Triggering Threshold Spacecraft Charging with Changes in Electron Emission from Materials

Abstract

Modest changes in spacecraft charging conditions can lead to abrupt changes in the spacecraft equilibrium, from small positive potentials to large negative potentials relative to the space plasma; this phenomenon is referred to as threshold charging. It is well known that temporal changes of the space plasma environment (electron plasma temperature or density) can cause threshold charging. Threshold charging can also result from by temporal changes in the juxtaposition of the spacecraft to the environment, including spacecraft orbit, orientation, and geometry. This study focuses on the effects of possible changes in electron emission properties of representative spacecraft materials. It is found that for electron-induced emission, the possible threshold scenarios are very rich, since this type of electron emission can cause either positive or negative charging. Alternately, modification of photon-or ion-induced electron emission is found to induce threshold charging only in certain favorable cases. Changes of emission properties discussed include modifications due to: contamination, degradation and roughening of surfaces and layered materials; biasing and charge accumulation; bandstructure occupation and density of states caused by heat, optical or particle radiation; optical reflectivity and absorptivity; and inaccuracies and errors in measurements and parameterization of materials properties. An established method is used here to quantitatively gauge the relative extent to which these various changes in electron emission alter a spacecraft's charging behavior and possibly lead to threshold charging. The absolute charging behavior of a hypothetical flat, twodimensional satellite panel of a single material (either polycrystalline conductor Au or the polymeric polyimide Kapton™ H) is modeled as it undergoes modification and concomitant changes in spacecraft charging in three representative geosynchronous orbit environments, from full sunlight to full shade (eclipse) are considered.

Low-Fluence Electron Yields of Highly Insulating Materials

“…The electron emission properties of electrically-insulating materials are central to modeling spacecraft charging, as a function of incident electron energy since insulating materials generally exhibit higher yields than conducting materials, and accumulated charge cannot be easily dissipated. Furthermore, electron emission in insulators is complicated by the fact that the yield itself is affected by accumulated surface and bulk charge [1]. In order to more accurately describe the electron-induced charging behavior of insulators used on spacecraft, we have developed a model that quantifies the response of the electron yield as a function of accumulated charge and material surface potential.…”

Spacecraft Coating-Induced Charging: Materials and Modeling Study of Environmental Extremes

“…Errors in the various input parameters have effects of varying degrees on the results; for example as expected, the resistivity has a large impact on the spacecraft potential, while the impact of the plasma densities is relatively small. 48 Consequently, errors in the input parameters (resistivity, electron emission, particle energies, and particle densities) will have the effect of shifting the potential peaks within that range. To look at the situation from another perspective, one might say, for example, that the transition point is , 47(1), 134-146, (2010).…”

“…This photoyield value depends appreciably on the absorptivity, angle of incidence, surface roughness, and contamination or surface degradation. 48,49 Values for the mean absorptivity over a range of 250 nm to 750 nm can be estimated from measured room temperature reflectivity curves, assuming R − = 1 α for the Feurerbacher data 46 as α Al2O3 ≈0.15 and for the Solar Probe samples as α Al2O3 ≈0.27, α PBN ≈0.33, and α BaZP ≈0. 28.…”

“…4 The photoyield is also expected to have some temperature dependence, as is the absorptivity. 4,48 Given these uncertainties and variabilities of the materials during the mission lifetime, it is a reasonable approximation to use the value for Al 2 O 3 photoyield (corrected for absorptivity ratios) as a surrogate for PBN and BaZP, since they are similar ceramic materials with roughly comparable optical absorptivity and band gaps. The corrections of the photoyield and absorptivity for grazing incidence 48,49 of the solar radiation on the heat shield cone with cone half-angle of Θ=15º have been included in the present calculations.…”

Strategies for Determining Electron Yield Material Parameters for Spacecraft Charge Modeling

“…The three most prominent codes are NASCAP‐2k (Mandell et al, 2006; Davis et al, 2000; Mandel et al, 1976; Katz, et al, 1977), European Space Agency (2018), and MUSCAT (Muranaka et al, 2008). Precise descriptions of the materials used in spacecraft construction, for the specific spacecraft design (Toyoda, 2014; Dennison et al, 2007). Relevant materials properties characterizing the interaction of these specific materials with the environment and how these properties may change with exposure to the space environment.…”