Physics Commons^™

Low Temperature Cathodoluminescence In Disordered Sio2, Amberly Evans Jensen, Jr Dennison, Gregory Wilson, Justin Dekany

Graduate Student Publications

In recent charging studies, a discernible glow was detected emanating from sample surfaces undergoing electron beam bombardment that resulted from a luminescent effect termed cathodoluminescence. This suggests that some of the materials used as optical elements, structural components, and thermal control surfaces in the construction of space-based observatories might luminesce when exposed to sufficiently energetic charged particle fluxes from the space plasma environment. A central focus of our experiments was the temperature dependence of the luminescent behavior. Here, an overview of our experimental results is given, as well as a qualitative model to describe the luminescent behavior. We look at …

Ultrahigh Vacuum Cryostat System For Extended Low Temperature Space Environment Testing, Justin Dekany, Robert H. Johnson, Gregory Wilson, Amberly Evans Jensen, Jr Dennison

Graduate Student Publications

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 from450 K, with long-term controlled stability of

Electron Transport Models And Precision Measurements With The Constant Voltage Conductivity Method, Justin Dekany, Jr Dennison, Alec Sim, Jerilyn Brunson

Graduate Student Publications

Recent advances are described in the techniques, resolution, and sensitivity of the Constant Voltage Conductivity (CVC) method and the understanding of the role of charge injection mechanisms and the evolution of internal charge distributions in associated charge transport theories. These warrant reconsideration of the appropriate range of applicability of this test method to spacecraft charging. We conclude that under many (but not all) common spacecraft charging scenarios, careful CVC tests provide appropriate evaluation of conductivities down to ≈10-22 (Ω-cm)-1, corresponding to decay times of many years.

We describe substantial upgrades to an existing CVC chamber, which improved the precision of …

Electron Energy Dependent Charging Effects Of Multilayered Dielectric Material, Gregory Wilson, Jr Dennison, Amberly Evans Jensen, Justin Dekany

Graduate Student Publications

Measurements of the charge distribution in electron-bombarded, thin-film, and multilayer dielectric samples showed that charging of multilayered materials evolves with time and is highly dependent on incident energy; this is driven by electron penetration depth, electron emission, and material conductivity. Based on the net surface potential’s dependence on beam current, electron range, electron emission, and conductivity, measurements of the surface potential, displacement current, and beam energy allow the charge distribution to be inferred. To take these measurements, a thin-film disordered SiO2 structure with a conductive middle layer was charged using 200-eV and 5-keV electron beams with regular 15-s pulses at …

The Effects Of Surface Modification On Spacecraft Charging Parameters, Amberly Evans Jensen, Jr Dennison

Graduate Student Publications

Charging of materials by incident radiation is affected by both environmental and physical conditions. Modifying a material’s physical surface will change its reflection, transmission and absorption of the incident radiation which are integrally related to the accumulation of charge and energy deposition in the material. General arguments for incident and emitted photons, electrons and ions are considered. An optical analysis of the effects of surface modification on spacecraft charging parameters on prototypical polyimide Kapton HNTM and Cu samples is presented. Samples were roughened with abrasive compounds ranging from 0.5 to 10 μm in size, comparable to the range of incident …

The Second Annual Space Weather Community Operations Workshop: Advancing Operations Into The Next Decade, Jennifer L. Meehan, Jared Fulgham, W. Kent Tobiska

Graduate Student Publications

No abstract provided.

Workshop Addresses Aviation Community, Jennifer L. Meehan, Joseph Kunches

Graduate Student Publications

No abstract provided.

The Density Factor In The Synthesis Of Carbon Nanotube Forest By Injection Chemical Vapor Deposition, Robert W. Call, C. Read, C Mart, T. C. Shen

Graduate Student Publications

Beneath the seeming straight-forwardness of growing carbon nanotube(CNT) forests by the injection chemical vapor deposition(CVD) method, control of the forest morphology on various substrates is yet to be achieved. Using ferrocene dissolved in xylene as the precursor, we demonstrate that the concentration of ferrocene and the injection rate of the precursor dictate the CNT density of these forests. However, CNT density will also be affected by the substrates and the growth temperature which determine the diffusion of the catalyst adatoms. The CNT growth rate is controlled by the temperature and chemical composition of the gases in the CVD reactor. We …

Approximation Of Range In Materials As A Function Of Incident Electron Energy, Gregory Wilson, Jr Dennison

Graduate Student Publications

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 …