NASACR - 174862

NASA-CR_ 174862

'_" 19850010711

ADVANCEDION THRUSTERRESEARCH

_ PREPAREDFOR

LEWIS RESEARCHCENTER

NATIONALAERONAUTICSAND SPACEADMINISTRATION

Grant NGR-06-O02-112

LIBRABY

_/\R_i.9 1985

NGLEYRESEARCH OENTER

LIBRARY,NASA

Annual Report HANP__O_N_=_RGI_IA

Jan. 1985

_ Paul O. Wilbur

I Departmentof MechanicalEngineering ColoradoStateUniversity

Fort Collins,Colorado

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MAJS: /*OEFLECTION/*D£VERGENCE;*HOLLOW CATHOOES/*rON EEAMS/*ION ENGINES/*ION

PROPULSION;*ION SHEATHS/*MAGNETIC FIELDS

MINS: / ANOOES/ CATHODES; ELECTRIC POTENTIAL; GAS PRESSURE; NITROGEN/ RING

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1. Report No. I 2. Government AccessionNo. 3. Recipient's Catalog No.

NASACR 174862 I

4. Title and Subtitle 5. Report Date

ADVANCEDION THRUSTERESEARCH Jan. 1985

6. Performing OrganizationCode

7. Author(s) 8. PerformingOrganization Report No.

Paul d. Wilbur

" , , 10. Work Unit No.

9. PerformingOrganization Name and Address

Department of Mechanical Engineering

Colorado State University 1t. Contract or Grant No.

Fort Collins, Colorado 80523 NGR-06-002-112

13. Type of Report and Period Covered

Annual, 12, SponsoringAgency Name and Address

National Aeronautics and Space Administration Oct. I, 1983 - Nov. I, 1984

Washington, D.C. 20546 14, SponsoringAgency Code'

15. Supplementary Notes

Grant Monitor - William Kerslake, NASALewis Research Center, Cleveland, Ohio 44135.

16. Abstra_

A series of experiments conducted on a ring cusp magnetic field ion thruster; in which

the anode, cathode and discharge chamberbackplate were movedrelative to the magnetic cusp;

are described. Optimumlocations for the anode, cathode and backplate which yield the lowest

energy cost per plasma ion and highest extracted ion fraction are identified. The results

are discussed in terms of simple physical models. The results of preliminary experiments

into the operation of hollow cathodes on nitrogen and xenon over a large pressure range

(<0.I to >I00 Torr) are presented. They show that the cathode discharge transfers from the

cathode insert to the exterior edge of the orifice plate as the interelectrode pressure is

increased. Experimental evidence showing that a new ion extraction grid concept can be used to

stabilize the plasma sheath at the screen grid is presented. This concept, identified by the

term "constrained sheath optics," is shown to hold ion beamlet divergence and impingement

characteristics to stable values as the beamlet current and the net and total accelerating

voltages are changed. The current status of a study of beamlet vectoring induced by displacing

the accelerator and/or decelerator grids of a three-grid ion extraction system relative to the

screen grid is discussed.

17. Key Words (Suggested by Author(s)) 18. Distribution Statement

E1ectrostatic Thruster Unclassi fled - Unl imi ted

Hollow Cathode

"_ 19. Security Classif.(of this report) 20. Security Classif.(of this page) 21. No, of Pages 22. Price*

Unclassified Unclassified 64

Forsale bytheNationalTechnicalInformatioSen rvice,Springfield,Virginia 22161

NASA-C-168 (Rev. 10-75) i

I

TABLEOF CONTENTS

Page

Abstract .......................... i

lon Thruster Performance Model ......... 1

lon Beamlet Steering for Two-grid Electrostatic Thruster . . 2

The Annular Flow Electrothermal Ramjet .......... 3

Ring Cusp Discharge ChamberStudies .......... 5

Apparatus and Procedure .................. 6

Results .......................... 14

Effect of Discharge Chamber Length ......... 14

Effect of Cathode Location .... 19

Effect of Anode Position ............... 23

Conclusions ........................ 32

Hollow Cathode Research .................. 34

Apparatus and Procedure ........... 34

Results ................. 36

Conclusions and Recommendations ............ 41

Constrained Sheath Optics ............ 43

Apparatus and Procedure ................. 45

Results ...................... 46

Concluding Remarks.................... 48

Three-Grid Beamlet Vectoring ................ 50

Apparatus and Procedure ......... 51

Results ...................... 55

Conclusion .................. 55

Appendix A - A Technique for Making Iron Filings Maps

in Strong Fields from Permanent Magnets .......... 56

References ............... . ............ 57

ii

LIST OF FIGURES

Figure No. Title

I. 8 cm dia Ring Cusp Thrusterwith Movable

Back Plate ................. 7

2. 8 cm dia Ring Cusp Thrusterwith Movable

Cathode.................... 8

3. 8 cm dia Ring Cusp Thrusterwith Movable

Anode....................... 9

4. 8 cm dia Ring Cusp Thruster Magnetic

Field Map.................. lO

5. Effect of DischargeChamber Lengthon 8 cm

dia Ring Cusp Thruster Plasma Ion EnergyCost. . 15

6. Effect of DischargeChamber Lengthon 8 cm dia

Ring Cusp Thruster ExtractedIon Fraction . .. 16

7. Effectof DischargeChamber Lengthon 8 cm dia

Ring Cusp Thruster Beam Ion EnergyCost.... 18

8. Effect of Cathode Positionon 8 cm dia Ring Cusp

Thruster Plasma Ion Energy Cost.......... 20

9. Effect of Cathode Positionon 8 cm dia Ring Cusp

Thruster ExtractedIon Fraction........ 22

lO. Effectof Anode Positionon Plasma Ion Energy

Cost Curve ................ 25

If. Effectof DischargeChamber Lengthon Plasma Ion

Energy Cost Curve - Anode Axial'Position

Optimized................... 27

12. Effectof Anode Diameteron Plasma Ion Energy

Cost Curve - Anode Axial PositionOptimized. . 28

13. Anode Positionsat Pointsof DischargeExtinction. 30

14. 8 cm dia Ring Cusp Thruster- Effect of Discharge

Voltage for OptimizedAnode....... 31

15. Hollow Cathode Configurations ........ 35

16. Effect of Interelectrode Pressure on Anode

Potential .................... 38

17. Typical interelectrode Potential Profiles ..... 39

iii

LIST OF FIGURES(Continued)

Fi gure Ti tl e

18. Qualitative Sheath/Beamlet Behavior ........ 44

19. Constrained/Free Sheath Optics Comparison.... 47

20. Three-Grid Deflected Beamlet Nomenclature ..... 52

21. Grid Translation Apparatus ........... 53

iv

ION THRUSTERPERFORMANCEMODEL

John R. Brophy

A model describing the performance of high flux density, cusped magnetic

o field ion thruster designs was developed during this grant period. This work

is described in detail in Ref. 1 so it will only be summarized here. The

model is formulated in terms of the average energy required to produce an

ion in the discharge chamber plasma and the fraction of these ions that are

extracted to form the beam. The direct loss of high energy (primary) electrons from the plasma to the anode is shown to have a major effect on thruster

performance. The model provides simple algebraic equations enabling one to

calculate the beam ion energy cost, the average discharge chamber plasma ion

energy cost, the primary electron density, the primary-to-Maxwellian electron

density ratio and the Maxwellian electron temperature. Experiments which

show the model correctly predicts the variation in plasma ion energy cost

for changes in propellant gas (Ar, Kr and Xe), grid transparency to neutral

atoms, beam extraction area, discharge voltage, and discharge chamber wall

temperature are described in Ref. I.

The model and experiments indicate that thruster performance may be

described in terms of only four thruster configuration dependent parameters

and two operating, parameters. The model also suggests that improved performance should be exhibited by thruster designs which extract a large

fraction of the ions produced in the discharge chamber, which have good

primary electron and neutral atom containment and which operate at high

propellant flow rates. In addition, it suggests that hollow cathode

_ efficiency becomes increasingly important to the discharge chamber performance as the discharge voltage is reduced. Finally, the utility of the
model in mission ana|ysiscalculationsis demonstrated. The model makes

it easy to determinewhich changes in thrusterdesign or operatingparameters have the greatesteffect on the payload fractionand/or mission

duration.

IONBEAMLETSTEERINGFORTWO-GRID

ELECTROSTATICTHRUSTERS

John M. Homa

An experimentalstudy of ion beamlet steering in which the direction

of beamletsemitted from a two-gridaperture system is controlledby relative

translationof the grids,was completedduring the grant period. This work

is describedin Ref. 2 so it will only be summarizedhere. The resultspresented in Ref. 2 can be used to design electrostatic,ion acceleratingdevices

for which the directionand focus of emerging beamletsare important. Deflectionand divergenceangle data are presentedfor two-gridsystems as a

function of the relative lateral displacementof the holes in these grids.

At large displacements,acceleratorgrid impingementsbecome excessiveand

this determinesthe maximum allowabledisplacementand as a result the useful

range of beamlet deflection. Beamlet deflectionis shown to vary linearly

with grid offset angle over this range. Values of deflection-to-offsetangle

ratio and useful range of deflectionare given as functionsof grid-hole

geometry,normalizedperveancelevel, and acceleratingvoltage levels. The

divergenceof the beamlets is found to be unaffectedby grid offset over the

useful range of beamlet deflection. The grids of a typical dished-gridion

thrusterare examinedto determinethe effects of thermallyinduced grid

distortionand prescribedoffset of grid hole centerlineson the characteristics of the emergingbeamlets. The results are used to determinethe
region on the grid surface where ion beamlet deflections exceed the useful

range. Over this region high accelerator grid impingement currents and

rapid grid erosion are predicted.

THEANNULARFLOWELECTROTHERMAL RAMJET

Ben D. Shaw

Theoretical analysis of the annular flow, electrothermal, plug ramjet

was completed and the results are described in Ref. 3 so the results of this

work will also simply be summarized here. In Ref. 3 the ramjet is examined

as a possible means of achieving rapid projectile acceleration to velocities

for such applications as direct launch of spacebound payloads. The performance of this ramjet device operating with hydrogen propellant is examined

for cases where this working fluid is treated (I) as a perfect gas and (2) as

o a gas that is allowed to dissociate and ionize and then recombine with finite

reaction rates in the nozzle. Performance results For these cases are compared

to the performance of a conventional ramjet operating with perfect gas} hydrogen

propellant. It is shown that the performance of the conventional ramjet is

superior to that of the annular flow, electrothermal ramjet. However, it is

argued that the mechanical complexities associated with the conventional

ramjet would make it impractical, and for this reason the annular flow, electrothermal ramjet is more desirable as a launch system. Models are presented

which describe both electrothermal_plug ramjet and conventional ramjet operation, and it is shown that for a given flight velocity there is a rate of

heat addition per unit propellant mass for which ramjet operation is optimized. In general, dissociation and ionization losses in the hydrogen

propellant are found to be small and nozzle flow is shown to be near
4

chemical equilibriumrather than the chemicallyfrozen state. It is demonstrated that diffuser shock losses,while significant,do not degrade performanceto unacceptablylow levels. Data are presentedin such a way that

they can be used in analyzingvariousmission profiles. It is demonstrated

that the residencetimes of fluid particleswithin the electrothermal:plug

ramjet are small comparedwith the times requiredfor flowfieldto change

significantly. It is argued, therefore,that errors introducedby assuming

quasi-steadyflow are not excessive. The thermal efficiencesover different

launch cycles are shown to be high (between30% and 40%), and the pressure,

temperatureand power demand requirementsare demonstratedto be reasonable.
RING CUSPDISCHARGECHAMBERSTUDIES

Jeff Hiatt

The ring cusp discharge chamber design proposed by Sovey for ion thruster

° applications in 1982 is attractive because it shows significantly lower beam

ion energy costs than its predecessor, the low strength, divergent magnetic

field thruster.4 The reason that the ring cusp thruster shows this improved

performance is, however, not clear because several design parameters were

changed to transform the low strength, divergent magnetic field thruster into

the ring cusp design. Specifically the ring cusp thruster differs from its

predecessor, the divergent field thruster, because a) it utilizes high magnetic

flux density magnets [a few thousand gauss at the magnet surface vs a few

hundred gauss for the divergent field design], b) it effects electron collection at an anode on the magnetic field cusp rather than at an anode between

cusps and c) it utilizes a hollow cathode exposed directly to the discharge

chamber plasma rather than one that couples through a baffle aperture to the

discharge plasma. This last design difference coupled with somewhat different

magnetic field shapes for the two designs suggests that primary electrons are

supplied to the discharge chamber along different magnetic field surfaces of

revolution for the two designs. In order to investigate the effects of changes

in magnetic field strength and points of electron emission and collection

independently and to determine if still greater performance improvements might

be realized in the ring cusp design, a series of experiments were conducted on

an 8 cm ring cusp thruster. The thruster used in the study was designed so

either the chamber length, the region of electron injection or the anode

location relative to the magnetic field cusp could be varied during operation.

Interpretation of the experimental results obtained in this study will

be couched in terms of the ion thruster discharge chamber model proposed by
6

Brophy. 1 By examining the key parameters associated with this model, namely

the plasma ion energy cost (_p) and the extracted ion fraction (fB), it will

be possible to determine whether improvements in performance caused by

changing discharge chamber length, region of electron injection or region of

electron collection actually come about because of improved utilization of

the energy supplied to discharge chamber electrons, an increase in the fraction

of the plasma ion extracted into the ion beam or both.

Apparatus and Procedure

The basic 8 cm diameter discharge chamber used in these studies is shown

schematically in Fig. I. In the configuration of Fig. I, the chamber is

arranged so the steel backplate can be moved axially under the action of the

lead screw, yoke and guideposts. In other configurations (Figs. 2 and 3) the

lead screw is connected through the yoke and appropriate insulators to either

the cathode or anode so their axial location can be varied during operation.

In each of these configurations, the steel cylindrical housing has a 9.2 cm

inside diameter and both it and the steel backplate are 6.4 mmthick. A single

1.9 cm x 1.3 cm x 0.5 mmthick samarium cobalt magnet on the thruster centerline and a ring of these magnets arranged end-to-end 3.7 cm upstream of the

grids drive the magnetic circuit (Figs. 1 and 3). For the cathode study

(Fig. 2) the ring magnet is 3.2 cm upstream of the grids. These magnets

have a flux density of 0.27 T at their surfaces and they are arranged so

the polarity on the outer surface of the ring magnet is opposite to that

at the outer surface of the centerline magnet. An iron filings map of the

resulting magnetic field for the case where the backplate is 11.7 cm upstream of the grids is shown in Fig. 4. Because of the high magnetic field

strengths in this chamber, the unique procedure described in Appendix A had

to be developed to make this iron filings map. It is noted that Figs. I, 2

and 3 show cooling coils on the backplate and sidewalls; they are provided to
PROPELLANTFEEl .COOLING

// WATER LINES

PERMANENTMAGNETRING

GUlDEPOSTS..

i

YOKE_ "COOLINGWATER PLATE /1

..:-:- LIN CATHODE Jl 8crn BEAM

_ _i_- )_/'_j; WIRES 6Cmll

__ _I_,j;,_ ..,

- - 9:2 cm

:)DE Ii

SCREW DISC CATHODE I i

i_ "•" ETS \CENTE MAGNET RLINE SUPPORT PO_ I I II , _ GRID

[ ANODi ti

INSULATOR EL GRID

....ous,.o,k - 8 cm _,_j 2.2

ca _ 1cm

.5__

Figure I. 8 cm dia Ring CuspThruster with Movable Back Plate
- - - - - iJt1t-'t1JJ1J!1l1L

_____iiUJlOOA.!lJA

QUARTZ

TUBING

COOLING WATER

LINE

PROPELLANT FEED) COOLING

WATER LINES

PERMANENT MAGNET RING

SCREEN GRID

co

Figure 2. 8 cmdia Ring Cusp Thruster with Movable Cathode
/ wATt. _,._s J

ALUMINUM OXIDE . I'I J-ol O PER.MANENT MAGNET RING

,: ..:..: . .-.-

" " I"

'_-__ O_L,._" _--- _'-_. _ c_oo_u_Po._--

5//_" LINE_Y/X/:_'V// ' POSTS II- " T Y/, _ _?} - _ " " II

-.. "0 . . jI1

ING o o o ,-,

:5.7cm

Figure 3. 8 cm dia Ring Cusp Thruster with Movable Anode
I0

PROPEL

\ RING MAGNE

\

_STEE L DISCHARGE

CHAMBER

Figure 4. 8 cm dia Ring Cusp Thruster Magnetic Field Map

7
II

insurethe magnetswill not be overheatedand to preventthermallyinduced

bindingof the movablebackplate.

In all configurationsthe grids have a cold spacingof 0.86 mm, the

screengrid holes are 1.9 mm dia, the accel grid holes are 1.7 mm dia and

the hole-to-holespacingon these grids is 2.2 mm. For all tests the screen

grid potentialwas 750 v, the accel grid voltagewas -250 v and argon propellantwas fed throughthe dischargechambersidewallin the manner suggested

in Figs. l, 2and 3. The tests were conductedat a dischargevoltageof 50 v

unlessotherwisenoted. All tests were conductedin a 45 cm dia vacuumbell

jar system in which the backgroundpressurevariedover the range 5 x lO-s

Torr to 2 x lO-4 Torr duringtesting. Neutral densitiesand propellantutilizations haveall been correctedfor neutralbackflowassociatedwith the bell

_ jar pressurethat prevailedduring each particulartest.

For the dischargechamberlengthstudies,the backplatecould be moved

over the range from If.7 cm to 5 cm upstreamof the grids during ion source

operation. The cold thrusterdiametricalclearancebetweenthe backplate

and the cylindricalhousingwas 0.13 mm. This valuewas selectedas sufficiently great to permitaxial motion of the backplatewithout imposinga

large reluctancein the magneticcircuitat the backplate-sidewallinterface.

- For these studies,two 0.25 mm dia tungstenfilamentcathodes,havinga

diametricalextentof 6 cm, heated by alternatingcurrentswere used. The

filamentswere located_ 1.5 cm upstreamof the screengrid. The anodesused

in this seriesof tests were 0.25 mm thick steel sheetsmagneticallyattracted

toward,but separatedfrom, the centerlineand ringmagnets by 0.76 mm thick

flexiblemica sheets--inthis configurationelectroncollectionoccurs at the

magneticfield cusp. For these tests the screengrid was maskeddown to produce

an 8 cm dia. beam.
12

For the study of the effectsof cathode positionon performance,the

backplatewas fixed at an ll.7 cm lengthand the guidepostsof Fig. l were

replacedby piecesof quartz tubing that housed copperwires supportinga

single0.75 cmdia cathode coil of 0.25 mm dia tungstenwire in the manner

suggestedby Fig. 2. The coil structureextended4 cm across the discharge

chamber;this configurationwas selectedto be sufficientlyflexibleso the

wire would not break as it was moved under the actionof the lead screw and

yoke over its axial range from 0.7 to 7 cm upstreamof the grids. Anodes

were also located at the magnetsurfaces for this study so electroncollection occurredat the cusps of these magnets. The screen grid was again

masked downto producean 8 cm dia. beam.

In studyingthe effectsof anode positionon performance,the anode

positioncould be varied continuouslyat each of three backplateaxial

locations(correspondingto chamberlengths of ll.7, 8.8 and 5.3 cm). The

dual, tungstenfilamentcathode positionwas however held fixed at 1.5 cm

upstreamof the grids. In order to conduct these tests, the sheet metal

anodes on the magnets were replacedby a single stainlesssteelwire loop

anode supportedfrom the movableyoke by copper wires housed in aluminum

oxide tubes after the manner suggestedin Fig. 3. Actuallyanode loops having

major diametersof 7.6, 6 and 4 cm were all testedand they could be moved

over the range from l cm to lO cm upstreamof the grids under the action of

the yoke and lead screw. For this test the screengrid was masked to produce

a beam diameterof 4 cm.

The basic test proceduresused in conductingall of the test were similar

and involvedestablishmentof the desiredargon mass flow rate (m) (I00-700

mA eq.) and a steadydischargeat a 50 v dischargevoltage (VD) and a moderate

dischargecurrent (JD). The dischargecurrentwas then changed in increments
13

and the dischargecurrent,ion beam current (JB) and ion productioncurrent

(Jp) were measuredat each increment. The parameterof interest(chamber

length, cathodeor anode location)was then variedand the procedurewas

repeated. If desired,the propellantflow rate could also be changedand this

procedurecould be repeated. Extractedion fractions(fB) and energycosts

per plasma ion (_p)were computedfrom the measureddata using the usual

1

equations.

fB = JB/JP (l)

Cp = (JD - JP) VD/JP (2)

The ion productioncurrentwas determinedin the conventionalwayI for these

tests. This is accomplishedby biasing the thrusterbody and screen grid

30 v negativeof cathode potentialto repel electronsso the ion currents

_ to each of these surfacescan be measured. The sum of the ion currentsto

these surfacesand into the beam approachesthe total ion productioncurrent

(Jp). It shouldbe noted that in conductingthe study of the effect of anode

positionon performancethe dischargewould go out when the anode was moved

too far from the axial locationof the magneticfield cusp. The axial location of the anode at which this occurredrepeatedlyfor a given anode loop

diameteris referredto as the extinctionpoint for that loop. In this report

the extinctionpoint will always be the one betweenthe cusp at the ring magnet and the grids becauseoperationof the thrusterwith the anode on the other

side of the cusp (upstream)always resultedin poorer performance.

The error between the true ion productioncurrent and the measuredcurrent

is due to the collectionof ions on the anode and the cathodewires and

supportposts. This error shouldbe small becausethe areas of these

surfacesare small romparedto those of the thrusterbody and grid.l

Iii i
14

Results

Effect of DischargeChamber Length

As dischargechamber length increases,the ratio of the path length for

a typical primary electronthat reaches an anode to its mean free path for

inelasticcollisionsalso increases. As a result the probabilitythat a

primary electronwill lose its energy in an ionizingcollisionincreases. Two

recent theoreticalmodels have employed these facts to show the energycost of

a plasma ion shoulddecreasewith dischargechamber length.1'5 Results showing

that this trend is also observed experimentallyare given in Fig. 5. While

these data were obtained at a flow rate of I03 mA eq., this general result was

typical over the limited flow rate range investigated. While the data of Fig. 5

suggest this trend is independentof propellantutilization,it should be noted

that the theoreticalmodel of the processI suggeststhat the plasma ion energy

cost should become independentof chamber length at very low utilizationsand

high flow rates.

The overall ring cusp thruster performance(beam ion energy cost) is

dependenton both on the plasma ion energy cost and the fractionof the plasma

ions produced that are extractedinto the beam. The typical effect of discharge

chamber length on the extractedion fractionmeasured in these tests is shown

in Fig. 6. It is observed to decreaseas the dischargechamber is lengthened,

and to decrease less rapidly as the propellantutilizationis increased. It

should be noted that the curves of Fig. 6 correspondto operationat constant

beam current (mass flow rate and utilizationare both constant). This leads

to the observationthat increasingthe chamber length causes the extracted

ion fraction to decrease because the ion productioncurrent increaseswhile

the beam current remains constant. The decrease in extractedion fractionwith

chamber length is expected because this fractionshould be directlyproportional
400

ARGON FLOW RATE = 103mAeq

^ 350 Vo =50- Ov

0=

30O .86

00

250

z

w

Z -_

===l

.4

200

ol

.JG.

0

I 150 0 r_u= o. 2

=L. ._ ! !

6 7 8 g 10 1! 12

D!SCHARGECHAMBERLENGTH (cm)

Figure 5. Effect of DischargeChambeLeng r thon 8 cm

dia Ring Cusp Thruster Plasma Ion Energy Cost
m

ARGON FLOW RATE=IO3mAQc::: I

• 9 VD--50. Ov

• 5 I I I I I I I

5 6 7 8 g 10 11 12

DISCHARGE CHAMBERLENGTH (cm)

Figure6. Effectof DischargeChamberLengthon 8 cm dia

RingCuspThrusterExtractedIon Fraction
17

to the ratio of beam cross sectional area to the surface area of the discharge

chamber and this ratio decreases with chamber length. The effects of Propellant

utilization on the extracted ion fraction vs. discharge chamber length curve

is not predicted by existing models.

The energy cost of a beam ion (CB) is described in terms of the plasma

ion energy cost (_p) and the extracted ion fraction (fB) by the equation I

_p l-f B

- fB+ vD (3)

for the present case where the fraction of the thruster interior surface area

at anode potential is small. Applying this equation to the results of Figs. 5

and 6 yields the plots shown in Fig. 7. These plots suggest there is an

optimum discharge chamber length for this 8 cm dia. ring cusp thruster (i.e.

the one of Fig. I) of 8-9 cm which will yield a minimum beam ion energy cost.

This minimum is, however, observed to be quite broad so chamber length does

not have a strong influence on performance. It is noted that a similar

optimum length was observed by Isaacson and Kaufman for a 15 cm dia. multipol_argon thruster. 6 It is also noted that the results of Fig. 7 apply to

the case where electron collection occurs at the magnetic field cusps and the

optimum length observed for this case will be shown later to be different

from the one obtained when the region of electron collection is moved axially

away from the midpoint of these cusps.
600

ARGON FLOW RATE=IO3mAoq

VO- 50. Ov

550

_B

. ._ ,r ° .

450

g 400

z 350 0

o

W 300 _ o

250

200 , , , , , , ,

5 6 7 8 9 10 11 12

DISCHARGECHAMBERLENGTH (cm)

Figure 7. Effectof DischargeChamber Lengthon 8 cm dia

Ring Cusp ThrusterBeam Ion Energy.Cost I

I
19

Effectof Cathode Location

The dramaticeffect of the axial locationof a filamentcathode on the

energy cost per plasma ion for an 8 cm dia., If.7 cm long ring cusp thruster

(the one of Fig. 2) is shown in Fig. 8. These particularresults were obtained at a propellantutilization(nu) of 41% and flow rate (m) of 150 mA eq.

but they are typical of all of the results obtainedin this study. They show

the lowest plasma ion energy cost when the cathode is close to the grids. The

plasma ion energy cost is observed to rise rapidly as the cathode is moved upstream from the vicinityof the grids until it peaks as the cathode moves

through an axial location near that of the ring magnet and its cusp. Further

upstreammovementof the cathode causes the plasma ion energy cost to drop

slightlyand then level out. The horizontalerror bars in Fig. 8 indicatethe

_ range over which emissionoccurredfrom the 0.75 cm dia. spiral cathode wires

at a given axial location. It is believedthat the highest energy cost per

plasma ion occurs in the Fig. 8 data when the cathode is near the cusp because

a substantialfractionof the primary electronsemitted from this locationcan

travel in a straightline along the axis of the cusp and its associatedconverging magnetic field to the anode. This convergingmagnetic field can be

seen in the iron filings map of Fig. 4. When the cathode is moved downstream,

toward the grids, direct access to the anode along the axis of the cusp is

denied, reflectionfrom the convergingmagnetic field increases7 and primary

electron losses to the anode decrease. Axial cathode movement into the region

upstreamof the ring cusp apparentlyinduces the same sort of decrease in

o, losses through the ring cusp but losses to the centerlinecusp anode apparently

increaseand the combined _esult is that the plasma ion energy cost remains

relativelyconstant. While it may be possibleto replace this centerline

anode with a cathode potentialsurface and stop the electronlosses to this

I!! i
45O ARGON

Vo= 50. Ov

_u= O. 41

r_=150 mAaq.

]

400

8

_ 350

d 300 MAGNET Z C

RING

250 I I i i

0 I 2 3 4 5 6 7

C^THODE-SCREENGRID SEPARAT]0N ¢=m)

Figure 8. Effect of Cathode Position on 8 cm dia Ring Cusp

Thruster Plasma lon Energy Cost i

i
21

cusp, this change was not attempted during this study of the effect cathode

location on performance.

" The percentage of ions extracted from the discharge chamber into the

beamwas found to decrease as the cathode was moved away from the screen grid

toward the magnetic ring cusp. This effect is illustrated in Fig. 9 where the extracted ion fraction is plotted as a function of the cathode-screen grid

separation distance. The fraction of ions directed into the beam is seen to

reach a minimum when the cathode is near the axial location of the magnetic

ring cusp. It is believed that this fraction depends on the diameter of the

surface of revolution of the magnetic field line most distant from the thruster

centerline at the grids onto which primary electrons are emitted. Whenthe

cathode is close to the grids this would be the surface of revolution having

_' the same diametricalextent at the grids as the cathode (_ 4 cm in the case of

Fig. 2). If the cathode were positionednear the cusp, however, Fig. 4 suggests

the electronswould be emitted onto field lines that convergeas they extend

toward the grids. This suggeststhat in this case the diametricalextent:of

the primary electron region at the grids would be small (<< 4 cm). It is also

postulatedthat the ion productionregion correspondsclosely to the region

occupied by the primary electrons. When the cathode is close to the grids,

the ion productionregion would be large and close to the grids and the extracted ion fractionwould, therefore,be high. When the cathode is near the

ring cusp, the ion productionregion is probablysmall, but more importantly

it would be far from the grids and as a result the extractedion fraction

< would be small. Although beam profiles were not measured in this stud_one

would expect the beam profile to be flatter if the cathode were near the grids.

It should also be noted that the curve of Fig. 9 is one of constant beam

current (flow rate and utilization are both constant). This means that

I I
.4 .

ARGON

3 ' 1 ' Vo= 50. Ov

_u=O. 41 '

o=

== 2

,- !

MAGNETIC

RING CUSP

,7

o , , frill/l/IliA , , , ,

0 1 2 3 4 5 8 7

CATHODE-SCREEN GRID SEPARATION (=m)

Figure 9. Effect of Cathode Position on 8 cm dia Ring Cusp

Thruster Extracted lon Fraction
23

operationwith the cathode at the cusp impliesa higher ion productionrate

than that realizedwhen the cathode is near the grids.

A phenomenonsimilar to the one describedin the preceedingparagraph

was observedwhen the diametricalextent of the cathode was decreasedfrom

6 cm to 4 cm. This induced a reductionin the general level of extracted

ion fraction from the 0.7 range (data of Fig. 6) to the 0.2 range (data of

Fig. 9). Again_it is argued that the decrease in the diametricalextent of

the cathode induceda reductionin the diametricalextent of the ion production volume near the grids and thereforea lesser ion extractionat the outer

radii of the grids.

Figure 9 also shows the extractedion fractionrising as the cathode is

moved upstreamof the magnetic ring cusp. This phenomenoncan be explained

in a similar manner assuming that a small fractionof electronsenteringthe

magnetic cusp region may be reflectedonto field lines on the opposite side

of the magnetic ring cusp.

Because the plasma ion energy cost decreasesand the extractedion

fraction increasesas the cathode is moved close to the grids, the best overall

performance(lowestbeam ion energy cost) is realizedwith the cathode close

to the grids. It should be noted, however,that the dischargebecomes unstablewhen the cathode is too close to the grids. The cause of this instabilityis not understood.

Effectof Anode Position

When the anode loop shown schematicallyin Fig. 3 is moved downstream

from the axial locationof the magnetic field cusp to a pointjust upstream

of the dischargeextinctionpoint a dramaticreductionin the plasma ion

energy cost is observed. The lowest (optimum)plasma ion energy cost is

realizednear the thresholdof dischargeextinction. This effect can be
24

seen in a plot of the plasma ion energy cost against the parameter [m (l-n )],

u

which is proportional to the discharge chamber neutral density. Figure 10

is a typical example of such a plot of data collected over a wide flow rate

range in these tests. The extent of the reduction in the plasma ion energy

cost induced by moving the anode from the cusp location downstream to that

point where extinction is approached (optimized anode location) is seen by

comparing the circular and square data symbols. These data

using the equation proposed by Brophyl

[

-COm ( 1- n ) ] - 1

£ = £* 1 - e u p p

can be fitted

(4 )

where £~, the baseline plasma ion energy cost, is the average energy required

to produce an ion when losses associated with excitation reactions and

Maxwellian electron energy losses to the anode are considered. The primary

electron utilization factor, Co' is given by the equation

(5)

where ab ;s the total inelastic collision cross section for primary electronneutral atom collisions, £e is the primary electron containment length (i.e.

the distance primary electrons would travel in the thruster before they would

reach the anode if they had no inelastic collisions), e is the electron charge,

Ag is the grid area, ~o is the grid transparency to neutral atoms, and Vo is

the velocity of the neutral atoms.

The values of the parameters £~ and Co that resulted in the best fits

of the two data sets in Fig. 10 are indicated on the figure. The increase in

Co caused by moving the anode to the optimized location reflects a corresponding

improvement in primary electron containment (an increase in £e). The reason

for the decrease in 8~ caused by this anode movement is not certain but it ;s
200 300 400 SOD 600

NEUTRAL DENSITY PARAMETER (mAeq)

N

O"l

•

700

C

ANODE POSITION

O-OPTIMIZED

C-ON CUSP

ARGON

VC =50.0v

';'=150-700 mA.q

7.6cm 011'. ANODE

S.Scm LONG CHAMBER

o

o

100

... E: =4S.V

p

Co =lBAeq-l

200

180

160

"c

a

"-- 140

>

CJ

I-

- 120 en

c

u

> 100 t.:)

a:

w

z

w 80

z

c

-

< 60

z

en

<

..J

a. 40

20

0

0

Figure 10. Effect of Anode Position on Plasma Ion Energy Cost Curve
26

probablydue to both reduced ion losses to the aq_de and reducedMaxwellian

temperaturesfor electronscollectedat the anode in the optimizedlocation.

When the anode was moved from the cusp locationto the optimizedlocation,no

significantchanges in the extractedion fractionswere observed. Hence the

minimum beam ion energy cost is realizedwhen the plasma ion energy cost is

a minimum (i.e. at the optimizedanode location).

The experimentalstudy of the effect of anode axial positionon performance was repeated for three differentdischargechamber lengths. Although

the axial locationof the anode loop at which the performancewas optimized

changed for each chamber length, the actual level of ion productionlosses at

the optimum performanceconditionwas independentof chamber length. This

result is demonstratedin Fig. II, where plasma ion energy cost/neutraldensity

parameterdata for three chamber lengths, are shown to fall on a common curve,

characterizedby CO = 17 (A eq.)-I and _ = 47 eV. This result is important

because it implies that by optimizingthe anode locationrelative to the magnet

cusp one can define an ion source in which the plasma ion energy cost is independentof dischargechamber length. This leaves the designerfree to adjust

dischargechamber length to maximize extractedion fraction.

A study was also conductedto determinehow the anode axial position

requiredfor optimum performancewas affectedby changes in anode diameter.

With the dischargechamber length held constant,the chamber was operated in

three tests, each characterizedby an anode loop with a differentmajor diameter. When the anodes were moved near the dischargeextinctionpoints in

these three tests, the performancewas again optimizedand the resultingplasma

ion energy cost/neutraldensity parameterdata pointsmeasured at these anode

positionsonce again fell on a common curve. This curve is shown along with

the data po_nt_.correspondingto each anode diameter in Fig. 12. It is
N

-.....J

600 700

(mAeq) •

7.6cm OIA. ANODE

VO =50.0v

m=150-700 mA.q

CHAMBER LENGTH

0-11. Scm

C-B.Scm

~-5. 3cm

ARGON

200 300 400 500

NEUTRAL DENSITY PARAMETER

100

-1 CO =17AGq

~- ... E:p

= 47sV

200

180

160

"c

0

"

- 140

>

II

- 120 (J)

0

u

>- 100 to:)

a::

UJ

z

IJJ 80

z

0

-

< 60

x

(J)

<

-'

Q. 40

20

0

0

Figure 11. Effect of Discharge Chamber Length on Plasma Ion

Energy Cost Curve - Anode Axial Position Optimized
200

180

1BO ARGON

11. Bom LONG CHAMBER

>_ 140 V0=50. Ov

m=150--7OO mAeq

uo ANODE DIAMETER

>u lOO _ = 4B. O _V 0-7. Bcm

u= Co= 1 7Aeq -z 0-6. O=m co

z 80 Z_-4. Ocm

IW=,l

< so

=<

.-' 40

20 .

O j

O 1DO 200 300 400 500 600 700

NEUTRAL DENSITY PARAMETER (mA,aq).

Figure12. Effectof AnodeDiameteron PlasmaIon Energy

CostCurve- AnodeAxialPositionOptimized
29

noteworthy that the parameters characterizing the curve of Fig. 12 are

essentially the same as those that characterized the curve of Fig. II. It is

. noted that the 4 cm a_ode loop was operated _ 0.3 cm upstream of the extinction point to prevent excessive discharge current oscillations. These oscillations were not observed with the Other anode loops,

The results of Fig. 12 indicate that the same discharge performance

characteristics can be achieved with anodes of different diameters if the

axial location of each anode is proper. Someinsight into which location is

proper can be obtained by plotting the optimum anode positions on an iron

filings map in the manner shown on Fig. 13. Careful examination of these anode

locations suggests that to first order they fall along a single magnetic field

line. This in turn suggests that the surface of revolution of this field line

serves as a "optimum virtual anode" for collecting electrons, In other words

the same performance is achieved in a discharge chamber for an anode positioned

to collect electrons at any place along the surface of revolution of a given

"virtual anode" field line. It is noted that the distance over which the anode

could be moved from the position where the minimum plasma ion energy cost was

first achieved to the point of discharge extinction was a function of neutral

atom density. At low densities the discharge would extinguished at the same

point that the minimum plasma ion energy cost was achieved. As the density

was increased it appeared that the anode could be moved through a small distance

at the minimum plasma ion energy cost before extinction occurred,

The ion source was also operated at a 32v discharge voltage in the configuration of Fig. 3 with the anode position optimized. The plasma ion energy

cost data associated with this test are compared to those at 50v by the square

and circular data symbols respectively in Fig. 14. Both sets of data were

characterized by baseline plasma ion energy costs (_) of 45 eV and by primary
30

NG,MAGNET

9.2 cm --,.. I.-U-'vcmj___--ANODE

CROSSSECTION

-" I"--0.32cm

O.

r'--0.16cm

Figure 13. Anode Positionsat Points of DischargeExtinction
200

ARGON

180

160 =;1=150--700 mAeq

11. 75cm LONG CHAMBER

140

>_ E:I= 45"V

]20 =11. 5 A==q -1 DISCHARGE VOLTAGE

u =1 7. 0 Aeq-1

loo 0 - 50. Ov

O-32. Ov _.

z eo

z

€3

< 60 .

==

<IL 40

2O

0 I I I I I I I

0 100 200 300 400 500 800 700

NEUTRAL OENSITY PARAMETER (mAeq)

Figure 14. 8 cm dia Ring Cusp Thruster - Effectof Discharge

Voltagefor OptimizedAnode
32

electronutilizationfactors (CO) of II.5 (A eq.)-I for the 32v dischargeand

17 (A eq.)-1fOr the 50v discharge. Using the 17 (A eq.)-I value for CO as a

referencevalue in Eq. 5 with deHeer_collision cross sectiondata8 one

predictsCO = 12.8 (A eq.)-I for 32v operationon argon. This value is reasonably close to the value of ll.5 (A eq.)-I determinedfrom the experimental

results of Fig. 14. This representsa corroborationof the dischargechamber

model for a differentdischargechamber diameter than the one BrophyI used in

his tests.

Conclusions

The most significantfinding from this study is probably that the efficiency of an 8 cm dia. chamber having one centerlineand one ring magnetic

field cusp is improvedsubstantiallyby moving the electroncollectionpoint

(anode) from the cusp to a point downstreamof the cusp. This finding suggests

that the improvementin thruster performancethat Sovey4 achievedwith the ring

cusp thrusterwas not due to the fact that he moved the electron collection

point from a field line off of the cusp to the cusp itself. Hence the improved

performancehe observedwas caused by either removalof the baffle or the increasedmagnetic field strength in his thruster. The results of the present

study also suggest that the regionof electroncollectionon the anode corresponding to optimum performancemay lie at any line of contact on the surface

of revolutionof the "optimumvirtual anode" field line on which incipient

dischargeextinctionoccurs. The sourceof this improvementis a reduction

in the plasma ion energy cost for the chamber. Extractedion fractionsdo not

seem to be affected by the locationat which electroncollectionoccurs so

long as it is effected in the region between the ring cusp and the grids.

Further,the optimum performancelevel that can be achieved by proper positioning of the anode is the same, regardlessof the length of the dischargechamber.
33

For a ring cusp dischargechamber in which electroncollectiondoes

occur at the cusps, the length of the dischargechamber and the positionof

the cathode in the chamber affect both the energy cost of plasma ions and

the extractedion fractions. The most significantbeneficialeffect associated

with these changes can be achievedby placing the cathode close to the grids.
34

HOLLOWCATHODERESEARCH

An experimental study of the suitability of hollow cathodes as electron

emitters for arc jets was initiated late in the grant period and some prelimi- _

nary data were collected. The basic philosophy of the study was to begin with

a hollow cathode operating in a regime, where it is known to perform well in

ion thruster applications, and then extend the operation gradually into the

arc jet operating regim_ where its performance is not well understood. The

transition between these operating regimes has involved changing the propellant

from one used in ion sources (xenon) to one suitable for arc jets (nitrogen)

and increasing the interelectrode pressure from the ion source regime (of order

0.I Torr) to that for arc jets (of order I000 Torr).

Apparatus and Procedure

The basic test configuration used for this study as illustrated in

Fig. 15a includes a conventional 6.3 mmdia. orificed hollow cathode and a

5 cm dia. cylindrical, stainless steel, sheet metal anode having its axis

concentric with that of the cathode. The hollow cathode, which has a 0.5 mm

dia. hole at the center of its thoriated tungsten orifice plate, contains a

_olled tantalum foil insert (4 layers of 0.013 mmthick foil) dipped in

chemical R-500. The assembly of Fig. 15a is installed in a 30 cm dia. by

45 cm high vacuum chamber that can be isolated from its vacuum pump and backfilled with various gases to _ 1 atmosphere through a valve connected to the

chamber. The interelectroderegion can be probed using a Langmuir probe that

can be swept from the cathode centerline to the anode surface at any axial

location downstream of the cathode orifice plate. The probe has a tantalum

electrode that is 0.79 mmdia. by 1.2 mmlong. Its output is recorded on an

X-Y plotter and analyzed using basically the same procedure as the one

9

developed by Beattie.
35

0.5ram DIA. 5 cm DIA. ANODE

• //ORIFICE

-'- ROLLED FOIL

6.3 mm DIA. INSERT

HOLLOWCATHODE

I'

IO cm

o. TEST CONFIGURATION

RG

b. SPOKE DISCHARGEFROM INTERNAL CATHODE DISCHARGE

/

_SPOKE

DISCHARGE

11

C. SPOKE ,DISCHARGE FROM EDGE OF CATHODE ORIFICE PLATE

Figure15. HollowCathodeConfigurations
36

The test procedureused in thisstudyinvolvedheating the cathode,

establishinga flow rate through it (_ 400 mA eq. for xenon,_ 5 A eq. for

nitrogen)and then applying a few hundred volt potentialto the anode until

the dischargestarted and the dischargevoltage and heater current could be

turned down. During this startup procedurethe bell jar pressurewas maintained in the range _ O.l - l Torr. After startup the cathode flow rate was

reduced to 92 mA eq. for xenon or maintainedat 5 A eq. for nitrogenand the

pressurewas maintainedin the range _ O.Ol to _ O.l Torr. The dischargewas

then allowed to run at IA dischargecurrent for a few hours to insureoperation had stablized. The chamber was then isolatedfrom the vacuum pump and

backfilledwith the desired gas in increments. After each backfillevent the

interelectrodepressurewas measuredusing either a Shultz Phelpsor

mechanical bellows gauge, the anode voltagewas noted and Langmuirprobe

traces were recorded. All tests reported here have been conductedat a IA

dischargecurrent.

Results

When 92 mA of xenon was flowing through the cathode orifice and no

backfillwas being used the interelectrodepressurewas typically_ O.Ol Torr

and the anode voltage required at IA dischargecurrent was _ 20v. At this

point the dischargeluminosityextendedfrom a bright spot at the orifice

into a uniform,axially symmetric,rather diffuse plume extendingabout 3 cm

downstreamof the cathode.This operatingconditionissimilar to thatobserved

in ion thrustersusing hollow cathodes.lO As the chamber was backfilledwith

xenon to a pressure in the range of l to lO Torr a luminoussheath became

visible at the anode surface and otherwisethe appearanceof the discharge

did not change. In the lO to 40 Torr range a very luminousspoke passing

from the orifice to the edge of the anode in the manner suggestedin Fig. 15b
37

\

became evident and the luminositydroppedoff rapidly away from the spoke.

As the interelectrodepressurewas increasedfurther no changes in the

appearanceof the dischargewere evident;this dischargecontinuedto pass

from the orifice to the edge of the anode. In Fig. 16 a plot of data obtained

in a typical run for a xenon-fed,xenon-backfilledcathode shows that the anode

voltage remainedessentiallyconstantas the pressurewas increasedover the

range from _ 0.1Torr to about four orders of magnitudeabove this value.

Figure 17a shows the potentialprofilemeasured through the discharge

spoke at the point identifiedon Fig. 16 by the symbolO (_ lO Torr).

These data suggest a _ 20v potentialrise at the cathode and no additional

significantvoltage drop either through the spoke plasmaor at the anode.

Essentiallythis same potentialprofile was observedover the completerange

of pressuresinvestigatedusing the xenon-fedcathode and xenon backfill.

When the same xenon-fedhollowcathode was operated in a chamber backfilled with nitrogenthe luminoussheath at the anode and the spoke again

developedin the same pressurerange that they had with xenon. In spite of

this similaritythe anode voltage required to sustainoperationchanged

dramaticallyfrom that for the xenon backfilledone. This differencecan be

seen by comparingthe circular and triangulardata pointsof Fig. 16. Although

these data points start out togetherat low pressurethe nitrogenbackfill

causes the anode voltage to rise with increasesin interelectrodepressure.

point identifiedby Qin Fig. 16 the potentialprofile through the

At the

spoke dischargefollowedthe triangularsymbols in Fig. 17a. By comparingthe

o data points in this figure one can see the increasein anode voltage induced

by an increasein interelectrodenitrogenpressureis requiredto overcome

potentialdrops at the anode and in the bulk plasmaof the spoke. The cathode

potentialdrop is however not affectedby the nitrogenbackfillpressure so it
INTERNAL CATHODE

DISCHARGEEXTINCTION

I A DISCHARGE CURRENT

ill, ? .

_ " -'=11 g

>,oo- / i, ! _,- I _/.' I /

I,l,I - / At I _'

,:I . f-I / A'_I ._/'_"'---5 Aeq NITROGEN-CATHODEFLOW

>0_ . ,'// /A/____Z_ NITROGEN BACKFILL . ///

o 5o ."a,' ..__92 mAeq XENON-CATHODE FLOW

<1: _/z / NITROGEN BACKFILL

El._.__._''" // /92 mAeq XENON-CATHODE FLOW

/ !XENON BACKFILL

l I I I I IIII l I I a l I'1 I I III

O.I I IO IOO IOOO

INTERELECTRODE PRESSURE (Torr)

Figure16. Effectof InterelectrodePressureon Anode Potential
39

h

IOOA

> f

v NITROGENBACKFILL I

-J 26 Torr I

50

z _ XENONBACKFILL

uJ _------_.----_'-_ _ {O.I-IOO Torr)

I-" _.....- ----- (_

o o , _ oo

a. XENON-FED CATHODE(92 mA eq. )

150 -

NITROGENBACKFILL I

_ _" INTERNALDISCHARGE I

>_ 30 Torr _ -I

,,_iO0

W

5o

""- NITROGENBACKFILL-NO INTERNAL

DISCHARGE 31 Torr 1_

O I I I I I

CATHODE I 2 A_DENO

RADIUS ( cm )

b. NITROGEN- FED CATHODE ( 5A eq.)

Figure 17. Typical Interelectrode Potential Profiles
40

appears that the cathode is continuingto operateas it did in the low pressure

ion thrusteroperatingregime. It should be noted that an ideal arc jet would

have zero voltage drop at the cathode and anode so all of the input power would

be going to heat the propellant. Hence the nitrogen-backfilledcathode would

be more attractivein this applicationthan the xenon-backfilledone. The

spoke observed in both of these tests is, however,undesirablebecause it

implies incompleteheatingof the propellantthat would be flowing through the

interelectrodegap of an arc jet.

As the interelectrodenitrogenpressureof the xenon-fedcathode was increased above about 50 Torr the electronemission site would move from the

cathodeorifice to the edge of the orifice plate over a time period on the

order of a second. As this movementoccurs, the anode voltage drops in the

manner suggestedby the triangularsymbols on Fig. 16. It is presumedthis

occurs because the distancelbetweentheelectron emissionsite and the anode

decreasewhen the transitionoccurs. The dischargegenerallystabilizedwith

the electroncurrent passing from a very hot spot on the edge of the cathode

orifice plate and the upstream edge of the anode (as shown in Fig. 15c). It

is noted that the pressureat which this occurredvaried from test to test.

The general trend was that the pressure that was required to induce the transition in emissionsites increasedwith cathode operatingtime. Because the

work functionof the insert generallyincreaseswith cathode operatingtime

it is argued that the pressurerequiredto induce the emission site transition

from the insert to the edge of the orifice plate increasesas the insert is

depleted of low work functionmaterial.

In a final series of tests the cathode was operatedwith 5 A eq. of

nitrogenflowing through the cathode and nitrogenbackfillwas used. In this

case the dischargebehaviorwas essentiallythe same as that describedfor the
41

xenon-fed,nitrogen-backfilledcathode discharge. The typical anode voltage

requirementas a functionof interelectrodepressureis shown by the square

symbols in Fig. 16. The interelectrodepotentialprofilesforpoints(_')and(_)

on this curve are given in Fig. 17b (solid squaresfor condition_where no

v

" internalcathode dischargewas present and open squares for conditionQ

where an internaldischargewas present). The data of Fig. 17b suggest that

there is a small cathode voltage drop (in additionto bulk plasma and anode

drops) that accompaniesincreasesin interelectrodepressure. This behavior,

which was not observedfor the xenon-fedcathode,can be seen by comparingthe

open and solid squares at the cathode location (_ zero radius). Still the

transitionof the emissionsite from inside to outside of the cathode occurred

in the same pressurerange whether the interelectroderegion was backfilled

with xenon or nitrogen. It is possiblethat the cathode voltage drop suggested

by the data of Fig. 17b was due in part to the fact that the Langmuir probe was

not exactly at the cathode orifice. It had to be located I-2 mm downstreamto

the orifice to insure that the probe would not affect the discharge. It is

also likely that the nitrogenflow rate could be lowered somewhatto minimize

this cathode voltage drop effect. The effects of changes in cathode flow

rate have not been investigatedat this time.

Conclusionsand Recommendations

As the interelectrodenitrogenpartial pressureof an orificed,hollow

cathode dischargeis increasedthe anode and bulk plasma potentialdrops

increase. The Voltage drop at the cathode does not seem to be affectedas

significantlyby this pressure. This leads to the conclusionthat most of

the energy dissipationin such a dischargeoccurs at the anode and in the

bulk plasma region of the discharge. This in turn suggests that the hollow
42

cathode should operate as effectivelyin the high pressureregime of the

arc jet as it does in the low pressureone of the ion thruster.

Unfortunatelythe electronemission site of an orificed hollow cathode

moves from inside the cathode to the outside edge of the cathode orifice plate

as the interelectrodenitrogenpressure passesthrough pressuresin the range

of several tens of Torr. When this transitionoccurs the anode voltage decreases. In order to prevent this transitionthe hollow cathode orifice plate

could be removed so the dischargecan be establishednormally between the edge

of the cathode tube and the anode. It is also noted that one reason for an

orifice plate (to maintain the internal pressurein the cathode when external

pressuresare low) is eliminatedwhen interelectrodepressuresare high.
43

CONSTRAINEDSHEATHOPTICS

Both experimental and theoretical studies II'12 have indicated that the

shape of the sheath from which ions are drawn to form ion beamlets in an ion

acceleration system, is altered by changes in the discharge plasma properties

and the ion extraction voltages. This apparently occurs because altering

either the discharge plasma without altering the accelerating voltage or visa

versa necessitates a readjustment of the sheath to ensure the Child-Langmuir

current density law will continue to be satisfied. For example, decreasing

the rate at which ions are supplied to the sheath from a value J to J causes

i 2

the sheath to move further from the accelerator grid so the space charge

limited capacity of the grid system will match and the reduced ion current

being supplied. The nature of this sheath motion is suggested in Fig. 18a by

the movement from the dashed sheath line (corresponding to J ) to the solid

I

_ one (corresponding to J ). Because the outer boundary of the sheath is con- 2

strained by the edge of the screen grid hole, the change in beam current from

J Ito J2results in a more concave sheath This increased concavity in turn

causes the qualitative change in the ion beamlet shape suggested by the transition between the dashed and solid beamlet envelope lines in Fig. 18a. If beam

current continues to be reduced the ion trajectories begin to cross over each

other and excessive direct ion impingement on the accelerator grid occurs.

If on the other hand, the current supplied to the grids is increased the

sheath becomes flatter, the beamlet envelope expands and if this trend is continued, direct ion impingement can again become excessive. It is this latter

condition that imposes a limit on the ion current that can be extracted at a

given accelerating voltage condition. These arguments suggest that one might

be able to realize either higher or lower ion beamlet currents at a given

accelerating voltage condition without excessive impingement currents if the
44

_SCREEN _ /ACCEL

_di •I _ _

;HEATH FOR Jl> Ja

BE PE

ts_ _g

a, FREE SHEATH OPTICS

/BEAMLET ENVELOPE

SHEATH .,)'_ / FOR J=_' J_ JI

FOR J ING SCREEN

FOR JI > Ja

b. CONSTRAINED SHEATH OPTICS

Figure 18. QualitativeSheath/BeamletBehavior
45

shape of the sheath could be controlled.

Several years ago, Byers proposedthe use of a fine mesh screen to

_ accomplishthe sheath controllingfunctionjust describedbut found that the

screen burned through quickly.13 More recentlyAston proposedthe decoupled

extractionand accelerationconcept (DEAC),I4which includesfocusingthe

•screen grids that appear to be serving this same function. The constrained

sheath optics system that is the focus of this research involvesa screen like

the one Byers proposed. The shape of this screen is selectedto match the

shape of the sheath of a properlyfocused beamlet.II The basic concept of

constrainedsheath optics is then to place a constrainingscreenon the screen

grid hole that would inducemany sheaths that would move in the manner suggested

in Fig. 18b as the beam current was changed from a high value J to a lower one

I

J . Because the overall sheath is constrainedto the basic concave shape

2

shown in Fig. 18b, it is argued that the beamlet enveloperemains relatively

unchangedas these beam current changes are introduced.

Apparatusand Procedure

In order to test the validityof this concept,a special 8 cm dia. ring

cusp ion source designedfor ion implantationapplicationsand equippedwith

a single 2.9 cm dia. screen grid aperturewas modified to serve as a test

bed. The source, which was operatedon argon propellantfor the tests,

utilizeda tungstenfilamentcathode. The acceleratorgrid had a single

2.3 cm dia. aperturealigned with the screen grid apertureon the ion source

centerline. The grids,which were both 1.5 mm thick,were separatedby 2.8 cm.

The test was conductedby operatingthe ion sourceover a range of beam

c currentsat various acceleratingvoltage conditionsfirst without the constrainingscreen and then with it. At each operatingconditionthe beam

current and impingementcurrent were recordedand ion beam was probedusing
46

a Faraday probe located 46 cm downstreamof the accel grid. From the Faraday

probe data the divergenceangle of the beam was computedusing the usual definition, namely the half angle of the cone enclosing95% of the beam current and

containingthe circumferenceof the accel grid aperature.

The constrainingscreen used for this study was made of 0.25 mm dia.

tantalumwires separatedby _ l mm and arrangedin a crossed pattern. It was

attached by spot welding it to the upstreamscreen grid surface. The constraining screen was then deformedmechanicallyuntil its positionon the hole centerline was 0.03 cm upstreamof its original,flat positionand its contour

II appeared to be the same as that for the sheath of a well-focusedbeamlet.

Results

Resultsobtained in the tests are presentedin Fig. 19 where a comparison

of the impingement-to-beamcurrent ratio and the beamlet divergencehalf angle

are shown for variousoperatingconditionsboth with the constrainingscreen

(open symbols and solid lines) and without it (solidsymbols and dashed lines).

Normalizedperveanceper hole (P) values were computed for Fig. 19 using the

equation

p=_..JBVT3/2 [_ss,_]2 (6)

where JB is the beam current, VT is the total acceleratingvoltage, and ds is

the screen hole diameter (definedin Fig. 18a). The effectiveacceleration

length _, was assumed to be distance between the acceleratorgrid plane and

an imaginaryplane tangent to the constrainingscreen at the hole centerline.

This same distance (_ = 3.25 cm) was used to compute the perveancefor both ,,

the free and constrainedsheath results. The net acceleratingvoltages (Vn)

and the net-to-totalacceleratingvoltage ratios (R) given on Fig. 19 can

be used to compute the total acceleratingvoltage needed in Eq. 6.
47

ds = 2,9 cm to ts = _ = 0.05

l_g/ds = 0.98 ds d$

do/d $ = 0.79 ARGON PROPELLANT

40 _ _FREE SHEATHOPTICS VN = 7.5 kV 14 r=__t

30 \ I 2 \_

\\ R =0.38 I0 ,,_.=__.._._o

20 \ 8

I 0 4 SHEATHOPTICS

2

o_ 0 , , , , , 0 ' ' ' ' '

"-" 40 =

o v. =20kV =0i4

- 30 _ R = 0.80 _1_,1(_,r'",,

20 _,\\ _ 8 _ \_ _

6

I0 _ _ 4 O <12

0 ' ' ' ' ' 0 , , , , i

i,i

'_ _LU 40

4

m VN = 40 kV _ 12 n_'e'o'l-e"e'\

2030 R = 0.89 _1_I 8 I"°'_1"

•_ I0 I

o=,0 :I

o ;o,

-- 4 _ FREE SHEATH

VN = 60 kV I12 ee4_e OPTICS

30 R = 0,92 I 0

20 8

I0 " 4 CONSTRAINED

I I I I I 2 I ISHEATH I OPTICS ' i

_' % I 2 3 4 5xlO-9 0 I 2 3 4 5xlO-9

NORMALIZED PERVEANCE PER HOLE( A/V 3/2)

Figure 19. Constrained/Free Sheath Optics Comparison
48

The data of Fig. 19, which have been collectedover a very wide range

of net acceleratingvoltages (7.5 kv to 60 kv), suggest two importantpoints.

They are:

l) the impingementcurrentsmeasuredwith the constrainingscreen are

either equal to or less than those obtainedwithout it, and

2) the beamletdivergenceangles remain relativelyconstant as either

the beam current (i.e. normalizedperveanceper hole) or the

acceleratingvoltage are changed with the constrainedsheath optics

while dramaticchanges in these angles are observed when the free

sheath optics system is used.

It is noted that operationat high perveancelevels was not attemptedat

the higher net acceleratingvoltage of Fig. 19. Such operationwas not # attempted because of the fear that the Faraday probe would be destoyed if it

were operated in the high energy flux environmentthat exists in this regime.

ConcludingRemarks

It should be noted that the net acceleratingvoltagesused in this study

are much higher than those that wouldbe used in typical ion thrusterapplications. Still, the trends shown support the physicalmodel presentedand

suggest the constrainedsheath optics concept could be used to maintain proper

ion beamlet focusingat high beam current densitiesand low net accelerating

voltageswithout the need for close grid spacings. Such operationwould be

accomplishedby operatingat a low net-to-totalacceleratingvoltage ratio.

It is also possiblethat a constrainedsheath, single screen grid hole covered

with a sheath constrainingscreen and a single hole accelgrid might be used

to design an ion optics system. It is noted that considerableadditionalresearch needs to be conductedto demonstratethe true potentialsof this concept for ion thruster applications. This research should include a study of
49

the effects of I) the webbing thicknessand spacing and the aperture shapes

in the constrainingscreen, 2) the surface contour of the constrainingscreen,

3) installationof a third (decel)grid, and 4) operationat the net accelerating voltage and beam current levels appropriateto ion thrusteroperation.

The followingpotentialadvantagesof constrainedsheathoptics over

conventionaloptics are suggested:

e Higher current densitiescould be achievedat low specific impulses,

e Close grid separationswould not be requiredso grid failuresdue

to thermal distortionand grid shortingby metallic flakeswould be

much less likely.

• Thrustershaving diametersgreater than those being tested today

-_guldbe operated.

e Stable grid system operationcould be achievedover a wider range

of beam current and accelerationvoltage operatingconditions.

c.

o The accelerationgrid support structurewould be simpler to both

design and build.

The concept also carrieswith it some disadvantagesrelativeto conventional designs that should be mentioned. These include:

• The screen grid fabricationrequirementscould be more complex.

• Dischargechamber performancewould be degraded if it were necessary

to make the screen grid less transparentto dischargechamber ions

than conventionaldesigns.

• The transparencyof the grid systemto neutralatoms could increase.

Consideredin total the potentialadvantagesof the concept are considered sufficientto outweighthe potentialdisadvantagesand the concept

is consideredsufficientlypromisingto warrant additionalstudy.
50

THREE GRID-BEAMLETVECTORING

Barry Andrews

The divergenceand deflectioncharacteristicsof ion beam optics are

importantparametersin the design of electrostaticthrusters. At the low net

accelerationvoltagesaSsociatedwith low specific impulse,inert gas ion

thrusteroperationit becomes necessaryto either use an extremelyclose grid

separationdistance or to operate at a very low net-to-totalaccelerating

voltage ratio. Operationat a low net-to-totalacceleratingvoltage ratio

requiresa three grid optics system to assure good optical performance. In

order to minimize the off-axis thrust associatedwith dished grid sets, hole

misalignmentsare used to redirectthe emergingbeamletsfrom these three

grid systems. A study that would yield either empiricalcorrelationsor a

theoreticalmodel of beamlet deflectionas a functionof hole misalignment

would facilitatedesign of these grids. Further,some ion thrusterappli- _

cations would benefit from a beam that could be vectoredand grid translation

is one method of producinga moderate thrust vectoringcapability.15 In

addition,beam focusingmay be desirableto increasethe current density in

such ground-basedapplicationsas sputtering.16 Again a model or empirical

correlationsdescribingthe effects of hole misalignmenton three grid optics

would be useful in designingthe hardwareneeded for such equipment. Conrad

has proposeda theoreticalmodel of these effects17 and one purposeof this

research i_to verify his results over the range of parametersof interest

for electric propulsionapplications A secondobjectiveis to determine

the preferredgrid system translationscheme for ion thrusterapplications.
51

Apparatusand Procedure

The apparatusand proceduresused for this study are identicalto those

which were used during a previousbeamlet vectoringinvestigation,2 except for

the additionof a deceleratorgrid and the additionalhardwareneeded to effect

its translation. Briefly,a mildly divergentmagnetic field 8 cm diameter

electron-bombardmention source is operatedon argon propellant. Tungsten

wire filamentsare used as both the main and neutralizercathode emitters.

The screen, accelerator(accel),and decelerator(decel)grids are made from

thin,flat sheet graphite. The grid aperture patterncomprisesa nineteenhole

hexagonalarray with a center-to-centerhole spacing of 2.5 mm. Variablegrid

separationis accomplishedby using various numbersof thin mica sheets (0.25 mm

thick). The assembledacceleratorsystem is placedon a masked-downdischarge

chamber,which ensures that the l cm2 hexagonalhole array extracts ions from

a near uniform plasma. The deceleratorgrid is biased 12v below ground

potentialto prevent neutralizationelectronsfrom reachingits surface.

The nomenclatureused to describe the geometryof one of the nineteen

hole sets in the three grids is shown in Fig. 20. As the figure suggeststhe

apparatusis designed so the accel, decel or both of these grids can be translated to inducebeamlet vectoring. The acceleratoror deceleratorgrids can

be translatedin a directionperpendicularto the grid hole axes during thruster

operationby mechanicalsystems coupled to micrometerassemblies,while the

screen grid is held stationary. With this type of arrangement,relative

screen grid deflectioncan also be achieved by translatingthe accel and decel

holes simultaneouslyin such a way that their axes remain aligned.

A schematicof this grid translatingapparatusis shown in Figure 21.

t,

Separateassembliesattached to the accel and decel grids are used to translate

them under the action of the micrometers,which are located outside of the bell
52

VT = TOTAL ACCELERATING VOLTAGE

VN = NET ACCELERATING VOLTAGE

VD = DISCHARGE VOLTAGE

R = VN/TV

SCREEN ACCEL DECEL

GRID GRID GRID

//// ////

//// ////

""//// //// //// EXITING

//// ////

HM //// BEAMLET €__

.... DECEL .H. //// ////

""" GRID ,,,,/////////I "-_--_'-'-__aS //....

$ -

y////_ " dd _

DISCHARGE CHAMBER __ GRI ACCEI D_ : _ SCR HOLEEEN

PLASMA .-// ._" I Y//'/_/I

.. _///_ BEAMLET

/ / ////

"" "/// ENVELOPE

/ / ////,

--- -- i

a - BEAM DIVERGENCE ANGLE

,8- BEAM DEFLECTION ANGLE

_. Xo, - ACCEL GRID TRANSLATION

Xd DECEL GRID TRANSLATION

Figure 20. Three-Grid Deflected Beamlet Nomenclature
I " I

I

I -'

I . I

I

/./ ""_ GRID GUIDE( INSULATING)

GRID

BLocKLOADING ._. /GRAPHITE ...

-DECEL GRID - /GUIDE HOUSING

" / GRAPHITE / " /EXTENSION '_' _

• _ _ . /ACCELGRID / / _.SPRING -

MICROMETER _ " /.._,. 1_.: 1 ' ': " " " . - 43.. .... ,---.-:-,.:--..-._;:] _TO MICROMETER

SUPPORT PLATE _ ' /_ ACTIVE __ ! " (EXTENSION / _INSULATING

SPRING \ CONNECTOR/GUIDE

. . GUIDE / GRID

• - / REGION GRAPHITE "PUSH ROD

MICA SHEET SCREEN

INSULATOR GRID

SECTION A-A

Figure 21. Grid Translation Apparatus
54

jar. Extensionsprings are employed to remove the backlash from their assemblies at the push rod/guideinterfaceand at the micrometer/insulating

connector-guideinterface. The grids are held in place at their sides by

grid guides and they are held at the proper spacingby a loading block that

compressesthe mica spacer/gridsandwichtogether. Testing of the apparatus

has shown it is possibleto maintain the desired positioningaccurracy

(< O.Ol mm) and achieve the needed electricalisolationbetween the various

componentsof the system that are at differentpotentials.

The procedureused in this study is similar to that employed by Homa2 in

an earlier experiment. In short, beam current density profilesare measured

using a movable probe rake containingthirty individualFaraday ion current

sensors which can be positionedat various distancesdownstreamfrom the

acceleratorsystem. The beam current density profilesare recordedand analyzed

using a Hewlett Packard3054 Data Logger in conjunctionwith a computer routine.

A detaileddescriptionof how the computer routinecalculatesdivergenceand

2

deflectionangles can be seen in the aforementionedstudy by Homa.

For the present study, the standardcase selectedis defined by the

followinggeometricaland operatingconditions:

Screen hole diameter (ds) 2.06 mm

Total acceleratingVoltage (VT) llO0 volts

Dischargevoltage (VD) 40 volts

Net-to-totalacceleratingvoltage ratio (R) 0.70

Screen grid thicknessratio (ts/ds) 0.19

Acceleratorgrid thicknessratio (ta/ds) 0.37

Deceleratorgrid thicknessratio (td/ds) 0.37

Acceleratorhole diameter ratio (da/ds) 0.64

Deceleratorhole diameter ratio (dd/ds) 0.83
55

Screen-to-acceleratorgrid separationratio (_g/ds) 0.49

Accelerator-to-deceleratorgrid separationratio (_d/ds) 0.25

The standardcase describedabove was selectedto define a conditionat

which the performanceobtained from this apparatuscould be comparedto that

18

measured previouslyin a study of the three inmovablegrids.

Results

The data that have been collectedso far in this study are considered

preliminary. The intentof this study is to determinethe preferredway to

achieve beamlet steeringand then to carry out a parametricstudy using this

preferredmethod. To this date, the experimentshave not yielded sufficiently

consistentresults so such informationas the ratio of deflectionangle to grid

translationdistance can be presented, Research has show_ however,that translationof the accel grid induces beamletdeflection in a directionopposite

to accel grid motion while decel or screen grid translationcause the beamlet

to be deflected in the same directionas eitherof these grids is moved. In

addition,data collectionhas consistentlyshown that the deflectionangle is

most sensitiveto accel grid translationand least sensitiveto decel grid

translation.

Conclusion

The apparatusneeded to test the beamlet deflectioninduced by translating each of the three grids relative to the other two grids has been developed. Researchhas indicatedthat precisiongrid alignmentand stable

o thrusteroperationare essentialif accuratedata are to be obtained.

Further work is needed to obtain continuousand consistentvalues of beamlet

deflectionas a function grid translation.
56

APPENDIX A

A Techniquefor Making Iron Filings Maps

in Strong Fields from PermanentMagnets _

In order to make magnetic field maps for ring cusp thrusters,lO0 mesh

iron filings are mixed with a two-componentacrylic and the mixture is placed

in a thin layer between two slides,made from 1.6 mm thick clear acrylic that

fit into the dischargechamber.-The acryliccement nearest the magnets is

heated prior to insertioninto the dischargechamber to speed curing in these

high field strengthareas. The slide assembly is then allowed to cool and

after severalminutes it is placed insidethe dischargechamber to cure. The

cement in the heated areas near the magnetic field cusps is more viscous than

the rest of the mixture and this preventsrapid loss of the iron filings to

the edges of the slide in these areas. The un-heatedcementis less viscous

and this facilitatesmovementof the iron filings in the lower flux density

regions of the source. After the cementhas set, the slide maybe removed

and photocopied. A typical map made in this way is shown in Fig. 4. The

time required to make a map is of course dependenton the setting time of the

cement being used. Cement having a l hour setting time was found to work well

in making the map of Fig. 4.

\
57

REFERENCES

I. Brophy, John R., "Ion Thruster PerformanceModel," NASA CR-174810,

Dec. 1984.

2. Homa, John M., "Ion Beamlet Steering for Two-grid ElectrostaticThruster,"

o NASA CR-174671,July 1984.

3. Shaw, Ben D., "The Annular Flow ElectrothermalRamjet,"NASA CR-174704,

July 1984.

4. Sovey, J. S., "ImprovedIon ContainmentUsing a Ring-CuspIon Thruster,"

AIAA Paper No. 82-1928, Nov. 1982.

5. Goebel, D. M., "Ion Source DischargePerformanceand Stability,"The

Physicsof Fluids, Vol. 25, No. 6., pp. I093-II03,June 1982-/--

6. Isaacson,G. G., and Kaufman,H. R., ,'15-cmMultipoleGas Ion Thruster,"

Journal of SPacecraftand Rockets,Vol. 14, No. 8, August 1977,

pp. 469-473.

7. Rose, D. J. and Clark, Jr., "Plasmaand ControlledFusion,"p. 216,

M.I.T. press, 1961.

e

8. deHeer, F. J., et. al., "Total Cross Sectionsfor ElectronScattering

by Ne, Ar, Kr and Xe," J. Phys.B: Atom_ Molec. Phys., Vol. 12,

_ No. 6, pp. 979-I002,1979.

9. Beattie,J. R., "NumericalProcedurefor AnalyzingLangmuir Probe Data,"

AIAA Journal, Vol. 13, No. 7, pp. 950-952,1975.

lO. Siegfried,D.S., "Xenon and Argon Hollow Cathode Research," appears in

NASA CR-168340,P. J. Wilbur, ed., Jan. 1984, pp. 76-132.

II. Aston, G. and P. J. Wilbur, "Ion Extractionfrom a Plasma,"J. Appl.

_., V. 52, No. 4, April 1981.

12. Brewer, G. R., Ion Propulsion,pp. 185-197,Gordon and Breach, New

York 1970.

13. Byers, David C., "An ExperimentalInvestigationof a High-Voltage

Electron-BombardmentIon Thruster,"J. of ElectrochemSoc.,V. ll6,

No. l, Jan. 1969, pp. 9-17.

14. Aston, G. and W. D. Deininger,"Test Bed Ion Engine Development,"

,_ NASA CR-174623,March 1984.

15. Latham, W. C. and W. B. Adam, "TheoreticalAnalysisof a Grid Translation

Beam DeflectionSystem for a 30 cm DiameterKaufman Thruster,"

_ NASA TM X-67911, 1971.
58

16. Kaufman,H. R., J. M. E. Harper, and J. J. Cuomo, "FocusedIon Beam

Design for Sputter Deposition,"Journal of Vac. Science & Tech.,

May/June 1979, pp. 899-905.

17. Conrad, J. R., "BeamletSteering by Aperture Displacementin Ion Sources

with Large Acceleration-DecelerationRatio," Rev. Sci. Inst. v. 51,

April 1980, pp. 418-424.

18. Aston, G. and Kaufman,H. R., "Ion Beam DivergenceCharacteristicsof

Three-GridAcceleratorSystems,"AIAA Journal, Vol. 17, Jan. 1979,

pp. 64-70.
59

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