Table of contents

Introduction

The low Earth orbit environment

Spacecraft/plasma interactions

Evaluation of in-orbit spacecraft/plasma interactions

Introduction

Spacecraft wake effects

Spacecraft contamination effects

Spacecraft power system effects

Spacecraft electromagnetic effects

Evaluation of in-orbit spacecraft charging

Spacecraft charging effects

Conditions required for spacecraft charging

Summary of previous work

The ESPIRE computer codes

Scope

Description of the ESPIRE codes

LEOPOLD

SOLARC

EQUIPOT

SAPPHIRE

PICCHARGE

The DICTAT computer code

References

Introduction

• effects caused by the supersonic spacecraft motion through the background ions in the plasma;
• effects caused by exposure to high energy auroral electron fluxes in the polar regions;
• effects on solar arrays operating in the relatively dense plasmas in low Earth orbit;
• effects of contamination, both of the spacecraft itself, and by the spacecraft on the ambient neutral and plasma environment;
• effects leading to the generation and emission of plasma waves and electromagnetic radiation.

Many of these phenomena will be affected by the move to large, active structures, which will be accompanied in their orbits by a self-generated cloud of contamination. The trends to larger size, longer lifetimes, higher power requirement and frequent thruster firings and water and other waste dumps will pose a series of important problems for the space system designer and operator.

Figure 1. Plasma interactions with a spacecraft in polar orbit

The low Earth orbit environment

However, since Debye lengths (plasma charge shielding distances) in the ionosphere are of the order of centimetres, as compared to metres or tens of metres at geostationary altitudes, the effective current collecting area of a spacecraft in LEO will be much smaller than that in GEO.

In addition, the spacecraft will create a large volume of disturbed plasma as it moves through the ionosphere. The spacecraft orbital velocity is hypersonic with respect to the plasma ions, and so phenomena such as bow shocks, ion voids, trailing wakes and density enhancements and depletions will occur. These aspects of the spacecraft/plasma interaction in LEO will not only cause a complicated, disturbed environment local to the spacecraft, but significant wake effects could introduce problems with differential charging.

This is liable to be a problem of some importance in the auroral precipitation zones at high magnetic latitudes. Here, energetic electrons, with energies typically in the range of 5-10 keV, precipitate down magnetic field lines in sufficient intensity to have the potential of greatly modifying the spacecraft charging situation.

Thus, in auroral regions a spacecraft moving along its orbit could suddenly experience a transition from a situation with a spacecraft charging voltage no more than a volt or two negative, to the case where it is intercepting a high energy stream of electrons. If this occurs in darkness, where photoemission is suppressed, and the electrons impinge on wake surfaces where ions are excluded by the spacecraft motion, then the potential for charging is high.

Spacecraft/plasma interactions

There are five broad areas where spacecraft/plasma interactions cause effects which may be of great concern for future missions.

1. Effects caused by the supersonic spacecraft motion through the background ions in the plasma. Large density and space potential disturbances will be set up in the wake of bodies, and density enhancement will occur in the ram. These disturbances will be many body dimensions in spatial extent, and could lead to the shadowing of surfaces, disruption to instrumentation, enhanced particle fluxes to other parts of the spacecraft, and many other related effects. The motion across magnetic field lines will also introduce electric fields, and the effects of these will be greater for large structures.
2. Solar arrays operating in the relatively dense plasmas in low Earth orbit will experience a current drain on the power system, as a result of losses through coupling to the plasma. This will become more severe as systems are enlarged and operating voltages are raised. Arcing of solar arrays in plasmas could lead to damage and electrical noise. Arcing becomes likely when the array is in a contaminated envrionment. Attraction of the ambient ions by negatively charged areas of the array could lead to sputtering and mass loss rates that may limit the lifetime of components, especially those designed for long duration operation.
3. Contamination will be a critical issue in future missions, and this can be divided into two areas of concern. The first is that of spacecraft contamination, where material properties may be changed, thermal contral systems affected, delicate sensing equipment damaged, etc. The relatively dense atmospheric pressure in LEO is important here. The second aspect is that of modification of the ambient atmosphere by outgassing from the spacecraft structure, thruster firings, water dumps, etc.
4. There will be large scope for the generation and emission of plasma waves, not just by active beam emissions experiments, but also by other spacecraft interactions. Plasma wakes, spacecraft power leakage and contaminant ions all provide a source of radiofrequency emissons. As spacecraft become larger and produce more contamination, the possibilities of disturbance, and of a wider range of generation mechanisms, frequencies and power levels, will all increase.
5. Exposure to high energy auroral electron fluxes in the polar regions can lead to high levels of charging on spacecraft, particularl if the current collectoin occurs in the ion-depleted wake zones. The orientation of the magnetic field will complicate the interaction mechanisms, with fields parallel to the collection surface inhibiting the escape of secondary emitted electrons and enhancing charging. Again, these effects will be more important for large structures, where voltages well in excess of the kilovolt level have been predicted to occur. A related problem is that of the charging of one body immersed in the wake of another. The dynamic nature of the charging mechanisms will materials dependent and differential charging between adjacent surfaces could be of concern, especially for large structures.

Evaluation of in-orbit spacecraft/plasma interactions

Introduction

While ions and electrons will be constrained by the magnetic field of the Earth, neutral particles generated by the spacecraft will be free to travel with the vehicle until disturbed by collisional processes. This will be especially of concern for large structures carrying out active emissions of neutral particles, e.g. water and waste dumps, thruster firings, atmospheric venting, etc. In one sense, a spacecraft carries its own atmosphere along with it in orbit.

Space power systems will have to operate in a plasma environment. For solar arrays, with exposed solar cell interconnects, the ionised plasma may be a cause of current leakage and power loss from the array. Operation of the array at high voltage, to reduce cabling losses and array mass, may experience problems with plasma induced breakdowns and arcing, and with anomalous current collection phenomena. Biasing of interconnects at negative voltages in excess of sputter thresholds could lead to ion-induced erosion of the interconnects and destruction of the integrity of the array.

The environment will be turbulent, not only as a result of plasma disturbances caused by spacecraft passage and perturbation of the ambient density, but also because of such activities as thruster firings and water dumps. The turbulence could lead to the generation of electromagnetic emissions with a wide range of frequencies, leading in turn to possible problems with sensors, instrumentation, communications and control systems.

All these effects will have an impact upon future programmes in LEO and polar orbits. The possible phenomena that could occur, the possible consequences of the interactions with the spacecraft, and ways of monitoring and alleviating the effects require detailed study and analysis.

Spacecraft wake effects

The filling of the wake behind a large body moving supersonically or the expansion of a plasma into a rarefied area has been studied. Multiple charged particle populations result in polarisation electric fields which control particle motion along with flow expansion in the collisionless case and diffusion in cases where collisions must be considered. From these processes the wake region becomes a source of electron heating and ion acceleration, preferentially of lighter ions and of minor constituents. Other processes involve plasma oscillations and instabilities, strong jump discontinuities in plasma parameters at the expansion front, and rarefaction wave propagation into the ambient plasma. These phenomena all depend on the ionic constituents and concentrations, ambient electron temperature and density gradients, and the size of the body relative to the Debye length.

Spacecraft contamination effects

Observations suggest that the Shuttle may be immersed in a large gas cloud, made of atoms and molecules from various outgassing sources, whose shape is governed by the Shuttle's interaction with the ambient neutral atmosphere and space plasma environment. Ionisation of contaminants and charge exchange ionisation processes lead to the formation of pick-up ions, tailing behind the spacecraft. Engine firings enhance the contaminant cloud and may produce their own characteristic contaminant cloud or plume that has an associated engine firing light flash which illuminates the Shuttle and enhances the surface glow phenomena. Particulate contamination is also enhanced when RCS engine exhaust plumes impinge directly on Shuttle surfaces. All of these observations suggest a close coupling between the various contaminant sources which contribute to the formation of a multispecious gas cloud surrounding the Shuttle.

Spacecraft power system effects

Spacecraft electromagnetic effects

1. spacecraft electromagnetic interference (EMI) associated with the hardware itself;
2. fields associated with the interaction of the spacecraft and its environment.

Analyses of the wave environment aboard the Space Shuttle have led to the emerging picture that the broadband noise environment is dominated not by the induced environment associated with the large body interaction (although these effects certainly have a role to plasy), byt by the production of waves by the neutral gas cloud as it expands and undergoes chemical interactions, such as charge exchange which results in an ion tail, and creates plasma waves presumed to be driven by the ion currents. If this is the case, then it should be possible to correlate the level of background noise with the density of the neutral cloud, and indeed this has been done in the case of water releases from the Shuttle.

Evaluation of in-orbit spacecraft charging

Spacecraft charging effects

Surface charging could disrupt environmental measurements on scientific spacecraft. For this application and others where control of electrostatic fields is required, meterial selection to minimise differential charging is mandatory. For operational spacecraft, surface charging can also cause problems. The hallmark of the spaceraft charging phenomena is the occurrence of electronic switching anomalies. These anomalies are believed to result from transients caused differential charging induced discharges. These anomalous events even seem to occur in systems that are supposedly immune to noise. The discharge induced transients, under very severe environmental conditions, can cause system failures.

Conditions required for spacecraft charging

1. The spacecraft must encounter an auroral electron plasma which imposes on it a sufficiently large ratio of high energy electron ambient flux to total ion (ambient or ram flux).
2. The ambient electron plasma has a relativel large fraction of its total flux at energies well above the secondary yield maximum of the spacecraft surface material, in order to suppress secondary emission effects which would tend to discharge the spacecraft.
3. The spacecraft must be in darkness in order to suppress photoemission effects, which would tend to discharge the spacecraft.
   In addition, theoretical work suggests that:
4. Charging is more likely for larger spacecraft because electron collection increases more rapidly with spacecraft size than ion collection does.

There are two aspects of the charging phenomena that should be kept in mind when assessing the impact upon future spacecraft systems:

1. Recorded charging levels were for spacecraft with typical dimensions of one metre. As noted above, the charging levels are expected to increase in relation to an increase in the spacecraft dimension. Thus, while maximum charging levels of the order of -0.5 kV have been observed on polar transiting spacecraft to date, levels of charging in excess of -1 kV are anticipated for larger spacecraft. This must be considered during the design of such systems.
2. Presumably, the charging levels recorded by the spacecraft instrumentation represented absolute charging levels, with the whole vehicle assuming the potential with respect to the plasma. Possible more serious in the context of future systems is the question of differential charging. Ion density depletion levels in the wake of a spacecraft, or components of a spacecraft, can be very large and very high levels of charge could accumulate upoon surfaces or equipment, if subject to high energy precipitating electrons. Again, this is an area that must be considered during the design of the systems.

Summary of previous work

Several of these problem areas have been investigated in laboratory experiments. However, it is important to appreciate that the simulation of a complex environment, such as the low Earth orbit ionosphere, in the laboratory is a difficult task. In general, there are five areas to which close attention should be paid:

• the scaling laws that must be applied to reproduce large scale phenomena in the laboratory;
• the need to avoid significant backgrounds of slow, charge exchange generated ions in test facilities;
• the production in the laboratory of monoenergetic ion streams, rather thatn ones with thermal velocity component;
• the need to minimise ion stream divergence and axial density variations;
• the simulation of the on-orbit magnetic field, and an understanding of its influence (or otherwise) on plasma interactions.

A comprehensive review of test facilities used in experimental simulation studies led to the following conclusions:

• In plasma wake studies, slow background ions can be controlled in a satisfactory manner, and resonable values of divergence can be achieved. However, these have not always been reproduced at the same time, and data from many facilities must be treated with caution. Little work has been reported on plasma wakes created by large bodies.
• In solar array studies, most facilities have not adequately simulated the correct ambient pressure. Operating pressures are often in the 10^-5 torr range and above, and this can lead to anomalous plasma conditions near solar array samples, distorting experimental results. In addition, most facilities have produced Debye lengths (plasma screening distances) of the same order as the solar cells. Only recently have Debye lengths of the same dimension as array interconnects been produced.
• In spaccraft charging studies, initial experiments relevant to conditions in low Earth and polar orbit have only recently begun, and information is limited. Again the presence of slow facility ions has caused distortion of the results.

The problems have also been studied using numerical and computational simulation methods. There are essentially two different philosophies that can be applied when writing computer codes to simulate the spacecraft/plasma interaction in low Earth orbit:

• the use of analytical fluid equations to represent the plasma;
• the use of numerical methods to compute the trajectories of particles by solving their equations of motion.

It is a feature of analytical codes that they describe only macro-processes (i.e. above the scale of individual particles) that are explicitly included by the author. Hence, in a sense, they tell the user only what he/she already knows and will not surprise him/her with unexpected behaviour.

There are generally less approximations in particle tracking codes than in analytical ones. Where approximations appear, they are, for example, in the form of the secondary electron emission spectrum and not in the particle trajectories.

In additon to this ground based work, account must be taken of space based experience. Over the past years, an ever increasing number of publications have appeared on the plasma modifications around a vehicle orbiting in the low Earth plasma. The mid-latitude, low Earth orbit region has been investigated fairly extensively by a series of satellites plus a few sounding rockets. In polar regions on the other hand, very few satellites have yet been flown which carried adequate instrumentation to monitor plasma modifications. However, many rocket payloads have been launched into the auroral zones.

Figure 2. Spacecraft interactions with ambient plasma

The current status of affairs can be summarised as follows:

• Of a wide range of spacecraft/plasma interactions and their effects (see Fig. 2), many hold serious implications for future large, long lived, active structures in space, and in some cases information is limited. Again, the presence of slow facility ions has caused distortion of the experimental results.
• Laboratory experiments can adequately simulate the space environment and its interaction with spacecraft, but attention must be paid to the validity of the simulation. Plasma wake studies have generally only coverd small to medium vehicles. A reasonable data base on solar array interactions exists. Spacecraft charging studies in polar orbit conditions are in their infancy.
• Numerical and computational studies can aid in interpretation of the interaction phenomena and, to a lesser extent, in spacecraft design. The assumptions inherent in sevral codes limit their application to design activities, until a more fundamental understanding of the basics behind the interactions has been achieved.
• There is a large literature concerned with space based experiences, but interpretation is, at times, difficult and contradictory. Vehicle charging in polar orbits has been indicated, and the adverse effect that active beam emissions can have on vehicles are well established. Again, a reasonable data base on solar array interactions exists, but it should be noted that this is in conflict with ground data.

The ESPIRE computer codes

Scope

Experimental work on plasma wake phenomena used simple metallic models in the shape of a disc and a rectangular plate, placed in a streaming plasma. The wake structure downstream of the model was studied as a function of the Debye length or body size, model potential and Mach number. A valid simulation requires control of the slow ion population, using high pumping speeds and low operating pressures, and this was successfully achieved in both facilities.

Experimental studies in support of spacecraft charging used high energy (5-20 keV) electron guns to bombard the wake side of a model placed in a streaming plasma. Isolated metal targets of different materials and isolated Kapton were placed in the wake position. The charging levels were measured as a function of such parameters as ambient ion density, background pressure, ion energy, and test geometry.

The final outcome has been the development of a flexible software suite to model important aspects of spacecraft/plasma interactions and spacecraft charging. The emphasis has been upon experimentally validated engineering tools for use in spacecraft design. The suite is constructed in such a way as to facilitate future additions tot he programs as a result of the continuing development of engineering software tools.

Description of the ESPIRE codes

The solution to this issue is the creation of a suite of computer programs each capable of analysing a part of the spacecraft/plasma interaction problem on the basis of a specific but restricted set of assumptions. While each code individually will allow only part of the problem to be addressed, the entire suite will allow a much more complete picture to be obtained. The ESPIRE program suite structure is illustrated in Fig. 3.

Figure 3. Structure of the Spacecraft/Plasma Interactions and Electromagnetic Effects Program Suite ESPIRE

LEOPOLD

Using empirical and theoretical relationships and in-built data bases, the code evaluates and outputs seventeen space environment parameters. The input menus have been replaced by HTML forms in the SPENVIS implementation of LEOPOLD.

SOLARC

SOLARC Is an O-D code which provides an assessment of the current collection and the power loss that would be experienced by a solar array in LEO and polar environments. For a given set of conditions, specified by the user, the code creates current-voltage characteristics derived from two sets of empirical formulae by finding the solar array voltage distribution that produces a zero net collected current.

The first set of equations (model 1) were based upon the results of ground-based solar array experiments undertaken at the NASA Lewis Research Center. The second set of equations (model 2) were derived from space based data obtained on the PIX-2 test flight.

The equations are fully integrated with the SOLARC code. Any of the program variables (such as solar array area, voltage relative to space plasma potential, electron and ion current densities and temperatures) may be altered independently within the code.

EQUIPOT

A number of options are available to cater for plasma regimes with Debye length large (GEO) or small (LEO), solar illumination or shadow, spacecraft velocity (ram and wake effects), and normal or isotropic incidence of particles. For small Debye lengths, it is reasonable to assume that current collection is sheath limited (plane probe assumption); for large Debye lengths, the geometry becomes more important and a spherical probe assumption offers an alternative limit.

SAPPHIRE

SAPPHIRE Is a two dimensional particle and potential code which calculates the ion and electron densities and the electrostatic potential about a body or bodies in the presence of a streaming plasma. It has the capability of representing a variety of spacecraft geometries and allowing different potentials to be specified. A number of ambient plasma conditions may be modelled including monoenergetic plasmas and plasmas with a Maxwellian temperature distribution.

PICCHARGE

The DICTAT computer code

DICTAT calculates the electron current that passes through a conductive shield and becomes deposited inside a dielectric. From the deposited current, the maximum electric field within the dielectric is found. This field is compared with the breakdown field for that dielectric to see if the material is at risk of an electrostatic discharge.

References

, , Final Report for ESA/ESTEC Contract No. 7989/88/NL/PB(SC), Vol. 1, 199

Soubeyran, A., and R. Floberhagen, ESA-DDC 1.1 User Manual, Matra Marconi Space, 1994.

Last update: Mon, 12 Mar 2018