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Larry Schultz, John Eckert, Tom Ralys,. David Wotford, ... Chung,P. M., Talbot, L., andTouryan,K.J. "ElectricProbesin StationaryandFlowing. Plasmas:Part1.

NASA Contractor AIAA-91-2339

Report

187165

A Preliminary Applied-Field Roger M. Myers Sverdrup Technology, Lewis Brook

Research Center Park, Ohio

David

Wehrle

Cleveland Cleveland,

State Ohio

Characterization MPD Thruster

of Plumes

Inc. Group

University

Mark Vemyi University of Akron Akron, Ohio James Biaglow University of Cincinnati Cincinnati, Ohio Shawn

Reese

Ohio University Athens, Ohio August

1991

Prepared for Lewis Research

Center

Under

NAS3-

Contract

25266

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7'491 - 3 0 2 0 1

1_ 2 1 H :(3.Sl Z O

A Preliminary

Characterization

of Applied-Field Roger

Sverdrup Lewis

M. Myers

Brook

David

Ohio

Plumes

Inc.

Center

Park,

Thruster

l

Technology,

Research

MPD

Group 44142

Wehrle 2

Cleveland

State

Cleveland,

University

Ohio

Mark

Vernyi 3

University Akron,

44115

of Akron Ohio

James

44325

Biaglow

University Cincinnati,

4

of Cincinnati Ohio 45221

Shawn Ohio

Reese 5

University

Athens,

Ohio

45701

Abstract Electric the plume applied

probes,

quantitative

characteristics

magnetic

importance

by the cathode

on the plume

though

spectral

temperatures

of applied-field

field plays

impact

lines ranged

effect

radial

current

geometry

of neutral from

and

species

field

5undergraduate student

always

on the electrical

in the plume.

student

student

The

in establishing

propellant. were

spectroscopy

The

measurements

anode

studied

radius

the plume

present. from

were

the plume

conductivity

used

to study

showed

structure, highly electron

to 20,000

the in

ionized, densities

indicate

and

K, respectively.

radial density gradients potential measurements and

that

followed

had no measureable was

Centerline 7500

confined by the magnetic field, with with applied field strength. Plasma

3undergraduate student, member AIAA 4undergraduate

thrusters.

For all cases

IPropulsion engineer, member AIAA 2undergraduate

role

2 x 1018 to 8 x 10 is m -3 and

of the magnetic conduction

and emission

MPD

the dominant

characteristics.

The plume was strongly increasing monotonically strong

imaging,

show

the presence

a of

Nomenclature Aik

transition

Ap

Bz

probe surface area, m 2 applied field strength at magnet

e

electron

charge,

Eu

energy

of upper

gi

ie

degeneracy of excited state electron saturation current, A

Ip

probe

Iik

intensity,

k

discharge current, A Boltzmann's constant,

me N

electron number

T

temperature,

v

average

Vb

probe bias voltage, V frequency of light from

Uik

probability,

current,

sec -1 exit plane,

T

C excited

state,

J

A

arbitrary

units J/K

mass, kg density, m -3 K

particle

speed,

ln/s i - k transition,

sec -1

Introduction Magnetoplasmadynamic handling

capabilties

which

(MPD)

thrusters

have

them

attractive

for use as the primai),

make

demonstrated

orbit raising and planetary missions I. Thruster efficiencies over 5000 seconds have been demonstrated at power levels with the low-power 500 kW operating While

these

questions

thrusters

performance

about

utilizing

standing

of the physics

cylindrical accelerated magnetic anode.

applied

MPD

While

This

axial

magnetic

thruster,

applied

a substantial

mode,

satisfy

certain

field shown

fields,

body

results data

and there

1, consists

exists

and thrusters

system

with powers

over

power

and those have been

is presently

fundamental level,

obtained obtained only

propellant, from from

a limited

steadyunder-

acceleration. of a central

cathode

with

a coaxial,

an insulating backplate, is heated, ionized, current and the self-induced or applied generated

concerning

2

on

of 1 - 2 milliseconds.

there remain

with thruster

in Fig.

is usually

of data

missions,

plasma

injected through of the discharge field

mode

thruster

and power-

propulsion

over 40% and specific impulses ranging from 30 kW to 5 MW,

with test times

parameters

the quasi-steady thruster all of the high-performance

of applied

anode. Propellant, via the interaction field.

might

of performance

and the relationship between state thrusters. In addition, thrusters

in a steady-state

or quasi-steady

levels

the scaling

A typical

operating

in a pulsed,

performance

using

solenoidal

the plasma

coils

external

of self-field

and

to the

MPD

thrusters,little work has beendone to characterizethe impact of the plasma

properties,

substantially measurements governing

This

is particularly

in light

of recent

applied

field on

res_Ults showing

improved performance with applied-field MPD thrusters2-5. Plasma property can be used not only to study the scaling properties of the thrusters via the non-dimensional

parameters,

Quasi-steady

to make thrusters

internal property measurements, due to the extremely high heat property

testing

but they

models.

plasma

important

an externally

permits

measurements

insertion

are also

essential

of diagnostic

for verification

probes

of computer

into the thrust

chambers

but this is currently impossible with steady-state fluxes experienced in the chamber. For this reason

in steady-state

thrusters

are usually

confined

to the plume

region. This

paper

in the plume

of several

spectroscopy, and thruster techniques excited-state presented

presents

the results

applied-field

MPD

to measure

thrusters.

the

Three

global

plasma

diagnostics

were

characteristics

used:

emission

quantitative imaging, and electric probes. In Section II the experimental facility designs are briefly described, followed by a detailed discussion of the diagnostic and their implementations. distribution, and electron in Section

for the dominant

III.

physics

II. Vacuum

of an effort

Results of the species identification, temperature and density distribution

Finally,

a brief

discussion

is given

and the work

Experimental

Facility and Power The thruster test stand,

of the implications

are

of the measurements

summarized.

Apparatus

Supplies shown in Fig.

global ionic measurements

2,

was

and Procedures

mounted

in a 3 m diameter,

3 m long

spool piece attached to a 7.6 m diameter, 21 m long vacuum chamber via a 3 m diameter valve. The facility was pumped by 19 oil diffusion pumps backed by three roots blowers two

mechanical

indicated given

pumps.

pressure

The tank pressure

during

all tests.

Details

was

maintained

of the facility

below

0.07

Pa (5

and performance

gate and

x 10 -4 Tort)

diagnostics

are

in Ref. 4-6. i

The thrusters

were

powered

connected in a series-parallel thruster current and voltage mately ripple.

The discharge of these

diagnostics. Thruster

welding

supplies

network providing up to 3000 amps at 130 volts. ripples are shown in Fig. 3 for a thruster operating

Typical at approxi-

15 kW. It is clear that the thruster discharge current and voltage had substantial The peak,to-peak amplitude for the case shown was 30%, though the ripple magni-

tude and frequency v:alues

using a set of six 66 kW Miller

appeared

currents

to depend

and voltages

parameters.

on both thruster

reported

As discussed

geometry

in the following

below

this ripple

and operating

sections had

a large

correspond

conditions. to the mean

impac t on the plume



and Applied Field Magnet Designs A schematic of the MPD thrusters used

in this study

3

is shown

in Fig.

1.

The thrusters

consistedof watercooled, cylindrical copperanodesandcoaxial, 2% thoriatedtungsten cathodes.The chamberbackplatematerialwas boron nitride. Dimensionsfor the thrusters usedin this study are given in Table 1. The letter designationscorrespondto thoseusedin Ref. 4, where performanceresultsfor thesetestsaregiven. The cathodesfor all geometries exceptG andI had hemisphericaltips. The cathodefor geometryG was conical. The hollow cathodeusedin geometryI has a flat tip. Propellant,eitherargonor an argon-hydrogen mixture, was injectedthrough an annulusat the cathodebaseand through holesnearthe chambermidradius. Theseinjector holes werespaced15 degreesapartto ensurea relatively uniform azimuthalpropellantdistribution. The applied magneticfields weregeneratedusing a solenoidexternalto the anode. To accomodatethe various thrustersizesandmaximizethe potentialapplied field strengthwith eachthruster,two solenoids,15.3and20.3 cm in borediameter,were needed. These solenoidsgeneratedmagneticfields rangingfrom 0 to 0.2 Tesla at the centerlineof the magnetexit plane. All geometriesexceptC, E, andF were testedwith the 15.3cm I.D. magnet. Calculatedmagneticfield strengthsare shown as a function of axial distancefrom the exit planefor the 15.3cm I.D. magnetat a currentof 1400ampsin Fig. 4. These calculationshave beencomparedwith detailedmeasurements madeof the field strengthand showedgood agreement.The measurements showedthat the fields scalelinearly with magnet current,yielding field strengthsat the centerof the exit planeof 1.66 x 10 4 Tesla/amp and 8.48

x 10 5 Tesla/amp

study

the MPD

solenoid.

All magnetic

of the magnet Plume

for the

thrusters

Emission compared

field

mounted strengths,

and

Data

B z, reported

magnets,

respectively.

exit plane

below

refer

flush

For this

with

the end of the

to its value

at the centerline

Reduction

spectroscopy,

the thruster

quantitative

plume.

Results

to evaluate

the plume

emission

spectroscopy

The

cm diameter

with the anode

exit plane.

Diagnostics

characterize

15.3 and 20.3

were

plume from

the thruster

flexibility

to make

resolution

was

measurements

plume

probes

independent

were

used

measurements

were

made

at a variety 0.008

using

a

1.25

to

were

m Czerny-Turner

with a 2400 grooves/mm grating detector. The optical arrangement,

onto the spectrometer

measurements

approximately

and electric

of these

physics.

spectrometer. The spectrometer was equipped nm and an intensified 1024 diode linear array Fig. 5, imaged

imaging,

each

of axial

nm per pixel,

blazed at 500 shown in

entrance

slit and

provided

and radial

locations.

The

depending

slightly

the spectral

on the wavelength.

The

data reported here were taken 5 mm from the anode exit plane across the centerline of the thruster. Results were used to identify the plume species and obtain preliminary excitation temperatures These regions

using

single-point,

of interest

using a CID automatically

line-of-sight

integrated

line-of-sight

for plume

imaging.

camera with an image moved a stepper-motor

line intensity integrated Relative

results

ratios. were

distributions

used

to identify

of excited

states

spectral were

obtained

acquisition board and software. The control software driven filter wheel to preselected filters and acquired

a

presetnumberof imageswith eachfilter. For this work two filters wereused:onecentered at 488.0 nm with a 1 nm bandpass,and a secondcenteredat 514.7nm with a 0.8 nm bandpass.Both filters passonly light from argonion transitions. Eachpicture consistedof 493 columnsand461 rows of pixels with 8-bit intensity resolution. The line-of-sight intensityintegralsrecordedin theseimageswerereducedto radial emissioncoefficient profiles usingthe Abel inversiontechniquedevelopedby Sudharsanan 7. No intensity calibration wasperformedon the optical system,so that only relative valueswere obtained. The Abel-inversionsoftwareloadedan image,interactively found the thrusterexit plane,automaticallyidentified the plume region, and performedthe inversionon the maximum numberof columnspossiblewithin the constraintsof the plotting software. The plume region was identified in orderto maximizethe spatialresolutionwithin the luminousplume andpreventinversionof dark regions. The plotting packagelimited the numberof columns to 54, which typically correspondedto a spatialresolutionin the axial direction of 0.1cm. The Abel integralequationwas solved for eachcolumn of intensity profiles by a multistep process. First, the discreteFourier transform(DFT) of the intensityprofile wascomputed using a fast-Fouriertransform algorithm. No attempt was made to reduce signal noise. The axis

of symmetry

of the data

was determined

by minimizing

the imaginary

component

of the

DFF. This component was then set equal to zero to force symmetry for the inversion. The inverse Hankel transform was then applied to the shifted Fourier transformed input profile to yield the relative emission coefficient for the observed transition as a function of radius. Inversion of 54 columns required about 40 minutes on a 386-based personal computer. The inversion routine was verified with several known test functions 8. However, all of these test functions off-axis

had

on-axis

peaks.

algorithm

peaks,

while

No test functions

under

those

much were

of the experimental found

which

data

could

consisted

be used

of profiles

to verify

the validity

Fig.

2.

This

with After

water

probe-positioning probe

holding four linear tables,

cooled

moving

currents, mounted L-shaped, attached

positioning

copper

system,

sheet

to the preset

A single The

system

was

mounted

shown

in front

schematically

of the thrust

in Fig.

quantitative electric probes. stand

6, had

as shown

the capability

in

of

probes and providing 91 cm of radial motion and 30 cm of axial motion with both driven by computer-controlled stepper motors. The tables were covered

a constant velocity the traverse.

ments.

of the

conditions.

Interpretation of the spectroscopic and plume-imaging results required distributions of electron density and temperature. These were obtained using A fast-moving,

with

axial

of 30 cm/sec

0.7 cm long,

probe

to prevent

dimensions

postion,

them

the probes

with acceleration

0.013 were

cm diameter, selected

and permit the use of thin-sheath in a 0.1 cm diameter, 4 cm long, water-cooled, to the vertical

stainless-steel portion

from

tube

of the steel

over-heating were

during

moved

radially

and deceleration

electric

to minimize

probe

thruster through

phases

was

used

end-effects,

operation. the plume

near the ends

for these

reduce

peak

measuresaturation

theory for the data reduction 9,t°. The probe alumina tube which was glued into the end with ceramic tube

adhesive.

to minimize

5

Triangular

vibration

while

at

of

braces the probe

was of an were was in

motion. Water cooling was requiredonly alongthe vertical portion of the probe supportto preventmelting of the wire insulation. Tungstenwire andceramicinsulation wereusednear the probetip, ratherthan water cooling, in orderto minimize the probe cross-section. A test wasperformedto establishthe impactof surfacecontaminantson the probe response.The probeswerecleanedduring the pumpdown of the 3 m diameterspool piece using an 800V, 0.5 mA glow dischargebetweenthe probetips and an electrodeplaced approximately8 cm awayfrom the tips. The areaaroundthe probeswasflooded with argon in an effort to preventresidualair from damagingthe surface. During testing with the cleanedprobesno evidenceof signaldegradationwas observed,indicating probe-surface contaminationhadlittle effect on the measurements.The probe-biascircuit, describedbelow, was left on throughoutall tests,so thatsome probe cleaningwould take place between periodsof dataacquisition. The probe was biasedwith respectto facility ground(the vacuumtank wall) using a bipolar amplifier driven by a function generator. The function generatorprovideda continuous triangularwave at 135Hz, which the bipolar supply amplified to +/- 15 volts. The circuit is shown in Fig. 7. The triangle-wavefrequencywas chosento minimize the time over which the probe hadto sustainthe electronsaturationcurrent andto provide one completevoltage - currentcharacteristicfor the probeevery 1 mm of radial motion. Several testswereperformedto verify the frequencyresponseof the probe electronics,andno distortionswere measurablebelow 500Hz. The softwareusedto control probe motionpermittedautomaticscanningof the plume properties. The axial distancesfrom the thruster at which radial profiles wereto be taken were preselectedandthe entire data-acquisitionsequencewas performedautomatically.Probe current andvoltagedata werecollectedcontinuouslyat either76 or 150kHz during the radial traverseandstoredoncethe traversewascompleted. Either 50,000or 100,000datasetswere takeneachtraverse,correspondingto either500or 1000full voltage - currentcharacteristics for probe. The large numberof points per ramp wererequiredto properly curvefit the data during datareduction. The resultinglarge datasetsrequiredup to 3 minutesto store,and limited the numberof radial distributionsobtainedat a given operatingcondition. In addition,the very high heatfluxes experiencedbythe probesprecludedregularmeasurements doserthan 15cm from the thruster,though somepasseswere madeas closeas 6 cm. The electric-probedata were automaticallyreducedusing softwarewhich loadedin the continuousvoltageand currentsignals,isolatedthe individual Vt, - Ir characteristics,performed the requiredcurve fits, and storedthe results. The softwarepermittedselectionof the portion of the radial profile to be reducedand automaticallyplotted the raw dataandthe curve fits to the semi-log plots usedto calculatethe electrondensity,temperature,andthe plasma potential. Simple electric-probetheory9'1°was usedto reducethe data, andno attempt was madeto accountfor the effect of the applied:magneticfield. Neglectingthe effectsof the applied magneticfield wasjustified by consideringthe ratio of the electrongyro radius to the probe radius1°. For all datashownexceptthat collectedlessthan 10 cm from the thruster

_

exit plane

this ratio

was

be small.

In this simple

greater

than

analysis,

five, values

the electron

for which

temperatures

and densities

were

the intersection of the lines of the probe characteristic.

characteristics were needed to verify that appropriate used for the curve fits. This was especially important for regions

effects

should

obtained

from

Ie eA;_T_/2_m_

Ne =

The plasma potential was established by finding electron-saturation and electron-repelling regions

had to be valid

field

(I)

(Ip)) -I; Te = k( dlndUb

ranges

the magnetic

in the plume

curve-fit to the The plots of the

ranges of the V_, - Ip characteristic were because the criteria used to select these

with densities

differing

by almost

two

orders-

of-magnitude.

III. An MPD applied

field,

thruster

and

test was

using

Experimental

initiated

a set of countdown

burst of propellant into the chamber, seconds of the test were characterized the plume. initial

No attempt

operating

thruster

was

condition.

was allowed

made Once

to remain

Results

by setting timers

the propellant

flow

to turn on the main

rate,

power

and cycle the high voltage ignitor. The by particulate emission and substantial to characterize

the discharge

the start-up had stabilized,

at one operating

condition,

behavior data

defined

turning

on the

supply,

inject

first 1 - 3 fluctuations

as a function

collection

characteristics

diagnostics geometry, sufficient tics.

trends

geometry

For all other operation Plume

are discussed I in Table

geometries

with

began.

The

by the propellant

flow

severe

complexity

turning particulate

required

Operating of the

for the plume

study of plume characteristics as functions of current, and applied-field strength. However, trends and begin to identify the plasma characteris-

in the following 1, could

and time

rate,

sections.

be succesfully

off the applied-field

tested magnet

Note

that

with

no applied

resulted

only

the hollow-cathode magnetic

in very

field.

unstable

ejection.

Species Plume

11. Spectra band width singly-ionized

species

were

identified

were most considerable lines,

by comparing

measured

spectra

with tabulations

in Ref.

were collected at 333.6, 356.0, 362.6, 407.2, 420.0, 433.3, and 488.8 nm with a of 8.19 nm. Figure 8 shows a typical spectrum collected at 356 nm where several argon

lines

are identified.

dominant spectral lines identified only observed when those species

argon

The

precluded a highly systematic propellant, flow rate, discharge data were obtained to establish

These

thruster,

and performance.

in

of the

discharge current, and applied-field strength, for several minutes to insure stability. points were always repeated at least twice over long intervals to ensure reproducibilty terminal

a

evident effort though

Listed

were

2 are the plasma

species

and

from these spectra. The argon and hydrogen lines were were used as a propellant. The copper and tungsten lines

at either high discharge current was made to find doubly-ionized weak,

in Table

evident

or high magnetic-field strength. argon lines, none were found.

for all operating

conditions.

As shown

While Neutral

in Fig. 9, for a

test with hydrogenpropellant on geometryA, the peak_ line intensity increasedlinearly with applied-field strength.It was not possibleto obtain moredetailedcorrelationsof line intensitieswith thrusteroperatingconditionsdueto the limited numberof data sets. Density

and

Temperature

Preliminary obtained Using

Distributions

estimates

assuming

of the electron

that the excited-state

this assumption,

temperature

populations

the temperature

near

the thruster

followed

was obtained

a Boltzmann

estimates

of the temperature

2 vs. the difference the slope shown

of the linear

in Fig.

intensities

10.

plane

data

levels

used

lines

of thruster

A, and with

energy

an applied

applied-field

0.038,

obtained

energies,

fit to the data.

listed

in Table

C using

T showed

of the plume

data

T.

shown

Values

Ref.

integrated

were

propellant

obtained

to is

0.5 cm from current

of

probabilities,

adequacy

of the

in Eqn.

line

at a discharge

for transition

l 1. The

no dependence

showed

of the linear

curve

for the case shown, at applied-field

electron

temperature

on

centefline.

Outside

the

in intensity

cone

direction

and the cone

both

the 514.7

nm filter field

the actual

geometry

and The

radial

12a and b.

for the 514.7

radius

level.

increased

turned

dependence

These

of constant The slightly.

images,

off.

pronounced

structure

existed

luminosity

intensity

of similar

of the emission

followed

of the plume

A similar

coefficient

were

in the

was observed

disappeared

images

by a rapid

decreased

structure

and the structure

A series

It is apparent that a cone of with a minimum along the

Abel

and correlate

when

with

the

inverted

to

this with thruster

conditions.

of the applied-field

nm argon

a very

flow rate of 0.1 g/s. the cathode diameter

is a plateau

and unfiltered was

operating

effect

there

to the background

axial

applied-magnetic

that

Shown in Fig. 11 are raw intensity profiles of the 488 nm argon ion distances from thruster geometry E with a discharge current of 1250

A, an:applied field of 0.042 T, and an argon high luminosity propagates from just outside

in Fig.

of this process

distribution. The electron temperature slope, was 12,200 K. Results obtained

and 0.034

studies

with the applied field. transition at three axial

extract

example

term

is proportional

the line-of-sight

The

from

logarithmic

strength.

Photographic

decrease

A typical

of 0.051

obtained

the

the temperature

0.1 g/s argon

strength

were

by plotting

were 3.

distribution.

(2)

where

in this analysis

field

and degeneracies

of 0.051,

state

geometry

fit justified use of the Boltzmann obtained from the inverse of the strengths

were

excited

least-squares

The

for the spectral

the exit 1000

in upper

were

from

T. _ k(Si 1- Zj) ln(IiggjAjlvJ--i) ZjlgiA kv kJ Improved

exit plane

are contour ion line.

of the upper

The

excited

stength plots

on the plume

of the relative-emission

emission state

structure

coefficient

of the transition,

coefficient

is directly

density

ground thruster

state density of argon ions and the excitation/de-excitation rates would be to the left of the plots, with the exit plane at an axial

is dependent

C is shown

in the plume

proportional

number

8

and

for geometry

to the

on both the

in the plume. The position of zero, and

the cathodetip at the (0,0) location. The lines of constantemissioncoefficient do not intersectthe thrusterexit plane (the y - axis) dueto the limitations of the plotting package. The presenceof the high-luminositycone is obvious,andit appearsthat the plasmanearthe cathodetip doesnot contribute significantly to the plume luminosity. From the two figures it is Clearthat increasingthe applied-fieldstrengthfrom .025to 0.064Tesla approximately doubledboth the peak emissioncoefficient andthe axial extentof a given emissioncoefficient. Note that the inner radius of the cone,~ 0.7 cm, correspondsclosely with the cathode radius of 0.64 cm, andthat the the inner surfaceof the coneappearsto slowly diverge in the downstreamdirection. The influenceof the dischargecurrenton the 514.7nm emission coefficient is shown in Fig. 13aandb for currentsof 750 and 1500 amps. For both cases there weresignificant off-axis peaksin the emissioncoefficient, but at the higher currentsthe plasmain front of the cathodecontributedsignificantly to the luminosity, whereasit did not at the low currents. To checkwhetherthe correlationof the plume-luminositydistribution with the cathode radius was spurious,a test was performedwith a 1.27cm radiuscathodewith the sameanode dimensions(geometryE). Shownin Fig. 14 is a contourplot of 487.9 nm emissioncoefficient for this thrusterwith an appliedmagneticfield of 0.030Tesla. It is clearthat for this geometrythe luminous conehadan inner radius of - 1.5cm, showing that the conedid arise from the cathodesurface. A final checkon the influenceof cathodegeometryon the excited statedistribution in the plume wasperformedby shorteningthe cathode. The thrusters, geometriesB and G in Table 1, hadanodeand cathoderadii of 3.81 and0.64 cm, respectively, andcathodelengthsof 7.6 and2.5 cm. The cathodeusedin geometryG had a conical, non-hemispherical,tip. As can be seenfrom Fig. 15athe plume for the long cathodetest (geometryB) had a similar structureasthat for thrusterwith the large anoderadius (geometry C) shown in Fig. 12, indicating that anoderadiusdid not havea fundamentaleffect on the plume characteristics.However, a dramaticdifferenceis seenin Fig. 15b with the short, conical-cathodethruster. For the shortcathode,the plume intensitypeakedalongthe centerlineanddecreasedmononoticallyfor increasingradius,no longer showingthe highluminosity cone. The influenceof propellanton the plume-speciesdistribution was studiedby adding small quantitiesof hydrogen( 1 to 10%)to the argon. This increasedthe terminal voltage and subtantiallychangedthe plume characteristics.As shownin Figures 16aand b, adding hydrogennot only increasedthe intensityof the 488 nm emissionbut also changedits distribution, with the luminosity now substantiallymore concentratednearthe center. To eliminatethe possibility of the 486.1 nm H_line contributingto the intensity measuredwith a 488 nm filter, severalcheckswith purehydrogenwereperformed. In none of theseexperimentswas _ emissiondetectedthroughthe 488 nm filter. Electron-densityandtemperaturemeasurements weremadeusing singleelectric probes 'swept

through

dependence complicated

the plume

to identify

the causes

for the observed

plume

structure

and its

on the thruster geometry and operating conditions. These measurements were by the large ripple in the thruster current and voltage shown in Fig. 3, which

9

inducedcorrespondingfluctuationsin the electrondensityandtemperature.Raw probe current andvoltage signalstaken at a reducedprobe-biaspower supply frequencyof 13.5Hz (27 V-I characteristics/second) areshown in Fig. 17. Thesedata weretakenwith the hollowcathodethruster(geometryI) outsidethe mainplume to reducepower to the probesresulting from maintainingthe electron-saturationcurrentsfor theseextendedperiods (seediscussionin SectionII). The probewas biasedbetween+/- 17 volts. The probe current was nearzero in the ion-saturationregion and reacheda peakof - 0.013ampsin the electron-saturationregion. As the probe currentincreased,the large thrustercurrentand voltageripples beganto impact the signal, The effect peakedat ~ 30% ripple of the meanelectron-saturationcurrent. The magnitudeof the probe-currentripple was comparableto the 30%dischargecurrentripple observedduring the test (Fig. 3). The frequenciesweredifferent, however,with the power supply oscillating at ~ 450 Hz andthe electronsaturationcurrent at N 360 Hz. During data acquisitionthe rampfrequencyof the biasvoltage was 135Hz, or an order-of-magnitudehigher thanthat usedin the abovedescribedtest. While this prevented large fluctuationsfrom appearingin the electronsaturationcurrentof a given V - I characteristic, over a seriesof rampsthe valuesdid fluctuate by the same30% previously measured. Shown in Fig. 18 areraw, probe-voltageandcurrentsignalstakenat onelocation over 0.03 seconds.It is evidentthat the electron-saturationcurrentfluctuatedsignificantly over the courseof the measurements.Thesefluctuationswerethe principal sourceof scatterin the electrondensity andtemperaturemeasurements presentednext. Showh in Fig. 19aandb areradial electrondensityprofiles taken 15 and 35 cm away from the hollow-cathodethruster,geometryI, both with and without an applied-magnetic field. For both casesthe propellantflow rate was 0.15 g/s argon andthe dischargecurrent was 1000 AI The confining effect of the applied field is evident. The centerlinedensities with Bz -- 0.1 T were significantly higherthan thosefor Bz -- 0, and the radial densityprofile with the applied field had a fiat-toppedcentralregionfollowed by a very rapid decreasein density. Specifically, 15 cm from the thruster,the densityfor the casewithout an applied field dropsfrom a peakof ~ 2 x 1018 m 3 on center to 4 x 1017 m 3 at a radius of 30 cm, while with the applied

field,

cm, but decreased

the density

was

approximately

to 1 x 1017 111-3 by a radius

the observed radial number density gradients, gradient of ~ 9 x 1019 m -a, while without the magnitude ing axial

to 6.6 x 10 _8 m -4. In addition, distance

the thrusters cm away field. were thruster

the

was

centerline

the density

These

trends

lower densities

with the field continued

less dramatic. geometry

much

Shown F with

when in Fig.

a discharge

constant

the rate

at which field

comparable

was six times the applied

the density than

field

of 1000

was

profiles

A, an argon

case

increased, obtained flow

applied

field.

The

electron

densities

differed

10

by almost

an order

7

in terms

of a

with increas-

At 15 cm from

and without

than for the

strength

density

it.

the field, with

but 35

no applied

but the changes 35 cm from

rate of 0.1 g/s,

with applied fields of 0.030 and 0.12 Tesla. The centerline electron density significantly.for the two field strengths, though the radial profile was much higher

cases

decreased

without

both with

greater

20 are electron current

the two

it appears that the applied field maintained applied field this dropped by an order-of-

with the applied were

at 4 x 10 _8 in -3 for the inner

of 12 cm. Comparing

and

did not change sharper with the

of magnitude

at a

radiusof

20 cm, showing

The magnetic

effect

field

that the higher

of the discharge

is shown

field

current

in Fig.

21.

sustained

much

on the electron

These

data

were

steeper

density

taken

radial

profiles

density

gradients.

for zero

25 cm from

applied

the hollow-cathode

thruster, geometry I, at an argon mass flow rate of 0.15 g/s. The centerline density increased from ~ 1 x 10 t8 m 3 at a current of 1000 A to ~ 4 x 1018 m -3 at a current of 1500 A. However,

increasing

profile,

but rather The

density have

the discharge increased

high luminosity

profiles

shown

relatively

cone

above,

broad,

current

the density

did not dramatically

across

observed

though

fiat-topped

the entire

using

the imaging

the profiles

peaks.

change

for cases

While

off-axis

ty cone did appear in the radial profiles of the electron reflect changes in the electron temperature rather than

the the radial

density

profile. was with

not reflected an applied

peaks

in the electron

magnetic

corresponding

saturation the density.

currents, Shown

field

did

to the luminosithey appear to in Fig. 22 is the

radial temperature profile for geometry B taken 25 cm from the thruster with a discharge current of 1000 A, an applied field of 0.1 T, and an argon flow rate of 0.1 g/s. The relatively fiat-topped density profiles are in sharp contrast to the off-axis peaks seen in the temperature results. The temperature reached a minimum of -10,000 K on centerline, increased to a peak of ~ 17,000 higher

K at a radius

radii.

scopically.

Note The

as the electron

radii

is not known, energy

increase affect

single

density though

probe has

been

have

with respect

radii

The

cause

been

due

explain

radial

experiments

electron

reduction

temperature

performed

using

profiles,

indicating

K obtained

was has

field

spectro-

same

of the

of an

shown

an error

intersecting

The

at higher rates

a result

been

been

not to due

to the

the probe

increase

that the result

technique was

12 showed

not due

tip, no

in tempera-

at the farthest axial distances more than 50 times the probe

electric-probe

at

in signal-to-noise

diffusion

increase

the result.

a triple

monotonically

in temperature

radial

it may have

of applied-magnetic

would

12,200

as this parameter While

rose

due to a decrease

for the increase

ture was observed at the lowest applied field strengths thruster, cases for which the electron gyro radius was In addition,

with the was

that the temperature

measurements.

which

K and then

to the higher

to the flow,

B z components found

are consistent

decreased. it may

temperature

to N 12,000

at the larger

It is unlikely

angle

of the B i. and

mechanism

magnitudes

in scatter

electrons.

in probe

presence

that these

increase

ratio higher

of 5 cm, decreased

from the radius. the similar

to the data

technique.

The effect of the applied field on the electron temperature for the hollow cathode thruster is illustrated in Fig. 23, which compares the results for B z --- 0 and Bz -- 0.1 T at an axial position 15 cm away from the thruster. about 7500 K are unaffected by the applied rises

rapidly

temperature

for the high distribution

applied

field

for the case

and

It is clear that the centerline temperatures of field while the temperature at increasing radii is flat for the case

with the applied

field

of zero

is shown

applied-field. in Fig. 24.

The Much

axial as was

found with the electron density, the centedine temperature did not change significantly 15 to 35 cm away from the thruster, though the radial distributions showed a substantial

from drop

in temperature

I with

no applied-field

at higher showed

radii.

The

no significant

axial temperature changes,

and

11

distribution increasing

for thruster the discharge

geometry current

from

1000 ture.

to 2000

A without

It was dramatic

shown

effect

states).

The

are shown

earlier

corresponding

without The

and

b.

the off-axis

to the

ground,

with long

of the plasma

not the thruster

cathodes

from

facility

close

to ground

ground.

Only

potential,

conditions

As discussed

However,

with the short-cathode a value

on the

above,

slight 26b

minima shows

near the centerline.

the potential

For the operating uniform

with

conditions

a magnitude

respect

with a minimum discharge

did not have

the radial

distribution

geometry

I for discharge

discharge scatter,

current and

for increasing potential The

in scatter

and associated The

distribution played

anode

the

Shown

from

the thruster

them

are clear, having

field observed

geometry role

and

which

at higher

of these

a global

distance

clearly

cathode

having

magnetic

fields field

was

not

was

currents

some

from

thruster

field.

Increasing

the

the data

contrasts rise

the

in Fig. 27 is

to the results

in the plasma

on the thruster

was

Tesla.

structure,

Increasing

decreased

a rapid

Figure

and 0.12

Shown

of 20 cm

This

a

approximately

there

to appear

fields.

of 0.03

no applied radii.

centerline.

due to the increase

in

ratio. no measurable

measurements.

both

higher

caused

between a local

on the centerline.

19.

effect

on the plasma-potential

However,

the magnitude

the cathode

and behavior

the potential

B and G, with B z -- 0.09

minima

0 and -1 V

on the plasma potential thruster shown in Fig.

potential,

at large

minimum

had

in Fig. 28 is a comparison

with the long

to

G) was the cathode

the field.

A with

discharge

of the thruster

for geometries

was

for thrusters

between

of 3 to 5 volts.

the centerline

measured

a potential

in determining

increase

2000

in signal-to-noise

distances

probe

to ground.

strengths,

as increasing

increased

strength,

and caused

fields

at an axial

of 1000

potentials

increase

at the axial

a significant

potential.

cathode

radii,

potential

A clearly

the applied

at large

decrease

density

to 2000

effect

with

F for applied at the lower

an overall

same

currents

did not affect

continued

At the higher

and

the

of plasma

trend

the potential

of ~ 5 V.

on the centerline,

current

is also

conditions was

(geometry

V with

plume

are with respect

for all operating

thruster

of-15.5

for geometry

shown,

on the

near zero throughout the plume, whereas when a the potential increased by 3 to 5 volts and showed

This

distributions

density

is peaked

the electric

potentials

Figure 26 shows the influence of the applied magnetic field distribution. For the zero-field case studied with the hollow-cathode 26a, the plasma potential was flat and magnetic field of 0.041 T was applied,

a

measurements

cathode

A, B, C, D, E, F, and I), the cathode

reaching

had

excited

on the electron

short

so that all reported

electrodes.

(geometries

effect

with the

length

of upper

and temperature

had little

and operating

wall,

tempera-

by the long cathode.

potential.

facility

electron

distribution

density

distribution

geometry

on the

12 - 16) that the cathode

length

exhibited

effect

(density

of electron cathode

peaks

vacuum

(Figures

distribution

The

of thruster

in the behavior

no measurable

the images

luminosity comparisons

25a

influence

relative

facility

had

at 25 cm, but the temperature

centefline

biased

field

using

on the plume

in Figures

distribution

evident

an applied

Tesla.

maxima Comparing

distributions The

geometry

of the plasma

differences

on the centerline

25 cm away between and the short

Fig. 28 with Fig. 25b

it is

apparentthat the minima in both distributionscorrespondwith the highesttemperaturein the high-densityplume. The axial potentialdistributionfor the short cathodegeometryis shown in Fig. 29, where it is seenthat both the depth of the potential well andthe rate of potential increaseoff axis decreasedawayfrom the thruster.

Discussion These

measurements

establishing

the plume

evidenced

by the great

direction

and the steep

dearly

show

properties.

The

reduction radial

that the applied

magnetic

field

in the rate at which

density

gradients

field

plays

strongly

confines

the plasma

observed

a dominant

as

decreased

in the

applied

field.

K.

The

centerline

properties

were

relatively

insensitive

to operating

axial

In general,

for the operating conditions and thruster geometries used in this study, the centerline density was between 3 x 1018 and 8 x 10 _8 m -3 and the temperature between 10,000 20,000

in

the plume,

density

with the

role

electron and

condition

so long

as the applied field was on, though the imaging clearly showed an increase in excited ion state population with increasing applied field and discharge current. Thruster anode geometry did not have Large

as large

changes

luminosity slowly only

cone

diverge slightly

an impact

in plume observed

the basis

pressure

field

the electron

electron electron

and

plasma

however,

on the plume

;

Hall

magnetic

was

length

and

not present.

shape.

The

to come

off the cathode

potential

distribution

with the

applied-field

plasma

high-

surface

was there

parameter,

electron

These

;

_NkT

_=

eBz

ion temperatures

can be qualitatively

pressure.

mev

re -

mere_ i

where

the field

appeared

The

the electron to the

eBz

-

field field;

as did the cathode

when

and

flat and was

a

in the potential.

parameters:

thermal

applied direction.

of the applied

of three

properties

observed

the applied

minimum

effect

the plasma

with the

without

central

The

were

in the downstream positive

pronounced

on the plume

structure

gyro

were

examined

radius,

calculated

on

and ratio

of

from (3)

B_/2_o

and densities

were

assumed

equal,

and the mean

velocity was set equal to the thermal speed based on the electron temperature. The - ion collision frequency was calculated from Spitzer's standard formulation 13. At

this point we have neglected the potential effects of plasma microturbulence, which would increase the effective collision rate and decrease the calculated Hall parameters. Values were calculated plume. was

for the range The

strongly

around

results, confined

the field

lines

of magnetic

presented by the before

field

strengths

in parametric magnetic undergoing

field.

form

and electron in Table

densities

4, clearly

Not only do the electrons

a collision,

but the gyro

radii

measured

show

in the

that the plasma

gyrate are much

many

times

smaller

than

the observed density gradient length scales. In addition, the fact that the magnetic pressure was much greater than the plasma thermal pressure supports the observation of strong radial density

gradients.

13

These field

lines

estimates

how the plume this calculation density

imply

and do not move

profiles

show

that the centerline

lines

Fig.

the vectors

30, where

were

strongly

cm, the field

lines

does

in the

the plasma

between

axial

radii

is separating

several

authors

It appears

from

did not decrease

by an axial

It is apparent

of 5 and

direction, from

the

parameters

have

studied

This

axially,

distance

applied

along

result

lack

between

the

explains

magnetic

field

discussion

radial. The

evidence

that as a result

corresponding

While

the plume

density

apparent

indicates dichotomy

unresolved, separation

field

of applying

in

of 25 and 35

remains

of plasma/magnetic

the

is shown

distances

in centerline lines.

and the experimental

though

This

magnitude

axial

of a drop

even

of 25 cm.

with the vector that

the phenomena

the preceding

predominantly

10 cm are predominantly

the apparent

Hall

moved

properties.

structure observed using the imaging. While of the lumosity cones, note that the electron

density

diverging

magnitude.

between

the calculated

though

in the plume

with different

B -- B,r + Bzz are plotted,

to the the magnetic-field

that

regions

can support the sharply defined appears to explain the presence

magnetic-field

diverge

that the electrons through

the

14'16.

magnetic-

field, electrons coming off the cathode surface will be partially confined to the region near the cathode. The corresponding reduction in radial electron flux would require an increase the radial electric field to maintain current conduction at a constant level. Thus, not only would

the plasma

density

by the fields to the electron temperature to the plume during

data,

thruster

near

the cathode

surface

increase

slightly,

but the energy

in

imparted

particles would increase. The latter phenomena may be the source of the structure observed with the long-cathode thrusters (Fig. 25b). In addition this argument

operation

is supported

with several

by observations

applied-field

made

strengths.

of the

It was

cathode

found

that

surface

the surface

temperature of the cathode increased monotonically across the entire surface with appliedfield strength. For a constant discharge current, this observation can only be accounted for by an increase

in the ion-impingement

performance Performance

of applied-field measurements

increasing

applied-field,

will certainly

magnetic

fundamental thrusters

The heating.

It was

centefline

preclude

potential

lines,

thrusters

current

isolation

Fig. 29, that

increases

the

rate and

to the

increasing

the magnitude

applied

the centedine

plume

field

of the radial

region

high

strength

this proposed

the applied

temperature

life.

While

currents

the

in self-field

the phenomena. mechanism

depressed

potential

for cathode

the downstream gradient.

These

was strongly affected behave somewhat like

is well connected,

it similar

required

in this study,

controlling

the perpendicular electrical conductivity speaking, the axial applied-field lines

so that

surface

thruster

magnetic-field

also support

that the life and

be fundamentally coupled. and specific impulse with

at the very

to those

of the physics

shows

cathode

may limit

operated

magnitude

measurements

and increased

measurements imply that magnetic field. Generally equipotential

MPD

of comparable

of the discharge

found,

potential

sublimation

in self-field

most likely

plasma

material

fields

coupling will

This result

thrusters with similar geometries may show a monotonic increase in efficiency

the

may occur

to achieve

to the cathode.

yet if this applied-field

increase

phenomena

current

electrically,

by the

to the

cathode tip, while at larger radii magnetic field lines that arise from the anode surface are well connected to the anode. The observation that the potential gradients decreased in the axial

direction

indicates

the presence

of radial

currents

14

in those

regions.

Conclusions Measurements distributions

show

partially ionized, 8 x 10 _s m 3 and structure

were

confine ment,

of plume that

very

density

of the electron field

flux tubes

applied

magnetic

of the

MPD

and temperature

thrusters

Hall

did

in the downstream

increased

indicate

studied

which here,

implying

to strongly of confine-

with applied-field increase.

the plasma

show that

were

was found the degree

monotonically

that

here

ranging from 2 x 10 _s to magnitudes and the plume

a commensurate

distributions region,

field,

studied

not show

parameter

the density

applied

conditions

gradients,

densities

lines,

While

is strongly

the plasma

that the plume

coupled

does

does

to the

not follow

separate

from

the

field.

Measurements the applied

to the presence

the centerline

magnetic

magnetic

electron-density,

of the applied-field

For the operating

by radial

though

applied

sensitive plasma.

as indicated

calculations

the plumes

excited-state,

with centerline electron densities and temperatures from 7,500 to 20,000 K, respectively. Both these

the plume

strength,

species,

field,

of the plasma-potential

which

lar to the magnetic

appeared

field.

This

distributions

to subtantially phenomenon

confirmed

reduce

the strong

the electrical

not only resulted

gradients, but also increased the cathode surface temperature. fundamentally couple the performance and Life of applied-field

influence

conductivity

in incroased

of

perpendicu-

radial

potential

The latter phenomenon may "tluxisters of similar geometry.

Acknowledgemems The David

authors

Wotford,

wish to thank John

John

for their inwduable of his magnetic

McAlea, support

field

Naglowsky,

Rob Buffer,

John

on this project.

calculation

Larry Miller,

Thanks

Schultz, Gerry

John

Eckert,

Schneider

and

also go to Dr. Michael

Tom Cliff

Ralys,

Schroeder

LaPointe

for use

code.

References 1.

2.

Sovey, J. and Mantenieks, M., "Performance Thruster Technology," Journal of Propulsion 71-83. Tahara, MPD

3.

H., Yasui, Arcjet

Kagaya,

H., Kagaya,

Thruster

for Near-Earth

Y., Yoshikawa,

Magnetic

Fields,"

Myers, RAM., "Applied June 1991.

5.

Mantenieks, "Performance

Yoshikawa,

Missions."

T., and Tahara.

AIAA

4.

M.A.,

Y., and

Paper Field

Sovey,

of a 100 kW

J.S., Class

Myers,

AIAA

Oct.

Thruster

Paper

Applied

of a Quasi-Steady

87-1001, MPD

Arc 1991,

Arcjets

May

1987.

with Applied

1985.

Geometry

R.M..

of MPD 1, Jan.-Feb.

T., "Development

H.. "Quasi-Steady

85-2001.

MPD

and Lifetime Assessment and Power, Vol. 7, No.

Haag,

Field

15

MPD

Effects," T.W.,

AIAA

Raitano,

Thruster,"

Paper

91-2342,

P., and Parkes, AIAA

Paper

J.E.,

89-2710,

pp.

6. 7. 8. 9.

10.

July 1989,seealsoNASA TM 102312,July, 1989. Haag,T., "Designof a Thrust Standfor High PowerElectric PropulsionDevices," AIAA Paper89-2829,July 1989,seealsoNASA TM 102372,July, 1989. Sudharsanan,S.I., "The Abel Inversion of Noisy Data Using DiscreteIntegral Transforms," M.S. Thesis,The University of Tennessee,Knoxville, August 1986. Cremers,C. andBirkebak, R.C., "Application of the Abel Integral Equation to SpectrographicData," Applied Optics,Vol. 5, No. 6, June 1966,pp. 1057-1064. Chung,P. M., Talbot, L., andTouryan,K.J. "Electric Probesin StationaryandFlowing Plasmas: Part 1. CollisionlessandTransitionalProbes,"and, "Part2. Continuum Probes,"AIAA Journal, Vol. 12, No. 2, Feb. 1974, pp. 133 - 154. Swift,

J.D.,

Elsevier 11.

12.

Wiese,

and Schwar,

Publishing W.L,

M.J.,

Co., Inc.,

Smith,

M.W.

"Electric

Probes B.M.,

"Atomic

Stand.

Myers,

of MPD

"Plume

Diagnostics,"

American

1970.

and Miles,

Sodium Through Calcium," National Standards, Vol. 22, Oct. 1969. R.M.,

for Plasma

Characteristics

Transition

Ref. Data

Series,

Thrusters:

Probabilities, National

Vol.

Bureau

A Preliminary

of

Examination,"

AIAA Paper 89-2832, July 1989, see also NASA CR 185130, Sept. 1989. 13. Spitzer, L., "Physics of Fully Ionized Plasmas," Interscience Publishers, Inc., 1956. 14.

Kosmahl,

H.G.,

"Three-Dimensional

Diverging Magnetic Jan. 1967. 15.

Walker, including

16.

Hooper, 1991.

E. and Thermal

Fields

Seikel,

G.,

based

on Dipole

"Axisymmetric

Conduction,"

E. B., "Plasma

Plasma

NASA

Detachment

Acceleration Moment Expansion

TN D-6154,

from

a Magnetic

16

through

New

York,

Axisymmetric

Approximation,"

NASA

of a Plasma

in a Magnetic

Feb.

II,

TN D-3782, Nozzle

1971.

Field,"

AIAA

Paper

91-2590,

June

Anode Radius

Geometry

Anode

Cathode

Length

Radius

Cathode

Length

La, cm

Re, , cm

Lc, cm

2.5

7.6

0.64

7.6

3.81

7.6

0.64

7.6

C

5.1

7.6

0.64

7.6

E

5.1

7.6

1.27

7.6

5.1

15.2

1.27

7.6

G

3.81

7.6

0.64

2.5 (conical)

I

3.81

7.6

Ra,

A

F

cm

3.00D,

1.0 ID

6.1

(Hollow) Table

Plasma

1: Dimensions

of MPD

Species

thrusters

Identified

used

in this study

lines,

nm

ArI

404.6,

419.8,

433.4,

436.4,

696.5

ArII

355.9,

358.8,

407.2,

433.2,

487.9

HI

486.1

WI

368.2,

Cull

Table

368.4,

491.8,

2:

Plasma

species

and the most

17

368.8

490.7

prominent

lines

identified

Wavelength(nm)

Eu (eV)

Gu

Aik x 108(secq)

410.3

22.76

4

1.3

408.2

19.73

6

0.027

407.2

21.56

6

0.57

405.3

23.86

4

1.50

404_.3

21.55

4

1.40

Table 3. Spectrallines andconstantsusedto determineelectrontemperature.

Bz,Tesla

N_ +

Electron

N i (m -3)

Hall

Parameter

Table

4.

Electron

gyro

radius

(cm)

1.76

x 10 -2

Thermal

P

Magnetic

.02

5 X 1017

176

.02

1 x 1018

88

"

.02

5 x 1018

18

"

.02

1 X 1019

9

"

8.7 x 10 .3

.05

5 X 1017

439

7.1 X 10 -3

7 x 10 .5

.05

1 X 1018

220

"

1.4 X 10 "4

.O5

5 X 1018

44

.05

1 X 1019

22

"

1.4 x 10 .3

.1

5 X 1017

878

3.5 X 10 -3

1.7 x 10 -5

.1

1 x 1018

439

"

3.5 x 10 .5

.1

5 x 1018

88

"

1.7 X 10 -4

.1

1 X 1019

44

"

3.5 X 10 "4

Electron

Hall

T e --- T i -- 10 4 K and

Parameter,

gyro

densities/magnetic

radius

observed

18

of thermal

X 10 "4

8.7 x 10 .4 4.33

7

and ratio

fields

4.33

P

to magnetic

in this work.

X 10 -4

x

10 -4

pressures

for

propellant input: ' ner, outer cathode clamp (watercooled'

water coooling passages Radius, cm

outer injection holes |nnulus ta

Axial distance, cm

boron nitride backplate

Fiberglas epoxy insulator

Figure 1. MPD thruster measurements. Applied given

in Table

schematic showing coordinate system used for plume field magnet not shown. Elecn'ode dimensions are

1.

3 reTEST

SECTION

3 m GATE

REFERENCE VACUUM

VALVE

STRUCTUR

FEED CALIBRATION

I_ECHANISM_

DISPLACEMEN3

PRIMARY

///////////////////////// FLOOR

t_

LINE

J

Figure 2. Schematic of MPD probe positioning system.

thruster 19

test facility

showing

thrust

stand

and

Discharge

).,,i



°

_>

_ •

Voltage,

_,

I



I

V



I

_

I



I

_., _,, ,

I



_

I_.

I



P
.

-d

10 _fi

O t_

magnetic plied

, I ,

E

' lrr"'P

1,

: _d

ta_

t_

5

01

0.03 T

-20

'

:' '

'

I

!

-10

0 Radius,

b. Geometry

'I"

| ''

I

l0 cm

F, 0.1 g/s argon

flow rate.

Figure 26. Plasma potential profiles for thruster geometries discharge current of 1000 A and two applied field strengths 33

20

I and F for a each.

d

0

[..,

ca

0 e_

o

cD ¢.,q

*-' o:1

E o

0 c'q

U'3

u_

eq_ t_

_d II

II = ¢)

II

o i

N i

o

¢q

A 'i-e.tlu01od etUSeld A 'le.tluolod

etUSeld

0

N

II

20 Axial distance from thruster:

15 6 cm

>

--d "1::: =o

10 25 cm

E

-15

-10

-5

0 Radius,

5

10

15

cm

Figure 29. Plasma potential distributions 6 and 25 cm from Jd = 750 A, Bz = 0.13 T, 0.1 g/s argon flow rate.

thruster

geometry

G.

0.25-

Ill,,,

_E "

.............. Ii,

................

IIs

................

II,

...............

t,,

................

0.15-

o

/

o < E_ O.lO< p,,

Jt

II/ II/ // // '11 /I/

o.oo 0.05

...............

I*,

...............

/

#/,,

............... ...............

//IJ,,

I/Ill

0.00

le, /

1111,,

/

0.05--

................

//4S

................

//#l,,

................ ................... ....................

o._o O.'lS o._o

' 0.25

o.;o

' 0.35

' 0.40

' 0.45

o._o

AXIAL POSITION (M)

Figure 30. Magnetic field vectors for 15.3 cm I.D. magnet with a coil current of 1400 A. Vector lengths are proportional to field stregths. Origin is at centerline of magnet exit plane. 35

IJ tSA

Report

NatlonalAeronauticsand SpaceAdministration 1. Report No.

NASA CR-187165 AIAA - 91- 2339

Documentation

Page

2. Government Accession No.

3. Recipient's Catalog No.

4. Title and Subtitle

5. Report Date

A Preliminary MPD Thruster

Characterization Plumes

of Applied-Field

August

1991

6. Performing Organization Cede

7: Author(s)

8. Performing Organization Report No.

Roger M. Myers, David Wehrle, Mark Vernyi, James Biaglow, and Shawn Reese

None

(E- 6426)

10. Work Unit No.

506-42-31 9. Performing Organization

Name and Address

Sverdrup Technology, Inc. Lewis Research Center Group 2001 Aerospace Parkway Brook Park, Ohio 44142

11. Contract or Grant No.

NAS3-

13. Type of Report and Period Covered

Contractor Report Final

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration Lewis Research Center Cleveland, 15. Supplementary

Ohio

44135-

25266

14. Sponsoring Agency Code

3191

Notes

Project Manager, James Sovey, Space Propulsion Technology Division, NASA Lewis Research Center. Prepared for the 27th Joint Propulsion Conference cosponsored by AIAA, SAE, ASME, and ASEE, Sacramento, California, June 24-27, 1991. Roger M. Myers, Sverdrup Technology, Inc.; David Wehrle, Cleveland State University, Cleveland, Ohio 44115; Mark Vernyi, University of Akron, Akron, Ohio 44325; James Biaglow, University of Cincinnati, Cincinnati, Ohio 45221; Shawn Reese, Ohio University, Athens, Ohio 45701. Responsible person, Roger M. Myers, (216) 433 - 8548.

16. Abstract

Electric probes, quantitative imaging, and emission spectroscopy were used to study the plume characteristics of applied-field MPD thrusters. The measurements showed that the applied magnetic field plays the dominant role in establishing the plume structure, followed in importance by the cathode geometry and propellant. The anode radius had no measurable impact on the plume characteristics. For all cases studied the plume was highly ionized, though spectral lines of neutral species were always present. Centerline electron densities and temperatures ranged from 2x 1018 to 8 x 1018 m -3 and from 7500 to 20,000 K, respectively. The plume was strongly confined by the magnetic field, with radial density gradients increasing monotonically with applied field strength. Plasma potential measurements show a strong effect of the magnetic field on the electrical conductivity and indicate the presence of radial current conduction in the plume.

17. Key Words (Suggested by Author(s))

18. Distribution Statement

Electric propulsion Magnetoplasmadynamics

Unclassified Subject

19. Security Classif. (of the report)

Unclassified NASA FORM1626OCT86

20. Security Classif. (of this page)

Unclassified

- Unlimited

Category

20

21. No. of pages

36

*For sale bythe NationalTechnicalInformation Service,Springfield,Virginia 22161

22. Price*

A03