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
ltl/ A National Aeronautics and Space Administration ( _'_! A q A- (. :t- ] !_? 1 £ _ ) tt C !,_it{:t AC 'IF _kt L A I .,r. 13_'! 0 7.: A P i:>t. [ ic D- F 17E I_{) i_4P i) T __7t!.J:JT C!7 Ik P iot I i_77! :,i F i ti{_l i_:_Ti;_C._l" _ ( Sver Jrup l:) C_
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
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22. Price*
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