Spectroscopic Behavior of Composite, Black Thermal Paint, Solar Cell, and Multi-Layered Insulation Materials in a GEO Simulated Environment

The population of objects orbiting Earth is dominated by orbital debris. The following study presents reflectance spectroscopic measurements and bidirectional reflectance distribution function (BRDF) evaluations taken on common spacecraft materials (Table 1), some of which are likely candidates in the orbital debris population. Their optical properties were assessed in their pristine conditions, as well as after exposure in a space environmental chamber used to simulate space weathering. The materials studied will prove that they have excellent properties in resisting the effects of damage that are common in both low Earth orbit and geosynchronous Earth orbit (GEO) based on the research discussed in this work

Spectroscopic Behavior of Composite, Black Thermal Paint, Solar Cell, and Multi-layered Insulation Materials in a GEO Simulated Environment

Materials currently populating Earth orbital regimes can be distinguished by comparing remote observational data to that of optical material measurements obtained in the laboratory. Experimentation for this research primarily involved the acquisition of spectroscopic measurements on materials of interest to the telescopic observational community for enhanced space situational awareness. Common spacecraft materials worthy of preeminent analysis for this investigation include a carbon-carbon (c-c) matrix composite, various black thermal paints, a GPS solar cell and three different cover glass components. These materials were subjected to a simulated geosynchronous Earth orbit (GEO) space environment with the intent of observing material optical property behavior over quantitative exposure time. The aforementioned materials have been measured in their pristine and GEO simulated exposed conditions. A reflectance spectrometer and a bi-directional reflectance distribution function (BRDF) optical system have been operated to perform material characterization, optical property analysis, and to further compare such data to telescopic observational data acquired on equal materials

Investigating the Photoyield of Spacecraft Materials

Understanding the photon-induced charging of spacecraft materials is necessary in modeling the overall charging of a spacecraft. Measuring the photoyields of insulators requires sophistication, since insulators\u27 electrons must overcome a greater potential energy barrier, than electrons in a metal, to move within a solid. In order to determine the photoyields of insulating and semiconducting materials for NASA\u27s Solar Probe Mission (PBN, Alumina) and James Webb Space Telescope project (SixPI-ExVDA), a chopper and lock-in amplifier were added to a photoyield measurement system. A standard (Au) photoemission spectrum was compared with Au spectrum taken before addition of the lock-in to verify the validity of the modified system. Two insulators (polyboron nitride and Alumina) under investigation for the NASA/APL Solar Probe Mission and materials for the JWST project (vapor deposited aluminum and silicon on substrate Kapton E) were then studied using the modified photoemission measurement system. The resulting spectra were used to calculate the solar photoelectron yield and work function of each of the materials

Methods to Determine Total Electron-Induced Electron Yields Over Broad Range ofConductive and Nonconductive Materials

The electron emission properties of a material subject to incident radiation flux are key parameters in determining to what equilibrium charge a spacecraft will established under given environmental conditions. However, there is a complex relation between these emission properties and the charge built up in spacecraft insulators. Complex modeling codes have been developed to predict the potential a spacecraft will adopt as a consequence of its interaction with the space plasma. These require correct models of the electron yields as a function of charge to accurately predict the both the charge build up and the equilibrium potential of spacecraft components. This paper focuses on different methods appropriate to determine the fundamental electronic material property of total electron yield as the materials accumulates charge. Three methods for determining the uncharged total yield are presented: (i) The DC Continuous Beam Method is a relatively easy and accurate method appropriate for conductors and semi- conductors with maximum total electron yield σmaxρ\u3c106 \u3eΩ-cm. (ii) The Pulse-Yield Method seeks to minimize the effects of charging and is applicable to materials with σmaxρ up to \u3e1024 Ω-cm. (iii) The Yield Decay Method is a very difficult and time consuming that uses a combination of measurement and modeling to measure the most difficult materials with σmax\u3e4 and ρ up to \u3e1024 Ω-cm. Data for high purity polycrystalline Au, Kapton HN, and polycrystalline aluminum oxide ceramic are presented. These data demonstrate the relative strengths and weaknesses of each method, but more importantly show that the methods described herein are capable of reliably measuring the total electron yield of almost any spacecraft material

Embedded Charge Distributions in Electron Irradiated Polymers – Pulsed Electroacoustic Method Reproducibility and Calibration

The pulsed electroacoustic (PEA) method has been used to measure the embedded charge distributions in electron irradiated polymers. The PEA method allows for non-destructive direct measurements of embedded charge distributions in dielectric materials. Samples of polyether-etherketone (PEEK) and polytetrafluoroethylene (PTFE) of 125 μm or 250 μm thickness were tested after irradiation with either a 50 keV or 80 keV electron beam. The reproducibility of the PEA method and the experimental conditions were studied by: (i) measuring each sample multiple times in a given mounting configuration, (ii) re-measuring each sample after repositioning them in the PEA test fixture, and (iii) measuring two similar samples of each of these eight different experimental configurations. For accurate absolute measurements of the charge distribution and deposition depths, calibration of charge position, charge density, and amplitude attenuation for the PEA system are required. Calibration is accomplished by measuring the speed of sound in each material and by observing the effects of applying a small DC voltage to use as a reference signal. A deconvolution of the measured waveform is performed with the reference signal to remove the effects of system response, resulting in only the charge distribution. Reproducibility of measurements before and after application of DC voltage identified any effects of the applied voltage

Electron-Induced Electron Yields of Uncharged Insulating Materials

This study presents electron-induced electron yield measurements from high-resistivity, high-yield materials to validate a model for the yield of uncharged insulators. These measurements are accomplished by using a low-fluence, pulsed incident electron beam and charge neutralization to minimize charge accumulation. Our measurements show large changes in total yield curves and yield decay curves, even for incident electron fluences of/mm2. We model the evolution of the yield as charge accumulates in the material in terms of electron re-capture based on the extended Chung-Everhart model of the electron emission spectrum. This model is used to explain anomalies measured in high yield ceramics, and to provide a method for determining the uncharged yield in highly insulating, high yield materials. Relevance of these results to spacecraft charging will also be discussed

Renyi Entropy based Target Tracking in Mobile Sensor Networks

This paper proposes an entropy based target tracking approach for mobile sensor networks. The proposed tracking algorithm runs a target state estimation stage and a motion control stage alternatively. A distributed particle filter is developed to estimate the target position in the first stage. This distributed particle filter does not require to transmit the weighted particles from one sensor node to another. Instead, a Gaussian mixture model is formulated to approximate the posterior distribution represented by the weighted particles via an EM algorithm. The EM algorithm is developed in a distributed form to compute the parameters of Gaussian mixture model via local communication, which leads to the distributed implementation of the particle filter. A flocking controller is developed to control the mobile sensor nodes to track the target in the second stage. The flocking control algorithm includes three components. Collision avoidance component is based on the design of a separation potential function. Alignment component is based on a consensus algorithm. Navigation component is based on the minimization of an quadratic Renyi entropy. The quadratic Renyi entropy of Gaussian mixture model has an analytical expression so that its optimization is feasible in mobile sensor networks. The proposed active tracking algorithm is tested in simulation. © 2011 IFAC

Comparison of Pulsed Electroacoustic Measurements and AF-NUMIT3 Modeling of Polymers Irradiated With Monoenergetic Electrons

Successful spacecraft design and charging mitigation techniques require precise and accurate knowledge of charge deposition profiles. This paper compares models of charge deposition and transport using a venerable deep dielectric charging code, AF-NUMIT3, with direct measurements of charge profiles via pulsed electroacoustic (PEA) measurements. Eight different simulations were performed for comparison to PEA experiments of samples irradiated by 50 or 80 keV monoenergetic electrons in vacuum and at room temperature. Two materials, polyether-ether ketone (PEEK) and polytetrafluoroethylene (PTFE), were chosen for their very low conductivities so that minimal charge migration would occur between irradiation and PEA measurements. PEEK was found to have low acoustic attenuation, while PTFE has high acoustic attenuation through the sample thicknesses of 125 and 250 μm for each material. The measurements were directly compared to AF-NUMIT3 simulations to validate aspects of the code and to investigate the importance of various simulation options, as well as to characterize the PEA instrumentation, measurement methods, and signal processing used. The measurement and simulation values for magnitude of charge deposition, penetration depth, and charge deposition spatial profiles are largely in agreement, though spatial and temporal distributions in incident electron flux and effects of radiation-induced conductivity (RIC) and delayed RIC during the deposition process complicate the process. This work provides an experimental validation of the AF-NUMIT3 deep dielectric charging code and insight into the accuracy and precision of the PEA method

On the Computation of Secondary Electron Emission Models

Secondary electron emission is a critical contributor to the charge particle current balance in spacecraft charging. Spacecraft charging simulation codes use a parameterized expression for the secondary electron (SE) yield delta(Eo) as a function of the incident electron energy Eo. Simple three-step physics models of the electron penetration, transport, and emission from a solid are typically expressed in terms of the incident electron penetration depth at normal incidence R(Eo) and the mean free path of the SE lambda. In this paper, the authors recall classical models for the range R(Eo): a power law expression of the form b1Eo n1 and a more general empirical double power law R(Eo)=b1Eo n1+b2E o n2. In most models, the yield is the result of an integral along the path length of incident electrons. An improved fourth-order numerical method to compute this integral is presented and compared to the standard second-order method. A critical step in accurately characterizing a particular spacecraft material is the determination of the model parameters in terms of the measured electron yield data. The fitting procedures and range models are applied to several measured data sets to compare their effectiveness in modeling the function delta(Eo) over the full range of energy of incident particle