The Space Plasma-High Voltage Drainage Experiment (SP-HVDE) was comprised of two identical experimental trays. With one tray located on the leading (ram facing, B10) edge and the other located on the trailing (wake facing, D4) edge of the Long Duration Exposure Facility (LDEF), it was possible to directly compare the effects of ram and wake spacecraft environments on charged dielectric materials. Six arrays of Kapton dielectric samples of 2 mil, 3 mil, and 5 mil thicknesses maintained at +/- 300, +/- 500, and +/- 1000 voltage bias formed the experimental matrix of each tray. In addition, each tray carried two solar cell strings, one biased at +300 volts and the other at -300 volts, to study current leakage from High Voltage Solar Arrays (HVSA). The SP-HVDE provides the first direct, long-term, in-flight measurements of average leakage current through dielectric materials under electric stress. The experiment also yields information on the long term stability of the bulk dielectric properties of such materials. Data and findings of the SP-HVDE are an extension of those from shorter term flight experiments such as the PIX-1 (Plasma Interaction Experiment) and PIX-2 and are therefore valuable in the design and evaluation of long-lived space systems with high voltage systems exposed to the low earth orbital environment. A summary of the SP-HVDE post flight analysis final report delivered to the LDEF Project Office under contract to the National Aeronautics and Space Administration is presented

Blakkolb, B. K.

Ryan, L. E.

Schurig, H. J.

Taylor, W. W. L.

Wong, W. C.

Yaung, J. Y.

English

NASA Technical Reports Server

N93-29690
A
LDEF SPACE PLASMA-HIGH VOLTAGE DRAINAGE EXPER/MENT
POST-FLIGHT RESULTS
J.Y. Yaung, B.K. Blakkolb, W.C. Wong, L.E. Ryan,
H.J. Schurig and W.W.L. Taylor
TRW, Space & Technology Group
One Space Park
Redondo Beach, CA 90278
INTRODUCTION
The Space Plasma-High Voltage Drainage Experiment (SP-HVDE) was comprised of two
identical experimental trays. With one tray located on the leading (ram facing, B 10) edge and the
other located on the trailing (wake facing, D4) edge of the LDEF, it was possible to directly
compare the effects of ram and wake spacecraft environments on charged dielectric materials. Six
arrays of Kapton dielectric samples of 2 mil, 3 mil, and 5 mil thicknesses maintained at +/- 300
+/- 500, and +/- 1000 voltage bias formed the experimental matrix of each tray. In addition, each
tray carried two solar cell strings, one biased at +300 volts and the other at -300 volts, to study
current leakage from High Voltage Solar Arrays (HVSA).
The SP-HVDE provides the first direct, long-term, in-flight measurements of average
leakage current through dielectric materials under electric stress. The experiment also yields
information on the long term stability of the bulk dielectric properties of such materials. Data and
findings of the SP-HVDE are an extension of those from shorter term flight experiments such as
the PIX-I (Plasma Interaction Experiment) and PIX-H (1,2,3) and are therefore valuable in the
design and evaluation of long-lived space systems with high voltage systems exposed to the low
earth orbital environment.
This paper is a summary the SP-HVDE post flight analysis final report delivered to the
LDEF Project Office under contract to the National Aeronautics and Space Administration.
MISSION PROFILE
The LDEF was placed in a 482 km, near circular, orbit of 28.4 degree inclination on
April 7, 1984 by the Space Shuttle Challenger. The LDEF was to have been retrieved from space
after a two-year mission, but due to a catastrophic Shuttle launch incident and subsequent two-year
hiatus from Shuttle launches, the LDEF was not retrieved until January 12, 1990 by the Columbia
orbiter at an altitude of 340 km. The SP-HVDE gathered data over the first 233 days of the LDEF
mission.*
Even though the objective of the SP-HVDE was not to characterize the space environment,
knowledge about that environment is helpful in understanding the experimental results, therefore a
summary of the LDEF environment experienced by SP-HVDE is included in Table 1. The ram-
facing tray of the SP-HVDE saw micro meteoroids and space debris (M/D) as well as energetic
atomic oxygen. On the wake-facing side of the LDEF, the trailing-edge tray saw a substantially
lower flux of atomic oxygen and space debris. Both trays incurred essentially the same natural
radiation dose.
* Harry Dursch, LDEF Systems Special Investigation Group, private communication, 1991.
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https://ntrs.nasa.gov/search.jsp?R=19930020501 2020-03-17T04:27:59+00:00Z
Table 1. SP-HVDE Space Environmental Exposure Summary
Atomic Oxygen
UV
M/D
Leading Edge (D- 10)*
7.88x1021/cm 2
10698 Eqv. sun hrs.
187 Impact sites
(l.3x10 -5)
(Frationai Area Coverage)
Trailing Edge (B-4) t
1.15x 105/cm 2
10458 Eqv. sun hrs.
23 Impact sites
(2.0x 10 -6)
(Frationai Area Coverage)
Tray location designation
s : :
SP-HVDE CONFIGURATION
A schematic of the SP-HVDE is shown in Figure 1. Separate coulombmeters were
employed to obtain time integrals for the leakage current and bias voltages for each set of samples.
These devices can record currents flowing in either direction by either deplating or plating the
electrodes. The Plessey coulombmeters used in the experiment had been plated to I0,_ mA-
seconds by the manufacturer. The duty cycle of the storage capacitor was 0.5%, with the power
processing unit, consisting of DC-DC power converters controlled by a switching circuit,
satisfying the duty cycle requirement.
Each tray contained six sets of four dielectric samples, with each set maintained at a
different bias voltage (i.e., +300, +500 or +1000 volts). The sets were comprised of various
combinations of Kapton/VDA film in one of three thicknesses (i.e., 2 mils, 3 mils, and 5 mils)
bonded to the fiberglass/epoxy substrate with a 60 wt% silver loaded epoxy. Thus, the long-term
average leakage current and resistivity were determined by using a matrix of dielectric samples of
different thicknesses and biases comprising 22 dielectric samples per tray. The adhesive and the
VDA (Vapor Deposited Aluminum) served as conducting layers of the dielectric stack for collecting
leakage current. The layered construction of the dielectric stacks is shown in Figure 2. Both
trays also contained two solar cell modules, one maintained at +300 and the other at-300 volts
bias. Each cell module consisted of three solar cells in a closed loop circuit with a load resistor.
POST-FLIGHT RESULTS
The striking contrast in the post-flight condition of the leading and trailing edge trays is
evident in Figures 3 and 4. The trailing edge tray appears essentially the same as it did when the
LDEF was deployed, but the leading edge tray shows the result of exposure to an atomic oxygen
fluence of 7.88x 1021/cm2. Of course the complete erosion of the Kapton material radically
changes the dielectric properties of the experiment, but over the intended functional period of the
experiment, approximately ! .5 mils of Kapton were eroded by the action of energetic atomic
oxygen. Leakage current data, taken during the 233 day design life of the SP-HVDE, are presented
below.
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Leakage Current Measurements
Average leakage currents versus sample thickness and bias voltage for both trays are
shown in Table 2. The average leakage current was obtained by measuring the current required to
deplate the coulombmeters back to the pre-flight condition of 10,000 mA-seconds. The deplating
process serves to integrate leakage current over time and yields an average over the operational life
of the experiment. During the course of post-flight measurement, 6 of the 152 coulombmeters
exhibited anomalous behavior, i.e., 3 showed "open circuit" such that the readout equipment
could not deplate the coulombmeter and 3 coulombmeters showed "short circuit" such that
deplating during readout did not terminate at the pre-flight endpoint.
Table 2. Average Leakage Current (micro amperes)
Trailing-edge Bias Voltage(V)
Thickness +300 +_00 +1000 -300 -500 -1000
(mil)
5 N/D 0.53 0.1 ! N/D 0.17 0.196
3 0.196 0.43 1.9 0.194 N/D 0.184
2 0.36 0.16 N/D 0.27 0.2 N/D
Leading-edge Bias Voltage(V)
Thickness +300 +;500 +1000 -300 -500 -1000
(rail)
5 N/D 1.3 0.61 N/D 0.496 0.48
3 0.598 1.0 ! .2 0.497 0.38 0.487
2 0.58 -0.65 N_ N_ N_ N_
N/D=No Data
Ground simulations predicted leakage current to increase with positive bias voltage and
decrease with greater dielectric sample thickness. Figure 5 shows the leakage current data for
positively biased samples on the leading and trailing edge trays. The data show that only the 3 mil
samples followed the predicted trend and that the 5 mil samples showed an unexpected decrease in
leakage current over the 500 to 1000 +volt range. The limited data obtained for the 2 mii samples
also seems to follow the atypical trend, although the 2 mil sample did demonstrate an expectedly
larger leakage current compared to the thicker samples at the 300 +volt bias.
Leakage current data shown in Figure 6, for negatively biased samples, are distinctly
different compared to those for positively biased samples. For the negatively biased condition, the
leakage currents obtained are lower than those measured for the positive bias condition by a factor
of three. Also of significance is the smaller change in leakage current across the voltage range. It
is speculated that for the negatively biased surfaces even the lowest voltage (i.e., -300 volts) is
beyond the threshold at which interaction with the space plasma results in localized but intense
electrodischarge events. The occurrence of such discharges is consistent with the lower post-flight
average leakage currents measured and also consistent with work performed by Thiemann, et al.
(ref. 4) which reported greater numbers of discharges for negatively biased surfaces during
ground simulations. The reason for the dip in the leakage current at -500 volts for the 3 mil sample
on the leading edge tray is unclear at this time, but it should be noted that a similar phenomenon
was seen in the data from the PIX-I experiment (see ref. I ).
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Dielectric Property Measurements
The leakage current time integrals and bias voltages permit the calculation of an average
resistivity across the dielectric sample stack. Tables 3 and 4 show the average voltages and
resistivities derived from post-flight measurements.
Table 3. Average Voltage (V)
Bias Voltage (V) +300 +500 -_00
Trailing-edge Tray 123.4 N/D 57.1
Leading-edge Tray 16.2 N/D 326.2
N/D=-No Data
Table 4. Derived Resistivity (ohm-cm)
Trailing-edge Tray
Thickness (mil) +300
Bias Voltage(V)
N/D
2.3 x 1013
2.0x 1013
Average: 2.2 x 1013 ohm-cm
-300
N/D
1.1 x 1013
1.2x 1013
1.2 x 1013 ohm-cm
Leading-edge Tray
Thickness (mil) +300
Bias Voltage(V)
-3OO
5
3
2
Average:
6.8 x 101 !
1.4 x 1012
12
5.5 x 10
3.8 x 1012 ohm-cm
N/D
6.2 x 1013
136.6x 10
6.4 x !013 ohm-cm
Unflown reference Kapton: p = 3 x 1015 ohm-cm at +300 V bias.
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SolarCell ModuleCharacterization
We wereverypleasedto be able to locate the original set of control samples for the SP-
HVDE solar cell modules after some ten years since the experiment was first conceived. These
control samples gave us the opportunity to compare not only the relative performance of leading
and trailing edge flight specimens but also to compare the flight data to a baseline reference.
Having found the control cells, there was however some question as to exactly what type of cells
they were and the type of coverglass used. Based on the age and configuration of the cells it is
reasonably certain that the cells are of a low efficiency, 10 ohm-cm, single crystal silicon type with
fused silica coverglass and conventional bar interconnects.
One of the cells on a leading edge module experienced an impact by a micro meteoroid or
debris particle which, from the data shown in Tables 5 and 6, resulted in a significant reduction in
performance. Scanning electron micrographs of the impact site are presented in Figure 7. The
particle penetrated the coverglass and the silicone coverglass adhesive and produced a raised and
melted spall zone on the silicon solar cell. The damage to the cell extends well beyond the
immediate impact site with radial cracks in the coverglass extending 0.25 inches from the impact
center. Elemental analysis of the impact site did not reveal any material that could be conclusively
identified as extraterrestial or as anthropogenic debris.
Environmental Interaction
The metal interconnect strips between the cells of each module were exposed to the ambient
space plasma, providing a path for current leakage. The area ratio of interconnect to coverglass is
2.8%. As with the other charged surfaces of the experiment, average current integrals were
obtained through coulombmeters. The average current integrals for each cell flight module are
presented in Table 6.
Table 5. Electrical Characteristics of
LDEF SP-HVDE Solar Cell Modules
Cell Module Voc{V) lsc(A)
Number/Type
# I. Trailing 1.63 0.285
# 2. Trailing !.63 0.286
# 3. Leading !.63 0.290
# 4. Leading 1.64 0.223
# 5, Control 1.64 0.287
# 6. Control 1.64 0.287
Vmp(V3 Imp(A) Pmp(W) Comment
1.36 0.271 0.369
1.36 0.272 0.370
1.36 0.272 0.369
1,51 0.222 0.336
!.37 0.275 0.377
1.37 0.273 0.374
Particle Impact
1347
Table 6. Derived Current Integrals & Average Currents
Trailing-edge Tray:
=,
Bias Volta_,e:+300 V .Bias Voltage: -300 V
Current Integral Iav Current Integral lav
(mA-second) (micro ampere) (mA-_cond) (micro ampere)
-469.74 -2.3 x 10-2 1509.85 7.5 x 10 -2
" zz : _s.
--------,;, -o- ---Leadin_-edeeTray: _ _ -_:
l_ias Voltage:+300 V
Current Integral Iav Current Integral
(mA-second) (micro ampere) (mA-second)
5411.92 2.7 × 10"1 355.30
_ias Voltage: 2300 V
Ia,,
(micro ampere)
1.8 x 10 -2
(one cell damaged by
M/D impact)
i
CONCLUSfONS_
The concept of the High Voltage Solar Array (HVSA) was created around 1973. The goal
of this power-efficient concept was to allow power conversion directly from the modular solar
array to high voltage systems. In the early 1980s, the solar array community envisioned that SDIO
would require many large solar power systems employing HVSA technology. Instead, the interest
in the HVSA concept has materialized in the form of the proposed Space Station Freedom. The
Space Plasma-High Voltage Drainage Experiment has extended the existing data base On sPacecraft
charging and current leakage phenomena first studied by the PIX-I and PIX-II experiments. The
SP-HVDE also makes a substantive contribution to the overall LDEF solar cell data package which
has confirmed the robustness of design practices and materials selection. _e information oiStained
from th_ retrieved the LDEF has made an incalculable contribution to the database of spac_e
environmental effects on materials and spacecraft aging.
1348
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Ii
@
=
=
REFERENCES
I. N.T. Grier and N.J. Stevens, "Plasma Interaction Experiment (PIX) Flight Results",
Spacecraft Charging Technology -1978, NASA CP-2071/AFGL-TR-79-0082, 1979, pp. 295-
314.(AD-A084-626/I)
2. N.T. Grier, "Plasma Interaction Experiment II(PIX-II): Laboratory and Flight Results", in
Spacecraft Environmental Interactions Technology - 1983, NASA CP-2359/AFGL-TR-85-0018,
1985, pp. 333-348.
3. N.J. Stevens, "Summary of PIX-2 Flight Results Over the First Orbit", AIAA-86-360, AIAA
24th Aerospace Sciences Meeting, Jan. 6-9, 1986, Reno, Nevada.
4. I. Thiemann and K. Bogus, "Anomalous Current Collection and Arcing of Solar-Cell Modules
in a Simulated High-Density Low-Earth-Orbit Plasma", ESA J., 10, 43, (1986).
1349
Front (exposed} face of film
Conducting layer
a_ rear face of ( dric film
dielectricfilm-,_J _:_::_-_ _ . _ _ -- -:=--:_: : =
/lodt-_
Coulometer \
determines /_ _ ....... ..........
"__ Coulometer
time integral (, ..j determines time integral
of applied
of drainage current
bias +
pote ntial ---._ l
c_L Power
j.vsdt__.. _R T _ processingunit
]series [ k
_7 LOEF frame \
" ground
Storage
capacitor
Battery
pack
Figure 1. Schematic of SP-HVDE
"__'___ConnectordetailsDielectricsample
and test fixture
t
Figure 2.
1350
LDEF exterior
surface
Electronics package
Dielectric Sample Construction
\
Sample details
(2-5 HIL)
FILM (.1 MICRON)
57C (SILVER
LOADED EPOXY)
-GLASS/EPOXY SUBSTRATE
ii
|
|
|
i
lange
i
|
i
7
!
!
|
Figure 3. Leading Edge SP-HVDE Tray (Post Flight)
i
)VDA-Kapton Dielectric
Bonded to Epoxy Fiberglass
Substrate with Silver-filled
Conductlve E
Figure 4. Trailing Edge SP-HV'DE Tray (Post Flight)
ORIGINAL. PAGE
BLACK AND WHITE PHOTQGRAPH
1351
Ram-Facing Tray
Z
E
i
o
o
g
f
g
t,4
1.2
I
0.8
0.6
0.4
0,2
0
2 mils
100 200 300 400 500 600 700 800 900 1000
Positive Bias Voltage (volts)
Wake-Facing Tray
E
?
P
o
g
¢o
I
-I
g
t_
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0 100 200 300 400 500 600
Positive Bias Voltage (volts)
2 mils
mils
f:---- I l-------------_
700 800 900 1000
1352
Figure 5. Average Leakage Current Data for Positive Bias Voltage Samples
_ = Y 2 _ _ 4-= =
Ram-Facing Tray
_k
E
?
o
._o
g
e-
,-i
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
3 mit 5 mils / z='-==_
-- _ -- -_----_ ..... -P_---------F------t _,, --I
200 300 400 500 600 700 800 900 1000
Negative Bias Voltage (-volts)
Wake-Facing Tray
E
6
E
v
c
E
,.d
0.3
0.25
0.2
0.15
0.1
0.05
...... I_--F--
= = .
0 100 200
3 mils E}--------
5 mils
-E
F - t_ --{ -- t _ ...... L -----t
300 400 500 600 700 800 900 1000
Negative Bias Voltage (-volts)
Figure 6. Average Leakage Cun'ent Data for Negative Bias Voltage Samples
1353
1354
Figure 7. impact site on Photovoltaic Cell Stack
ORIGINAl PAGE
B EACK AND WHITE PHOTOGRAPH


LDEF Space Plasma-High Voltage Drainage Experiment post-flight results

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