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Browsing by Author "de la Venta Granda, Jose, committee member"

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    Iodine compatible hollow cathode
    (Colorado State University. Libraries, 2019) Thompson, Seth Joseph, author; Williams, John D., advisor; Farnell, Casey C., advisor; Yalin, Azer P., committee member; de la Venta Granda, Jose, committee member
    Most electric propulsion (EP) systems utilize xenon gas as a propellant, which is expensive and must be stored in heavy high-pressure tanks, within which the storage density is still lower than desired. The halogen iodine (I2) has risen as a leading alternative propellant with the potential to overcome these drawbacks with its lower cost, higher storage density, and significantly reduced tank pressure. Hall-effect thrusters have been operated with iodine propellant in the range of a hundreds of watts to greater than ten kilowatts [1], [2], with performance comparable to that of devices operated on xenon; however, due to the reactive nature of iodine, the hollow cathode electron sources used with these thrusters, have been operated on xenon. Without being able to operate cathodes on iodine, the consideration of iodine propellant for many space missions is not possible. This research aims to develop and examine hollow cathode assemblies capable of operating on iodine propellant. We propose that a cathode can be constructed with iodine resistant materials and with an insert capable of participating in a tungsten-iodine life cycle that is utilized in halogen lamps to increase filament lifetime. Results from this work demonstrate that a cathode with a graphite tube and a tungsten-based ceramic-metal composite insert is capable of being operated on iodine for longer than any currently published operation time. This type of cathode has the potential to be operated on iodine for over 3,000 hours, a lifetime approaching the minimum requirement of EP systems currently being used.
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    Low work function, long lifetime filament for electron beam-based, wire-fed metal additive manufacturing
    (Colorado State University. Libraries, 2018) Nguyen, Bao Gia, author; Bradley, Thomas, advisor; Williams, John, advisor; de la Venta Granda, Jose, committee member
    Tantalum filaments are used in electron beam additive manufacturing to thermionically emit electrons that are used to build near-net shape, metal parts. High operating temperatures are required to emit electrons which consequently limits the lifetime of these filaments. This thesis presents the thermionic emission characteristics of drop-in filament replacements that incorporate barium calcium aluminate cermets. Barium calcium aluminate is a low work function material used with hollow cathodes in electric propulsion devices to provide very long service lifetimes by acting as a moderate temperature, electron source. A marriage of these two technologies may limit downtime and increase the productivity and output of electron beam additive manufacturing. Results of extended runtime tests are presented from configurations that immerse the modified filament in plasma and operate it as a vacuum emitter. The effect of contamination by air and fabrication methods are examined and evaluated based on effective work function and current density measurements. The latter includes formation methods for barium diffusion orifices as well as surface preparation methods for cermets. The experimental data collected were used to validate a predictive model that evaluates emission current densities, in both temperature and space-charge limited conditions, and effective work functions based on the fractional surface coverage of barium over a tantalum substrate.
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    Studies of magnetization dynamics in magnetic recording media and patterned yttrium iron garnet films
    (Colorado State University. Libraries, 2018) Richardson, Daniel, author; Wu, Mingzhong, advisor; de la Venta Granda, Jose, committee member; Kabos, Pavel, committee member; Krueger, David, committee member; Marconi, Mario, committee member
    Exchange coupling and damping are studied in magnetic media materials for applications in current perpendicular magnetic recording (PMR) technology as well as future heat assisted magnetic recording (HAMR) media technology. Damping and exchange coupling are directly related to magnetization switching time in writing operation and the signal-to-noise ratio in reading, both critical to the performance of hard disk drives. Intergranular exchange is studied in current PMR media to see how exchange is altered in the presence of SiO2 based segregant. By varying the segregant by as much as 30%, there is strong tunability of the exchange field between the grains. The damping in future FePt-based HAMR media is studied near the curie temperature (725 K) of FePt where the writing stage in the recording media takes place. The trends of ferromagnetic resonance (FMR) linewidth varying with the sample temperature, the volume fraction of carbon in the media, and the angle of the external field indicate that the overall damping includes strong contributions from intrinsic magnon-electron scattering as well as extrinsic two-magnon scattering between the grains. Interlayer exchange coupling and damping were studied in magnetic layered systems consisting of a soft ferromagnetic transition metal or alloy layer and a hard FePt layer at room and elevated temperatures. It was found that exchange coupling and damping are strongly dependent on temperature, the soft layer thickness, and the choice of material of the soft layer. Spin waves are studied in the linear and non-linear regimes using magnonic crystals consisting of yttrium iron garnet (YIG) thin film strips with periodic etched lines or periodic metallic lines deposited on top of the YIG strip, as well as YIG strips with randomly spaced metallic lines deposited on top. The various media provide ways of controlling the dispersion by altering the interference of the spin waves, allowing for a wide range of interesting phenomenon to be observed. Spin-wave fractals are observed for the first time in a YIG strip with periodic etched lines. The etched lines serve as position dependent potentials to increase dispersion in the YIG strip large enough for fractal formation in the nonlinear regime. This is also the first time fractals of any type that have been observed without the formation of time-domain solitons. Spin-wave localization is observed in the linear regime for the first time in YIG strips with randomly spaced metallic lines where the metallic lines serve as potential barriers for causing spin wave interference. Magnonic crystals consisting of YIG strips with periodically spaced metallic lines are used to compare a standing wave state with the localized state. The localized state is much stronger and much more confined to a smaller physical space than the standing wave state.