NASA Technical Reports Server (NTRS) 20030015491: The Neutral Gas Desorption and Breakdown on a Metal-Dielectric Junction Immersed in a Plasma

7 Pages·4.4 MB·English

by  NASA Technical Reports Server (NTRS)

AIAA 2002-2244
THE NEUTRAL GAS DESORPTION AND BREAKDOWN ON A MET AL
DIELECTRIC JUNCTION IMMERSED IN A PLASMA
. *
Bons Vayner
Ohio Aerospace Institute and NASA/Glenn Research Center
Joel Galofaro"", and Dale Ferguson§
NASA/Glenn Research Center, Cleveland, OH
ABSTRACT INTRODUCTION
New results are presented of an experimental study The current paper is a continuation of our study [I
and theoretical analysis of arcing on metal-dielectric S] of arc inception on a triple-junction with a
junctions immersed in a low-density plasma. Two conductor negatively biased with respect to the
samples of conventional solar arrays have been used surrounding plasma. Previous experiments have
to investigate the effects of arcing within a wide clearly demonstrated that an electrostatic discharge
range of neutral gas pressures, ion currents, and occurs when the electric field strength is
electron number densities. All data (except video) approximately 1 MY/m, which is almost one order
were obtained in digital form that allowed us to of magn itude lower than is needed for in itiation of a
study the correlation between external parameters vacuum arc [6]. In order to explain this fact we
(plasma density, additional capacitance, bias voltage, suggested that the ion ization of adsorbed gas (water
etc) and arc characteristics (arc rate, arc current pulse vapor) plays the main role in the generation of
width and amplitude, gas species partial pressures, initial plasma cloud and the plasma sheath which
intensities of spectral lines, and so on). Arc sites were lead to the enhancement of an electric field. Two
determined by employing a video-camera, and it is field enhancement mechanisms, protrusions on a
shown that the most probable sites for arc inception metal surface and plasma polarization, can provide
are triple-junctions, even though some arcs were a high enough electric field (1-2 GY/m) to generate
initiated in gaps between cells. The effect of surface cold emission current with the density 1011_1012
conditioning (decrease of arc rate due to outgassing) A/m2. The plasma conductivity allows an electric
was clearly demonstrated. Moreover, a considerable current of 10-50 A, depending on the conditions of
increase in arc rate due to absorption of molecules the experiments, to discharge a negatively biased
from atmospheric air has been confirmed. electrode for short time intervals (10-50 /-ls).
The analysis of optical spectra (240-800 nm) A simple model of a discharge quenching due to
reveals intense narrow atomic lines (Ag, H) and wide water ions dissociative recombination has been
molecular bands (OH, CH, SiH, SiN) that confirm a developed, and its prediction of the relation
complicated mechanism of arc plasma generation. between pulse width and capacitance (rIXCI12) is
The rate of plasma contamination due to arcing was confirmed for a wide range of parameters (C=O.O1 -
measured by employing a mass-spectrometer. These 1 /-IF, U=300-800 V). Moreover, hydrogen and
measurements provided quite reliable data for the hydroxyl spectral lines were observed in emission,
development of a theoretical model of plasma and measurements revealed strong correlations
contamination. In conclusion, the arc threshold was
between the intensity of Hex line and the value of the
increased to above 350 Y (from 190 Y) by keeping a
initial electrical charge. The observation of metal
sample in vacuum (20 /-lTorr) for seven days. The (Ag, Cu) spectral lines is an unarguable argument
results obtained are important for the understanding in favor of a high density Fowler-Nordheim current
of the arc inception mechanism, which is absolutely heating a metallic protrusion to vety high
essential for progress toward the design of high temperatures. Together with the observed decrease
voltage solar arrays for space applications. of arc rate with number of arcs (conditioning) and a
considerable rise of arc threshold for a sample that
* has been kept under high vacuum for a few days, all
Ohio Aerospace Institute, Senior Scientist, Member AIAA
these details can be incorporated in a cohesive
'Physicist, Member AIAA
Senior SCientist, Member AIAA model of arc inception.
§
It should be noted that the role of electron impact
Copyright © 2002 The American Institute of Aeronautics aJ1d desorption of water molecules was thoroughly
Astronautics Inc. All rights reseIVed
discussed almost ten years ago [7,8], and the
. . IT his is!n~re~rlnt or reprint of a paper intended for presentation at a
American Institute of Aeronautics <lner on Weause changes may be made before formal
comer e. . h . '11
publication, this is made avai.'ab'e with the u.nderstand1l1g t at It WI
not be cited or reproduced Without the permisSion of the author.
increase of arc rate due to increase of water vapor excited atoms can be derived from these
partial pressure was confirmed by experiment [9]. measurements:
The current paper is devoted to the analysis of x = N*(H) = 1(656) . 7](328)· A
experimental data containing optical emission spectra (1)
of arc plasmas and mass spectra of a low-density N*(M) 1(328) 7](656)·A32
plasma contaminated by species emitted from arc
where N*(M) is the number density of exited
sites. The results obtained with these techniques are
metal atoms (Ag), I(A,) is the observed line
in a quite good agreement with each other. Both
methods revealed the presence of water-ion intensity, Aik is the transition probability, and 77 (It)
is the photocathode quantum efficiency.
dissociative recombination products - hydrogen and
hydroxyl - in respective plasmas. The incorporation
of these new data in a simple model of ionization Substituting measured intensities in Eq.(I) results
in the following estimate: X=0.6-0.7. The degree of
balance in an arc plasma made it possible to estimate
ionization of water molecules Y and silver atoms Z
the number of water molecules emitted in the
surrounding plasma due to the discharge on triple can be estimated from the simple model of
ionization balance: Y=0.01-0.04, and Z=0.1-0.2
junction.
(see Ref.S for details). According to experimental
study of vacuum arcs with silver cathode, an
2.CHEMICAL COMPOSITION OF average ion charge is Q=2.14, and an erosion rate
is about 1017 ions(Ag)/C [II]. On the other hand,
ARC PLASMAS
computer simulations and direct measurements
Experimental setup is described in all details in
show erosion rates about 10 0 ~lglC [6,12], which
Refs. [4 , I 0]. Optical spectra measurements were
correspond to a degree of ion ization of silver atoms
performed in a small vacuum tank (45 cm diameter
Z=0.15 in very good agreement with the estimate
and 75 cm height) installed at the NASA Glenn
obtained from optical spectra. Taking into account
Research Center. Vacuum equipment provided
differences between the ionization degrees for
pressures as low as 10-7 Torr. A Penning source
metal atoms and water molecules, one can estimate
generated an argon plasma with the electron density
the rate of water em ission from an arc site as R=
17.=(2-20) I 05 cm-3 , temperature Te=2-5 eV, and (3-6)1018 moVC. However, according to Ref. 13,
neutral argon pressure p=(5-80) 10-5 Torr, which
the electron impact desorption efficiency is about
could be adjusted during the experiment. Two solar
0.02-0.05 molecules/electron, which corresponds
array samples consisting of nine Si cells each were
to at least one order of magnitude lower R. The
prepared. These 2x4 cm cells were connected in three
integrations of current pulse wave forms over time
parallel branches with three cells in series for each
provide numbers for lost charges (Fig.2). The
one. All cells were mounted on a fiberglass base. One
analysis of a few hundred measurements shows
of the two samples was mounted vertically in the
that a simple estimate for a lost charge
center of the chamber, and it was biased by a high
i1q = fI(t}it = (0.8 - 0.9)· CU (2)
voltage power supply through a 10 kD resistor.
Diagnostic equipment included one spherical is valid within a wide range of capacitances C and
Langmuir probe with diameter d=2 cm, one current bias voltages U . Thus, it seems possible to
probe to measure arc current, a voltage probe, an estimate the number of water molecules N ejected
intensified CCD camera with spectrometer, and a from the arc site for each experiment. For example,
video camera for recording arc images and arc sites. arc plasma optical spectra (Fig. I) and mass spectra
The exposed sides of contacts and connecting wires of low-density plasma (shown below) were
were insulated by RTV and Kapton strips. Because obtained with bias voltages U=250-1000 V and
the sample itself had a low capacitance, an additional capacitance C=l ~F. In these conditions, simple
capacitor was installed between the sample and calculations result in the following:
ground. This allowed us to locate arc sites visually. N = R· i1q = (0.5 - 2.5) .1015 (3)
The measurements of mass spectra were performed
in a large vacuum chamber (1.8 m diameter and
The last result is very important because it
2.5m length) by employing a quadruple mass
demonstrates with a high degree of celtainty that
spectrometer. It should be stressed that the plasma
the water adsorbed on a dielectric side surface does
parameters were practically identical in both
not form a monomolecular layer, and the exposure
chambers. One example of arc plasma optical spectra
of an array to a high vacuum and/or high
is shown in Fig.l. The ratio of number densities of
temperature can cause "drying" of triple junctions
that leads to a rise of arcing threshold voltage.
2
American Institute of Aeronautics and Astronautics
Two more observations SUppOlt the hypothesis of To obtain an estimate for the number of excited
desorbed water ionization as the main source of the hydrogen atoms one can disregard the differences
arc development: a) one triple junction can in arc pulse widths for different intensities. Then,
experience 15-20 arcs being biased at fixed voltage Eq.8 transforms into the simple relation
when the capacitance is low (C=O.O I f..lF), but the N: =107 .! (9)
same junction arcs on Iy 3-5 times with bigger
capacitance (C=I f..lF); b) the intensity of the
Thus, measurements of optical spectra indicate the
hydrogen spectral line (Ha) increases with
number of excited hydrogen atoms of 101°-1011
increasing lost electrical charge (Fig.3).
(see Fig.3). This last result is in excellent
It seems reasonable to estimate the number of
agreement with an estimate of the number of
hydrogen atoms in the arc plasma by using
desorped water molecules (Eq.3) and the degree of
measurements of H a line intensities. The number ionization (~1O·2) calculated in Ref.5.
of quanta radiated by excited hydrogen atoms per
The rate of the background plasma
second is
contam ination caused by arcing was also measured
by employing the quadruple mass spectrometer.
(4)
Several examples of mass spectra are shown in
FigA. It seems reasonable to develop a simple
The number of quanta entering the spectrometer
analytical model of this process to compare the
window during one pulse can be calculated as the
results of optical and mass spectrometry. The rate
following:
of plasma contamination by ejected molecules
-. hd
~a = A23N M · 27rD2 . j3 (5) (atoms) is determined by the following equation:
H
dN(t) = R . .6q . r - a . N(t)
where d=50 f..lm and h=I.2 cm are the width and dt (10)
the height of the slit respectively, LIt is the arc
where N(t) is the number of ejected molecules
current pulse width, D=40 cm is the distance
(atoms) in surrounding plasma, and ex is the
between the arc site and the slit, and ~=0.9 is the pumping rate.
transparency of the vacuum chamber window.
The solution of this equation is simple
The number of quanta entering the CCD is
expressed by the formula [14]: N (t ) = R . .6q . r . (1 _ e -al ) , (I I)
a
~ell = ~ h· .6,1 (6)
and it provides the possibility to determine
~a 16f·s parameter ex from the measured dependence of
p(t) N(t)
partial pressures vs. time: ex; (Fig. 5).
where Ll4 is the spectrometer resolution,j=25 cm is
It is clearly seen that the paltial pressure of atom ic
the spectrometer focal distance, ~ ",0.1-0.3 is the
hydrogen grows considerably, while the partial
factor accounting for light absorption/reflection,
pressures of other components stay practically
and s= 1/ 1200 mm is the spectrometer grating.
steady. The increase in paltial pressure of species i
due to arcing can be expressed by the following
The measured intensity can be written as
formulae:
! = 7J' g . j . ~ell (7)
( 12)
where lF3 is the photocathode efficiency for
4=656 nm, g=128 is the electronics gain, andj=30
is the number of CCD horizontal rows used in where Tb is the background gas temperature, Pib is
current measw·ements. partial pressure of respective component in the
16
Finally, after simple algebra, the average number background gas, k=1.38* 10. ergs/K is the
of excited hydrogen atoms can be expressed by the Boltzmann constant, and V is the vacuum chamber
following formula: volume.
N: 327r' D2I s
2 ! (8) The first stage of measurements was performed
7Jgj~fJdh .6,1. A23M with bias voltage U=-600 V, and arc rate r=0.5-1
3
American Institute of Aeronautics and Astronautics
arc/so As one can see in Fig.6., a real chance to Netherlands, pp. 469-474 (NASA/TM-2001-
register an increase in partial pressure exists for 211070).
species with background partial pressure lower 4. Vayner, B., Galofaro, 1., Ferguson, D., and Degroot,
than 2.S flTorr. This observation explains the well W. "Arcing Onset on a Metal-Dielectric Junction
expressed trend registered for hydrogen partial Immersed in a Low Density Plasma", Proc. XXVth
pressure (Fig.7). At the same time, it is difficult to International Conference on Phenomena in Ion ized
measure changes in water vapor paltial pressure Gases, 2001, July 17-22, Nagoya, Japan,
because its initial concentration is high: p=2.S Vol.3, pp.293-295.
j..lTorr. Thus, the measurements of mass spectra 5.Vayner, B., Galofaro, 1., Ferguson, D., and
confirm the presence of atom ic hydrogen in the arc Degroot, W. "Electrostatic Discharge IJ1ception on a
plasma. Moreover, the measured increase of H igh-Voltage Solar Array", 40th Aerospace
hydrogen paltial pressure is in a quite good Sciences Meeting & Exh ibit, January 14-17,
agreement with the analytical estimate (12), and 2002/Reno, NV (AIAA Paper 2002-0631).
this result can be considered as one more argument 6.Handbook of Vacuum Arc Science and
in favor of the arc inception model developed in Technology, Eds. R.L. Boxman, D.M. Sanders, and
Ref.5. P.J. Martin. NP, New Jersey, USA, 1990.
7. Hastings, D.E., Weyl,G., and Kaufman, D.
3.CONCLUSJON "Threshold Voltage for Arcing on Negatively Biased
Solar Arrays", Journal of Spacecraft and Rockets, 1990,
The considerable decrease of arc rate due to solar Vo1.27, No.5, pp.539-544.
cell heating was demonstrated almost ten years ago 8.Mong, R., and Hastings, D.E. "Arc Mitigation
[9]. In that paper, the authors suggested that the on High Voltage Solar Arrays", Journal of
evaporation of water molecules from the dielectric Spacecraft and Rockets, 1994, Vol.3l, No.4,
side surface could be responsible for the observed pp.684-690
effect. Recently, new evidence was obtained to 9.Upschulte, B.L., Marinelli, W.1., Carleton, K.L.,
confirm the role of water vapor in the arc Weyl, G., Aifer, E., and Hastings, D.E. "Arcing on
initiation process [4,S]. The results shown in the Negatively biased Solar Cells in a Plasma
current paper leave no doubts that adsorbed water Environment", Journal of Spacecraft and Rockets,
must be removed from the dielectric surface in 1994, Vol.3l, No.3, pp.493-501.
order to achieve a high arcing threshold. Hence, it 10. Galofaro, 1., Vayner, B., Ferguson, D., and
seems possible to raise operating voltages up to Degroot, W. "A Desorbed Gas Molecular Ionization
300 V for conventionally designed solar arrays, Mechanism for Arcing Onset in Solar Arrays
and to avoid arcing on their surfaces, by Immersed in a Low-Density Plasma", 33rd AIAA
thoroughly outgassing all dielectric surfaces. Of Plasmadynamics and Lasers Conference, May 20-
course, much work lies ahead to determine the rate 23, 2002, Maui, Hawaii (AlAA Paper 2002-2262).
of outgassing, critical temperature, influence of II. Anders, A. "Ion charge state distributions of
radiation, and other parameters that depend on the vacuum arc plasmas: The origin of species",
specific array design. Phys.Rev.E, 1997, Vo1.55, No.1, pp.969-98I.
12. Anderson, R.A., and Brainard, J.P.
"Mechanism of pulsed surface flashover involving
REFERENCES electron-stim u lateddesorption", Journ. A pp I. Phys.,
1980,VoI.51, No.3, pp.1414-1421.
I.Ferguson, D.C. "Solar Array Arcing in 13. Miller, H.C., and Ney, R.1. "Gases released
Plasmas", NASA CP-3059, 1989, p.509. by surface flashover of insulators", 1.AppI.Phys.,
2. Vayner,B., Galofaro, J., Ferguson, D., Degroot, 1987, Vo1.63, No.3, pp.668-673.
W., and Vayner, L. "Arcing Onset on a Solar 14. Krasnopol'sky, B.A. "The Physics of
Array Immersed in a Low-Density Plasma", 39[h Atmosphere Rad iation of Planets and Comets",
Aerospace Sciences Meeting & Exhibit, JanualY 8- Nauka, Moscow, 1987, p.77.
11,200 I, Reno, NV (AIAA Paper 200 J -0400)
3.Vayner, B., Galofaro, J., and Ferguson, D.
"Arc Inception Mechanism on a Solar Array
Immersed in a Low-Density Plasma", 7[h
Spacecraft Spacecraft Charging Technology
Conference, 200 I: A Spacecraft Charging
Odyssey, April 23-27, Noordwijk, The
4
American Institute of Aeronautics and Astronautics
Sample
,00 (C=1 IJF; U=-250 V) ,00 o0 1 r-----r--,---,---,---------.--,-~'o 01
0OO375~~1 ~~~~L..;.~~~~~~~,OOO375
OH
75 75 ·00025 -00025 I
1-
?:50 50 ~ 000975 -000875
.~ c L:: :, f
2E 25 25 EQ) _+-If--+-I4-_!l--I_ _- I--If---I--1-D 015
~lll .II.~~ . U r- -001125u
o ~~ r'f'f""~'t·A ·00275 ·00275
·003315 -003375
'~7~5----~~~5~--~2~95~--~3~~~--~3'5~
Wavelength (nm) -O~~~~--~~~~---.-~I~~~~'-~~=----~~D~5----~~~·M
Time(s)
Sample
AOO (C=1 ~F; U=-250 V) AOO 0000102,, 1, - I I - -, - 0000011 25
350 350 .0007 ~c ~I 00075
300 300 ::~5i
250 Ha 250
f200 - -
~150 215000~c~ ,5 --f-- I-- 0033751
100 iJ.l:.J~'iI..;I ~ 100 -00<, - -01''''
50 . I 50 -00512 5 H f-- -005125
0
·5603 0 640 W6a5v0e length &(niom ) 670 68·05 0 ·01·151e ·OS 1 • Ie-OST imefs) Ie-OS 3e-05 ,.."oo e
Fig.2. Arc current pulse and capacitor voltage are
Sample
shown for the discharge with bias voltage -700 V
(C=1 ~F; U=-600 V)
3500 ,------- 3500 and capacitance 1 fLF.
3000 3000
2500 2500
",2000 2000",
;; Ag ;; Halpha Intensity vs.charge
~1500 1500 ~
y;a+bx2
-'000 -- 1000- ,2=0 667; &""525, b>=O 0222
500 .} ---.) 500 I ,
·50301 0 320 330 lAO 35·05 00 17500 - l- I - ,7 500
Wavelength (nm) ,500) I ,5 00)
;"'2500
~ r / ,
S:g tiDD I /
,.:j 7500 V 7
500) - I-~ f-- 5000
---.
2500 ----- 2500
Fig.1. Arc spectra demonstrate the presence of
o 0
hydroxyl, hydrogen, and silver in the arc plasma. '00 3lJ 500 700
Lost charge (~C)
Fig.3. The intensity of ff a spectral line depends on
bias voltage and capacitance rather strongly.
5
American Institute of Aeronautics and Astronautics
4e·O 6 4e-06 5. .0 6 5e-06
35. .0 6 "2P 350·06
_ 30·0 6 t- ---- I--. - - - 3,·06 _ 4. .0 6 4e -OS
is ~ t: ~
t!e:. 2 5 ..0 6 N2 2 5,·06I~- -~5;3 e-O6 3e -OB~
20·0 6 2e -OS iell m
~
~., 1 5e-O6 1. 5e-os~ e~.,2 e.OB 2e -OB eC.D,
a. ~ a. a.
1. .0 6 1e -OS 1e·OS - 1e -06
5e-O 7 ~ ~t tJ .u-"--. Ie. ..... 1 .-~: e-07 A IA- II
0 40 eo 120 oo 40 eo 120 0
Mass/Charge Mass/Charge
a) c)
3. .0 8 3,·06
5e-OS 5e-OB
2.5. .0 8 2.5,·08
4e-OS - f- - I- - - 4 e·OS 1; 1t:.;
F ~e 2e-06 I 2e-OB e
~;- 3e-OB - - - .+ 1- 3 ~ 1 5e·06 1 5e-06 ~
~ ~ ~
ml5 . 2e-08 -- - 2 (~ij ie-OB ~ 1e -OS ~(ij
e
IE
1e-OS ft .. - - - - - 1e -OS 5,·07 j 5,·07
fl .rI ~
I o ~ ~ ~O
oo 40 eo 120 0 o 20 Mass/Charge 40 60
Mass/Charge
d)
b)
FigA. Examples of mass spectra are shown in the following sequence: a)background vacuum;
b)chamber filled with xenon; c)xenon plasma with n.=106 cm-3; d)arcing on the sample with rate 1 arc/so
___
H O ____
r--~ 2 ___
u u ..............................................
,,,Torr) 1
1.1 L...._ __ _ _____ ______.. ..J
o :l00 Ela.. p00s ed time (s6)0 0 900 LOOO oaL ....-----200- ----4-00 -----6-00 -----8-00- ------10'0 0
Elapsed tim. (s)
Fig.S. Changes in partial pressure are shown for hydrogen atoms (left panel) and the
most abundant molecules (right panel).
6
American Institute of Aeronautics and Astronautics
-,
Hydrogen partial pressure
.7 2.7
=/. I
- - 1 .-
26 5 I 265
.6 26
~25 5 • 255 ~
W/]J. ,r) I I ~
,b 'I-"""';;;;:~"""".-;-;-:;--:----------------l ~ 2.5 1----. 2.5 ~
.... ..... ...... '':""J :ilJl
...... . ai". 245 2. 45 ai".
o.,r------------------=~ 2.4 2
2.35 1-. 2 35
I
•
.0.01:------':------10-----',- ___- ----1 2.3 2 3
0 500 700 900 1100
Partial pressure (p. Torr) Bias voltage IV)
Fig.6. The relative inc/-ease of partial pressure due ~ig.7_ Th~ p.artial pressure of atomic hydrogen
mcreases slglllficantly with increasing bias voltage.
to arcing is shown for two arc rates_
7
American Institute of Aeronautics and Astronautics