Secondary Electron Emission and Spacecraft Charging

Spacecraft charging due to the natural plasma environment found in all orbits is known to produce many of the observed spacecraft anomalies and failures. A primary factor in adverse spacecraft charging is the secondary electron emission of differing materials on the spacecraft. Precipitating electrons and ions from the plasma to spacecraft surfaces can result in varying amounts of charge being released, depending on the secondary electron yield of the materials; this can lead to arcing between surfaces. NASA\u27s Space and Environments Effects (SEE) program has recognized the need to improve their current materials database for modeling spacecraft charging and have chosen the surface science group at Utah State University to carry out electron emission studies on spacecraft materials as well as other research related to spacecraft charging. The instruments being used at USU are specifically designed to study the problem of spacecraft charging and the contributions of the group will continue after my research on secondary electron emission funded by the Rocky Mountain Space Grant Consortium is completed. In addition to improving NASA\u27s ability to model spacecraft charging, my secondary electron research has the potential to benefit numerous other fields, such as scanning electron microscopy

The Role of Bandgap in the Secondary Electron Emission of Small Bandgap Semiconductors: Studies of Graphitic Carbon

The question of whether the small bandgaps of semiconductors play a significant role in their secondary electron emission properties is investigated by studying evaporated graphitic amorphous carbon, which has a roughly 0.5 eV bandgap, in comparison with microcrystalline graphite, which has zero bandgap. The graphitic amorphous carbon is found to have a 30% increase in its maximum secondary electron yield over that of two microcrystalline graphite samples with comparable secondary electron yields: highly oriented pyrolytic graphite and colloidal graphite. The potentially confounding influence of the vacuum level has been isolated through the measurement of the photoelectron onset energy of the materials. Other less significant materials parameters are also isolated and discussed. Based on these measurements, it is concluded the magnitude of bandgap may have an appreciable effect on the magnitude of the secondary electron yield and further studies of this effect with annealed graphitic amorphous carbon are warranted. In support of this work, a hemispherical two-grid, retarding field electron energy analyzer has been designed, constructed, and characterized for the present work. The advantages and disadvantages of the analyzer are discussed in comparison to other methods of measuring secondary electron emission. The analyzer has a resolution of ±(1.5 eV + 4% of the incident electron energy). A novel effort to derive theoretical, absolute correction factors that compensate for electron losses within the analyzer, mainly due to the grid transmission, is presented. The corrected secondary electron yield of polycrystalline gold is found to be 30% above comparable experimental studies. The corrected backscattered electron yield of polycrystalline gold is found to be 14% above comparable experimental studies. Corrected secondary yields for the microcrystalline graphite samples are found to range from 35-70% above those found in five experimental studies in the literature. The theoretical correction factors are estimated to have a 4-6% uncertainty. Reasons for the large discrepancy in yield measurements with the analyzer are discussed and thought to be due mainly to the lack of similar corrective factors in the previous studies. The supporting instrumentation is fully characterized, including a detailed error analysis

Secondary Electron Emission and Spacecraft Charging

Spacecraft charging due to the natural plasma environment found in all orbits is known to produce many of the observed spacecraft anomalies and failures. A primary factor in adverse spacecraft charging is the secondary electron emission of differing materials on the spacecraft. Precipitating electrons and ions from the plasma to spacecraft surfaces can result in varying amounts of charge being released, depending on the secondary electron yield of the materials; this can lead to arcing between surfaces. NASA\u27s Space and Environments Effects (SEE) program has recognized the need to improve their current materials database for modeling spacecraft charging and have chosen the surface science group at Utah State University to carry out electron emission studies on spacecraft materials as well as other research related to spacecraft charging. The instruments being used at USU are specifically designed to study the problem of spacecraft charging and the contributions of the group will continue after my research on secondary electron emission funded by the Rocky Mountain Space Grant Consortium is completed. In addition to improving NASA=s ability to model spacecraft charging, my secondary electron research has the potential to benefit numerous other fields, such as scanning electron microscopy

Applications of Secondary Electron Energy- and Angular-Distributions to Spacecraft Charging

Secondary electron (SE) emission from spacecraft surfaces as a result of energetic electron bombardment is a key process in the electrical charging of spacecraft. It has been suggested that incorporating more complete knowledge of the energy- and angular-distributions of secondary electrons is necessary to fully model how SE emission and spacecraft charging are affected by re-adsorption of low energy electrons in the presence of charge-induced electrostatic fields and ambient magnetic fields in the spacecraft environment. We present data for such energy- and angular-distributions from sputtered, polycrystalline gold surfaces. The data are compared to empirical SE emission models and found to agree well. We also discuss at what level inclusion of such energy- and angular-distributions will affect models of spacecraft charging for both positive and negative surface charging

Applications of Secondary Electron Energy- and Angular-Distributions to Spacecraft Charging

Secondary electron (SE) emission from spacecraft surfaces as a result of energetic electron bombardment is a key process in the electrical charging of spacecraft. It has been suggested that incorporating more complete knowledge of the energy- and angular-distributions of secondary electrons is necessary to fully model how SE emission and spacecraft charging are affected by re-adsorption of low energy electrons in the presence of charge-induced electrostatic fields and ambient magnetic fields in the spacecraft environment. We present data for such energy- and angular-distributions from sputtered, polycrystalline gold surfaces. The data are compared to empirical SE emission models and found to agree well. We also discuss at what level inclusion of such energy- and angular-distributions will affect models of spacecraft charging for both positive and negative surface charging

Absolute Electron Emission Calibration: Round Robin Tests of Au and Polyimide

Accurate determination of the absolute electron yields of conducting and insulating materials are essential for models of spacecraft charging and related processes involving charge accumulation and emission due to electron beams and plasmas. Apparatus using low-fluence pulsed electron beam sources and various methods to minimize charge accumulation have been developed at facilities around the world. This study presents a round robin comparison of such tests performed in CSIC at Instituto de Ciencia de Materiales de Madrid, LaSeine at Kyushu Institute of Technology, DESSE at ONEREA, and the Space Environment Effects Materials (SEEM) test facility at Utah State University. The primary objectives of the study were to determine the consistency and uncertainties of these absolute yields measurements, and to investigate the effects of the similarities and differences of the diverse facilities. Measurements were made of the absolute total, secondary and backscattered electron yields at normal incidence over the full range of incident energies accessible with each group’s instrumentation (a full range of ~5 eV to ~30 keV). Electron emission spectra at specific incident electron energies were also measured. Measurements were made for identical samples with reproducible sample preparation of two standard materials: (1) the elemental conductor Au (25 μm thick 6N high purity Au foils) and (2) the polymeric insulator polyimide (25 μm thick Kapton HNTM). Studies of the effects on electron yield of Au surface contamination—as measured with Auger Electron Spectroscopy and other techniques—were made for samples: (1) as received, (2) subject to a simple standard cleaning procedure, (3) subsequently baked out under ultrahigh vacuum, and (4) after Ar ion sputter cleaning and thermal annealing. Similarly, studies of the effects of absorbed water and volatile compounds on electron yield were made for polyimide samples: (1) unbaked and (2) subjected to a vacuum bake out. An outline of measurement and analysis techniques used by each laboratory is presented, along with methods used to calibrate the incident energies and absolute yields measured with their different electron detectors. The effects of different charge minimization and neutralization methods are compared. Various empirical and physics-based models to characterize the electron yield curves are used to parameterize the yield data. The values determined at each laboratory for the maximum yield and energy at this yield, the first and second crossover energies, and the asymptotic yield at high energy are also compiled and compared

Instrumentation and Measurement of Secondary Electron Emission for Spacecraft Charging

Secondary electron emission is an important physical mechanism in the problem of spacecraft charging. The NASA Space Environments and Effects branch is currently revising NASA’s strategy for mitigating damage due to spacecraft charging. In an effort to substantially improve the modeling of spacecraft charging, measurements of secondary electron emission parameters are being made. The design of the apparatus needed to measure these parameters is discussed in detail. Various measurement techniques are explained and conclusions are drawn about the suitability of the final design

Instrumentation and Measurement of Secondary Electron Emission for Spacecraft Charging

Secondary electron emission is an important physical mechanism in the problem of spacecraft charging. The NASA Space Environments and Effects branch is currently revising NASA’s strategy for mitigating damage due to spacecraft charging. In an effort to substantially improve the modeling of spacecraft charging, measurements of secondary electron emission parameters are being made. The design of the apparatus needed to measure these parameters is discussed in detail. Various measurement techniques are explained and conclusions are drawn about the suitability of the final design

Effects of Spacecraft Potential on Secondary Electron Yields in Geosynchronous Orbit

Surface charging due to interactions with the earth=s plasma is a hazard for orbiting spacecraft. Secondary electron (SE) emission is an important physical process in spacecraft charging. Current spacecraft charging models do not consider the SE energy or angular distributions and their implications for estimating the return of SE to the spacecraft. Comprehensive work on the application of SE energy and angular distributions to spacecraft charging has been published [Nickles et al., 1999] and part of that work is summarized here. The application of SE energy distributions to the case of positive charging in geosynchronous orbit is discussed and shown to impact the cutoff voltages required to assume that secondary electron yields are effectively zero. The ramification of the SE angular distribution for cases of negative charging in geosynchronous orbit is also briefly discussed