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Full text of "NASA Technical Reports Server (NTRS) 19810017271: The simulation of the geosynchronous Earth orbit plasma environment in Chamber A: An assessment of possible experimental investigations"

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William Bernstein 
Center for Space Physics 
Dept, of Space Physics & Astronomy 
Rice University, Houston, TX 77001 

A Final Report of the 
Research on NASA Contract NAS9-16166 


January 1981 

Introduction and Summary 

This report considers the possible use of Chamber A for the replication 
or simulation of space plasma physics processes which occur in the Geosyn- 
chronous earth orbit (GEO) environment. It is shown that replication is not 
possible and that scaling of the environmental conditions is required for 
study of the of the important instability processes. Rules for such experi- 
mental scaling are given. At the present time, it does not appear techno- 
logically feasible to satisfy these requirements in Chamber A. 

It is, however, possible to study and qualitatively evaluate the problem 
of vehicle charging at GEO. In particular. Chamber A is sufficiently large 
that a complete operational spacecraft could be irradiated by beams and 
charged to high potentials. Such testing would contribute to the assessment 
of the operational malfunctions expected at GEO and their possible correction. 
However, because of the many tabulated limitations in such a testing programs, 
its direct relevance to conditions expected in the GEO environment remains 

Philosophy of Simulation Experiments 

Block (1976) has succinctly summarized both the relevance and limitations 
of laboratory experiments in the space plasma physics area as follows: 

a) New theories should as far as possible be tested in labora- 

b) Theoretical processes which cannot or have not been reproduced 
in the laboratory should often be met with scepticicm when 
applied to space plasmas. 

c) Most plasma processes observed in the laboratory are probably 
of importance in space. 

d) Proper application of a laboratory process to space conditions 
requires a theoretical understanding of its dependence on all 
plasma parameters and boundary conditions. 

These rules may seem self-evident, but they are unfortunately not always 
obeyed, since too many theoreticians are not aware of the multitude of plasma 
phenomena not understood theoretically, and too many experimentalists are 
unaware of appropriate theories. There is of course at least a partial 
excuse: paper proliferation. 

Because of the immensity of space plasma configurations, scaling must be 
applied to reduce the experiment size such that it can be accomodated in a 
laboratory system. Block [1976] also gives a quantitative tabulation of the 
the various scaling dependences shown in Table I, 


Length, time, resistivity vary as L'*’^ 

Particle energy, velocity, potential, 

temperature vary as L° 

Particle density and pressure, electric 

and magnetic field, frequency vary as 

Magnetic pressure, space charge density, 

current density vary as L"^ 

Table 1. Plasma Scaling Laws 

It seems reasonably clear that, in most instances, quantitative scaling 
will not allow technologically feasible laboratory configurations; therefore 
the concept of qualitative, limited scaling has been introduced. This means 
that the relevant dimensionless numbers [such as the ratio of the electron- 
neutral collision frequency to the electron cyclotron frequency] should be 
kept qualitatively the same in the laboratory as in space. Ratios mucn 
smaller or larger than unity retain this property but not necessarily the same 
order of magnitude. Problems of course remain with ratios that are near 

In some cases, the need for even qualitative scaling can be relaxed still 
further and idealized geometrical configurations and idealized particle dis- 
tribution functions can be employed in the laboratory experiments. The objec- 
tive here is tht: test of theoretical concepts: the experimental conditions 

need only satisfy the constraints of the specific theory, not duplicate the 


total complex set of phenomena. The direct relevance of the specific theory 
under study to space plasma physics phenomena cannot be established by the 
laboratory experiments alone. 

The use of a very large experiment system offers several important advan- 

1. The range of the parameter scaling is reduced, and therefore the 
uncertainties associated with dimensionless ratios near unity are reduced. In 
fact, the existing work that has been performed in Chamber A has been per- 
formed under conditions where only minor scaling has been required. It should 
be noted, however, that only extremely small scale l ocal phenomena have been 
studied in the configuration. The study of longer range phenomena (several 
meters) would obviously require at least qualitative scaling. 

2. Operation at smaller magnetic field strengths and particle densities 
reduce the pertinent electron plasma and cyclotron frequencies to a very 
convenient range. 

3. The temporal evolution of plasma phenomena is slowed. 

4. Insertion of various diagnostics into the plasma is possible without 
severe perturbations, which modify the properties under study. 

The utility of scaling experiments is clearly shown in the work of 
Bernstein et al . (1979) describing the Beam Plasma Discharge. These experi- 
ments represented an inverse scaling, in which phenomena first observed in 
small laboratory configurations were demonstrated to occur for the large 

dimensions available in space experiments. They noted that the critical beam 
current for ignition scaled quantitively with earlier work in the small 

Bo 3 

laboratory systems despite the very large change in parameters (^ = lO'^, 

4 ^ 

Neo/^ej " * ^o^^I ~ 0*02). where I represents near ionosphere conditions. 

Although Soviet investigators did not produce the BPD under laboratory condi- 


tions approximating the ionosphere, they independently interpreted the 
confusing results of several electron gun rocket flights in terms of the BPD. 

In this report we will first summarize the plasma characteristics 
existing at GEO and then determine whether it is possible to perform meaning- 
ful laboratory experiments relevant to processes occurring at GEO. 

Plasma Characteristics at GEO 

Fig. 1 is an overly complex schematic representation of the magnetosphere 
taken from DeForest (1978). The location of GEO at all local times is indi- 
cated. Obviously, GEO lies in a fixed geographical position with respect to 
the Earth, but all the illustrated plasma boundaries show large temporal vari- 
ations in radial location not only in assocation with diurnal and seasonal 
effects, but transiently with the level of geomagnetic activity. Thus some- 
times GEO may be located within the high density, co-rotating plasmasphere, 
the ring current-plasma sheet region, and even, during severe geomagnetic 
storms, within the unperturbed solar wind beyond the bow shock. Fig. 2, given 
by Bernstein et al. (1974) shows the correlated dependence of the plasmapause 
(the boundary between the co-rotating plasmasphere and the ring current 
region) and the equatorial boundary of isotropic ion precipitation on geo- 
magnetic activity characterized by the Kp index. Thus for quiet conditions, 
GEO (L-6.6) lies partially within the plasmasphere; for more active condi- 
tions, GEO lies entirely within the ring current region. 

Chappell (1972) has given a comprehensive report on the distribution of 
cold plasma (Tg < 10 eV) within the magnetosphere. Within the plasmapause, 
the dependence of plasma density on radial distance varies as R"^. The plas- 
mapause boundary is characterized by an abrupt drop (1-2 orders of magnitude) 
decrease in plasma density; beyond twe plasmapause this cold density typically 


varies from 0.1 - 1 cm” . As represented in Fig. 1, the plasmapause is cir- 
cular in shape; a more detailed representation is shown in Fig. 3 showing a 
pronounced outward bulge in its location in the dusk sector (Chappell, 1972). 
Mass spectrometric measurements indicate the dominant ion to be with some 
He"^ present within the plasmapause. At times localized regions of high cold 
plasma density (detached) are observed outside the plasmapause. 

The region outside the plasmapause is populated by a low density, hot 
plasma. Garrett (1979) has tabulated the hot plasma characteristics in this 
region; the measurements considered were conducted predominantly from geo- 
synchronous spacecraft. Typically, electron temperatures (based on the 
assumption of a Maxwellian velocity distribution) are in the rar ge of a few 
KeV; ion temperatures are ~ 20 KeV. Typical hot plasma densities lie in the 
range of 1-2 cm” ; thus the ratio can be as small as 0.1. As might be 

expected, the assumed maxwellian distributions represent only a qualitative 
approximation; careful comparison indicates the presence of non-Maxwellian 
distributions. Large (order of magnitude) variations in density together with 
smaller variations in temperatures occur. The ion composition is highly 
variable; at times or O'*’ may be the dominant ion: both He"*"^ and He"^ are 
also observed but usually not as the dominant ion. The presence of He"^ and O'*’ 
(higher charge states of 0 have not been observed) ions indicate an iono- 
spheric origin; He"*"^ ions are attributed to a solar wind source. Thus the hot 
plasma population originates from both the solar wind and ionosphere with 
large temporal variations in the respective abundances. 

The particle angular distributions are not completely isotropic. Because 
of the mirror magnetic field geometry, particles with large V||/Vj^ relative to 
the magnetic field are preferentially lost to the atmosphere (within the loss 
cone) ~ i 6° at GEO. Williams and Lyons (1974) report that during the 


recovery phase of large geomagnetic storms, the ion population in the region 
extending 1-2 R^, beyond the plasmapause may show a totally depleted loss cone 
for ions. The empty loss cone indicates the absence of pitch angle scattering 
processes. At large L, the ring current does, in fact, show a completely 
isotropic pitch angle distribution (full loss cone). This isotropy for ener- 
getic electrons and ions is evidence for the occurrence of strong pitch angle 
scattering which almost surely results from a variety of plasma instabilities 
(the electromagnetic ion cyclotron (Cornwall et al., 1970), electromagnetic 
electron cyclotron (Kennel and Petschek, 1966), and electrostatic electron 
cyclotron (Kennel et al., 1970) instabilities. It appears likely that 
electrostatic ion cyclotron modes are important at large L, but experimental 
verification is lacking (Bernstein et al., 1974). These particle losses to 
the atmosphere are the origin of the diffuse aurora and provide a mechanism 
which limits the energetic particle population in the outer magnetosphere. 
Conversely, Mcllwain (1975) has reported transient observations of KeV field 

aligned (within the loss cone) electron and ion beams at GEO. Thus the 

''x ''i 

angular pitch angle distribution can range from -^ > 1 to isotropy to — < 1. 

''ll ''ll 

Auroral arcs cannot be mapped to the equatorial plane on the auroral field 
lines. General conclusions are that the arcs are generated in field aligned 
accelerating regions located in the vicinity of 1-3 Re. The equatorial plane 
ion beams may be associated with this acceleration, however. 

Other important parameters at GEO include the solar UV flux together with 
transient periods of eclipse and the dipole (mirror) magnetic field configura- 
tion. The field strength at GEO is typically 100 + 50 x 10"^ gauss. Typical- 
ly, the hot plasma region is characterized by large P where 3 = « 0.25. 

We note again that GEO can lie beyond the magnetosphere (at tne sub-solar 
point) so that the vehicle is immersed in the solar wind (DeForrest, 1973). 


Table Eslimuics of Minimum, Typical, and MaAimum Values for the First Four Moments of the Distribution Function [OrFurrst und 

Mcliwain, 1971) as a Function of Local Time 















Numhtr Density, Forliclvs cm'' 

























7 7 















Energy Flux, erg cm 

j-i j^.i 








































Number Flux, KF Particles cm"’ s~' 


Minimum ' 




15 ' 





































IO-“ dyft 









































Data, from ATS 5, are for the energy range 50 eV to 50 keV and 1970. 

Table II, from Garrett (1979), shows some of the wide range of plasma 
conditions encountered at GEO. Note that this table is limited to particles 
with E > 50 eV. Thus the cold plasma component is omitted. Because the 
plasmapause also represents the equatorial boundary of the hot component, the 
simultaneous occurrence of larg^fe cold plasma densities together with the hot 
component is unlikely except within a localized boundary region. Superimposed 
on the diurnal variations are the even larger variations associated with geo- 
magnetic activity. 

The important plasma instabilities are basically characteristic of the 
hot plasma component. In general, the free energy source lies in the pitch 
angle anisotropy arising from the presence of finite loss cones. The end 
result of the instabilities is the establishment of an isotropic pitch angle 
distribution; however, because of particle loss, complete isotropy may not be 
realized and the instabilities may persist in steady state. On the other 


hand, the instabilities may, in fact, be transient at a fast enough occurrence 
frequency so that they appear steady state in the measurements. 

The occurrence of the instabilities depends critically upon the total 
plasma density (hot and cold components) although large cold plasma densities, 
such as exist within the plasmapause, quench instability growth. It is for 
this reason, that the density gradient at the plasmapause has been postulated 
to be a likely region of ion cyclotron wave growth (Cornwall et al., 1970) 
Unfortunately, although certain features of the ion pitch angle distribution 
are indeed consistent with the occur»'rence of this mode, the close asociation 
of ion cyclotron waves with changes in the pitch angle distribution has not 
been well established. A particularly interesting active experiment (Project 
Fi rewheel— this payload was launched in Spring 1980 but failed due to vehicle 
malfunction) attempts to destabilize the hot ion and electron components near 
GEO through the artificial increase in cold plasma density produced by large 
chemical releases. The present space measurements have not conclusively esta-^ 
blished the required close association between waves and particle energy and 
angular distributions inherent in the theoretical treatments. If these insta- 
bilities could be successfully studied in a laboratory configuration, confi- 
dence in the applicability of the theoretical treatments to space processes 
would be greatly improved. 

A second consequence of the presence of hot plasma coupled with the 
absence of cold plasma is the charging of GEO Spacecraft to high potentials. 
Charging itself leads to the gross distortion of some scientific measurements, 
particularly those of the characteristics of the low to moderate energy 
plasma. In fact, several techniques to reduce charging potential have been 
proposed and tested in space. Unfortunately these techniques lead, in turn, 
to the distortion of other ambient measurements, particularly plasma waves and 


consequept'ly have not been employed on scientific missions where charging is 
anticipated such as Galileo (Jupiter Orbiter). Of greater operational signi- 
ficance is the occurrence of arcs from ona point of the vehicle to another or 
to the plasma. The large transient currents in the spacecraft structure 
arising from these arcs represent a source of intense electromagnetic inter- 
ference and may even produce component failures. Their occurrence has been 
clearly demonstrated to be associated with periods of high geomagnetic 

Charging results because an isolated, equi potential object, immersed in a 
plasma, assumes a potential relative to the plasma such that the net current 
to and from the object equals zero; + ^from “ 

Current away from the object [negative charge to and positive char 3 from 
the object] is produced almost entirely by the flux of ambient energetic and 
cold electrons impacting the object. 

Current to the object [negative charge from and positive charge to the 
object] arises from a variety of sources including: 

1 ) Positive ions from the plasma impacting the object. In general, 

unless T^ > T^, this flux is small compared to electron flux to the 
object because v^ = /(mg/M^) 7g. 

2) Secondary electron emission + backscatter. 

Energetic electrons and ions can eject low energy (< 50 eV) electrons 
when they strike surfaces. For most clean materials the secondary yield is 
< 1 although, of course, selected surfaces with yields > 1 have widespread 
applications. The secondary yield is energy dependent with the maximum yield 
for electrons occurring at ~ 200-500 eV; at higher energies, the secondary 


yield decreases with increasing energy approximately as KE""’ (Wall et al., 
1977). Similarly a fraction of the incident enegetic electron flux will be 
backscii^tered from surfaces. Wall et al . (1977) suggest the following empiri- 
cal relationship for the backscatter yield, B AE"^. For both equations the 
quantities K, A, m, n are characteristic of the particular surface material 
and the angle of incidence of the primary particles. 

3) Photo electron emission 

Surfaces irradiated with UV and shorter wavelength light emit photo- 
electrons. The photoelectron yield again depends upon the surface material 
and the photon angle of incidence. 

Both t!)e secondary and photoelectron emission are primarily surface 
phenomena. In space, after prolonged exposure in the ultra-high vacuum envi- 
ronment coupled with energetic particle and solar photon irradation, it is 
likely that the surfaces will be clean with yields characteristic of the 
materials themselves. Under poor vacuum conditions (> 10"^ torr) surfaces 
will be covered with monomolecular layers of contaminant materials; in general 
such contamination results in increased yields. Such layers cannot be removed 
by prolonged pumping alone; rather baking at > 200“C or intense energetic 
particle bombardment is required. Furthermore, at poor operating pressures, 
the layers will be rapidly redeposited once they are removed. 

Because it has been assumed that the surface is an equi potential (conduc- 
ting) surface, spatial non-uniformities in the distribution of charging and 
discharging currents do not lead to differential charging. Rather the final 
uniform potential distribution arises from the total charging and discharging 


The magnitude of the charging and discharging currents and the potential 
distribution are not uncoupled parameters however. This situation arises 
becsusG the dimensions of the sheath surrounding the charged object increase 
with increasing potential. It is the dimensions of the sheath rather than 
that of the object itself which determine the effective area for the collec- 
tion of charge neutralizing current from the ambient plasma. Secondarily, the 
energy angular distribution of particles striking the object surface will be 
modified in their transit of the sheath region with consequent modification of 
the secondary emission and backscatter yields. Complex computer programs have 
been developed (see, for example, Katz et al , 1977) for this parameter inter- 
dependence. If ion and electron gyroradii are large compared to sheath dimen- 
sions, the configuration can be treated as unmagnetized. At lower altitudes 
where B is greatly increased, gyroradii may be < sheath dimensions and conse- 
quently the magnetic field modifies the neutralizing current collection con- 
figuration. However, the stability of such sheath configurations for condi- 
tions where (typical of the ionosphere) has not been well established; 

P ^ 

the consequence of such sheath turbulence has been proposed to produce 
enhanced crossed field diffusion of plasma. At the present time most theore- 
tical treatments of the neutralization at GEO do not consider magnetic field 

Differential charging of various regions of the object surface results if 
the surface is not an equipotential ; that is if regions of the , surface are 
insulators. Because of the different photoelectron, secondary electron, and 
backscatter yields coupled with both photon and energetic particle shadowing 
arising from the object geometric configuration, differential charging of 
various portions of the surface can be produced. Obviously the bulk conduc- 
tivity of the insulating regions determines the conduction charge loss and the 

magnitude of the potential differences which can be maintained. Thus the bulk 
conductivity represents another surface material propr 'ty influencing the 
final potential distributions. It is these large potential differences 
between nearby regions which produce the vacuum arcs and both malfunction and 
damage of spacecraft components. 

Reference to Fig. 1 indicates that this high potential phenomenon is not 
restricted to GEO. It can occur throughout the entire region of the magneto- 
sphere beyond the plasmapause where the hot plasma of the ring current and 
plasma sheet dominate the cold plasma. Charging, of course, occurs throughout 
the magnetosphere; in the high density region within the plasmapause, the 
floating potential remains low (few volts) even in the presence of large 
energetic particle fluxes such as the aurora. In sunlight, photoelectron 
emission may dominate plasma effects and a positively charged object results. 
Such charging has negligible operational impact, but can severely impact 
measurements of the local low energy plasma characteristics. 

Charging patterns also are produced when the object emits either electron 
or ion beams. Usually beam currents are planned to be far in excess of the 
maximum return (neutralizing) total current from the ambient medium even 
within the high density lower ionosphere. In the case of the space shuttle, 
the collecting surface is ~ 80% insulating tiles. It is possible that very 
large potential differences may be produced between different insulating areas 
and between these and the conducting surfaces. However, intense beams have 
been launched from rockets with little evidence for charging to excessive 
potentials. It is believed that charge neutralization is achieved through the 
local production of a very high density plasma surrounding the vehicle which 
can supply the required neutralizing current. This can be accomplished by 



either the Beam-Plasma-Discharge or an E x B (Penning) discharge (Galeev _e^ 
a1 . , 1976) in which the ambient neutral gas is ionized. Success of beam 
experiments planned for Spacelab require such "non-classical" local sources of 
ionization. Other possible neutralization schemes include the use of plasma 
sources and the deployment of very large area collecting surfaces, nor'e of 
which are planned for early flights. Measurements (Bernstein et al., 1980) 
with an isolated electron gun payload in Chamber A have shown that BPD igni- 
tion does neutralize beam emission currents of ~ 100 ma. Obviously, if neu- 
tralization does occur, large differential charging potentials are eliminated. 
What Can Be Studied in the Large Vacuum Chamber 

From the previous discussion, it is concluded that the most interesting 
plasma phenomena are those associated with the hot plasma region. Although 
conditions there are highly variable, we can assume the following to be repre- 
sentative: Ne = 1 cm"^, Te = = 10 keV, and B = 10"^ gauss. We therefore 

obtain the following important dimensions: 

Debye length, " 7 x 10^ cm 
Proton Gyroradius « 4.7 x 10® cm 
Electron Gyroradius Pg « 1.1 x 10® cm 

Treatment of the ions and electrons as a magnetized plasma requires that these 
be « the dimensions of the system. Thus, even if the ambient magnetic field 
in the chamber could be reduced to 10" gauss, exact simulation of the GEO 
plasma environmenc is not possible. Scaling according to the rules given 
earlier is required. 



The plasma density and magnetic field strength required to reduce the 
above dimension to < 10 cm for a 10 keV thermal plasma are 


Plasma density “ 5 x 10' cm 

" 4.7 X 10^ gauss 
Pg ^ 11 gauss 

At the present time, I cannot identify any method of producing this plasma 
density at 10 keV with a loss cone angular distribution in the chamber. This 
is not reant to imply that scaled experiments are not possible; rather we 
conclude that it is not feasible to produce even the qualitative scaling 
conditions in Chamber A. Several laboratory experiments in small magnetic 
mirror confinement systems have demonstrated the occurrence of the electro- 
magnetic electron cyclotron instability, one of the critical instabilities 
limiting the energetic electron population in the outer magnetosphere (Ikegami 
et al., 1969; Jacquinot et al., 1969). 

Simulation of charging phenomena present a very different set of con- 
straints. As noted earlier, quantitative simulation of the charging process 
requires quantitative duplication of all environmental and surface conditions. 
Even if this could be accomplished, the finite dimensions of any laboratory 
configuration (R < Xp) produces gross modifications in the sheath configu- 
ration and hence the trajectories of the incident particles. 

It is possible however to study the charging process in idealized experi- 
ments which do not duplicate or scale the space environment. Here the objec- 
tives are limited to the following: 

1. To determine whether differential charging to large potential 
difference can occur. 


2. To determine whether these large potential differences will produce 
arcs, and at what potential difference arcing will occur. 

3. To identify the operational consequences of such arcing. 

The use of the large chambers offers several important advantages as 

1. The large dimensions of the chamber allow the irradiation of larger 
and more complex structures than can be studied in the presently 
used laboratory experiments. 

2. The availability of the solar simulator could allow photoelectron 

emission to be included as an experimental parameter. However, 

because the simulator does not duplicate the solar EHV spectrum (La 
and shorter wavelengths), the value of the simulation is doubtful. 

3. The role of electrically grounded conducting boundaries in the 
breakdown process is reduced because of the greater distance from 
the surface to the wall. 

For these qualitative studies of charging, low density energetic electron 
and ion beams from wall mounted accelerators, can provide a reasonable 

approximation of the natural energetic electron. The characteristics of these 
electron beams together with the techniques for electron beam generation 

outlined in th=* Spire report [1979] appear adequate. Energetic ion beams are 

unnecessary because vehicle charging by energetic ions in nature is 
unlikely. A low energy ion source would be desirable. 

The poor vacuum conditions in Chamber A will provide some uncertainties 
in the significance of the results for the following reasons: 

1. Both the secondary electron and photo electron fields are dependent 
on surface properties. The presence of contaminant layers modifies 

the surface characteristics severely. Also the presence of such 
layers may modify breakdown characteristics. Cleaning by baking or 
prolonged electron bombardment together with ultra high vacuum tech- 
niques are usually employed to ensure clean surfaces. At pressures 
> 1 X 10"® torr, with significant H2O vapor partial pressures, it is 
questionable whether clean surfaces can be maintained after clean- 
ing. Also bombardment cleaning may be dangerous to complex struc- 

2. The passage of energetic particle beams through the residual neutral 
gas will produce significant ionization. The fraction of primary 10 
kV electrons producing 1 ion pair in a 20M path length, L, is given 

L L 
MFP ■ NqO. 

1-2 X 10"^ 

The ionization electrons will eventually impact the chamber walls; the ions 
will reach the target surface however and can modify the charge balance. At 
P “ 10"® torr, this effect is unimportant; at 10"® torr and greater, the 
steady state ion flux can be extremely important in modifying the charging 

The following problems in simulation of plasma phenomena in general are 

1) The electromagnetic cyclotron instabilities are characteristic of 
high e plasmas; at Lower 3, electrostatic instabilities become dominant. 
Almost all laboratory plasma devices (Tokamaks, Stellarators, mirrors, etc.) 
currently operate in the low 3 (<10"^) regime. As noted in the scaling 
relationships, quantities such as density and magnetic field strength scale as 


L"^ whereas magnetic pressure scales as L"^. This implies severe difficulties 
in the direct scaling of high B GEO plasma processes where * 0.25. 

2) The magnetic field particle anisotropy configuration is difficult to 
simulate. The hot plasma is confined in the mirror field geometry with a 
mirror ratio equatorial plane is only 
a few degrees. Yet, it is the particle spatial anisotropy arising from the 
finite but small loss cone which provides the energy source for the instabi- 
lities. The use of a smaller mirror ratio increase the magnitude of the loss 
cone and in turn modifies the vi/vi ratio from that at GEO. 

3) A cold plasma density slightly larger than the hot plasma density 
quenches most postulated GEO instabilities. Cold plasma will be produced by 
two processes: (a) charge exchange of energetic ions with ambient neutral 
gas and (b) collisional ionization of the ambient neutral gas by the energetic 
particles. Although the lifetimes of the hot and cold plasma components in 
the mirror geometry will be different, a relatively dense cold component will 
accumulate at large neutral gas densities. 

4) No obvious techniques exist for the production of the hot plasmas/ 
electron and ion components in the density range > 10^ .equired by simple 
dimensional scaling within the mirror geometry. 

For these reasons, it is concluded that the simulations of GEO-plasma 
processes do not seem possible in Chamber A within the limitation imposed by 
present technology. It should be noted that the EM electron cyclotron insta- 
bility has been apparently produced in some experiments with fusion oriented 
mirror devices, but the instability characteristics have not been studied in 
sufficient detail to demonstrate their relevance to space plasma physics 
processes (Ikegami et a1., 1969; Jacquinot et al., 1969). 

Simulation of charging phenomena presents a very different set of circum- 


stances. As noted earlier, the quantitative simulation of the charging 
process requires quantitative duplication of all environmental and surface 
conditions. Even slight deviations from exact scaling would obviate results 
from scaled experimen'^.s, and here quantitative simulation rather than scaling 
would be the preferred approach. Of course, the finite dimensions of any 
laboratory configuration with sheath dimensions larger than chamber dimensions 
produces gross modifications in the overall sheath configuration because of 
the grounded boundaries. The presence of grounded boundaries in near proxi- 
mity allows the occurence of arcs from charged surfaces to ground; however, 
because the walls are reasonably removed from the object in the large chamber, 
and more closely spaced regions of large potential difference will be present, 
the simulation improves with chamber size. It seems reasonable to derive the 
following from a charging simulation experiment. 

1) To determine whether differential charging to potential differences 
exceeding a few KV can occur with maximum particle energies in the range of 
20-30 K«?v (the range expected at GEO). These measurements do not necessarily 
imply the magnitude of the potential differences which will be encountered at 
GEO because of the limited nature of the simulation. Rather they indicate 
that the surface and volume conductivity characteristics are such that they 
will allow the existence of such large potentials. 

2) To determine whether these large potential differences will produce 
vacuum arcs, and at least, qualitatively, the minimum potential difference at 
which arcing occurs. 

3) To identify operational consequences of the arcing; these include 
component failure (infrequent) and transient operational malfunctions. 

The use of the large chamber offers some major advantages in the assess- 
ment of the charging problem for large compl exstructures as follows: 


1) It is possible to test a large complex spacecraft; present testing 
procedures in small vacuum systems can only accomodate small objects and 
therefore a total system test is impossible. Therefore, experiments in small 
systems have been limited to tests of the surface and volume properties of 
isolated surface materials. These data are subsequently included in the large 
computer programs to evaluate the system charging problem. 

2) The presence of the solar simulator allows for inclusions of photo- 
emission effects in charging. However, the chamber does not adequately repro- 
duce the space geometry so that shadowing and angle of incidence is changed. 

3) The roles of electrically conducting boundaries have been discussed. 

4) Complete systems can be irradiated so that operational effects can be 
evaluated rather than estimated. 

Present approach to the charging problem 

At the present time, the vehicle charging problem at GEO appears reason- 
ably well understood. Several approaches to the problem have been attempted 

1) Ideally the use of conducting material for the spacecraft surface so 
that the entire surface is an equipotential . For some cases when insulator 
properties are required, insulating glass cloths have been developed which 
retain their insulating characteristics at small applied potential differ- 
ences, but where conductivity increases at potential differences in the few KV 
range. Effectively use of such materials limits differential charging to 
potential differences less than the vacuum arc threshhold.. 

For cases where the surface is the ideal equipotential, active techniques 
can be employed to prevent charging of tha entire vehicle to large potential 
differences with respect to the ambient plasma. These techniques include 


field and/or thermionic electron emission sources and plasma generators. 
Obviously neither of these techniques will eliminate differential charging 
effects caused by the presence of Insulating surfaces; rather only the poten- 
tial of the conducting surfaces will be clamped to the plasma potential. 

2) In the case of insulating surfaces, the secondary electron and photo- 
electron yields, together with the bulk conductivity (the conductivity may be 
a function of the potential difference) can be determined in laboratory 
experiments. Furthermore laboratory experiments also yield the threshold 
voltage required for vacuum arcs. These data, coupled with assumed plasmas 
distributions allow estimates of vulnerability of various insulating regions 
of a spacecraft to charging problems. 

3) Inclusion of the resultant charging probabilities into t'~e computer 
codes for EMI susceptabi 1 ity then allow an evaluation of arcing consequences. 

4) External arc sources can be used to experimentally simulate the EMI 
effects of vacuum arcs. 

To date, no attempt has been made to develop a test procedure for space- 
craft to simulate the arcing problem. However, such a test is planned in the 
near future at TRW in which an operation spacecraft will be irradiated by 
energetic electron beams (A. Rosen Private Communication). However, it was 
the general consensus of opinion at both TRW (F. Scarf and A. Rosen) and the 
Aerospace Corp. (R. Holzworth and J. M. Cornwall) that this planned test is 
unique and that routine similar spacecraft testing to evaluate charging 
effects would not be an operation requirement for future GEO flights. 



Bernstein W, B Hultqvist and H Borg "Some duplications of low altitude 
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Fig. 2 Dependence of the UATTruDiNic location {£,) of the point where the 6 keV 

. At-nviTy (Kf), 

/Jso shown (•) are plasmapaiise locations delermined by Chappell ei at. (1970) for equivalent 

geomagnetic conditions. 


Fig. 3. An L, local-time plot showing the different local- 
time sectors ot the p]asr.^asphe^e (da.vside, nighlside, and 
bulge). The solid line shows the average plasmapausc posi- 
tion determined frorn more than 150 Ogo 5 profiles IChap- 
pell et al., 19716).