We use a 1-D bulk charging model to evaluate dielectric charging at cryogenic temperatures relevant to space systems using passive cooling to <100K or extended operations in permanently dark lunar craters and the lunar night

Blackwell, W. C., Jr.

Coffey, V. N.

Garrett, H. B.

Jun, I.

Minow, J. I.

Parker, L. N.

NASA Technical Reports Server

Bulk Charging of Dielectrics in Cryogenic Space Environments
J.I. Minow1, V.N. Coffey', W.C. Blackwe11 2, Jr., L.N. Parker, I. Jun 3 , and H.B. Garrett3
'NASA, Marshall Space Flight Center, Huntsville, AL 35812
2Jacobs Engineering, ESTS Group, Huntsville, AL 35812
3The Jet Propulsion Laboratory, The California Institute of Technology, Pasadena, CA 91109
35 Word Abstract
We use a 1-D bulk charging model to evaluate dielectric charging at cryogenic temperatures relevant to
space systems using passive cooling to <100K or extended operations in permanently dark lunar craters
and the lunar night.
Corresponding (and presenting) Author:
Joseph I. Minow, NASA, Marshall Space Flight Center, EV 13/Natural Environments Branch,
Spacecraft and Vehicle Systems Department, Huntsville, AL 35812 (USA), phone: 256-544-2850,
fax, 256-544-0242, e-mail: joseph.i.minow@nasa.gov
Contributing Authors:
Victoria N. Coffey, NASA, Marshall Space Flight Center, EV I3/Natural Environments Branch,
Spacecraft and Vehicle Systems Department, Huntsville, AL 35812 (USA),
phone: 256-961-7635, fax, 256-544-7216, e-mail: victoria.n.coffey@nasa.gov
William C. Blackwell, Jr., Jacobs Engineering, ESTS Group, Marshall Space Flight Center,
Huntsville, AL 35812 (USA)
phone: 256-544-6741, fax, 256-544-0242, e-mail: william.c.blackwell@nasa.gov
Linda N. Parker, Jacobs Engineering, ESTS Group, Marshall Space Flight Center, Huntsville, AL
35812 (USA)
phone: 256-544-5313, fax, 256-544-0242, e-mail: linda.n.parker@nasa. og_v
Insoo Jun, The Jet Propulsion Laboratory, The California Institute of Technology, 4800 Oak Grove
Drive, Pasadena, California, 91109, USA
phone: 818-354-7107, fax, 818-393-4699, e-mail: insoo.iun@ipl.nasa.aov
Henry B. Garrett, The Jet Propulsion Laboratory, The California Institute of Technology, 4800 Oak
Grove Drive, Pasadena, California, 91109, USA
phone: 818-354-2644, fax, 818-393-4699, e-mail: henry.aarret@jpl.nasa.gov
Session Preference: Space, Atmospheric, and Terrestrial Radiation Effects
Presentation Preference: Oral
^9https://ntrs.nasa.gov/search.jsp?R=20090028719 2019-08-30T07:44:20+00:00Z
INTRODUCTION
Internal electrostatic discharges (IESD) originating in dielectric materials charged during exposure to the
space radiation environment accounts for over half of the anomalies and failures of spacecraft and space
systems that have been attributed to the space environment [1]. Charge accumulation is particularly
important at cryogenic temperatures because the electrical conductivity a for semiconductors and
insulating materials generally exhibits a — 1/T44 temperature dependence where N depends on the material
and physical mechanism for electrical conductivity (which varies with temperature). The reduced
conductivity at low temperatures greatly limits current flow from the charged dielectric allowing charge
densities to accumulate for longer periods of time. This effect increases the risk of dielectric breakdown
and IESD due to the enhanced electric fields generated by the buried charge.
Space systems operating at cryogenic temperatures are found in a number of applications [2]. One
example is infrared and microwave astronomy missions because low operating temperatures reduce the
background of long wavelength photons and increase the sensitivity of infrared and microwave sensors.
Passive cooling to —70K is currently used by the Wilkerson Microwave Anisotropy Prove spacecraft in
orbit about the Sun-Earth L2 point and will also be used to cool the instrument systems on the James
Webb Space Telescope to —40K (also bound for L2). Passive cooling technologies represent a particular
threat to charging because the requirement that systems be exposed to the cold background of space to
achieve low operating temperatures also means they are exposed to the space radiation environment
responsible for charging. Cold environments will also be encountered in future lunar exploration where
lunar night time temperatures of approximately 85K are observed immediately before sunrise [3,4] and
temperatures as low as 40K to 50K will be encountered in the permanently dark craters at the lunar poles
[5,6,7]. Radiation environments at lunar and L2 distances are generally considered relatively benign
compared to the extreme bulk charging environments within the Earths radiation belts. However,
evaluation of bulk charging is an important step in the design and qualification of space systems. This is
particularly true for systems at cryogenic temperatures because of the potential threat of enhanced
charging due to the reduced conductivity of insulating materials at cold temperatures.
We first describe a bulk charging model developed for evaluating electrostatic discharge risk in insulating
materials exposed to radiation environments. We use the model to evaluate the electric fields generated
in cold insulators exposed to interplanetary radiation environments applicable for both the earth-sun L2
and lunar destinations. The model results are used as a screening tool; demonstrating the order of
magnitude charging risks anticipated for standard aerospace insulating materials at both ambient and cold
conditions.
BULK CHARGING MODEL AND SIMULATION RESULTS
The 1-dimensional bulk charging model is a modified version of the NUMIT (for "numerical integration")
code originally developed by the late Dr. Robb Frederickson [8,9] using the radiation induced conductivity
approach [10] for solving the charging equations
V•E=^/e 	 (1)
ap
at 
= —V • (JR + JC)	 (2)
where p is the charge density, E the electric field, J R
 and Jc the incident radiation current density and
conduction currents, respectively, and the dielectric permittivity s is related to the permittivity in free space
so and dielectric constant x through the relationship 6 = xso. Conduction currents are defined as
JC = 6E = a"dark + 4 dt) a E	 (3)
where the bulk conductivity a is divided into two terms: the aaazk conductivity in the absence of exposure to
radiation and a radiation induced conductivity (RIC) term which depends on the dose rate dy/dt and material
dependent conductivity coefficient K and exponent a. Material electrical properties which are temperature
dependent include 6, x, and to a lesser extent the RIC parameters K and a.
NUMIT solves equations (1) to (3) numerically yielding self-consistent solutions for the electric fields
generated by the charge density deposited in insulating materials exposed to the space radiation
environment. We have modified NUMIT to extend the original fixed radiation current and mono-energetic
electron energy input to allow for reading a time series of electron flux data from spacecraft measurements.
In addition, options for electron flux input to the model includes use of mono-energetic flux from individual
energy channels as well as spectral fits to extend the spectra to arbitrary energies or use of a complete
spectrum. The mono-energetic flux option is used for its simplicity in the examples given here. Radiation
current inputs are derived from —9.13 years of 245 keV electron flux measurements at L1 by the Deflected
Electrons (DE) detector component of the Energetic, Proton, and Alpha Monitor (EPAM) instrument on
board the Advanced Composition Explorer (ACE) spacecraft. Transport of the radiation environment into
the dielectric material required to obtain the charge deposition and dose rates are accomplished using look-
up tables for dose as a function of depth [11,12]. This method provides a computationally efficient method
for updating dose rates and charge deposition at each simulation time step.
We adopt the representative electrical parameters given in 	 Table 1. Dielectric Electrical Properties
Table 1. Dielectric conductivity at cryogenic temperatures
(T<100K) compared to values at ambient 'temperatures Property 300K 100K
(T-300K) suggest conductivity ratios for polymers of a 1.00x10"16 S/m 1.00x10"19 S/m
interest to	 aerospace applications	 are x 3.71 7.42
6B(T<100K)/aB(T-300K) — 10"2 to 10.5 [13,14] and the K 2.76x10-16 S/m 2.76x10"16 S/m
dielectric constants increase over the same temperature a 1.00 1.00
range by factors of KTsloox /KT-soox ~ I to 2 [15,16,17,14].
Output from NUMIT for T-300K conditions is shown in Figure 1. Electric field magnitude and charge
density as a function of depth in the insulator is given in the top two panels. Electron current density (blue)
of the 139 keV charging electron beam (black) is given in the third panel. The bottom panel is the
maximum (black) and minimum (blue) electric field extracted from the results in the top panel. The orange
and red lines provide guidance on the approximate nominal and extreme electric fields, respectively, where
dielectric failure may occur based on --1x10 8 V/m dielectric strengths (red) reported in the literature for
many polymers [18] and a more conservative _1x10 7 V/m value (orange) suggested for use in space
applications [19]. In this example we demonstrate that exposure to 139 keV electrons at T-300K may
charge the material to values approaching the lower breakdown strength but never exceeding critical values
for dielectric failure. Charge densities and electric fields within the material are elevated only when directly
exposed to the high flux events and the electric fields rapidly decay with time constants on the order of days
when the electron flux decreases to background values.
Charging results for the T-100K electrical properties from Table 1 are given in Figure 2. Reducing the
conductivity and increasing the dielectric constant to values appropriate for cryogenic conditions increases
the charging time constant to such a large value that the dielectric integrates charge over the
complete exposure period and electric fields ,approach the breakdown strengths of polymers in
environments generally considered relatively benign for bulk charging.
_ 0-10
E
E
x 0.01
0-10
E
E
X
0.01
S
1000.0
100.0
10.0
1.0
W	 01
10'
E 108
106
W
104102
8.o 
-1E-6.6
5.2
3.8 W
2.4 O^
1.0 s
1.0
0-2 U
-1 -4 O
-2.6 O=[
-3-a o
-5-O 0
t2
O v
-2 c
_q o
-10 0
0-10
E
E
x 0.01
_ 0-10
EE
X 0.01
S 1000.0100.0
10.0
1.0
"'	 011010
E 108
a 106w 104
102
8.0 E
6.6 ^ii;
5.2
3.a e
2.41.0 -
1.0 G
-0-2 L)
;7-
-1-4 O
-2-6 O=C
-3_a a
-5-0 m0
+2
o
_2 Q
a
-4 o
-6 3
-10 p
500	 1000	 1500	 2000	 2500	 3000
Time (day)
Figure 1. NUMIT T - 300K Results. A -0.23 mm thick dielectric is exposed to 245 keV
electrons for -9.13 years with only moderate charging results during the high electron flux events.
500	 1000	 1500	 2000	 2500	 3000
Time (day)
Figure 2. NUMIT T - 100K Results. Conductivity and dielectric constants are modified to the
T-100K values in Table 1 with more extreme charging due to the greater charging time constants
at lower temperatures. Equilibrium electric fields are still less than breakdown in this example.
CONCLUSIONS
We have described an application of the NUMIT 1-dimensional bulk charging model for evaluating
radiation charging of dielectric materials in space environments. Novel features of the modified version
of the NUMIT model include the capability of reading long (years) electron flux time series for use in
screening environments for potential risk of electrostatic discharge when dielectric materials are exposed
to the radiation environment at both ambient and cryogenic temperatures. The model facilitates bulk
charging evaluations for spacecraft design and operations support as well as anomaly analyses when
appropriate environment data is available. In addition, the model provides a useful tool for evaluation of
total radiation dose, dose rate, and electric field effects. This information can be used to define
parameters for incident electron beams in laboratory test protocols for the qualification of materials for
space environments.
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[3] Mendell, W.W., and F.J. Low, Preliminary results of the Apollo 17 infrared scanning radiometer,
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g of Dielec in Cryogenic Space Environments IRI 
3. Blackwd12, Jr., L.N. Psrke6, L Jun3, and BB. Garrett3 
CaIifmh InstRute of Technology, Pasadana, CA 91 109 
Example Malerlals Propsrties as Function d Temperaturn 
Bu!k r c r i . ( l v i i  (=lkonduclhHyl 
. I.. 
. a-~.id;* , 4 ; .  .. - . ; a 
a. .* f 
-7 J. .- 


Bulk Charging of Dielectrics in Cryogenic Space Environments

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