**1. Introduction**

When a solid dielectric interacts with plasma, it is subject to a number of different destructive and non-destructive processes. A charged particle impinging on a surface with sufficient

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

energy will penetrate the surface and lose energy into the bulk of the material. The deposition of this kinetic energy will result in electronic and vibrational excitations. If sufficient energy is deposited into a single bond to excite it beyond its dissociation energy, the chemical bond can be broken. The dominant mechanism of energy deposition depends strongly on the mass of the particle. Massive particles such as heavy ions will impart large amounts of ballistic energy over a relatively small depth, displacing nuclei in the solid, exciting phonons, and vibrational transitions sufficient to rupture any chemical bond and create radicals. A less massive particle such as an electron can be expected to deposit energy primarily in the form of electronic excitation. Sufficient electronic energy deposition will also rupture bonds and create radicals, but the damage will be deposited over a longer trajectory and the chemical damage will be more bond-selective. Ions, electrons, and photons incident on a surface will also eject secondary electrons, initiating charge imbalance near the surface. After the kinetic energy of a charged particle has been exhausted, the ion or electron can imbed itself into the bulk, creating a local charge imbalance at the penetration depth of the particle. When considering the interaction of space plasma with solid dielectrics, it is important to distinguish between energy deposition and charge deposition [1, 2].

The following chapter will focus on the interaction of electrons, which comprise the most damaging species in the Geosynchronous Earth Orbit (GEO) environment in terms of energy deposition [3–5], with dielectric materials such as polymers and solar array cover glass [6]. The results of this plasma/material interaction are characterized in terms of modification of the materials' optical and electrical properties. It will be shown that these materials change dramatically in the plasma environment and that these changes can have profound effects on the performance of spacecraft components.

The thermal plasma in GEO (6.6 Earth radii, or RE) is predominantly quasi-neutral plasma consisting of atomic hydrogen ions (replenished mainly by the solar wind and ionized by Lyman-alpha radiation from the Sun) and electrons. It is typically at a very low-density (0.1–1 cm−3) but high plasma temperature (typically 4–10 keV). During geomagnetic storm conditions, the plasma temperature can increase dramatically (to 16–30 keV), as magnetic reconnection in the magnetotail accelerates trapped electrons, and the accompanying ions. The second population of particles exists at GEO in the highly non-thermal outer radiation belt. These highly energetic electrons (0.1–10 MeV) form a toroidal belt extending out to about 60,000 km (10 RE) from Earth, with a maximum intensity typically at about 3–5 RE at the equator. Thus, although GEO orbit is beyond the distance of maximum intensity, it is well within the limits of the outer belt, and satellites orbiting within it are subjected to significant penetrating electron fluxes, as can be seen in **Figure 1** [3–5].

penetrated by electrons of energy > 2 MeV, while electrons of > 0.25 MeV energy can fully penetrate the MLI layers [3]. However, surface materials are subject to deterioration by electrons of even 0.1 MeV, which have the greatest flux due to the steep outer belt electron spectrum, and are typically stopped within the outermost polymer layer [5]. In addition to damage caused by energy deposition, non-penetrating electrons deposit charge in the

Monitor on the Advanced Composition Explorer (ACE EPAM) [5, 8].

**Figure 1.** Plot of mean annual electron flux (>100 keV electrons) orbit averaged, experienced by a satellite in a circular geocentric orbit as a function of altitude and maximum latitude (inclination for prograde orbits, a supplement of inclination for retrograde orbits). Representative orbits are shown as dashed lines for reference, as are the positions of the moon, International Space Station (ISS), and several other satellites. Figure based on AE9 V1.5 mean radiation belt model, [7] with mean solar wind values outside AE9 coverage from data collected by the Electron, Proton, and Alpha

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The material properties of greatest interest to spacecraft engineers are those that influence temperature and differential charging. Surface temperature is dependent on the selectivity of the surface, defined as the ratio of incident light absorbed (absorptivity, α; heating) to radiated infrared flux (emissivity, ε; cooling). Any change in color or surface roughness will change one or both of these quantities, and lead to a change in equilibrium temperature. Unfortunately, both quantities may be changed by polymer degradation due to radiation.

Optical properties of materials are also important for surveillance and health-monitoring purposes. Satellites at GEO are not spatially resolvable with ground-based optics, even with adaptive optics techniques. Therefore, the observable quantities are brightness, position, color

material.

The exterior of spacecraft is typically comprised of materials that regulate temperature by reflecting visible sunlight and radiating infrared radiation. Among the most common of these materials is multi-layer insulation (MLI), which usually consists of many thin layers of aluminized (or silvered) polymers such as Kapton® (polyimide or PI). Tenets of modern spacecraft design dictate that radiation-sensitive electronics be positioned in the bulk of the spacecraft in a shielded conductive "Faraday cage". These Faraday cages can only be

energy will penetrate the surface and lose energy into the bulk of the material. The deposition of this kinetic energy will result in electronic and vibrational excitations. If sufficient energy is deposited into a single bond to excite it beyond its dissociation energy, the chemical bond can be broken. The dominant mechanism of energy deposition depends strongly on the mass of the particle. Massive particles such as heavy ions will impart large amounts of ballistic energy over a relatively small depth, displacing nuclei in the solid, exciting phonons, and vibrational transitions sufficient to rupture any chemical bond and create radicals. A less massive particle such as an electron can be expected to deposit energy primarily in the form of electronic excitation. Sufficient electronic energy deposition will also rupture bonds and create radicals, but the damage will be deposited over a longer trajectory and the chemical damage will be more bond-selective. Ions, electrons, and photons incident on a surface will also eject secondary electrons, initiating charge imbalance near the surface. After the kinetic energy of a charged particle has been exhausted, the ion or electron can imbed itself into the bulk, creating a local charge imbalance at the penetration depth of the particle. When considering the interaction of space plasma with solid dielectrics, it is important to distinguish between energy deposition

226 Plasma Science and Technology - Basic Fundamentals and Modern Applications

The following chapter will focus on the interaction of electrons, which comprise the most damaging species in the Geosynchronous Earth Orbit (GEO) environment in terms of energy deposition [3–5], with dielectric materials such as polymers and solar array cover glass [6]. The results of this plasma/material interaction are characterized in terms of modification of the materials' optical and electrical properties. It will be shown that these materials change dramatically in the plasma environment and that these changes can have profound effects on

The thermal plasma in GEO (6.6 Earth radii, or RE) is predominantly quasi-neutral plasma consisting of atomic hydrogen ions (replenished mainly by the solar wind and ionized by Lyman-alpha radiation from the Sun) and electrons. It is typically at a very low-density (0.1–1 cm−3) but high plasma temperature (typically 4–10 keV). During geomagnetic storm conditions, the plasma temperature can increase dramatically (to 16–30 keV), as magnetic reconnection in the magnetotail accelerates trapped electrons, and the accompanying ions. The second population of particles exists at GEO in the highly non-thermal outer radiation belt. These highly energetic electrons (0.1–10 MeV) form a toroidal belt extending out to about 60,000 km (10 RE) from Earth, with a maximum intensity typically at about 3–5 RE at the equator. Thus, although GEO orbit is beyond the distance of maximum intensity, it is well within the limits of the outer belt, and satellites orbiting within it are subjected to significant

The exterior of spacecraft is typically comprised of materials that regulate temperature by reflecting visible sunlight and radiating infrared radiation. Among the most common of these materials is multi-layer insulation (MLI), which usually consists of many thin layers of aluminized (or silvered) polymers such as Kapton® (polyimide or PI). Tenets of modern spacecraft design dictate that radiation-sensitive electronics be positioned in the bulk of the spacecraft in a shielded conductive "Faraday cage". These Faraday cages can only be

and charge deposition [1, 2].

the performance of spacecraft components.

penetrating electron fluxes, as can be seen in **Figure 1** [3–5].

**Figure 1.** Plot of mean annual electron flux (>100 keV electrons) orbit averaged, experienced by a satellite in a circular geocentric orbit as a function of altitude and maximum latitude (inclination for prograde orbits, a supplement of inclination for retrograde orbits). Representative orbits are shown as dashed lines for reference, as are the positions of the moon, International Space Station (ISS), and several other satellites. Figure based on AE9 V1.5 mean radiation belt model, [7] with mean solar wind values outside AE9 coverage from data collected by the Electron, Proton, and Alpha Monitor on the Advanced Composition Explorer (ACE EPAM) [5, 8].

penetrated by electrons of energy > 2 MeV, while electrons of > 0.25 MeV energy can fully penetrate the MLI layers [3]. However, surface materials are subject to deterioration by electrons of even 0.1 MeV, which have the greatest flux due to the steep outer belt electron spectrum, and are typically stopped within the outermost polymer layer [5]. In addition to damage caused by energy deposition, non-penetrating electrons deposit charge in the material.

The material properties of greatest interest to spacecraft engineers are those that influence temperature and differential charging. Surface temperature is dependent on the selectivity of the surface, defined as the ratio of incident light absorbed (absorptivity, α; heating) to radiated infrared flux (emissivity, ε; cooling). Any change in color or surface roughness will change one or both of these quantities, and lead to a change in equilibrium temperature. Unfortunately, both quantities may be changed by polymer degradation due to radiation.

Optical properties of materials are also important for surveillance and health-monitoring purposes. Satellites at GEO are not spatially resolvable with ground-based optics, even with adaptive optics techniques. Therefore, the observable quantities are brightness, position, color (or reflected spectrum), and polarization. Any change in these quantities may lead to satellite misidentification, or be indicative of a change in satellite health.

changes manifest themselves as changes to a number of different physical properties including absorptivity and emissivity, electrical conductivity, and mechanical properties of the materials. Because it is not experimentally convenient to study on orbit spacecraft *in situ*, the spacecraft charging and instrument calibration laboratory in the space vehicles directorate of the US air force research laboratory have constructed a space weather simulation chamber, nicknamed Jumbo, in which electron and VUV photon damage can be inflicted on a variety of spacecraft materials and the effects of this damage can be quantified. The results of these simulated spaceaging experiments can be used to refine existing models for spacecraft differential charging [10], predict, and characterize anomalous satellite behavior, identify space debris, and even be used

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The Jumbo space weather simulation chamber consists of a 6' long, 6' diameter cylindrical vacuum chamber, energetic particle sources, and various probes used for material characterization [11]. Pressures of < 10−6 Torr can be achieved from the atmosphere in ~1 h using a combination of mechanical pumps and a turbo- and/or a cryopump. In GEO, the electromagnetic radiation most likely to damage polymer materials is the Lyman-α line of hydrogen at 121.6 nm. These VUV) photons have enough energy (~10 eV) to break bonds in polymer materials. In order to mimic Lyman-α line of hydrogen, the chamber is equipped with four Krypton lamps (Resonance Ltd.) with emission lines at 123.6 and 116.5 nm, providing approximately 10 suns of VUV radiation. The prime electron source is a Kimball physics EG8105-UD electron flood gun with a range of 1–100 keV. This electron gun is used both for material aging (high-energy, deeply penetrating electrons) and for charging of materials (low-energy, shallowly penetration electrons) in order to perform SPD experiments from which material bulk

In addition to the particle sources, which are used to age common spacecraft materials; jumbo is equipped with a number of probes, which are used to characterize physical characteristics of the chosen material before, during, and after irradiation. The primary characterization tools used *in vacuo* are an integrating sphere with fiber optic coupled external light source and spectrometer, which is used to characterize optical changes and a non-contact electrostatic voltmeter, which is used to investigate material charge transport characteristics using the

Optical transmission/reflectance spectra are recorded using an in-vacuum integrating sphere so that the measured quantity is the directional hemispherical reflectance (DHR). For each measurement, an ASD FieldSpec Pro spectroradiometer operating over a 350–2500 nm range collects first a white reference spectrum from a piece of in-vacuum calibrated Spectralon®. The electron beam is then temporarily extinguished and the motion system moves the integrating sphere to a sample carousel where it measures samples as they are rotated into the measure-

The SPD method utilizes charge injection via a low-energy electron beam to induce an electric potential near the surface of the material. To perform an SPD experiment, the front surface of the material is dusted by a beam of 5 keV electrons for 1–2 s immediately, after which the non-contact voltmeter is positioned 1–2 mm from the surface and begins to record the surface potential of the dielectric. After the charged body induced by the beam has reached the

to diagnose the health of satellites on orbit from ground-based observations.

conductivity/resistivity can be derived [12].

SPD method.

ment position [13].

Spacecraft charging is the potential a spacecraft or spacecraft component will assume as a result of the balance of incoming charged particles from the environment and particles removed via material conduction or electron emission. Interaction with space plasma can result in potential gradients between different spacecraft components (differential charging) of thousands or even tens of thousands of volts, which can seriously affect the operation of the spacecraft. Differential charging is most pronounced in GEO where the environment is dominated by electrons and the flux of mitigating ions is relatively low. Material properties also affect the charging behavior of a spacecraft. For instance, secondary electron emission and photoelectron emission tend to discharge surfaces, so spacecraft charging is very sensitive to these material properties. Also, dielectric materials can lose surface charge to interior chassis "ground" by electron conduction; this property also changes as the dielectric interacts with the ambient plasma. This is important due to the fact that in the presence of space plasma, electric arcs can jump from a negative surface to a positive surface if the potential difference between them is great enough. Depending on the amount of energy stored in the "surface capacitance," these arcs can lead to contamination of adjacent surfaces, radio frequency interference, sharp current transients and electronic upsets, and in the worst case can develop into "sustained arcs" that can destroy entire solar arrays or other electrical circuits.

In general, charge buildup anywhere on or in the spacecraft can lead to arcing. Changes in surface and bulk conductivity, secondary electron emission, and photoemission will lead to changes in the local electric fields and therefore, changes in the susceptibility of spacecraft to arcing. Unfortunately, although these electrical properties are known to change with radiation exposure, the magnitude of the changes is poorly characterized at present.

In the laboratory materials degradation tests reported here, the incident electrons are 90 keV electrons unless otherwise noted. This energy is chosen because the outer belt non-thermal electron flux is highest at lower energies, so they are most important from an energy deposition standpoint. Finally, 90 keV electrons can easily be produced by commercial electron guns and are less dangerous from an X-ray production standpoint than higher energy electrons. High-energy ions can also produce material degradation, but the fluxes of these particles in GEO are much lower than for electrons [5, 7].
