Samples of Mylar and Teflon film were exposed to combinations of monoenergetic electron and lithium ion fluxes in various ratios. The samples' discharge rates and strengths were found to diminish as the ion proportion increased. Various types of capacitors were exposed in air to beta irradiation from a 100 mCie Strontium-90 radioisotope source located at distances ranging from 2 cm to 5 cm from the capacitors. In these preliminary experiments, no evidence of spontaneous electrical breakdown was noted, nor was any change in RF impedance detectable using the available instrumentation. A decrease in DC resistance was noted, apparently due to radiation-induced conductivity. A cylindrical glass vacuum chamber is being assembled. Its inside dimensions are 44 cm diameter by 100 cm length. All necessary associated components and instruments have been acquired, including electron and ion guns, Trek surface potential probe and turbo-molecular pump. A mass-spectrometer detector for leaks and evolved gases will be ordered shortly

Balmain, K. G.

English

NASA Technical Reports Server

ELECTRON BEAM CHARGING AND ARC DISCHARGING OF
SPACECRAFT INSULATING MATERIALS
Semi-Annual Status Report
NASA Grant NSG-7647
Submitted to NASA Lewis Research Center
21000 Brookpark Road, Cleveland, Ohio
March 1983
K.G. Balmain
Department of Electrical Engineering
University of Toronto
Toronto, Canada M5S 1A4
Funding agencies:
U.S. Air Force Weapons Laboratory
U.S. National Aeronautics and Space Administration
https://ntrs.nasa.gov/search.jsp?R=19830012728 2020-03-21T03:40:30+00:00Z
ELECTRON BEAM CHARGING AND ARC DISCHARGING OF
SPACECRAFT INSULATING MATERIALS
Semi-Annual Status Report
NASA Grant NSG-7647
Submitted to NASA Lewis Research Center
21000 Brookpark Road, Cleveland, Ohio
March 1983
K.G. Balmain
Department of Electrical Engineering
University of Toronto
Toronto, Canada M5S 1A4
Funding agencies:
U.S. Air Force Weapons Laboratory
U.S. National Aeronautics and Space Administration
TABLE OF CONTENTS
Page
1. Summary of Research 1
2. Personnel 1
3. Publications 2
4. Incident Ion Effects on Polymer Surface Discharges 3
5. High-Energy Beta-Irradiation of Capacitors 7
6. References 10
-^
Table 11
Illustrations 12-16
1. SUMMARY OF RESEARCH
Samples of Mylar and Teflon film were exposed to combinations of mono-
energetic electron and lithium ion fluxes in various ratios. The samples'
discharge rates and strengths were found to diminish as the ion proportion
increased.
Various types of capacitors were exposed in air to beta irradiation
from a 100 mCie Strontium-90 radioisotope source located at distances ranging
from 2 cm to 5 cm from the capacitors. In these preliminary experiments, no
evidence of spontaneous electrical breakdown was noted, nor was any change in
RF impedance detectable using the available instrumentation. A decrease in
DC resistance was noted, apparently due to radiation-induced conductivity.
r
A cylindrical glass vacuum chamber is being assembled. Its inside di-
mensions are 44 cm diameter by 100 cm length. All necessary associated com-
ponents and instruments have been acquired, including electron and ion guns,
Trek surface potential probe and turbo-molecular pump. A mass-spectrometer
detector for leaks and evolved gases will be ordered shortly. All this
equipment has been paid for on grants from the Natural Sciences and Engineer-
ing Research Council of Canada.
2. PERSONNEL
K.G. Balmain, Principal Investigator.
M. Gossland, Research Engineer.
T. Nozaki, Engineering Technologist.
D.D. Loh, Summer Student (undergraduate).
3. PUBLICATIONS
1. A paper entitled "Incident Ion Effects on Polymer Surface Discharges",
by M. Gossland and K.G. Balmain has been submitted for the IEEE
Nuclear and Space Radiation Effects Conference, to be held on
July 18-21, 1983, at Gatlinburg, Tenn.
2. Two papers have been published in the December 1982 issue of the IEEE
Transactions on Nuclear Science. They are: "Optical Measurement
of the Velocity of Dielectric Surface Arcs", by K.G. Balmain,
M. Gossland, R.D. Reeves and W.G. Kuller, pp.1615-1617; "Barriers
to Flashover Discharge Arcs on Teflon", by M. Gossland and K.G.
• Balmain, pp.1618-1620.
3. The paper "Dielectric Surface Discharges: Effects of Combined Low-
Energy and High-Energy Incident Electrons", by K.G. Balmain and
W. Hirt, has been accepted conditionally for publication in the
IEEE Transactions on Electrical Insulation. This is a slightly
augmented version of a paper which appeared in Spacecraft Charging
Technology 1980.
4. INCIDENT ION EFFECTS ON POLYMER SURFACE DISCHARGES
4.1 Introduction
The charging by electrons of spacecraft insulating surfaces in
synchronous orbit must be accompanied by a contribution from ambient posi-
tive ions. Experiments in space (References 1, 2, 3) indicate that proton
fluxes are normally less than 1/20 of the electron fluxes, but under charg-
ing (eclipse) conditions the relative proportion of protons can rise to 1/10,
or sometimes as high as 1/2 in some energy ranges. The laboratory experi-
ments to be described here make use of electron-to-ion current density ratios
of 20, 10 and 5 in an attempt to observe ion effects on the properties of the
surface arc discharges which result from charge accumulation. The current
density magnitudes used in the laboratory experiments are much higher (of the
order of 50 times higher) than would be observed in synchronous orbit. How-
ever the materials tested (Mylar and Teflon) have been shown (Reference 4) to
exhibit discharge properties which are independent of incident electron
current density over the relevant range, and so it is reasonable to postulate
independence of combined electron and ion current densities provided their
ratio remains fixed. Such a postulate can then be tested, using the experi-
mental data to be acquired.
v
4.2 The Experiment
Two types of dielectric film were used, 75 urn plain Mylar and 125 urn
silvered Teflon. Samples were cut from sheet stock, rinsed in trichloro-
ethylene, blown dry, and mounted in the sample holder depicted in Fig. 1.
2The samples were sandwiched between a copper mask with a 1 cm circular
aperture and a copper substrate, each connected separately through a 2.5 ohm
resistor to ground. The current flowing through the substrate resistor
during discharge provided a trigger for pen motion on a chart recorder, re-
sulting in a permanent record of discharge occurrences in time.
The electron source was a hot filament followed by a 20 kV accelera-
tor electrode; the beam was magnetically deflected through 90 degrees to
permit arc photography. The ion source was an alkali-metal oven intended
for application as a surface neutralizer in scanning electron microscopes.
This source produces positive lithium ions which have been accelerated
through a potential difference of approximately 100 V. Upon close approach
to a specimen which has been pre-charged by an electron beam, the ions would
be further accelerated.
Each sample was exposed to a constant electron flux for the entire
duration of the irradiation sequence. After 5 min of electron exposure (10
min on a few occasions with less responsive samples), the ion beam was
turned on at a constant flux for the same interval, then off, then on, then
off and finally on again. Thus the only difference between adjacent inter-
vals was the presence or absence of the ion beam.
The electron and ion fluxes quoted in the experiment were defined as
those incident on a grounded Faraday cup located behind the mask aperture
(see Fig. 1). Because this position was normally occupied by the sample
itself, it was unavailable during the tests. Therefore initially the quoted
fluxes were calibrated against currents which were available for monitoring
during the experiment. The electron flux was calibrated against the current
into the side Faraday cup and the ion flux was calibrated against the ion
current intercepted by an accelerating grid in the ion source.
For Mylar, the following combinations of fluxes were used: electrons
2 2
at 100, 50, 25 and 50 nA/cm with ions at 5, 5, 5 and 10 nA/cm respectively,
giving electron-to-ion ratios of 20, 10, 5 and 5. For Teflon, the combina-
2 2tions were electrons at 100, 50 and 50 nA/cm with ions at 5, 5 and 10 nA/cm
respectively, giving the electron-to-ion ratios of 20, 10 and 5.
4.3 Results
The numbers of discharges occurring in each time intervals were -ex-
pressed as "rates", by dividing them by the time interval itself. Typically
these numbers ranged from 3 to 18 per 5-min initial interval (electron
exposure), with an average of 7 or 8. The extremes of discharge activity
encountered with each combination of material and electron and ion flux are
shown in Table 1 which lists the minimum, maximum and average numbers of dis-
charges occurring on a sample during the first time interval. Any samples
discharging fewer than three times in this interval were excluded from the
results. Aside from fatigue effects and random fluctuations in discharge
occurrences, changes in discharge rates from one period to the next were
attributed to the ions. Randomness was reduced in the final results by ave-
raging all samples subjected to like conditions. Before averaging, the dis-
charge rates were normalized so that the rates during the first interval were
set to unity. This enabled easier comparisons and gave equal weight during
averaging to the rates themselves, rather than to the individual discharges.
The normalized discharge rates for three electron-to-ion ratios on
Mylar are shown in Figs. 2 to 4. The horizontal plateaus represent the rates
and the oblique joining lines enable one to trace the samples' histories.
The dots represent the averages of the rates of all of the samples.
In Fig. 2 the electron-to-ion ratio is 20 and no ion effect is visible.
The slight overall decline in the average rates demonstrates sample fatigue.
In Fig. 3 the ratio is 10 and in addition to fatigue a consistent reduction
in discharge rate upon ion exposure is evident. In Fig. 4 the ratio is 5 and
the ion effect is increased.
In Fig. 5, the electron-to-ion ratio is the same as in Fig. 4, but both
the electron and ion current densities have been doubled. In Fig. 5 the ion
effect is approximately as strong as in Fig. 4, apparently confirming the
tentative postulate previously mentioned, that for Mylar the ion reduction in
discharge rate depends on the electron-to-ion flux ratio and not on the flux
magnitudes.
For Teflon Fig. 6 shows that at an electron-to-ion ratio of 20 produces
little or no ion effect; in fact the only effect in evidence is a slight
increase in discharge rate with ion exposure. However as the electron-to-ion
ratio is reduced to 10 and 5 as shown in Figs. 7 and 8, the ion effect of
reduced discharge rate is once more clearly demonstrated. It is worth noting
that the results of Fig. 8 for Teflon are similar to those of Fig. 4 for
Mylar, the same electron-to-ion ratio having been employed in both experiments.
It should also be noted that the substrate discharge peak currents
were somewhat lower in the presence of incident ions; measurements on compara-
tive discharge strengths are continuing.
4.4 Conclusions
The results suggest that low-energy ions when drawn to electron-charged
regions on exposed spacecraft dielectrics may serve to reduce both discharge
frequency and strength, probably due to near-surface conduction processes.
5. HIGH-ENERGY BETA-IRRADIATION OF CAPACITORS
5.1 Introduction
The objective of the experiments to be described was to explore the
effects of high-energy, broad spectrum beta radiation from a 100 mCie
Strontium-90 radioisotope on a selection of capacitors. In previous work
it was noted that such a source when spaced from 2 cm to 6 cm from a target,
provides an approximate simulation of the high-energy part of the synchro-
nous-orbit electron environment, ranging from nuclear-enhanced to natural
conditions. In this work, a preliminary search was carried out for three
phenomena: electrical breakdown, RF conductivity and DC conductivity.
5.2 Electrical Breakdown
The following capacitor types were tested: Phillips capacitors de-
signated as solid aluminium foil;metallized film (342, 344 series); film
foil (347, 352 series); ceramic (type 2); ceramic disk; Micronics capacitors
designated as dipped silver mica. The procedure used was to expose the
2
capacitor in air to an estimated 4 pA/cm current density beam from the source
which was placed at a distance of 5 cm from the capacitor. The estimated
dose rate was of the order of 1 rad/sec. The voltage across the capacitor was
monitored by a high-impedance-input JFET type of operational amplifier con-
nected to a chart recorder. The normal voltage across the capacitor changed
very slowly with time, so that any sudden change could be readily identified
as a possible breakdown.
All the capacitors listed above were tested in this manner for approxi-
mately 30 minutes; and no sudden changes in voltage were recorded. The time
available did not permit higher dose rates through closer spacing of the
source, nor longer exposure, nor conduct of the experiment in a vacuum. Such
additional experiments in vacuum under higher-dose conditions would be
necessary to establish clearly whether or not discharges can be caused by
the 100 mCie Strontium-90 source.
5.3 RF Conductivity
The experiments were carried out using a Wayne Kerr Model B602 impe-
dance bridge, over the frequency range of 500 kHz to 2 MHz. The bridge
measures capacitance and parallel resistance to ± 1% accuracy. The capaci-
tors tested were: dipped silver mica (3.29 nF, 63.1 pF, 1.21 nF); film foil
(20.9 nF); metallized film (10 nF); ceramic (2500 pF, 0.01 uF).
Tests were carried out via a coaxial cable connected from the bridge
to the capacitor in its exposure cell made of lead bricks lined with
Plexiglas. Tests were done by connecting the capacitor directly to the coa-
xial cable, and also by adding an inductor to permit cancellation of the
reactance, thus allowing the bridge to be balanced under a wider range of
its internal settings. All tests were carried out with the source located
both at 4 cm and at 2 cm from the capacitor. No effect on capacitance or
conductance due to irradiation was observed.
5.4 DC Conductivity
In this experiment only the 2500 pF ceramic disk capacitor was tested.
The voltage across the capacitor was measured using a high-impedance opera-
tional amplifier, and the current through the capacitor was measured using a
picoammeter. The radioisotope source was located 4 cm from the capacitor.
The operational amplifier had an equivalent circuit consisting of a bias
voltage of the order of thirty volts in series with an internal resistance of
the order of 300 Gft, so that when the capacitor was attached it charged
slowly, coming to a steady state in about 15 minutes. At steady state the
internal resistance of the capacitor could be deduced from the measured vol-
tage and current.
Before irradiation the resistance of the capacitor was 138 Gft; during
irradiation it fell to 101 Gft; after irradiation it returned to 127 GO.
Repetition of the experiment caused the resistance to return always to 127 GO,
so if indeed there was a slight permanent effect of radiation on the capaci-
tor then certainly this effect was not cumulative.
Calculations based on the literature (see references 5, 6 and 7) and
on an estimate of the ceramic capacitor dimensions indicated that the observed
radiation-induced conductivity was comparable to typical computed values
(within one to two orders of magnitude) for a variety of insulating materials,
although figures for ceramics were not available. Therefore it is reasonable
to ascribe the effect observed as due to radiation-induced DC conductivity.
5.5 Conclusions
Under the specific experimental conditions of exposure of various
capacitors in air to a Strontium-90 radioisotope source, no electrical break-
down was observed and no change in RF impedance was detectable. For a cera-
mic disk capacitor, a 37% increase in DC conductivity was observed, the
change having been identified with reasonable certainty as radiation-induced
conductivity.
The phenomena searched for in these preliminary tests appear to be
either non-existent or weak. Nevertheless it would still be reasonable to
repeat the breakdown search and the DC conductivity measurements in vacuum,
using higher dose rates and longer exposures. In particular, the breakdown
10
tests should be done with the capacitor at its maximum normal operating
voltage.
6. REFERENCES
1. S.E. DeForest, J. Geophys. Res., Vol. 77, pp.651-659, 1972. See also
E.G. Whipple, Rep. Prog. Phys., Vol. 44, pp.1197-1250, 1981.
2. J.B. Reagan et al, Spacecraft Charging Technology 1980, NASA CP 2182/
AFGL-TR-81-0270, pp.74-85.
3. J. Wilkenfeld et al, RADC-TR-81-198, July 1981.
4. K.G. Balmain and W. Hirt, IEEE Trans. Nucl. Sci., Vol. NS-27, pp.1770-
1775, 1980.
5. B. Gross, M.G. Sessler, and J.E. West, J. Appl. Phys., Vol. 45, No. 7,
pp.2841-2851, 1974.
6. B. Gross, J.E. West, H. von Seggern, and D.A. Berkley, J. Appl. Phys.,
Vol. 51, No. 9, pp.4875-4881, 1980.
7. R.D. Reeves and K.G. Balmain, IEEE Trans. Nucl. Sci., Vol. NS-28, No. 6,
pp.4547-4552, 1981.
11
Table 1
SUMMARY OF INITIAL DISCHARGE RATES
Number of discharges on a sample
Figure
2
3
4
5
6
7
8
Material
Mylar
Mylar
Mylar
Mylar
Teflon
Teflon
Teflon
Electron
Flux
(nA/cm2 )
100
50
25
50
100
50
50
Ion
Flux
(nA/cm2 )
5
5
5
10
5
5
10
during first interval
to ion exposure)
Minimum Maximum
6
3
3
3
6
3
4
12
16
10
7
18
13
12
(prior to
Average
9'
9
6
5
7
8
8
12
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Fig. 1
1-Sample, 2-Faraday Cup,
3-Mask, 4-Insulator,
5-Substrate.
During calibration, a
Faraday cup" was behind
the mask aperture (1)
and the substrate was
absent.
L _ A 3 4 5
13
75 urn plain Mylar
100 nA/cm2 electrons
5 nA/cm2 ions
• averages
ions off on off on off
Time (5 min intervals)
on
Fig. 2 Normalized discharge rates for Mylar with an electron-to-ion
flux ratio of 20.
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60
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75 ym plain Mylar
50 nA/cm2 electrons
ions off on off on off
Time (5 min intervals)
on
Fig. 3 Normalized discharge rates for Mylar with an electron-to-ion
flux ratio of 10.
14
<u
75 ym plain Mylar
25 nA/cm2 electrons
5 nA/cm2 ions
• averages
ions off on off on off
Time (5 min intervals)
on
Fig. 4 Normalized discharge rates for Mylar with an electron-to-ion
flux ratio of 5.
2.5
0)
<u
60
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CO
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CO
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N
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Bh
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75 ym plain Mylar
50 nA/cm2 electrons
10 nA/cm2
averages
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Time (5 min intervals)
Fig. 5 Same as Fig. 4 but both currents increased by a factor of 2,
15
0)
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125 ym silvered Teflon
100 nA/cm2 electrons
5 nA/cm2 ions
• averages
Fig. 6
ions off on off " on " off
Time (5 min intervals)
on
Normalized discharge rates for Teflon with an electron-to-ion
flux ratio of 20.
<u
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125 pm silvered Teflon
50 nA/cm2 electrons
5 nA/cm2 ions
• averages
ions off on off on off
Time (5 min intervals)
on
Fig. 7 Normalized discharge rates for Teflon with an electron-to-ion
flux ratio of 10.
16
1.5'
•a
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01
N 0.5-
125 ym silvered Teflon
50 nA/cm2 electrons
10 nA/cm2 ions
• averages
ions off on off on off
Time (5 min intervals)
on
Fig. 8 Normalized discharge rates for Teflon with an electron-to-ion
flux ratio of 5.


Electron beam charging and arc discharging of spacecraft insulating materials

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