Author(s)
Evert, Richard William
Title
Arcjet plume ionization effects on exposed solar array conducting surfaces
Publisher
Monterey, California. Naval Postgraduate School
Issue Date
1991-09
URL
http://hdl.handle.net/10945/43761
This document was downloaded on May 04, 2015 at 23:00:20
jQQ'a
NAVAL POSTGRADUATE SCHOOL
Monterey, California
THESIS
ARCJET PLUME IONIZATION EFFECTS ON
EXPOSED SOLAR ARRAY CONDUCTING
SURFACES
by
Richard W. Evert
September, 1991
Thesis Advisor:
Approved
Dr. Richard C. Olsen
for public release; distribution
is
unlimited.
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PROJECT
TASK
WORK
NO.
NO.
ACCESSION NO.
UNIT
ARCJET PLUME IONIZATION EFFECTS ON EXPOSED SOLAR ARRAY CONDUCTING
SURFACES
12.
personal author(S) Evert, Richard
13a.
TYPE OF REPORT
13b.
W.
TIME COVERED
FROM
Master's Thesis
14.
TO
DATE OF REPORT
(Yr.,
Mo.. Day)
15.
PAGE COUNT
September 1991
72
SUPPLEMENTARY NOTATION
16.
The views expressed in
Department of Defense
this thesis are
or the U.S.
those of the author and do not
COSATI CODES
17.
18.
GROUP
FIELD
reflect
the
official
policy or position of the
Government.
SUBJECT TERMS
(Continue on reverse
if
necessary and
identify
by block number)
SUB-GROUP
Arcjet, Electrothermal propulsion.
19.
ABSTRACT (Continue on reverse necessary and identify by block number)
High efficiency arcjet propulsion will be used in the near future for satellite orbit adjustment and eventually for orbit transfer. Testing Is
currently being conducted to explore spacecraft Interface difficulties with this method of propulsion. This thesis looks at one aspect of this
Interface. Since most earth orbiting spacecraft use solar arrays for power generation, it Is of Interest to Investigate how exposed, biased
conducting surfaces will Interact with the slightly ionized plume environment of the arcjet thruster. It was found that with the arcjet thruster
if
25.4 cm above the solar array, firing along its axis at a 20 degree cant angle, electrical currents were indeed collected. The effect of
having a constricted area exposed to the plume was to Increase the current density. The electron densities at typical solar array distances
10
12
3
were found to be 10 -10 /m
An estimate of the total power lost for an array In this configuration showed that 0.05% of the overall power
mounted
.
Is
lost
due
to collected currents.
DISTRIBUTION/AVAILABILITY OF ABSTRACT
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INDIVIDUAL
RPT.
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Arcjct
Plume
Ionization Effects on
is
unlimited.
Exposed
Solar Array Conducting Surfaces
by
Richard William Evert
Lieutenant, United States
B.S., University of
Submitted
Navy
Southern California, 1984
in partial fulfillment
of the requirements for the degree of
MASTER OF SCIENCE
IN PHYSICS
from the
NAVAL POSTGRADUATE SCHOOL
September 1991
Karlheinz E. Woehler, Chairman, Department of Physics
ABSTRACT
High-efficiency arcjet propulsion will be used in the near future for satellite
orbit
adjustment and eventually for orbit transfer.
conducted to explore spacecraft interface
This thesis looks
at
one aspect of
difficulties with this
is
it
is
currently being
method of propulsion.
Since most earth orbiting spacecraft
this interface.
use solar arrays for power generation,
Testing
of interest to investigate
how
exposed,
biased conducting surfaces will interact with the slightly ionized plume environment
of the arcjet thruster.
above the solar
It
was found that with the
array, firing along
currents were indeed collected.
the
plume was
The
an array
axis at a
in this configuration
in
mounted
The
-10
showed
12
/m
that
to collected currents.
in
3
.
cm
25.4
20 degree cant angle, electrical
effect of having a constricted area
to increase the current density.
array distances were found to be 10
for
its
arcjet thruster
exposed
to
electron densities at typical solar
An
estimate of the total power lost
0.05% of the
overall
power
is
lost
due
brp
TABLE OF CONTENTS
INTRODUCTION
I.
II.
III.
A.
ARCJET THRUSTER USE
B.
SPACECRAFT INTEGRATION
C.
THESIS ORGANIZATION
1
,
1
2
ELECTROTHERMAL PROPULSION PRINCIPLES
3
A.
RESISTOJET THRUSTERS
3
B.
ARCJET THRUSTER
6
PLASMA PRINCIPLES
9
A.
PROBE THEORY
B.
PREVIOUS ARCJET PLUME INVESTIGATIONS
9
Plume Ionization
1.
Arcjet
2.
Plume Ionization
Investigations by
Langmuir Probe ...
Investigations by Spectral Emission
14
14
18
SOME EXPECTATIONS
23
EXPERIMENT DESCRIPTION
24
C.
IV.
1
A.
EXPERIMENT OVERVIEW
IV
24
B.
HARDWARE
Vacuum Chamber
25
2.
Arcjet
25
3.
Solar Cell Emulators and Reference Plates
28
4.
Measuring Equipment
28
C.
LAYOUT OF EXPERIMENT
28
D.
PROCEDURE
33
1.
Chronology
36
2.
Aspects of the
1.
V.
VI.
VII.
24
Low
Pressure Firing
Run
RESULTS
37
38
A.
CONDUCTING PLATE DATA
38
B.
HIGH PRESSURE DATA
43
ANALYSIS AND DISCUSSION
46
A.
MEAN FREE PATH
B.
COLLECTED CURRENTS DATA
47
C.
CURRENT DENSITIES
52
D.
ANALYSIS OF CURRENT COLLECTED BY SOLAR ARRAY
57
46
-
CONCLUSIONS
A.
SOLAR ARRAY PERFORMANCE
59
59
B.
lis r oi
I
URTIILR RISLARC
rli
59
II
lrhnchs
60
INITIAL DISTRIBUTION LIST
62
VI
ACKNOWLEDGEMENTS
I
would
like to
thank
all
the people at
A
instrumental to this experiment.
this
special thank
experimental opportunity possible
Jaime Valles whose
insight
effort
and time on
and experience were key
to
Rocket Research Company helped
arcjet
and the
specifics of
for his guidance
its
to
whose help and guidance was
you to Sidney Zafran who made
in the first place.
this
Thank you
also goes to
experiment were greatly appreciated. His
overcoming several hurdles. Charlie Vaughn of
in
many ways with background information on
operation.
and time given
TRW
Very special thanks go
producing
vn
this thesis.
to Professor
the
Olsen
INTRODUCTION
I.
A.
ARCJET THRUSTER USE
Propulsion technology has advanced so that
many electrothermal propulsion
systems have become attractive operational alternatives to the usual auxiliary
The
chemical thruster system.
efficiency of these electrothermal systems
higher than their chemical counterparts allowing for a larger payload.
electrothermal devices
is
the arcjet thruster.
The
One
is
much
of these
principle of operation for the arcjet
involves the heating of a propellant within a high temperature arc discharge and
subsequent expansion through a nozzle to produce
testing are
up
kW power and
to 2
Newton
thrust.
Systems
in
kW
10
to
Current systems under
have 450-600 second specific impulse and 0.13-0.44
This type of thruster
the 3
thrust.
its
kW
is
excellent for station keeping purposes.
power range
will
be useful for orbit transfer.
Investigation of the effects arcjet thrusters have on other spacecraft systems has been
ongoing and intensifying as deployment on spacecraft nears.
B.
[Ref.
1]
SPACECRAFT INTEGRATION
For thrusters that
integration
question
will
is.
be used for station keeping purposes the main system
how
communications and other
will
satellite
the
arc
systems?
electromagnetic
interference
affect
But as systems of higher power are
considered other potential spacecraft integration problems become of great concern
Effects that the arcjet thruster
also.
surfaces
is
of particular interest.
ionized gas that
may have on exposed
The exhaust plume
may generate conduction
solar array conducting
of the arcjet
is
a partially
paths to ground for the solar arrays thus
reducing their power generation efficiency.
Arcjets used for orbit transfer would
require significant power, operating for long periods of up to several months.
implies that
it
is
important to take a close look
at arcjet
plume
effects
This
on solar
arrays.
The opportunity
1.4
kW
arcjet
to
do so arose during a spacecraft integrated
test firing of
on an operational qualification model communications
vacuum environment
(1.2 x
10" 3
Pa)
at
TRW
in
a
satellite in a
Redondo Beach, CA.
Exposed
conducting surfaces (with areas typical of solar array connectors and junctions) were
placed in various positions on the solar panel. These surfaces were biased to varying
voltages,
and the current measured
conduction.
to
determine losses due to ionized plume
Additional conducting surfaces having larger areas were also used in
order to estimate the plasma character at these positions.
C.
THESIS ORGANIZATION
This thesis
will describe the arcjet thruster
operating principles and discuss the
interaction principles governing the processes being investigated.
This will be
followed by a description of the experiment and the results obtained.
conclusions and follow on areas of investigation will be presented.
Finally
II.
ELECTROTHERMAL PROPULSION PRINCIPLES
Electrothermal propulsion
chemical system where the gas
is
is
based on the same principles for propulsion as a
heated and expanded through a nozzle. However
instead of chemical combustion providing the heat, heat
is
generated
electrically.
Electrothermal propulsion provides lower thrust than a chemical system, but the
specific impulse can
be much higher.
launch, due to their low thrust.
maneuvering. Orbit transfer
The higher
Electrothermal systems are not suitable for
Their
electrical
specific impulse of electrothermal
system but
power
much
propulsion systems
A.
unit weight are
less propellant
is
orbit station
keeping and
be possible with bigger systems under development.
will
perform the same mission as a chemical system using
and
on
utility is in
is
comparable
required.
The
systems will allow them to
less total weight.
to the
first
The
thruster
hardware of a chemical
generation of electrothermal
the resistojet.
RESISTOJET THRUSTERS
The
system.
(NH 3 ),
resistojet
The
shown
thruster propellant
or hydrazine (N 2
chamber, via the
propellant).
Figure
in
Here
H
4 ).
catalyst
it
is
is
The
bed
in
decomposed
1
below
is
a typical arrangement for this type of
either hydrogen
propellant
the
case
is
(H 2 ),
nitrogen (N 2 ),
ammonia
delivered into the gas generator
of hydrazine
into nitrogen, hydrogen,
intended
flight
and ammonia.
Since
(the
4&
U*
s
I
2
<
>
I
LJ
a
2
k
5
E
I?
2
Z
a.
tug
_p <
Figure
1
Resistojet [Ref.
1]
3
i
s
-j
T
Uj
z
7
n
LU
T
^
CD
>
3
-J
<
a
UJ
U o
IA
s
?
the decomposition reaction
From
the gas generator
is
exothermic, the propellant also gains heat at this stage.
chamber
the gas delivery tube transfers the propellant to the
heat exchanger where the electrical heater element heats the gas prior to
entry
its
into the nozzle.
Through the
This
thrust.
generated
is
is
nozzle, the gas undergoes an adiabatic expansion to generate the
The
identical to operation of a chemical thruster nozzle.
calculated from the
thrust
temperature and other characteristics of the
The
propellant gas at the entry to the nozzle.
efficiency of the system
dependent on the efficiency of the heat exchanger. In the
is
very
much
resistojet the heat transfer
occurs in a cavity around the augmentation heater.
This design has been used on
satellites
[Ref.
1].
The
SATCOM, GSTAR
on these
thruster used
[Ref.
is
0.0590-0.127 g/sec.
The mass of
SPACENET
I-IV
operates on hydrazine
satellites
propellant and provides a steady state thrust of 0.178-0.334
flow rate range
and
I-IV,
N
thrust.
The
the arcjet thruster
propellant
is
0.816 kg.
1]
The
SATCOM G
1983, and
SPACENET
and
I
H
and
satellites
II
were launched
were launched
in
in
April and September of
May and November
of 1984.
As
of August 1990 these resistojet thrusters have accumulated from 80 to 141 hours of
burn time for station keeping.
resistojets for orbit transfer.
[Ref.
1].
As
of August
GSTAR
GSTAR
III
III
was launched
in
September 1988.
It
used
has 180 hours of accumulated burn time
1990, a total of 20 satellites have
resistojet thrusters for station keeping, with all firings
been launched with
nominal [Ref.
1].
The
arcjet principles of operation are the
However, the
leads to
some
arcjet differs
from the
same
as those for the resistojet.
method of heat
resistojet in the
transfer,
and
this
additional complications, especially concerning integration with other
spacecraft systems.
B.
ARCJET THRUSTER
In the arcjet thruster, the heat transfer to the propellant
The
convection and conduction from a plasma arc.
differences are
shown
in
Figure
is
gas
stabilize the arc, cool the walls
arc to the gas.
is
also functioning as
smaller.
is
established between the cathode probe and nozzle/anode as
The
configuration
significant
resistojet in that the nozzle
the anode for the arc and the region of heat transfer
2.
accomplished by
2.
Note the difference from the
portion of Figure
is
shown
The
in the
arc
is
lower
injected tangentially to the cathode in order to
and
to increase the
The power required
time for heat transfer from the
for establishing
and maintaining the arc
is
provided by the power conditioning unit (PCU).
The
first
TELSTAR
in this
operational use of the arcjet thruster
is
scheduled to be on the
4 satellite scheduled for launch in late 1993 [Ref.
experiment
is
same
Research on
The
arcjet thruster
a prequalification, mature, engineering-development
the thrusters planned for the
aspects are the
1].
TELSTAR 4 satellite.
as the flight
model
model of
All significant engineering design
thruster.
arcjets of varying designs
and power
levels
was conducted
in the
RESISTOJET HEAT TRANSFER REGION
HEAT TRANSFER
•
RADIATION HEAT TRANSFER
FROM HOT FILAMENT TO HEAT
EXCHANGER WALLS
•
ARCJET HEAT TRANSFER REGION
(ENLARGED TO SHOW DETAIL)
•
HEAT TRANSFER
CONVECTION, CONDUCTION
FROM PLASMA ARC REGION
•
(> 20,000
CONVECTION. CONDUCTION HEAT
TRANSFER TOGAS
ELEMENTS
'K;
TOGAS
GAS INJECTED TANGENTIALLY
PROVIDES ARC STABILIZATION.
KEEPS WALL COOL
Anode/Nozzle
— r^tgl
Spark gap
ARCJET BODY
POWER CABLE
)
i
PROPELLANT
IN
FLUID
THRUSTER
GENERATOR
RESISTOR CONTROL
VALVE
Figure 2
Arcjet / Resistojet Comparison [Ref.
1]
early 1960's [Refs. 2-4].
This work showed them to be a promising alternative to
chemical systems although the thrust levels were lower than desired for the low
power versions. Research was focused on magneto-plasma-dynamic (MPD)
throughout the 70's and into the mid 80's [Ref.
the arcjet thruster used in this experiment.
arcjet thruster that achieves
much
The
5].
The
MPD
MPD arcjet
is
thrusters
thruster differs
from
an evolution from the
higher ionization of the propellant, and relies on
electromagnetic forces for thrust rather than the thermal expansion through a nozzle.
A survey of the literature in the Journal
of Aeronautical and Astronautical Abstracts
reveals that the field
was devoted soleley
when thermal
came back under
arcjets
Recent work on thermal
to the
arcjets includes studies of the
(nozzle) and cathode [Refs. 6-8].
The
now being on low power
The main concerns
to spacecraft.
on
is
arcjet
plume
characteristics
and the design of the thruster anode
recent studies are similar to earlier
arcjets since they are
work with
near deployment.
at this point are issues involving the integration of the arcjet
One concern
is
the electrical currents collected by the solar arrays
from the ionized exhaust gas produced by the
process
thruster design until 1985,
study.
(ionization, enthalpy, temperature, composition),
the focus
MPD
arcjet.
The
basic physics of this
considered in the next chapter, along with a review of previous lab work
plasma
characteristics.
III.
A.
PLASMA PRINCIPLES
PROBE THEORY
A
solar array has exposed biased conducting surfaces.
these surfaces will collect currents from that plasma.
surfaces in plasmas
potential
is
interactions
for idealized
10].
The
plasma,
in a
interaction of biased
explained well by Debye theory for small potentials.
If
the
on the order of or larger than the electron temperature, then these
become more complex. The
theoretical
development
spherical and cylindrical objects such as
Assuming an
compared
is
Immersed
infinite quasineutral
is
quite complete
Langmuir probes
[Refs. 9
plasma with a mean free path,
to other system dimensions, a charged
probe placed
in a
plasma
A,
and
large
will create
a potential for the charged particles in the plasma and they will redistribute
according to
this potential
and the probe geometry. This
will result in a shielding
out
of the probe by attracted ions or electrons that form a sheath around the probe.
Figure 3 depicts
defined by
=
this shielding
relative to the
plasma potential. Outside of this surface the particles
do not sense the charged spherical
The
out principle. There will be a spherical surface
object.
current density of the probe
is
given by
+
4>=0 Surface
Figure 3
Debye Shielding
of a Spherical Charged Object in a Plasma
10
J=-nw
where
Js
is
the current density, n
quasineutrality), q
velocity [Ref.
If
1
1
the ion or electron
is
the species charge in coulombs,
and v s
number
is
the
density (by
mean
species
9].
the probe
of the probe
probe
is
(1)
is
spherical with radius
is
small
9].
and the absolute value of the potential
,
to kT/e, then the sheath thickness
be small compared to
will typically
lengths, k D [Ref.
compared
r
r
p
and
will
around the
be on the order of a few Debye
This will also leave the bulk of the plasma undisturbed.
charged particle species (electrons or ions)
Maxwell-Boltzman velocity
in this
If
the
sheath are characterized by a
then the current density would be the
distribution,
familiar
J=n oq
where k
is
the
{
kT
of plasmas [Ref.
9].
is
(2)
m
1
s
is
the species
mass
in
kg [Ref.
9].
used extensively with Langmuir probe investigations
In the experiment
of planar geometry and
Debye theory and
is
exp '*£)
kT
Boltzman constant, and
This familiar equation
\j
documented by
this thesis, the
biased such that the sheath region
is
much
conductor
different
thus will take on a character like that pictured in Figure
plasma source (the
arcjet thruster)
spot source which expands and
is
is
4.
is
from
The
generating a maxwellian plasma from a small
affected by the potential of the plate once electrons
11
Charged
Particle
Paths
(J)=0
Contour
Biased Plate
Figure 4
Plasma Charged Particle Interaction with an Oppositely
Charged Plate with a Pinhole of Exposed Surface
12
=
or ions cross the
potential surface.
The assumption
experiment.
This
This will
e0
compared
small
is
implicit
mean
in
Equation 2
is
Nevertheless Equation
is
not valid
Equation
that the plate's potential will effect the bulk
1
will
1.
is
plasma and
overall density of the plasma.
it
is
applied to the
in
be determined from the assumption that the velocities
the sheath region are due to the potential of the
This leads to a current density of
(
J=en.
2qq>
Mr
where v has been related
i-
(3)
I
to the potential by
—1 mv 2 =q<p.
.2.
is
this
Therefore to obtain qualitative estimates of the electron
of the charged particles
Equation 4
for
conductor geometry
holds for this experiment, so long as
1
region near the plate.
conducting surface.
kT
that the
no precise information can be gained about the
density, v in
to
a basic assumption to obtain Equation 2 from Equation
is
Another assumption
spherical.
that
just
an expression of the conservation of energy and as long as
the particles remain outside the
and their kinetic energy
is
just
=
=
contour then they are not
Vknv
as this applies to this experiment
of potential out to the
(4)
is
2
.
in
An
approximate sketch of what
Figure
4.
The charged
is
occurring
plate creates a region
contour where the plate potential
13
in a potential field
is
shielded out as
described above.
surface but
is
The contour depicted
intended to indicate further complexity beyond the simple spherical
arrangement of Figure
straight paths
not necessarily representative of the actual
is
With large
3.
A, the particles will
be traveling
from the source of the plasma generation. As these
contour they are either repelled from or attracted to the plate.
is
appropriate, they will be collected on the plate.
density n can be evaluated by Equation
We
3.
It is
in
random
particles cross the
If their
entry angle
at the plate surface that the
might expect that
this density at the
plate surface will vary with the density outside the sheath.
B.
PREVIOUS ARCJET PLUME INVESTIGATIONS
plume has been of considerable
Investigation of the arcjet
of spacecraft integration are being
worked
out.
The plume has been
the region very near the arcjet thruster exit plane
1.
Arcjet
Plume Ionization
The plume
investigated [Refs.
cylindrical
6,
Langmuir probes
thruster propellant
decomposed hydrazine.
to
stationary probes
[Refs. 2, 6,
7,
11].
Langmuir Probe
low power
(1.1
kW)
dc arcjet have been
determine plasma number density and temperature
plume and
was a
A
investigated in
These studies were conducted using spherical and
11].
as a function of position in the
The
(<35 cm)
Investigations by
characteristics of a
interest as the details
2:1
as a function of arcjet operating condition.
ratio
of hydrogen to nitrogen to simulate
vacuum tank pressure of
were positioned
at 30.5
cm down
14
0.05
the
Pa was maintained. Two
plume
axis with
one probe on
the axis centerline and one 15.2
18.4
cm down
the
plume
cm
off centerline.
A moveable probe was positioned
axis.
In these investigations the probe potentials
were kept small
Equation 2 applies and the electron density can be determined
at
relative to
kT e
.
plasma potential
by
/2
J /27tm V
n=-
I
(5)
kT
where the electron temperature
is
determined by the slope of the linear portion of
the characteristic curve and knowing
/>—
kT
—(In
dV
which
is
(6)
e
e
obtained by taking the logarithm of Equation 2 and differentiating with
respect to the probe voltage.
The I-V
centerline
is
[Ref.
6,
11]
characteristics of the spherical
shown
in
Figure
5.
From
probe
this voltage
at 30.5
cm on
the
plume
versus current plot, using the
slope of the linear portion of the curve, Equation 6 yields an electron density of
3.5
x107cm 3
[Ref. 11].
Figure 6 shows the electron
off centerline
number
density found as a function of angle
and the electron temperature as a function of the same angle. These
measurements are
for an axial distance of 18.4 cm.
the density and note that the density
is
Note the exponential decay of
half of the centerline value at 25° [Ref. 11].
15
lOOp-
—
—
60
80
10
t
10 hA/\n
1.0NA/1N
—
20
5
D
A
8
L
—
6
—
.1
—
Cx
s
.2-
-2
2
PtOK vaTAtf
Figure 5
«T
(). V
I-V Characteristic Graph of Arcjet Plume [Ref. 11]
16
8x10
+9
>
>—
E
J
I
(A) ELECTRON NUMBER DENSITY AS A FUNCTION OF ANGLE OFF
CENTERLINE.
LLl
1
O
Si
-
-
8
i-
o
o
.6
10
20
30
J
L
40
50
60
ANGLE OFF CENTERLINE. DC6
Figure 6
Electron Temperature and Density from Langmuir Probe
Investigations as a Function of Angle Off Centerline [Ref.
17
1
1]
The value
for electron
temperature
angle off centerline [Ref.
The
is
0.8
This would be reperesentative of the case for large X.
11].
potentials used in this experiment vary from -200 to
The measured
plume centerline
is
electron
shown
in
number
Figure
7.
investigations of the
arcjet
thesis.
plume
volts
axial distance varies
and q0> >kT.
from 12
to
32 cm.
These studies discuss further
distance.
characteristics as a function of propellant
mass flow rate
power, but the graphs presented are the subset of the data germane to
Carney's overall findings were that the plume ionization
percent and that the plasma potential
2.
+200
density as a function of distance along
The
Note the exponential decrease with
and
±0.2 eV, and does not show variation with
Plume Ionization
is
very near
facility
ground.
is
less
[Refs.
this
than one
6,
11]
Investigations by Spectral Emission
Arcjet performance and characteristics have been investigated by the use
of spectroscopy [Refs.
2,
7].
Determining electron density by the method of
spectroscopy relies on spectral line broadening due to the Stark effect.
broadening of the spectral
Stark
lines
and
caused by the
interactions of the (spectral) emitters with charged particles.
...produces
Lorentzian
line
shapes
is
[Ref. 7]
The
full
width
at half
maximum broadening (AX)
by
18
is
related to the electron density
4x10
10
RP, CM
I
X
o
O
0.316
O
0.562
g
to
«
2
PS
UJ
_
;
UJ
10
20
15
AXIAL DISTANCE, CM
Figure 7
Electron Density
vs.
Axial Distance [Ref. 11]
19
2/3
AX
where n c
1/2
(A)=2.5xlCT 9 a 1/2 n,
3
the electron density in cm"
is
and a 1/2
maximum. This technique can be used on
spectrum.
Langmuir probe and
power range of
Arcjets in a
The
only.
several different lines of the emission
6
-
others included a study of
spectral line broadening
42
kW
at the centerline
power of the
is
arcjet
The graph
in
is
in
exit plane.
Figure 9
is
dip in electron density
more recent
studies, but the
the electron density as a function of arcjet
line
broadening due
The
densities are about five times that found by the
Due
to techniques
and technical
difficulties the results
than those in the previous study [Ref.
The work by Manzella and
arcjet operating
The
good agreement.
and were obtained by the method of
kW
kW
involved in this experiment) and are
not reflected in any of the other
overall electron density
on a
power
to the Stark effect [Ref. 2].
Langmuir method
in
Figure
8.
presented are more qualitative
2].
others was conducted solely using spectroscopy
2:1
hydrogen-nitrogen mixture [Ref.
investigation found an electron density of 1.8 * 10
exit plane.
to the Stark effect.
were studied. The propellant was hydrogen
determined by Langmuir probe 2 cm from the
1
due
plume electron
electron density measurements presented in Figure 8 are for 30.3
(over 20 times the
on a
the theoretical width at half
is
[Refs. 7, 9]
The work by VanCamp and
density by
(7)
13
electrons/cm on centerline
This was determined by a measured A X
20
3
7].
in the
HB
This
in the
emission line of 0.34
—
2.0. 10 13
i
1
Mosi How
E
J *
rct«
,•4
IW/t.c
3.6 > 10
1.1
«
r
-4-
1.6
Ml—
Powtf
Powir input
30.3 kW
input
19.3 kW-
1.0
t
O.t
1.0
0.1
I
sorxlt txit diomttor
0.6
0.4
0.2
0.2
0.4
Rodiol position (inches)
Figure 8
Electron Density Measurements [Ref.2]
21
0.6
0.1
1.0
1.1 * 10
u
-r
r-
3.6 k
o
4.1 >
10*" 4
^
,u
"
4
4.3 a 10"*
I
1.2
1
J
i
10~ 4
*
-S
—
1
*Ut, flow (lb/t«c)
1
—
1
l
».-
.
— -_
J
.
*»*^0
„.*.
->^
!
0'
I
1
^^^^^1
]
1
0.1
20
30
25
33
40
Powar input (kW)
Figure 9
Electron Density Determined by the Stark Effect [Ref. 2]
22
[Ref. 7]
A.
SOME EXPECTATIONS
C.
Based on the above
experiment.
yield
cm
Given
studies,
that the
we can
plume
measurable currents [Refs.
6,
is
ionized to about 1%, this experiment should
11].
With an electron density of
3
and that
at
1
x 10 16 m"3 at 18
4x
10
12
/m 3
there will be
remain
0.8
to within
little
1
falls
an angle of 20° off of centerline the density
roughly half of the centerline value as in Figure
to
the results from this
distance from the source as in Figure 7 and assuming that the plasma density
off as a function of 1/r
of
some of
estimate
order of magnitude.
If
6,
then
the
we would expect
mean
free path
is
is
densities
large, then
exchange of energy and we can expect the electron temperatures
±0.2
eV
over
all
distances in the
23
vacuum chamber
[Ref. 11].
IV.
EXPERIMENT DESCRIPTION
EXPERIMENT OVERVIEW
A.
The
arcjet test firing at
plume/biased
surface
TRW that provided the opportunity to look at the arcjet
interaction
Development (ASID) program.
The
FLTSATCOM
Integration
ASID program
are to
(EMI) and plume impingement.
These
were accomplished by operating the
powered
the
System
Arcjet
objectives of this
investigate electromagnetic interference
objectives
the
of
part
is
arcjet in a
vacuum environment near
satellite [Ref. 12] by:
•
Measuring
EMI
coupled into the spacecraft.
•
Measuring
EMI
radiated with antennas.
•
Measuring plume characteristics with radiometers, calorimeters, and witness
plates positioned in various stations.
Along with these
test goals, the interaction of the
biased conducting surfaces was investigated.
cell
As
ionized
the arcjet
plume with exposed
was operating, the solar
emulators and reference plates were biased to various voltages between -200 and
+ 200 volts and the current was measured.
B.
HARDWARE
The
principle hardware in the test firing
was the vacuum
test facility, the arcjet
system, the emulators and reference plates, and the measuring equipment.
24
Vacuum Chamber
1.
The vacuum chamber
vacuum pumps
is
a 30 foot
diameter sphere.
rated at 25,000 liters/second for
H
2
or
N2
It
has four cryogenic
In order to
[Ref. 11].
3
achieve and maintain chamber pressures of 10" Pa, the chamber also has a liquid
nitrogen cooled inner wall and liquid helium cooled cryopumping array.
operating the chamber can
liters/second for
N
2
pump
7.5
x
10
6
liters/second for
H
2
When
fully
or 3.8 x 10
6
These gases are the principle propellant constituents
[Ref. 12].
of the arcjet.
Arcjet
2.
The
arcjet
was a
1.4
kW
thruster operating
on hydrazine propellant.
consisted of three principle subsystems: the arcjet thruster, the
unit
It
power conditioning
(PCU), and the interconnecting power cable between the thruster and the PCU.
These items are shown
The
PCU
in
Figure 10 and Figure
11.
stepped up the spacecraft bus voltage to approximately 100 volts
dc and regulated the arc current during operation.
It
volt pulse to initiate the arc during arcjet start up.
also
would provide a
The power
to the
brief 4000
PCU
was
provided by a 28 volt dc power supply external to the chamber.
The power
cable between
PCU
and thruster was
triaxial
with center
conductor connected to the thruster cathode. The inner shield was connected to the
thruster
anode or nozzle and the outer shield was connected
The
NASA
arcjet thruster
to chassis ground.
was made by Rocket Research Company under the
Lewis Research Center sponsored Arcjet Research and Technology Program.
25
Figure 10
Arcjet Subsystems [Ref.l]
26
Figure 11
Picture of Arcjet Thruster [Ref.
27
1]
was rated
It
seconds.
to provide 0.169 to 0.200
The
was rated
at
thruster
mass was
1260 watts.
0.25
1.36 kilograms.
The
electrical
Figure
13.
test
The conducting
mm thick stainless steel
surfaces of the emulators and reference plates
a 0.20
mm
damaged
actual areas
plate
nominal
thick cover glass with a
were
as follows: (1)
was mounted on
9.5
mm
damaged,
(2) 1.59, (3) 2.27,
thick plexiglass
1.5
cm 2
.
mm
Emulator one was
main experiment so no data was gathered from
prior to the
were
of the reference plates was 3.14
diameter hole, exposing an area of approximately 1.77 mm".
Each
arcjet
items are in Figure 12 and a picture of one plate
The exposed area
The emulators were covered by
The
power of the
450
[Ref. 12]
Diagrams of these
in
thrust with a specific impulse of
Solar Cell Emulators and Reference Plates
3.
is
Newton
and
and attached
that position.
(4) 1.89
mm
2
.
to the solar panel
with adhesive tape.
Measuring Equipment
4.
Kiethley 617 programmable electrometers were used to measure the
collected currents.
an
C.
HP 3455A
A Hewlett
digital
Packard 6448B dc power supply provided the bias and
voltmeter measured
it.
LAYOUT OF EXPERIMENT
The arrangement
Figure
15.
of the equipment in the
The emulator
plates
chamber
is
shown
in Figure 14
and
were placed along the length of the solar array panel
parallel to the arcjet thruster axis 42.5
cm
28
off the axis center line.
The reference
3
mm
t
Voltage source cnctr
Stainless Steel Plate
i
2
cm
*
Cover Glass
2mm
in
6
cm
hole
cover
glass
Nylon screw
Solar Cell Emulator Plate
Nylon Screw
2
Exposed Conductor
Reference Plate
Figure 12
Diagram of Reference and Emulator Plates
29
cm
5.7
cm
Figure 13
Picture of Emulator Plate
30
LID
HARD POINTS
•SPOOL
30'
DIAMETER
. VACUUM TEST
L>»
FACILITY
-RGA
•ARCJET
cab:e
SOLAR PANEL
SUPPORT FIXTURE
FLTSATCOM
TEST
FIXTURE
Figure 14
0IVt22C90 01i
Vacuum Chamber
Configuration Side View [Ref. 12]
31
SOlAAPANt;
QIACNOSnCS
XT DIAMrTTR
VACUUM TIST
FAOUTY
CHAMBU COMSOU
Figure 15
Vacuum Chamber
Configuration
32
Top View
[Ref. 12]
plates
were placed on the opposite
The
centerline.
in
Table
1.
PLATE DISTANCES ALONG ARRAY PANEL AND
SLANT RANGE TO ARCJET
I
Emulators
The
E3
E4
R2
R3
R4
165
104
165
Slant (cm)
172
259
263
113
171
259
263
#
115
maneuvers
is
typical of
Figure 16
[Ref. 12].
were connected
plates
is
is
Figure
in
17.
geometries for north-south station keeping
a photograph of the
to the
via a connector in the wall of the
circuit
satellite.
was 25.4 cm above the solar panel centerline, canted 20° from the
This configuration
axis.
The
E2
system was mounted to a support platform just above the
arcjet
arcjet thruster
panel
References
Distance (cm) 104
Plate
The
off of the thruster axis
distances along the panel for both types of plates and the slant
ranges from the arcjet nozzle are
TABLE
cm
side of the panel 36.2
chamber
layout.
power supply and the measuring equipment
chamber
to a switching box.
Although the connector allowed
The diagram
of this
for a floating ground,
no
noticeable difference in measurements of the two conditions was observed so the
circuit
D.
was grounded
to the
common chamber
ground.
PROCEDURE
Once
the
chamber pump down was complete
voltages from -200 to
above 0.006
/.iA
+200
volts
were measured
a survey of line currents at
on the conducting plates was taken.
at
No
currents
the extreme voltages, which established the
33
Figure 16
Picture of
Chamber Ready
34
for Firing
Run
St
•
J
o.
i
Reference Plates
Jumper
C-l
^ 1
V
>
/
'
Jumper
Figure 17
Emulator Plates
Schematic for Plate Biasing Circuit
35
&-
baseline at less than 0.02% of any measured value in the arcjet environment.
the arcjet was firing, the pressure on the
diffusion
N2
pumps, and
Each
shroud.
chamber was maintained by the He
plate
was biased
Once
pane!-,
and
to varying voltages
current readings
c recorded.
Chronology
1.
The
3
2.13 x 10"
firing
first
run was attempted on 24 July 1990.
Pa was achieved.
However
supply line prevented firing on
The following day
the pressure had
brought up to 15.1 Pa and the propellant was no longer frozen
chamber and attempting
insulation immediately, the decision
in
pressure of
freezing of the propellant in the propellant
this day.
pressure. Rather than opening the
A
was made
to
do a
firing
been
in the line at this
to fix the propellant line
run
at the
high pressure
order to collect some data in case a later attempt at a low pressure run was also
unsuccessful.
It
was during
one was cracked.
sufficient
this arc
from
Due
this
high pressure run that the cover of emulator plate
to the high pressure, a voltage of less
this position to establish
than 200 volts was
arcing to ground, and the resultant heat from
broke the glass cover.
After repairs to the insulation on the propellant feed
3
pressure of 1.20 x 10" Pa was achieved and the firing run
1990. This
is
the run
thesis chiefly deals.
from which the main body of data
The data from both runs
chapter.
36
line,
commenced
is
the desired low
at
1015 26 July
drawn and with which
this
are presented and discussed in the next
2.
Aspects of the
Low Pressure
Firing
During the main (low pressure)
Run
firing run, data
were
first
the emulator plates at 10 to 20 volt intervals over the -200 to
obtained from
+200
volt range.
Collecting data by hand was time consuming. Since the supply of helium was low for
maintaining
this
pressure (due to the previous problem with the propellant feed line),
the time for data collection on the reference plates
repeat this experiment,
it
would be advisable
37
to
was
limited.
In any attempt to
automate the data collection process.
V.
RESULTS
CONDUCTING PLATE DATA
A.
The main
results
from the low pressure run are presented
in Figure 18, the
nearest set of plates, through Figure 20, the furthest set of plates (see Table
logarithm of the current
all
taken with the arcjet
is
plotted versus voltage for each plate.
firing
reference
and
3
at 1.2 ±0.1 x 10" Pa.
emulator
plates
plotted
Substantially higher electron currents than ion currents
As
The
The readings were
nominally under stable vacuum pressure conditions.
The chamber pressure was maintained
corresponding
I).
These graphs have the
on
the
were found
same
graph.
as expected.
the distance between the plate and arcjet increases the current density
This
decreases.
is
expected since, as was found in previous research, the plasma
density decreases as a function of distance and angle off of arcjet axis.
[Refs.
6, 7,
11].
Note
that in each case the emulator current density
steeper slope than the corresponding reference plate.
densities are plotted
on a
emulator current density
plate at the
The
same
The
and
this effect is
more than two times
higher and has a slightly
In Figure 21 the current
more
obvious.
In fact the
the current density for the reference
distance.
ability to
were made.
is
linear scale
is
repeat and check readings was limited, however
some
spot checks
electron current readings (plate at positive potential) were very
38
Current Collection
Emulator and Reference 2
100
D
10
c
,
°
°
°
'
^
j$
1
k
<
E
-
^
.1-
r.
—
%
D
<c
o
JE2
•
JR2
.01
±
001
j
T
3
o
-.01
4
#
-.1c
-1
-2 25°
"
-°150
75
-75
150
Plate Bias Voltage (volts)
Figure 18
Emulator and Reference Plate 2 Data
39
225
Current Collection
Emulator and Reference 3
100
o
10-
~
c
O
D
-1-
o
JE3
oi
Q
c
£
3
°
D
E
i
°
°
1-
<
£
D
r
JR3
±.001
o
D
-.01
-I-
-.1-
o
D
D
D
D
D
1-z 25
D
-150
75
-75
150
Plate Bias Voltage (volts)
Figure 19
Emulator and Reference Plate 3 Data
40
225
Current Collected
Emulator and Reference 4
100
10
1
D
D
D
6
.1
to
.01
c
s
Q
+-*
c
S
t
JE4
+
JR4
D
=j
o
°
-.01
-4-
-225
*
—
-150
D
D
•
-75
75
150
Plate Bias Voltage (volts)
Figure 20
Emulator and Reference Plate 4 Data
41
225
Electron Current Collection
Emulator and Reference 2
° JE-»
F
»
**
.•
1Bn«J-1 KT1IO TOO
Ptel> Bi*a
VoAaga
ZO
(volti)
Electron Current Collection
Emulator and Reference 3
° JE-3
r
20
f
16
4
JR-3
i
I
D
10
5
**
+,.k
"1OT120
Plit»
"140
'
lBC^tKr~2O0 ~Z2D
Buu Vottig* («*j!
Electron Current Collection
25
Emulator and Reference 4
JE-3
e
2o
JR-3
E
n
5
^*x****
80
100~l20 140 HOT"180 200 220
Plot* Bias Voltag* (volts)
Figure 21
Linear Representation of Data
42
repeatable, but the ion current readings were
current readings
The
±20%
is
unstable.
for potentials of -200 to -100 volts,
The
error in the ion
and
±2%
otherwise.
electron current readings are within ±0.5%.
HIGH PRESSURE DATA
B.
1.2X10" 3 Pa, there was a run at a higher
Prior to the arcjet firing runs at
pressure of 15.1 Pa.
Due
80
more
During
to the high pressure, arc
volts,
although
to this arc
this
this run,
data was taken on
breakdown was achieved
was not consistent
breakdown
that the
emulator
all
four emulator plates.
at voltages
somewhat below
for any position or condition.
1
It
was due
coverglass was broken prior to the low
pressure firing runs.
The
common
arcjet
facility
arrangement
potential.
grounding scheme allowed for the anode
ground or
for the arcjet
left floating.
be grounded to the
During the data acquisition the grounding
anode was switched from
There was a marked change
to
facility
ground
to the floating
in the collected currents at this point, as
indicated in the graphs of this data (Figure 22 and Figure 23).
After this high
pressure run, the relay which allows a change in arcjet grounding configuration
became inoperable and
all
other data were obtained with the arcjet anode at
ground.
43
facility
Pa
Current Density at 15.1
0.1;
Emulator
^
1
E
M
0.01;
Encircled data are with arcjet ground floating.
All
C
g
3
other data are with arcjet at
facility
ground.
D
o
001
(D
20
6b
40
Voltage
80
Current Density at 15.1
0.1 2
120
100
(volts)
Pa
Emulator 2
D
CM
<
E
D
-
Boxed data are
V)
c
CO
0.01
All
Q
with arcjet ground floating.
other data are wtth arcjet at
facility
ground.
D
c
CO
b
3
o
D
O
001
()
40
20
60
Voltage
Figure 22
Emulator
1
&
80
2 Current Density
44
100
1^0
(volts)
vs.
Plate Voltage
Data
Current Density at 15.1 Pa
U.l
Emulator 3
E
c
tp
a
D
0.01
1
-
D
Encircled data with arcjet ground floating.
/
°
!
All
c
I
other data with arq'et at
facility
ground.
3
o
o
001
)
40
20
80
60
Voltage
100
Current Density at 15.1
0.1
Pa
Emulator 4
'
D
)
Encirded data with ard«t ground
All
E
120
(volts)
other data arcjei at
facility
floating.
ground.
0.01
co
c
<D
Q
3
0.001
-
0001
)
40
20
60
Voltage
Figure 23
Emulator 3
&
80
4 Current Density
45
100
120
(volts)
vs.
Plate Voltage
Data
VI.
A.
ANALYSIS AND DISCUSSION
MEAN FREE PATH
In Chapter
III
it
of Equation 2 a large
required.
A
was pointed out
mean
that to evaluate the electron density by
free path (k)
basic definition of k
compared
—
]
the ideal gas law:
«=-T-
(
kT
we
is
(8)
[nnd2
By using
dimensions
is,
k=
[Refs. 11, 13].
to other physical
means
9)
obtain
kT
k=-^—
(io)
pud
Assuming
The value
for
that
T is
T = 5000 K
and d =
10" 9
m, then k
a conservative estimate based
46
is
on the order of 20 meters.
on data from Reference 8 which has
the electron temperature average at
large, then there will be little
.6
±.1 electron volts.
temperature decrease
mean
the arcjet up to the distance of the
free path length.
does meet the requirement as stated
COLLECTED CURRENTS
B.
A curve
of
J
vs
V
as
in
Chapter
III
N-,.
mean
the
the gas as
in
estimate for the largest collision cross section, that of
If
it
free path
gets further
The value
for d
10
12
m"
3
,
which
is
This value for X certainly
that k»r
p
»k d
.
DATA
-
determined by Equation 3
is
presented in Figure 24. This
representative of the data from reference plate
used for the ion mass,
m,,
is
1.1
26
x 10"
kg.
constituents of hydrazine, namely
times the mass of
H
+
2
,
and
this
from
a large
is
modeling of the current voltage characteristic assumes an electron/ion density,
4x
is
A
l
of the
2.
n,
of
The value
This value comes from the decomposed
mass of
N2
value gives the best
+
,
fit
Vi
the
mass of
to the data.
N+
,
and 3
Figure 25
through Figure 27 compare the data from the reference plates to the modeled curve.
The
solid line in these figures represents the
The agreement
modeled
curve.
of the data with the modeled curve from Equation 3
good with a few exceptions. For reference plate 2 the knee
current density
is
less sharp,
in the
is
quite
measured electron
and the measured ion current density
is
quite divergent
from the modeled or predicted curve. The other ion and electron current densities
are in excellent agreement with the model.
47
Modeled Current Collection
100
J=qn(2qV/m)\5
10
12
-3
n-2.6x10m
CO
<
m,=1.1xld kg
E
£w
.01
c
ec
±.001
3
o
.01
-
.1
-1
-150
-75
75
Conductor Voltage
Figure 24
150
Plot of Current Density vs. Voltage
48
225
(volts)
from Equation 3
Modeled Data
100
vs. Actual
Data
Reference Plate 2
10
-r-
CO
<
E
2
n-4.5xld m"
1-
3
m.-1.1x10 kg
<r
.01
,§±.001
c
2
J-R2
-.01
o
-1
-150
75
-75
Voltage
Figure 25
I-V Comparison to
150
225
(volts)
Model
49
for
Reference Plate 2
Modeled Data
^
100
Data
Reference Plate 3
10
1
vs. Actual
-
-^
)K
12
n=2.6x10 m"
/
p
^
^
&
.01
noi
c
©
E
-.01
____-^
o
-.1
.1
-225
-150
75
-75
Voltage
Figure 26
3
mj=1.1x10 kg
.1
m-t
Q
J-R3
150
225
(volts)
I-V Comparison to Model for Reference Plate 3
50
Modeled Data
100
vs. Actual
Data
Reference Plate 4
10
*
J-R4
2
3
n=1.65xld m'
CO
<
m^l.lxlO" kg
E
-3
.01
®
±.001
c
<D
fc
-01
o
-.1
-225
-150
75
-75
Voltage
Figure 27
I-V Comparison to
150
225
(volts)
Model
51
for
Reference Plate 4
C.
CURRENT DENSITIES
The
shown
electron densities at the plate surface are
in Figure 28.
The number
voltages as a saturation value
Presented
is
approached.
Figure 29
in
is
way
to a leveling off at higher
These number densities are derived
and hold for the density local
3,
Note the
densities are plotted versus plate bias voltage.
rapid increase in density at low voltage which gives
from Equation
for each reference plate
to the plate.
the average over the voltage of the calculated electron
densities at the plate surface (from Figure 28), plotted against the distance
arcjet exit plane.
The
density
is
approximately proportional to
3/2
r"
from the
and not
3
r"
since
the density near the plate depends on the surface area of the sheath, which varies
from collector
To
to collector.
estimate the density in the undisturbed plasma at the sheath edge, the
physical process that
size of the
sheath
when
Initially,
boundary
is
is
is
related to the sheath as described in Chapter
characterized by the geometry of the
the voltage
=
III
applies.
The
potential surface.
very low (near the plasma potential), the sheath
is
very near the plate surface.
As
the potential
is
varied from the value
near the plasma potential, the sheath boundary expands.
The
"radius"
surface area associated with this sheath boundary expands as the sheath
expands (Figure
dimensions of
30).
this sheath.
It
Radius
in
is
no way implies that
the current collected on the plate this
sheath surface.
If
in quotations
and
it is
is
used only to relate the
rigidly spherical.
Whatever
same current must pass through the
the sheath expands significantly enough,
52
it
will
be more
=
in the
Number Density
Estimated Electron
1.00e+13
n=J\2?v)
n ref2
CO
n ref3
I
<
E
:
^
*
•
C
D
%
n ref4
-
+
+
+
1.00e+12
E
3
ts
LU
1.00e+11
~~
25
50
75
100
Voltage
Figure 28
Electron
Number
125
150
175
200
225
(volts)
Density of Reference Plates
53
Electron Density at the Plate Surface
6.00e+12-
\
\
-3/2
curve
r
-f\
5.00e+12-
y\
<
c
J4.00e+12
\
0)
2
1 3.00e+12
x^
\.
c
o
1
^"^-^
2.00e+12
Hi
1.00e+12-
I
0.5
1
I
1
I
I
1.5
2
2.5
3
Distance from Thruster (m)
Figure 29
Electron Density as a Function of Distance
54
Arcjet
<t>=0 for
Plume Centerline
high voltage
4>=0 for low voltage
Reference Plate
Side View
Arcjet
Plume Centerline
4>-0 for high voltage
4>=0 for low voltage
Reference Plate
Top View
Figure 30
Sheath Geometry
in
Relation to Arcjet Plume Axis
55
plume
arcjet
a
much
geometry of
axis for the
higher electron density [Refs.
charged particles to pass through
This would occur
plate.
If
this
this
experiment (see Figure 30). This area has
This will cause significantly more
sheath surface and hence be collected on the
expanded
the sheath size
if
11].
2, 6,
the sheath size remained less than this 50
to
cm
much more than 50 cm.
size,
then the increase in
current collected at the plate would be related to the sheath boundary contour area.
Knowing
that
it
that the sheath starts out very near the plate at low voltage
does not increase beyond the 50
centerline
cm
may be estimated
I=ns eA
is
From
prior to entering the
is
then
1/2
(ID
m
1.5
cm 2
A
for
xlO 11 m" 3
.
A
is
the area of the sheath.
is
approaching some
This would indicate the sheath has stopped expanding
centerline.
T
The
level will suffice.
a value of 2500
ns
plume
can be obtained. For
power
kT\
e
appears that the current density
it
limiting or saturation value.
for n s
in current, the electron density at the
the electron density at the sheath edge, and
the graphs
plume
by
/
where n s
limit at high voltage to enter the
and thus experience a huge jump
sheath edge
and assuming
e,
Using
this
assumption, a qualitative number
the values of previous investigations of arcjets at this
electron temperature was 0.6 ±0.1 ev [Ref.
and a value of 2
mA for
I
11].
Using
as found at reference plate 2,
This compares reasonably well with the estimate from
56
chapter
III
based on extrapolation from previous work, which was a factor of ten
higher.
ANALYSIS OF CURRENT COLLECTED BY SOLAR ARRAY
D.
Solar arrays are constructed from individual solar cells that are connected in
These
series strings to boost voltage.
Along
current.
the edge of each cell
approximately 0.8
mm wide
A
same
method with
its
strips
would be 0.367
across the panel.
the panel
centered
is
in
The
750 watts would have two panels of very nearly the
experiment [Ref.
14].
The
volts, at the first set
edge of the array. The increase
length of each 0.8
To determine
the
mm wide
power
lost at
voltage on the exposed
near the spacecraft body,
at
each series connection
connector
strip
would be 124
each step or connector
strip,
divided into thirds so that an emulator from this experiment would be
each
third.
linearly interpolated
strip.
In
a solar panel in this configuration, the
The
current density values measured for the respective
emulator are applied to the exposed connector
each
boost
an exposed connector strip which measures
plume on
would vary from zero
volts.
in parallel to
assumptions and approximations was used.
size as the panel in this
to 25.7 volts, at the outer
cm
this
silicon type solar array of
connector
connected
but this depends on the specific cell manufacturer.
order to estimate the effect of
following
is
strings are
The
strip areas.
The
between the measured voltages and applied
current density J n of each strip
57
is
found by
current density
is
for the voltage of
Jn =Jm2,-
m2
where subscripts ml and
(vm2 -K)
<Vm7 -vml )
indicate the
(12)
V*2"JJ)
measured values of
appropriate emulator that bracket the connector
strip's actual
the connector strip
the arcjet plume.
by the panel.
percentage
The
power values are summed
loss in this
The whole
in this
example
is
to
V
power
n
is
0.05%.
58
.
determine the power
for the
Jn
lost.
is
then
Finally,
lost
due
to
0.39 watts of the 375 watts produced
array (both panels) produce 750 watts so the
example
V
and
voltage
multiplied by the area and the voltage at each strip to determine
all
J
power
loss
CONCLUSIONS
VII.
A.
SOLAR ARRAV PERFORMANCE
The use
power
of arcjets should cause very
loss of
0.05%, as estimated
arcjet/array configuration.
If
significantly lessened, then the
surface
more
significantly.
directly
in
the
in
little
this
degradation to solar arrays due to the
experiment.
This was only for one
the angle between arcjet and solar array surface
may be much
is
power
losses
arcjet
plume the currents collected would jump
higher.
With the array
This would be for very limited configurations and can be avoided
during the design of the spacecraft.
B.
FURTHER RESEARCH
Investigation
of the
far-field-arcjet
plume
is
a
natural
preliminary look at the effects of this slightly ionized plume.
density
as
a
function
calculations of the
of angle
power
follow up
Knowing
to
this
the electron
and range within the plume would
facilitate
losses for any given firing configuration with respect to the
array surfaces.
59
LIST
1.
OF REFERENCES
Rocket Research Company, Report 90-H-1478, Electric Propulsion Technology
W. W. Smith and R. J. Cassidy, August 1990.
Status, by
2.
NASA
al,
3.
CR-54691, Study of Arcjet Propulsion Devices, by W. M. Van Camp,
March
Giannini Scientific Corporation, APL-TDR-64-58, 30
Research, by
4.
NASA
J. P.
6.
kW
Arc-Jet
J.
Thruster
Todd, March 1964.
CR-54035, Research and Advanced Development of a 2
Thruster, by O.
5.
et.
1966.
McCaughey,
et.
kW Arc-Jet
al, 1963.
kW
Institut fur
Raumfahrtsysteme, Universitat Stuttgart, Report 88-107,
Experimental Arcjet, by H. L. Kurtz, et. al., 1988.
A
NASA TM-89924, Langmuir Probe Surveys of an Arcjet Exhaust, by L.
M. Zana,
15
July 1987.
7.
NASA
TM-103241, Preliminary Plume
D. H. Manzella,
et. al.,
Characteristics of
an Arcjet
Thruster,
by
July 1990.
8.
NASA TM- 102284,
9.
L. Schott, Electric Probes,
The Effects of Arcjet Operating Condition and Constrictor
Geometry on the Plasma Plume, by L. M. Carney and J. M. Sankovic, July 1989.
Plasma Diagnostics edited by W. Lochte-Holtgreven,
,
North-Holland Publishing Co., 1968.
Chen, Electric Probes, Plasma Diagnostic Techniques edited by R. H.
Huddleston and S. L. Leonard, Academic Press, 1965.
10.
F. F.
1 1
NASA TM-
12.
,
An
Experimental Investigation of an Arcjet Thruster Exhaust
Using Langmuir Probes, by L. M. Carney, April 1988.
1
00258,
TRW Inc., Test Plan No. 54732-6004-UT-00, Test Plan, Arcjet System Integration
Development Program, by R. J. Glumb, 13 April 1990.
60
13.
F. F.
Chen, Introduction
to
Plasma Physics and Controlled Fusion, 2d
Plenum Publishing Corporation,
14.
ed., v. 2,
1980.
B. N. Agrawal, Design of Geosynchronous Spacecraft, Prentice-Hall Inc.,
Jersey, 1986.
61
New
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Code 1114 SP
800 North Quincy Street
Arlington, Virginia 22217-5000
63
Thesis
Evert
E82
c.l
Arcjet plume ionization
effects on exposed solar
array conducting surfaces,
Thesis
E82
c.l
Evert
Arcjet plume ionization
effects on exposed solar
array conducting surfaces.