**3.1. Electrostatic charge and discharge**

Interaction of space plasma with materials on orbit has been shown to drastically and permanently change spacecraft materials' charging properties. There is strong evidence that Galaxy 15, a GEO communications satellite, was incapacitated for 8 months due to arcing caused by the space environment, [16] even though in its 5 years of previous flight it had experienced similar environments several times with no ill effects. Charging in the auroral streams or when satellites exit eclipse can cause arcing on solar arrays, single event upsets in electronics, or as in the case of ADEOS-2, arcing in power lines leading to a complete loss of power [17].

Different parts of a satellite have different susceptibilities to charge accumulation. For example, a space-like test of a four-cell GPS-like solar array showed that most arcs occurred on the SCV10-2568 room-temperature vulcanization rubber (RTV) used to glue the cells and wiring to the Kapton substrate or near solar cell edges or corners. Each arc discharged about the coverglass area of the array. From these results, it is hypothesized that at least some of the arcing that occurs on GPS satellites on orbit are from the RTV and that some of the contamination that degrades the GPS solar array performance over time comes from arcs on the RTV as well as from the arcs from silver interconnects [18].

The likelihood of electronic discharge on a satellite on orbit is a function of the differential potentials that develop throughout the craft. That potential is dictated by the balance of incoming charge from the environment and charge that is removed via conduction and secondary electron emission. Since incoming charge cannot be controlled, the only way to predict what electric potential a material will adopt is to control the rate that charge is removed.

**Figure 2** is a plot of the SPD rate as a function of polyimide resistivity [12]. When that decay rate exceeds the orbital period (1 day for GEO) the material can gather charge for the entire mission and the likelihood of discharge increases. The red and green stars in **Figure 2** show the resistivity of pristine polyimide and polyimide that has been radiation damaged, respectively. The irradiated material has become far less susceptible to charge accumulation, which is important to take into account in the design of spacecraft and the characterization of anomalous spacecraft behavior.

grounded backplane, the dissipation of surface potential is primarily determined by the loss of electrons from the material and is directly proportional to the material conductivity. SPD measurements were performed in darkness to eliminate the possibility of optically enhanced conductivity and photoemission. Since SPD is measured immediately following the charging electron beam, persistent radiation-induced conductivity (RIC) is still active [14]. However, it is only in effect between the surface and the penetration depth of the electrons, which for the case of 5 keV electrons in polyimide is less than a micron [3]. This minimizes the effect of RIC,

In addition to in-vacuo characterization, a portable vacuum pumped window is used to transport aged materials to a commercial UV/visible transmission/reflection spectrometer (Perkin-Elmer Lambda 950) and a Fourier transform infrared reflection spectrometer (Surface Optics SOC-400T). The portable vacuum window was designed to enable characterization of air sensitive materials using existing bench-top instruments without subjecting the space-weathered samples to unnecessary air exposure. Post-irradiation air exposure has been shown to modify

Interaction of space plasma with materials on orbit has been shown to drastically and permanently change spacecraft materials' charging properties. There is strong evidence that Galaxy 15, a GEO communications satellite, was incapacitated for 8 months due to arcing caused by the space environment, [16] even though in its 5 years of previous flight it had experienced similar environments several times with no ill effects. Charging in the auroral streams or when satellites exit eclipse can cause arcing on solar arrays, single event upsets in electronics, or as in the case of ADEOS-2, arcing in power lines leading to a complete loss of power [17].

Different parts of a satellite have different susceptibilities to charge accumulation. For example, a space-like test of a four-cell GPS-like solar array showed that most arcs occurred on the SCV10-2568 room-temperature vulcanization rubber (RTV) used to glue the cells and wiring to the Kapton substrate or near solar cell edges or corners. Each arc discharged about the coverglass area of the array. From these results, it is hypothesized that at least some of the arcing that occurs on GPS satellites on orbit are from the RTV and that some of the contamination that degrades the GPS solar array performance over time comes from arcs on the RTV as well

The likelihood of electronic discharge on a satellite on orbit is a function of the differential potentials that develop throughout the craft. That potential is dictated by the balance of incoming charge from the environment and charge that is removed via conduction and secondary electron emission. Since incoming charge cannot be controlled, the only way to predict what

electric potential a material will adopt is to control the rate that charge is removed.

which is assumed to be negligible for the bulk of the material in this measurement.

certain materials' chemistry extensively and on a very short time scale (minutes) [15].

**3. Effect of space plasma on satellites**

230 Plasma Science and Technology - Basic Fundamentals and Modern Applications

as from the arcs from silver interconnects [18].

**3.1. Electrostatic charge and discharge**

Charge transport characteristics of materials exposed to space plasma are defined by their fundamental material properties; in particular for insulators, density, and energy distribution of electron trap states within the band gap. Under the approach described by Dennison and Hoffmann, [20] as materials are bombarded with a flux of penetrating high-energy radiation, energy is shared with many bound (valence) electrons within the material, which are excited into energy levels scattered in the conduction band. These excited electrons quickly thermalize to shallow localized trap states just below the conduction band edge. Next, electrons can, among other processes, (i) be thermally re-excited into the conduction band, leading to thermally assisted charge transport, and termed radiation-induced conductivity (RIC); [21] or (ii) hop to an adjacent trap, termed thermally assisted hopping conductivity or dark current (DC) conductivity [22]. In addition, as a material age, more traps are generated [23].

**Figure 3** Presents a representative charge–discharge curve of electron-irradiated PI material during and after bombardment with non-penetrating electrons. This curve can be divided into three regions: (I) charging section driven by the balance of electron deposition, secondary electron generation, RIC and DC conductivity; (II) pre-transit discharge section dominated by the RIC and DC conduction, as is the time regime when the deposited charge body is traversing the material but has not made contact with the grounded backplane; (III) post-transit discharge section dominated DC conduction.

**Figure 2.** The plot of charge decay time versus resistivity value for PI. The red star indicates the resistivity of pristine PI and the green star that of laboratory-aged Kapton-H®. Figure adapted from [19].

is a dispersive term, taking into account the disordered structure of polyimide; μ0

a certain rate α, s−1. The remainder of the deposited charge (1-a<sup>1</sup>

of the charges are initially placed in the surface traps, from where they move to the bulk at

The post-transit region of the surface potential decay curve (region III) starts when a charged body has reached the grounded back plane. This can take place within a fraction of a second for high conductivity polymers or years in the case of low conductivity polymers such as PTFE (Teflon®). After the front of the charge, body has reached the grounded backplane the dissipation of charge is primarily determined by the loss of electrons from the material. This region is fit by Eq. (3), from which a decay time and the dark resistivity of the material may be derived [30]:

Vs(τdecay) = m τdecay

tivity of free space and relative permittivity of the PI material, respectively. The conductivity of the material is then calculated as inversely proportional to the resistivity of the material,

Interaction of PI with highly energetic particles of space plasma will modify its chemical structure. The extent of this modification is a function of several simultaneous kinetic processes, namely, damage (interaction of material with highly energetic particles, resulting in broken chemical bonds), healing (formation of bonds identical to those damaged, returning the material to its pristine state), and scarring (formation of new chemical bonds in the damaged material, which are different from those in the pristine material) [15]. Often, macroscopic properties are measured making it difficult to distinguish healing from scarring; hence we

**Figure 4** Shows the photographs of a radiation-damaged PI sample taken immediately after irradiation with 90 keV mono-energetic electron flood gun to the dose of 5.6 × 10<sup>7</sup> Gy and a pristine PI control sample. This energetic dose is equivalent to that experienced by PI during approximately 8 years in GEO orbit [3, 5, 7]. The damaged sample has a deep brown color, which differs from the characteristic amber color of pristine PI. From **Figure 4**, it is clear both electron damage and subsequent exposure to air have a significant effect on the optical

The effect of damaging radiation on the optical properties of the irradiated material is evident from the transmission spectra of radiation-damaged PI, as shown in **Figure 5**. The red-shift of the absorption edge indicates an effective shrinking of the PI's "band gap" to ~1.8 eV in the damaged material due to the emergence of radiation-induced electronic states. Compared to the measured "band gap" of Kapton of 2.3 eV, these states are energetically shallow [31].

where τdecay is charge decay time, seconds; m and b are fitting parameters; ε0

refer to the sum of healing and scarring as recovery.

properties of PI in the visible spectrum.

b <sup>ρ</sup>dark <sup>=</sup> <sup>τ</sup> \_\_\_\_ decay ϵ<sup>0</sup> ϵ<sup>r</sup>

mobility between traps, m2 V−1 s−1; a<sup>1</sup>

bulk immediately after the discharge.

a1

1/Ω∙cm.

**3.2. Material changes**

is the carrier

233

(3)

are permit-

) is injected directly into the

Space Plasma Interactions with Spacecraft Materials http://dx.doi.org/10.5772/intechopen.78306

and ε<sup>r</sup>

and β are fitting parameters. It is assumed that a fraction

**Figure 3.** Representative charge/discharge curve of PI material bombarded with a non-penetrating electron. Shaded areas represent the three regions of the charge/discharge curve. See text for further details.

Theoretical models developed over the past several decades allow the extraction of many material parameters from a material's charge/discharge curve, including the density of trapped states (region I), trapping and de-trapping rates, and effective electron mobility (region II), and dark resistivity and conductivity of the material (region III) [24–28].

In particular, the charging part of the charge/discharge curve (region I) may be modeled with the following equation developed by Sim [24]:

$$V\_s(t) = \frac{q\_v d \, N\_i}{\varepsilon\_0 \varepsilon\_r} \left\{ 1 - \frac{R(\varepsilon\_i)}{d} \right\} R(\varepsilon\_b) \left[ 1 - \exp\left\{ \frac{s\_c l\_s \tau\_{met} (1 - \sigma\_{yield})}{q\_c (1 - m)} \right\} \left[ 1 - \left( 1 + \frac{t}{\tau\_{met}} \right)^{1 - m} \right] \right] \tag{1}$$

Here q<sup>e</sup> is the charge of an electron, C; ε0 and ε<sup>r</sup> are the permittivity of free space and relative permittivity of the chosen material, respectively; J<sup>b</sup> is the electron beam flux, nA/cm<sup>2</sup> ; d is a sample thickness, cm. The secondary electron yield, σyield, the number of electrons emitted per incident electron, may be estimated based on the measurements and models of Song et al. [29]. The range, R (ε<sup>b</sup> ), is the maximum distance an electron of a given incident energy can penetrate through the material before all kinetic energy is lost and the electron comes to rest. Free parameters for Eq. (1) are the density of states, N<sup>t</sup> ; capture cross-section s<sup>c</sup> ; characteristic onset time for the current decay to occur, τonset; and a power parameter m, with 0 < m < 1.

The pre-transit discharge section (region II) may be described using a model based on original work of Toomer and Lewis [25] supplemented by Aragoneses *et al* [26].

$$\frac{\mathbf{V}(t)}{\mathbf{V}\_{0}} = \mathbf{1} - \frac{\mathbf{V}\_{0}\mu\_{0}}{2\,d^{2}\,\mathrm{R}} \left( r\_{\mathrm{i}}\,t + \frac{r\_{\mathrm{i}}}{R} [\mathbf{1} - e^{-Rt}] + \frac{r\_{\mathrm{r}}a\_{\mathrm{i}}^{2}}{2a} [\mathbf{1} - e^{-2at}] \right) - \frac{\mathbf{V}\_{0}\mu\_{0}}{2\,d^{2}\,\mathrm{R}} \left( \frac{r\_{\mathrm{i}}a\_{\mathrm{i}}^{2}}{R + 2a} [\mathbf{1} - e^{-(R + 2at)}] \right) - \lambda\ \,t^{\mathrm{f}} \tag{2}$$

where d is the sample thickness, μm; V<sup>o</sup> is the initial surface potential; R is the parameter describing charge transport dynamics; r<sup>r</sup> and rt are the probabilities of charge per unit time to be released from the trap and to be re-trapped in different trapping center, respectively, s−1; λ is a dispersive term, taking into account the disordered structure of polyimide; μ0 is the carrier mobility between traps, m2 V−1 s−1; a<sup>1</sup> and β are fitting parameters. It is assumed that a fraction a1 of the charges are initially placed in the surface traps, from where they move to the bulk at a certain rate α, s−1. The remainder of the deposited charge (1-a<sup>1</sup> ) is injected directly into the bulk immediately after the discharge.

The post-transit region of the surface potential decay curve (region III) starts when a charged body has reached the grounded back plane. This can take place within a fraction of a second for high conductivity polymers or years in the case of low conductivity polymers such as PTFE (Teflon®). After the front of the charge, body has reached the grounded backplane the dissipation of charge is primarily determined by the loss of electrons from the material. This region is fit by Eq. (3), from which a decay time and the dark resistivity of the material may be derived [30]:

$$\begin{aligned} \mathbf{V}\_s \{ \mathbf{r}\_{\text{decay}} \} &= \mathbf{m} \, \mathbf{r}\_{\text{decay}}^b \\ \mathbf{Q}\_{\text{dark}} &= \frac{\mathbf{r}\_{\text{decay}}}{\mathbf{e}\_0 \, \mathbf{e}\_r} \end{aligned} \tag{3}$$

where τdecay is charge decay time, seconds; m and b are fitting parameters; ε0 and ε<sup>r</sup> are permittivity of free space and relative permittivity of the PI material, respectively. The conductivity of the material is then calculated as inversely proportional to the resistivity of the material, 1/Ω∙cm.
