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volts, relative to plasma potential. For cells with silver-coated interconnects, a threshold ... ground and. LEO experiments. The arc rate beyond the threshold voltage depends .... ground tests, and x is about. 8 for the fronts and backs of. 5.9x5.9 cm cells ... ground and orbital data. This threshold may be a function of solar cell.

N'Sb-2555s SOLAR

ARRAY

ARCING

IN

PLASMAS

Dale C. Ferguson Space Environment Effects Branch NASA Lewis Research Center 21000 Brookpark Road Cleveland,

Ohio

44135

ABSTRACT Solar cells known to interconnects of a few potential.

in arc

hundred For

interconnects, arcing exists

ther tions,

space plasma conditions are into the plasma when the are at a negative potential volts, cells a at

V,

voltage as found

called

by arcs in cells isolation from a Models for the on the choice

conductor and possibly

material on the

for in

copper

at

a few relative

hundred to the

volts local

of flying with it

rounding (Kennerud

low

cells, voltages,

which are

individually typically

in space

will

contact

the

the insulating electrons from

plasma, 1974)

tion about arcing arc rates, as well electron currents, Because arcing systems of some

large. another

standard

The are

techno-

space

plasma.

the

first and

in later

cover glasses plasma as

if

effect, known understood in generated on over to be

of a few plasma, into the ground in

hunsolar sur-

tests orbital

of

the

voltage thresholds and as about the "snapover" in LEO conditions. obvious

implications

and anomalous current exposed to the space interest to understand

collection plasma, these

of on it is plas-

ma interactions with spacecraft systems. This paper reviews current progress in understanding solar array arcing in plasmas.

generate strung

In

applicaacross

flight tests (PIX-I and PIX-II, Grier and Stevens 1978, and Grier 1983). PIX-I, because of limited plasma diagnostics, essentially only confirmed that arcing was "not an effect caused by the plasma chamber walls. PIX-II, however, yielded informa-

power systems capable of generating large amounts of power. To keep cable masses low, with no loss of efficiency, high voltages must be used; much higher than the 28 V systems typically orbited until now, and even higher than the occasional i00 V used on Skylab.

Solar

quite cell to

At high negative potentials dred volts relative to the cells were observed to arc

negative plasma.

large structures a need for

may become from one

power voltage

they were conductors. This as "snapover", has been terms of secondary electrons the cover glasses "hopping" collected at the conductor.

INTRODUCTION

The prospect space brings

high total

interconnects.

ma, even collected

to the plasma and choice of

adjacent insulator material. Finally, new technology solar cells use copper for the cell interconnects, a material which may have a lower arcing threshold voltage than silver. It is expected, from ground tests of simulated solar cells, that any junction of conductor and insulator exposed to space plasma conditions will arc into the plasma potential,

for the

Early plasma testing of solar cells in simulated low Earth orbit (LEO) plasmas (Cole et al 1968, Stevens 1978) revealed that at high positive array potentials of a few hundred volts relative to the plas-

having insuffisource Of high arcs are highly of interconnect

exposed geometry

series that

logy solar arrays, the interconnects are coated with silver, for ease in soldering, and are exposed to the surrounding environment. Newer technology cells are welded to a copper trace from the back, so that little conductor is exposed in front. If the cell backs are not well insulated,

both ground and LEO experiments. The arc rate beyond the threshold voltage depends nearly linearly on plasma density, but has a strong power-law dependence on voltage, such that for small increments in operating voltage there is a large increment in arc rate. The arcs generate broadband radio interference and visible light. In ground tests, interconnects have been damaged cient current. dependent

so

the array connections

relative to plasma with silver-coated

threshold about -230

in

toge-

509

PRECEDING PAGE BLANK NOT FILMED

ARC

RATE

AND

Ferguson

THRESHOLD

(1986)

MEASUREMENTS

shows

that

the

onset

of

arcing in solar array plasma tests may not accurately reflect the voltage threshold. If, for example, the arc rate at some combination of plasma conditions and bias voltage is move on to is observed. ground tained 1984), depended ing

not depend or exposed For a large nects, one

very low, the experimenter may higher voltages before arcing When arcs were observed in

tests, and arc rates could be ob(Miller 1983, Leung 1985, and Grier it was found that the arc rate on the conditions in the follow-

approximate

array designs. Figure 1 shows the arc rate behavior found for several arrays of 2x2 cm and 2x4 cm cells in ground and orbital tests, normalized by the plasma parameters in equation (i). It is worth mentioning that the arc rate observed does strongly on interconnects array with pinhole in

thus arc effectively interconnects were space plasma.

the

number of cells at high voltage. insulated interconthe insulation will

as all

much exposed

as

if to

the the

way: I0o

R

=

C 1

n

T 0"5

m -0"5

where C 1 is a constant, density, T is the plasma the plasma ion mass, V

V x,

(i)

n is the plasma temperature, m is is the interconnect

voltage relative to the plasma and x is approximately equal to cm cells, 2x4 cm cells, and the 5.9x5.9 about

cm 8

5.9x5.9

cells in for the

cm

cells

tests. The a value for in orbit. fronts

of

x and

contrasted tests.

If the observed voltage between uous shown

tests, and

5.9x5.9

cm

in

x

ground

cells

only

and

the

together may be caused the exposed conducting cell fronts and backs later. The difference

other

10-I

is of

results yielded for 2x4 cm cells between the

for the 2x4 cm x in space may of atomic oxygen with

and backs

together,

PIX-II flight x of about 3 The difference

fronts and backs a difference in materials on the will be discussed between tests presence

ground fronts

potential, 5 for 2x2 fronts of

by

10-2

as

cells in ground be due to the in space, as

gases

used

in

ground

voltage at which arcing is first in a test is interpreted as that at which the average time interval arcs becomes less than the contin-

test that

time at that (Ferguson,

voltage, 1986):

it

may

be O • • t

10-4 Von where C 2 is

=

C 2

n (-I/x),

Von is the a constant,

(2)

apparent and n

fined before. Thus, an dependence of the arcing fact be simply a reflection voltage dependence of the

The 2x4

true voltage cm cells with

nects, which standard polated found plasma, orbital

the arc below

from higher be about -230 from data.

onset voltage, x are as de-

apparent density threshold may in of the steep arc rate.

threshold for silver-coated

defined as the measured deviations to

and

PIX II flight PIX IIground Leung Miller

arcing for intercon-

potential rate is the rate

voltages, V, relative

all available This threshold

function of solar cell geometry ials, and should not be taken the threshold for arbitrary

i0-5

below several extra-

has

ground may

to

1 Threshold 10 .6

been the

i00

and be

' 200

I ,I,I,I 600 -V.V

I I000

__

2000

a Figure standard LEO ram

and materto represent or new solar

510

1

Arc rate interconnect conditions.

versus cells,

voltage normalized

for to

rent EFFECTS Of arc

OF

SOLAR

course, occurrence

craft (1985),

ARRAY

it

is if

operations as part

experiment ofrequency cells. found.

only the of

necessary arcs may

or the

to harm

systems. now-defunct

avoid spaceLeung VOLT

been

on

repeatedly

plasma arrays,

produce with

to

visible optical

spectrum of been measured.

the as

capacitance of also found by EMI generated may be significant. For to generite the high

radiofrequency

light, which experiments. the

If arcs occur insufficiently

arcs

has

in solar isolated

2_

early

experiments

EMI, may The not,

negative self.

In

(diodes,

where

potentials this case,

array in

arrays from

a

I

arcs

which high

large source

his

experi-

experimental resistance array to space of the

be

solar high

may be the array the strength of

on the internal

total array

itthe

array capaconnections

etc.).

Adverse gated

Since standard a large and the

For the

arcs will depend citance and on

arcing

several

effects

may

be

miti-

ways:

Design

the

system

so

that

high

negative potentials relatiye to the plasma will exist nowhere in the system. This will mean, in practice, one of two design solutions. Either the total array voltage, from end to

interfere visible to

arced.

tested. however,

i. addition

in

ence, it has become technique to place between bias sources

potentials necessary for arcing, EMI may be significant if it couples to spacecraft electrical systems. Communications between spacecraft, or between spacecraft and telerobotic systems may be disrupted. In

as

a high voltage power supply was used to bias the arrays, the large arc currents may damage materials at the arcing point. Miller (1983) found partially melted interconnect material in arrays which had

development, measured the radinoise spectrum of arcing solar Figure 2 shows the spectrum he Depending on the strength of the

arcs (which depends the array to space, Snyder in 1985), the negligible or quite arrays large enough

source,

ARCS

date,

end, must be current-collecting

are cur-

at

I

the

I

negative

limited, area end

I

of

or is the

a large provided array,

I

so

I

--.x-- 2000 pF CAPACITOR -'D0

_

W ITHOUT CAPAC ITOR

"--_._. e'--"e"

E .

"_,,,"x-----x-,,_

..

COMPOS ITE PAYLOAD

",--..,...._-..._ ''

IN PAYLOAD BAY

....

100

LOCATED ON AFT FL IGHT DECK 50 O.Ol

I O.1

",

I 1

I I0 FREQUENCY

Figure ated cells, with

2

- Spectra of RF a 4 cell array with welded-through and without an added by

511

I I00 (MHz) radiation generof 5.9x5.9 cm interconnects, capacitance.

i i,000

_ I I0,000

there that

will end.

be

a The

low potential first solution

drop

at will

As

mean that large_ cable masses must be used to limit I_R losses. The second solut4on will push the positive end of the array up to high positive potentials relative to that snapover problems important. 2.

Insulate

the

thoroughly

that

the

no For

EMI, etc., in mind,

pendent array unit, so the up linearly

is

so that may be This may

to tolerable though, that

each

insu-

levels. inde-

All the on at

one go arc

4. Use materials for interconnects, coatings, etc., which will not arc a_ the high negative potentials likely to exist on parts of the array. This option will be further examined in the next section.

electric on the

Interesting

exist

in

arc rates between surface materials. tests (Thiemann and have shown that

arcing arrays For Bogus arrays

layer surface

cell or system.

flight arrays oxide atomic testing

threshold. tion techniques

is

Obviously,

of of

part

more

ground tests to investigate dependences

testing. In this case, the orbital were likely covered with a thin coating from interaction with the oxygen in LEO, whereas pre-flight was done in an argon plasma.

materi-

Copper in his the local copvalue less than

indicating a for copper

lower than

be understood if arc rate depend of the materials are two popular

of In

arcing one, a

contaminant the conductor,

of

is

at thin

high di-

built up and suf-

work

Center,

TRW,

some

other

needs

to

spacecraft

be

done

in

and

space flight experiments, the material and geometry of the arc rate and arc

Only be

then can employed.

Shuttle experiment on solar arrays in Solar Array Module Experiment, a joint manifested for late ting continues at

Miller (1983) showed that in ground tests the solar array arc rate started out high, and then decreased to a constant level on

ground tests using plasma. PIX-II in orbit (Ferguson

possibly voltage

for the onset potentials.

Solar arrays for the PIX-II experiment had a much higher voltage for the onset of arcing in pre-flight ground test experiments than they exhibited in orbit (Ferguson, 1986). The voltage dependence of the arc rate of the PIX-II arrays was also very different than found in pre-

effect in background this effect

insulating

and the observations they are meant to explain, arcs may be expected at negative potentials of a few hundred volts negative, relative to the plasma, at conductor-insulator junctions, regardless of whether the junction occurs on a solar

arcing.

a time scale of a few hours, as sites were cleaning themselves testing. Leung (1985) observed

of

tests at and backs

insulator under electron bombardment may lead to an avalanche into the plasma if the electric fields are high enough (Hastings et al 1989). In both of these models material properties play a strong role, as does the presence of high electric fields near conductor-insulator interfaces. On the basis of these models

having the interconnects coated with an "insulating" material sometimes arced at lower voltages than they did before the interconnects were coated. The effect amounted to hundreds of volts in the onset of

fronts

ficiently high electric fields may be produced in the vicinity of the insulator to punch through the layer, triggering an arc (Jongeward e__t al 1985). In the second model, breakdown of gas emitted by the

DEPENDENCES

differences

thresholds and having different instance, ESA 1986 and 1988)

with

of these resu!ts may arcing threshold and the surface properties the arc sites. There

models negative

MATERIAL

covered

about -120 V, arcing threshold for silver.

Segments.

POSSIBLE

the

old found in other tests. tests arced so as to bring per potential in arcs to a

only a dumped limit

segment will arc as total arc rate will as the number of

mentioned,

cells arced in ground rate than the fronts

al, to simulate the conductor-insulator junctions on solar cells. Silver in his tests arced so as to bring the local conductor potential down to about a -230 V level, coincident with the arcing thresh-

so

plasma contact will be made. long-lived arrays, micrometeoroids and/or debris may puncture the lation, nullifying this solution.

the Bear

cm

partially

certain

3. Design the arrays small amount of charge when an arc occurs.

already

together. Here, the significant fact may be that the cell backs had copper exposed to the plasma, rather than silver. Supporting this contention are the ground tests performed by Snyder (1986) on metals

plasma, so may become

interconnects it

was

5.9x5.9 a slower

and

proper One

mitigaapproved

to LEO

investigate arcing is the SAMPIE, or Plasma Interaction NASA/ESA venture now in 1994. Ground tesNASA Lewis Research

elsewhere.

if the arc up during the same CONCLUSIONS

a different also showed 1986).

EMI generated in generate radiofrequency

512

solar

array arcs noise which

may might

disrupt negative surfaces

telerobotic potentials are a

communications. High on other spacecraft possible threat to the

successful operation of spacecraft tems, including automation and electronics and communications. necessary to consider and solar-array-type

solar arcing

Grier,

sysrobotics It is

array in the

arcs may potentials

also of

occur the

during docking

Material thresholds

in system design, and them relies to a great spaceflight experiments, remain to be done.

N.J.

(1978),

Experiment

(PIX)

Spacecraft

1978,

NASA

Charqin_

CP-2071,

pp.

295-

Hastings, D.E., Weyl, G., and Kaufman, D. (1989), "A Simple Model for the Threshold Voltage for Arcing on Negatively Biased High Voltage Solar Arrays", Journal of

docking, vehicles

dependences are important

Stevens,

Results",

Technoloqy 314.

arcing design

differ sufficiently. Thus, control of spacecraft potentials is important in spacecraft design, if reliability and communications are important to spacecraft systems. rates and

and

Interaction

Flight

of sPacecraft power systems and other systems which may be affected by arcing. Systems should be designed to mitigate the incidence and effects of arcing, whenever possible. Although not the topic of this paper, if the

N.T.

"Plasma

of arc factors

Spacecraft

and

Jongeward, Unneutralized

G.A.

submitted.

et al Surface

Potential vol. NS-32,

Arcing", no. 6,

Kennerud,

K.L.

Array

our knowledge of extent on lab and some of which

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(1985), Ions

IEEE Dec.,

(1974),

Experiments",

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Trans. pp. "High

NASA

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Nucl. 4087-4091.

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1985.

Miller, W.L. (1983), "An Investigation of Arc Discharging on Negatively Biased Dielectric-Conductor Samples in a Plasma", a_r_ Environmental Interactions Technoloqy 377. REFERENCES

Snyder, D.B. Arc Currents

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Charged-Particle Char_inq Technopp. K.

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