Author(s)
Norwood, Christopher W.
Title
Ions generated on or near satellite surfaces
Publisher
Monterey, California. Naval Postgraduate School
Issue Date
1988
URL
http://hdl.handle.net/10945/23061
This document was downloaded on May 04, 2015 at 23:18:34
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NAVAL POSTGKADOATE SCHOOL
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NAVAL POSTGRADUATE SCHOOL
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THESIS
N ^M-<4Z.»-|
IONS GENERATED ON OR NEAR
SATELLITE SURFACES
by
Christopher William Norwood
' '
June 1988
*
Thesis Advisor:
Richard
C
Olsen
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IONS GENERATED ON OR NEAR SATELLITE SURFACES
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Norwood, Christopher W13b TIME
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6.
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GROUP
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SUBJECT TERMS {Continue on reverse
Ions, SCATHA, Sputtering,
SUB-GROUP
if
necessary and identify by block numper)
Ion Emission
1
1
ABSTRACT (Continue on reverse
9
if
necessary
and
number)
identify by block
Observations of positively charged particles that are generated on or near
satellite surfaces have been made on several spacecraft. This thesis postulates
sputtering of the satellite surface due to ambient ion impact as the generating
mechanism- Calculations are made using the Sigmund-Thompson sputtering theory to
determine the response at the satellite particle detectors/i
These calculations
indicate that surface sputtering creates a sufficient flux to account for the
observed phenomena.
The NASA Charging Analyzer program was run to determine the
trajectories for measured particles.
The calculated trajectories were determined
to lead to the spacecraft surface, again indicating that surface emission was the
source.
The sputtering flux as calculated was insufficient to cause any significant
short-term damage to the spacecraft, beyond thin coating erosion.
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Ions Generated on or Near Satellite Surfaces
by
Christopher W. ^Norwood
Lieutenant, United States Navy
B.S., Villanova University, 1981
Submitted in partial fulfillment of the
requirements for the degree of
MASTER OF SCIENCE IN PHYSICS
from the
NAVAL POSTGRADUATE SCHOOL
June 1988
ABSTRACT
Observations
on or near satellite surfaces have been
generated
several
the
spacecraft.
satellite
generating
S
of positively charged particles that
are
made
on
This thesis postulates sputtering
of
surface
mechanism.
due to ambient ion
Calculations
are
impact
made
as
the
using
the
igmund-Thompson sputtering theory to determine the response
at the particle detectors.
These calculations indicate that
surface sputtering creates a sufficient flux to account
the observed phenomena.
The NASA Charging Analyzer Program
was run to determine the trajectories for actually
lead
to
surface
observed
The calculated trajectories were determined
particles.
the
spacecraft
for
surface,
emission was the source.
again
indicating
The sputtering
calculated was insufficient to cause any significant
term damage to the spacecraft,
Ill
to
that
flux
as
short-
beyond thin coating erosion.
// 1*
CI
TABLE OF CONTENTS
I
.
INTRODUCTION
1
A
.
OVERVI EW
1
B
.
SCATHA SPACECRAFT
2
C
D
1.
Spacecraft Construction
2
2
Spacecraft Orbit and Environment
5
3.
Instruments
9
4.
Ion Gun
.
OTHER
.
13
13
2.
I
SEE
15
SPACECRAFT CHARGING
15
SPUTTERING
20
1.
The Sigmund Theory
22
2.
The Thompson Theory
27
3
Compar ison with Experiment
29
Limitations of the Sigmund Theory
38
.
4.
.
TES
ATS -6
E.
II
I
1
.
F.
S ATELL
13
NASA CHARGING ANALYZER PROGRAM
OBSERVATIONS
A.
39
41
PREVIOUS OBSERVATIONS
iv
41
MOaiEEEY, CALlFOaUIA
1.
ATS -5
41
2
ATS -6
4 4
SEE
4 6
.
3.
P73-2 (SCATHA)
B.
III
48
1.
Day 83 of 1979
43
2.
Day 86 of 19 79
53
3.
Day 94 of 1979
58
4.
Day 200 of 1979
60
5.
Day 92 of 19 81
53
DISCUSSION
.
A.
B
IV.
I
.
5 6
POSSIBLE ION SOURCES
56
1.
Outgassing
67
2.
Sputtering
70
CALCULATION OF THE SPUTTERING FLUX
AT THE DETECTOR
CONCLUSIONS
81
101
APPENDIX
104
LIST OF REFERENCES
115
INITIAL DISTRIBUTION LIST
119
V
93943-60L
LIST OF TABLES
1.
Magnetospher ic Ions and Their Sources
104
2.
Storm Time Ring Current Composition
105
3.
Storm Time Magnetosphere Composition
106
4.
Values of the Reduced Stopping Power
107
5.
Calculated and Measured Sputtering Yields for
Various Materials
Calculated Sputtering Yields using a
Modified Binding Energy
Ratio of Sputtering Yield per Atom for Molecular
and Atomic Ion Bombardment
SCATHA Particle Detector Energy Channels
108
6.
7.
8.
9.
10.
The Sputtering Yields of 10 keV Ion Beams on
Silicon and Silicon Dioxide
Ambient Plasma Values for Days 83,86, 200
of 1979 from the Lockheed Ion Mass Spectrometer
VI
109
... 110
Ill
113
114
LIST OF FIGURES
Figure
1.
SCATHA Satellite
4
Figure
2.
SCATHA Orbit
6
Figure
3.
Regions of Space near Earth
7
Figure
4.
SC-9 Detector Arrangement
11
Figure
5.
SC-9 Detector Angular Coverage
12
Figure
6.
ATS-6 Satellite
14
Figure
7.
Charging Current Schematic
16
Figure
8.
Sputtering Yield for Aluminum
21
Figure
9.
a{H^/H^) vs. M2/M1.
26
Figure 10. The Function E/(E +U0
^
30
)
Figure 11. Energy Distribution of Sputtered Gold Atoms.. 31
Figure 12. Calculated And Experimental Sputtering
Yields for Copper
Figure 13. Mass Dependence of the Sputtering
Yield for Various Atoms
Figure 14. Typical Sputtering Yields for H+ and 0+
on a Silicon Target
Figure 15. The Dependence of the Sputtering Yield
on Angle of Incidence
Figure 16. ATS-5, Spectrogram, Day 274 of 1970
33
34
35
37
42
Figure 17. ATS-6, Spectrogram, Day 248 of 1974
45
Figure 18. ISEE-1, Mass Spectrometer, March 17,1978
47
Figure 19. SCATHA, Spectrogram, Day 83 of 1979
50
Figure 20. Day 83 of 1979, Ion Distribution Funct ion
.
.
.
.
51
52
Figure 21. Day 83 of 1979, Vehicle Potential,
Triangle Peak Energy, and Look Angle vs. Time
54
Figure 22. SCATHA, Spectrogram, Day 86 of 1979
Figure 23. Day 86 of 1979, Ion Distribution Funct ion .... 55
VI
1
Figure 24. Day 86 of 1979, Vehicle Potential
57
Triangle Peak Energy, and Look Angle vs. Time
59
Figure 25. SCATHA, Spectrogram, Day 94 of 1979
Figure 2G. SCATHA, Spectrogram, Day 200 of 1979
61
Figure 27. Day 200 of 1979, Ion Distribution Funct ions
.
.
Figure 28. SCATHA, Spectrogram, Day 92 of 1981
62
64
Figure 29. Day 92 of 1981, Ion Distribution Funct ion .... 65
Figure 30. Detector Beam Viewing Geometry
73
Figure 31. Oxygen-Silicon Yield Curves
75
Figure 32. Lindhard's Universal Function
83
Figure 33. Incident, Emitted, and Ion Fluxes
84
Day 86 of 1979
Figure 34. Channel Responses for a 35 V Di f f erent ial
86
Charge, 0+ Incident, Day 86, 1979
Figure 35. Channel Response for a 35 eV Di ff erent ial .... 87
Charge, 0+ Incident, Day 200, 1979
Figure 36. Channel Response for Zero Differential
90
Charge
Figure 37. NASCAP Trajectories, Day 86 of 1979
91
.
.
.
.
Figure 38. NASCAP Trajectory, Day 94, Lo Detector
93
Figure 39. NASCAP Trajectory, Day 94, Lo Detector
94
Figure 40. NASCAP Trajectory, Day 94, Hi Detector
96
Figure 41. NASCAP Trajectory, Day 94, Hi Detector
97
Figure 42. NASCAP Trajectory, Day 94, Fix Detector
98
Figure 43. NASCAP Trajectory, Day 94, Fix Detector
99
VI
1 1
I
A.
INTRODUCTION
.
OVERVIEW
charging
Spacecraft
between
difference
environment.
since
It
is
the development of
space
a
vehicle
of significance
is
and
plasma
its
for scientific purposes,
the potential may affect measurement of
properties of the environment,
potential
a
ambient
the
and for practical
purposes,
physical
since it may lead to anomalous command signals and
damage to the affected spacecraft.
Although predictions of
spacecraft
previously,
reports
charging
had been made
high level spacecraft charging were
of
Applications Technology Satellite (ATS)
events
5
{Ref.
first
the
those
Similar
1}.
were subsequently noted on ATS-6 and P78-2
for
(SCATHA)
by various observers {Ref. 2}.
Generally, negative charging events are identified by an
the measured
ion spectra
intense
peak
energy,
with an absence of ions of lesser
intense
peak
distribution
in
is
most
clearly seen on
function versus energy,
this work.
at
a
specific
This
energies.
the
plot
as will be
This peak is known as
the
of
the
displayed
charging
later
in
peak.
Such observations result from the acceleration of the
ambient plasma ions into the detector by the potential
between the plasma and the
fluxes
spacecraft.
are seen at energies below the
Occasionally,
peak
energy,
drop
ion
which
appear
would
col 1 is ionless
violate
to
conservation
energy
of
These ion distributions
plasma.
for
a
been
have
and tentatively ascribed to sputtering
reported previously,
from spacecraft surfaces {Ref.
They have been
3}.
termed
"spacecraft generated ions" {Ref. 2}.
of this particular phenomenon
Analysis
ions may contaminate the charging
as these
interest,
satellite
to an underestimation of the
leading
contamination of the charging peak may be
The
active
important
SPACELAB
experiments in low
Additionally,
.
practical
of
is
earth
potential.
particularly
orbit, such
since we believe the source to be
vehicle's
the damage to the satellite surface caused by the
environment.
coatings
as
the flux of these particles is an indication of
sputtering,
satellite
peak,
Also,
surfaces,
may
the
e.g.
expectancy
life
optical
surfaces,
be directly affected by the
precision
of
surface
and
sputtering
rate
appropriate to their environment.
B.
SCATHA SPACECRAFT
Most of the experimental data and calculations performed
in
this thesis are based on the P78-2
The construction,
environment,
(SCATHA)
spacecraft.
and sensors of this vehicle
will be discussed in the next several sections.
1
.
Spacecraft Construction
Air Force P78-2 spacecraft was launched
The U.S.
January
Charging
1979
AT
to
collect data for
High
Altitudes
a
study
(SCATHA),
of
in
in
Spacecraft
a
joint
The satellite body was
NASA/Department of Defense Program.
cylindrical
approximately
orbit
shape
in
with
spacecraft
the
diameter
and
provide
to
depicts the SCATHA spacecraft.
1
satellite in the figure
is
the
is
in
isolation
experiments from charging effects on the satellite
Figure
of
deployed
Seven booms were
1.75 meters.
from
length
a
surface.
The top
the forward end,
for
the
of
bottom
and the
The experiments and communications equipment
aft.
satellite are primarily located on equipment
The University of
the center of the cylinder.
for
decks
in
California,
San Diego, charged particle detector was on the forward end,
and
ion gun was on the aft
the
below.
a
Additionally,
lightweight
provide
between them to
sections
various materials,
The
(Ref.
of
surface
the
access
were
such as kevlar and mylar,
three
aft,
panels.
covered
for use
in
in the
4}
spacecraft
materials,
constructed
was
as
its orbit was
capability of the launch vehicle.
spacecraft
was
magnesium
aluminum.
The
solar
fiberglass
(Si02)
covered
described
aro
one forward and one
two solar arrays,
bellyband
experiments.
both
outer cylinder surface was divided into
The
general areas;
and
end;
and
arrays
outer face.
at
of
special
the
maximum
The central tube of
the
equipment
were aluminum
The bellyband
with thermal paint and second surface
decks
core
the
were
with
panels
a
were
mirrors, and
Figure
1.
SCATHA Satellite {Ref.
4}
The forward cylinder end was
acted as waste heat radiators.
coated with gold, except for some sample patches.
2
{Ref.
Spacecraft Orbit and Environment
.
The vehicle was
inserted into a near-geosynchronous,
elliptical
orbit with a perigee of 5.5 R« and an apogee
7.7
The
R«
.
period
orbit
and
hours,
23.5
was
to the equatorial plane was
inclination
degrees.
7.8
The
and
The satellite
the vehicle had a spin period of 59 seconds.
encountered 40 day eclipse periods in both the fall and
spring.
pole
.
the
During these times, the spacecraft was shadowed for
approximately
the
1
hour per day.
orbit.
Figure
gives
2
The view is from above the
a
schematic
north
earth's
{Ref 5}
.
The
environment of the SCATHA spacecraft
of two main regimes,
the plasmasphere and the
consisted
plasmasheet.
particularly
nature
of
these
composition,
is
important in determining the magnitude
The
regimes,
the vehicle's interaction with the environment,
the
(the
of
the
satellite axis was perpendicular to the earth-sun line,
of
4}
sputtering yield.
plasmapause)
is
activity and solar wind parameters.
depicted
in
hemisphere.
The
density
is
Figure
{Ref.
3.
The
of
for example,
The boundary between these
well defined,
ion
the
but varies
regimes
with
solar
The spatial regions are
view
depicts
the
northern
6}
innermost
generally
region
greater
is
the
than
plasmasphere.
10®
m~^.
and
The
the
Figure
2.
SCATHA Orbit {Ref.4}
Figure
3.
Regions of Space near Earth {Ref.
6}
temperature
0"^
and
less than
is
The source of these
ionosphere.
to be the
{Refs.
density) while 0*
quiet
During
energy
considered
is
ring
the
current.
with densities normally
the principal
times
constituent
the
with
energy
the
of
keV.
10
and He* are typically minor constituents.
indicates
source
these
of
During active times,
appears to be the solar wind.
0*
of
H"^,
6,7,8}
dominant species (93%
the
is
is
and temperatures greater than
10® m~^
H"^
20°6
ion
currently
is
thin plasma,
Tt consists of a hot,
Again,
ions
the plasmasphere
Overlapping
than
The dominant
eV,
contributing approximately
He"^
density.
less
1
ionospheric
ions
large
a
sources.
{Refs.
6,7,3}
The
plasmasheet
plasmapause,
region
the
is
Densities on the order of 10*
large
0"^
negative
1
H"^
and 10%
0"^
and
.
variability
.
This is a mixing zone,
the
is
in
origin.
in
the
eclipse.
magnetosphere,
and
6,7,8}
interesting
of the
the
At active times,
particularly when the satellite
their sources {Refs
a hot,
times,
potentials are found on spacecraft
shows the various ions in
An
He"^.
the
10^ m~^ and
particles of both terrestrial and solar
plasmasheet,
Table
90%
ratios (>50%) are observed.
containing
Large
is
-
At quiet
temperatures near 10 keV are common.
composition
consists of
and like the ring current,
diffuse plasma.
outside
feature
of
these
areas
ion composition of the plasmas
is
the
observed
during magnetically active periods.
it has been observed that
example,
contributes
0"^
much
more.
Also,
the
acceleration
adiabatic
contribution
He*
for storm
values
{Refs.
and
particles
may
Table
gives
2
current
time ring
minimum
increases,
of these solar wind
leave them with energies up to 32 keV.
typical
a
for
and may contribute
23% of the storm time energy density,
of
ring current,
In the
some
composition.
7,8}
The plasmasheet region has a direct source of 0*,
field
aligned
H'^/0"^
ratio
region (e.g.,
it
has been observed that
that of
0"^
(The Mcllwain
H"^.
6/1
in the
[Eef.
magnetic
'L'
scale is quas i -logar ithmic,
and
end
density
values
.
the high.
On
several
for
the
storm
{Refs.
time
Table
magnetospher ic
bulk
is
the
the
lists
3
9
The Kp
with
occasions,
by
general
the
of
and runs from 0-9,
has been found to reach 70%.
composition.
3
9
-
high
parameter is defined
K^ is a measure
latitude.)
G
density becomes comparable
level of magnetic activity caused by the solar wind.
low
=
L
where R is the distance in earth radii, and
R=L cos^S,
the
the
The
during periods of
the SCATHA orbit),
magnetic activity (Kp~5),
to
ionosphere.
the
of the beams has been found to be
Additionally,
8}.
3
beams streaming from
in
0"^
some
plasma
5,7,0}
Instruments
The data for this experiment came primarily from the
University
of
California,
San
Diego
auroral
particles
}
experiment,
SC-9
experiment was composed
This
.
five
of
detectors; two pairs of rotating ion and electron detectors,
Figure
and one fixed ion detector.
depicts the
4
The rotating detectors scanned in
arrangement.
detector
orthogonal
and were designated the Hi set and Lo set.
planes
detector rotated from -20 deg to 200 deg,
but it was more symmetrical,
line parallel to the spin axis.
with 70 deg along
with its midpoint on
Thus,
it
radially away from the
looked
Figure
5.
of
{Ref.
The
81
Hi
The
keV.
scanned from
1
by
5
spacecraft
The geometry is
deg.
7
64
in
the
in
4}
set covered an energy spectrum from
Lo rotating set,
eV to
2
and
the
was approximately
20*%.
eV
to
complete
a
series
a
The energy resolution at each step
Additionally, the detectors could be
ordered to dwell at fixed positions and/or
detectors
1
detector,
fixed
For all detectors,
keV.
logarithmic steps.
The
,
illustrated
energy scan required 16 seconds and was covered in
of
The fixed
The detectors had an angular
same plane as the Lo detector.
resolution
a
could not depress
more than 20 degrees below the forward end plane.
detector
Lo
The Hi detector scanned the same
the spacecraft spin axis.
range,
The
were composed
of
energies {Ref
.
three
parts;
.
4
an
electrostatic analyzer, an electrostatic focussing lens, and
a
spiraltron particle sensor.
The analyzer
provided
energy differentiation for the system through curved
and an applied voltage.
the
plates
The lens focussed the particles on
10
J3
E
a>
CO
CO
<i:
__
o
>-%
CP
rO
"^
LU <_;>
53
QJ
di
^"'-
o
o
c
5
o
'
^
_J
-
en
-O
E
CD
CO
tn
<X
=>-»
o
en
C
-Q
\
F
O)
«/1
CO
4_
o
d
c:
o>
X
o
o
^1
e^c^
a>
-n
^ca^
.«-~
>^—
OJ
c_:>
-C=
CT^
ni
_o
CJ)
O)
CZi
OJ
CD
\
CO
C^
GO
cr
UJ
5
o
_l
"~~^
Figure
4.
SC-9 Detector Arrangement {Ref.
11
4}
CO
o
o
o
-—
c
o
« eac^
-*—
o
o
..^—
rr
le:
^ ^
o
c?
CJ
ft
en
c
.<«>
CO
o
CD
^_
O
c
-e-
I
CO
CD
CD
Figure
5.
SC-9 Detector Angular Coverage
12
{Ref.
4}
•
cz
•
M»
C/3
o
O)
^_
o
CD
O
-^
one at ground
the sensor center by means of two grids,
to
counts per second.
10"^
4
{Ref.
rated
The sensor was
the other at the analyzer potential.
and
4}
Ion Gun
.
An ion gun was installed on the SCATHA spacecraft to
investigate
the
In particular,
modifying satellite potentials.
used
negative voltages
develop
to
on
the
system
and accelerated by either a
1
or
vehicle.
kV potential
2
2.0 mA
drop.
The
mA
The package also included an electron source
.
could
The
discharge
current could be varied incrementally from 0.3
beam
be configured to neutralize the particle beam
provide an electron beam.
(Ref.
in
could be
it
utilized xenon gas ionized by cathode
experiment
C.
emission
efficiency of an ion
to
that
or
to
9}
OTHER SATELLITES
Although SCATHA was the primary source of data for
thesis,
These satellites,
information.
briefly discussed
1.
satellites
other
two
in the
significant
provided
will be
ATS-6 and ISEE-1,
following paragraphs.
ATS-6
ATS-6
was a large three-axis stabilized
placed in geosynchronous orbit in 1974.
are shown in Figure
since
this
it
6
.
spacecraft
Its major
features
The large antenna was significant,
fostered differential charging by shadowing
spacecraft
measurements
surfaces.
package
The satellite had
above
the
13
an
spacecraft
other
environmental
dish.
This
UCSO AURORAL
PARTICALS EXPERIMENT
EAST-WEST SENSOR
HEAD
MAGNETOM
ENVIRONMENTAL
MEASUREMENTS
EXPERiMENT
SOLAR PANEL
{ONE OF TWO)
DIAMETER
PARABOLIC
REFLECTOR
30 FT
Y
(NORTH)
^
J
*Z (EARtH)
Figure
6.
ATS-5 Satellite {Ref. 12}
14
package contained rotating particle detector sets similar to
those found on SCATHA.
2.
I
{Ref.2}
SEE
The ISEE-1 satellite was launched in 1977 as part of
a
three spacecraft mission to investigate the magnetospher ic
With this in mind,
plasma.
absolute
minimize
conducting
and
materials
important of these,
composition
experiment
spectrometer.
{Ref.
for our purposes,
contained
which
is
an
The
plasma
the
mass
ion
10}
SPACECRAFT CHARGING
The theory of spacecraft charging will now be
differential
charging will play
later discussions.
In general,
a
a
"floating"
potential
is
deciding the potential are;
the
These
fluxes
probe immersed in
plasma
medium.
This
the
the
in
the ambient plasma electron and
due to secondary and
of
of
The primary currents
the photoelectron emission due to
half
in
a
sunlight,
backscattered
currents are schematically illustrated in
left
role
determined by the balance
currents incident upon the probe.
ion flux,
reviewed,
significant
will develop some potential relative to that
The
to
measure
plasma waves, and plasma composition.
electric fields,
as
using
by
connections
impedance
low
and
charging
The spacecraft was equipped to
several occasions.
D.
differential
to
Regardless, the satellite was observed to charge on
ground.
most
the satellite was designed
figure
15
depicts
typical
and
electrons.
Figure
7.
eclipse
^r
V.3
f-=
C)
T
>-
03 i^
CL
C£
rr
w
<
<
->£
<
—
uo
1/1
O
O
T
2" CD t_>
»—
UJ
t- J^
VJ
^
•J <
o cr
u.
UJ
h-
'n.
"<
^
z
z
o
7.
^
z
<
s:
t/1
k/>
<
//
^- rr
5
I •- 3
a <
!•>
^'
Xo
cr
«^
>- r
CO v^
UJ
<
<
Figure
o
oc
^
o
A
14.
>
——
1
'^ii?
Charging Current Schematic {Ref. 12}
16
conditions
satellite),
(negative
currents present in sunlight.
daylight,
In
photoe lectrons
material
\lA/m^
electron
dependence
which
eclipse.
temperature.
the
dependent,
has
secondary
normal
A
greater
5-50
than
energies.
in
have
a
range
and yields less than one at
higher
The point in the high
the yield decreases to one
point
and
is
called
energy
the
critical in determining the
secondary emission current.
range
cross-over
effect
of
the
The energy at which this point
and the peak yield, varies with target composition.
energy
spacecraft
ambient distributions (T<5 keV}
will
create large
secondary
greater than the ambient electron flux,
will charge positively.
(T>5 keV)
even
charging,
middle
some
in
where
occurs,
material
energy
one
and
is
material
the
this
is
and
electron
secondary emission curve will
50-500 eV),
lower
It
differential
allows
(typically
impacting
fluxes,
and the
in
is
fact
spacecraft
will produce lower secondary emission yields,
the satellite
the
High energy electron distributions
the vehicle will gain negative charge.
of
of
that
for electrons,
current is highly dependent on both
the
yield
1
The value of the
ions.
for
]lA/m-
emission
Low
is
current densities in the range
which is generally below
plasma,
and
the
This current density overshadows that of the ambient
pA/m2.
.1
half
11,12,13}
current
although
which,
saturation
typical
{Refs.
dominant
the
and the right
Thus,
the potential
determined by the high energy
17
and
electron
The threshold electron temperature for which charging
flux.
may occur
is
typically in the 5-10 keV region.
the critical energy,
additional citeria,
This
met.
which also must be
an upper energy bound on
is
There is an
distribution
the
function resulting from magnetospher ic convection processes.
It
is also
termed the Alfven boundary, and must exceed 15-20
keV for charging to occur.
11,12,13,14,15,16}
{Refs.
A simplistic equation to represent the situation may
be
given by:
(1-1)
Iri«-t:
during
Thus,
dominates,
daylight,
net
the
spacecraft),
and
positively.
+
I
when
» • <=
current
is
spacecraft
the
+
p hi o t o
positive
current
(toward
start
will
the flow of
the spacecraft will be
electrons from,
I
photoelectron
the
this happens,
As
value of In.t
Iamb
—
the
charge
to
ions
to,
and
inhibited,
and
the
When the value reaches
will decrease.
zero,
the currents are balanced and the spacecraft will remain
For eclipse
that potential until the balance is disturbed.
events,
between
Ip^oco
the
is
zero,
and
the balance
low
values
effectively
is
ambient and secondary electrons.
energy electron distribution, the potential
(+2 to +5 V),
while
for
is
high
at
For
a
low
restricted to
energies,
the
spacecraft
may
potential may reach -1 to -20 kV.fRefs. 2,11}
In
addition
to absolute
charge differentially.
of
the
spacecraft
charging,
a
That is, separate insulated portions
surface
13
may
charge
to
different
potentials.
If
environment,
then
charge in the form of
develop
will
ability to
their
a
between them.
Also,
pitch
angle
different
a
spacecraft in
Additionally,
a
of
one
portion of
the
the
may
cause
regions.
shadows
spacecraft
the shadowed area will not
photoelectrons and will charge negatively compared
emit
rest
electric
of the satellite.
Also,
in
orbits,
lower
distributions.
variations
cause
{Refs.
Differential
the
in
2,12,13}
charging
is
believed to be the
differential potential increases,
for
it
to discharge
in the
an
electromagnetic
pulse
even
areas.
cause
there will be a
form of an arc.
which
as a command signal.
is
read
17,18}
19
of
As the
tendency
This arc creates
by
the
For large discharges
be physical damage or destruction to
(Refs.
an
particle
ambient
numerous control logic upsets on various spacecraft.
circuits
to
reference
field induced in the satellite frame of
may
(E=vxB)
may
are
Frequently,
ways.
spacecraft
separate
another continually (see ATS-6),
the
portions
they will each charge to
number
impact
to
if
potential
a
anisotropies in the ambient plasma
fluxes
excess
The environment may differ over
different potentials.
of
two
if
emission
the
current will differ, and
exposed to differing environments,
range
shed
same
the
to
secondary
different
have
but
characteristics,
insulators are exposed
two
the
control
there
arcing
out on ATS-S and SCATHA
carried
Passive
active
by
and
Active trials using ion and electron guns
passive measures.
were
may be controlled
charging
Spacecraft
attempts,
such
with
as those on the
success.
some
satellites,
ISEE
using low impedance connections and conducting surfaces were
also moderately successful.
10,19,20}
{Refs.
SPUTTERING
E.
The general theory of sputtering is very significant
this
thesis,
as
postulated to be the source
is
it
Sputtering
observed spacecraft generated ions.
under
(number of target atoms sputtered per incident ion) are;
its dependence on atomic number,
table,
its
c)
b)
correlation
with
momentum
and its dependence on
such
decreased.
aluminum as
21}
and
.
that
Figure
a
e)
increased to
it
8
is
the
heat
of
its relation to the efficiency of
d)
transfer,
a)
its correlation with the
sublimation of solids,
energy,
ion
The observed features of the sputtering yield
bombardment.
periodic
the
defined
is
the ejection of material from solid surfaces
as
of
to
a plot
a
incident
maximum
and
of the sputtering yield
ion
then
for
function of incident ion energy and mass {Ref.
We see that the yield maximum increases with
shifts to higher energies.
ion mass,
These observations led
theories based on atomic collision cascades.
to
Such cascades
are created by one incident ion impinging on the lattice and
transferring
energy
to
participate in collisions.
other
atoms,
which
in
turn
These theories are based on the
20
>
c
a>
c
o
O
to
p|3|X
Figure
8..
5uuaunds
Sputtering Yield for Aluminum {Ref. 21}
21
of
classical
mechanics,
satisfy the observed dependence on atomic
therefore
momentum
and
collisions
elastic
simple
sublimation
is
explained by assuming the
binding
surface
dependence
depth,
energy
beyond which the ability of the
cascade
collision cascade theories
in
some
21,22,23}
{Refs.
detailed, and successful of the
is
the Sigmund
-
Thompson theory.
theory is actually two theories developed
complement one another
they
facets
sputtering.
of
treats
the
Thompson's
yield
atoms.
experimental results.
1
.
of
different
by
amorphous
an
The development of both
be briefly discussed,
will
explaining
by
The portion developed
sputtering
separately,
theory explains the energy distribution
sputtered
{Refs.
followed by a
these
of
the
theories
with
The Sigmund Theory
for
sputtering
is
particles.
the
yield
depending
F
target.
22,23,24,25}
deposited in the surface layers of the solid as the
and
Sigmund
comparison
The Sigmund theory focusses on the amount of
factor
some
of
optimum
postulating
by
The most comprehensive,
but
of
energy
The
particles to reach the surface decreases.
This
heat
the
effect
maximum
number
the
energy.
occurs
deposition
The correlation to
transfer.
and
yield.
is
The
then S=rF,
basic
where
expression
r
is
only on the properties and state of
a
a
the
function describing the interaction of
In order to evaluate this
22
expression,
energy
driving
for
the
constant
target,
the
two
Sigmund
as his starting point the sputtering of an
chose
amorphous
target,
and used transport theory to describe the collision
cascade
in
random media.
assumes a planar surface
theory
The
starting its motion at time
initial
23,24}
{Refs.
and position x
=
t
quantity of interest
and
is
G
(
x,a/i3
,
=
dx
v, t )d^Va
the average number of atoms moving at time t
is
with
(x,dx)
a
velocity (v's/d^Va) due to an
single atom at velocity v.
an
atom
0.
The
This
.
in a
impact
layer
by
This quantity is then used in
a
a
Boltzmann transport equation that equates the initial number
G
probability
with a later G which is dependent on the
collision.
some
of
equation
The
the
simplify
then integrated
is
variables.
functions
Two
eliminate
to
defined
are
of
to
Integration.
the
(1-2)
F(x,V(a,V)
(1-3)
H(X,V)
=
=
P
r
G(X,Va,^,t)dt
F( X, Va, V)
I
Vax d^Va
I
J
The function F is the total number of atoms which
penetrate
plane x with a velocity (v'a^d^va) during the
collision
the
cascade development.
plane
x
function
H(x,v)
is
the sputtering yield at the
for a source of sputtered particles at x
may
sputtering.
sputtering.
be
For
our
Backward
purposes,
sputtering
we will
is
This
backward
choose
backward
emission
opposite to the initial direction of inc idence
23
0.
or
forward
defined for either
=
.
{
Re f s
directed
.
23,24]
remaining expression in
The
of
is
H
expanded in
Legendre polynomials and then transformed into
terms
moment
a
equation by multiplying each term by x" and integrating over
X.
With
electronic
separated
and
{Ref.26}.
collisional
cross
cross
cross section for elastic
differential
losses
to
collisions,
nuclear
sections
may
section,
be
which
expression
This yields a final
the electronic stopping
includes
electronic
that the
are small and isolated from
electrons
the
assumption
the
and
scattering.
the
{Refs.
23,24}
next
The
step
determine
to
is
functions
the
describing the electronic and elastic cross
sections.
electronic
E"*,
constant
cross
{Ref.27}.
classical
elastic
The
mechanics,
Thomas-Fermi
used is S«
determined
is
K
section
and
section
power
a
K
where
interatomic potential.
is
the
arguments
Thomas-Fermi
using
cross
=
The
derived
approximation
from
the
of
The equation is
then
given by;
(1-4)
dCT(T)
where C
the
is
a
initial
=
C E-m-p-X-m
constant,
energy.
T
is
(3^1
the transferred energy,
The variable m comes from
and
the
E
power
approximation. m=l corresponds to Rutherford scattering, and
m=0 approximates scattering from a Born-Mayer potential.
these
values
are substituted in the
expression
previous paragraph, the following is achieved;
from
If
the
H(x,E,n)
(1-5)
3_ F(x.E,B)
=
N Cb Ub
4ii=
where
is
3
F(x,E,f3)
the direction cosine of the ejection vector,
and
function describing the distribution
the
is a
energy deposited in the solid.
Ca
is
and Ua is
the function F(x,E,3)
In general,
the binding energy.
constant,
a
of
is
found to obey the following relation;
F(x,E,0)
(1-6
where
a
M2/M1,
is a
N
dimens ionless constant dependent on the
the atom density,
is
9
.
function
The
P
T da.
a
is
is
ratio
nuclear
the
Additionally, we
the
unit
normal
in
Figure
illustrated
{Refs.23,24}
Using
all of the previous relations,
some general expressions for the
define
For low energies
S(E,3)
(1-7)
Sn(E)
in
we
then
can
sputtering
yield.
(E<lkeV), we have
=
E_
4M^M^
3_ a
4Tt2
a
=
ourselves to sputtering in
restrict
direction.
and Sr»(E)
given by Sn(E)
stopping power,
now
a N Sr^(E)
=
(Mi + Ms)^ Ua
this equation was evaluated by integrating
The Born-Mayer potential
value of m=0.
is more
with
dcr
accurate
at lower energies than the Thomas-Fermi approximation.
The
expression for higher energies is;
(1-8)
S(E,(3)
=
3
.
56
a
'
25
+
2/ 3
Z2^^3
\
)
Vfc
Ml
Sr.( C
(M1+M2
Ua
1
M
il
1
1
I
1
1
1
/)
1
1
1
i
;
M
ii
I
1
1
j
1
1
j
1.0
/
I
'
1
'
t
I
\i'
i
iii'i
r
!i
1
f
II!
/
Of
I
/I
i
i
'
1
il
/
Ii
0.5
i
i
i!!
^'!
/
!i
/
1
i
1
i
1
!
1
i
1
1
i
:
!
1
1
1
0,0
1
iMi:
!
!
10
0.1
M^/Mj
Figure
9.
a(Mi/Mi)
vs. Mi/Mi {Ref.
25
23}
The function Sr»(E)
universal
function
Thomas-Fermi
for
variable
The
24,27}.
the reduced stopping power.
is
aM^E
E
This is a
interactions
the reduced energy,
is
{Refs.
defined by
where
are
the
atomic
Zi and
Zs
(ZiZ^e^ (MX+M2)
Tabulated
number,
and a is the Lindhard screening radius.
e=
,
values for Sn(£) are shown in Table
dependence
was roughly determined to
on
dependence
This
The sputtering yield
4.
derived
was
(cos(9))~^.
be
empirically
Sigmund.
by
{Refs. 22, 23, 24, 27}
2
.
The Thompson Theory
Thompson
The
sputtering
a
theory
of
involving
the
complete
The portion
of the sputtering yield was
overshadowed
but the energy spectrum
Sigmund Theory,
determination
primary
solid with recoils at energy E2 from some
infinite
Then,
unit time,
by
Thompson started his derivation by assuming an
has endured.
event.
was
published in 1968.
determination
the
Theory
with a density of recoil
atoms q(Ez)dEi per
each generating their own collision cascade, the
total number of atoms slowing down through some energy range
E'
is
(1-9)
n(E')
=
q(E=) M(E2,E')dE=
PJ
h;
'
where v
The mean rate of energy loss is given by vdE'/dx,
the velocity.
Assuming an isotropic distribution,
def ine
(1-10)
n(E' ,r )dE'd2'
= p-
r
aiEz,)U(E^ .E'
V dE'/dx
27
)
dE^ dE
'
d^'
4Tt:
one
is
may
From this, the flux may be determined.
*(E' ,r )dE'dii'= v n E ,r cosGdE
(1-11)
in
half
'
)
dii
now assumes that the infinite
The theory
cut
'
(
and
flux
the
is
observed.
Also,
expressions are needed for the functions above.
the
number
(BEz/E',
is a
(3
constant of order
is
some
H(E2",E')
approximated
of displaced atoms and may be
where
solid
is
by
Similarly,
unity.
dE'/dx may be replaced with E'/D, where D is the interatomic
spacing.
Using these relations we obtain
(1-12)
^(E' ,r )dE'dil'=i3_D
p-q(E=)E2dE2 cos9
7?P
This function
dE
dfl'
Ti:
the flux inside the surface,
is
<!>
4
and must be
This is accomplished
transformed to that observed outside.
by assuming a binding force normal to the surface which will
decrease the normal kinetic energy,
component unaffected.
Thus,
the particles trajectory.
parallel
but leave the
the effect will be a bending of
With the energy
equations,
and
the relationship between the inner and outer angles in hand,
we come to the
(1-13)
final expression
'i'(E.0)dEd2=
In this equation,
Ua
particle energy,
and
the
surface
proportionality
4ti(
cos0
P- q(E2)E2dE2 dEd^l
l+Ua/E) ^E^Jb-
is
the binding energy, E is the emitted
B
is
normal.
to
D
the angle of the trajectory
The
E/(E+U0)^
28
chief
and
features
the
cos*
are
from
the
dependence.
Figure
10
is
a
plot of this function
versus
energy.
the spectrum will fall off as l/E^,
high energies,
For
and for
the binding energy will have an impact.
low energies,
maximum of this curve occurs at U0/2.
Figure 11 is
The
plot
a
of the measured energy d istr ibut ion, with the model overlaid.
It can be seen that up to
matches
approximately 1000 eV,
observed spectrum quite
the
model
the
closely.
Since
the
majority of the sputtered particles are in the energy region
this theory is quite effective
that follows the 1/E^ curve,
in
describing the data.
3
{Refs.
22,24}
Comparison of Theory with Experiment
.
To evaluate the applicability of these theories,
must
determine
closely
function of ion energy,
target
they
masses,
dependence
of
and
the
binding
surface
sputtering yield
energy.
the
various
calculated
and
measured yields for various
listed
Table
in
5
{Ref.
correlation is reasonable,
The solid
23}.
by
geometric
23}.
shapes.
It can
is
In
the
a
line
plot
are
addition,
materials
are
that
the
seen
be
normally within
29
the
experimental
The experimental results
for the low energy equation.
marked
in
Calculated yields
and the dotted line
the high energy result,
energy
The
contained
is
equations 1-7 and 1-8 are compared with
values in Figure 12 for copper {Ref.
is
incident and
angle of incidence,
energy dependence of the stopping power.
using
experimental
recreate
Therefore, we will look at the sputtering yield as
results.
a
how
we
factor of two.
+
>
OJ
M
uoT-i^ounj
Figure 10. The Function E/(E
30
+
\3,)^
1-0
700 "C
zee
10''
I
10'
-£.
10'
0-1
10
10^
lO-'
10'
Sputtered ion energy, eV
Figure 11. Energy Distribution of Sputtered Gold Atoms
{Ref.
23}
31
Some
questionable,
the experimental yields are
of
sputtering
yield
is
known to have
a
the
as
dependence.
dose
The
Sigmund Theory also tends to overestimate sputtering at very
low energies
indicates
(
<100 eV).
that
region {Refs
.
.
yield goes to zero in
the
50-100
the
yield is a function of
and in the factor aCMs/Mi).
eV
directly
Mi
Due to the above mentioned dose
yield data for masses are generally normalized
effects,
sputtering
sputtering yield or the argon
self
for various targets are presented in Figure
Data
data
22,23,24}
The sputtering
the
experimental
In most cases,
to
yield.
{Ref.
13
The top plot displays results for impacts on silicon.
23}.
along
the
horizontal axis and the yield of the ions normalized to
the
mass
The
argon
of
the
sputtering
sputtering
ion
AMU
in
yield on the vertical
silicon is quite good,
for
heavier targets.
The agreement
but decreases
Sputtering yields for H* and
are illustrated in Figure 14.
considered
is a
to
1.7.
target,
a
{Refs.
we
t.o
be proportional to
(l/cos8)".
variable quantity ranging from
For
the situations we will
32
in
be
will
the
22,23,24}
angular dependence of the sputtering
The
the
on silicon
the lack of agreement for heavier targets
model which we develop.
n,
0"^
for
with
Since in general we will
discussing silicon or silicon dioxide as
disregard
other
The
axis.
plots are similar for copper and gold.
theory
is
a
be
The
yield
is
exponent,
little less than
discussing,
1
with
30
1
•
25
I
M
*
Dupp
I
I
I
I
i
i
Ml
et al.
Guseva
o Alme'n et
20 -
r
1
Wehner
^
al.
Kr"*"
- Cu
et al.
~ Keywell
S
15
10
**^
-
5
-^
'•
10
11
I'M
I
III'
I
-1
10^
30
III
25
•
Wehner
»'
Guseva
i_LL
10-
I
III
et al.
Aime'n et
S
I
10'
Ar"^
al.
20
~
*
15
—
1
Southern et
Dupp
et
•
Yonts
et al.
-
Weijsenfeld
-
Cu
al.
al
10
5
TTf.'.-'^*"
10
-1
E (keV)
Figure 12. Calculated and Experimental Sputtering Yields
for Copper {Ref. 23}
33
Silicon
-
2.0
^<-
<
^
III!
T
i
1
5.0 _
^
T
•
1.0
/'
0.5
.
h
N
Andersen and Bay
•
— Sigmund Theory
C/5
0.2
0.1
.(If
1
1
1
10
20
30
40
60
50
3-0
2.0
80
70
"!
—
- Copper
3
O
3
0.7
0.5
-
0.3
-
en
•
Andersen and Bay
\
Measuring Uncertainty
— Sigmund Theory
0.2
0.1
0.
10
20
40
30
60
50
70
80
90
100
Gold
<
2,0
t
<
CO
1.0
3
<
0.5
t
•
Andersen and Bay
— Sigmund Theory
N
C/3
0.2
0.1
10
20
30
40
50
60
70
80
90
Figure 13. Mass Dependence of the Sputtering
Yield
for Various Atoms {Ref, 23}
34
+
>
o
o
o
O
o
.o
o
o
o
C
O
o
o
+
a.
>
«
03
c
+
o
o
o
o
o
(4-1
CO
o
o
lO
OJ
C\i
d
d
lO
-^
o
d
uo| jsd
p|SiA 6ujj9j|nd3
Figure 14. Typical Sputtering Yields for H+ and 0+
on a Silicon Target
35
M=/Mi
1,
>
Figure
15
the value of
n
generally taken to
is
from experimental data {Ref.
23}.
The dependence ranges up
Beyond this point, the
incident angle of 70 degrees.
to an
incidence
plot of the dependence on angular
is a
sputtering yield decreases rapidly to
yield
the
by
attempt
If we
0.
to
we will probably be
model using only the 0-70 degree range,
underestimating
1.7.
be
approximately
20^0.
{Refs.
22,23,24}
binding
depends directly on the
The yield
quite
the determination of this energy is
thus
energy,
important.
Sigmund suggested that the appropriate value for the binding
energy
generally
This
slightly
for
be the sublimation energy
would
a
material.
the
leads to sputtering yield values
that
Recent work has given theoretical
high.
are
backing
higher value, but no definite numerical quantity
put forward {Ref.
providing
29}.
numbers in Table
It has been
28}.
results
best
the
energy {Ref.
5,
are much
closer
to
calculated for Table
use
is
was
suggested that the value
17/10
the
of
sublimation
this value is used to recalculate the
If
we get the results
obvious that in general,
will
of
calculated
the
the experimental
5.
in Table
the larger value of the
22,23,24}
36
It
yields in Table
yields,
For calculations
6.
in this
binding
than
is
6
those
thesis, we
energy.
(Refs.
3.0
E = 1.05
keV
l/COSfJ
o
to
r:
2.0
to
r
30'
60-
90°
30^
Angle of Incidence
60
Angle of Incidence
Figure 15. The Dependence of the Sputtering Yield
on Angle of Incidence {Ref.23}
37
4
Limitations of the Sigmund Theory
.
theory
The
bombarded
targets
devised
was
amorphous
for
by atomic ions.
It
not
is
monatomic
so
applicable to the bombardment of molecular solids,
necessary in later calculations.
be
molecular oxygen,
of
be
bombardment
with
relevant to discuss this
type
be treated through computer simulation,
may
complicated
extensive,
cases,
it
it
is
an
is
calculated.
for
In
most
more appropriate to utilize experimental
data
available.
Such data is
often
difficult
to
Only recently has an effort begun to systematically
obtain.
tabulate
with
is
but it
as the probabilities
project,
combination must be
collision
each
the
The case of molecular ions on molecular targets
impact.
if
it will
will
as
Since several of
data points are concerned with
reference
readily
the
known experimental results and
The presently tabulated
theory.
the
atomic ions on monatomic targets {Ref. 30}.
compare
data
them
concerns
A great deal
data is also distributed randomly through the literature
of
on
other ion-target combinations.
An additional bit of information is
the
yields of monatomic molecules.
several
atoms
molecular ions
{Ref.
are
31}.
greater
silicon
they
that
is
the
sputtering
available
It has been
yield
per
for
found
for
atom
for
slightly higher than that for atomic
ions
The results are listed in Table 7.
The yields
by approximately a factor of
while
range
from
1.15
38
to
1.30.
two,
There
is
for
a
difference, but it
is
not drastic.
While it is difficult to
accurately model the more complicated situations,
be possible to obtain some approximate results
molecules
are within a factor of two
that
should
it
for monatomic
actual
the
of
results
F.
NASA CHARGING ANALYZER PROGRAM
Charging
NASA
The
developed
realistic
to
environments
magnetosphere
geometrically,
utilizes
static
steps.
in both
The
NASCAP
can
laboratory
and
that
are
calculating
timestep procedure,
a
spacecraft
of
objects
of
was
and electrically complex.
materially,
program
dynamics
accumulation from external sources,
conduction
(NASCAP)
environments.
plasma
simulate the charging,
effectively
Program
accurately model the dynamics
to
response
Analyzer
driven
are
charge
The
quasicharge
by
depletion,
and
Each timestep includes a
full
three dimensional electrostatic potential calculation.
The
calculation
staggered
in dielectrics.
of
with
particle fluxes,
Poisson's
equation over
procedure
a
in
leakage currents,
induced
spacecharge
effects
current
quasistatic
conditions.
are
which
a
grid
incident
computed
NASCAP
based
is
dimensions
of the examined object.
and
the
to
to
the
This condition is
met
NASCAP can output
39
on
limited
where the Debye length is long compared
the SCATHA spacecraft.
charged
emission currents,
situations
for
time-
is
a
time history
of spacecraft charging,
potential contours, charge contours,
current contours, and particle trajectories.
40
II
A.
OBSERVATIONS
.
PREVIOUS OBSERVATIONS
satellites,
Data taken from three
ISEE-1,
will
(SCATHA)
results.
be
prior
discussed,
Observations
on satellites prior to SCATHA will be
diffuse background)
illustrated.
ATS-5
presence of
The
first
reported
ions below the charging
for ATS-5
ATS-5
{Ref.l}.
data
eclipse in 1969 and 1970 showed ion fluxes in
of
P78-2
primary
three
the
of
and
ATS-6,
presenting
to
shadow peak,
phenomena (triangle peak,
1.
AT3-5,
energies below the charging peak.
such data is shown in Figure 16.
grey scale spectrogram format,
the data concisely.
three components.
a
taken
broad
range
of
The data is presented
in
common method of displaying
The figure is vertically separated into
The top primarily displays magnetic field
The
remaining
two sections indicate the count rates of electrons and
in the detector versus
time and energy.
defines increasing time,
the
hours
associated
(3i50
eV)
sections,
of
day
and in this case is labelled
274 of
1970.
The
and
vertical
the energy axis is located
energy increases upward for
41
ions
The horizontal axis
with the energy of the particles.
of
in
A typical example
unnecessary for our purposes.
information,
a
was
peak
The
between
axis
with
is
minimum
the
two
electrons,
and
ill
« E njt
'mimmitmmm
SO
-:=:^
Q. N OEN
» CD*
«
n
ELEcraoNSi
12000
E^€P^Gr
IN EV
"WWB 120X3
.^.....„,...„....^....,„... .„....„„. .„.„.„...^.„....^....._^..„._^....^_..„„.,_^
a
1
HOURS IN DAT 274 OF 1970
I
Figure 16. ATS-5, Spectrogram, Day 274 of
1971
42
1
downward for ions to
a
maximum of 50 keV.
detected countrates
the
is
The magnitude of
indicated by the shading of
the
figure from black to white, with black corresponding to zero
white
and
counts,
saturated
at
spectrogram
maximum.
certain time in
a
will
contain
a black
the
If
an
energy
spot at
detector
channel,
that
time
is
the
and
feature such as this can usually be distinguished
energy.
A
from
zero
a
with
count
rate
saturation
because
points
are
revealed
as
The data displayed in Figure 16 are from day 274
of
normally
surrounded by intense count regions,
bright white areas.
The satellite enters a region of hot plasma at 04 5 0,
1970.
as
indicated
(>4500
seen
eV)
by
the increased intensity
electrons.
between 0620 and 0720.
can
high
energy
The satellite charging peak may
the intense white region in
as
of
the
proton
be
spectrum
From the energy of this region,
infer a satellite potential of approximately .75-2
we
kV.
The black spots on the peaks are an example of how
detector
saturation
ions
evident
is
in the
displayed.
The broad spectrum of
energy bands below the charging peak as
grey area that clearly extends from
1
kV to about 300 V,
may even continue to lesser energies.
not
the
and
These particles
did
so the source
was
to determine and no complete explanation was
put
appear during every charging event,
difficult
is
forward
43
ATS-6
2.
also
ATS-6
contamination
of
observed
were
potential.
at
non-ambient
showed evidence of
source
Secondary or shadow
the ion data.
energies below
An example of such data
satellite
inferred
the
peaks
illustrated in Figure
is
This figure is a grey scale spectrogram similar to the
17.
The data presented are
shown in Figure 15.
one
from
the
The first is
the
North-South detector on ATS-6 on day 243 of 1974.
This plot displays two phenomena.
which are clearly presented elsewhere {Ref. 2}.
peaks,
ion
ions were denoted "spacecraft generated
These
The
ions".
second is an apparently analagous phenomenon in the electron
In addition to the electron and
spots".
"Minnesota
These observations are the so-called
spectrum.
ion spectra,
thi.^
figure displays the detector pitch angle in the top section,
which
related to the rotation angle.
is
between
The spots are seen in the electron spectrum
100
and 300 eV as intense white points that appear to
triangle
shapes.
If
one compares the periodicity
form
the
of
electron triangles with the detector look angle periodicity,
a
correspondence
direct
this time,
secondaries
electrons
charged
evident.
As the
where the detectors are located,
spacecraft,
at
is
with
the
surface
been emitted from
respect
to
the
44
of
a
the
was in shadow
detectors,
to
vehicle.
surface
the
of
the source electrons were determined
from
had
top
be
These
differentially
and
thus
the
2CQ-3aE-2i
a3P:l.2
085=0050 StPE: 3 P»«: 2 NSc
1-0
^-380,
_ „„
3SCi C3Mr 124373210 Sfte -20 LNG;2I5
9u
-
*
^
:0QA
h
h
—H
3x
S
n
/\
/\
l\
;'
H
rr
.
A
'
'
•\
:
•
'
\
^
=tc
/v
-5C-
^v
yV
?*.'!'.
Vv
^
,
',
--90
^
.
Vv
VV
vv
v\i
vy
Vv
Vv
v^/
^^-f
r-C
,
WyH[^j|i|,i
l»Xip[|.pi «JMp!jf;W.p
UCSO.R'5-S
NU
I
\
\
QflY
248 OF 1374
^
W,A
i.l
^^^^^^^
f
I
|"Hip iniii y wii w
i
fui'
n
V z/m
of 1974 {Ref.2}
Figure 17. ATS-6, Spectrogram, Day 248
45
charged
respect
with
distribution
emissions
detectors
{Ref.
detectors,
the flux was eventually
of
secondary
the
and
thus
been accelerated by a potential
had
source
The
to
from
also located on the satellite
drop
determined
University
the
the
<t>
to
be
Minnesota
of
measurements
box.
2}
3.
I
SEE
Negative charging events on the I3EE satellites were
relatively
rare,
the
as
satellites
specifically
were
designed to avoid the problem of spacecraft charging.
1
did,
ISEE-
charge to significant negative levels on
however,
During one such event,
few occasions.
analysis of the ion
mass composition revealed some interesting features.
18
is a
Figure
presentation of the ISEE-1 mass spectrometer reading
from March 17,
The vertical axis is a count rate, and
1973.
horizontal axis
the
a
is
the applicable
channel.
mass
The
mass channel is related to the atomic mass by.
AMU
(2-1)
=
3615
Mass Channel^ -^"^^
Thus, mass channel 54 corresponds to
0*.
{Ref.
0533
with
(Universal
UT
a
corresponding
channel
54,
H*.
Time),
instrument
the
broad peak in the high Atomic
(AMU=30-100) channels.
10
and 14 to
15 to N*,
10}
At
responded
H"^,
to
This peak had a maximum in
32 AMU.
There was also
The normal value of the
46
Unit
Mass
a
ambient
channel
peak
at
atomic
|Iillll
I
1
|IIIU.IJ
1
pillll
j
I
|I1IIJII
1
J
-
o
CD
•«
+
•• •
X
•
•
• « •
•
IT)
to
• •
• •
•
o
• • •
• • •
ICL
mX
• •
•
«
o
• • •
too
•• •
•
to
t
I'CL
o
••
c\j
•
• •
O
o
O
I
<
LO
LU
a.
Z
2
<
X
o
<
X
O
LLi
LU
C^J
o
CM
• • •
•
• •
CO
00
CO
C/D
ir>
C/5
<
<
lO
• • •
•
—
I
lllllli
o
o
o
o
<
X
o
CO
CO
<
UJ
-
• •
CN4
CO
CO
CD
CO
• • •
00
CO
8x
•^
3
<
.
LLI
• • •
I
I
I'lilITT
t
\
Hilll
o
o
o
o
o
I
I
o
o
seLS'o/siNnoo
Figure 18. ISEE-1, Mass Spectrometer, March 17, 1978
{Ref.
10}
47
oxygen
indicated by the solid black line under the
is
except that at this
The data from 0721 UT are quite similar,
point the maximum has shifted to channel 11,
larger than the indicated ambient oxygen.
is
time
these
that
these molecules were probably hydrocarbons
observations were first made
products.
sputtering,
light
In
however,
one
of
At
concluded
was
it
spacecraft
and
different
may postulate a
The masses of these are 28,
0-.
respectively,
numbers
observed data.
{Ref.l0}
flux
which
would
fit
IG,
Si,
and 32
AMU
into
the
nicely
P78-2 (SCATHA)
B.
Observations of spacecraft generated ions are
five
for
events
1979,
for
the second pair are for
eclipse
ion
gun
presented
charging
induced
and the last is for eclipse charging in 1931.
Day 83 of 1979
1.
This
peak
The first two are
days.
in
charging,
first eclipse event illustrates
and its associated angular dependence.
1979.
This
is
a
the
triangle
Figure 19
is
one hour period of day
S3
composite of the electrons from the
Hi
grey scale spectrogram for
detector,
the
surface
the ejected particles would largely be
dioxide (SiO-),
of
other
and
Since the surface is primarily silicon
spacecraft surface.
the
the
That is, these particles are ions emitted from the
source.
0,
Again,
AMU.
28
this
outgassing
peak.
a
and the ions from all three detectors.
ion displays has zero energy at the top
and
Each
of
increases
energy
The electron presentation
downward.
the triangle structures can be seen in the Lo
1740 and 1820,
that
From the Hi detector data,
data.
the
at
v
and Increases rapidly to
Multiple shadow peaks are
V at 1825.
300
seen
can be
it
spacecraft charges negatively to about 200
decreases to 100 V at 1815,
1750,
Between
increasing upward.
with low energies at the bottom,
detector
reversed,
is
during
visible
this period in both the Hi and Fix detector data.
Figure
function
is a
plot of the
log
distribution
ion
space density) versus energy
(phase
The
this
shadow peak
approximately
parallel
Figure 21 is
triangle
peak
a
parked
is
the
to
at
eV.
At
spin
diagram of the spacecraft
energy,
250
degrees,
92
spacecraft
at
(f)
the secondary peak at 78
detector
the
time,
is
and Lo detector look
Hi
in eclipse,
indicated by the peak in phase space density
eV.
the
for
The spacecraft is charged to -250 V,
detector.
as
20
axis.
potential,
angle
versus
time.
The energy of the peak varies directly with the look
angle,
with
maximum,
(where
the
that is,
it
energy
minimum occurring
angle
the
at
looking down and away from the spacecraft
approaches closest to the
spacecraft
surface).
The shadow peak energy maintains a relatively constant ratio
with satellite potential.
and
7,
In
this case, Ep/Et
where Ep is the satellite potential,
triangle peak minimum energy.
is
between
and Et is
This is significant,
49
a^
S
the
we
10000
10000
>
OS
Hi IONS
1
w
10
100
1000
Lo IONS
1
10
17^0
Fix IONS
18^0
Figure 19. SCATHA, Spectrogram, Day 83 of 1979
50
>
0)
cr
UJ
LU
o
o
o
o
00
h-
to
in
o
^
o
o
ro
C\J
Figure 20. Day 83 of 1979, Ion Distribution Function
51
(A3)
ADa3N3 MV3d
o
O
O
O
O
C\J
ro
"T
lO
CO
O
K
o
O
OB
a>
6aa )e
Figure 21. Day 83 of 1979, Vehicle Potential, Triangle
Peak Energy, and Look Angle vs. Time
52
would expect
constant
2.
a
differentially charged surface to maintain
ratio
time with
over
mainframe.
Day 86 of 1979
second
The
similar
that
to
charging
eclipse
of day 83.
a
day
event,
Figure 22
spectrogram of the ion data for
of
satellite
the
is
fluctuation
in
electron
from
From the Hi
data
Alfven
boundary
a
period,
the
throughout
the
the satellite potential due to a
achieves
spacecraft
During
widely
a
36
visible
we see that charging begins at approximately 1640,
varies
is
scale
one hour period on day
1645 to 1740 in the Lo detector display.
potential
36,
grey
a
The triangle structures are again
1979.
the
a
and
the
period.
A
change
in
1705.
The
maximum potential of -500 V at
1725.
is
seen
at
clearly
shadow peak is
visible
at
energies lower than the charging peak in both the Hi and Fix
detector data
Figure
23
the charging peak.
the
charging
Frequently,
spectrograms,
between
There is
In addition,
100 eV,
peaks.
the
Hi
The satellite potential is -340 V as indicated by
detector.
below
plots the ion distribution from
well defined shadow peak
there are various other sm.all
peak that may
shadow
other
maintaining
each other,
a
a
indicate
peaks
constant
other
appear
separation
and varying as the main
peak
at
peaks
shadow
in
the
ratio
varier;.
The detector is again parked parallel to the spacecraft spin
ax is
.
53
10000
1000.
100 _
^'-
l^W!^
5
---*'-4:f
^^
sum
10
•--'f- -+**:-:''•
KT:f^
:•'*--''---
^-•r
'--
^B ^^^^g
i^^niMHM
Hi ELECTRONS
10 ^
l!"!.:->*^
100 .i,#:r:
1000
10000
Hi IONS
9
(X
1
10
.
-T
.1'.
.«-^i;
•;
*..^,.
100
:^
tr.
1000
Lo IONS
10
V*^if-
1635
Fix IONS
1735
Figure 22. SCATHA, Spectrogram, Day 86 of 1979
54
o
o
o
O
CO
N-
CD
lO
O
^
o
o
lO
CVJ
Figure 23. Day 86 of 1979, Ion Distribution Function
55
minimum
dependence between
strong
The
triangle
the
illustrated
energy and spacecraft potential is
It
minimum
increases
the triangle peak
of
increasingly negative vehicle potential.
is
maintained between
shadow
achieving
minimum
extent.
The
the
the energy of the
detector
angle,
energy at the detectors maximum
angular
directly
varies
peak
with
The ratio, Ep/Et,
Again,
and G.5.
6
is
by
clear that the
Figure 24 for this more variable day.
energy
peak
shadow
with
the
peaks usually lose
definition
their
below 110 degrees.
These two
during
eclipse
occurred.
passages
comparison
By
observations
similar
events.
ATS-6,
with
a
charging
negative
which
for
during
however,
solar array to provide
an eclipse.
observations
examples illustrate typical
ATS-6,
daylight
we
expect
would
negative
charging
was stabilized and had its large
shadowed area which would
3CATHA had no such array,
simulate
and in addition was
spinning so that no area was shadowed long enough to
charge
Observations of daylight charging
showed
to a high level.
that
spacecraft potential was highly
the
making
However,
it
difficult to use these data
for
spin
this
modulated,
project.
induced charging events with the ion gun provide an
additional
range of daylight data.
experiments are presented.
55
Two examples
of
such
x
-200
X
X
X X
>^X
X
-300
XX
X
SPACECRAFT
POTENTIAL
>^x
^x ^
SCATHA
UCSD
LO DETECTOR
x\
DAY 86 OF 1979
CO
T
X
-400 h 1717-1725 UT
X
>$<>$<x
X
X
-500
30
45
>
<u
~
f
'
>-
SHADOW
X PEAK
ENERGY
O
\
(T
UJ
•
UJ
<
UJ
Q.
in
01
<u
o>
Q
<D
-20
1717
1720
1723
TIME
Figure 24. Day 86 of 1979, Vehicle Potential,
Triangle
Peak Energy, and Look Angle vs. Time
57
Day 94 of 1979
3.
The first example of ion gun induced charging is day
1979.
94 of
Figure 25
is
that
from
Fix
1
kV and
the grey scale spectrogram for a
1
hour
The
day.
detectors,
the
The ion gun was in operation at
presented
data
with the Lo detector in the upper
lower.
the
in
The
gun
ion
period
two
ion
diagram,
and
from
are
turned
is
mA
1
on
at
approximately 1405 and the spacecraft charges promptly to
final value of -G0 V.
a
Shadow peaks occur at 11 and 20 eV in
the Lo detector between 1420 and 1430,
and between 1440 and
1455.
curious feature
A
is the
appearance of twin shadow
peaks in the Lo data at 1440, with bands between in which no
ions were detected.
been
just
had
spacecraft
detector
When this phenomena began, the detector
parked at an angle of
spin axis,
had
11
eV,
degrees
away from the satellite
previously
printouts are examined,
20
been
scanning.
to
body.
Txhe
the
data
When
three bands clearly emerge;
another at 22 eV,
and the last at 30 eV.
infer that certain energies,
or trajectories,
the
one at
We
may
do not reach
the detector at that angle.
These events indicate something
about
emitting
the
striations
non-d
i
f
location of the
in
the Lo data,
surfaces.
From
the
we would expect non-emitting
or
erent ially charged surfaces to be sandwiched between
emitting regions, thus giving the dark regions where no ions
are detected.
Importantly, the Hi detector is viewing in
5
a
C4^> ADH3Na
(AS) A3HaNa
Figure 25. SCATHA, Spectrogram, Day 94 of 1979
59
direction
similar
reports
Hi,
a
exists.
completely
indicates
This
similar
a
viewing 70 degrees from the Lo
different
however, show a similarity in that
does,
gives
and
time,
this
The Fix detector,
response.
and
at
environment.
large energy gap
a
differentially
mixed
It
non-
and
different ial ly charged surfaces as above.
Day 200 of 1979
4.
Our fourth example is during an ion gun operation at
1
kV,
1
mA
.
Figure 25
timeframe.
hour
In this
700
V,
is
a
grey scale spectrogram for
The data are taken from the Hi
a
detector.
figure, the spacecraft is charged to approximately
except during the two five minute periods when
ion gun was
in
Trickle mode
"trickle" mode.
defined
is
the gun in operation with the accelerating voltage off,
current
still applied to the diode.
visible in the ion display.
Thus,
Ep/Et,
indicating
for
these
figures is in
that the Hi detector is looking at
9-10
an
the
as
but
clearly
detector.
the
-
generated
This day is one of the few
these peaks become evident in the Hi
ratio,
the
Triangle peaks are
ions leak out at low energies.
which
two
in
The
range,
emission
point different from that seen by the Lo detector on days 83
and 86 (or different processes are at work).
Figure 27 shows the distribution functions when
Lo
and Fixed detectors are looking approximately
radially away from the spacecraft.
its
maximum rotational angle.
60
the
parallel,
The Hi detector is near
The satellite potential
is
T^
<c
O
u
o
o
o
}-
O
o
9*
o
O O
CV4
O"
o
<N
'
(A^) ADaaNa
Figure 26. SCATHA, Spectrogram, Day 200 of 1979
61
—
ION DISTRIBUTION FU NCTIONS
8.0
1
1
\
I
II
I
1
I
I
~I
I
I
I
I
I
I
I
I
SCATHA
UCSD
DAY 200 OF 1979
'
X
2208 30-2208 44
KV
ION GUN
=
=
^ X
7.0
1
DETECTOR
HI
<o
e
mA
1
X
172'-I91'»
'
en
«.
6.0
o
XX
BACKGROUND
COUNT/ACCUMULATION
5.0
,^
v
X
„X xx
8.0
/
4.0
7.0
;r
£
cn
6.0
X^
X
o
X
7.0
-
LO DETECTOR
^ - 150*-138*
-X
\x
6.0
uj
X
L
4.0
X
£
Xj^
X
X
BACKGROUND
"*"
xx \/
5.0
^nIx
o
4.0
'
10
'
t
5.0
I
r
I
I
I
_L
I
L_J
1
I
1
I
I
1000
100
ENERGY
"^
X
(eV)
Figure 27. Day 200 of 1979, Ion Distribution Functions
62
-700 V.
approximately
The Hi and Fix detectors exhibit
but such a spectrum is
broad array of low energy ions,
The Lo detector does
in the Lo detector.
evident
throughout
period,
this
not
show
The data have a similar
sharply defined shadow peak.
with
variations
subtle
amplitude and energy of the diffuse
spectrum,
a
a
form
the
in
particularly
with angle
Day 92 of 1981
5.
Figure 28
is the
eclipse charging event.
from
the
The figure displays
detector for both
Hi
charging
eclipse
event
is
between 0810 and 0945 UT,
to -400 V.
grey scale spectrogram from
visible
29
is
functions for day 92.
a
a
plot
is
Thus
the
of
the
ion
a
detected
countrates
are
detected
as
86
high
to
the
observed
phenomena
63
1979.
those
as
This indicates that
particles
is
not
satellite
a
This
of
the
dependent
satellite lifetime, and hence that outgassing is not
contributor
is
distribution
value of 6.07.
comparable to those on days 83 and
the
illustration
The log distribution function has
encountered early in the mission.
source
the
The
definite shadow peak
of
charging peak at 0820 at 90 eV with
value
in
electrons.
-60 to -150 V region.
in the
Figure
and
hours of data
with potentials varying from -100
During this period,
also evident,
ions
4
1981
a
on
a
major
generated
ion
Figure 28. SCATHA, Spectrogram, Day 92 of 1981
64
11.0
10.0
9.0-
8.0
s
7.0ao
o
6.0
5.0
J
4.0
0.1
1.0
100
10
1000
10000
Energy (eV)
Figure 29. Day 92 of 1981, Ion Distribution
Function
65
Ill
A.
POSSIBLE ION SOURCES
diffuse
the
observations have
ions
potential.
Thus,
spacecraft
where
common
in
is
that
spacecraft
the
by
they do not experience the
outgassing
emission
from
There are two prime candidates for the
Outgassing
sputtering.
and
the satellite of contaminant
primarily
atmospheric molecules.
the inclusion of
plasma-
full
is
the
molecules
and
with energies near the temperature of the
are
the
they must be generated in regions near the
source,
These
peak,
the appearance of the subject
below that provided
satellite potential.
atoms
feature
The prime
spectrum.
energies
at
the shadow or mirror
the triangle peaks,
phenomena,
and
different
ion source must be able to explain three
The
by
DISCUSSION
.
trapped
hydrocarbons
satellite.
and
other
The outgassing rate may be increased
efflux from the attitude control jets,
Sputtering
emission
of
charged and neutral atoms from the satellite surface due
to
and
ion
the
ion beam system.
bombardment,
previously.
follows,
of
ions
theory
its
has
the
described
been
A qualitative discussion of both possibilities
followed by
that
and
is
a
numerical calculation of the
would reach the detectors
due
to
number
Sigmund-
Thompson sputtering from differentially charged surfaces.
66
1
Outqassinq
.
contaminant particles leave
the
As
they are subject to ionization from photons,
ions and electrons.
ambient
with
return
and collisions
molecules
the
from the spacecraft they
at various distances
ionized
As
in a broad range of energies.
This is
how many particles outgas,
then,
ionized?
these are
problem
The
and what percentage of
outgassing
of
The dominant mechanism for the ionization of the
been
has
photoionizat ion,
electrons.
and
determined
in
eclipse
to
in
and
outgassing
to
collision
be
and
the
probability,
slightly
collisions
in
electron
the
eclipse,
become
dominant.
the
spacecraft
all
as the electron density will
increase
The ionization rate, however,
{Refs.
will be the width
of
the
2,32}
returned
distance within which the ions may be
plasma sheath.
is
In
increase
will not exceed that of the photoionization.
to
rate.
will
ionization rate
due to space charge effects.
The
with
the photoionization rate goes to zero
however,
electron
be
approximately
is
two orders of magnitude less than the photoionizat ion
In eclipse,
the
32,33}.
sunlight
electron ionization rate
The
will
The question
outgassing yield has been studied previously {Refs.
molecules
are
possible
a
explanation of the observed diffuse spectrum.
is
spacecraft
the
surrounding
The sheath distance at geosynchronous orbit
approximately 100
With this
m.
ionization rates above,
it
information
may be shown that, at
67
and
a
the
maximum.
approximately
.02%
ionized.
a
In
outgassing silicon
of
central force field,
atoms
will
be
with conservation
of
calculations reveal that
all
the particles ionized in the sheath will be returned to
the
momentum and energy,
angular
spacecraft.
The
is
that .02%
32}
When the spacecraft is first launched,
number of particles at the detectors.
extremely
an
dependent
time
ionized
the
significant
flux may be sufficient to provide a
outgassing
is
the
of
satellite.
molecules vill be reattracted to the
outgassing
{Ref.
final result then,
Outgassing, however,
phenomenon.
Once
the
so
the
leave there is no way to replenish them,
particles
outgassing
must
rate
decrease
with
Various
time.
experimental data have placed the outgassing as proportional
where n varies from 0.8 to 1.6,
to t~",
to exp( -t/41
e-folding
.
6
)
with
,
t
in days
{Refs.
or as proportional
The longest
32,33}.
Even
time then would be 41.6 days.
would
this
lead to a decrease in the outgassing flux by three orders of
magnitude over the period of
1981 above,
over
time
conclusion
however,
in
a
year.
As noted
day 92
in
the shadow peaks have not
any significant
way.
This
that the outgassing flux may be
diminished
leads
a
of
to
the
contributor,
but is not the prime source of the ions.
What
this
SCATHA
effect
conclusion?
do the thrusters and ion gun
An experiment was performed
spacecraft to measure the contamination
68
have
aboard
on
the
(impurities
mixed with the hydrazine fuel)
thruster
operation.
on the satellite due to
sensors
Two
were
placed
the
the
on
satellite, one in the bellyband, and one on the forward end.
The
were capable of detecting
sensors
and a current as low as 10"^^ A.
deposition
particle
ng/cm^
5
were located on the aft end of the vehicle,
The thrusters
and were
once per week during the bulk of the flight time to
for
that
precession.
measurable
no
flux
gun
ion
contaminants
is
indicated
was
{Ref.34}
also
unlikely
an
the detectors.
to
correct
contaminants
thruster
of
returned to the spacecraft.
The
experiment
The results of the
fired
noted
As
located on the aft end of the spacecraft
source
above,
it
and directed
of
is
away
During most normal modes of operation, the energy
from it.
and angular momentum of the exiting particles would prohibit
them
from reaching the detectors.
some
ions
but
satellite
mode,
at low energies,
may
be
operation
continuous
are
potential
reattracted,
contribute to the flux observed at the detectors.
probably
they
that
There is a possibility that ions emitted during
35}.
trickle
detectors,
at energies greater than the
observed
{Ref.
returned to the
are
There is evidence
only
a
small
of
the
ion
or
frequent.
intermittent
gun
in
Thus,
the
the
component,
above
mode
outgassing
primarily composed of the emitted contaminants.
69
This
and
is
as
the
was
not
flux
is
With just the emissions of the contaminant flux,
outgassing
still numerically conceivable that the
is
particles
sufficient
produce
could
observations.
However,
it
Additionally,
data.
the
it
flux
the
for
outgassing
certain that the
is
and this is not observed
does decrease over time,
flux
account
to
focussed
into
a
detector.
Moreover,
should
such that
be
beam one energy channel
it
is
the
vide
in the
the
at
detector
the
angle yields such a definite change in the observed
as
flux
unreasonable that the focussing
small change in
a
in
hard to conceive that
is
satellite potenials are such that the randomly ionized
is
it
phenomena of the triangle peaks.
look
energy,
In sum then,
it
does not appear likely that ionization of outgas products is
the source of the observed flux.
2
Sputtering
.
When this work began,
As the work progressed,
the ion source was outgassing.
became
initially assumed that
it was
apparent that there were serious drawbacks
assumption,
as
noted
above.
advantages
While
it
in
this
searching
for
alternatives,
the
became clear.
First, besides a small dose dependence, there
is
of sputtering
as
no reason for the sputtering yield to decay
This
attribute
significant
emission
charged
of
was
very
important
shadow peaks observed
sputtered
surface
would
particles
lead
to
70
in
from
over
light
in
1981.
a
observed
source
a
of
Second,
time.
the
the
differentially
beams
at
the
detectors.
various
the particles left the surface
As
initial
accelerated
angles
energies,
and
they
orbit.
following
specific trajectories would reach
Thus,
for each
energy,
limiting
observation
channels
have a resolution of 20 %,
channels
increases with energy.
angles.
channels.
the
detector,
observed
data
a
energy
the bandwidth
a
8.
which
peak
energy
lower
narrow peak in the
It seemed
"secondary"
SCATHA
it could
also
spots
reasonable that if such
could occur for secondary electrons,
essentially
high
The similarity between the
and the data from the Minnesota
became evident.
the
of
low and high energy channels of the
The
detectors are listed in Table
were
particles
Therefore,
can be compressed into
almost
an
detector
Since the
be
only
appear broad when distributed over the
channels,
their
would
by the existing electric field into
parabolic
would
with
a
process
occur for what
Additionally,
ions.
the
sputtering theory could explain the diffuse spectrum as
particles sputtered from
receipt
of
charged
surface.
a
non
Without the charging,
the
differentially
the ions are
not
focussed into the higher channels, and can appear as a broad
low energy distribution.
peak,
can be described by two mechanisms.
motionless detector,
one
The third phenomena,
trajectory,
not scanning in angle,
i.e.,
energy.
one
the
shadow
The first is
and
a
observing
Alternatively,
the
detector could be scanning laterally across an emitted beam.
71
as
and thus be picking up the
opposed to vertically,
Figure 30 illustrates the geometry.
energy at each angle.
The problem with sputtering as a source,
was that it
never seemed to be able to provide an adequate flux
had
of
However, the only previous mention of sputtering had
ions.
been prior to the advent of the Sigmund theory,
it
same
acknowledged
was
that
and
magnetospher
the
at this point indicated that the
regions
ic
Preliminary
contained large numbers of oxygen ions {Ref.l}.
calculations
before
due
flux
to
oxygen sputtering was sufficient to account for the observed
phenomena.
determine
It
remained to model the
then
situation
more accurately the flux at the detectors due
and
to
sputter ing
It
was
characteristics
spacecraft.
with
now
necessary
evaluate
to
of the environmental
interaction
with
the most likely target of the
ions would be amorphous silicon dioxide.
time
period,
ion flux
the
primarily of oxygen and hydrogen.
the
reasonable to expect
heavier ion.
See,
a
incident
For a magnetically
would
be
composed
The Sigmund theory has a
strong dependence on the mass of the incident ions,
is
the
As the bulk of the vehicle surface was covered
solar panels,
disturbed
physical
the
and
much larger sputtering yield
for example.
Figure 14 for H* and
it
for
0"^
on amorphous silicon.
The next step was to use the equations
8)
to calculate the yield.
(1-7)
and (1-
One roadblock to this was
72
the
Detector
Scamng
Vertically
Through Energy
Detector Scanning Across Angle Receiving
Constant Energy
Figure 30. Detector Beam Viewing Geometry
73
inability
theory
Sigmund
the
of
available for impacting molecular oxygen on
{Ref.36}.
we
see
If
they
and
dioxide^
silicon
and
treat
There are some
sputtering from mult i -atomic targets.
points
adequately
to
listed
are
the data from Table
7
data
silicon
Table
in
9
are applied to Table 9,
that the sputtering yield for
silicon
on
0"^
will
yield
on
silicon dioxide will range from .23 to .44 atoms per ion
at
range from
10
.1
for atomic oxygen on silicon is too high,
accurately
however,
approximates
the yield
we will
To do this,
SiOz.
Sigmund
and
will,
and calculate
the
increase the binding
from
two yield curves are displayed in
The
more
We
in line with those
to give us values more
experiment.
from
the worst case approach,
take
yield for silicon.
energy,
the
The sputtering yield calculated from the
keV.
theory
Similarly,
10 keV.
.18 at
to
the
Figure
The new binding energy is 26.0 eV.
31.
Given
that this new curve now reasonably
the problem is to calculate how
the sputtering yield,
particles emitted from a surface will reach
charged detector for
describes
a
a
many
differentially
realistic incident flux.
The
total
yield of sputtered particles will be described by
(3-1)
In
this
particle
Y
=
r^
s(v,e) V f(v) d^v
expression,
can have,
transferred.
This
r
is
the maximum energy
sputtered
the maximum amount of
that is,
maximum
a
is
74
calculated by
energy
+
Ci3
00
o
+
o
•O
o
o
o
>^
!^
0)
;^
>
w
CO
>
c
Ul
•
I—
o
o
o
;^
0)
a
00
o
o
CO
d
d
d
d
uoi |U9piou| jsd p\Q[X 6uij9|,|nd3
Figure 31. Oxygen-Silicon Yield Curves
75
r
=
M2
M:l
4
Em,
(Mx+Mz)^
and
equivalent to the maximum energy transferred
is
The function f(v)
collision.
binary
and S(v,9)
function of the incident ions,
y ield
the
is
a
distribution
the sputtering
c
ve may now attempt to
With the total yield in hand,
The total
calculate the countrate at the detector.
of
is
in
described
the detector receives may be
counts
number
by
the
following equation.
Counts=Pdt PdAPf
(3-2)
This
area,
/N
(V) (\?.n)d^v
differential
that the detector is
assumes
and energy.
solid angle,
If we
time,
in
integrate over
the
area increment and time, we get,
Counts = T
(3-3)
Thus,
pdii
(3A
the countrate
pv^
rr
f
(
v
)
(
v-
ri
)
dv
is
Countrate = Counts/Time=3Arrv^f(v)(^. n)dvdil
(3-4)
If
we must assume that limits of
To continue,
are
not
a
function of energy,
and that the value
integration
This is not actually true,
depend strongly on energy,
fill the entire
5
by
7
of
the
characteristic of
integral may be given by a simple number,
the detector.
integration on d£
as the limits on
since f(v) may
not
degree aperture of the detector.
The
76
inaccuracy of this assumption will surface later.
With the
assumption, ve derive.
Countrate=CR = (5A 62
(3-5
fv^
f
(
v
)
(
v-
h dv
)
r
constants,
two
The
geometric factor
detectors,
G,
G = 1.6
combined
into
the
constant for the detector.
For
our
and
(3A
a
10~=*
*
are
Due
m= ster.
aperture of the detector,
is
6Sl^
to the differential
the value of the dot product v- h
approximately v (i.e. cos 9~1).
We must now determine the distribution function
From the earlier discussion of
the emitted particles.
Thompson Theory,
it
is
the
evident that the energy distribution
flux is proportional to E/(E+Ua)^.
of the yield
of
That is, we
have the function
dY
dE
(3-6)
E
(E+Ub)^
The flux Y,
is
(3-7)
=
Y
A
=
also given by
p« E
B
f(E) dE
J
where
energy
f(E)
the distribution function as a
is
and B is a constant
of the emitted particles,
involves the integral over the solid angle.
these
two
relationships,
it
constants) that
(3-8)
f(E)
=
1
dY
E dE
=
1
(E +
Ub)^
77
function
is
evident
If we
of
that
combine
(disregarding
normalization
We will now determine the appropriate
From
constants.
f
definition
the
distribution
the
of
unct ion.
(3-9)
-r.
d^v
we assume that the yield
the calculation,
Continuing
function
distribution
the
and
f(v)
dependencies.
Thus,
E
(
v, 9
)
=f v cose
(
)
.
angular
similar
have
If we
flux
break down d^v,
and substitute for energy, we get
(3-10)
n = APd0Psin(e
cose depf(E)i2Ei^ ^^
1
In the case of emission from a surface,
to
2x1,
and
9
will vary from
k/2.
to
range from
* will
Integrating
over
these limits, the equation reduces to
(3-11)
dE
n = ATt(2/m3)^
(E
J
This
integral
energy
1
extend the full energy
will
emitted particles,
Ub)^
+
that is,
transferred
energy to
from
substitutes a dummy variable, x^,
standard form {Ref. 37}.
n=ATt(2/m3)^
for E
If
dE
P''
(E
J
(X^
+
U.
n=ATt(8/m3)^
dx
+
78
Ub)3
s
of
the
the maximum
one
then
integral has a
the
Solving, we then get
Ja
<a
r,
collisions.
the
in
range
=
r'
n = ATi(8/m=)^
{(-x)/(4(x2
(l/8(Ua^)^)
+
tan-=^
n=An:(8/m^)^ {(r^/(8(U0
f3
)
^
x/(8U0(x2
+
)
(x/(U0)^)
T))){l/U<^
+
+
Ua
)
^
}-a
2/(Ue,
-
+
D)
(l/(8Ua^^^)) tan-^ (r/Ua)^}
+
Defining
U0
+
large constant on the right side, ve get
as the
n=ATt(8/m^)"^
or
(3-12)
A = n m = ^2/T^38^
Using this normalization constant, the distribution function
becomes
(3-13)
f(E,9)= (n m^/2/T^08v*) ^^^q
1
(E + Ue,)^
If we
then calculate the yield flux, Y, ve obtain
Y=(l/2m)^
(n/13)
(3-14)
Y=(l/2m)^ (n/3){-(r
Y=(l/2m)^ (n/3)
dE
E
P*"
Jo
(E + U,
+
=
)
Ua/2)/(r
+
U0
)
=
+
(I/2U0)}
6
or
(3-15)
n=(2m)^0Y/a
Therefore, substituting in the distribution function.
(3-16)
f(E,e)
=
ml
Y
2k
6
cos9
1
(E
+
Ub)
79
)
now
We
an expression
have
This expression can then
function of the emitted particles.
be
inserted
with
modification
one
modification
into
accounts for the differential
the emitting surface and the detectors.
to
be
charged
eqn
detectors.
the countrate at the
determine
accelerated to the detector,
.
The
For the
+
q*
to
necessary
between
particles
surface
the
Since E will map to E
(3-5)
charge
positively relative to the detectors,
will be negative.
distribution
the
for
must
therefore
be
*
(Liouville's
Theorem), we will obtain
(3-17)
f
2Tt
(5
(
(E-*)
we have assumed that the
Here,
<t>
cos9
(E,9,0)= ml Y
will
+
countrate expression
is
(3-18)
CR=G(2/m=)
(3-19)
CR=2GY cose
V
on
this
basis.
The
then.
f(E,e,
P
3
ions are singly charged, and
converted to eV from
be
Ub)
<t>)
E dE
dE
J
To
find the countrate,
we
must now integrate for each
the detector energy channels over
resolution
is
0.9Ec, where
The
20%,
Ec=
is
its energy range.
the upper and lower limits are
of
As this
1
.
lEc and
the central energy of the energy channel.
integral in eqn.
(3-19)
is a
standard
countrate may finally be written as.
80
form,
and
the
CR = 2GY cosQ{- (2. 2E.^ ^ U=, - <i>) +
2(1. 1E= + Ua - 0)=
71(5
(3-20)
the energy channel
If
and
1
.
8E^
+
2(.9E=
+
Ua
Ua
limits are both less than 0,
countrate over that channel
below 0,
(
zero.
is
-
0)
0)
then the
the lover limit
If
}
=
is
then the integral is taken between the upper limit
.
CALCULATION OF THE SPUTTERING FLUX AT THE DETECTOR
B.
Using
derived
Theory
Sigmund
the
expression for the countrate at
developed
a
program
to calculate
incident and emitted flux,
changes
to
charge,
available
83,
86,
developed
in
the
and
sputtering
the
channel
the
detector,
and the reponse of the
both the energy
Data
0.
above,
and
we
yield,
detector
differential
for the ambient plasma compositions
was
from the Lockheed Ion Mass Spectrometer for
days
The values utilized are listed in
and 200 of 1979.
Table 10.
The first portion of the program was the calculation
This value is
the total yield or emitted flux.
of
calculated
from the following expression;
(3-21)
Y =
P- V
f(v)
S(v,9) d=v
I
In this relation,
a
as
v f(v)
function of velocity,
a
is
the flux of the ambient ions as
and S(v,9)
is
function of velocity and angle.
S(v,e) exists in tabulated form,
simple continuous function.
the sputtering
yield
The factor Sr.(£)
in
and is not described by
a
fitted to
a
If the points are
ve arrive at the equation;
curve,
S„(£)= .35874
(3-22)
+(
-(
-(
equation
This
in
function,
the
(.17885)log£ -(. 14359
)(
log£
)
.06193) (loge) = +
.04228) doge)
.00541) (log£ )= -(. 00630) (logE)«
.00092) loge)^
(
and
tabulated
available
the
Figure
displayed
-
Since Sri(£)
32.
integral
evaluated
was
is
points
complicated
a
numerically.
that
depended
temperature
on the
ambient
the
of
The
energy
function was calculated at a thousand points over an
range
are
distribution,
the
high energy was 50000 eV and the low energy was 100 eV.
100
Maxwellian distribution.
For a 4000 eV
as most sputtering yields
eV was chosen as the lower limit,
approximately
are
zero by that point.
At
50000
the
eV
point, the value of the distribution function has reduced by
a
factor of 10"^" from the peak value.
The yield obtained in this method was then multiplied by
.02,
{Ref.
as approximately 2%-4% of the emitted flux is
the
This ion yield is substituted in the countrate
29}.
to obtain the detector responses.
integral
were
integrals
varied
over
differential charge to determine
narrow
emitted,
are
incident
similar.
flux
is
energy
The values
and
the responses were
very
shows the values
From
a
of
this it is very
clear
much greater than the emitted
the
function
The figures for days 83
1979.
of
channel
and ion fluxes for oxygen as
of energy for day 86 of
200
if
Figures 33
as observed.
incident,
ionized
that
ion
and
the
flux.
82
J
^
^
^
)<^
)i^
)¥:
M
m
c
o
•I—
o
r—
CO
CD
;^
0}
C
>
*s
U
-d
c
3
s
55
>
a
(0
_i_
CO
d
d
d
j9M0<j 6uiddo|3
paonpay
Figure 32. Lindhard's Universal Function {Ref.27}
83
u>
c
Figure 33
Incident, Emitted, and Ion Fluxes
Day 86 of 1979
84
Thus there is
net accumulation of positive charge on the
a
spacecraft due to the ambient ion current.
Figures
function
35 are displays of
34 and
energy
of
differential charge,
for
energy channels.
concentrated
constant
a
This
is
only one channel.
in
in fact not true.
However,
constant in eqn
viewing
center)
will
from
charges is quite similar.
a
narrow
become
varying
generate
due
to
a
range
Similar calculations for the
large
the
contributions
significant flux at the
of
depending
amount
these two ambient plasma
The
it
can
simply
relative
components very
on the temperature and density of
85
of
ambient
detectors,
present.
given
a
population on the same days indicate that
also
of
cone
the
That is,
certain
a
reduce
the peak will most likely
only respond for
is
as
<3il
The response for a specific channel and
differential charge.
widely,
cone
as noted previously, and we
cone (with different energies
differential
hydrogen
counted.
are
and since particles at the edges
will be reduced,
narrower.
channel
plate,
flat
This factor will serve to
(3-3).
.
the countrate overall,
the
plate
a
the error due to treating the factor
seeing
the
above
the
The detector viewing
probably not completely filled,
are
eV)
(35
The data indicates that it in fact should
all particles reaching that
that
a
high in 3-4 of
is
equations assume that the detector surface is
and
as
for days 86 and 200 for oxygen induced
We see that the countrate
emission.
be
channel
the countrate
their
O
>
o
o t
o
o
CO
+
a
+
O
+
+ >
O
+
S
+
03
c
5
+
CO
O
>
<j\
p>.
o
©
««r
V
c
Oi
"5
CO
00
(/)
V
V
5^
o
"c
•o
w
u
"c
*-
•^
Q
u
II
o
"y
0)
>
in
o
©
CO
o
o
©
O
o
4J
«n
fO
©
>^
o
«;
>
a,
_c
+
©
O
©
©
O
CM
SjDJjUnOQ
Figure 34. Channel Response for a 35 V Differential Charge
0+ Incident, Day 86, 1979
86
o
>
a;
o
o
o
i
^^
;o
+
+
+
+
+
+ H
+
+
a
o
+
+
h
o
^
o
o
o
m
O
a
o
>
>
03
c
en
a;
V
C6
j:
</l
V
w
:3
41
D
o
o
in
fO
II
o
u
o
c
V
o
o
o
o
o
,0J
c
>
^^
O
^
"G
--
a;
c
in
o
o
O
o
CO
O
O
O
o
02
ajOjjunoQ
Figure 35. Channel Responses for a 35 V Differential Charge
0+ Incident, Day 200, 1979
87
respective
distributions.
contribution
is
general,
In
oxygen
the
dominant.
The countrates from the detectors are available for
three
above
days.
countrates varied from 29
average at 360.
with
measured
-
800 counts per second,
with the
For day 86,
the average at 300.
213,
the
day
On
1981,
of
83
the range was 24
-
1080, with
Day 200 was much lower, between 25 and
average value of 120.
an
the
available
The
spectrometer data was averaged over one hour periods,
mass
so we
will generally calculate only one representative number
charging time frame.
the
and 400 for days 83,
325,
380,
The calculated
86,
countrates
responses compare favorably with
with
data,
already
the
noted
been
indicated
a
exception of day
were
and 200, respectively.
Considering the approximate nature of the calculations,
theoretical
for
200.
that this data from
the
However,
the
Hi
the
measured
it
has
detector
sputtering source different from those seen
the Lo detector.
on
This further supports that observation, as
the sputtering yield for this event is clearly much lower.
As the assumptions
the
in this
calculation tend to
number of detected particles,
conclude
that
sputtering
from
reasonable
it seems
a
differentially
sufficient to account for the
minimize
narrow
charged
triangle
surface
is
peaks.
These discrete responses will serve to explain
triangle
and
background.
shadow
peaks.
This
leaves
We require a broad response,
the
to
the
diffuse
with the particles
equally across the energy
scattered
channels.
gives the log distribution function as
emission
an
for
energy
channels,
and
numerically compares favorably with Figures 21 and 24.
spectrum
broad
described
then be
may
the
as
36
distribution
The
energy
has its maximum in the low
function
function of
a
degrees.
angle of
Figure
The
returning
emissions from non-d i f f erent ially charged surfaces.
We must now explain the shadow peaks.
a
combination
of
occurrences,
two
differentially charged surface to
acceleration
scanning
detector
opposed
from
to
geometry.
a
a
These appear to be
acceleration
motionless detector,
differentially charged
laterally across the
vertically.
surface
emitted
to
beam,
See Figure 31 for a view
this is then a variation of the
Since
from
a
or
a
as
the
of
triangle
peaks, the source and results would be the same.
The
the
recreated
trajectories to the detectors can be
NASCAP
Figure
program.
37
displays
the
particle
trajectories for the observed angles and energies at the
From top to bottom,
detector on day 86.
a)84 eV,
follows;
spacecraft
degrees,
plane,
d)
145 degrees,
b)
and 53 eV,
the lines are
or 12.5 degrees above
152 degrees,
70 eV,
188 degrees.
c)
60
portion
expected.
These
points are all located in the
of the spacecraft surface.
as
the
175
eV,
different
array
solar
These results
are
The solar array has charged differentially,
89
Lo
One can see that the
particles track back to the spacecraft surface to
points.
by
as
and
o
-1
o
o
o
o
00
a;
o
o
o
>
0)
c
UJ
o
O
C\2
Figure 36. Channel Response for
Charge
90
a
Zero V Differential
CO
M
X
<
><
<X Mw
Figure 37. NASCAP Trajectories, Day 86 of 1979
91
the sputtered particles follow specific trajectories
in
the
The limited viev cone of the detector only-
electric fields.
picks up specific energies.
was also run for the potentials observed on
day
The code was run to simulate the responses of the
Lo,
NASCAP
94.
Hi,
and
detectors
Fix
energies.
to a range
energies ranged from 2,0 to
The
noted above in the observations,
to -60 V,
the
incoming
of
90.1
particle
eV.
As
the spacecraft was charged
and peaks were noted at various lower energies on
detectors.
The Hi detector was parked at 67
and the Lo at 91 degrees.
Thus,
degrees,
detectors
Hi and Lo
were
sampling approximately the same region of space.
Figures 38 and 39 depict the Lo detector trajectories in
two
different
planes.
spacecraft surface.
returned
are
corresponding
to
asterisk.
The
left
side
the
indicate
to
first shows the
The
top
of
Particles at energies less than 43
spacecraft.
the
observed
The
particles are
the
eV
trajectories
marked
with
second figure is a complementing view
an
from
These
two
figures
that the actually observed particles
are
emitted
of the
first
figure.
from the satellite's forward end.
Additionally,
gaps from which no particles are seen at the
there are
detector.
An
patches
of
different nonconducting material on the forward surface
for
explanation
use
in
for
this
is
the
existence
of
the Satellite Surface Potential Monitor
these patches (kapton,
silvered teflon,
92
(SSPM).
When
and quartz) charge
Figure 38. NASCAP Trajectory, Day 94, Lo Detector
93
31.
=
2 9.
•
2 7.
'
25.
23.
•
21.
19.
•
^
17.
•
15.
%<"
U
'
•^
^««
13.
11.
.
9.
.
-
7.
5.
3.
1.
12345678
—H—H
9
10
1112
h-
h
N"
13 14 15 16 17
X-AXIS
Figure 39. NASCAP Trajectory, Day 94, Lo Detector
94
differentially,
the
particles reaching the
each will follow different trajectories,
energies.
Additionally,
forward end
is
the same striations.
We see that the origin
is
satellite
sheath
is
approximately
is
vicinity,
fields expected in
potential
distort
the
shows
The detail
100 V,
1
this
extremely
strong
region.
then the field due
V/m.
the
In
If
the
to
the
surface
forward
with a differential potential of 30 V, the fields
be on the order of
will
the
Again, the only nonconducting surfaces
not sufficient to study the
electric
local
of
and the data
on the forward end are the SSPM and the antenna.
on NASCAP
the
and may be a particle source.
also on the upper surface,
is
on
and 41 are similar views for a model of
detector trajectories.
particles
from
and have different
S-band antenna mast
the
nonconducting,
Figures 40
Hi
detector
30-60 V/m,
the local trajectories
of
significantly
and will
particles.
Thus,
the
NASCAP trajectory plotting routine is somewhat unreliable in
this region.
Looking at both figures, however, the origins
for the two detectors are reasonably close,
the local fields,
and allowing for
may possibly come from the same
As a working hypothesis,
surface.
we will assume that the SSPM is the
source of the emitted particles.
Figures 42 and 43 model the fixed detector.
are
similar
viewed
regions
to those above.
We see
by the fix detector come from
of the spacecraft.
views
the
particles
completely
different
that
All the obviously
95
The
reattracted
910111213 14151S17
Figure 40. NASCAP Trajectory, Day 94, Hi Detector
96
Figure 41. NASCAP Trajectory, Day 94, Hi Detector
97
33
31
29
27„
25„
23
HI 3
Figure 42. NASCAP Trajectory, Day 94, Fix Detector
98
I
s
Figure 43. NASCAP Trajectory, Day
94, Fix Detector
99
particles have energies of less than 43 eV, as did the other
two
detectors.
particles
skip
would
the
bellyband
mainframe potential,
not
trajectory
the
over
can see that the trajectories
region.
of
the
This
is
as the bellyband would be expected to be at the
reasonable,
satellite
We
theory
be accelerated by a
and thus particles
differential
calculations for the Fix detector
of
sputtering
from
sur faces
100
emitted
charge.
also
differentially
The
support
charged
IV.
It
reasonable to conclude that sputtering
is
source
CONCLUSIONS
observed phenomena.
of the
explain
the flux and structure of the triangle
peaks.
It also can account
detectors,
the
shadow
and
for the observed broad spectrum
These fluxes are large enough to be measured
ions.
of
likely
a
satisfactorily
can
It
is
they
are
but
enough
large
by
cause
to
substantial damage to the spacecraft surface?
this data set,
For
incident
the
flux,
the sputtering yield was less
accretion on the surface.
thesis
higher energy days,
result
net
energy
low
days.
However,
depletion.
surface
the
Assuming
damage caused by this process appears to be minor.
a
On
the sputtering yield increases, and the
surface
is
particle
was
Coincidentally, the data in this
taken from relatively
were
result
so that the net
than
higher energy day, with emission on the order of twice the
incident
silicon
cells
flux,
one
atomic layer will be
dioxide surface
are
covered
every
8
glass
by the
years.
removed
Since
shields,
from
solar
the
they
a
will
be
impervious to the surface sputtering.
Another
facet
of
this
is
conducting coatings on spacecraft.
the
damage
to
surface
These coatings are only
several atoms thick and may be expected to sputter from
surface within several months of the satellite launch.
101
the
This
account
may
satellite
quite
rapidly,
and
are usually not large,
charges
charging
ISEE-1
the
on
sputtered
energy spectrum of the
the
off
contamination
great
observed
.
Since
dies
the
for
observed
the
it
is
differential
not likely
charging
of the
particles
peak
results.
sputtering in general will probably be uniform over
Surfaces
material.
opposed
Therefore,
it
The
given
a
will be worn down over large areas
experiencing
to
any
that
significant
localized
as
damage.
appears that no short term damage will result
from sputtering on spacecraft.
Further
work
required to accurately
is
calculate
the
sputtering yields from various surfaces to prove or disprove
these conclusions.
of
At this point,
the sputtering field,
certainty
any
ambient
it
is
given the current state
not possible to state
whether extensive damage will
ion bombardment.
result
There appears to be no
work
with
from
in
progress anywhere to determine experimentally the sputtering
yields for most ion-target combinations.
Since the Sigmund
Theory
is
these results are necessary for
any
actual
calculations of the effect of the ambient heavy
ion
bombardment
so limited,
on space vehicles.
This thesis demonstrates that sputtering is the probable
cause
of the observed
low energy
ion
fluxes.
Additional
work is required to refine the NASCAP trajectory information
and
couple it with the theory developed here to
102
model
the
detector response accurately.
It
is
recommended that
work be continued when and if further applicable
data become available.
103
this
sputtering
APPENDIX
TABLE
1
MAGNETOSPHERIC IONS AND THEIR SOURCES {Ref.
7}
Origin
Source
Ions
Sun
Solar Wind
U*,Ue**,0^*
Earth
Ionosphere
H*,He*,0*
Earth
High Altitude
Thermal Plasma,
Plasmasphere
H*, He*, He**, 0*'
104
o**,0^*
._
TABLE
2
STORM TIME RING CURRENT COMPOSITION {Ref.
07 July
Date
1977
H* Flux
la-'cm-^s-1
Nov
1977
26
Mar
1978
09
8}
30 May
1978
1.26
4.20
13.0
6.0
H*
11
44
143
58
He*
.72
4.3
10.9
6.6
0*
8.3
91
228
107
H-^
8.3
5.7
5.1
6.4
He*
4.5
4.3
5.4
2.7
0*
4.9
3.8
5.3
4. 4
Dens i ty
10-=cm-3
Mean Ener gy
keV
105
TABLE
3
STORM TIME MAGNETOSPHERE COMPOSITION {Ref.
July
8}
04 May
29
29
1977
1977
Jan
1978
02 June
1978
3.3
2.4
10
2.8
20
10
27
He** 1.5
.16
9.3
.37
He*
1.8
.9
.3
.79
0*
3.0
13
4.9
60
8.5
4.2
6.8
He** 20.3
15.9
8.6
14.9
He*
4.8
3.9
6.4
4.8
0*
4.5
4.7
5.4
4.1
Date
H* Flux
10^cm-^s-^
Density
10-=cm-^
H*
26
Mean Energy
keV
H*
9.6
106
TABLE
4
VALUES OF THE REDUCED STOPPING POWER {Ref.27}
Sr,(£)
e
0.120
0.002
Sn(£)
0.403
0.20
0.154
0. 004
0. 405
0. 40
0.211
0.01
0.356
1.00
0.261
0.02
0.291
2.00
0.311
0.04
0.214
4.00
0.372
0.10
0.128
10.0
0.393
0.15
0.0813
20.0
0.0493
40.0
107
TABLE
5
CALCULATED AND MEASURED SPUTTERING YIELDS FOR VARIOUS
MATERIALS {Ref. 23}
THE NUMBERS IN PARENTHESES ARE THE YIELDS CALCULATED
FROM THE SIGMUND THEORY
SDutterinq Ra t io
TARGET
MATERIAL
UB(eV)
Z^
Ne*
Pb
2.01
82
3.6
Ag
2.94
47
4.5
Sn
3.11
50
1.8
Cu
3.46
29
Au
3.79
79
3.2
(3.7)
3.6
Pd
3.87
46
2.5
Fe
4.29
26
1.3
Ni
4.43
28
1.4
V
5.3
23
0.3
Pt
5.82
78
1.9
Mo
6.82
42
0.6
Ta
8.06
73
0.7
W
8.70
74
1.0
108
Ar*
10.5
(29.7)
10.8
(12.7)
4.3
(12.5)
6,8
(6.7)
10.2
(16.1)
5.3
[9.24)
2.3
:5.15)
3.5
5.35)
1.0
3.85)
5.3
9.35)
1.5
5.14)
1.6
6.62)
2.3
5.07)
(
ateDms/ion
Kr*
24.0
(44.0)
23.5
(20.1)
8.5
(19.5)
11.8
(11.9)
24.5
(23.9)
10.5
(14.1)
4.0
(8.92)
5.6
(9.01)
1.7
(6.21)
11.3
(15.3)
2.7
(7.70)
3.1
(10.0)
4.7
(9.31)
)
Xe^
44.
5
(74.6)
36.2
(24.0)
11.8
(24.9)
19 ,0
(15.5)
39.0
(27.8)
14.4
(18.1)
4.9
(11.7)
7.6
(11.8)
1.9
(8.9)
16.0
(19.2)
3.8
(10.1)
4.0
(12.3)
6.4
(11.9)
TABLE
6
CALCULATED SPUTTERING YIELDS USING A
MODIFIED BINDING ENERGY.
TARGET
MATERIAL
1.7 Ua
Pb
3.42
17.5
25.9
43 .9
Ag
5.00
7.47
11.8
14 .1
Sn
5.29
7.35
11.5
14.7
Cu
5.88
3.94
7.00
9.12
Au
6.44
9
.47
14.1
16.35
Pd
6.58
5.44
8.3
10.7
Fe
7,29
3.03
5.25
6.9
Ni
7.53
3.15
5.3
6.94
V
9.01
2.26
3.55
5.24
Pt
9.89
5.50
9.00
11.24
Mo
11.59
3.02
4.53
5.94
Ta
13.7
3.89
5.88
7.24
14.79
2.98
5.48
7.00
W
SDutter inq Ratio
Ar-^
Kr^
109
(
Atoms/I on
Xe^
TABLE
7
RATIO OF SPUTTERING YIELD PER ATOM FOR MOLECULAR
AND ATOMIC ION BOMBARDMENT {REF. 31}
Projectiles
Si
Ag
CI-CI2
...
1.09
Se-Sez
1.15
1.44
1.44
Te-Te^
1.30
1.67
2.15
110
Au
TABLE
8
SCATHA PARTICLE DETECTOR ENERGY CHANNELS {Ref.
4}
Lo Detector
Step
Energy eV)
-0.334
(
7
-0.29
-0.23
-0.16
-0.08
0.00
0.09
0.22
8
0. 34
9
0.50
0.67
0.87
1.10
1.36
1.66
2.00
2.39
2.83
3.34
3.94
4.60
5.37
6.25
7.25
8.40
9.72
11.23
12.96
14.96
17.23
19.83
22.80
1
2
3
4
5
6
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
Step
Energy eV
32
33
34
26.26
30.10
34.57
35
39 .70
36
37
38
39
45.55
52.22
60.00
68.77
78.55
90.10
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
111
(
103. 32
118.32
135.65
155.43
178.09
203.98
233.42
267.20
306.30
350.74
401.96
457.73
525.50
601.05
687.71
787.70
902.13
1033.23
1184.33
1355.42
1529.85
1777.60
TABLE
8
CONTINUED
Hi
step
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
Energy
{
Detector
Step
eV)
-4,68
-2.35
0.32
3.38
6.88
10.89
15.49
20.75
25.77
33.66
41.56
50.60
60.95
72.80
86.37
101.91
119.71
140.08
163.41
190.12
220.70
255.72
295.81
341.72
394.29
454.45
523.39
602.30
692.65
796.11
914.56
1050.19
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
112
Energy (eV)
1205.49
1383.30
1586.90
1820.02
2086.94
2392.57
2742.51
3143.19
3601.97
4127.28
4728.76
5417.45
6206.00
7108. 89
8142.70
9326.41
10681.76
12233.63
14010.53
16045.08
18374.64
21041.98
24096.09
27593.05
31597.06
36181.66
41431.02
47441. 54
54323.58
62203.53
71226.06
81556.86
TABLE
9
THE SPUTTERING YIELDS OF 10 keV ION BEAMS ON
SILICON AND SILICON DIOXIDE {Ref. 36}
Target
Ar*
Incident Ion Beam
O2*
Si
0.94
0.20
SiOz
0.69
0.48
113
TABLE 10
AMBIENT PLASMA VALUES FOR DAYS 83, 86, AND 200 OF 1979
DATA FROM THE LOCKHEED ION MASS SPECTROMETER.
Day
83/
1979
Particle
H+
Hs^
He
0*
86/
1979
H*
Ha-"
He
0^
200/
1979
H^
H2*
He
0*
Number Dens i
3.845
2.487
2.865
3.188
Energy(keV)
10»
10^
10 =
10 =
8.852
120.778
33.992
4.217
10»
103
103
10»
5.889
43.728
3.662
4.035
*
10^
10*
*
10 =
*
10 =
5.271
7.279
0.420
1.957
*
*
*
*
2.606
2.850
2.410
1.635
*
9.064
4.209
2,169
4.093
*
114
(m~^)
*
*
*
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INITIAL DISTRIBUTION LIST
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5.
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Department of Physics
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Monterey, CA 93943-5000
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40.
Pedersen
Space Science Department
ESA/ESTEC
Noordwijk, The Netherlands
41.
W.F. Denig
Space Physics Division
Air Force Geophysics Laboratory/PHG
Hanscom AFB, MA 01731
42.
Lt.
Dr.
A.
Dr.
C.W. Norwood
1524 St. James PI.
Roslyn, PA 19001
123
NAVAL POSTGRADUATE
aCTTnm
Thesis
N94424
c-1
Norwood
Ions; generated on or
near satellite surfaces.