Scott D. Kovaleski

QSS Group, Inc., Cleveland, Ohio

Michael J. Patterson and George C. Soulas

Glenn Research Center, Cleveland, Ohio

Correlation of Hollow Cathode Assembly

and Plasma Contactor Data from Ground

Testing and In-Space Operation on the

International Space Station

NASA/TM—2001-211293

December 2001

IEPC–01–252
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NASA/TM—2001-211293

December 2001

National Aeronautics and

Space Administration

Glenn Research Center

Correlation of Hollow Cathode Assembly

and Plasma Contactor Data from Ground

Testing and In-Space Operation on the

International Space Station

Scott D. Kovaleski

QSS Group, Inc., Cleveland, Ohio

Michael J. Patterson and George C. Soulas

Glenn Research Center, Cleveland, Ohio

Prepared for the

27th International Electric Propulsion Conference

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

Pasadena, California, October 14–19, 2001

IEPC–01–252
Available from

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Available electronically at http://gltrs.grc.nasa.gov/GLTRS
NASA/TM2001-211293 1 

Correlation of Hollow Cathode Assembly and Plasma Contactor Data from Ground 

Testing and In-Space Operation on the International Space Station

Scott D. Kovaleski 

QSS Group, Inc. 

Cleveland, Ohio 44135 

216–433–3735 

scott.kovaleski@grc.nasa.gov 

Michael J. Patterson and George C. Soulas 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Charge control on the International Space Station (ISS) is currently being provided by two 

plasma contactor units (PCUs). The plasma contactor includes a hollow cathode assembly 

(HCA), power processing unit and Xe gas feed system. The hollow cathode assemblies in 

use in the ISS plasma contactors were designed and fabricated at the NASA Glenn 

Research Center. Pre-qualification testing of development HCAs as well as acceptance 

testing of the flight HCAs is presented. Integration of the HCAs into the Boeing North 

America built PCU and acceptance testing of the PCU are summarized in this paper. 

Finally, data from the two on-orbit PCUs is presented. 

Introduction 

Electron current collection by the high voltage solar 

arrays is expected to lead to charging of the 

International Space Station (ISS) habitat modules, 

structure and radiators. The high voltage solar arrays 

of the ISS power system are designed to operate at 

output voltages of typically 140 to 160 V. The 

negative tap of the ISS solar arrays is electrically tied 

to the habitat modules, structure, and radiators. With 

the solar arrays providing a large surface for electron 

current collection from the ambient space plasma, the 

ISS habitat modules, structure and radiators are 

predicted to float as much as 120 V negative with 

respect to the ambient space plasma potential [1]. If 

uncompensated, this large negative potential could 

lead to arcing through insulating surfaces on the ISS, 

ion bombardment resulting in sputter erosion of 

surfaces, as well as unsafe conditions for astronauts 

during extra-vehicular activities. 

To mitigate the effect of these large negative voltages, 

a plasma contactor unit (PCU), which includes a 

hollow cathode assembly (HCA) current source, has 

been developed to actively control space station 

charging [1]. The HCA was chosen for its ability to 

emit copious electron current at low voltages and 

under variable demand. The high degree of reliability 

and long life of HCAs also make them desirable for 

the ISS PCU application. 

HCA development at the NASA Glenn Research 

Center (GRC) has encompassed manufacture of 

engineering model, qualification model, and flight unit 

HCAs as well as extensive qualification and life 

testing of HCAs. The details of the HCA designed by 

GRC are given by Patterson [1]. The PCU HCA is a 

direct outgrowth of the ion thruster development 

program at GRC [2]. Two primary life tests of the 

HCA have been run. A single cathode life test reached 

28,000 hours of operation before the cathode failed to 

ignite [3]. A four cathode life test was performed with 

a mission-like emission current profile that 

approximated the predicted demand from the ISS. One 

of the four cathodes was voluntarily stopped for 

destructive analysis [4]; the other three have reached 

from 12,000 to 19,000 hours of cathode operation [5]. 
NASA/TM2001-211293 2 

Ignition testing of a flight-like cathode reached 42,000 

ignitions before heater failure [6]. The cathode heater 

reliability has been the subject of testing as well [5,7]. 

This paper summarizes the performance of the two 

PCUs currently aboard the ISS and the efforts at GRC 

to develop and manufacture HCAs for the ISS PCU. 

Description of the Plasma Contactor Unit 

The space station PCU is a self-contained system 

providing power processing and expellant flow to 

generate a low impedance plasma bridge for spacecraft 

charge control. A photo of the ISS PCU is shown in 

Figure 1. The power processor includes separate 

power supplies for the HCA heater, keeper anode, and 

cathode ignitor. The PCU gas feed system provides 

high purity xenon expellant to the HCA at a fixed flow 

rate. Electron current emission is performed by the 

HCA itself. 

The HCA design was chosen for its long life and 

effective, self-regulated operation under variable 

emission current demand. A drawing of the ISS HCA 

is shown in Figure 2. An enclosed keeper cathode was 

chosen since this configuration has been proven to 

efficiently provide a low voltage, electron current 

coupling to the ambient plasma. A small diameter 

orifice was employed on the downstream end of the 

cathode tube to increase internal cathode pressures for 

cathode operation, thereby reducing the expellant 

requirements for the PCU. A low work function, 

impregnated tungsten thermionic emitter insert was 

placed in the cathode tube at the downstream end to 

reduce cathode operating temperatures, to facilitate 

starting, and to produce the dense plasma in the 

cathode interior from which the HCA electron current 

is extracted. A cathode heater was included in the 

design to provide for fast, reliable HCA ignition and to 

evolve contaminants on the emitter surface after 

atmospheric exposure. The cathode assembly is 

fabricated largely from refractory metals and alloys for 

a rugged, durable assembly. Each of these 

components works together to yield a reliable and 

efficient electron source for the ISS PCU. 

Requirements for the International Space 

Station 

There are several requirements placed on an ISS 

plasma contactor system. An ISS plasma contactor 

system is required to control station charging to within 

+/- 40 V of ambient space plasma potential. The effect 

of v x B charging as the ISS moves through the earth’s 

magnetic field tightens the requirement to within 

+/- 20 V of ambient space plasma potential. 

Additionally, practical requirements include long life, 

reliable ignition, and effective operation after 

environmental exposure during manufacture and 

integration [1]. 

Clamping voltage and emission current 

The HCA must supply sufficient current to control the 

ISS potential with respect to the ambient space plasma 

potential. To meet this requirement, the HCA was 

restricted to an emission current of at least 10 A at less 

than 20 V. The peak emission current demand of the 

ISS is expected to be much less than 10 A [1]. 

HCA life and reliability 

The HCA is also required to provide durable and 

reliable charge control for the ISS. HCA lifetime must 

meet or exceed 18,000 hours, which is the expected 

life of the PCU system given the PCU xenon storage 

capacity. PCU lifetime can be optionally extended by 

operating the HCA only during periods of active ISS 

charging. In order to facilitate this option, the HCA 

was required to be capable of 6000 ignitions with at 

least 99 percent reliability. 

Hollow Cathode Assembly Integration into the 

Plasma Contactor Unit 

Integration of the HCA into the Boeing developed 

PCU is supported by GRC engineers and technicians. 

Figure 3 shows a photo of an HCA installed in the 

third PCU manufactured by Boeing. This PCU will 

serve as a spare for the two PCUs currently on the ISS. 

Installation of the HCA into the PCU is supported by 

GRC technicians and connection of the HCA to the 

PCU gas feed system is performed by GRC 

technicians. Any issues which may arise during HCA 

installation are resolved cooperatively by Boeing and 

GRC engineers. 

Ground Testing of Flight Hollow Cathode 

Assemblies 

Each flight HCA was acceptance tested before 

delivery to Boeing by GRC. The acceptance test 

procedure includes a confidence test of the cathode 

heater and a plasma test. The heater confidence test 

consists of a 150 cycle burn-in and a hot resistance 

measurement to verify heater operation and 

workmanship. The plasma test includes a test in idle 

mode (with emission current to the keeper anode only) 
NASA/TM2001-211293 3 

at several flow rates. Figure 4 shows the keeper anode 

voltage during acceptance testing as a function of flow 

rate for the flight cathodes. Note the high degree of 

repeatability between the newly fabricated cathodes. 

Additionally, a clamping mode test is run to verify that 

the cathode is capable of emitting at least 10 A of 

emission current at less than 20 V with a keeper anode 

current of 3 A. The cathodes were also ignited 

repeatedly to demonstrate that the cathodes are capable 

of a minimum of 10 ignitions in 6.0 minutes or less. 

After integration of the HCA, the PCU is acceptance 

tested to demonstrate that the system is capable of 

meeting the ISS charge control requirements. The 

acceptance test includes 15 cathode ignitions, a 

clamping mode test, and a 24 hour test in idle mode. 

Results of the clamping mode tests for the QUAL, 

FM.01, FM.02, and FM.03 PCUs are shown in Figure 

5. The upper bound on clamping voltage of the PCU 

is also shown to illustrate the performance of the PCU 

with respect to the ISS requirements. 

Plasma Contactor Operation on the 

International Space Station 

The PCUs FM.01 and FM.02 were delivered to the ISS 

in October 2000. The PCUs were carried to the ISS 

aboard space shuttle flight STS–92. Figure 6 shows 

the FM.01 and FM.02 installed on the ISS Z1 truss. 

FM.01 was first ignited aboard the ISS on October 27, 

2000. FM.02 was ignited on November 17, 2000. 

Since that time FM.01 and FM.02 have undergone 10 

ignitions each. Time to ignition for each of these 

ignitions is plotted in Figure 7. Time to ignition is 

defined as the time from the application of high 

voltage pulses between cathode and keeper to the time 

at which a 2.5 A current is measured at the keeper. 

Ignition times ranged from 2 seconds to 90 seconds; 

the time to ignition requirement for the PCU is 30 

minutes. FM.01 and FM.02 have operated for 

accumulated times of 2123 hours and 2824 hours, 

respectively. Keeper anode potentials have ranged 

between 13.1 and 13.9 V. These keeper anode 

voltages and ignition times indicate that the two PCUs 

themselves are operating as expected. 

Summary and Ongoing GRC Involvement 

Efforts toward the development of HCAs by the 

NASA Glenn Research Center have been instrumental 

in the successful deployment and demonstration of the 

Plasma Contactor system on-board the International 

Space Station. Extensive testing of the PCU HCAs 

has been conducted at GRC to address issues of life, 

reliability, and cathode operation [5]. The eventual 

outcome of this testing was a finalized design for the 

HCA used in the ISS PCU system. To date, twelve 

flight HCAs have been built by GRC and rigorously 

acceptance tested. A total of three of these HCAs have 

been installed and tested in flight PCUs, and currently 

FM.01 and FM.02 PCUs are providing charge control 

upon the International Space Station. FM.01 and 

FM.02 are both operating as expected with 10 

ignitions each and a total of 2123 hours and 2824 

hours of accumulated operating time, respectively. 

The third PCU, FM.03, has been acceptance tested, 

and is available as a spare for the two units already onorbit. 

NASA Glenn Research Center continues to support the 

ISS plasma contactor program in a number of ways. 

Life testing of three flight-like HCAs in a mission-like, 

cyclical profile is continuing until the life test articles 

have reached 1.5 times their rated life. Integration of 

the HCA into the Boeing developed PCU is also 

supported by GRC engineers and technicians. Flight 

model PCU acceptance testing to verify the charge 

control capabilities of the completed system is 

performed at GRC in its world class vacuum facilities. 

HCAs with reduced requirements for expellant flow 

are also being developed at GRC as an option to 

extend the operating life of on-orbit PCUs. 

References 

[1]. Patterson, M.J., Sarver-Verhey, T.R., Soulas, 

G.C., Zakany, J. “Space Station Cathode Design, 

Performance, and Operating Specifications,” 25th 

International Electric Propulsion Conference 

Proceedings, IEPC–97–170, August 24–28, 1997. 

[2]. Rawlin, V.K., Pawlik, E.V., “A Mercury PlasmaBridge Neutralizer,” J. Spacecraft and Rockets 5, 814–

820, July 1968. 

[3]. Sarver-Verhey, T.R., “28,000 Hour Xenon Hollow 

Cathode Life Test Results,” 25th International Electric 

Propulsion Conference Proceedings, IEPC–97–168, 

August 24–28, 1997. 

[4]. Soulas, G.C., Sarver-Verhey, T.R., “International 

Space Station Cathode Life Testing,” 25th 

International Electric Propulsion Conference 

Proceedings, IEPC–97–166, August 24–28, 1997. 
NASA/TM2001-211293 4 

[5]. Kovaleski, S.D., Patterson, M.J., Soulas, G.C., 

Sarver-Verhey, T.R., “A Review of Testing of Hollow 

Cathodes for International Space Station Plasma 

Contactors,” 27th International Electric Propulsion 

Conference Proceedings, IEPC–01–271, September 

15–19, 2001. 

[6]. Zakany, J., Pinero, L., “Space Station Cathode 

Ignition Test Status at 32,000 Cycles,” 25th 

International Electric Propulsion Conference 

Proceedings, IEPC–97–167, August 24–28, 1997. 

[7]. Soulas, G.C., “Status of Hollow Cathode Heater 

Development for the Space Station Plasma Contactor,”

30th AIAA/ASME/SAE/ASEE Joint Propulsion 

Conference Proceedings, AIAA–94–33109, June 27–

29, 1994. 
NASA/TM2001-211293 5 

Hollow Cathode 

Assembly 

Figure 1. Photo of an ISS flight model PCU. The PCU is covered by a multi-layer insulation blanket. The HCA 

has been pointed out in the image. 

Cathode Tube 

Enclosed Keeper 

Emitter Insert 

Heater 

Figure 2. Drawing of a flight HCA (drawing not to scale). 
NASA/TM2001-211293 6 

HCA with protective cover 

Figure 3. Photograph of flight HCA.004-F being integrated into the ISS PCU box. Image shows the PCU with its 

top cover removed. The large white sphere is the xenon storage tank. The HCA is pointed out in the picture. 

12.00

13.00

14.00

15.00

16.00

17.00

18.00

19.00

20.00

4.50 5.50 6.50 7.50 8.50

Xenon Flow (sccm)

Anode Voltage (V)

HCA.QC HCA.QUAL

HCA.001-F HCA.002-F

HCA.003-F HCA.004-F

HCA.005-F HCA.006-F

HCA.007-F HCA.008-F

HCA.009-F HCA.010-F

HCA.011-F HCA.012-F

Figure 4. Flight HCA keeper anode voltage in idle mode, as a function of flow rate, during HCA acceptance 

testing. 
NASA/TM2001-211293 7 

0

2

4

6

8

10

12

4 6 8 10 12 14 16 18 20 22

Clamp Voltage (V)

Clamping Current (A)

HCA.QUAL in PCU.QUAL

HCA.001-F in FM.01

HCA.003-F in FM.02

HCA.005-F in FM.03

Clamp Voltage

Requirement

Figure 5. Clamping voltages of the PCUs QUAL, FM.01, FM.02, and FM.03 during flight PCU acceptance 

testing at NASA Glenn Research Center. Performance in the GRC vacuum facility is expected to match 

performance on-orbit. 

Plasma Contactor 

Units 

Figure 6. PCUs FM.01 and FM.02 installed on the ISS Z1 truss. 
NASA/TM2001-211293 8 

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12

Ignition Number

Time to Ignition (seconds)

ISS PCU 1

ISS PCU 2

Figure 7. Time to ignition for on-orbit ignitions of PCU 1 and PCU 2 (the FM.01 and FM.02 PCUs). A gap 

indicates time to ignition data was not obtained for that ignition. 
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IEPC–01–252

E–13097

WU–478–88–10–00

14

Correlation of Hollow Cathode Assembly and Plasma Contactor Data from

Ground Testing and In-Space Operation on the International Space Station

Scott D. Kovaleski, Michael J. Patterson, and George C. Soulas

Hollow cathode; Plasma contactor

Unclassified -Unlimited

Subject Category: 20 Distribution: Nonstandard

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

JPL, MSFC, and NASA, Pasadena, California, October 14–19, 2001. Scott D. Kovaleski, QSS Group, Inc.,

21000 Brookpark Road, Cleveland, Ohio 44135; Michael J. Patterson and George C. Soulas, NASA Glenn Research

Center. Responsible person, Michael J. Patterson, organization code 5430, 216–977–7481.

Charge control on the International Space Station (ISS) is currently being provided by two plasma contactor units

(PCUs). The plasma contactor includes a hollow cathode assembly (HCA), power processing unit and Xe gas feed

system. The hollow cathode assemblies in use in the ISS plasma contactors were designed and fabricated at the NASA

Glenn Research Center. Prequalification testing of development HCAs as well as acceptance testing of the flight HCAs is

presented. Integration of the HCAs into the Boeing North America built PCU and acceptance testing of the PCU are

summarized in this paper. Finally, data from the two on-orbit PCUs is presented.