Jr Dennison

Evolution of the Electron Yield Curves of Insulators as a Function of Impinging Electron Fluence and Energy

Electron emission and concomitant charge accumulation near the surface of insulators is central to understanding spacecraft charging. A study of changes in electron emission yields as a result of internal charge buildup due to electron dose is presented. Evolution of total, backscattered, and secondary yield results over a broad range of incident energies are presented for two representative insulators, Kapton and Al2O3. Reliable yield curves for uncharged insulators are measured, and quantifiable changes in yields are observed due to <100-fC/mm2 fluences. Excellent agreement with a phenomenological argument based on insulator charging predicted by the yield curve is found; this includes a decrease in the rate of change of the yield as incident energies approach the crossover energies and as accumulated internal charge reduces the landing energy to asymptotically approach a steady state surface charge and unity yield. It is also found that the exponential decay of yield curves with fluence exhibit an energy-dependent decay constant alpha(E). Finally, physics-based models for this energy dependence are discussed. Understanding fluence and energy dependence of these charging processes requires knowledge of how charge is deposited within the insulator, the mechanisms for charge trapping and transport within the insulator, and how the profile of trapped charge affects the transport and emission of charges from insulators

Approximation of Range in Materials as a Function of Incident Electron Energy

A simple composite analytic expression has been developed to approximate the electron range in materials. The expression is applicable over more than six orders of magnitude in energy (<; 10 eV to >; MeV) and range ( 10-9-10-2 m), with an uncertainty of ≤ 20% for most conducting, semiconducting, and insulating materials. This is accomplished by fitting data from two standard NIST databases [ESTAR for the higher energy range and the electron inelastic mean free path (IMFP) for the lower energies]. In turn, these data have been fit with well-established semiempirical models for range and IMFP that are related to standard material properties (e.g., density, atomic number, atomic weight, stoichiometry, and bandgap energy). Simple relations between the IMFP and the range, based on the continuous-slow-down approximation, are used to merge results from the two databases into a composite range expression. A single free parameter, termed the effective number of valence electrons per atom Nv, is used to predict the range over the entire energy span.

Methods for High Resistivity Measurements Related to Spacecraft-Charging

A key parameter in modeling differential spacecraft-charging is the resistivity of insulating materials. This parameter determines how charge will accumulate and redistribute across the spacecraft, as well as the timescale for charge transport and dissipation. American Society for Testing and Materials constant-voltage methods are shown to provide inaccurate resistivity measurements for materials with resistivities greater than ~1017 Omegamiddotcm or with long polarization decay times such as are found in many polymers. These data have been shown to often be inappropriate for spacecraft-charging applications and have been found to underestimate charging effects by one to four orders of magnitude for many materials. The charge storage decay method is shown to be the preferred method to determine the resistivities of such highly insulating materials. A review is presented of methods to measure the resistivity of highly insulating materials-including the electrometer in resistance method, the electrometer in constant-voltage method, and the charge storage method. The different methods are found to be appropriate for different resistivity ranges and for different charging circumstances. A simple macroscopic physics-based model of these methods allows separation of the polarization current and dark current components from long-duration measurements of resistivity over day- to month-long timescales. Model parameters are directly related to the magnitude of charge transfer and storage and the rate of charge transport. The model largely explains the observed differences in resistivity found using the different methods and provides a framework for recommendations for the appropriate test method for spacecraft materials with different resistivities and applications

Low-Fluence Electron Yields of Highly Insulating Materials

Electron-induced electron yields of high-resistivity high-yield materials - ceramic polycrystalline aluminum oxide and polymer polyimide (Kapton HN) - were made by using a low-fluence pulsed incident electron beam and charge neutralization electron source to minimize charge accumulation. Large changes in the energy-dependent total yield curves and yield decay curves were observed, even for incident electron fluences of < 3 fC/mm2. The evolution of the electron yield as charge accumulates in the material is modeled in terms of electron recapture based on an extended Chung-Everhart model of the electron emission spectrum. This model is used to explain the anomalies measured in highly insulating high-yield materials and to provide a method for determining the limiting yield spectra of uncharged dielectrics. The relevance of these results to spacecraft charging is also discussed.

Effects of Evolving Surface Contamination on Spacecraft Charging

The effects of evolving surface contamination on spacecraft charging have been investigated through (i) ground-based measurements of the change in electron emission properties of a conducting surface undergoing contamination and (ii) modeling of the charging of such surfaces using the NASCAP code. Specifically, we studied a Au surface as adsorbed species were removed and a very thin disordered carbon film was deposited as a result of exposure to an intense, normal incidence electron beam. As a result of this contamination, we found an ~50% decrease in secondary electron yield and an ~20% reduction in backscattered yield. The type and rates of contamination observed are similar to those encountered by operational spacecraft. Charging potentials of an isolated panel of the material were determined under both sunlit and eclipse conditions in geosynchronous orbits for typical and extreme environments. In all environments studied, just monolayers of contamination lead to predictions of an abrupt threshold effect for spacecraft charging; panels that charged to small positive values when uncontaminated developed kilovolt negative potentials. The relative effect of NASCAP parameters for modeling secondary and backscattered electron emission and plasma electron distributions were also investigated. We conclude that surface contamination must be considered to avoid the serious detrimental effects associated with severe spacecraft charging.

Temperature and Electric Field Dependence of Conduction in Low-Density Polyethylene

A traditional constant voltage conductivity test method was used to measure how the conductivity of highly insulating low-density polyethylene (LDPE) polymer films depends on applied electric field, repeated and prolonged electric field exposure, and sample temperature. The strength of the applied voltage was varied to determine the electric field dependence. At low electric field, the resistivity was measured from cryogenic temperatures to well above the glass transition temperature. Comparisons were made with a variety of models of the conduction mechanisms common in insulators, including transient polarization and diffusion and steady-state thermally activated hopping conductivity and variable range hopping conductivity, to determine which mechanisms were active for LDPE and to provide a better picture of its electrical behavior.

Ultrahigh Vacuum Cryostat System for Extended Low-Temperature Space Environment Testing

The range of temperature measurements have been significantly extended for an existing space environment simulation test chamber used in the study of electron emission, sample charging and discharge, electrostatic discharge and arcing, electron transport, and luminescence of spacecraft materials. This was accomplished by incorporating a new two-stage, closed-cycle helium cryostat which has an extended sample temperature range from to , with long-term controlled stability of . The system was designed to maintain compatibility with an existing ultrahigh vacuum chamber (base pressure ) that can simulate diverse space environments. These existing capabilities include controllable vacuum and ambient neutral gases conditions , electron fluxes (5-30-keV monoenergetic, focused, and pulsed sources over 10-4-1010 nA-cm-2), ion fluxes ( monoenergetic sources for inert and reactive gases with pulsing capabilities), and photon irradiation (numerous continuous and pulsed monochromated and broad band IR/VIS/UV [0.5-7 eV] sources). The new sample mount accommodates one to four samples of 1-2.5-cm diameter in a low-temperature carousel, which allows rapid sample exchange and controlled exposure of the individual samples. Custom hemispherical grid retarding field analyzer and Faraday cup detectors, custom high speed, high-sensitivity electronics, and charge neutralization capabilities used with , , and electrons/pulse pulsed-beam sources permit high-accuracy electron emission measurements of extreme insulators with minimal charging effects. In situ monitoring of surface voltage, arcing, and luminescence (250-5000 nm) have recently been added.

Experimentally Derived Resistivity for Dielectric Samples From the CRRES Internal Discharge Monitor

Resistivity values were experimentally determined using charge-storage methods for six samples remaining from the construction of the internal discharge monitor flown on the Combined Release and Radiation Effects Satellite (CRRES). Three tests were performed over a period of three to five weeks each in a vacuum of ~5times10-6 torr with an average temperature of ~25degC to simulate a space environment. Samples tested included FR4, polytetrafluoroethylene (PTFE), and alumina with copper electrodes attached to one or more of the sample surfaces. FR4 circuit-board material was found to have a dark-current resistivity of ~1times1018 Omegamiddotcm and a moderately high polarization current. Fiber-filled PTFE exhibited little polarization current and a dark-current resistivity of ~3times1020 Omegamiddotcm. Alumina had a measured dark-current resistivity of ~3middot1017 Omegamiddotcm, with a very large and more rapid polarization. Experimentally determined resistivity values were two to three orders of magnitude more than found using standard American Society for Testing and Materials (ASTM) test methods. The 1-min wait time suggested for the standard ASTM tests is much shorter than the measured polarization current-decay times for each sample indicating that the primary currents used to determine ASTM resistivity are caused by the polarization of molecules in the applied electric field rather than charge transport through the bulk of the dielectric. Testing over much longer periods of time in vacuum is required to allow this polarization current to decay away and to allow the observation of charged-particle transport through a dielectric material. Application of a simple physics-based model allows separation of the polarization current and dark-current components from long-duration measurements of resistivity over day- to month-long time scales. Model parameters are directly related to the magnitude of charge transfer and storage and the rate of charge transport

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(E_o) as a function of the incident electron energy E_o. 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(E_o) and the mean free path of the SE lambda. In this paper, the authors recall classical models for the range R(E_o): a power law expression of the form b₁E_o ⁿ¹ and a more general empirical double power law R(E_o)=b₁E_o 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(E_o) over the full range of energy of incident particles

Engineering Tool for Temperature, Electric Field and Dose Rate Dependence of High Resistivity Spacecraft Materials

An engineering tool has been developed to predict the equilibrium conductivity of common spacecraft insulating materials as a function of electric field, temperature, and adsorbed dose rate based on parameterized, analytic functions derived from physics-based theories. The USU Resistivity Calculator Engineering Tool calculates the total conductivity as the sum of three independent conductivity mechanisms: a thermally activated hopping conductivity, a variable range hopping conductivity, and a radiation induced conductivity using a total of nine independent fitting parameters determined from fits to an extensive data set taken by the Utah State University Materials Physics Group. It also provides a fit for the temperature dependence of the electrostatic breakdown field strength, in terms of a tenth independent fitting parameter related to an interchain bond strength. The extent of F, T and measured in the experiments were designed to cover as much of the ranges typically encountered in space environments as possible. This Mathcad worksheet calculates the total conductivity and the individual contributions from each conductivity mechanism based on user inputs for F, T and D. It also plots 2D and 3D graphs of the conductivities over the appropriate full ranges of F, T and .