Browsing by Author "Williams, John D., advisor"
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Item Open Access A high-speed mass spectrometer for characterizing flash desorbed species in pulsed power applications(Colorado State University. Libraries, 2022) Ossareh, Susan J., author; Williams, John D., advisor; Yalin, Azer P., committee member; Roberts, Jacob L., committee memberSandia National Laboratories operates the largest pulsed power facility in the world that hosts the Z machine that is utilized for research in fusion, energy, and national security. It can simulate extreme environments in these research areas in a single "shot" or "pulse of power," where large capacitor banks are rapidly discharged simultaneously, sending power to the center of the machine where a load is compressed into a z-pinch. A shot on the Z machine occurs in 150ns with peak currents on the order of 26 mega-amperes. However, there is a power flow obstacle that limits its ability to reach these extreme conditions. Approximately 1-3 MA of current is lost per shot. This could be partially attributed to chemisorbed contaminants on the cathode and anode stack in the center section of the machine being liberated in a flash desorption process, forming a conductive plasma between the anode and cathode electrodes that causes current to bypass the load and limits the power flow into the load. This project is focused on the design and development of a high-speed mass spectrometer to make measurements of the gasses evolved from the electrodes that are heated to 1000°C in 100 nanoseconds. The measurements from this diagnostic would allow for more accurate predictive modeling of current loss for Next Generation Pulsed Power Drivers, such as the Z machine. Since a probe does not exist commercially, the project requires the development of new mass spectrometry technology, however a pre-existing probe was used to begin the design process. This probe is known as the Energy and Velocity Analyzer for Distributions of Electric Rockets (EVADER) probe, which combines an electrostatic analyzer and a Wien velocity filter. Within this study, two different plasma sources were used separately to simulate the plasma generated in the Z machine, and steady state measurements were made of the ions produced while working towards taking transient measurements. The design and development efforts described in this thesis were guided by: (1) using the EVADER to collect steady state data in its original configuration as a basis of comparison, (2) then replacing an ammeter in the experimental system with a transimpedance amplifier (TIA) circuit to speed up the data sampling rate over that of the ammeter, (3) incorporate a micro-channel plate within the probe to amplify the current feed to the TIA and enable even faster data sampling rates, and (4) design a high speed electric shutter to quickly turn "on" and "off" ion flow to the probe to enable measurement of the temporal response of the probe with the transimpedance amplifier and micro-channel plate elements. The end goal of the project is to improve transient performance of a probe from 10s of seconds to 10s of micro-seconds in a stepwise manner to support pulsed power research.Item Open Access An experimental investigation of heaterless hollow cathode ignition(Colorado State University. Libraries, 2020) Ham, Ryan K., author; Williams, John D., advisor; Yalin, Azer P., committee member; Marconi, Mario C., committee member; Tomasel, Fernando G., committee memberA hollow cathode is a specially designed plasma source that is capable of driving a large electron emission current throughout the course of a remarkably long lifetime. Given these characteristics, hollow cathodes are commonly used as electron sources in state-of-the-art plasma thrusters. Modern advancements in small-satellite technology have led to an increased demand for low-power electric propulsion systems. Given the high thrust-to-power ratio and flight-proven heritage of Hall-effect thrusters, efforts are currently being made to downsize these thrusters to a considerably small scale. By forgoing the use of a heater, heaterless hollow cathodes provide several advantages that are best realized in miniaturized Hall-effect thrusters. Unfortunately, the lack of a cathode heater gives rise to nontrivial complications in the process of igniting a plasma discharge, along with reason to believe that life-limiting cathode erosion could occur during ignition. These concerns have resulted in a lack of confidence that heaterless hollow cathode technology can endure the rigors of spaceflight qualification. In this research, heaterless hollow cathode ignition behavior was characterized. In doing so, it was found that repeatable and reliable instant start ignition behavior can be achieved when using a high propellant mass flow rate. To provide this flow condition without placing a large burden on a propellant feed system, a novel gas flow mechanism was developed and characterized. To investigate whether instant start ignition causes cathode erosion, a series of tests were performed in which heaterless hollow cathodes were subjected to a large number of ignition cycles. Microscopy revealed no indication of cathodic arc activity, and no other evidence of life-limiting erosion were observed. The instant start ignition process appears to be a viable approach to heaterless hollow cathode ignition, and we believe it provides a means for heaterless hollow cathode technology to be integrated into spaceflight propulsion systems.Item Open Access Characterization of a plasma reactor device for photovoltaic applications(Colorado State University. Libraries, 2012) Metz, Garrett Eugene, author; Williams, John D., advisor; Mahoney, Leonard, committee member; Sampath, Walajabad S., committee member; Robinson, Steve, committee memberHeated pocket deposition (HPD) sources are used for the rapid manufacture of thin film CdS/CdTe photovoltaic devices. Standard lab devices produced at CSU by the HPD process have achieved efficiencies of 13%. New process methods are required to further improve the quality of the films, increase cell efficiency, and reduce production costs. A plasma-enhanced, close-spacing sublimation (PECSS) technique has recently been developed as a candidate process method. It has been successfully used to eliminate pin holes, to dope CdS with oxygen, and dope CdTe absorption layers; all of which have resulted in higher device efficiencies. In this work we present measurements describing the properties of the PECSS plasma. Specifically the uniformity of the ion current flux to the substrate is presented for nitrogen/oxygen and argon feed gases by means of in situ surface probes fabricated by segmenting a transparent conductive oxide film that is laid over the glass. Plasma properties within the PECSS processing chamber are also presented including plasma density, electron temperature, and plasma potential. Operational characteristics and scaling of PECSS are presented for pressures of 100-300 mTorr and surface areas of 160 - 1700 cm2. A three-dimensional model was developed to calculate plasma production and transport processes, and to gain a greater understanding of the role of energetic primaries versus bulk cold electrons on spatial ionization rates that develop within the PECSS plasma as a function of gas pressure and geometry. Comparisons between the model and experimental measurements are presented and good agreement has been observed when the appropriate spatially varying ionization rates are estimated. This work also presents the development of a diagnostic test bed that will be useful for future work in the development and understanding of the PECSS technique.Item Open Access Development of a novel additive manufacturing method: process generation and evaluation of 3D printed parts made with alumina nanopowder(Colorado State University. Libraries, 2017) Hensen, Tucker Joseph, author; Williams, John D., advisor; Prawel, David A., advisor; Wang, Qiang, committee memberDirect coagulation printing (DCP) is a new approach to extrusion-based additive manufacturing, developed during this thesis project using alumina nanopowder. The fabrication of complex ceramic parts, sintered to full density, was achieved and the details of this invention are described. With the use of additive manufacturing, complex features can be generated that are either very difficult or unattainable by conventional subtractive manufacturing methods. Three unique approaches were taken to create a slurry suitable for extrusion 3D-printing. Each represented a different method of suspending alumina nanopowder in a liquid; a bio-polymer gel based on chitosan, a synthetic polymer binder using poly-vinyl acetate (PVA), and electrostatic stabilization with the dispersant tri-ammonium citrate (TAC). It was found that TAC created a slurry with viscosity and coagulation rate that were tuneable through pH adjustment with nitric acid. This approach led to the most promising printing and sintering results, and is the basis of DCP. Taguchi and fractional factorial design of experiments models were used to optimize mixing of the alumina slurry, rheological properties, print quality, and sinterability. DCP was characterized by measuring the mechanical properties and physical characteristics of printed parts. Features as small as ~450 μm in width were produced, in parts with overhangs and enclosed volumes, in both linear and radial geometries. After sintering, these parts exhibited little to no porosity, with flexural modulus and hardness comparing favorably with conventionally manufactured alumina parts. A remarkable aspect of DCP is that it is a completely binderless process, requiring no binder removal step. In addition, DCP can employ nanopowders, allowing for enhanced mechanical properties as observed in nano-grained materials. Perhaps most importantly, any material that acquires a surface charge when in aqueous media has the potential to be used in DCP, making it a method of additive manufacturing using many metals and ceramics other than alumina.Item Open Access 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 memberMost 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.Item Open Access Performance and lifetime simulation of ion thruster optics(Colorado State University. Libraries, 2007) Farnell, Cody Coffman, author; Williams, John D., advisor; Wilbur, Paul J., advisorA simulation code, ffx, was developed to study various aspects of ion thruster optics. Information concerning sheaths, impingement limits, perveance, electric potential, charge exchange, and sputtering is covered. Electron backstreaming and pit and groove wear are discussed in detail as two grid failure mechanisms. The code was used to study the effects of several parameters on grid performance and lifetime, including grid spacing, aperture diameter, and grid thickness. An evolutionary algorithm was used with the ffx code to design grid sets, utilizing net accelerating voltage and current density as primary input parameters. Validation of the ffx code was accomplished through comparison to other ion optics codes and to experimental data obtained from both gridlet and full thruster testing. Gridlet test comparisons included simulations of finite aperture grid sets. The NSTAR thruster was studied in detail with regard to lifetime. The methods used for accurate and efficient optics simulation are discussed, including the multigrid method for solving for electric potential.Item Open Access Performance and plume characterization of a laboratory krypton Hall thruster(Colorado State University. Libraries, 2020) Andreano, Thomas Malachi, author; Williams, John D., advisor; Marchese, Anthony J., committee member; Roberts, Jacob L., committee memberHall thruster research has been in progress at the CSU Electric Propulsion and Plasma Engineering (CEPPE) lab for the past decade, however, a full performance and plasma plume characterization has not been conducted with the laboratory Hall thruster available, which recently was modified to be configured as magnetically shielded as well as non-magnetically shielded. Additionally, heaterless cathode geometries that could benefit scaling of Hall thrusters to either much larger or much smaller designs have been undergoing development at the CEPPE lab. One of these cathodes, named the postage stamp, was designed to mount to the outer pole piece on the front of the thruster in the seperatrix of the magnetic field, and fits in the space between the outer pole piece and the backplate of the thruster. To further the research on Hall thrusters at CSU, a baseline of the laboratory thruster performance is necessary, and performance characterization of the operation using different cathodes is necessary to further the cathode design. To these ends, performance of the thruster was characterized with: (1) the center mounted cathode, providing a baseline for all future Hall thruster research at the CEPPE lab, (2) with the postage stamp cathode, to determine the potential performance differences between operation with the two cathodes, and (3) in the magnetically shielded configuration, to verify proper operation and investigate any potential performance differences compared to the traditional configuration. Thrust measurement, along with data from an Electrostatic Analyzer (ESA), ExB probe, and Faraday probe were collected to determine the performance characteristics of the thruster as well as the characteristics of the ion beam in each of the three cases outlined above. Additionally, a preliminary study of an anomalous operation mode providing higher than usual performance was conducted using these probes, as well as a combined ESA/ExB called the EVADER probe.Item Open Access Plasma flow field measurements downstream of a hollow cathode(Colorado State University. Libraries, 2007) Farnell, Casey Coffman, author; Williams, John D., advisor; Wilbur, Paul J., advisorThe focus of the research described herein is to investigate and characterize the plasma produced downstream of a hollow cathode with the goal of identifying groups of ions and possible mechanisms of their formation within a plasma discharge that might cause erosion, especially with respect to the hollow cathode assembly. In space applications, hollow cathodes are used in electrostatic propulsion devices, especially in ion thrusters and Hall thrusters, to provide electrons to sustain the plasma discharge and neutralize the ion beam. This research is considered important based upon previous thruster life tests that have found erosion occurring on hollow cathode, keeper, and ion optics surfaces exposed to the discharge plasma. This erosion has the potential to limit the life of the thruster, especially during ambitious missions that require ultra long periods of thruster operation. Results are presented from two discharge chamber configurations that produced very different plasma environments. Four types of diagnostics are described that were used to probe the plasma including an emissive probe, a triple Langmuir probe, a remotely located electrostatic analyzer (ESA), and an ExB probe attached to the ESA. In addition, a simulation model was created that correlates the measurements from the direct and remotely located probes.Item Open Access Sputter deposited hydroxyapatite thin films to enhance osseointegration(Colorado State University. Libraries, 2010) Riedel, Nicholas Alfred, author; Williams, John D., advisor; Popat, Ketul, advisor; Prieto, Amy L. (Amy Lucia), committee memberAs the demand for hip and knee replacements continues to grow, researchers look to increase the operational lifetimes of these implants. Many of these implants fail as a result of aseptic loosening caused from repeated loading of these joints. It is thought that implant life could be extended by improving the interface between the implant and natural tissue. To this effect, hydroxyapatite coatings have been demonstrated to improve implant to bone bonding and allow a more natural integration of the metallic substrates. This work explores the potential of using ion beam etching and sputter deposition to produce a hydroxyapatite thin film with a unique surface topography that would potentially enhance osseointegration. First, the effects of ion etching bare titanium were evaluated. Three ion energies (300 eV, 700 eV, and 1100 eV) were used to etch either as-received or polished substrates. Topographical changes were examined by scanning electron microscopy. Rat mesenchymal stem cells were differentiated to osteoblasts to test the biocompatibility of the surfaces with bone cells. It was found that ion etching the titanium increases cellular activity, and an ion energy of 700 eV appears to create the most beneficial topography. Hydroxyapatite thin films were then sputter deposited on titanium substrates etched at 700 eV. After the coatings were deposited, some of the hydroxyapatite films were re-etched in efforts to induce a unique topography. It was found that the hydroxyapatite coatings improved short term cell response but degraded over the course of the culture. Further investigation showed the as-sputtered coatings were amorphous. To prevent degradation of the coatings, annealed films were then prepared by heat treating at 600 °C for 2 hours. X-ray diffraction was used to confirm the presence of a crystalline hydroxyapatite phase. Films were immersed in culture media for four weeks, showing no signs of degradation. Ion etching performed on the substrates post annealing yielded a unique topography in the hydroxyapatite film. A final study was conducted evaluating the MSC response to the annealed and post-anneal etched films. It was found that the post-anneal etched hydroxyapatite coating had the highest cellular activity, indicating that this preparation may be an effective means to enhance osseointegration on medical implants.Item Open Access The development of a thin film sputter deposition system using a novel hidden anode ion source and motion control(Colorado State University. Libraries, 2020) VanGemert, Jack J., author; Williams, John D., advisor; Farnell, Casey, committee member; Menoni, Carmen, committee member; Wilson, Jesse, committee memberThin films consist of metallic or dielectric materials that are commonly deposited onto surfaces where properties, intrinsic to the thin film, are desired. Ranging from a single atomic layer to several microns in thickness, thin films are found to be useful for a broad range of applications. Most thin film applications desire uniform, durable, and adherent coatings with specific optical, electrical, or tribological properties. Therefore it is important that deposition systems can produce thin films with properties suited for the application at hand. The development of a thin film sputter deposition system is presented. The system has been shown to produce large area art pieces at a low cost compared to current deposition systems. The deposition system uses a novel hidden anode ion source (HAIS) to sputter target material, assist film growth, and to clean substrates prior to deposition. To the author's knowledge, an ion source of this design has not been implemented in a deposition system prior to the one discussed. The characterization of a novel ion source is presented in detail along with the other system components. Deposition rates and thin film profiles are used to validate experimental results and predict thin film properties for various operating conditions. Coatings produced by the system are studied and used to determine film characteristics of interest to the application of outdoor art. Structural thin film properties of interest for long outdoor lifetime art work include film adhesion, density, and residual stress. Visual thin film properties important for the artwork are related to optical properties such as reflection, transmission, and absorption. The plasma-based deposition system is shown to be a tool of high potential for creating engaging, long lifetime art pieces.Item Open Access The effects of a realistic hollow cathode plasma contactor model on the simulation of bare electrodynamic tether systems(Colorado State University. Libraries, 2013) Blash, Derek M., author; Williams, John D., advisor; Bradley, Thomas H., committee member; Robinson, Raymond S., committee memberThe region known as Low-Earth Orbit (LEO) has become populated with artificial satellites and space debris since humanities initial venture into the region. This has turned LEO into a hazardous region. Since LEO is very valuable to many different countries, there has been a push to prevent further buildup and talk of even deorbiting spent satellites and debris already in LEO. One of the more attractive concepts available for deorbiting debris and spent satellites is a Bare Electrodynamic Tether (BET). A BET is a propellantless propulsion technique in which two objects are joined together by a thin conducting material. When these tethered objects are placed in LEO, the tether sweeps across the magnetic field lines of the Earth and induces an electromotive force (emf) along the tether. Current from the space plasma is collected on the bare tether under the action of the induced emf, and this current interacts with the Earth's magnetic field to create a drag force that can be used to deorbit spent satellites and space debris. A Plasma Contactor (PC) is used to close the electrical circuit between the BET and the ionospheric plasma. The PC requires a voltage and, depending on the device, a gas flow to emit electrons through a plasma bridge to the ionospheric plasma. The PC also can require a plasma discharge electrode and a heater to condition the PC for operation. These parameters as well as the PC performance are required to build an accurate simulation of a PC and, therefore, a BET deorbiting system. This thesis focuses on the development, validation, and implementation of a simulation tool to model the effects of a realistic hollow cathode PC system model on a BET deorbit system.