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2000

All Physics Faculty Publications

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Full-Text Articles in Physics

Applications Of Secondary Electron Energy- And Angular-Distributions To Spacecraft Charging, Neal Nickles, R. E. Davies, John R. Dennison Sep 2000

Applications Of Secondary Electron Energy- And Angular-Distributions To Spacecraft Charging, Neal Nickles, R. E. Davies, John R. Dennison

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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 …

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Utah State University Ground-Based Test Facility For Study Of Electronic Properties Of Spacecraft Materials, W. Y. Chang, John R. Dennison, Neal Nickles, R. E. Davies Sep 2000

Utah State University Ground-Based Test Facility For Study Of Electronic Properties Of Spacecraft Materials, W. Y. Chang, John R. Dennison, Neal Nickles, R. E. Davies

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Materials used for spacecraft and space structures in near-Earth orbit are subject to severe environmental effects including high vacuum conditions, hot and cold extremes temperature, strongly oxidizing atomic oxygen environments, and high fluxes of energetic electrons, ions, neutrals and photons. Instrumentation developed at Utah State University is designed to simulate, at least to some level, all of these conditions and to study charged particle and photon interactions with spacecraft surfaces. The facilities are particularly well suited to study electron emission as related to spacecraft charging, including secondary and backscattered yields, energy-spectra, and angleresolved measurements as a function of incident energy, …

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Evolution Of Secondary Electron Emission Characteristics Of Spacecraft Surfaces: Importance To Spacecraft Charging, Robert Davies, John R. Dennison Sep 2000

Evolution Of Secondary Electron Emission Characteristics Of Spacecraft Surfaces: Importance To Spacecraft Charging, Robert Davies, John R. Dennison

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A sample of oxidized aluminum was placed inside an ultra-high vacuum (UHV) chamber alongside a piece of PTFE (Teflon®) coated wire and continuously bombarded with 1-3 keV electrons for ~30 hours. The SE yield of the surface was monitored as a function of time throughout the electron bombardment. Oxidized aluminum was chosen as a typical material comprising spacecraft surfaces, while outgassing of the Teflon wire contaminated the UHV environment, simulating the microenvironment surrounding an operating spacecraft. Continuous electron bombardment resulted in two effects—( i) the removal of the oxide layer, and (ii) the deposition of a thin (~1 nm-thick) layer …

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Terdiurnal Oscillations In Oh Meinel Rotational Temperatures For Fall Conditions At Northern Mid-Latitude Sites, W. R. Pendleton Jr., Michael J. Taylor, Larry Gardner Jun 2000

Terdiurnal Oscillations In Oh Meinel Rotational Temperatures For Fall Conditions At Northern Mid-Latitude Sites, W. R. Pendleton Jr., Michael J. Taylor, Larry Gardner

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High‐precision (∼0.5 K) measurements of OH Meinel (M) (6,2) rotational temperatures above the Bear Lake Observatory, UT (42°N, 112°W) during October 1996 have revealed an interesting and unexpected mean nocturnal pattern. Ten quality nights (>100 h) of data have been used to form a mean night for autumnal, near‐equinoctial conditions. The mean temperature and RMS variability associated with this mean night were 203 ± 5 K and 2.4 K, respectively, and compare very favorably with expectations based on Na‐lidar measurements of mean tidal temperature perturbations over Urbana, IL (40°N, 88°W) during the fall 1996. Furthermore, this comparison shows that …

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Instrumentation And Measurement Of Secondary Electron Emission For Spacecraft Charging, Neal Nickles, John R. Dennison May 2000

Instrumentation And Measurement Of Secondary Electron Emission For Spacecraft Charging, Neal Nickles, John R. Dennison

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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.

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Snapover: Anomalous Plasma Current Collection By Positively Biased Conductors When Surrounded By A Dielectric, Clint Thomson, John R. Dennison May 2000

Snapover: Anomalous Plasma Current Collection By Positively Biased Conductors When Surrounded By A Dielectric, Clint Thomson, John R. Dennison

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Over the last decade, high-powered spacecraft have been designed that will operate at voltages greater than 100 V. At these voltages, the solar arrays can undergo both destructive arcing at negative biases, and plasma electron current collection at positive biases. Furthermore, above some critical positive bias voltage (~100 V), the electron current collected by the array interconnects increases dramatically through a phenomenon termed Asnapover@. During snapover, large portions of the solar array cover glass charge positively, and begin to draw electron current from the plasma as if it were a conducting surface. This leads to substantial power losses for the …

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Surface Core-Level Phonon Broadening Of Li(110), D. Mark Riffe, G. K. Wertheim Jan 2000

Surface Core-Level Phonon Broadening Of Li(110), D. Mark Riffe, G. K. Wertheim

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High-resolution core-level photoemission data from the 1s level of Li(110) have been obtained between 77 and 280 K. Analysis of the data reveals a significant difference in the zero-temperature phonon broadening between the bulk and surface atoms but only a small difference in the effective surface and bulk Debye temperatures. This latter result is in good agreement with an embedded-atom-method calculation of the bulk and surface Debye temperatures of Li. Implications of these results to surface core-level phonon broadening and surface lattice dynamics of the heavier alkali metals are discussed.

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Modeling The Evolution Of Meso-Scale Ionospheric Irregularities At High Latitudes, Jan Josef Sojka, L. Zhu, M. David, Robert W. Schunk Jan 2000

Modeling The Evolution Of Meso-Scale Ionospheric Irregularities At High Latitudes, Jan Josef Sojka, L. Zhu, M. David, Robert W. Schunk

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The modeling of ionospheric plasma density irregularities (ΔN/N) is a necessary first step towards forecasting ionospheric scintillation effects. This modeling challenge is difficult not only because the linear‐nonlinear saturation levels of instabilities are not completely understood but also because ΔN/N often evolves on time scales which are long compared to the characteristic time constants of the instability drivers. In this study, the linear 1‐D gradient drift instability (GDI) is used in a high latitude simulation of ΔN/N evolution. For the first time snapshots (maps) of ΔN/N are presented demonstrating that it is possible to use …

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Extended Conformal Symmetry, James Thomas Wheeler Jan 2000

Extended Conformal Symmetry, James Thomas Wheeler

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We show that the grading of fields by conformal weight, when built into the initial group symmetry, provides a discrete, non-central conformal extension of any group containing dilatations. We find a faithful vector representation of the extended conformal group and show that it has a scale-invariant scalar product and satisfies a closed commutator algebra. The commutator algebra contains the infinite Heisenberg and Virasoro algebras. In contrast to the classical treatment of scale invariance, covariant derivatives and gauge transformations automatically incorporate the correct conformal weights when the extended symmetry is gauged.

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Group Invariant Solutions Without Transversality, Ian M. Anderson, Mark E. Fels, Charles G. Torre Jan 2000

Group Invariant Solutions Without Transversality, Ian M. Anderson, Mark E. Fels, Charles G. Torre

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We present a generalization of Lie's method for finding the group invariant solutions to a system of partial differential equations. Our generalization relaxes the standard transversality assumption and encompasses the common situation where the reduced differential equations for the group invariant solutions involve both fewer dependent and independent variables. The theoretical basis for our method is provided by a general existence theorem for the invariant sections, both local and global, of a bundle on which a finite dimensional Lie group acts. A simple and natural extension of our characterization of invariant sections leads to an intrinsic characterization of the reduced …

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Model Study Of Ionospheric Dynamics During A Substorm, Lie Zhu, Robert W. Schunk, Jan Josef Sojka, Michael David Jan 2000

Model Study Of Ionospheric Dynamics During A Substorm, Lie Zhu, Robert W. Schunk, Jan Josef Sojka, Michael David

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A global substorm electrodynamic model and a global ionospheric model were coupled in order to study ionospheric dynamics during substorms, with the focus on small-scale substorm electrodynamic and plasma structures. The simulation results show that in the expansion phase, structured precipitation and channeled field-aligned currents quickly develop in the substorm onset region. The Hall and Pedersen conductance ratio in the region increases significantly, and the magnetospheric field-aligned currents are mainly closed by highly structured Hall currents. Correspondingly, the plasma in the ionosphere also undergoes significant changes during a substorm and is highly structured in both the horizontal and vertical directions. …

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Sensitivity Curves For Spaceborne Gravitational Wave Interferometers, Shane L. Larson, William A. Hiscock, Ronald W. Hellings Jan 2000

Sensitivity Curves For Spaceborne Gravitational Wave Interferometers, Shane L. Larson, William A. Hiscock, Ronald W. Hellings

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To determine whether particular sources of gravitational radiation will be detectable by a specific gravitational wave detector, it is necessary to know the sensitivity limits of the instrument. These instrumental sensitivities are often depicted (after averaging over source position and polarization) by graphing the minimal values of the gravitational wave amplitude detectable by the instrument versus the frequency of the gravitational wave. This paper describes in detail how to compute such a sensitivity curve given a set of specifications for a spaceborne laser interferometer gravitational wave observatory. Minor errors in the prior literature are corrected, and the first (mostly) analytic …

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Using Binary Star Observations To Bound The Mass Of The Graviton, Shane L. Larson, William A. Hiscock Jan 2000

Using Binary Star Observations To Bound The Mass Of The Graviton, Shane L. Larson, William A. Hiscock

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Interacting white dwarf binary star systems, including helium cataclysmic variable (HeCV) systems, are expected to be strong sources of gravitational radiation, and should be detectable by proposed space-based laser interferometer gravitational wave observatories such as LISA. Several HeCV star systems are presently known and can be studied optically, which will allow electromagnetic and gravitational wave observations to be correlated. Comparisons of the phases of a gravitational wave signal and the orbital light curve from an interacting binary white dwarf star system can be used to bound the mass of the graviton. Observations of typical HeCV systems by LISA could potentially …

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Inception Of Snapover And Gas Induced Glow Discharges, J. T. Galofaro, B. V. Vayner, D. C. Ferguson, W. A. Degroot, C. D. Thomson, John R. Dennison, R. E. Davies Jan 2000

Inception Of Snapover And Gas Induced Glow Discharges, J. T. Galofaro, B. V. Vayner, D. C. Ferguson, W. A. Degroot, C. D. Thomson, John R. Dennison, R. E. Davies

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Ground based experiments of the snapover phenomenon were conducted in the large vertical simulation chamber at the Glenn Research Center (GRC) Plasma Interaction Facility (PIF). Two Penning sources provided both argon and xenon plasmas for the experiments. The sources were used to simulate a variety of ionospheric densities pertaining to a spacecraft in a Low Earth Orbital (LEO) environment. Secondary electron emission is believed responsible for dielectric surface charging, and all subsequent snapover phenomena observed. Voltage sweeps of conductor potentials versus collected current were recorded in order to examine the specific charging history of each sample. The average time constant …

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A Comprehensive Study Of Dielectric-Conductor Junctions In Low Density Plasmas, B. V. Vayner, J. T. Galofaro, D. C. Ferguson, W. A. De Groot, C. D. Thomson, John R. Dennison, R. E. Davies Jan 2000

A Comprehensive Study Of Dielectric-Conductor Junctions In Low Density Plasmas, B. V. Vayner, J. T. Galofaro, D. C. Ferguson, W. A. De Groot, C. D. Thomson, John R. Dennison, R. E. Davies

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In this paper, results are presented of an experimental and theoretical study of snapover, glow discharge, and arc phenomena for different materials immersed in argon or xenon plasmas. The effect of snapover is investigated for several metal-dielectric junctions: copper-teflon, copper-Kapton, copper-glass, aluminum-teflon, aluminum-Kapton, steel-teflon, anodized aluminum with pinholes, and copper-ceramics. I-V curves are measured, and snapover inception voltages, essential parameters (increase in current and collection area due to secondary electrons), and glow discharge inception thresholds are determined. Optical spectra are obtained for glow discharges in both argon and xenon plasmas. These spectra provide information regarding atomic species entrapped in the …

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Measurements Of Electronic Properties Of Conducting Spacecraft Materials With Application To The Modeling Of Spacecraft Charging, W. Y. Chang, John R. Dennison, Parker Judd Jan 2000

Measurements Of Electronic Properties Of Conducting Spacecraft Materials With Application To The Modeling Of Spacecraft Charging, W. Y. Chang, John R. Dennison, Parker Judd

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This paper describes the results of the first stage of this project, measurements of the electronic properties of conducting spacecraft materials. We begin with a description of the required measurements and specifics of the experimental methods used. A complete list of the conducting materials studied, justification of their selection for study, and a summary of the important results of the measurements is presented. This is followed by detailed measurements and analysis for one representative conductor, namely polycrystalline Au. We end with a description of incorporation of these measurements into the NASCAP database.

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Investigation Of The First Snapover Of Positively Biased Conductors In A Plasma, C. D. Thomson, John R. Dennison, R. E. Davies, D. C. Ferguson, J. T. Galafaro, B. V. Vayner Jan 2000

Investigation Of The First Snapover Of Positively Biased Conductors In A Plasma, C. D. Thomson, John R. Dennison, R. E. Davies, D. C. Ferguson, J. T. Galafaro, B. V. Vayner

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We describe a systematic experimental investigation of the phenomenon termed "snapover." In snapover, the current collected by a positively biased conductor, surrounded by a dielectric and immersed in a plasma, increases dramatically when the conductor potential is raised above some threshold value. The phenomenon is particularly relevant to the case of high-voltage solar arrays in Earth orbit. Our experiments examined the importance of conducting material, insulating material, size and shape of the conductor, sample history, biasing rate, and condition of the dielectric surface (contamination and smoothness) to the onset potential and current jump. In addition to a primary snapover occurring …

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Actions For Biconformal Matter, Andre Wehner, James Thomas Wheeler Jan 2000

Actions For Biconformal Matter, Andre Wehner, James Thomas Wheeler

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We extend 2n-dim biconformal gauge theory by including Lorentz-scalar matter fields of arbitrary conformal weight. For a massless scalar field of conformal weight zero in a torsion-free biconformal geometry, the solution is determined by the Einstein equation on an n-dim submanifold, with the stress-energy tensor of the scalar field as source. The matter field satisfies the n-dim Klein-Gordon equation.

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Effects Of Evolving Surface Contamination On Spacecraft Charging, W. Y. Chang, John R. Dennison, Jason Kite, R. E. Davies Jan 2000

Effects Of Evolving Surface Contamination On Spacecraft Charging, W. Y. Chang, John R. Dennison, Jason Kite, R. E. Davies

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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 …

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Measurement Of Silicon Surface Recombination Velocity Using Ultrafast Pump-Probe Reflectivityin The Near Infrared, A. J. Sabbah, D. Mark Riffe Jan 2000

Measurement Of Silicon Surface Recombination Velocity Using Ultrafast Pump-Probe Reflectivityin The Near Infrared, A. J. Sabbah, D. Mark Riffe

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We demonstrate that ultrafast pump–probe reflectivity measurements from bulk Si samples using a Ti:sapphire femtosecond oscillator (λ=800 nm) can be used to measure the Si surface recombination velocity. The technique is sensitive to recombination velocities greater than ∼104 cm s−1

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