Charging characteristics of polyimide (Kapton) of varying thicknesses under irradiation by a very-low-curent-density electron beam, with the back surface of the sample grounded are reported. These charging characteristics are in good agreement with a simple analytical model which predicts that in thin samples at low current density, sample surface potential is limited by conduction leakage through the bulk material. The charging of Kapton in a low-current-density electron beam in which the beam energy was modulated to simulate Maxwellian and biMaxwellian distribution functions is measured

Dulgeroff, C. R.

Hymann, J.

Viswanathan, R.

Williamson, W. S.

English

NASA Technical Reports Server

q• -2251 3
KAPTON-CH_dLGINGCHARACT.ERLSTIC$: [/FECT_ OF MATERIALTHICKNESS
ANO ELECTRON-ENERGYDIS.TRIBU]ION ......
H. S. Wllliamson, C. R. Oulg_roff, and O. Hymann
Hughes ResearchLabora_ortes
,.: Maltbu, California 90265
R. Vtswanathan
Hughes. Space and CommunicatiOns Group_._
Los-AngeleS, California 90009
._. We report charging characteristics of polyimide (Kapton) of
_; varying thicknesses under irradiation by a
_ very-low-current-density electron beam, with the back surface of
_i the sample grounded. These charging characteristics are in good
_-_- agreement with a simple analytical model which predicts that in
_ thin samples at low current density, sample surface potential is ii
_ limited by conduction leakage through the bulk m_te_ial.. .
_ ; In a second investigation, we measured the cha_ging of
=_;--°_ Kapton in & low-cu_rent-density (3 uAm -2) electron beam in Which {I
__ the beam energy was modulated to simulate Maxwellian and :
i biMaxwellian distribution functions, i "
:i
INTRODUCT ION _ij
°i _.
The charging characteristics of dielectric thermal-blanket
materials in the geosynchronous-earth-orbit (GEO) plasma
environment is a subject of considerable importance in
_ _ spacecraft-charging studies. Polyimi_e (Kapton) is one of the
most commonly used materials in large-area thermal blankets, and
this material has been the subject o_ numerous previous
_xperimental investigations. Many previous studies, however, have
employed elect_on-beam current densities substantially higher
than the 3 uAm TM valOe which we believe represents a reasonable
upper bound in the GEO environment.
A second area of concern over ground simulations is that the
electron beams that are used are generally monoenergetic, and the
charging Characteristics that Would result in the
• distributed-energy GEO environment must be inferred from
monoenergetic charging data analytically. A model for
_i. accomplishing this inference is imbedded in the NASCAP (NASA
-_ Charging Analyzer Program) computer code, for example Since
_i _[
-,,_ NA_ _AP is currently being used by spacecraft designers to predict
'_ 547
p_U_,_3J{NG PAGE BLANK No.r t'ILMED
O0000006-TSG07
https://ntrs.nasa.gov/search.jsp?R=19850014202 2020-03-22T18:44:24+00:00Z
!i_ the- spacecraft-charging consequences of their designs,
experimental ver&flcatlon of the NASCAP modelling is very timely.
A credible distributed-energy electron source is needed, to
perform these experiments, and we describe a Simple mea,s ford vising such a source below.
S_S USED
I.B " electron-beam current density
(no sample in place)
Ip electron-beam current density!
(incident on charged sample)
CR Electron reflux coefficient
(including backscatter  Secondanies)
o bulk conductivity of sample
A sample area
d sample thickness
V Sample surface potential
i e_ electron-beam energy
_R curren_ of refluxed electrons (___Ip)
=dJBa-_
JB electron beant current dens/ty
fl (VB) electron--current distribution function
Only Sl units are used in this paper.
v, Tos caG,.a CSACTSST,CS
i': I_ _NOEI_RGETIC ELECTRON BEAMS
._ In this section we describe the experimental apparatus and
measurements and then present a simple analytical model for
_ .... comparison with the experimental results. _.
_ Eri_rlmental Apparatus and Measurements
_ .... conducted in a 0.6-m-diameter vacuum- _ The experiments were ..... =
!_ chamber, which i_ shown in Figure i. A divergent-beam electron
&-i\ is used to irradiate the sample_ _ f_ood gun, shown in Figure 2,
,%_. This electron gun is built to a design developed at NASA Lewis
_'.... Research Ce,_ter. It produces a broad uniform-current-density beam
and exhibits excellent stability over a wide range of current
densities. The sa:,Iple is housed in a separate antechamber which
can be isolated from the main chamber by a vacuum gate valve. The
geometry of the sample and sample holder is shown in Figure 3.
_ '4e have measured the charging characteristics of Kapton
_ unde. Condi tions which differ significantly from previous
.... practice: we employed an electron-beam current density of 3
-i ,Am -_, and we mounted the sample by wrapping it around the edges
* i of the metal sample-holder and clamping it at the rear. These two
_ !' innovations are more representative of the electron current
i
/:I
•,, 548
.............. .... .-_ . -...._ ................ -=.; .............................
"..... 00000006-TSG08
.i" density which exists at GEo, and of the geometry with w_hich
thermal-blanket materials are exposed to elect_on bombardment on
GEO spacecr_/t. We exposed Kapto_ samples of two thicknesses (25
,,,._: .m-and 127 pm)_ to irradiation by monoenergetic electron, beams
With energies _ to 14 keV_ A-typical charging characteristic is
shown in Figure 4, where the surface potential of 127-,m-thick
Kapton is shown as a f_nction of time, during continuou_
bombardment by 14-keV electrons. This plot reveals the relatively
long time constant involved in charging at those low current
densities.
t Figure 5 shows the equilibrium surface potential of
127-pro-thick Kapton as a function of incident beam-current energy
from 2 keV to 14 keV. The straight-line characteristic intercepts
"_ the beam-energy axis at an energy of about i_6 keY; this energy
,. corresponds to the "second-crossing '_ energy, i•e•, the energy at
which the secondary=electron yield of the material is unity.
F_ure 6 shows the corresponding characteristic of 25-_m-thick
Kapton, under the same test conditions• In this curve, the
:.!_ second-crossing intercept is the same, but the surface potential
saturates at were
_! much lower values than observed in the
127-pm-thick material. This saturation effect is produced by
condilction losses through the bulk Kapton, aS evidenced by the
fact that the saturation effect disappears when the ground
connection on the rear of the sample is removed; the
_._ characteristic then remains linear within the limits of our
beam-energy capability, We have developed a simple analytical
model which successfully predicts the surface potential at which
the saturatioR_effect occurs, This- model is similar to previoUs
_. models due to Purvis, et. al. (Reference i) and Reeves and_
Balmain (Reference 2), except that it conforms to the specific
!_ geometry of the test environment that we used. This model
__,ir_ predicts that the saturation surface potential of K.apton depends
•_,_: on the parameter k=dJ0 -_, where d is the sample thlckness, J is
-ti_ the electron-beam current density, and o is the material
-_:- conductivity. Material testing with thick (la'zge-d) samples and !
_/: high current densities (large J) raise the saturation potential
=_ beyor_d usual test limits. This explains why the saturation effect
._ shown in Figure 6 is not often seen in published
_,, dielectric-chargin9 results; the saturation is produced by the
_;._ use of thinner materials and lower electron-beam current
_._. densities than are. commonly used in material-charging te_tS.
__iii _tica]L Charg!n_ .Modeli
_";_i_I Xn t.hi_ section we present the simple model of samplecharging which we will evaluate by comparison with experimental
-_" data• This model is highly simplified and applies to an
equilibrium-charge condition (i•e., a condition in which
displacement currents and stored charge can be neclected). It
,_", , also assumes that cylindrical Langmuir-probe theory correctly
-_'_'_' . calculates the reduction in current that is collected by the
- sample &s it charges more and more negative
= ," ,'_ 549
00000006-TSG09
In equilibr-ium, the on, rent that is incident on a charged
sample is _lven by
• where the first term corresponds to re.fluxed electrons that are
ejected from. the surface by backscatte_._or secondary-elect_on
emission, and the second term corresponds, to conduction loss of
electrons through the sample to the grounded rear surface..
ACcording tO cylindrical Langmuir probe theory (Reference 3), the
current that is col_ected by a charged sample is related to the
incident current (i.e., current that would be collected by an
uncharged sample) by
VB-Vs
: Ip = IB _ (2)
mS,_
__ The foregoing expressions can be combined• to yield-the surface
_i_ voltage as a function of beam voltage:
__.- VBk (I_¢R)
Vs
= _ _ ,, (3}
-: l-e
-. Of course, the reflux coefficient _R is a function of the impact
/ energy of electrons which str_ke the surface, e (Vn-Vs) I this
function, along with the constants contained withiYi k, are in
general known only numerically. Equation (3) does not represent a
solution for Vu; it can, however, be solved for V s numerically by
_" a simple itera_ive procedure.
. We have solved Equation (3) numerically, using the Kapton
reflux-coefficient and conductivity data that is contained within
-_ NASCAP, and values of material thickness and beam current density
that are appropriate to our experimental conditions. The results
• are shown in Figure 7, where we compare the actual equilibrl.um
_ charging voltages with the values predicted by the model, for
..... several thicknesses of Kapton. The model predictions provid_
reasonably accurate descriptions of the shape of the charging
characteristics and the location of the beam energy at which
conduction-induced saturation of the charging voltage begins.
Figure 8 shows the solutions to Equation (3) plotted in a
different manner: here the parameter k is used as the independent
variable, and electron-beam energy is a parameter. Notice that
the curves are spaced evenly in surface voltage V_ for large
values of k (i.e., large current density and/or low co_ductance_,
and that ......the surface potential is independent of k in this
sso
-/i -
00000006-TSG10
F _L
• ' region. This case corresponds to the situation in which the
impactirg electro_ current, is balanced totally by electron
_ refiux. For low values of k, conduction losses beaoae
._ : signif/cant, causing the curves tO pack tog_ther_-at higher values
of VB. Figure 8 is a convenient tool- fo_ estimating Kapt_on
thatching; it could be easily reproduued fOr other dielectrics £o_
which cR is known by so_ving Equation (3).
L KAPTON (N_qRGING UNDER
ii_ DLqTRIBUTRD-ENBRGY ErJCTRON--BEAM II_qADIATION
[! As described in the Introduction, realistic simulation of
spacecraft charging should include .th e use of a
_i distributed-energy electron source. In .prznc_ple' of.course,
multlple exper._ments can be performed usz.ng monoenergetzc beams
and calcu.lating the expected surface potential uslng a model such
as that glven above.. Thi_ calculation entail._ solving Equation
(3) as before, but using an effectzve reflux_coefficient-such as ii
<c_>(v.)= (V..)dV. (4)
.
"_' xnstead of the monoenergetic ¢_ used above. This approach is '
hazardous because the effective c'" ma differ from that iven b I
Equation (4:); that is, the electrRon reflux which results _z Y'om. the I
simultaneous presence of two electron-energy species may differ
from the sum of the refluxes that would result from each energy
species being separately present. Such nonlinearities are well !
iil known in the case of sputtering of solids by ion bombardment, but _'
_,_"- we are unaware of electron-refluX data which would-, either
validate Equation (4] or provide a useful alternative.
In order to provide an empirical foundation for these
dlstributed-energy electron distribution considerations, we have
_ developed a simple and novel means of simulating the bLoad
distribution of electron energies which simultaneously bombard a
<<- spacecraft in GEe. We have used this tool to study the charging
? of 25-_m- and 127- _m-thick Kapton. We find that the thinner
material, which is in the conduction-dominated (saturated
_"!_ surface-potential) regime charges to essentially the same surface
potential regardless of the electron distribution function. The
thicker mate[ia!, however, experiences more severe charging when
exposed to electrons which are distribute_ in energy
corresFo_ding to "moderate" and "severe" charging conditions than
it _oes when exposed to a "quiescent" distribution of electron
energies.
Figure 9 illustrates the arrangement o_ our
_ distributed-energy electron source. The electron source, shown in
i, ,, Figure 2, is a simple hot-cathode Source with multiple wlr_-mesh
grids to extract a broad, spatially uniform @lectron beam. The
!
t
" _": 551
,, .. :_ _:"" " ....... 22.... "_ 2 ..... 2__2.......
00000006-TSG 11
_,W • !
e-gun is powered by a prOgrauuuable high-voltage power _upply
"" which operates over the range of 0 to 30 kV. Th_ power supply i_
programmed by a microcomputer; the microcomputer causes the powar
,_ supply to generate a repetitive sequence of electro_-beam
voltages. This repetitive sequence,_ which is defln_d in the
microaompster firmware is arranged in suah a manner that the
time-_ve_aged current produced by the e-gun- _t a given energy is
equal to that which exists in the GEe environment. That is# the
system selects an electron-head energy and caases the power
supply to output the corresponding voltage fo_ a time duration
•" that is proportional to the value of the desired current
_,. dist_.ibution functiOn at that energy. The energies a_e output in
_ _ an ascending-energy sequence, but we have found that the same
sample charging is produced- for descending- or random-energy
.! sequences.
This simulrtion is reasonable if the time scale within which
the electron-be& energy is varied is short compared to the
sample charging time. This condition is satisfied in our tests_
because the electron-beam energy is varied over a 5-s time
period, while the sample typically requires an hour or more to
reach equilibrium potential (in cases in which the sample is not
.. grounded at the rear surface, equilibration still requires
_ several minutes). The microcomputer is programmed to generate any
of three electron-energy distribution functions (which it selects
by reading the position of a front-panel switch); the programs
are written in a commercially available integer BASIC and a_e
L stored in ROM for conVenience. _
,J
The three distribution tdnctions we use are a biMaxwellian
distribution characteristic of quiescent conuitions (Reference 4)
and two single-Maxwellian distLibutions recommended by Stevens
! (Reference 5) to simulate the moderate and solar-substorm GEe
environmentS. The three distribution functiOns have identical
',"_  totalcurrents. These distributions are plotted in Figure I0.
" Figure II shows the response of both 25-_m- and 127-urn-thick
Kapton under irradiation by the multienergetic-beam apparatus.
The thicker Kapton charges to substantially higher voltages and
:_ evidences a larger difference betwee_ the three distribution
:¢ functions than does the thinner material. It is clear that the
_!:' single-Maxwellian distribution that simulates "moderate" char_ing
'_ conditions is significantly more severe than the biMaxwellian
._; "quiescent" condi tion.
+_+
.... REFERENCES
"" I. C.K. Purvis, N.J. Stevens, and J.C. Oglebay, "Charging
:,, Characteristics of Materials: Comparison of Experimental Results
_: " precedings of the spaceccaft. with Simple Analytical Models, ....
_ Charging Technology Conference, AFGL-TR-77-0051, NASA TMX-73537 , ....
: p. 459 (1977).
i
_:_.. 5,52
.::._,,_ ! ..__'_._%::',." . ...... _........_-_-..:.
,,:',',:,....:, :.... : "'' "...... 00000006-TsGI2
2. R.D. Reeves and. K.G. Balmai_, "_wo-Dimen_ional El_ct_on Beam
" IE_ '_ans. on _uC. Sai_Ch_gin_ _todel for Polymer Films, . -
') Pure ap.rl Applied t_h_.qic_ 21_.3.-F,F.- Chen_ "Electric Probes, ......... ...o: ... ............
Plasma Diagno:_tic To__ ods.- R. It.. Hucld].estone and S.L.
.... 4. H.B. Garrett and _. E. DoFo_rest, " An Analytical Simulation
of the Geosynchronous Plac_na F.nvironment_" Plano t. Space S_i. ?.7,
ll0z (1979),
5. N.J. Stevens, "Design P_actices for Control].in_ Spacecraft
Char_ino Interactions," AIAA _aper NO. 82-0_15, presented at the
AIAA 20th Aerospace Scier=ces Confe_ea_ce, Orlando, Florida, 11-14
January 1982.
i_: i
i_¸
i__ -----_ l.,iim
.r----c --------------- = - -_ - ..."_'--1 ]1 SAMPLE; BEA'i lPl i:l., :', II 'n ./_i,._:"u<M_""
CONTROLS_ ]]l ,l ISOURCEL-: I I ]] ,,-GHI __
I HIGii'VO-L'I'-'iT'GIlII _J H l--¢-Oki_i,,TR--
. _, wi,N_="-_I '-----u"" / SU_LVIII :. i
,, iiu__--=-="-=: :: ..... -,cup _ ..
" o.,,.--
i , BEAMSUPPLY MAINVACUUMCHAMBER SAMPLETEST nECOROING
SECTION EQUIPMENT
'_ EJgure l. - Hughes spacecraft-Charging-simulation facility: overview.
• 553
00000006-TSG13
I|
6nin
% \ I"I!V[I11_1;NT
\ \ [ LECTII(]N
_ATltO0[StofloUf_\ '_, ,_FI_,._FCTOnO[I
. I..S£m.vJ / I / ,' /
I l.nOUo_ / ... t ,.
Figure 2. - E!ectron source (NASALewis design).
:-'.l'f •
SHIELD \ I 3- J i
- .\ , -_, ----_.
i_" ALUMINUMCLAMP_.-'---_q ----I \
_-_;_ FARAOAY e--------
At203 INSULATOR _ i!I CUP
--, .... _----_-,_- ------_ _C_.,140 191t_' e--.---
" IOACK_) :J "------
16 _ TREK PROBE.--_ _-
•:, _.---- 150---_ L
'WALL I _vSAULL_TED 'WITNESS
Figure3. - Hughesspacecraft-charging-simulationfacility:sampietest
sectiondetails.Dimensionsar'einmillimeters.
i
•. 554
........................ _ ................................ .................................... _............ ........ -_ ._=.. 5...... .___.;=;,-.... = ...............................
O0000006-TSG14
I14 T T
_:Z_ > lo12 O
•J 8
' _ e - !
.! _ li
I
!
• 2 - J
_.2 o( 1 1
----'_. 0 0.5 1.0 1.5
_:- CHARGINGTIME, At, hr
-
_ Figure 4. - Surfa_cepotentialof 127-.m-thickKapton under irradiationby a
14-keV, %uAm-" electron beam.
" 14 I I I i I I I
12 -
ii'; >
_', _< e
z
_ 6 -
_: 4 -
2
J t t t t I0
0 2 4 6 8 10 12 14 16
ELECTRON BEAMENERGY oVB.keV
Figure5. - Eq.ilibriumsurfacepotentialof 127-.m-thickKaptonas a function
of electronbeam energy.
• _ 555
C -7
00000007
[ >
.' 0 0 2 4 6 6 t0 12 t4 16
ELECTRON-BEAMENERGY OVo. key
".;v Figure 6. - Surfacepotentialof 25-,m-thickKapton.
_C_' 16
_o, _: SYMBOL THICKNESS
;"-'-]'.7"]
:"=._:' 0 127M_m
--' _, 14
;-__.- O 25M,m
.J
i:.5
i-__ < 0e, M,
_!,% nc 6
0 2 4 6 O I0 12 14 16
' r ELECTRON-BEAMENERGY,eVB, keV
!i Figure7. - Comparisonof predictionsof analyticalmodel with experimental
- resultsfor chargingof Kapton of severalthicknesses.
' 555
.............. 00000007-TSA03
20 ----t--_ _,rf'.| • , ,-t-,,,,, I , -, ,,,,.| t r_rl_,,,,
18
t6
14
14t2
10 12
i S 10 ,
S
e
6
' !i4
,4- 2 i.t
0
1 10 100 1000 10,000
:... k "d J6 0''1, kV
Figure 8.-Analytical charging model for Kapton, showing surface potential as
a function of k = dJBo-1, with beam voltage VB as a parameter.
0 t
IOtSTRtBUTtON-FUNCTtON 0 5s 10S '_
SELECT "_ I I
!! RONT PANEL SWITCH)
o..,o vIMICROCOMPUTER },,--,,=-I_ DAC _ PROGRAMMABLE/ _ I POWERSUPPLY '
i STORED
OISTRIBUTION
FUNCTIONS:
1) QUtESCENT
2) MODERATE
3) SEVERE ELECTRONGUN
Figure 9. - Block diagram of multienergetic electron beam.
'IlxlOI[_'_'_T_'_T ..... I-- I .... 1 - I -1 1 - ii
I ,/DOUBLEMAXWELLIAN- I
l / (0,25. OkeV) . /
;_"_ ! X to°L/ GARRETT DATA (0EST FIT) .-I
,a e I1#-l_'- SINGLEMAXWELLIAN5 /|
.. _| -- MODERATE 18ke'V) J
I .,/_SEVERE.1 _ov_ /
_ 1 x _0-2
i I xt0 "4
_ 1xlo-s L I I i i I I I l
:.: 0 5 10 15 20 25 30 35 40 45- 50
i_ , ELECTRON ENERGY.keV
!_: Figure 10. Electron energy distribution for simulation experiments
14
'_. 12 -
>
I_ .j
'r.. r- _o-
- 0
_-- ¢w
< 8 --
i_. 2
O
_-:, uJ 4 2S-#m THICK KAPTON _
i
i 2 -
i- 0
Figure 11. - Equilibrium surface potential of 25- and 127-urn-thickKapt,o,nirradiated by 3-_Am- multienergetic electron beam: (1) = "quiescent ,
i (2) = "moderate", (3) = "severe" charging environments.
' 558
_-- c"
i
00000007-TSA0i


Kapton charging characteristics: Effects of material thickness and electron-energy distribution

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