Search

Your search keyword '"Hoffmann, Ryan"' showing total 153 results
Start Over Author "Hoffmann, Ryan"
153 results on '"Hoffmann, Ryan"'

14. Spacecraft materials degradation under space-simulated low Earth orbit (LEO) environment

Author

Plis, Elena, primary, Bengtson, Miles T., additional, Engelhart, Daniel P., additional, Badura, Gregory, additional, Cowardin, Heather, additional, Hoffmann, Ryan C., additional, Sokolovskiy, Alexey, additional, Reyes, Jacqueline, additional, Ferguson, Dale C., additional, Shah, Jainisha, additional, Collman, Sydney, additional, and Scott, Timothy, additional

Published
2023
Full Text
View/download PDF

17. Ground Testing of the 16th Materials International Space Station Experiment Materials

Author

Plis, Elena A., primary, Bengtson, Miles T., additional, Engelhart, Daniel P., additional, Badura, Gregory P., additional, Cowardin, Heather M., additional, Reyes, Jacqueline A., additional, Hoffmann, Ryan C., additional, Sokolovskiy, Alexey, additional, Ferguson, Dale C., additional, Shah, Jainisha R., additional, Collman, Sydney, additional, and Scott, Timothy R., additional

Published
2023
Full Text
View/download PDF

20. Characterization of novel spacecraft materials under high energy electron and atomic oxygen exposure

Author

Plis, Elena, primary, Bengtson, Miles, additional, Engelhart, Daniel P., additional, Badura, Gregory, additional, Scott, Timothy, additional, Cowardin, Heather, additional, Reyes, Jacqueline, additional, Hoffmann, Ryan, additional, Sokolovskiy, Alexey, additional, Ferguson, Dale C., additional, Shah, Jainisha, additional, and Horne, Sydney, additional

Published
2022
Full Text
View/download PDF

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

Author

Gibson, Zachary, Dennison, JR, and Hoffmann, Ryan

Subjects
Embedded Charge Distributions, Physics, PEA, Pulsed Electroacoustic Method
Abstract

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.

Published
2021

24. Low-fluence electron yields of highly insulating materials

Author

Hoffmann, Ryan, Dennison, John R., Thomson, Clint D., and Albretsen, Jennifer

Subjects
Polyimides -- Analysis, Aluminum compounds -- Analysis, Space ships -- Evaluation, Space vehicles -- Evaluation, Business, Chemistry, Electronics, Electronics and electrical industries
Abstract

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/[mm.sup.2]. 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. Index Terms--Charging, dielectrics, electron, emission.

Published
2008

25. On the computation of secondary electron emission models

Author

Clerc, Sebastien, Dennison, John R., Hoffmann, Ryan, and Abbott, Jonathan

Subjects
Electrons -- Emission, Electrons -- Research, Space vehicles -- Propulsion systems, Space vehicles -- Research, Business, Chemistry, Electronics, Electronics and electrical industries
Abstract

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.sub.o]) as a function of the incident electron energy [E.sub.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.sub.o]) and the mean free path of the SE [lambda]. In this paper, the authors recall classical models for the range R([E.sub.o]): a power law expression of the form [b.sub.1][E.sup.n1.sub.o] and a more general empirical double power law R([E.sub.o]) = [b.sub.1] [E.sup.n1.sub.o] + [b.sub.2][E.sup.n2.sub.o] 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.sub.o]) over the full range of energy of incident particles. Index Terms--Electron emission, secondary electron (SE) yield, surface charging.

Published
2006

36. Space Plasma Interactions with Spacecraft Materials

Author

Engelhart, Daniel P., Plis, Elena A., Ferguson, Dale, Robert Johnston, W., Cooper, Russell, Hoffmann, Ryan C., Engelhart, Daniel P., Plis, Elena A., Ferguson, Dale, Robert Johnston, W., Cooper, Russell, and Hoffmann, Ryan C.

Abstract

Spacecraft materials on orbit are subjected to the harsh weather of space. In particular, high-energy electrons alter the chemical structure of polymers and cause charge accumulation. Understanding the mechanisms of damage and charge dissipation is critical to spacecraft construction and operational anomaly resolution. Energetic particles in space plasma break molecular bonds in polymers and create radicals that can act as space charge traps. These electron-induced chemical changes also result in changes to the spectral absorption profile of polymers on orbit. Radicals react over time, either recreating identical bonds to those in the pristine material, leading to material recovery, or creating new bonds, resulting in a new material with new physical properties. Lack of knowledge about this dynamic aging is a major impediment to accurate modeling of spacecraft behavior over its mission life. This chapter first presents an investigation of the chemical and physical properties of polyimide films (PI, Kapton-H®) during and after irradiation with high-energy (90 keV) electrons. Second, the deleterious effects of space plasma on a spacecraft component level are presented. The results of this physical/chemical collaboration demonstrate the correlation of chemical changes in PI with the dynamic nature of spacecraft material aging.

Published
2018
Full Text
View/download PDF

43. Effect of Electron Bombardment on Polyimide Film

Author

Engelhart, Daniel P., primary, Plis, Elena, additional, Cooper, Russell, additional, Humagain, Sunita, additional, Koch, Andrei, additional, Brunetti, Matthew, additional, Greenbaum, Steven, additional, and Hoffmann, Ryan, additional

Published
2018
Full Text
View/download PDF

46. Synergistic Models of Electron Emission and Transport Measurements of Disordered SiO2

Author

Dennison, JR, Corbridge Gillespie, Jodie, Andersen, Allen, Evans Jensen, Amberly, Wilson, Gregory, Dekany, Justin, Sim, Alec, and Hoffmann, Ryan C

Subjects
Physics, Condensed Matter Physics
Abstract

An important way for the spacecraft charging community to address the expanding necessity for extensive characterization of electron emission and transport properties of materials is to expand the role of more fundamental materials physics. This includes the development of unifying theoretical models of the charge transport equations based on the creation, distribution, and occupancy of defect densities of states. Models that emphasize the synergistic relation between fitting parameters for diverse measurements can also lead to a better understanding of materials and facilitate solutions to spacecraft charging issues. As an example of this approach, we present results of many different measurements on similar samples of a single common insulating spacecraft material, disordered silicon dioxide. Measurements include time-, field-, and temperature-dependent conductivity, radiation induced conductivity, electron emission yields and spectra, surface voltage accrual and decay, cathodoluminescence, electrostatic discharge, endurance time, and optical transmission.

Published
2016

Catalog

Books, media, physical & digital resources
See catalog results