NASA / CR-2001-211290 IEPC-01-276 

Life Model of Hollow Cathodes Using a 

Barium Calcium Aluminate Impregnated 

Tungsten Emitter 

S.D. Kovaleski 

QSS Group, Inc., Cleveland, Ohio 

Prepared for the 

27th International Electric Propulsion Conference 

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

Pasadena, California, October 14-19, 2001 

National Aeronautics and 

Space Administration 

Glenn Research Center 

December 2001 

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Available electronically at http://gltrs.grc.nasa.gov IGLTRS 
Life Model of Hollow Cathodes Using a Barium Calcium Aluminate Impregnated 

Tungsten Emitter 

S.D. Kovaleski 

QSS Group, Inc. 

Cleveland. Ohio 44135 

216--433-3735 

scott.kovaleski@grc.nasa.gov 

Hollow cathodes with barium calcium aluminate impregnated tungsten emitters for 

thermionic emission are widely used in electric propulsion. These high current, low power 

cathodes are employed in ion thrusters, Hall thrusters. and on the International Space 

Station in pla'ima contactors. The requirements on hollow cathode life are growing more 

stringent with the increasing use of electric propulsion technology. The life limiting 

mechanism that determines the entitlement lifetime of a barium impregnated thermionic 

emission cathode is the evolution and transport of barium away from the emitter surface. A 

model is being developed to study the process of barium transport and loss from the emitter 

insert in hollow cathodes. The model accounts for the production of barium through 

analysis of the relevant impregnate chemistry. Transport of barium through the 

approximately static gas is also being treated. Finally, the effect of temperature gradients 

within the cathode are considered. 

Introduction 

Hollow cathodes find extensive use in electric 

propulsion. Hollow cathodes are used in ion thrusters 

and Hall thrusters [1] to provide electrons for both 

propellant ionization and neutralization. International 

Space Station plasma contactors [2] use hollow 

cathodes to emit electron current, as demanded, to the 

ambient space plasma at low coupling voltages, thus 

compensating for spacecraft charging. For all of these 

applications, long cathode lifetime is critically 

important. 

The entitlement, or theoretical maximum, lifetime of 

the cathode is dependent upon the chemistry of the 

oxide impregnated tungsten emitter insert. The 

cathode insert serves as a low work function 

thermionic electron source for internal ionization of 

the cathode expellant [I]. The tungsten emitter work 

function is lowered by a surface coating of barium on 

oxygen on tungsten [3]. Barium is supplied to the 

tungsten surface by chemical reactions of the 

BaO:CaO:Ah03 mixture with which the porous 

tungsten emitter is impregnated. In addition to 

supplying barium, products of the impregnate 

reactions will clog the emitter pores impeding the 

progress of barium to the surface. The impregnate can 

NASA/CR-2001-211290 

also react with impurities to produce "poisons" on the 

emitter surface. These "poisons" can increase the 

surface work function by displacing the barium-onoxygen monolayer. Thus, it is apparent that an 

understanding of the chemistry of the impregnate is 

critically important. 

The BaO:CaO:Ab03 impregnated tungsten cathodes 

have been widely studied since their development by 

Levi [4]. Common cathode impregnate compositions 

include 5:3:2, 4: I: I and 3: 1: 1, where the notation 

indicates the stoichiometric coefficient of BaO: CaO: 

Ah03 [5]. The chemistry of operation of impregnated 

cathodes was studied originally by Rittner, Rutledge, 

and Ahlert [6]. The chemistry of barium aluminate 

impregnates with added CaO were examined by 

Rittner [7]. Suitch examined the thermochemistry and 

kinetics of barium aluminate and barium calcium 

aluminate compounds in detail [8]. Lipeles and Kan 

generalized a thermochemical model for the operation 

of impregnated cathodes [9], and it is upon their work 

that the model reported here is based. 

Sarver-Verhey reported experimental results of a 

xenon hollow cathode destructive analysis, after 

28,000 hours of cathode operation [10]. The results 
indicated significant chemical evolution of the emitter 

at fixed operating conditions had resulted in cathode 

failure. The results indicated that cathode electron 

emission was sustained only at the downstream end of 

the emitter. Depositions of BaW04 and Ba2CaWOo 

were found on the surface of the upstream 70 percent 

of the emitter. Ample amounts of barium were found 

within the emitter, indicating that either the cathode 

was no longer able to create free barium due to 

progression of the internal chemical reactions, or that 

the surface of the cathode had been poisoned by 

tungstate formation. The results of these experiments 

shed new light on the end-of-life mechanisms of the 

impregnated emitters.used on xenon hollow cathodes. 

In light of the results of Sarver-Verhey' s experiments, 

and in order to support continuing effort to improve 

the lifetime of ion thruster and plasma contactor 

cathodes, a need for a better understanding of the life 

limiting chemical phenomena in xenon hollow 

cathodes was identified. The models of cathode 

operation presented above were usually applied to 

cathodes used in vacuum tubes. Gaseous products 

evolved from these cathodes freely expand into 

vacuum. The situation in a xenon hollow cathode is 

quite different, where a few Torr of xenon is present 

over the emitter, impeding transport of gaseous 

products away from the surface and encouraging 

additional reactions at the emitter surface. Significant 

temperature gradients are also present in hollow 

cathodes that are not present in other dispenser 

cathode applications. For these reasons, a 

thermochemical model of the emitter coupled to a 

diffusion model of gas phase transport within a hollow 

cathode has been developed. The model includes a 

dependence on empirically derived cathode emitter 

temperature profiles. In this way, a simple analysis of 

the chemical processes within hollow cathodes has 

been performed. 

Model Description 

The orificed hollow cathode, used extensively in 

electric propulsion technology and in the International 

Space Station plasma contactor, is the device this 

model attempts to describe. Orificed hollow cathodes 

are characterized by a refractory metal tube through 

which an inert gas flows. At the downstream end of 

the tube, a small diameter orifice is employed to 

reduce the cathode gas flow necessary for low voltage 

cathode operation by increasing the internal pressure. 

NASA/CR-2001-211290 2 

These hollow cathodes generally use a porous tungsten 

emitter impregnated with work function lowering 

barium calcium aluminate to generate an internal 

plasma. A diagram of an orificed hollow cathode is 

shown in figure 1. 

The model of orificed hollow cathodes that has been 

developed couples a thermodynamic model of the 

barium calcium aluminate chemical system with a 

diffusion model to estimate the barium supply and loss 

rates in the cathode. Empirical transfer functions are 

used by the model to estimate the hollow cathode 

internal pressure and emitter temperature profile based 

on discharge current, orifice diameter, and gas flow 

rate. The emitter temperature profile then serves as an 

input to the thermochemistry model to determine the 

partial pressures of gaseous barium and barium oxide 

within the emitter pores. The partial pressures in the 

interior of the emitter pores and the partial pressure in 

the interior of the cathode determines the rate of free 

molecular flow of barium and barium oxide gas out of 

the emitter. Diffusion through the inert gas flowing 

through the cathode determines rate of loss of barium 

and barium oxide leaving the cathode orifice and 

diffusing upstream of the cathode. 

The use of thermodynamically derived gaseous Ba and 

BaO partial pressures to estimate free molecular flow 

rates from the emitter pores implies that the applicable 

reaction rates in the emitter are fast compared to the 

rates of loss from the emitter pores. The use of a 

thermochemical approach should at a minimum 

provide an upper bound on the loss rate of Ba and BaO 

from the emitter, and may be quite accurate given the 

small reaction volumes within the emitter pores and 

long residence times of the diffusing Ba and BaO gas. 

As long as the time rate of change of critical 

parameters like composition and partial pressure of Ba 

and BaO are slower than the characteristic time of 

reaction kinetics, this thermodynamic approach should 

be a good one. The model also implies that other 

supply mechanisms of Ba and 0 to the cathode 

surface, like surface diffusion along the pore walls, are 

small. 

Thermochemistry model 

The thermodynamic model of the barium calcium 

aluminate impregnated tungsten emitter used here was 

first reported by Lipeles and Kan [9]. The model starts 

with the assumption that first, gaseous BaO is formed 

by thermal decomposition of the impregnate materials. 
Barium gas is then formed by reduction of gaseous 

BaO by tungsten according to the reaction, 

If the partial pressure of BaO is known, the barium 

vapor pressure can be calculated from the equilibrium 

constant of Equation 1, 

where PBa=PB)Ps1d signifies a pressure of barium 

relative to standard pressure, and PBaO has a similar 

definition. The equilibrium constant is given by the 

exponential; the arguments of the exponential, &I and 

L\S, are the enthalpy and entropy of reaction, 

respectively. 

Barium oxide gas is produced by the thermal 

decomposition of the barium-containing phases of the 

impregnate material. The phase with the largest partial 

pressure of gaseous BaO will determine the vapor 

pressure of BaO above the impregnate. The vapor 

pressures of BaO produced by the thermal 

decomposition of pure phases are calculated using the 

equation 

PBaO (!'!J{ (,s) PBaO =p=exp -Rf+T ' SId 

(3) 

where the definition of the variables are analogous to 

those of equation 2. The thermodynamic constants, 

&I and L1S, for the reactions used by this model were 

tabulated by Lipeles and Kan or were calculated by the 

method of Lipeles and Kan [9J. 

The phase composition of the impregnate determines 

which thermal decomposition reactions are potentially 

active in the production of BaO gas. For a given 

stoichiometric composition of the impregnate, the 

phase composition is determined from the BaO-CaOAb03 phase diagram [11]. The relative molar 

percentages of BaO, CaO, and AhO) in the impregnate 

are determined, and the position of the impregnate on 

the diagram is found. The relative molar amounts of 

the constituent phases are then determined by the lever 

rule, as described in [9]. The thermal decomposition 

reaction of each phase is assessed for the partial 

pressure of barium oxide it produces. For 

compositions consisting of three pure phases, the 

NASA/CR-2001-211290 3 

partial pressures above the phases are calculated from 

equation 3. For compositions consisting of two 

phases, either a pure phase in equilibrium with a solid 

solution or two solid solutions in equilibrium, the solid 

solutions are assumed to be ideal. The vapor pressure 

above an ideal solution is the sum of the vapor 

pressures of the component pure phases weighted by 

the mole fractions of each pure phase in the solution. 

Thus the vapor pressure above a solid solution is given 

by 

(4) 

where Xl and Xc are the mole fraction of phase I and 2 

and Xl + Xc = 1. Once the vapor pressure above each 

phase present, either pure phase or solid solution, is 

calculated. the vapor pressures are compared. The 

phase with the highest vapor pressure of gaseous BaO 

is the active phase. It will produce BaO and suppress 

the other decomposition reactions. 

The supply of gaseous BaO and Ba from the interior of 

the emitter pores to the interior volume of the hollow 

cathode was modeled as free molecular flow. 

Assuming cylindrical pores on a typical cathode 

emitter, the pore area is -20 j.lm2

, and the pore 

diameter is then -5 j.lm. The mean free path of a 

barium atom or barium oxide molecule in 10 Torr of 

Xe gas is about 20 j.lm. Thus, since the mean free path 

is approximately 4 times larger than the pore diameter, 

free molecular flow was assumed to be a relatively 

valid model for gaseous species escape through the 

emitter pores. Note that, for this model, only the 

inward-facing surface of the emitter cylinder was 

assumed to supply gaseous species. Supply from the 

surface facing the cathode tube wall was neglected. 

The mass flow rate out of a pore is given by 

mow =Apo"c~'):[Je:- ~], (5) - T,nserl T ~a\ 

where Apm. is the cross-sectional area of the pore, M is 

the molecular mass of the gas, PilLlcrt and Tin,crt are the 

pressure and temperature of the gas within the emitter 

pore, and P calhode and T~as are the temperature and 

pressure of the gas in the cathode interior. For the 

purposes of this model, the emitter temperature was 

calculated using transfer functions derived from the 

data of Salhi [12]. The data in Table 1, taken from 
[12], were fit by multiple linear regression to generate 

the transfer function for emitter temperature. The 

pressures of gaseous species within the emitter pores 

were calculated by Equations 2 and 3. The gas 

temperature, both within the emitter pores and within 

the cathode interior volume, was assumed to be 

approximately equal to the emitter temperature. The 

pressure of gaseous species in the cathode interior was 

determined using a simple one-dimensional diffusion 

model of the cathode. 

Diffusion model 

A simple, steady-state. one-dimensional diffusion 

model with advection was used to estimate the loss of 

Ba and BaO gas from the interior of the cathode. In 10 

Torr of Xe at 1373 K, the mean free path of Ba is 

approximately 20 Jlm. Thus, the mean free path is 

much smaller than the 6.35 mm diameter cathode tube. 

For a 7 sccm flow rate of Xe gas at 1373 K and 10 

Torr, the gas velocity in the interior of a 0.25 inch 

diameter cathode is 4 mls. Given this velocity, the 

advective flux is estimated to be an order of magnitude 

greater than the diffusion flux, requiring inclusion of 

the advection term. Since the velocity is derived from 

a regulated Xe flow, and since the source of Ba and 

BaO varies relatively slowly with time, a steady-state 

model was employed. 

A schematic of the cathode diffusion model is shown 

in figure 2. The loss of Ba and BaO through the 

cathode orifice was approximated as the flow rate of 

Xe through the orifice times the mole fraction of Ba or 

BaO in the Xe. Ba and BaO were assumed to 

condense on the cathode tube upstream of the emitter. 

Using a thermodynamic equation analogous to 

Equation 3 for the vapor pressure of either Ba or BaO 

above a surface at temperature T and the ideal gas law, 

a density gradient could be related to the temperature 

gradient along the surface. Thus, the diffusion flux at 

the upstream end of the emitter was given by 

-D n =-- -1 - Dn [/)J{ ""up l~ dx x:L T RT dx x:L ' 

(6) 

where D is the diffusion coefficient, 11 is the gaseous 

species density, T is the cathode tube surface 

temperature, Me\ap is the heat of evaporation, and L 

denotes the upstream end of the emitter insert. All 

gradients and spatially dependent variables are 

evaluated at the upstream end of the emitter. Ba and 

NASA/CR-2001-211290 4 

BaO gas release from the pores on the emitter surface 

into the cathode interior was approximately treated as 

generation within the cathode volume. The diffusion 

equation was solved by the finite difference method. 

Thermochemistry and diffusion model solution 

The thermochemistry and diffusion models are solved 

serially and then repeated to march the solution 

through time. Initially the cathode current, orifice 

diameter, flow rate, pore count and emitter density, 

and emitter dimensions are input. In addition, the 

relative molar percentages of BaO, CaO, and Al 20 3 in 

the impregnate are given. Next, the cathode internal 

pressure and emitter temperature profile are calculated 

from transfer functions derived from the data of Salhi 

[12]. The Xe pressure is used to calculate the 

diffusion coefficient. The emitter temperature and 

impregnate composition are fed into the 

thermochemistry model. The impregnate 

stoichiometric composition is applied to the BaOCaO-Ah03 phase diagram to determine the impregnate 

phase composition. The partial pressure of BaO above 

each phase is determined from the decomposition 

reaction equilibrium constant. The phase with the 

largest partial pressure is assumed to be the active 

phase, and the BaO pressure is set to the partial 

pressure of its decomposition reaction. This pressure 

is then used in equation 2 to determine the pressure of 

gaseous Ba in the emitter pores. Given the pressure 

inside the emitter pores, the diffusion model can be 

self-consistently solved to calculate the Ba and BaO 

gas densities within the cathode. In addition, the flow 

rate of gaseous Ba and BaO into or out of the emitter 

pores is calculated. From the flow rate, the amount of 

Ba and BaO lost from or gained by the emitter can be 

calculated, and a new impregnate composition is 

computed. The time step is indexed, and the solution 

is repeated. In this manner, the model is stepped 

through time, to the specified end point. 

Model Results 

Since the emitter temperatures and cathode internal 

pressures are derived from an empirical expression, 

unless otherwise noted, model results are for a cathode 

geometry set to match that of Salhi [12]. As such, the 

modeled cathode inner diameter was 5.32 mm, the 

emitter inner diameter was 3.8 mm, and the emitter 

length was 25.4 mm. Cathode currents were allowed 

to vary between 1 and 10 A, xenon flow rates could 

vary from 7 to 13 sccm, and cathode orifice diameters 
could range from 1.2 to 2.0 mm. Except for the 

NST AR cases, cathode current was 3.0 A, the flow 

rate was 7 sccm, and the orifice diameter was 1.2 mm. 

For the initial runs of this hollow cathode emitter 

chemistry model, an emitter initial composition of 

4BaO: CaO: AhO) was modeled. This composition 

falls in a three phase region of the phase diagram and 

is composed of BaO, CaO, and BaH~Cao.~6Ah07' 

Figure 3 shows the partial pressures as a function of 

temperature of the barium containing species of the 

impregnate. The thermal decomposition of BaO is the 

highest vapor pressure reaction, and will therefore be 

the dominant reaction of the impregnate initially. As 

the BaO is depleted, the impregnate composition will 

cross into a two phase region, with CaO in equilibrium 

with a solid solution varying in composition between 

Ba374Cao.26Ab07 and Ba3508CaoA92Ah07. The BaO 

vapor pressure from the thermal decomposition 

reaction of both of these compounds is similar to that 

of Ba3.74Cao.26AI~07' The thermal decomposition of 

the solid solution is the dominant reaction in this 

region. The vapor pressure will range between the 

vapor pressures of the two pure phases determined by 

the mole fraction of each in the solid solution, as given 

by Equation 4. The vapor pressure of BaO above both 

compounds is much lower than that above solid BaO, 

as can be seen in Figure 3. As a consequence, the loss 

rate of Ba from the emitter drops precipitously once 

the boundary between the three and two phase regions 

is passed. 

The effect of this sharp drop in BaO pressure is seen in 

the loss rates of Ba and BaO from the modeled 

cathode after 2000 model hours of operation. Figure 4 

shows the rate at which Ba and BaO are lost from and 

returned to the emitter as a function of axial position 

relative to the orifice plate. Note the sharp drop in 

loss rate between the orifice at ° mm and 

approximately 12 mm. In the portions of the cathode 

emitter close to the orifice, the temperatures are higher 

and consequently the Ba and BaO pressures within the 

emitter are higher. This increases the rate gaseous 

species exit the emitter pores. Thus, the BaO content 

of the impregnate in the downstream region is more 

quickly depleted than the upstream region. Eventually 

the impregnate composition is such that a new region 

on the phase diagram is entered. The pressure of Ba 

and BaO gas then drops precipitously. If the partial 

pressure of Ba and BaO in the cathode, but exterior to 

the emitter, is large enough, the process is reversed 

NASA/CR-2001-211290 5 

and BaO is returned to the emitter. This is seen in 

figure 4 between 9 and 13 mm from the orifice plate, 

where the return rate exceeds the loss rate. 

As mentioned above. the largest change in the Ba 

content of the impregnate occurs in the downstream 

sections of the emitter. Figure 5 shows the fraction of 

BaO lost from the emitter as a function of position and 

time. Examining figure 5, a steep composition 

gradient is observed in the first few millimeters of the 

emitter early in time. Upstream of the gradient 

(toward positions further from the orifice), the 

composition of the emitter remains relatively 

unchanged. Downstream of the gradient (toward 

positions nearer the orifice). significant amounts of 

BaO have been lost. This steep gradient moves 

upstream with time, but decelerates as cooler regions 

of the emitter begin contributing to the loss of BaO. 

The BaO lost from the emitter enters the cathode 

interior as gaseous BaO or as gaseous Ba. From the 

gas phase. it is available for deposition on the insert 

surface for work function lowering. 

To assess the differences between cathodes operated 

under different conditions, two NSTAR-like cases 

were compared. The NST AR discharge cathode at 

throttle level THO, or minimum power, and at THIS, 

or maximum power, was modeled though 10,000 

hours of operation [13]. The temperature distribution 

and cathode internal pressure calculations were 

modified from that of [12] to more accurately 

represent the NST AR cathode. Figure 6 compares the 

fraction of BaO lost from the emitter insert after 

10,000 model hours of running at conditions 

simulating the THO and TH 15 operating points. After 

10,000 hours of operation at THIS, the model predicts 

up to 90 percent of the BaO originally present in the 

downstream end of the emitter has been depleted. By 

comparision, at most 16 percent of the BaO is depleted 

in 10,000 model hours at THO. This difference is 

likely due to the increased operating temperature of 

the cathode at TH 15. 

Assuming the barium responsible for lowering the 

emitter work function is deposited from the gas phase 

barium in the cathode interior, the axial barium density 

in the cathode should be related to cathode 

performance. Examination of Figure 7 shows that the 

barium density in the TH IS cathode is still at least one 

order of magnitude higher than the barium density 

within the THO cathode at the downstream end. So 
even though the fraction of barium lost from the 

emitter is larger for the TH 15 cathode, it is still 

expected to have ample gas phase Ba available to 

lower the emitter work function. 

An interesting difference is noted when a modeled 

cathode run for 8,000 hours at THIS is compared to a 

modeled cathode run for 8,000 hours at THO. When 

both cathodes are compared at THO after 8,000 hours 

at different conditions, Figure 8 shows that the gaseous 

Ba density is different for these two cathodes. At the 

downstream end of the cathode, where work function 

lowering is most important, the cathode run at THO for 

8,000 hours has a barium density 4 times higher than 

its counterpart run at THIS. Compositional 

differences arise due to the difference in cathode 

operating power, which results in lower densities when 

the cathodes are subsequently compared at the same 

power. The compositional differences result in 

differing decomposition reactions between the two 

cathodes, which in tum results in different Ba supply 

rates. The gas phase Ba density difference could 

affect the performance of cathodes operated at low 

power, which were run previously at high power for 

extended periods. These results suggest that cathode 

operating-power history also affects cathode life and 

operability. 

Conclusions 

A coupled thermochemistry/diffusion model has been 

developed for xenon hollow cathode operation that 

shows good qualitative agreement with observable 

phenomena in hollow cathodes. The thermochemical 

model from Lipeles and Kan [9] has been used to 

estimate the supply of gaseous Ba and BaO from the 

impregnated tungsten emitter. This supply is coupled 

to a simple, one-dimensional diffusion model of the 

cathode to examine the transport of these gases 

through the xenon gas flowing through the cathode. 

The model has been used initially to examine the 

evolution and loss of Ba from the emitter impregnate. 

Model results at differing cathode power levels have 

also been compared. 

Several key results have been found during the 

inaugural model runs. The effect of emitter 

composition on the supply of Ba and BaO gas to the 

interior of the cathode has been quantified. The higher 

temperature downstream region of the emitter, near the 

orifice, has been shown to be most active in the 

NASA/CR-2001-211290 6 

production of Ba and BaO. The fraction of barium lost 

from the emitter is largest near its downstream end. 

The region of BaO loss moves upstream with time, 

decelerating as cooler regions of the emitter are 

reached. 

An NSTAR-like cathode operated at THIS and THO 

was also modeled. The fraction of BaO lost from the 

modeled cathode run for 10,000 model hours at THIS 

was shown to be much greater than the cathode run at 

THO. This was attributed to the higher operating 

temperature of the cathode at THIS. In spite of its 

compositional differences, the cathode run at THIS 

was shown to produce more gaseous Ba than the THO 

cathode. By contrast, after 8,000 hours of operation, a 

lower Ba density was produced internally in a cathode 

run at TH IS compared to a cathode run at THO, when 

both were compared at THO. The lower Ba density 

was due to differences in the emitter chemistry 

between the two cathodes. This suggested that the 

history of cathode operating power could be a 

significant parameter in determining useful cathode 

life. 

References 

[I]. Kaufman, H.R, "Technology of Electron 

Bombardment Ion Thrusters," Advances in Electronics 

and Electron Physics, vol. 36, New York: Academic 

Press, Inc., 1974. 

[2]. Patterson, MJ., et aI., "Space Station Cathode 

Design, Performance, and Operating Specifications," 

25th International Electric Propulsion Conference 

Proceedings, IEPC Paper No. 97-170, August 24-28, 

1997. 

[3]. Springer, R.W., Haas, T.W., "Auger Electron 

Spectroscopy Study of Cathode Surfaces During 

Activation and Poisoning I. The Barium-on-Oxygenon-Tungsten Dispenser Cathode," J. Appl. Phys. 45. 

5260-5263, December 1974. 

[4]. R Levi, "Improved "Impregnated Cathode," J. 

Appl. Phys. 26. 639, May 1955. 

[5]. Cronin, J.L., "Modem Dispenser Cathodes," 

IEEE Proc.128. 19-32, February 1981. 

[6]. Rittner, E.S., Rutledge, W.e., Ahlert, RH., "On 

the Mechanism of Operation of the Barium Aluminate 

Impregnated Cathode," J. Appl. Ph,Vs. 28. 1468-1473, 

December 1957. 
[7]. Rittner, E.S., "On the Mechanism of Operation of 

the Type B Impregnated Cathode," J. Appl. Phys. 48. 

4344-4346, October 1977. 

[8]. Suitch, P.R., Doctoral Dissertation, Georgia 

Institute of Technology, Atlanta, Georgia, 1987. 

[9]. Lipeles, R.A., Kan, H.K.A., "Chemical Stability 

of Barium Calcium Aluminate Dispenser Cathode 

Impregnants," Appl. Swi Sci. 16.189-206, 1983. 

[10]. Sarver-Verhey, T.R., "Destructive Evaluation of 

a Xenon Hollow Cathode After a 28,000 Hour Life 

Test," 34th A1AAIASMEISAEIASEE Joint Propulsion 

Confercncc and Exhihit Proceedings, AIAA Paper No. 

98-3482, July 13-15,1998. 

[11]. Wolten, G.M., "An Appraisal of the Ternary 

System BaO-CaO-AhO)," SDTR-80--67, Space 

Division, Air Force Systems Command, Los Angeles, 

California, October, 1980. 

NASNCR-2001-211290 7 

[12]. Salhi, A., Doctoral Dissertation, The Ohio State 

University, Columbus, Ohio, 1993. 

[13]. Rawlin, V.K .. et al., "NSTAR Right Thruster 

Qualification Testing," 34th A1AAiASMEISAEIASEE 

Joillf Propulsion Confercncc Proccedings, AIAA 

Paper No. 98-3936, July 13-15, 1998. 

[14]. Soulas, G.c., "Multiple Hollow Cathode Wear 

Testing for the Space Station Plasma Contactor," 30th 

A1AAIASMEISAEIASEE Joillf Propulsion COl!fercncc 

Procecdings, AIAA Paper No. 94-3310, June 27-29, 

1994. 
Table I. Data taken from [12] to generate a transfer function for emitter insert temperature as a function of 

position, discharge current, and orifice diameter. Values in parentheses are interpolated values. 

Emitter Insert Temperature Profile (K) 

Orifice Distance from Orifice (mm) 

Diam. = 

1.21 mm 1 6 16 25 

10 1328 (1268) 1149 1075 ~ 

0-< e?-- 7 1259 (1203) 1092 1020 

...c ~ -::: Co) 0 

.~ t:: 4 1178 (1127) 1025 955 o =s U 

2 1117 (1066) 963 896 

Orifice Distance from Orifice (mm) 

Diam. = 

0.76mm 3 6 13 22 

10 1342 1232 ( 1160) 987 Q)~ O/J __ 

7 1291 1188 1101 967 a - ...c ::: Co) 0 

VJ t:: 4 1247 1152 1077 944 .- =s °u 2 1160 1079 1009 865 

Shea1hed Huler 

Electrical Leads 

XenonAow ,.. 

CathodeTubc ~~\ 

Radiation 

Shield 

Orifice 

Plate 

Figure 1. Diagram of a xenon hollow cathode (drawing not to scale) [14]. The direction of xenon flow is 

indicated. 

NASNCR-2001-211290 8 
I 

I 

! 

Cathode tube 

... I ! I ....:ld~f+On up~tre::.r ~T / dr J _ 

I I I 

n=N n=N-l 

Orifice plate 

I 

nll/LINr I 

I I i I 

Ii· \ _. ! P~al l 

morlti·cc - '11 Xc * -- __ I -;~~ 

.L •• _._.~_. I t .J. __ 

P Yc I i. 

I I I ! I 

n=3 n=2 n = I 

••• 

Figure 2. Schematic diagram of the hollow cathode diffusion model. The slices indicate slices of the 1-0 

diffusion model. Upstream and downstream boundary conditions are indicated. Ba or BaO gas supply from the 

emitter insert is treated as volume generation within a slice. 

e1.E-07 

a; 1.E-08 Thermal Decomposition of BaO 

-; 1.E-09 +------------------:::;;;;0.--=-------:::.1 

:; 1. E-1 0 +--------------= __ =-------=::;0..-=------1 

tn 

~ 1.E-11 

a: 1.E-12 +--------:::0.1~----~--='-------------1 

ca 1.E-13 -f-----,..OJIIII!!"'--------c-"""""---------------1 i 1. E-14 +--~"""--------::;;.,.-.=-----------------I 

a. 1.E-15 +-----::;;;o~----------------_I 

~ 1.E-16 +-~~- Thermal Decomposition of Ba4<lal:Pa~lp7 aJ l.E-17 ~-----r------,.----r__---~---_t 

1000 1100 1200 1300 

Temperature (K) 

1400 1500 

Figure 3. Partial pressure of gaseous BaO for the thermal decomposition of pure BaO and of a B14Ab07 with 

substitutional Ca. These are the phases in 4: 1: 1 dispenser cathode impregnates active in production of gaseous 

BaO. 

NASA/CR-2001-211290 9 
1.E+12~--------------------~~~~-----------. 

cn1.E+10 

:ti: 

-

c:: 'm 1.E+08 ~~ ...... =- C) 

... 

o 

tn 1.E+06 

tn 

.2 

'01.E+04 

Ci) 

~ -l.E+02 -Ba effusion rate (#/s) -Ba return rate (#/s) 

i-BaO effusion rate (#/s) -BaO return rate (#/s) 

1 .E+OO +-=::;:::::;;;;::;:::::;;;;~::;:::::;;;;::;:::::;;;;~~::;:::::;;;;::;:::::;;;;;;;r;::::;:::::;;;;::;:::::;;;;~::;:::::;;;;::;:::::;;;;::;:::::;;;;~;;;;;;:""---1 

0.00 5.00 1 0.00 15.00 20.00 25.00 30.00 

Position (mm) 

Figure 4. Rate of outward flow of Ba or BaO from the emitter or rate of return of those gases back to the emitter 

after 2000 hours of model operation. Rates stated in number per second. Discontinuity in outward flow rate is 

due to a localized phase change after loss of BaO from the impregnate. Dominant BaO producing phase changes 

from a high vapor pressure phase to a low vapor pressure phase. 

25 

20 

E 15 

S 

c 

o 

; 

.; 

~ 10 

5 

o 1000 2000 3000 4000 5000 6000 7000 8000 9000 

Time (hours) 

18 

16 

14 

"D 

II 

.. 

() 

CD 

::I 

... 

1/ 

CD 

II 

0 

-

CD 

1/ 

0 

r 

0 

en 

... 

Figure 5. Fraction of the original barium oxide lost from the emitter over 9500 model hours. The color key 

indicates the fraction, with gray representing zero barium oxide lost and white representing 18 percent barium 

oxide lost. 

NASNCR-200I-211290 10 
0.4l 

0.2l 

o 5000 

Time (hours) 

16 

14 

12 "IJ 

<II 

.. 

(") 

<II 

::I 

10 ;; 

0 

6 

4 

2 

o 

10000 

cc 

<II 

So 

ttl 

'" 0 

r0 

~ 

a 

-I 

g 

l 

O.Ol 

0.6l 

C 

.g 

'en 

0 

C. 

0.4l 

O.2l 

o 5000 

Time (hours) 

10000 

20 

10 

Figure 6. Comparison of BaO loss from the emitter insert of an NST AR-like discharge cathode modeled at 

throttle levels THO and THIS. Cathode operation over 10,000 hours at each throttle level was modeled. Color 

keys to the right of each contour plot indicate the percentage of BaO lost. Position given relative to L, which 

represents the NST AR emitter insert length. 

logarithm of Ba Density at ThO (log(IImJ» logarithm of Ba Density at Th15 (log(IImJ » 

l 

,.' ~ l 

13 'r 

,,4 , O.Ol 

,~ O.Ol 

I 

I 

I 

0.6l 14 14J c c I 

.g 12--, 0 

;; 

'en j ~ 0 0 c. c. 

0.4l 

13 

15 15J 

0.2l 0.2l 

~ :15 J 

0 2000 4000 GOOO 0000 10000 0 2000 4000 6000 8000 10000 

Time (hours) Time (hours) 

Figure 7. Comparison of gaseous Ba density in the interior of an NST AR-like discharge cathode modeled at 

throttle levels THO and THIS. Cathode operation over 10,000 hours at each throttle level was modeled. Position 

given relative to L, which represents the NST AR emitter insert length. 

NASA/CR-200I-2II290 II 
16 .......... Ba Density after 8000 h at THO 

-Ba Density at THO after 8000 h at TH15 

15 

10 

9 +-----~------~------~----~----~ 

0.00 0.20 0.40 0.60 0.80 1.00 

Relative Position on Emitter (x/L) 

Figure 8. Comparison of gaseous Ba density in the interior of an NST AR-Iike discharge cathode modeled at 

throttle levels THO and THI5. Ba densities are produced at THO for both cathodes, after 8000 hours of operation 

at their individual operating points. Position given relative to L, which represents the NST AR emitter insert 

length. 

NASA/CR-2001-211290 12 
REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188 

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1. AGENCY USE ONLY (Leave blank) 

1

2

. 

REPORT DATE r' REPORT TYPE AND DATES COVERED 

December 200 I Final Contraclor Repon 

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS 

Life Model of Hollow Cathodes Using a Barium Calcium 

Aluminale Impregnated Tungsten Emitter 

6. AUTHOR(S) 

WU-4 77-72-10-00 

NAS3-00145 

SoD. Kovaleski 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION 

REPORT NUMBER 

QSS Group. Inc. 

21000 Brookpark Road E-13094 

Cleveland. Ohio 44135 

9. SPONSORINGIMONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORINGIMONITORING 

AGENCY REPORT NUMBER 

National Aeronautics and Space Administration 

Washington. DC 20546-0001 NASA CR-2001-211290 

IEPC--OI-276 

11. SUPPLEMENTARY NOTES 

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. Project Manager. Tom Burke. Systems 

Engineering Division, NASA Glenn Research Center, organization code 7800, 216-433-51 n 

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE 

Unclassified - Unlimited 

Subject Category: 20 Distribution: Nonstandard 

Available electronically at htlJl:Ugltrs.grc.nasa.gov/GLTRS 

This publication is available from the NASA Center for AeroSl'ace Infonnation. 301--621--D390. 

13. ABSTRACT (Maximum 200 words) 

Hollow cathodes with barium calcium aluminate impregnated tungsten emitters for thermionic emission are widely 

used in electric propulsion. These high current, low power cathodes are employed in ion thrusters, Hall thrusters, and 

on the International Space Station in plasma contactors. The requirements on hollow cathode life are growing more 

stringent with the increasing use of electric propulsion technology. The life limiting mechanism that determines the 

entitlement lifetime of a barium impregnated thermionic emission cathode is the evolution and transport of barium 

away from the emitter surface. A model is being developed to study the process of barium transport and loss from the 

emitter insert in hollow cathodes. The model accounts for the production of barium through analysis of the relevant 

impregnate chemistry. Transport of barium through the approximately static gas is also being treated. Finally. the 

effect of temperature gradients within the cathode are considered. 

14. SUBJECT TERMS 15_ NUMBER OF PAGES 

Hollow cathode; Barium dispenser cathode; Barium calcium 18 

aluminate thermochemistry: Cathode life 16. PRICE CODE 

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT 

OF REPORT OF THIS PAGE OF ABSTRACT 

Unclassified Unclassified Unclassified 

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

Prescribed by ANSI Std. Z39-18 

298-102