Jonathan L. Van Noord, Hani Kamhawi, and Heather K. McEwen

Glenn Research Center, Cleveland, Ohio

Characterization of a High Current,

Long Life Hollow Cathode

NASA/TM—2006-214095

February 2006
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Jonathan L. Van Noord, Hani Kamhawi, and Heather K. McEwen

Glenn Research Center, Cleveland, Ohio

Characterization of a High Current,

Long Life Hollow Cathode

NASA/TM—2006-214095

February 2006

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NASA/TM—2006-214095 1

Characterization of a High Current, Long Life Hollow Cathode 

Jonathan L. Van Noord, Hani Kamhawi, and Heather K. McEwen 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The advent of higher power spacecraft makes it desirable to use higher power electric propulsion 

thrusters such as ion thrusters or Hall thrusters. Higher power thrusters require cathodes that are capable 

of producing higher currents. One application of these higher power spacecraft is deep-space missions 

that require tens of thousands of hours of operation. This paper presents the approach used to design a 

high current, long life hollow cathode assembly for that application, along with test results from the 

corresponding hollow cathode. The design approach used for the candidate hollow cathode was to reduce 

the temperature gradient in the insert, yielding a lower peak temperature and allowing current to be 

produced more uniformly along the insert. The lower temperatures result in a hollow cathode with 

increased life. The hollow cathode designed was successfully operated at currents from 10 to 60 A with 

flow rates of 5 to 19 sccm with a maximum orifice temperature measured of 1100 °C. Data including 

discharge voltage, keeper voltage, discharge current, flow rates, and orifice plate temperatures are 

presented. 

Nomenclature 

A cross-sectional area of insert region 

C fit coefficient 

di insert inner diameter 

e elementary charge 

Isp specific impulse 

Jth current from thermionic emission

kb Boltzmann’s constant 

kth thermal conductivity 

Kvl virtual thermal loss term 

N loss multiplier 

Q heat transfer rate 

T temperature 

t lifetime 

Va energy of activation 

x position along cathode insert length 

y depth into insert of barium depletion 

φe insert work function 

( )1 reference cathode parameters 

( )2 design cathode parameters 

( )100 μm 100 μm depth from insert surface 

( )depth depth from insert surface 

( )l minimum insert temperature 

( )loss thermal loss out of cathode insert region 

( )t peak insert temperature 

( )vl virtual loss parameters 
NASA/TM—2006-214095 2

Introduction 

Some electric propulsion systems that operate at higher power offer additional efficiencies and higher 

thrusting than the current state-of-the-art systems. Two types of higher power thrusters under 

development are the ion and Hall thrusters. These higher specific impulse and higher power thrusters will 

be key for planetary exploration and efficient cargo delivery to Mars (refs. 1 and 2). NASA’s Project 

Prometheus has been supporting the development of high specific impulse (Isp), high power ion thrusters 

(6000 to 8000 sec and 20 to 30 kW respectively) and Hall thrusters (2500 to 4000 sec and 20 to 50 kW 

respectively) for use with a nuclear powered spacecraft (refs. 3 to 5). These thrusters would require 10 to 

100 khr of operation for the missions envisioned (ref. 4). The longest ion thruster test to date was 

approximately 30 khr at specific impulses up to 3100 sec and powers up to 2.3 kW (1100 V beam voltage 

and 1.76 A beam current)(ref. 6). Missions such as these would also require a substantial increase in 

lifetime for Hall thrusters (ref. 5). 

A component that can limit the lifetime for these thrusters is the hollow cathode. Inert gas hollow 

cathodes have been operated for 28 khr at a constant current and over 30 khr at various currents (refs. 6 

and 7). Mercury ion thrusters and their hollow cathodes lasted from 13 to over 26 khr (refs. 8 to 10). 

Previously developed hollow cathodes typically operated below 35 A. However, the higher power 

thrusters would require hollow cathodes that operated from 50 to 100 A (refs. 11 and 12). 

This paper will detail an approach for a higher current, longer life hollow cathode that can be applied 

regardless of required current along with test results from a cathode that was built to satisfy 50 A of 

discharge current for 100 khr. Initially, a prediction of the hollow cathode insert lifetime will be presented 

that is primarily dependant on the temperature and thickness of an insert impregnated with a specific 

molar ratio of barium. Next, a model will be presented that predicts the insert temperatures based on the 

change in thermal conductivity in the insert region for a particular design. This model, along with the 

temperature requirement from the lifetime model, will be used to establish a long life design. A hollow 

cathode based on this design was built and tested. The purpose of the testing was to determine the 

operational temperatures of the cathode for comparison with the models, and also establish expected 

lifetime. Discharge voltages and cathode operational modes are presented for various cathode flow rates 

and currents, but these values are dependent on the cathode and anode configuration, so they are most 

relevant for establishing trends and confidence in the cathode design, whether for ion or Hall thrusters. 

Cathode Lifetime Requirement 

As previously mentioned, the lifetime required for these cathodes will have an upper bound of around 

100 khr. The life limiters for a hollow cathode include insert life, which is driven by impregnate 

depletion, sputtering wear to the orifice plate, orifice walls, and heater (ref. 6). The orifice plate and 

heater are protected by a sacrificial keeper. The life of the keeper has been dramatically increased by 

changing from molybdenum to graphite which has a lower sputtering yield. The new keeper material then 

removes wear on the orifice plate and heater as life limiters. This leaves the cathode insert life as the 

primary life limiter and this will be the focus presented here. There are several factors that determine the 

life of an impregnated insert in the hollow cathode. They include the formation of tungstates, which bind 

up the barium and inhibit diffusion, and depletion of barium from within the insert (ref. 13). Lifetime data 

has been compiled for planar impregnated cathodes used in vacuum tubes (ref. 14), however, due to the 

different operating environments and mechanical designs (i.e., cylindrically enclosed vs. planar) it is 

unclear how well the planar vacuum data corresponds to the hollow cathode. The current hollow cathode 

life models are either based on prediction of chemical processes and vapor pressures (refs. 15 and 16), or 

on lifetime data from planar impregnated cathodes operated in a vacuum tube (ref. 17). The chemistrybased model is limited due to the complexity of the chemical processes in the insert and the planar 
NASA/TM—2006-214095 3

cathode-based model is limited due to the difference in operating environments and physical construction 

of the hollow cathode. However, emitter temperature is the major factor for determining critical reactions 

and, therefore, the insert’s lifetime (refs. 13 and 18). Thus, first order estimates for hollow cathode emitter 

life can be obtained by using the known temperature and lifetime data from planar cathodes. 

The model used in this paper is based on a relationship outlined by Palluel and Shroff that is shown in 

eq. (1) (ref. 18). This model examines the relationship between cathode lifetime and temperature. This 

model is developed based on data for impregnated planar cathodes that are used in the vacuum industry. 

k T

eV t C

b = + a ln( (1) )

Reference 14 further elaborates on the model to determine the time to particular barium depletion 

depth on hollow cathode impregnated inserts. This relationship is given in eq. (2) relating the time to a 

particular depletion depth to the 100 μm depletion depth time. The relationship between the 100 μm 

depletion depth time and temperature following the relationship of eq. (1) is given in reference 18. 

2

100 μm

depth depth 100 μm ⎟

⎠

⎞ ⎜

⎝

⎛ = y

y t t (2) 

While it is difficult to estimate the error using this model, it does provide a starting point for design. 

While eq. (2) will provide a life estimate to any depth, there is likely a depth where the barium will 

not be easily accessible. At the completion of the NSTAR ELT test (30+ khr) both the neutralizer and 

discharge cathodes were still operational (ref. 6). The measurements taken to determine barium depletion 

in that insert were only to a depth of 500 μm. The ELT results showed some barium depletion at a depth 

of 100 and 300 μm at the downstream end of the insert, but not at 500 μm. Based on those measurements, 

it would appear that barium is accessible to at least a 300 μm depth. Figure 1 shows the relationship 

between emitter temperature and lifetime for both the 300 and 500 μm barium depletion depths as derived 

from the model. In order to achieve a 100 khr lifetime the emitter must be between 1050 to 1100 °C. This 

is consistent with the recommended operating temperature of 950 to 1150 °C for a long life impregnated 

insert (ref. 13). 

To further determine a usable depletion depth, the 0.6 cm outer diameter hollow cathode that was lifetested for 28 khr (ref. 7) can be examined. A similar cathode had an insert peak temperature measured at 

1180 °C (ref. 19). In order for an insert with a peak temperature of 1180 °C to have a predicted lifetime of 

30 khr, figure 1 shows the allowable barium depletion depth would be 500 μm. 

This can also be examined from a slightly different approach. Equation (1) can also be used to 

compare similar cathodes when lifetime and temperature are known for one cathode and only temperature 

is known for a second cathode. To compare the two lifetimes based on different temperatures eq. (1) can 

be rearranged to the ratio in eq. (3). 

⎥

⎦

⎤ ⎢

⎣

⎡

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = − 1 2 1

2 1 1

exp k T T

eV

t

t

b

a (3) 

Again using the data from the hollow cathode tested for 28,000 h, a similar insert (Va = 2.82 eV) 

(ref. 17) operating with a peak temperature of 1100 °C would last 104 khr. Even though the results are 

based on the same model, this result gives validity to the design point of a maximum temperature of 

1100 °C for the insert, which results in 100 khr of operation. 
NASA/TM—2006-214095 4

10

100

1000

10000

100000

800 900 1000 1100 1200

Temperature, oC

Lifetime, khrs

500 μm depth 

depleted

300 μm depth 

depleted 

Figure 1.—Graph showing lifetime estimate depending 

on depletion depth for an impregnated insert 

operating at various temperatures. 

Background 

Affect of Temperature Gradient on Peak Temperature of Hollow Cathode 

The major requirements for an initial higher current, long life hollow cathode design are for a nominal 

current of 50 A with operation at or below 1100 °C. In order to design a hollow cathode that will operate 

with those requirements with a minimal insert diameter, it is essential that the entire insert largely 

contribute to the overall current emitted. The major mechanism for current production in the hollow 

cathode is thermionic emission. Since this emission is related to the exponential of the inverse of the 

temperature, the current that is emitted is very sensitive to its temperature. In previous hollow cathodes 

the temperature distribution along the length of the insert varied by as much as 200 °C (ref. 19). Figure 2 

depicts this measured temperature profile for a cathode running at 12 A. Figure 3 shows the current 

density and percent of overall current produced along the length of an insert corresponding to the 

temperature distributions shown in figure 2. It can be seen that for the 200 °C temperature profile, roughly 

75 percent of the current is produced in the first third of the insert. While these profiles do not account for 

the current that is produced from the orifice or any plasma effects, they do demonstrate that by reducing 

the temperature gradient, the insert should more evenly produce the current necessary. Figures 2 and 3 

also show the representative temperature profile and current production characteristics for a comparable 

cathode (i.e., same insert inner diameter), but with a temperature gradient of only 50 °C. It can be seen 

that the peak temperature is predicted to drop by about 73 °C and produce current densities on the order 

of 2:1 from front to back instead of over 20:1 for the previous profile. Using the relationship given in eq. 

(2), an insert with a peak temperature reduced by 73 °C would yield a lifetime increase of 330 percent of 

the original 28 khr lifetime that was demonstrated. 

The ability to decrease the peak temperature of an insert by decreasing the temperature gradient was 

demonstrated by Beattie (ref. 20). For cathodes with comparable inner diameters and an emitted current 

of 16 A, one with approximately a 240 °C temperature gradient and peak temperature of 1140 °C was 

reduced to a peak temperature of 985 °C and a temperature gradient of 64 °C. 
NASA/TM—2006-214095 5

950

1000

1050

1100

1150

1200

0% 20% 40% 60% 80% 100%

Length of Insert from Orifice

Temperature, o

C

 Measured Profile with 

200 °C Gradient and 

12 A Emitted

Predicted Profile 

Assuming 50 °C Gradient 

and 12 A Emitted

73° 

Change

Figure 2.—Hollow cathode insert temperatures as a function of length 

for 12 A of emission with differing temperature profiles. 

0

1

2

3

4

5

6

7

8

9

10

0% 20% 40% 60% 80% 100%

Insert Length from Orifice 

Emmited Current Density, A/cm2

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Percent Total Current 

Current Density 

(200 °C Gradient)

Current Density 

(50 °C Gradient) 

Total Percent Emitted 

(50 °C Gradient)

Total Percent Emitted 

(200 °C Gradient) 

Figure 3.—Current produced along the length of a hollow cathode insert 

for 12 A of emitted current with differing temperature profiles. 
NASA/TM—2006-214095 6

Model to Predict Hollow Cathode Temperatures 

The main approach to decreasing the temperature gradient along the insert is to increase the thermal 

conductivity in the insert region. It is important not to increase this conductivity outside of the region as 

to increase the thermal losses out of the cathode insert region. A simple model used for initial designing 

purposes was to consider that thermal conduction along the length of an insert is the major component to 

determining the temperature gradient along an insert. While there are other components that contribute to 

this gradient such as the emission of electrons and radiation, thermal conduction is the major driver 

(ref. 21). The variation in temperature gradient in the insert region can be estimated by determining the 

ratio of heat transfer rates based on a change in thermal conduction in the insert region. This ratio is 

shown in eq. (4). 

1

2

2

1

2

1

2

1

2

2

2 2

1

1

1 1

2

1

x

x

T

T

A

A

k

k

dx

dT k A

dx

dT k A

Q

Q

th

th

th

th

Δ

Δ

Δ

Δ = ≅ (4) 

In order to estimate the change in heat transfer rates resulting from designs with different thermal 

losses out of the insert region, a virtual thermal conductive loss term is determined. This virtual term 

approximates all thermal losses out of a cathode as conductive. It is assumed that for a given cathode, the 

conduction term is constant and the loss is determined by the difference between the lowest temperature 

of the insert and some heat sink at very a low temperature at a fixed distance away. This concept is 

represented in eq. (5) where Kvl is some conductive type term that represents thermal losses out of a 

cathode. 

( ) 1 0

insert loss 1

1

1 1 1 ≅ − ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ Δ

Δ

≅ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = vl vl l

vl

thvl vl

th T K T

x

k A

dx

dT Q k A (5) 

For a cathode where there are no design changes to the cathode outside of the insert region, the ratio of 

heat transfer rates in eq. (4) would be a ratio of the lowest insert temperatures since Kvl will be constant. 

However, a change external to the insert region, such as increasing the cathode tube diameter, can be 

approximated by a multiplier, N, as shown in eq. (6). 

( ) ( 0) Q2 loss ≅ NKvl Tl2 − (6) 

Then a comparison of heat transfer rates between a known cathode and temperature profile to a 

different design can be represented by taking the ratio of eq. (5) over eq. (6) to give eq. (7). 

( )

( ) 2

1

2

1

loss 2

1

2

1

0

0

l

l

vl l

vl l

NT

T

NK T

K T

Q

Q

Q

Q = −

− ≅ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = (7) 

Equation (7) can then be substituted in eq. (4) resulting in eq. (8). 

2

1

1

2

2

1

2

1

2

1

l

l

th

th

NT

T

x

x

T

T

A

A

k

k = Δ

Δ

Δ

Δ (8) 

Equation (8) can then be rearranged to determine the ratio of the peak to minimum temperatures for the 

design as shown in eq. (9). 
NASA/TM—2006-214095 7

1

2

1

1

2

1

2

1

2

2 1

x

x

T

N T

A

A

k

k

T

T

th l

th

l

t

Δ

Δ Δ = + (9) 

Once that ratio of peak to minimum temperatures is determined, a temperature profile similar to the one 

shown in figure 2 is fit to this ratio and a corresponding emission current can be determined using the 

Richardson-Dushman’s equation shown in eq. (9). 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ φ = − π insert

2

insert 120 exp k T

e T

d dx

J

b

e

i

th (10) 

The temperatures can then be iterated until the current produced using eq. (10) matches the desired 

emission current. There are additional effects that are often considered when evaluating hollow cathode 

current emission including the Schottky effect that lowers the effective work function, the current from 

ion bombardment, electron backstreaming, and current from the orifice plate (ref. 21). Based on 

measurements of a cathode with a segmented insert, it was assumed that roughly 10 percent of the 

discharge current comes from the orifice plate (ref. 22). The contribution of ion current will be ignored 

for this model as a second order effect, but it would effectively lower the amount of thermionic current 

for a given emitted current. The work function used varied with temperature, and was generated to 

provide a fit to the data in figure 4. This work function fit, along with other published values used for the 

work function of these inserts, is shown in figure 5. The slope for the work function used is steeper than 

the other slopes, but the overall values fall within the other published numbers. Since a fit was used for 

the work function to determine the insert temperatures, the Schottky effect is inherently accounted for. 

Using this approach, a set of peak temperatures versus currents were generated for the cathode with the 

measured insert temperatures (ref. 19). This comparison is given in figure 4 and demonstrates very good 

agreement with reasonable work functions. 

1000

1050

1100

1150

1200

1250

1300

1350

0 5 10 15 20

Discharge Current, A

Maximum Insert Temperature, o

C

High Measured

Predicted

Low Measured

Figure 4.—Hollow cathode model compared to test peak insert 

temperatures for the 0.6 cm tube outer diameter hollow cathode. 
NASA/TM—2006-214095 8

1.8

1.9

2

2.1

2.2

2.3

2.4

800 900 1000 1100 1200 1300

Temperature, oC

Work function, e

V

Model Fit

Fomenko

Mikellides

Palluel

Figure 5.—Work function used to fit temperature data compared to other published 

values (Fomenko (ref. 23), Mikellides (ref. 24), and Palluel (ref. 18)). 

Application of Model to Long Life, High Current Requirements 

Several candidate geometries were implemented in the model. The goal of the cathode design was to 

operate with a peak insert temperature of 1100 °C while operating at a current of 50 A. Two approaches 

were used to minimize the temperature. The first was to allow sufficient thermal conduction through the 

insert region for significant emission contribution over the entire insert. A second simultaneous approach 

is to increase the inner radius of the insert creating a larger emission area and lowering the emitted current 

density required. However, allowing the inner diameter to increase unbounded would create a cathode 

much larger than practical and one that is significantly larger, and perhaps different enough from existing 

experience that additional unforeseen issues could arise. Therefore, an additional requirement was to 

minimize the amount the cathode diameter could grow. 

The model presented in the previous section was used for a variety of scenarios whereby the cathode 

insert inner diameter was increased to lower the emitted current density requirements, while the external 

diameter of the cathode tube was increased to provide sufficient thermal conduction along the insert 

region. It is important to note that for a given insert outer diameter, there is an optimal insert inner 

diameter such that the temperature is minimized. The minimal peak temperature is driven by a smaller 

inner diameter for increased thermal conduction, and is driven by a larger inner diameter to decrease the 

emitted current density. Also kept in mind were standard tube outer diameter sizes in order to facilitate 

standard gas feed system fittings. Figure 6 shows the predicted temperatures for the selected cathode 

design with a cathode tube outer diameter of 1.9 cm and an insert thickness in excess of 1 mm. The N

thermal losses multiplicative number is estimated relative to the 0.6 cm outer diameter cathode. The 

0.6 cm cathode provided the baseline internal temperature measurements for the model. A rough 

approximation would put the loss multiplier N between 2 to 3 times the 0.6 cm cathode thermal losses. 

However, without a comprehensive study, a better estimate of losses cannot be determined. 
NASA/TM—2006-214095 9

800

850

900

950

1000

1050

1100

1150

1200

0 10 20 30 40 50 60 70

Anode Current, A

Orifice Plate Temperature, o

C

N=3

N=2

N=1.33

Greater Thermal 

Losses

Figure 6.—Peak insert temperature model predictions for candidate 

high current, long life hollow cathode are depicted with differing 

amount of thermal losses for a variety of emitted 

High Current Cathode Test 

Test Setup 

A laboratory model (LM) version of the hollow cathode was designed to operate at the temperatures 

shown in figure 6 and was fabricated with a graphite keeper. This cathode tube diameter facilitates a large 

emission area on the insert to reduce the current density required from the insert. The impregnated insert 

wall thickness is in excess of 1 mm to facilitate thermal conduction along its length and provide ample 

amounts of barium for the life of the insert. This cathode is shown assembled in figure 7. 

The cathode was installed in the NASA Glenn Research Center Vacuum Facility 61 (VF61). This 

vacuum facility has two ports that are configured for hollow cathode testing. The two ports are 0.33 m in 

diameter and 0.41 m long. The overall facility is 1.0 m diameter by 1.5 m long. VF61’s pumping speed is 

13,000 l/s with xenon. The base pressure is 5.3×10–7 Pa. This facility has been used for extensive hollow 

cathode testing in the past including the space station hollow cathode (ref. 25). 

Figure 8 shows the setup for the hollow cathode in a vacuum facility port. A cylindrical anode was 

used to allow the current to attach radially and the neutral gas to flow unimpeded downstream. The anode 

had a diameter of 0.25 m and was constructed of molybdenum, since large anode temperatures were 

expected. Two thermocouples were placed on the anode to monitor its temperature. Additional 

thermocouples were located on the cathode orifice plate weld, immediately behind the insulator on the 

cathode tube and also on the keeper cylindrical wall immediately behind its face. The power supply used 

for discharge could provide a 100 A current at 40 V. A flow controller capable of flow rates up to 

100 sccm was used to regulate the flow of xenon to the cathode. The cathode keeper collected no current 

and its potential was allowed to float. The potential was monitored to determine when the cathode 

transitioned into plume mode for this configuration. All NASA GRC procedures were followed for 

storing, conditioning, and igniting the cathode (ref. 26). 
NASA/TM—2006-214095 10

 

 Figure 7.—Pretest photograph taken Figure 8.—Picture showing 

 of the lab model hollow cathode VF61 port 1 test setup. 

 assembly with graphite keeper. 

Figure 9.—Picture taken of a LM 

hollow cathode discharge. 

Test Results 

The hollow cathode was operated at discharge currents from 10 to 60 A with xenon flowrates of 5 to 

19 sccm. Figure 9 shows the high current discharge from the LM hollow cathode. The hollow cathode 

required approximately 25 hours of operation to condition itself, during which the cathode orifice plate 

temperatures decreased by ~100 °C. Initially, the orifice plate will not have any barium on its surface and 

the barium in the insert will be migrating to its surface. This type of lowering in temperature over the first 

tens of hours of operation is typical in these cathodes and is also seen in the results from the 28 khr 

cathode (ref. 7). A comparison of predicted temperatures and test temperatures after conditioning is 

shown in figure 10. 

It can be seen that the insert temperature in figure 10 predicted is up to 150 °C hotter than the 

measured orifice plate temperatures. Either the model is overpredicting the insert temperatures or the 

insert is operating hotter than the orifice temperature. Temperature measurements of an insert as seen in 

the smaller cathode showed that the orifice plate could operate up to 100 °C cooler than the insert 

(ref. 19). Either way, it is very likely that the model is the upper bound of the temperature in the insert and 

the lifetime below 50 A could be well in excess of 100 khr. If orifice temperatures are fairly close to the 

insert temperatures then this cathode insert could exhibit a lifetime on the order of 100 khr at currents up 

Anode VF 61 

Port

Cathode 

Mount
NASA/TM—2006-214095 11

to 60 A. It will be valuable in future work to measure the insert temperatures for comparison to the model 

and a further assessment of life. The model predicts the trend of how the temperature changes quite well. 

While the anode voltages can vary based on the location and design of the anode, the discharge 

voltages are a useful parameter to evaluate cathode operation. The keeper voltages and discharge voltages 

are shown in figure 11 for cathode flow rates of 9 and 15 sccm. Hollow cathodes can operate in either 

spot or plume mode, with spot mode typified by a very bright spot downstream of the orifice and keeper 

voltage transients below 5 V peak-to-peak. When cathodes transition into plume mode, the keeper 

transient voltages shift above 5 V peak-to-peak and the spot becomes more diffuse with a hazy plume 

becoming more prominent. It should be noted that this hollow cathode would exhibit spot to plume mode 

type transition in the discharge chamber of an ion thruster differently because of the presence of a 

magnetic field and differing anode geometry. However, the discharge voltages are still very useful to 

establish trends and characterization of the cathode on a component level. As shown in figure 11, the 

hollow cathode tested transitioned into plume mode for current above 40 to 45 A at 9 sccm. With a flow 

rate of 15 sccm the cathode operates entirely in spot mode for currents up to 60 A. The discharge voltage 

also varies only a few volts for the 15 sccm flow rate. 

Figure 12 shows how the orifice plate temperatures vary with cathode xenon flow rate. This data was 

taken prior to cathode conditioning, so the temperatures will vary a bit from the temperatures shown in 

figure 10. Even though the temperatures are higher, the trends they demonstrate are still valid. It can be 

seen that in figure 10, the temperatures vary minimally between 9 and 15 sccm below 35 A and show 

divergence at higher currents just as shown in figure 12. At higher currents and lower flow rates the 

cathode was transitioning into plume mode where peak-to-peak keeper voltages above 5 V where seen. 

This transition is also seen in figure 12 by the orifice plate temperatures quickly increasing with lower 

flow rates. 

Figure 13 shows how the discharge voltage changed with flow rate, and corresponds to the same data 

shown in figure 12. The discharge voltages at higher flow rates were fairly similar, with the 10 A 

emission exhibiting a slightly higher discharge voltage. The discharge voltage increased with all currents 

tested as the flow rates were decreased. For currents above 20 A, a significant increase in discharge 

voltage is seen for low flow rates. This is also an indication of plume mode operation. It can be seen that 

as the current increases a higher minimal flow rate is required to establish spot mode operation. 

600

700

800

900

1000

1100

1200

0 10 20 30 40 50 60 70

Anode Current, A

Temperature, o

C

Model Insert Peak

Temperatures N=2

15.0 sccm Orifice

Plate Temperatures

9.0 sccm Orifice

Plate Temperatures

Figure 10.—Graph showing cathode orifice plate 

temperature for varying anode currents.
NASA/TM—2006-214095 12

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70

Discharge Current, A

Discharge Voltage, V

0

2

4

6

8

10

12

Keeper Voltage, V

15 sccm

9 sccm

15 sccm

9 sccm

Keeper Voltage Peak-to-Peak 

transitioning above 5 V

Figure 11.—Graph showing discharge and keeper 

voltages for varying discharge currents. 

750

800

850

900

950

1000

1050

1100

1150

1200

5 7 9 11 13 15 17 19

Flow Rate, sccm

Orifice Plate Temperatures, o

C

50 A

40 A

35 A

30 A

20 A

10 A

Figure 12.—A graph that shows how the of orifice plate 

temperatures varies with cathode flow rates. 
NASA/TM—2006-214095 13

0

5

10

15

20

25

30

35

40

45

5 7 9 11 13 15 17 19

Flow Rate, sccm

Discharge Voltage, V

50 A

40 A

35 A

30 A

20 A

10 A

Figure 13.—A graph that shows how the discharge 

voltages vary with cathode flow rate. 

Conclusion 

An approach has been outlined for long life, higher current hollow cathodes. A relationship between 

the insert temperature and lifetime is approximated to establish design requirements. A model was 

developed to predict peak insert temperatures for various currents using a known temperature profile as a 

baseline. Predictions of new temperature profiles were based on the change in thermal conductivity in the 

insert region and thermionic emission necessary for a given discharge current. The approach presented to 

minimize the peak insert temperature was mainly a twofold approach. First, increasing the thermal 

conductivity along the insert region length reduced the temperature gradient and lowered the peak 

temperature. Second, the insert inner diameter was increased to provide additional area for emission and, 

thereby, lowering the maximum current density required. However, the overall diameter of the cathode 

was minimized to prevent the cathode from becoming extraneously large. Based on this approach, a 50 A 

nominal hollow cathode with a predicted minimum lifetime of 100 khr was fabricated. This cathode was 

tested for flow rates from 5 to 19 sccm and discharge currents ranging from 10 to 60 A. The maximum 

orifice plate temperature measured for currents up to 60 A was 1100 °C and is consistent with a long life 

of 100 khr of operation. 

References 

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Human Exploration,” 1st Space Exploration Conference: Continuing the Voyage of Discovery,

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and Technologies Conference, AIAA–96–4470, Sept. 1996. 
NASA/TM—2006-214095 14

3. Oleson, S.R., “Herakles: The Electric Propulsion Element of Prometheus 1,” 41st 

AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA–2005–3888, Tucson, AZ, 

July 10–13, 2005. 

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Design,” 41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA–2005–

3889, Tucson, AZ, July 10–13, 2005.

5. Jacobson, D.T., et al., “An Overview of Hall Thruster Development at NASA’s John H. Glenn 

Research Center,” 41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA–

2005–4242, Tucson, AZ, July 10–13, 2005. 

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Results from the 30,000 Hr Life Test of Deep Space 1 Flight Spare Ion Engine,” 40th 

AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA–2004–3608, Fort 

Lauderdale, FL, July 11–14, 2004. 

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Electric Propulsion Conference, IEPC 97–168, Cleveland, OH, Aug. 24–28, 1997. 

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TM X–2785, June 1973. 

9. Wintucky, E.G., “A 20,000-Hour Endurance Test of a Structurally and Thermally Integrated 5-cm 

Diameter Ion Thruster Main Cathode,” 11th AIAA International Electric Propulsion Conference, 

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Propulsion Conference, AIAA–79–2116, Princeton, NJ, Oct. 31–Nov. 1, 1979. 

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July 10–13, 2005. 

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AIAA–2005–4244, Tucson, AZ, July 10–13, 2005. 

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Naval Surface Warfare Center, Crane, IN, March 2002. 

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Tungsten Emitter, 27th International Electric Propulsion Conference, IEPC–01–276, Pasadena, CA, 

Oct. 14–19, 2001. 

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Thrusters,” AIAA International Electric Propulsion Conference, AIAA 76–985, Key Biscayne, FL, 

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2004. 

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Life,” Journal of Applied Physics, vol. 51, no. 5, 1980, pp. 2894–2902. 

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NASA/TM—2006-214095 15

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3312, Indianapolis, IN, July 27–29, 1994. 
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WBS–22–800–92–71

21

Characterization of a High Current, Long Life Hollow Cathode

Jonathan L. Van Noord, Hani Kamhawi, and Heather K. McEwen

Ion thruster; Hollow cathodes

Unclassified -Unlimited

Subject Category: 20

Prepared for the 29th International Electric Propulsion Conference sponsored by ERPS, Princeton University,

NASA Glenn, NASA Jet Propulsion Laboratory, Aerojet, EPPDYL, IEPC, Busek Company, Inc., and Mitsubishi

Electric, Princeton, New Jersey, October 31–November 4, 2005. Jonathan L. Van Noord, e-mail: Jonathan.L.Van

Noord@nasa.gov, Hani Kamhawi, e-mail: Hani.Kamhawi@grc.nasa.gov, and Heather K. McEwen, e-mail:

Heather.K.McEwen@nasa.gov, NASA Glenn Research Center. Responsible person, Jonathan L. Van Noord,

organization code RPP, 216–433–5310.

The advent of higher power spacecraft makes it desirable to use higher power electric propulsion thrusters such as ion

thrusters or Hall thrusters. Higher power thrusters require cathodes that are capable of producing higher currents. One

application of these higher power spacecraft is deep-space missions that require tens of thousands of hours of operation.

This paper presents the approach used to design a high current, long life hollow cathode assembly for that application,

along with test results from the corresponding hollow cathode. The design approach used for the candidate hollow

cathode was to reduce the temperature gradient in the insert, yielding a lower peak temperature and allowing current to

be produced more uniformly along the insert. The lower temperatures result in a hollow cathode with increased life.

The hollow cathode designed was successfully operated at currents from 10 to 60 A with flow rates of 5 to 19 sccm

with a maximum orifice temperature measured of 1100 °C. Data including discharge voltage, keeper voltage, discharge

current, flow rates, and orifice plate temperatures are presented.