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N89 - 15802 


Arcing and Discharges in High-Voltage 
Subsystems of Space Station 


N. Singh 

Department of Electrical and Computer Engineering 
University of Alabama in Huntsville 
Huntsville, AL 35899 


Abstract 

Arcing and other types of electrical discharges are likely to occur in high-voltage 
subsystems of the Space Station. Results from ground and space experiments on the arcing 
of solar cell arrays are briefly reviewed, showing that the arcing occurs when the 
conducting interconnects in the arrays are at negative potential above a threshold, which 
decreases with the increasing plasma density. Furthermore, above the threshold voltage 
the arcing rate increases with the plasma density. At the expected operating voltages 
(~200V) in the solar array for the space station, arcing is expected to occur even in the 
ambient ionospheric plasma. If the ionization of the contaminants increases the plasma 
density near the high-voltage systems, the adverse effects of arcing on the solar arrays and 
the space stations are likely to be enhanced. In addition to arcing, other discharge 
processes are likely to occur in high-voltage subsystems. For example, Paschen discharge 

is likely to occur when the neutral density N n >10 cm , the corresponding neutral 
pressure P > 3 * 10 — 5 Torr. 


1. Introduction 

The purpose of this paper is to report on the possible effects of contaminant gases on 
the arcing and other discharge processes occurring near Space Station subsystems operating 
at relatively high voltages. The subsystem which is of primary concern here is the solar cell 
array, which is the heart of the Space Station Power System (SSPS). Under normal 
operating conditions the SSPS will operate at 160V, but during the cold starts the 
operating voltage is likely to double to about 320V. One of the main concerns here is, 
whether or not, at such voltages arcing and other discharge processes will occur in the 
array. These processes are likely to produce several unwanted effects on the power system 
and the space station, some of which are: (i) degradation of the solar cells, (ii) transients 
in the power system, even leading to the power disruptions, and (iii) electromagnetic 
interference, which can be detrimental to communications and telemetry. 

In the following section we briefly review the existing knowledge on arcing in solar 
cell arrays and then we use it to predict the allowable contaminant densities near the 
arrays. 


2. Arcing in Solar Cell Array 

The information on arcing in solar cell arrays has been obtained from both ground 
and space-flight tests. However, the latter tests are limited to only two flights known as 


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•; * 




PIX-1 and PIX-2, where PIX stands for Plasma Interaction Experiments. Results from 
both ground-based and space-flight experiments have been summarized by Stevens [1986], 
and Ferguson [1986] and Purvis et al [1988]. Some of the questions which have been 
attempted to be resolved using the experimental data are as follows: (i) Is there a voltage 
threshold for arcing? (ii) How does this threshold vary with the local plasma density?, 
(iii) How does the arcing rate for voltages above the threshold vary with the plasma 
density and other plasma parameters? 

Figure 1 shows a summary plot of threshold potential as a function of the ambient 
plasma density. PIX-1 data are limited and they show that arcs occur at potentials 
between -700 and -1000 volts for all plasma densities [Purvis et al, 19SS]. On the other 
hand, the more complete PIX— 2 data set shows that threshold voltage decreases with the 
increasing plasma density. Furthermore, a comparison of the ground test data with the 
PIX— 2 data shows that both the data sets predict the general trend of decreasing threshold 
voltage with the increasing plasma density, but the threshold voltages for the former set 
are higher than those for the latter data set based on space experiments. 

The above conclusion drawn regarding the arcing threshold voltage is based on a 
very limited data set. Unfortunately, there are no theoretical basis so that the 
applicability of the data set can be extended to conditions for which the measurements 
have not been performed. 

We use here PIX— 2 data to decide whether there is a possibility of arcing in Space 
Station solar cell arrays. Barring transients, and cold starts after eclipse, the maximum 
voltage on solar cells will be near -160 volts, for which arcing is likely to occur at densities 

N > 2 x 10 5 cm -3 (see Fig. 1). 



Fig. 1. Threshold voltage for arcing versus plasma density. The ground test data 

show a higher threshold than the PIX— 2 data from a flight experiment 
(Steven, 1986). 


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In the altitude range of the space station the ambient plasma density is likely to be 

A O 

in the range 10 — 10 cm , indicating the possibility of arcing even in the ambient 
plasma. The ionization of the contaminant molecules and atoms is likely to increase the 
plasma density above the ambient density. This may further aggravate the arcing 
problem. 

The contaminant molecules and atoms are generated by outgassing, leakage, venting 
and thruster firings. In addition, the phenomenon of ram pile— up enhances the neutral 
density in front of the vehicle. This enhancement can be as large as 20 times the ambient 
neutral density. 

The neutral densities of the contaminants and that associated with the ram pile— up 
have been calculated by the Science and Engineering Associates (SEA) contamination 
model [Rantanen, 1988]. For example, Table 1 shows the total density of the neutrals in 
the ambient environment and the enhanced density due to the ram pile— up. 

The production of plasma from the neutrals depends on the efficiency of the 
ionization processes, which include photoionization and charge exchange processes, and also 
on the transport of plasma. Thus, the determination of the total plasma density around 
the vehicle is a difficult task. However, at the altitude range of the space station it can be 

roughly assumed that about one out of 10 4 molecules or atoms are ionized. Thus, the ram 

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plasma density can be as high as 10 or more and Fig. 1 shows that arcing is quite likely to 
occur.' 

Since the solar arrays for the- space station are likely to have very large surface 
areas, the ram effects can be very pronounced. Thus, if the solar cells are exposed to the 
ram plasma, the arcing is expected to occur at smaller voltages (and over a larger portion 
of the array) than those at which the solar cells arc in the ambient plasma. 

Recent analysis of data from both ground and space experiments show that the 
arcing rate (R) depends on the plasma properties as follows [Ferguson, 1986]: 


R a n(T/m)^ 2 (1) 

where n and T are the plasma density and (ion) temperature, respectively, and m is the ion 
mass. The proportionally constant and the dependence on the voltage is found to vary 
from one set of experiments to another. From space data, it is empirically found that 

R~ 2.8 x 10 _13 |V| 3,1 n(T/m)^ 2 (2) 


where V is in volts, n is in cm , T is in eV and m in amu. The above relation is found to 
be true above a threshold at about —230V. However, this threshold is true for the 
prevalent ionospheric plasma densities. When the plasma density is enhaneed either by 
ionization of the contaminants or by ram pile— up the threshold is likely to be reduced and 
the arc rate is likely to go up. However, a quantitative estimate of the plasma density 
enhancement associated with the enhancement in the neutral density remains an unsolved 


problem, and its solution must include both ionization and transport processes. 


There is another issue involved here dealing with the effect of neutrals on the high 
voltage systems. At very low pressures nearing vacuum conditions discharges are difficult 
to occur. However, when the pressure increases so that the mean free path for the electron 


collision with the neutrals become of the order of the inter-electrode spacing d, the 


Paschen discharge occurs. Fur the Space Station sub-systems at high voltages the 
inter— electrode spacing is roughly the sheath size, which is roughly of the order of 10 cm or 
so at the voltages of about hundred volts. Thus, the condition for Paschen discharge 
becomes 


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( 3 ) 


where a is the collision cross section. Since the molecule size r >> size of an electron, 
a ~ nir 2 . Assuming r;3« 10 — 9 m, we find 


N n > 10 12 cm 3 


(4) 


5 

This neutral density amounts to a neutral pressure > 3 x 10 torr, which is two order of 
magnitudes or more larger than the ambient neutral pressure. But such enhancements of 
the neutral pressure have been observed aboard space shuttle during thruster firings [Wulf, 
1986]. 

Since Paschen discharge and associated plasma processes may lead to arcing, it is 
recommended that neutral pressure must be controlled to < 10 torr or equivalently 

N n < 10 12 cm -3 . 


3. Summary 

At the operating voltages for the space station solar cell arrays, arcing is expected 
to occur even with the ambient plasma. If the plasma density is enhanced by the 
ionization of the contaminants, the voltage threshold for arcs is likely to be reduced, 
causing arcing over a larger portion of the array. Furthermore, the arcing rate goes up 
with the plasma density. Thus, the detrimental effects of arcing on the array and the space 
station are likely to be enhanced by increase in the plasma density due to the ionization of 
the contaminants. 

High neutral densities (N >10 cm ) near high voltage systems are likely to 

cause discharge processes other than arcing. Such discharges generate plasma and are 
likely to create conditions for increased arcing. The ram pile— up (Table 1) at low altitudes 
(~200 km) appears to generate neutral densities comparable to this value. 

Finally, we state that our theoretical understanding of arcing and discharges is far 
from complete. Thus, it becomes very difficult to draw general conclusions from the 
limited set of data from laboratory and space tests on arcing of solar cell arrays. It is 
recommended that systematic investigations involving both theory and experiments be 
carried out so that arrays characterizations be carried out with confidence. Such an 
investigation warrants a global model of space station based on generation and transport of 
both neutrals and plasma 


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References 


Ferguson, D. C., "The Voltage Threshold for Arcing for Solar Cells in LEO — Flight and 
Ground Test Results", AIAA Paper 86-361, 1986. 

Purvis, C. K., D. C. Ferguson, D. B. Snyder, N. T. Grier, J. V. Staskus, and J. C. Roche, 
"Environmental Interaction Considerations for Space Station and Solar Array Design", to 
be published, 1988. 

Rantanen, R. 0., "Neutral Environment for Space Station, Space Station Contamination", 
Workshop Proceedings, ed. by M. R. Torr, J. F. Spann and T. W. Moorehead, 
NASA/MSFC Reprint Series #88—113, p. 1, 1988. 

Stevens, N. J., "High Voltage System — Plasma Interaction Summary", Space Technology 
Plasma Issues in 2001", ed. by H. Garret, J. Feynmann and S. Gabriel, JPL Publication 
86-49, p. 167, 1986. 

Torr, D. G., "Space Station Contamination Study: Assessment of Contaminant Spectral 
Brightness", Space Station Contamination Proceedings, ed. by M. R. Torr, J. F. Spann and 
T. W. Moorehead, NASA/MSFC Reprint Series #88—113, p. 43, 1988. 

Wulf, E. and U. Von Zahn, "The Shuttle Environment: Effects of Thrust Firings on Gas 
Density and Composition in the Payload Bay", J. Geophys. Res., 19, 3270, 1986. 


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