NASA/TMm2001-210669/REV1

Ion and Electron Transport in an

NSTAR-Derivative Ion Thruster

John E. Foster

Glenn Research Center, Cleveland, Ohio

June 2001
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NASA /TM--2001-210669 / REV 1

Ion and Electron Transport in an

NSTAR-Derivative Ion Thruster

John E. Foster

Glenn Research Center, Cleveland, Ohio

National Aeronautics and

Space Administration

Glenn Research Center

June 2001
Acknowledgments

The author would like to thank George Soulas and Michael Patterson for maintaining stable thruster operation

during these experiments. Additionally, the author would like to thank James Sovey for useful discussions

regarding the data. Finally, the author would like to thank Robert Roman for the fabrication and

installation of the Langmuir probes used in this test.

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Available from

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at httt2:Z / gltrs.grc.nasa.gov / GLTRS
ION AND ELECTRON TRANSPORT IN AN

NSTAR-DERIVATIVE ION THRUSTER

John E. Foster

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

SUMMARY

Diffusion of electrons and ions to anode surfaces between the magnetic cusps of an NASA Solar

Electric Propulsion Technology Application Readiness ion thruster has been characterized. Ion flux

measurements were made at the anode and at the screen grid electrode. The measurements indicated that

the average ion current density at the anode and at the screen grid were approximately equal. Additionally,

it was found that the electron flux to the anode between cusps is best described by the classical cross-field

diffusion coefficient.

B

de_

De

DX

e

Es

Je

J scm.grd

Jwall B

k

m

tl e

r

tea

rob

re

AV

g

V e

W

NOMENCLATURE

transverse magnetic field

constant of proportionality

electron diffusion coefficient, B=0

electron cross-field diffusion coefficient

elementary charge of an electron

sheath electric field

electron cross-field current density

ion current density at the screen grid

ion current density at wall probe B

Boltzmann's constant

mass of an electron

electron number density

radial displacement

electron temperature at probe A

electron temperature at probe B

electron temperature

voltage drop across plasma sheath

electron mobility

total electron collision frequency

electron cyclotron frequency

NASA/TM--2001-210669/REV 1 1
_RODUCTION

Controllingionandelectrontransport todischargechambersurfacesisakeyaspect ofionttu'uster

design. _2Indeed,atagiventhrusterpower,performancecanbeoptimizedbymaximizingthefractionof

dischargeionsthatreachtheionextractionoptics.Maximizingtheextractedionfractioncanbeachieved

byminimizingionlossestotheanode.Reducingtheflowofionstosurfacesotherthantheextraction

opticsusingamagneticfieldisusuallynotpractical duetotheverystrongfieldstrengthsrequiredto

magnetizeionmotion.Instead, thrustermagneticcircuits,suchastheringcusp,utilizemagneticfield

geometriesthatcontainelectronsinamannersuchthatthedischargeproducedislocatedjustupstreamof

theionoptics. 3Thecloseproximityofdischargetothecollectionopticsincreasesthelikelihoodthatanion

generatedinthisvolumewillbeextracted.

Ithasbeensuggestedthatmanyionthrusterdischargechamber configurations operatewith

preferentialonflowtotheionoptics)WhetherornottheringcuspmagneticcircuitoftheNASASolar

ElectricPropulsionTechnologyApplicationReadiness(NSTAR) 4ionthrusterinvestigatedinthisworkcan

effectionmotiontotheanodeandtherebyimproveionflowtotheopticsisaquestionthatisaddressedin

thiswork.Anunderstanding ofthenatureofionflowfromthedischargetotheionopticscanbeusedasa

guideinbothoptimizingtheperformanceofthisthrusterandalsoincreasingthethruster'spowerange.

Unlikeionmotion,electronmotionisseverelyaffectedbythepresenceoftheinter-cuspmagnetic

field.Inthiswork,electrondiffusiontotheanodesurfacesbetweenthemagneticcuspswasalsostudied.

Themagneticfieldbetweencuspsprovidesafairlyhighimpedancepathforelectronflowtotheanode,

therebyreducingelectronlossestotheanode.Thecharacter ofthecross-fieldelectrondiffusioncoefficient

isameasureofthisimpedance. A large cross-field diffusion coefficient would lead to a lower discharge

voltage and consequently, a reduction in the bulk plasma ionization rate. Previous investigations of electron

diffusion between cusps have assumed anomalous Bohm diffusion. 5 In this work, the nature of the crossfield electron diffusion coefficient in a NSTAR-derivative ion thruster is determined.

EXPERIMENTAL APPARATUS

The ion thruster used in this investigation was mechanically identical to the NSTAR thruster. 4 A

schematic of the ion thruster and internal discharge chamber wall probes is illustrated in Figure I. A screen

grid and accelerator grid comprised the high-voltage ion extraction optics. _The voIt/lges at the screen and

accelerator grid electrodes ranged from 650 V to 1500 V and -250 V to -180 V, respectively, as thruster

input power varied from 0.5 kW to 4.6 kW. It should be noted that for this engine, the ratio of peak to

average ion beam current density as measured in the plume varied from 0.45 to 0.55 over the range of

conditions investigated. Additional details regarding the 30-cm engine, the power console, and the

propellant feed system can be found elsewhere. 4'6"7As can be seen in Figure 1, the aluminum discharge

chamber was conical with a downstream cylindrical section. The magnetic circuit consisted of only three

permanent magnet rings: l) the cathode ring, located at the back-plate of the conical section; 2) the

cylindrical section ring, located at the junction between the cylindrical section and the conical section; and

3) the downstream ring, iocated at the most downstream flange of the cylindrical section. For these

investigations, xenon propellant is used.

The planar molybdenum wall probes used in this investigation each measured 6.4 mm in diameter. The

wall probes were flush mounted with the surface of the anode at positions indicated in Figure 1. The probes

were designated according to the letters as indicated in Figure 1. Wall probe A was located near the

midpoint between the cathode and cylinder magnetic cusps and wall probe B was located near the midpoint

between the cylinder and pole piece magnetic cusps. The transverse magnetic field component across the

surface of wall probes A and B was approximately 0.0021 T and 0.0023 T, respectively. The probes were

electrically isolated from the anode wall and were held in place using modified compression fittings. The

wall probes were biased relative to cathode potential using a variable DC voltage source to obtain

Langmuir current-voltage characteristics from which plasma density and electron temperature were

calculated. In order to obtain the average ion current density at the screen grid, the grid was negatively

biased -25 V with respect to the discharge cathode. For this investigation, thruster discharge power ranged

between 120 W and 465 W, which corresponded to a total thruster input power between 0.5 and 4.6 kW.

NASA/TM--2001-210669/REV 1 2
RESULTSANDDISCUSSION

Background

Betweenthecusps,themagneticfieldlinesrunparalleltothesurfaceoftheanode. Thisfieldlinfitsthe

cross-fielddiffusionofelectronstotheanodesurface. Figure2aand2billustratethevariationinthe

normalizedtransversemagneticfieldcomponent atheanodeasafunctionofpositionrelativetothe

magneticcusps.Thenormalizedtransversefildintheconicalsectionofthedischargechamberis

normalizedtothetransversefildintensitynearthecathodecusp.Forreference, thetransversemagnetic

fieldintensityattheanodeonecentimeter fromthecathoderingmagneticcuspwas0.053T.The

normalizedtransversefildinthecylindrical sectionisnormalizedtothetransversefildintensitynearthe

cylindrical sectionmagneticcusp.Themagneticcuspsarelocatedatpositions0and18-cmofFigure2a

andpositions0and12-cmofFigure2b.Along the anode, at distances greater than 2.5-cm away from the

magnetic cusps, the transverse field does not vary appreciably. As can be seen in the figures, a significant

portion of the anode surface lie in these regions of relatively constant transverse magnetic field. Indeed,

over 70% of the anode's surface area lies in these regions of relatively constant transverse magnetic field. It

therefore follows that the magnitude of the magnetic field at the anode between the cusps plays a critical

role in determining discharge efficiency. The transverse magnetic field component in these regions is less

than 20% of the transverse component measured near the cusps. Plasma collection at anode surfaces

located close to the magnetic cusps is associated with magnetic cusp physics and is not the subject of this

investigation.

The focus of this work is to ascertain the effect that the inter-cusp magnetic field has on ion and

electron transport to anode surfaces. Accordingly, this work is divided into two sections: 1) ion transport

and 2) electron transport.

Ion Transport

Ion loss to the anode between the magnetic cusps and to the screen grid was determined by biasing

wall probes A and B and the screen grid negative of the cathode potential to obtain the ion saturation

current density. Figure 3 depicts a schematic of ion losses to bounding discharge chamber surfaces in the

cylindrical section. The screen grid, which has a 67% open area, is the most downstream boundary of the

discharge chamber's cylindrical section. If preferential ion drift to the ion optics occurs in the NSTARderivative thruster, then it can be expected that the ion current density measured at the cylindrical section

sidewall as measured by wall probe B should be less than the average ion current density measured at the

biased screen grid. A plot of the ratio of ion current density measured at the screen grid to the ion current

density measured at wall probe B is shown in Figure 4. For the discharge powers investigated, the ratio

was nearly unity. This result is surprising in that it does not indicate a preferential flow of ions to the optics

at any discharge power as has been suggested by an earlier study. _It should be pointed out that in the

earlier study, the magnetic circuit as well as the anode geometry differed significantly from the NSTARderivative ion thruster studied in this present investigation. As determined from measurements presented

here, the ion losses to the cylindrical section boundaries and the screen grid are approximately equal. It

should be pointed out that such ion flow is consistent with the work of Masek. 8

Past studies have suggested that the extracted ion beam fraction (defined as the ratio of the ion beam

current to the sum of the ion current lost to discharge surfaces and the ion beam current) is considerably

larger than that that would be expected from the ratio of screen grid physical open area to total discharge

chamber surface area._'9'l°This result is attributable in part to an increase in screen grid transparency during

high voltage extraction, s'n The remaining disparity can be addressed through an examination anode wall

probe data. Figure 5 illustrates the behavior of the ion saturation current ratio of wall probe B to wall probe

A as a function of discharge power. In all cases, the ion current measured at probe B was approximately a

factor of 1.55-times larger than that measured at probe A. If the discharge ions escaping to the ion beam

optics and to the cylindrical section walls obey the Bohm Criterion, the flow to the walls is at the ion

acoustic velocity. Ion escape rates to anode surfaces should be proportional to the product of the squareroot of the local electron temperature and the plasma density. Because the plasma density near the anode in

both the conical and the cylindrical section of the discharge chamber was determined to be approximately

equal, _° the disparity between the ion wall flux measured at probe A and probe B must be due to the

NASA/TM--2001-210669/REV 1 3
electrontemperature. In this regard, the ratio between the two probes' current densities should be

approximately equal to the ratio of the square root of the electron temperature measured at the _'o probes.

Plots of these ratios as a function of discharge power are shown in Figure 6. As can be seen from the plot,

the average value of the ratio of the square root of the electron temperature is approximately 1.45, which

within experimental error (10 to 15%) accounts for the difference in ion currents measured in the conical

and cylindrical sections. This agreement supports the notion that a much higher percentage of ion

production occurs just upstream of the ion optics and that the ion flow depends primarily on local plasma

density and electron temperature. Because of the electron temperature disparity between the two discharge

chamber sections, ion loss rates to the anode are largest in the cylindrical section, which in this case is just

upstream of the optics. In this respect, ions are lost to the beam extraction optics at a rate larger than they

are lost to anode surfaces in the conical section. Because ion losses to different sections of the discharge

chamber are not uniform, the extracted ion fraction is not well represented by the ratio of screen grid open

area to total discharge chamber surface area.

The nature of ion flow suggests the importance of understanding plasma conditions, particularly the

local electron temperature and density, within the discharge chamber. Care must be taken such that the

discharge is not only localized near the optics, but also that the ratio of the open area of the screen grid to

the anode surface area near the optics is sufficiently large.

Electron Transport to Anode Surfaces between Magnetic Cusps

The discharge chamber magnetic field controls the flow of electrons to the anode. Ideally, in a ring

cusp discharge chamber, most of the electron current collection occurs at the magnetic cusps. Under these

conditions, the magnetic field between the cusps minimizes the flow of energetic electrons to other anode

surfaces. In practice, depending on the field strength between the cusps, a significant fraction of the

discharge current can be collected at the anode between the cusps. It was found in an earlier NSTARderivative thruster study that up to 30% of the discharge current can be collected at anode surfaces between

the cusps) ° Because minimizing the flow" of electrons to anode surfaces between the cusps improves

thruster discharge efficiency, a understanding the nature of electron diffusion to anode surfaces between

cusps is important.

In general the radial flow of electrons to the anode in the presence of a magnetic field may be

expressed as "12

Je =-e.,u.n_ .E, -e.D i dn_

• " dr (1)

Here mobility is defined as:

e

/d - (2)

m "v e

Of interest is the functional behavior of the cross-field diffusion coefficient. In general, plasma flow across

a magnetic field is a complicated problem. Under some conditions, plasma flow obeys classical formulism

in which the cross-field diffusion coefficient can be described by:

De

(3)

NASA/TM--2001-210669/REV 1 4
Heretheelectrondiffusioncoefficient underconditionswherethemagneticfieldiszeroisdefinedas:

k.L

De _mm

tii •V e

(4)

Under certain conditions, potential oscillations driven by plasma instabilities give rise to anomalous

cross-field diffusion that is substantially larger than classical predictions. In a variety of cases, such

anomalous diffusion is best described by the Bohm diffusion coefficient: 13-_s

kL (s)

D.j_ - 16.e.B

In this work, using Equation 1 and measured plasma properties at wall probes A and B, the electron

current collected at and above plasma potential is calculated as a function of wall probe bias voltage. The

total electron collision frequency, which is the sum of the electron-neutral collision frequency and the

electron-ion collision frequency, was calculated based on ensemble averaged momentum exchange

between the electrons and the background plasma using low energy electron-neutral and electron-ion cross

section data. 16 Here the relaxation rate is determined by test particle (ion and neutrals) interactions with a

Maxwellian electron distribution described by the measured electron temperature._7 Neutral densities inside

the discharge chamber were calculated from the propellant utilization efficiency. The calculation of the

electron current is done assuming two separate cases: 1) Bohm diffusion and 2) Classical diffusion. The

calculated electron current was then compared with the measured electron current to determine whether

Bohm or classical best describes electron diffusion to anode surfaces between the magnetic cusps.

In order to calculate the electron current at the wall probes as a function of probe potential, the electric

field at the sheath edge as well as the electron density gradient must be determined. As illustrated in

Figure 7, for all cases investigated in this work, the electron current at and above the apparent plasma

potential was a linear function of the potential of the probe relative to the plasma potential. The constancy

of the slope suggests the notion that the electric field in Equation 1 is proportional potential difference

across the sheath. Equation 1 can then be recast:

Je = -e. 1.1"ne. •AV. dee - e. D± •

dne (1 ')

dr

Here, the electric field at the sheath edge, may be expressed as the product of the sheath potential, AV,

and the constant of proportionality, dee • The slope of the line described by equation 1',

- e. ].1• ne • de:_, , can be obtained from a linear fit (see Fig. 7). From the slope, dee can be calculated and

thus be used to determine the effective electric field at the sheath edge. This effective electric field is a

consequence of potentials that leak out of the sheath and give rise to electron drift toward the probe's

surface. Such fields cause the collected electron current to increase as a function of increasing sheath

potential.

The second term in equation (1') is associated with cross-field diffusion. Classically, in the presence of

the transverse magnetic field, the characteristic diffusion step is a Larmor radius. 12 This scaling provides a

simple means for estimating the electron density gradient. The cross-field spatial derivative of the electron

density was taken to be the difference between plasma density in the bulk plasma (outside the sheath) and

the density at the probe's surface divided by the electron Larmor radius at the probe, -- .

tr/_j

Using equation (1'), the electron current was calculated for both the Bohm diffusion coefficient and the

classical cross-field diffusion coefficient. Figures 8 and 9 illustrate the behavior of the ratio of the

calculated electron current to the measured electron current as a function of sheath voltage for the two wall

NASA/TM--2001-210669/REV 1 5
probesatthe302Wdischargepowercondition(2.3kWthrusterinputpower)andatthe228Wdischarge

powercondition(1.7kWthrusterinputpower).Ascanbeseenfromthefigures,theelectroncollection

betweenthecuspsisbestdescribedbytheclassical cross-fielddiffusioncoefficient.Towithin

experimental error(approximately 35%),theclassical relationpredictsthemagnitudeoftheelectron

currentoverawidedischargepower(200to465W)range.ElectroncurrentcalculatedassumingaBohm

diffusioncoefficientwasoveranorderofmagnitudelargerthanthatwhichwasmeasured. Indeed, forthe

magneticfieldintensitiespresent betweencusps,if cross-fielddiffusionwereBohm-like, thentheutilityof

magneticcontainment wouldbedefeatedduetoexcessiveelectronflowtoanodesurfacesbetweenthe

cusps.It shouldalsobepointedoutthattheclassical relationfortheelectroncurrentalsotrackedthe

functional behavioroftheelectroncurrentwithincreasingprobevoltage.Therateofchangeintheelectron

currentwithincreasingprobevoltageascalculatedusingtheBohmdiffusioncoefficient casewaslessthan

thatactuallymeasuredasevidencedbythemonotonicallydecreasingratiospresentedinFigures8and9.

Finally,it shouldbepointedoutthattheratiooftheelectricfielddiffusiontermtothemagneticfieldcrossdiffusionterminequation1rangedfromapproximately 0.1to1forclassical diffusionwithincreasing

probevoltage.

Thefactthatelectroncross-fielddiffusiontointer-cuspanodesurfacesisclassical impliesanumberof

interestingconsequences: !)Electrondiffusiontotheanodesurfacebetweencuspsisseverelyreduced.

2)Operationathigherpropellant flowratesshouldincreaseelectronlossestotheanodebecauseofthe

classical diffusioncoefficient'sdependence ontheelectroncollisionfrequency. 3)Modestincreasesin

magneticfieldstrengthshouldleadtosignificant improvements inelectroncontainment.

Agreement between measured the electron current and the calculated classical current was best at the

higher thruster power conditions (>1,4 kW). It should be pointed out that deviations from classical crossfield diffusion tended to increase at the lower power operating conditions. At these lower powers, the

calculation for the electron current using the classical cross-field diffusion coefficient tended to underestimate the measured value. The under-prediction trend at probe A ranged from 1% at the 2.3 kW input

power condition to over 20% at the 1.4 kW input power. The discrepancy at wall probe B was somewhat

larger than at probe A with the under-prediction trend ranging from t0% at the 2.3 kW input power

condition to over 30% at the 1.4 kW input power.

Prior research suggests that deviations from the classical cross field diffusion may be due a plasma

• ..,- [" 18

mstaoruty. -" Additionally, ionization along the flux lines between the cusps may become more important

with decreasing discharge power. Drainage of inter-cusp flux lines via cross-field diffusion or a plasma

instability would also tend to increase the magnitude of cross field current collected at the probe. _9 These

mechanisms may be necessary to sustain the magnitude of the discharge current at the lower thruster

powers. The fact that measured electron diffusion to the anode between the magnetic cusps increases above

the classical rate at the lower thruster power conditions suggests that electron containment between the

cusps also degrades in concert. This degradation is consistent with the increased discharge losses at the

lower power conditions. In this respect, characterization of electron diffusion phenomena to the anode is a

key aspect for understanding discharge performance.

CONCLUSIONS

It was found that ion flow to anode surfaces just upstream of the ion optics is approximately equal to

the average ion current density at the ion optics. Additionally, because ion losses to different sections of the

discharge chamber are not uniform, the extracted ion fraction is not well represented by the ratio of screen

grid open area to total discharge chamber surface area. Inter-cusp electron flow to the NSTAR-derivative

ion thruster anode was found to follow classical diffusion predictions. This data suggests that modest

increases in magnetic field intensity between the cusps can significantly improve electron containment.

NASA/TM--2001-210669/REV 1 6
REFERENCES

1. Kaufman, H.R.,Robinson, R.S.,andFrisa,L.A.,"IonFlowExperimentsinaMultipoleDischarge

Chamber," AIAA Journal, vol. 22, no. 11, Nov. 1984, pp. 1544-1549.

2. Arakawa, Y. and Hamatani, C., "Reduction of Plasma Loss to Discharge Chamber Walls in a RingCusp Ion Thruster,'" J. Propulsion and Power, vol. 3, no. 1, Jan.-Feb. 1987, pp. 90-91.

3. Sovey, J.S., "Improved ion containment using a ring-cusp ion thruster," J. of Spacecraft and Rockets,

vol. 21 no. 5, May 1984, pp. 488-495.

4. Christensen, J.A., et al., "Design and Fabrication of a Flight Model 2.3 kW Ion Thruster for the Deep

Space 1 Mission," AIAA Paper 98-3327, July 1998.

5. Kaufman, H.R. and Robinson, R.S., "Plasma Processes in Inert-Gas Thrusters," J. Spacecraft and

Rockets, vol. 18, no. 5, Sept.-Oct. 1981, pp. 470--476.

6. Soulas, G.C., Foster, J.E., and Patterson, M.J., "Performance of Titanium Optics on a NASA 30 cm Ion

Thruster," AIAA Paper 2000-3814, July 2000.

7. Sovey, J.S., et al., "Development of an Ion Thruster and Power Processor for New Millennium's Deep

Space 1 Mission," AIAA paper 97-2778, December 1997.

8. Masek, T.D., "Plasma Properties and Performance of Mercury Ion Thrusters," AIAA Journal, vol. 9,

no. 2, Feb. 1971, pp. 205-212.

9. Poeschel, R.L., "Development of Advanced Inert-Gas Ion Thrusters," NASA CR-168206, June 1983.

10. Foster, J.E., Soulas, G.C., and Patterson, M.J., "Plume and Discharge Measurements of an NSTARType Ion Thruster," AIAA Paper 3812-2000, July 2000.

11. Brophy, J.R., "Simulated Ion Thruster Operation without Beam Extraction," AIAA paper 90-2655,

July 1990.

12. Chen, Francis, h_troduction to Plasma Physics and Controlled Fusion, Plenum Press, NY, 1984,

pp. 169-174.

13. Bohm, D., "Qualitative Description of the Arc Plasma in a Magnetic Field," The Characteristics of

Electrical Discharges in Magnetic Fields, edited by A. Guthrie and R.K. Wakerling, McGraw-Hill

Book Co., NY, 1949, pp. 1-12.

14. Chen, bltroduction to Plasma Physics and Controlled Fusion, pp. 190-195.

15. Kaufman, H.R. and R.S. Robinson, Operation of Broad Beam Ion Sources, Commonwealth Scientific

Corp., Alexandria, VA, 1984, pp. 143-144.

16. Frost, L.S. and A.V. Phelps, "Momentum-Transfer Cross Sections for Slow Electrons in He, Ar, Kr,

and Xe from Transport Coefficients," Physical Review, vol. 136, no. 6 A, Dec. 1966, pp. 1538-1545.

17. Kaufman, Operation of Broad Beam Ion Sources, pp. 137-142.

18. Hoh, F.C. and Lehnert, B., "Diffusion Processes in a Plasma Column in a Longitudinal Magnetic

Field," Phys. Fluids, vol. 3, no. 4, 19601 pp. 600-607.

19. Bemstein, I.B., Frieman, E.A., Kulsrud, R.M. and Rosenbluth, M.N., Phys. Fluids, vol. 3, no. 3, 1960,

pp. 136-137.

NASA/TM--2001-210669/REV 1 7
Downstream

magnet ring -7

Wall probe B --_

Cylinder _

magnet ring . _

Wall "_e_nn" _I "!i1!

probe A --_\

Cathode

magnet .

ring

"7

I

I

/_-- Discharge I

cathode l

/ I

// and keeper 1

jl/ ._

/

/

Accelerator grid --/

Figure 1 .--Half-plane cross-section of NSTARderivative thruster revealing wall probes and

magnetic circuit.

NASA/TM 2001-2 I0669/REV1 8
10

O

"o

,m

,w

<D

c"

E

(n

J=

N

O

Z

1.2 --

1o0 --

0.8-

0.6-

0.4-

0.2-

(a)

0

-2

,-- Cathode ring Cylindrical section /

// magnetic cusp magnetic cusp

Inter-cusp region

"_ Location of /

_. wall probe A /' •

0 2 4 6 8 10 12 14 16 18

Location along conical section downstream of cathode cusp, cm

I

20

13

O

¢-.

-a

tE

C

O

Z

1.2

//-- Cylindrical section Downstream

/ gnetlc cusp magnetic cusp --_

1.0

0.8

0.6 _ Inter-cusp region

0.4 _ B_

Location of

[b)

0 I I I I I

-2 0 2 4 6 8 10 12

Position along cylindrical section downstream of cylinder ring, cm

Figure 2(a).--Normalized transverse magnetic field between the cathode ring magnetic

cusp and the cylindrical section magnetic cusp. This profile illustrates the transverse

field behavior in the conical section of the discharge chamber. (b) Normalized transverse magnetic field between the cylindrical section magnetic cusp and the downstream magnetic cusp. This profile illustrates the transverse field behavior in the

cylindrical section of the discharge chamber.

NASA/TM--2001-210669/REV 1 9
\1 Aooeerator0

/ "_ / end-wall / / _ /X

(_._ Side-wall/

- i losses

,.-\,, / X ,, --,.-.,¢x,,\ /

potential / d _ \_ _L- Screen grid

surface --' (_ _ \\ (cathode potential)

_-- Cylindrical

section plasma

Figure 3.--Depiction of ion losses to the bounding discharge chamber anode

surfaces and the ion optics.

1.4 --

1.2

_1.0

0.8

0

0,6 --

• • •

0.4 I I I I I I I I

100 150 200 250 300 350 400 450 500

Discharge power, W

Figure 4.--Ratio of screen grid ion current density to ion current density measured at wall probe B

as a thruster discharge power.

NASA/TM--2001-210669/REV 1 10
2.0

O

1.8-

_=_ 1.6 -

_e el

0

"-_ 1.4 -

c 1.2-

,2o

m_ • •

1.0 I I I I I I I I

100 150 200 250 300 350 400 450 500

Discharge power, W

Figure 5.nVariations in the ratio ion saturation current measured at probe B to probe A as a function

of discharge power.

1.4

I=_1_

L_1.2

1.0

1o6 --

• •

• •

0.8 I ! I I I I I I

100 150 200 250 300 350 400 450 500

Discharge power, W

Figure 6.n Variations in the square-root of the electron temperature ratio of wall probe B to wall

probe A.

NASA/TM--2001-210669/REV 1 11
¢.)

o

2.2 --

2.0-

1.8-

1.6

1.4

O

1.2 o

1.0 •

0.8

[]

[] •

0.6 5 •

0.4-

0.2-

I 0

0 2

O

[]

O

0 •

0 •

[]

[] •

[] •

CI

[]

[] •

0

[]

Probes

• A, 1.7 kW

[] A, 2.3 kW

• B, 1.7 kW

o B, 2.3 kW

O

[]

= _ :± :i ........... [ I m

4 6 8 10

Probe voltage above plasma potential, V

1

12

Figure 7.--Electron current collected at wall probes A and B as a function of probe

voltage above the plasma potential.

20-

[]

._18 -

c

o=16

C

o

_14

_12

¢10 E

"5

_ 6

_ 2

121

r"l

0

DI +morn

0 2

O

0

[]

o

o

Probe A

• Classical/measured, 1.7 kW

• Classical/measured, 2.3 kW

[] Bohrn/measured, 1.7 kW

o Bohrn/measured, 2.3 kW

D

[]

[]

O

,3 []

O

[]

[]

O

Cl

om .m •m mom om ¢m • em •

I l I I I

4 6 8 10 12

Probe voltage above plasma potential, V

Figure 8o_Compadson between classical and Bohm electron flow to the anode

between magnetic cusps at probe A (1.7 and 2.3 kW input thruster power).

NASA/TM--2001-210669/REV 1 12
2O

,_18

r-

_16

r2

_10

E

.2

n- 2

[]

0

[]

0

• i|

I

[]

[]

0 o

o []

0

Probe B

• Classical/measured, 1.7 kW

• Classical/measured, 2.3 kW

o Bohm/measured, 1.7 kW

o Bohm/measured, 2.3 kW

[]

I"1

o o

O

o

[]

o

0

0 2

• • • OI 01 | I I • •

I t ] I

4 6 8 10

Probe voltage above plasma potential, V

Figure 9._Compadson between classical and Bohm electron flow to the anode

between magnetic cusps at probe B (1.7 and 2.3 kW input thruster power).

I

12

NASA/TM--200 l-210669/REV 1 13
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

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1. AGENCY USE ONLY (Leave b/ank) ! 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

June 2001 Technical Memorandum

4. TITLE AND SUBTITLE

Ion and Electron Transport in an NSTAR-Derivative Ion Thruster

6. AUTHOR(S)

John E. Foster

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

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

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

National Aeronautics and Space Adnfinistration

Washington, DC 20546-0001

5. FUNDING NUMBERS

WU-755-B4-04-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-12535-1

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--2001-210669-REV 1

11. SUPPLEMENTARY NOTES

Responsible person, John E. Foster, organization code 5430, 216-433-6131.

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

Unclassified - Unlimited

Subject Categories: 75 and 20 Distribution: Nonstandard

Available electronically at htl#://zltrs._rc.nasa.__ov/GLTRS

This publication is available from the NASA Center for AeroSpace Information, 301-621-0390.

13. ABSTRACT (Maximum 200 words)

Diffusion of electrons and ions to anode surfaces between the magnetic cusps of a NASA Solar Electric Propulsion

Technology Application Readiness ion thruster has been characterized. Ion flux measurements were made at the anode

and at the screen grid electrode. The measurements indicated that the average ion current density at the anode and at the

screen grid were approximately equal. Additionally, it was found that the electron flux to the anode between cusps is best

described by the classical cross-field diffusion coefficient.

14. SUBJECT TERMS

Ion thruster; Magnetic field; Inter-cusp; Cross field diffusion; Electron diffusion; Anode

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OF REPORT

Unclassified

NSN 7540-01-280-5500

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Unclassified

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Unclassified

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19

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