7 0 to 0.105 

112 April 19, 2013 15.0 4.5 9664.4 13.1 to 15.3 0 to 0.135 

Conclusion 

NASA GRC continues to play a critical role in supporting the ISS PCUs through monitoring hollow 

cathode assembly operation aboard the ISS. As of May 31, 2013 HCA.001-F has been ignited 123 times 

and has accumulated 8072 hours of operation, whereas, HCA.003-F has been ignited 112 times and has 

accumulated 9664.4 hours of operation. During this time period, HCA.001-F and HCA.003-F were 

operated for a total duration of 106.8 and 108.9 hr, respectively. The monitored ignition time, anode 

voltage, and clamping current values during the period from July 1, 2011 to May 31, 2013 indicate that 

the hollow cathode assemblies continue to operate and function consistent with their respective groundbased tests and earlier operation on the ISS. 

References 

1. Patterson, M.J., et al., “Plasma Contactor Development for Space Station,” IEPC 93-246. 

2. Moorehead, R.W., Deputy Director, Space Station Freedom Program and Operations, 

communications to Work Packages 1-4 Directors, April 3, 1992. 

3. Patterson, M.J., et al., “Plasma Contactor Technology for Space Station Freedom,” AIAA 93-2228. 

4. Patterson, M.J, Verhey, T.R., Soulas, G.C., and Zakany, J., “Space Station Cathode Design, 

Performance, and Operating Specifications,” IEPC 97-170. 

5. Verhey, T.R., “28,000 Hour Xenon Hollow Cathode Life Test Results,” IEPC 97-168. 

6. Soulas, G.C., “Multiple Hollow Wear Testing for the Space Station,” AIAA 94-3310. 

7. Zakany, J., Pinero, L., “Space Station Cathode Ignition Test Status at 32,000 Cycles,” IEPC 97-167. 

8. Soulas, G.C., “Status of Hollow Cathode Heater Development for the Space Station Plasma 

Contactor,” AIAA 94-3309. 
NASA/TM—2014-216579 9 

9. Kamhawi, H., Patterson, M.J., and Dalton, P., “On the Operational Status of the International Space 

Station Plasma Contactor Hollow Cathodes,” AIAA 2009-5092. 

10. Carpenter, C.B., “Comparison of On-Orbit and Ground Based Hollow Cathode Operation,” 

AIAA 2002-4098. 

11. Carpenter, C.B., “Comparison of On-Orbit and Ground Based Hollow Cathode Operation,” 

AIAA 2003-5145. 

12. Carpenter, C.B., “On the Operational Status of the ISS Plasma Contactor Hollow Cathodes,” 

AIAA 2004-3425. 

13. Kamhawi, H., and Patterson, M.J., “On the Operational Status of the International Space Station 

Plasma Contactor Hollow Cathodes,” AIAA 2005-3676. 

14. Kamhawi, H., and Patterson, M.J., “On the Operational Status of the International Space Station 

Plasma Contactor Hollow Cathodes,” AIAA 2007-5190. 

15. Kamhawi, H., Patterson, M.J., and Dalton, P., “Update on the Operational Status of the International 

Space Station Plasma Contactor Hollow Cathodes,” AIAA 2011-5990. 
 
NASA/TM—2014-216579 11 

Appendix—Spacewalk List for July 2011 to May 2013 

TABLE 2.—LIST OF SPACEWALKS SINCE JULY 1, 2011 1 

Mission Date Spacewalk 

duration 

Description PCU 

operated 

Expedition 28 

EVA1 

July 12, 2011 6 hr 31 min Last spacewalk performed while the Space Shuttle was 

docked to the ISS. During spacewalk a failed pump 

module was retrieved and MISSE 8 experiment was 

also deployed. 

Yes 

Expedition 28 

EVA2 

August 3, 2011 6 hr 23 min Spacewalkers installed: laser communication 

equipment, a student experiment, and the BIORISK 

experiment. 

Yes 

Expedition 30 

EVA1 

February 16, 2012 6 hr 15 min Spacewalkers repositioned the Strela 1 equipment crane 

in preparation for the undocking of the Pirs docking 

compartment. In addition an experiment was retrieved 

and a new one installed. 

Yes 

Expedition 32 

EVA1 

August 20, 2012 5 hr 51 min Spacewalkers relocated the Pirs docking compartment 

to prepare for its undocking in preparation for the 

docking of the Russian Multipurpose Laboratory 

Module. Other tasks were also performed including 

deployment of small satellite to track space debris. 

Yes 

Expedition 32 

EVA2 

August 30, 2012 8 hr 17 min Spacewalkers replaced the Main Bus Switching Unit 

(MBSU) and they also rerouted some cables. 

Yes 

Expedition 32 

EVA3 

September 5, 2012 6 hr 28 min Spacewalkers installed the MBSU. Astronaut Sunita 

Williams broke Peggy Whitson’s record for most time 

spacewalking by a woman. 

Yes 

Expedition 33 

EVA1 

November 1, 2012 6 hr 38 min Spacewalkers reconfigured and isolated a leak in the 

ammonia cooling system. 

Yes 

Expedition 33 

EVA1 

April 19, 2013 6 hr 38 min Spacewalkers installed the Obstanovka plasma waves 

experiment, replaced a faulty retro-reflector, and 

retrieved the BIORISK experiment. 

Yes 

Expedition 35 

EVA1 

May 11, 2013 5 hr 30 min Spacewalkers inspected and replaced a pump flow 

control subassembly. 

No 

 

1

 http://en.wikipedia.org/wiki/List_of_ISS_spacewalks#2011