John E. Foster and Michael J. Patterson

Glenn Research Center, Cleveland, Ohio

Plasma Emission Characteristics From a High

Current Hollow Cathode in an Ion Thruster

Discharge Chamber

NASA/TM—2002-211876

November 2002

AIAA–2002–4102
The NASA STI Program Office . . . in Profile

Since its founding, NASA has been dedicated to

the advancement of aeronautics and space

science. The NASA Scientific and Technical

Information (STI) Program Office plays a key part

in helping NASA maintain this important role.

The NASA STI Program Office is operated by

Langley Research Center, the Lead Center for

NASA’s scientific and technical information. The

NASA STI Program Office provides access to the

NASA STI Database, the largest collection of

aeronautical and space science STI in the world.

The Program Office is also NASA’s institutional

mechanism for disseminating the results of its

research and development activities. These results

are published by NASA in the NASA STI Report

Series, which includes the following report types:

• TECHNICAL PUBLICATION. Reports of

completed research or a major significant

phase of research that present the results of

NASA programs and include extensive data

or theoretical analysis. Includes compilations

of significant scientific and technical data and

information deemed to be of continuing

reference value. NASA’s counterpart of peerreviewed formal professional papers but

has less stringent limitations on manuscript

length and extent of graphic presentations.

• TECHNICAL MEMORANDUM. Scientific

and technical findings that are preliminary or

of specialized interest, e.g., quick release

reports, working papers, and bibliographies

that contain minimal annotation. Does not

contain extensive analysis.

• CONTRACTOR REPORT. Scientific and

technical findings by NASA-sponsored

contractors and grantees.

• CONFERENCE PUBLICATION. Collected

papers from scientific and technical

conferences, symposia, seminars, or other

meetings sponsored or cosponsored by

NASA.

• SPECIAL PUBLICATION. Scientific,

technical, or historical information from

NASA programs, projects, and missions,

often concerned with subjects having

substantial public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific

and technical material pertinent to NASA’s

mission.

Specialized services that complement the STI

Program Office’s diverse offerings include

creating custom thesauri, building customized

databases, organizing and publishing research

results . . . even providing videos.

For more information about the NASA STI

Program Office, see the following:

• Access the NASA STI Program Home Page

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

• E-mail your question via the Internet to

help@sti.nasa.gov

• Fax your question to the NASA Access

Help Desk at 301–621–0134

• Telephone the NASA Access Help Desk at

301–621–0390

• Write to:

 NASA Access Help Desk

 NASA Center for AeroSpace Information

 7121 Standard Drive

 Hanover, MD 21076
John E. Foster and Michael J. Patterson

Glenn Research Center, Cleveland, Ohio

Plasma Emission Characteristics From a High

Current Hollow Cathode in an Ion Thruster

Discharge Chamber

NASA/TM—2002-211876

November 2002

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

38th Joint Propulsion Conference and Exhibit

cosponsored by the AIAA, ASME, SAE, and ASEE

Indianapolis, Indiana, July 7–10, 2002

AIAA–2002–4102
Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

This report contains preliminary

findings, subject to revision as

analysis proceeds.

Available electronically at http://gltrs.grc.nasa.gov

Trade names or manufacturers’ names are used in this report for

identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.
NASA/TM2002-211876 1 

Plasma Emission Characteristics From a High Current Hollow Cathode in an 

Ion Thruster Discharge Chamber 

John E. Foster and Michael J. Patterson 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

The presence of energetic ions produced by a hollow cathodes operating at high emission currents (>5A) has 

been documented in the literature. In order to further elucidate these findings, an investigation of a high 

current cathode operating in an ion thruster discharge chamber has been undertaken. Using Langmuir 

probes, a low energy charged particle analyzer and emission spectroscopy, the behavior of the near-cathode 

plasma and the emitted ion energy distribution was characterized. The presence of energetic ions was 

confirmed. It was observed that these ions had energies in excess of the discharge voltage and thus cannot be 

simply explained by ions falling out of plasma through a potential difference of this order. Additionally, 

evidence provided by Langmuir probes suggests the existence of a double layer essentially separating the 

hollow cathode plasma column from the main discharge. The radial potential difference associated with this 

double layer was measured to be of order the ionization potential. 

Nomenclature 

D⊥ = Electron cross-field diffusion 

coefficient 

e = Elementary charge of an electron 

E = Ion energy 

Er = Sheath electric field 

e J = Electron cross-field current 

density 

e n = Electron number density 

μ = Electron mobility 

L = Distance between ESA and 

entrance 

e n = Electron number density 

R = Electrostatic energy analyzer 

mean radius 

r = Radial location

1

r = Radius of inner sector of 

 electrostatic energy analyzer 

2r = Radius of outer sector of 

 electrostatic energy analyzer 

Vp = Plasma potential measured with 

 respect to cathode potential 

U = Upper level excited state energy 

∆E = ESA energy resolution 

∆V = Potential difference between ESA 

 spherical sectors 

φ = Sector angle 

ω = Entrance aperture diameter 

Introduction

 Deep space missions utilizing ion thrusters will 

require very long thrusting times. Such long operation 

times place demanding requirements on the lifetime of 

thruster components. Components such as the ion 

optics and the discharge and neutralizer hollow 

cathodes are continuously subjected to erosion 

processes driven primarily by ion bombardment via 

either direct impingement or charge exchange. Hollow 

cathodes are peculiar in that not only are they subjected 

to ion bombardment from the surrounding discharge 

plasma, but also, as evidence suggests, they may 

actually be the source of very energetic ions that can 

enhance erosion of itself as well as erosion of upstream 

surfaces such as the screen grid. Indeed it was the 

erosion of internal discharge chamber structures such as 

the cathode baffle that led to the first suppositions 

hinting at the existence of a energetic heavy particle 

beam emanating from the discharge cathode. Studies 

led by Manteneiks,1

 Rawlin,2 and Brophy/Garner3 all 

observed severe baffle erosion that was suggestive of 

energetic ion bombardment. Brophy/Garner3 postulated 

that the observed severe cathode baffle erosion was due 

to energetic heavy particle bombardment, presumably 

energetic xenon ions. Findings from these studies 

eventually led to the initiation of studies aimed at 

determining the underlying mechanism for the observed 

erosion. The bell jar studies of Wilbur4

 determined that 

there were indeed very energetic ions originating at the 

discharge hollow cathode. These studies indicated that 

the magnitude of the energetic ion component increases 

with increasing discharge current. To date, the 

production of energetic ions in hollow cathode 
NASA/TM2002-211876 2 

discharges has been documented by a number of 

studies.4,5,6 Theories attempting to describe the 

production mechanisms of these ions range from the 

existence of a potential hill,4,6,7 to 

magnetohydrodynamic5

 effects to hydrodynamic drag 

driven primarily by electron-ion collisions.8 Presently, 

however, there has been no satisfactory complete 

explanation offered for the production of the energetic 

ions. 

 The purpose of the study presented here is to add to 

the body of knowledge regarding energetic ion 

production in high current hollow cathode discharges. 

The study presented here is unique in that the ion 

energy distribution is measured in a multi-cusp 

magnetic environment characteristic of ion thruster 

operation. In this respect the findings are likely more 

representative of ion thruster operation in regards to 

plasma production mechanisms and in particular plasma 

containment by the axial magnetic field in the nearcathode region.9 

 In addition to the analysis of the ion efflux from the 

cathode, the near-cathode plasma was probed with 

Langmuir and emission spectroscopy probes to measure 

local plasma properties such as density and plasma 

potential. Langmuir probe measurements were used to 

survey axial and radial plasma potential profiles. The 

emission spectroscopy measurements were used to 

detect the presence of doubly charged xenon ions. 

These data provides additional insight into the possible 

physical mechanisms of energetic ion production as 

well as useful insight into other possible erosion 

phenomena at the cathode. 

Experimental set-up

 The experiments were conducted in a 91 cm long by 

48 cm in diameter cryopumped bell jar shown in 

Figure 1. The ìbellî, which was made of Pyrex, 

allowed for excellent visual inspection of the discharge. 

The discharge itself was generated using a 12.7 mm 

diameter hollow cathode. The cathode assembly 

included a coaxial keeper electrode. This electrode was 

not biased during this investigation. The hollow cathode 

was installed in a three-magnet ring, 30-cm discharge 

chamber of similar geometry to that of the NASA Solar 

Electric Propulsion Technology Application Readiness 

(NSTAR) thruster. Again, the choice of conducting the 

investigation of energetic ion efflux using a hollow 

cathode in a ring cusp discharge chamber was made 

primarily to determine the nature of the ion efflux in a 

thruster-like environment. The magnetic field near the 

cathode tip was approximately 90 G. For this study the 

hollow cathode was operated grounded. 

 The energy distribution of the ion flux emanating 

from the hollow cathode was characterized using an 

electrostatic spherical sector energy analyzer (ESA). 

The ESA was located on axis with the hollow cathode, 

separated by approximately 23 cm. A schematic of this 

experimental set-up is illustrated in Figure 2. The 

analyzer consisted of two spherical sectors (141 degrees 

in this case) in which the potential of each relative to 

ground was varied. The potentials on these surfaces 

determine the energy of which an entering particle must 

have in order to pass through from entrance slit to exit 

slit. In this respect, the ESA can be operated as an 

energy selector. The energy of the particles that can 

pass through can be calculated based on the geometry 

and potential difference between the spherical surfaces: 

1

2

2

1

r

r

r

r

V E

−

∆ = (1) 

with an energy resolution also determined by geometry 

and energy: 

φ φ

ω

⋅(1− cos ) + ⋅sin ≈ ∆E R L

E

 (2) 

 The ESA was operated in a constant transmission 

energy mode. A schematic of the electrical connections 

for operation and data acquisition from the ESA is 

illustrated in Figure 2b. The transmission energy as 

determined by the potential difference between the two 

spherical surfaces was set at 10 eV. With this 

transmission energy, the energy resolution was 

approximately 0.347 eV. The input slit varied in 

potential so that ions with energies between 0 and 

approximately 100 eV could be scanned. This factory 

recommended mode of operation is similar to that used 

by Kameyama and Wilbur in their investigation of ion 

efflux from high current hollow cathodes.6

 The ESA 

was housed in an iron box so as to reduce the effect of 

the local magnetic field generated by the thruster 

discharge chamber. The magnetic field at the entrance 

slit of the ESA was less than 1 G. In order to minimize 

the flow of plasma into the device, a cup made of very 

fine, electro-etched nickel was placed over the entrance 

slit. The mesh make-up was 20 lines per mm with a 

0.039 mm spacing between lines. The nickel wire that 

made up the mesh was 0.01 mm in diameter. The mesh 

and the iron box were fixed at ground potential. The 

electrostatic energy analyzer was suspended over the 

discharge chamber via a perforated aluminum 

termination plate. The plate, which located roughly 

8.5 cm downstream of the discharge exit plane, 

contained 8ñ5.5 cm diameter holes and one 8.5 cm in 

diameter center hole. This plate was operated at 

cathode potential during this investigation. This open 

geometry was chosen because of its higher conductivity 

of neutrals away from the discharge chamber as 
NASA/TM2002-211876 3 

compared to discharge studies with a simulated grid 

terminating the exit plane. The open geometry of the 

test apparatus allows the discharge pressure to be 

maintained at values similar to that of the bell jar. 

Because the bell jar pressures are similar to those that 

prevail in ion thrusters (~0.013 Pa (~ 4 1 10− ⋅ Torr)), the 

discharge characteristics are expected to be similar to 

actual ion thruster operation, thereby circumventing 

issues associated with gridded discharge studies.10 This 

notion is supported by the modest discharge voltages 

measured (many of which were similar in magnitude to 

NSTAR engine operation (~26 V)). 

 The near-cathode plasma in the discharge chamber 

was characterized using three Langmuir probes located 

axially in the near vicinity of the plane that contains the 

keeper face (see Figure 2). The near-cathode probes 

tips were made from tungsten wire and were 0.38 mm 

in diameter and 3 mm long. Two of the probes also 

contained optical fibers to record line-integrated 

emission in this plane. This was accomplished using 

double bore alumina tubing. One bore contained the 

Langmuir probe tip while the other contained a 

recessed optical fiber. The fibers were recessed 

approximately 1 cm upstream of the bore opening to aid 

in the collimation of the collected light. The optical 

fiber had a core size of 400 micrometers and an 

aluminum protective outer sheath. These probes were 

positioned at a variety of locations ranging from near 

the anode wall to nearly on discharge axis in order to 

determine the potential and density distribution in the 

plane of the keeper. A fourth probe, 5.5 mm long, 

0.38 mm in diameter was used to measure plasma 

properties near the entrance slit of the ESA. 

 The plasma induced emission collected by the 

optical fibers was channeled to a 0.25 m spectrometer 

fitted with a photo-multiplier tube. The spectrometer 

contained a 1200 lines/mm grating blazed at 350 nm. 

Experimental Results and Discussion

Ion Energy Distribution Measurements

 The ion energy distribution was measured as a 

function of discharge current at a fixed flow rate of 4 

standard cubic centimeters per minute (SCCM). For all 

operating conditions, the bell jar pressure never 

exceeded 1.5 × 10ñ4 Torr. The discharge conditions 

investigated varied from 10 A to 22 A corresponding to 

discharge voltages varying between 26 V and 29 V. 

Before plasma measurements were taken, the discharge 

was allowed to stabilize at each operating condition. In 

all cases, the peak-to-peak oscillations in the discharge 

voltage were less than 3 volts. 

 Figure 3a shows typical variations in voltage as a 

function of discharge current. As can be seen here, 

initially the discharge voltage increases linearly with 

increasing discharge current up to nearly 16 A, at which 

point the discharge voltage appears to saturate. 

Figure 3b shows the variations in the ion energy 

distribution function at the different discharge currents. 

As can be seen here, the collected current at energies 

less than 8 eV did not have a strong dependence on 

energy. The slowly varying ion current collected in this 

range is likely due to low energy charge exchange ions 

generated between the nickel mesh and input aperture 

of the ESA. Such low energy ions have been observed 

in ESA spectra under similar conditions elsewhere.6 

The location of the primary peak of the ion current 

distribution did not vary appreciably with increasing 

discharge current. Its average value was approximately 

14 eV. The ions associated with this peak are due to 

ions accelerated into the detector due to the potential 

difference between the discharge plasma and the 

detector. In this case, the data suggests that the fall 

voltage between the detector and the plasma (on axis) 

was of order 14 V. This value should be representative 

of the potential difference between the discharge 

plasma and cathode potential surfaces. Interestingly, the 

potential difference between the plasma and the 

detector was not the discharge voltage. Consistent with 

this observation, Langmuir probes located near (~2 cm 

off axis) the ESA entrance slit indicated plasma 

potentials ~10 V less than the discharge voltage. These 

data suggest that the potential difference between the 

screen and discharge plasma in an actual ion thruster 

may also be less than the discharge voltage. In this case, 

ions do not enter screen grid apertures at the discharge 

voltage, but instead at a reduced energy. 

 It is interesting to note that the primary peak of the 

ion energy distribution was in all cases Gaussian in 

shape. The width of the peak tended to increase with 

increasing discharge current, from 14 eV at 10.5 A to 

over 21 eV at 18.3 A. Apparently the energy spread 

increases with increasing discharge current. Also 

according to Figure 3b, as the discharge current 

increases, the overall magnitude of the collected ion 

current at a given energy increases. Additionally, as 

with the increase in width of the primary peak, the 

extension of the edge of the distribution into higher 

energies increases with increasing discharge current. 

This implies that with increasing discharge current, the 

population of energetic ions increase. In this respect 

the ion current distribution may be characterized best as 

a Gaussian with a superthermal tail that increases in 

magnitude with increasing discharge current. The 

primarily Gaussian nature of the ion current distribution 

differs significantly from that observed in similar 

hollow cathode studies conducted in the absence of an 

ion thruster discharge chamber.4,6 In those cases, the 

distribution consisted of multiple peaks, some of which 
NASA/TM2002-211876 4 

were centered at energies well above the discharge 

voltage. It is likely that the presence of the magnetic 

field in the ion thruster discharge chamber of this 

present study, particularly the axial component, plays a 

key role in accounting for the differences. 

 In order to assess the changes in the energetic 

component of the ion current distribution, the ratio of 

energetic ion current associated with energies 27 eV 

and above to the total ion current collected over the 

energy range investigated was calculated. The value, 

27 eV, was chosen because it represents the lower limit 

for threshold sputtering of molybdenum by xenon 

ions.12 Molybdenum is a metal typically used in the 

construction of the cathode keeper and screen and 

accelerator grid electrodes. Because the low energy, 

charge exchange ion (see Figure 3b) flux incident upon 

the ESA slit are an artifact of the experimental set-up 

and are therefore not representative of actual ion flux 

emanating from outside the protective mesh, their 

contribution to the total ion current collected over the 

energy range investigated is neglected. The energy 

range containing the low energy ions was determined 

by taking the derivative of the distribution function to 

locate the region of the curve where the low energy ions 

contribution ended. It was assumed that the relevant ion 

distribution began at the knee of the derivative as 

illustrated in Figure 4a. Figure 4b shows the variations 

of ion current distribution function at energies just 

beyond the knee. 

 Figure 5 illustrates the behavior of the energetic ion 

current fraction (E>27eV) as a function of discharge 

current. As can be seen here the fraction of energetic 

ions on axis with energies capable of eroding cathode 

potential surfaces such as the screen grid tends to 

increase monotonically with discharge current. This 

finding is also similar to that observed by Kameyama 

and Wilbur.6 Also shown on the plot is a trendline 

which indicates a nearly quadratic increase in the 

energetic ion fraction with increasing discharge current. 

As can be seen in the plot, at discharge currents over 18 

A, nearly 15% of the ion current on axis have energies 

capable of eroding discharge chamber surfaces such as 

the screen grid. It should be pointed out that the ESA 

does not distinguish between different ion charge states. 

The energetic ion current detected is likely comprised 

of some fraction of doubly charged ions. This is to be 

expected because the discharge voltage under the 

operating conditions investigated was sufficiently high 

to generate doubly charged xenon. With this in mind, 

the energetic ion current fraction as determined here 

could be used to determine an upper bound on the axial 

energetic ion particle flux used in simple erosion 

calculations. 

Near-Cathode Plasma Properties

 Discharge plasma conditions were documented in 

the near-cathode region using Langmuir probes and 

emission spectroscopy. The Langmuir probes were 

used to measure the plasma density and potential 

distribution radially in the plane of the keeper and 

axially just downstream of the keeper. The emission 

spectroscopy was used primarily for the detection of the 

various xenon charge states in the discharge. 

B-field effects

 The axial magnetic field profile associated with the 

ring cusp magnetic circuit influences the nature and 

coupling of the hollow cathode discharge to the main 

discharge plasma. It is expected that the magnetic 

environment should significantly influence discharge 

plasma properties particularly in the near-cathode 

region. The axial field near the cathode tends to 

confine the hollow cathode external positive column. 

This field can significantly reduce radial electron 

diffusion and therefore plays a role in determining 

discharge impedance and stability. Axial plasma 

electrical conductivity is enhanced by the presence of 

the axial magnetic field. Electrons resulting from 

ionization along the axis cannot readily diffuse radially 

because of the constraining effects of the magnetic 

field. This effect coupled with the enhanced axial 

electrical conductivity would tend to smooth out 

potential structures on axis particularly in the region 

close to the cathode where the axial field is large (~90 

G). This reasoning is supported by Langmuir probe 

measurements made near the axis at 3 mm and 9 mm 

downstream of the keeper. It was found that the plasma 

potential at these probes probes was within 

experimental error virtually the same. In this regard, 

potential hill phenomena may not be as important in the 

production of energetic ions in actual ion thrusters due 

to the smoothing effects of the axial magnetic field. 

 Finally, as mentioned earlier, electron ìfrictionî was 

described as a possible mechanism for the formation of 

energetic ions. This theory postulates that energetic 

ions are formed due to a high rate of electron-ion 

collisions.8

 According to the theory, if the electron-ion 

collision frequency is sufficiently high, then ions can 

absorb significant amounts of energy, well in excess of 

the discharge voltage. This effect should depend on 

axial magnetic field particularly in the near cathode 

region. The containment of the hollow cathode plasma 

by the axial magnetic field would tend to increase the 

electron-ion collision frequency. 
NASA/TM2002-211876 5 

Potential distributions

 Qualitatively, visual observation of the discharge 

chamber operating suggests that the discharge plasma 

actually consists of two components: a hollow cathode 

plasma contained primarily by the axial magnetic field 

and a main discharge. Figure 6 qualitatively illustrates 

this notion. The hollow cathode external positive 

column appears as a conical plume fanning out from the 

orifice of the discharge cathode. The discharge glow is 

brightest here. Again, this conically shaped discharge 

emanating from the hollow cathode is a consequence of 

the axial magnetic field confining the external positive 

column of the hollow cathode. With increasing axial 

distance the axial magnetic field decreases and 

therefore the fanning effect (weaker containment) is 

observed. 

 As mentioned earlier, local to the near-cathode 

region, no appreciable axial potential gradient was 

detected. Interestingly, however, it was found that a 

significant radial potential gradient existed in the plane 

of the keeper. This potential gradient (and associated 

electric field, Er ) is apparently associated with the 

transition from the hollow cathode positive column to 

the main discharge plasma and is required to aid the 

diffusion of electrons across the axial magnetic field to 

the anode: 

dr

dn J e n E e D e e e r = − ⋅ ⋅ ⋅ − ⋅ ⋅ μ ⋅ ⊥ (3)

It is also interesting to note that this potential gradient 

was of order the ionization potential of xenon. In this 

regard, the gradient imparts radial energies to electrons 

formed in the hollow cathode external positive column 

so that ionization can take place as the electrons move 

radially toward the anode. This free-standing gradient 

structure indicates the presence of a double layer.13 

Double layers are free-standing potential structures that 

form the transition between two plasmas (in this case 

the cathode and the bulk discharge) that are at two 

different potentials. In this case the cathode column is 

at a lower potential than the main discharge. Figure 7a 

illustrates the variations in the plasma potential at 

various radial positions in the plane of the cathode 

keeper. As can be seen in the figure, there is a voltage 

rise of order the ionization potential of Xenon between 

points near the cathode centerline and the region of 

plasma just off axis (~1 cm). Beyond this rise, the 

plasma potential off axis, however, does not vary 

appreciably with increasing radial distance. As 

illustrated in Figure 7b, the plasma density profile also 

follows the trends similar to the potential. Plasmas 

density drops very sharply across the presumed double 

layer. In this respect, the hollow cathode plasma 

represents the high-density core of a composite 

discharge. 

 The voltage gradients are expected to be highest at 

the cathode orifice because the axial magnetic field 

(radial impedance) is strongest there. The effect of the 

double layer potential structure would be to accelerate 

ions that happen to enter this region toward keeper and 

cathode structures, possibly giving rise to a radially 

directed erosion component. It is possible that the 

double layer structure may have played a role in the 

anomalous keeper erosion recorded during the NSTAR 

ion thruster long duration life test conducted at the 

NASA Jet Propulsion Laboratory.14 If the cathode was 

operating in a starved condition, then it can be expected 

that the double layer voltage drop would be quite large, 

perhaps of order the discharge voltage. A double layer 

potential difference of this magnitude would accelerate 

electrons to energies necessary to create singly and 

doubly charged ions directly. The double layer 

potential difference would accelerate these ions into the 

cathode and keeper electrodes. 

Emission Spectroscopy

 Emission spectra were obtained from fibers that 

collected collimated light along the radius of the 

discharge chamber in the plane of the cathode keeper. 

In all cases the most intense lines were the Xe II 

transitions. Figure 9 illustrates the behavior of the 

484 nm Xe II line and the 467 Xe I line as a function of 

discharge current. In general, all emission lines tended 

to increase in intensity with increasing discharge 

current. This behavior is likely associated with the 

corresponding increase in plasma density and thus 

associated excitation rates. One primary function of the 

emission spectroscopy diagnostic was to investigate the 

presence of doubly charged ions. At all discharge 

currents investigated, no Xe III lines were found in the 

spectra. This finding suggests that the absence in signal 

associated with these lines may be due to the absence of 

electrons with the energy necessary to excite them. It 

should be pointed out that Xe III lines with low 

excitation energies (~12 eV) can be found in the 

vacuum ultraviolet. These lines should be present. Such 

measurements, however, were beyond to capability of 

the spectroscopy system used in this investigation and 

therefore were not made. Instead, the spectra was 

analyzed for the presence of xenon singly charged lines 

with comparable excitation energies as Xe III ions in 

the visible range. In the visible, the excitation energy 

for the Xe III 472.3 nm line is 18.2 eV. The Xe II 431 

(U=18.14 eV) and Xe II (U=17.9 eV) have comparable 

excitation energies as the Xe III 472.3 nm line.15 

These singly charged ion lines were observed in the 

spectra, suggesting that electrons with energy 
NASA/TM2002-211876 6 

comparable to that required to excite the Xe III lines in 

the visible were indeed present. The spectra of these 

energetic lines are illustrated in Figure 10. It was 

observed that these lines increased in intensity 

monotonically with increasing discharge current 

suggesting that the energetic electron population also 

grew with increasing discharge current. These findings 

suggest that the doubly charged xenon population is 

likely present, but their emission is below the 

sensitivity of the spectrometer system used. 

Conclusion

 The energy distribution of ions flowing axially 

downstream from a hollow cathode was investigated 

using a NSTAR-type ring cusp discharge chamber. It 

was found that the energy distribution extended to 

higher energies as the discharge current increased. At 

the higher discharge currents, the fraction of ions on 

axis with energies above the sputtering threshold for 

molybdenum, a typical ion optics material, was nearly 

15% at the higher discharge currents investigated. 

Near-cathode plasma potential measurements revealed 

the presence of a double layer separating the hollow 

cathode plasma from the main discharge plasma. This 

potential difference contributes to electron diffusion 

across field lines to the anode. This same potential 

difference, however, could drive ion bombardment 

erosion of cathode and keeper surfaces. Doubly 

charged xenon ion lines were not observed in the 

spectra. The absence of such emission lines in the 

measured spectra may be due to insufficient optical 

sensitivity. 

References

1

Mantenieks, M.A. and Rawlin, V.K., ìStudies of 

Internal Sputtering in a 30-cm Ion Thruster,î 11th 

Electric Propulsion Conference, New Orleans, 

Louisiana, March 19ñ21, 1975. 

2

Rawlin, V.K., ìInternal Erosion Rates of a 10-kW 

Xenon Ion Thruster,îAIAA Paper 88ñ2913, 24th Joint 

Propulsion Conference, Boston, Massachusetts, July 

11ñ13, 1988. 

3

Brophy, J.R. and Garner, C.E., ìTests of High Current 

Hollow Cathodes for Ion Engines,î AIAA Paper 88ñ

2913, 24th Joint Propulsion Conference, Boston, 

Massachusetts, July 11ñ13, 1988. 

4

Friedly, V.J. and Wilbur, P.J., ìHigh Current Hollow 

Cathode Phenomena,î Journal of Propulsion and 

Power, Vol. 8, No. 3, MayñJune, 1992, pp. 635ñ643. 

5

Latham, P.M., Pearce, A.J., and Bond, R.A., ìErosion 

Processes in the UK-25 Ion Thruster,î IEPC Paper 91ñ

096, 22nd International Electric Propulsion Conference, 

Viareggio, Italy, October 14ñ17, 1991. 

6

Kameyama, Ikuya and Wilbur, P.J., ìMeasurements of 

Ions from High Current Hollow Cathodes Using Electrostatic Energy Analyzer,î Journal of Propulsion and 

Power, Vol. 16, No. 3, MayñJune 2000, pp. 529ñ535. 

7

Davis, W.D., and Miller, H.C., ìAnalysis of the 

products emitted by dc arcs in a vacuum ambient,î J. 

Appl. Phys, Vol. 40, pp. 2212ñ2221, 1969. 

8

Hantzsche, E., ìA Hydrodynamic Model of Vacuum 

Arc Plasma,î IEEE Transactions on Plasma Science, 

Vol. 20, No. 1, February 1992. 

9

Delcroix, J. and Trindade, A.R., ìHollow Cathode 

Arcsî in Advances in Electronics and Electron Physics, 

Marton, L. ed., Vol. 35, Academic Press, NY, pp. 109ñ

145, 1974. 

10Brophy, J.R., ìSimulated Ion Thruster Operation 

without Beam Extraction,î AIAA, DGLR, and JSASS 

21st International Electric Propulsion Conference, 

Orlando, FL, 1990, AIAA Paper 90ñ2655. 12Stuart, R.V. and Wehner, G.K., ìSputtering yields at 

very low bombarding energies,î Journal of Applied 

Physics, vol. 33, no. 7, July 1962, pp. 2345ñ2352. 

13Coakley, P. and Hershkowitz, N., ìLaboratory double 

layers,î Phys. Fluids, vol. 22, no. 6, June, 1979, pp. 

1171ñ1181. 

14Polk, J.E. et al., ì An overview of results from the 

8200 hr Wear test of NSTAR ion thruster,î AIAA 

Paper No. 99ñ2446, Los Angeles, 1999. 

15Striganov, A.R. and Sventitskii, N.S., Tables of 

Spectral Lines of Neutral and Ionized Atoms, IFI 

Plenum, New York, 1968, pp. 571ñ607. 
NASA/TM2002-211876 7 

Figure 1. Bell jar used in this investigation. Note position of discharge chamber and electrostatic energy analyzer. 

NSTAR-Type 

Discharge Chamber 

Electrostatic 

Energy Analyzer 

Pyrex Bell 
NASA/TM2002-211876 8 

Hollow

Cathode

Keeper

Electrode

 NSTAR Ion

Thruster Anode

 Charged

Particle

Electrostatic

Energy

Analyzer

Perforated

Cathode Potential

Discharge

Termination

Near-Cathode

Langmuir

Probe

Near ESA

Langmuir

Probe

Fine Nickel

Mesh Plasma

Screen

Near-Anode

Langmuir

Probe

Figure 2a. Schematic of experimental set-up. Note location of the Langmuir probes and the ESA.

1

2 

3 

4 
NASA/TM2002-211876 9 

To

Scope

Ramp Power

Bipolar

Power

Supply

Oscillator

Entrance

Slit

Delta-V

Voltage

Source

Exit Slit

Ion Collector

Plate

Battery Bias

Supply

Current

Amplifier

Outter

Spherical

Sector

Inner

Spherical

Sector

Figure 2b. Pictorial representation of the electrical connections to ESA for constant transmission energy mode. 

Figure 3a. Discharge characteristics of NSTAR discharge chamber at fixed flow rate. 

25

25.5

26

26.5

27

27.5

28

28.5

29

29.5

30

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Discharge Current, A

Discharge Voltage, V

Pressure: 1.5 x 10-4 Torr

Flow = 4 SCCM
NASA/TM2002-211876 10 

Figure 3b. Variations in the ion energy distribution with increasing discharge current. 

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

-0.005

-0.004

-0.003

-0.002

-0.001

0.000

0.001

0.002 Derivative relatively constant

at points before this knee

Ion Energy, eV

DI/DE, Arbitrary units

Figure 4a. Derivative of the ion current signal as a function of ion energy. The slowly varying region at low 

energies is due to background charge-exchange ions. Discharge current=10.5 A. 

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 10 20 30 40 50 60 70 80 90 100

Ion Energy, eV

Intensity, Arbitrary Units

10.5 A

12.5 A

14.5 A

16.5 A

18.2 A
NASA/TM2002-211876 11 

12 14 16 18 20 22 24 26 28 30 32

0.0

0.1

0.2

0.3

0.4

0.5

Line Represents Gaussian Fit

Intensity, Arbitrary Units

Ion Energy, eV

Figure 4b. Gaussian fit to 10.5 A case with low energy ion contribution removed.

 

Figure 5. Variation in the energetic ion current fraction (ions with kinetic energies above 27 eV) with increasing 

discharge current. 

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

10 12 14 16 18 20

Discharge Current, A

Energetic Ion Current Fraction
NASA/TM2002-211876 12 

Figure 6. Discharge operation: Notice cathode plasma (plume emanating from discharge cathode) and main 

discharge plasma, which appears more diffuse and weaker in intensity. 

Cathode Plasma 

Positive Column
NASA/TM2002-211876 13 

Figure 7a. Radial plasma potential variations at different discharge currents. Profiles are indicative of a double 

layer. Probe located 3 mm downstream of keeper plane. 

Figure 7b. Radial plasma density variations as a function of discharge current. Probe located 3 mm downstream of 

keeper plane. 

1.0E+08

1.0E+09

1.0E+10

1.0E+11

1.0E+12

1.0E+13

0 10 20 30 40 50 60 70 80 90 100

Radial Position with respect to Thruster Centerline, mm

Density, #/cm^3

10.5 A

12.5 A

14.5 A

16.5 A

10

12

14

16

18

20

22

24

0 10 20 30 40 50 60 70 80 90 100

Radial Position with respect to Thruster Centerline, mm

Plasma Potential with respect to Cathode Potential, V

10. 5 A

12.5 A

14.5 A

16.5 A
NASA/TM2002-211876 14 

Figure 8. Variation in ion and neutral emission with increasing discharge current. 

Figure 9. Variations in xenon ion lines requiring energetic electrons (~18 eV) for excitation of transition with 

discharge current.

0

1

2

3

4

5

6

7

8

9

10

10 11 12 13 14 15 16 17 18 19 20

Discharge Current, A

Intensity, Arbitrary Units

U = 18.14 eV (431 nm)

U = 17.90 eV (439 nm)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

10 11 12 13 14 15 16 17 18 19

Discharge Current, A

Intensity, Arbitrary Units

Xe II (484 nm)

Xe I (467 nm)
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

REPORT DOCUMENTATION PAGE

2. REPORT DATE

19. SECURITY CLASSIFICATION

 OF ABSTRACT

18. SECURITY CLASSIFICATION

 OF THIS PAGE

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this

collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

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

Prescribed by ANSI Std. Z39-18

298-102

Form Approved

OMB No. 0704-0188

12b. DISTRIBUTION CODE

8. PERFORMING ORGANIZATION

 REPORT NUMBER

5. FUNDING NUMBERS

3. REPORT TYPE AND DATES COVERED

4. TITLE AND SUBTITLE

6. AUTHOR(S)

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

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT

13. ABSTRACT (Maximum 200 words)

14. SUBJECT TERMS

17. SECURITY CLASSIFICATION

 OF REPORT

16. PRICE CODE

15. NUMBER OF PAGES

20. LIMITATION OF ABSTRACT

Unclassified Unclassified

Technical Memorandum

Unclassified

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135–3191

1. AGENCY USE ONLY (Leave blank)

10. SPONSORING/MONITORING

 AGENCY REPORT NUMBER

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546–0001

Available electronically at http://gltrs.grc.nasa.gov

November 2002

NASA TM—2002-211876

AIAA–2002–4102

E–13558

WU–755–B4–04–00

20

Plasma Emission Characteristics from a High Current Hollow

Cathode in an Ion Thruster Discharge Chamber

John E. Foster and Michael J. Patterson

Hollow cathode; Ions; Double layer; Plasma; Electric propulsion; Ion thruster

Unclassified -Unlimited

Subject Categories: 20 and 75 Distribution: Nonstandard

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

Indianapolis, Indiana, July 7–10, 2002. Responsible person, John E. Foster, organization code 5430, 216–433–6131.

The presence of energetic ions produced by a hollow cathodes operating at high emission currents (>5A) has been documented in the literature. In order to further elucidate these findings, an investigation of a high current cathode operating in

an ion thruster discharge chamber has been undertaken. Using Langmuir probes, a low energy charged particle analyzer

and emission spectroscopy, the behavior of the near-cathode plasma and the emitted ion energy distribution was characterized. The presence of energetic ions was confirmed. It was observed that these ions had energies in excess of the discharge

voltage and thus cannot be simply explained by ions falling out of plasma through a potential difference of this order.

Additionally, evidence provided by Langmuir probes suggests the existence of a double layer essentially separating the

hollow cathode plasma column from the main discharge. The radial potential difference associated with this double layer

was measured to be of order the ionization potential.