Browsing by Author "Roberts, Jacob L., committee member"
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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 A new measurement of the 2S1/2-8D5/2 transition in atomic hydrogen(Colorado State University. Libraries, 2021) Brandt, Adam D., author; Yost, Dylan C., advisor; Yalin, Azer P., committee member; Roberts, Jacob L., committee member; Field, Stuart B., committee memberHigh-precision spectroscopy of simple atoms provides input data that can be used to extract fundamental constants and to test Standard Model theory. Hydrogen, the simplest element, has played a historically significant role in the development of fundamental theory and, more recently, provides important data for the proton radius puzzle. In this thesis, we will describe a new measurement of the 2S1/2-8D5/2 transition on a cryogenic hydrogen beam. We will overview the measurement scheme and experimental apparatus, then present analysis and systematic characterization important to the spectroscopy. Finally, we will present our preliminary determination of the proton radius and the Rydberg constant using our value for the 2S1/2-8D5/2 combined with the previously measured 1S-2S transition.Item Open Access Development of a petawatt class Ti:Sapphire laser for the excitation of extreme radiation sources(Colorado State University. Libraries, 2020) Rockwood, Alex Pratt, author; Rocca, Jorge J., advisor; Lee, Siu Au, committee member; Roberts, Jacob L., committee member; Marconi, Mario C., committee memberThis dissertation describes the design, construction and characterization of a high peak power, high repetition rate, Titanium-Sapphire laser system. This chirped-pulse amplification (CPA) laser delivers femtosecond pulses of up to 0.85 PW peak power. By utilizing pump laser amplifiers with a slab configuration high repetition rate are achieved, 3.3Hz, the highest at which Petawatt-class lasers have been operated to date. This 800nm laser also has a high power, ultra-high contrast 400 nm beamline. By frequency doubling the 800 nm with a KDP crystal at ≥ 40% conversion we are able to achieve a contrast of > 1 × 10-12. The ability to focus this second harmonic beam to ~1.2 μm Full Width at Half Maximum (FWHM) spot size made it possible to achieve intensities up to ~ 6.5 ×1021 W/cm2. With these high intensities and high contrast this laser is a powerful tool in many applications especially in the study of laser/matter interactions at relativistic plasmas. This Ti:Sapphire laser was used for the excitation of plasma based soft x-ray (SXR) lasers. prior to this work compact, repetitively fired, gain-saturated x-ray lasers had been limited to wavelengths above λ = 8.85 nm. We were able to demonstrate SXR lasers operating in the gain-saturated regime down to wavelengths as low as λ = 6.85 nm in Ni-like Gd. Gain was also observed at λ = 6.4 nm, and λ = 5.8 nm in Ni-like Dy. As an application of plasma-based SXR lasers, single shot Fourier holograms covering a large area of view were demonstrated using an 18.9nm laser with high spatial coherence based on dual plasma amplifier. Compact SXR lasers have made possible applications in nano-scale imaging, dense plasma diagnostics and a variety of new studies of materials and surfaces. Other applications that were enabled by this Petwatt-class laser discussed elsewhere include the study of the interaction of relativistic laser pulses with aligned nanostructures, producing record conversion efficiency of optical laser light into picosecond x-ray pulses with photons of > 1 KeV energy and flashes of deuterium-deuterium fusion neutrons.Item Open Access Investigation of optical prechamber spark plug and dual laser pulses for ignition(Colorado State University. Libraries, 2011) Loccisano, Frank Christopher, author; Yalin, Azer P., 1972-, advisor; Marchese, Anthony John, 1967-, committee member; Roberts, Jacob L., committee memberLaser ignition has the potential to increase efficiency and reduce pollutant emissions of natural gas engines. The work presented in this thesis focuses on investigate the reasons behind lower indicated mean effective pressures (IMEPs) obtained in laser ignition tests of Caterpillar G3516C engine when operated with hollow core fibers, and experimentally investigating the effect of using dual laser pulses to increase the total amount of energy deposited in a laser spark. To address the low IMEP, succinct tests were performed on the Caterpillar G3516C engine with a non-fueled electric prechamber plug, a non-fueled laser prechamber plug, and an open chamber laser plug. Test data showed that the open chamber laser plug exhibited a high degree of combustion instability, while the prechamber electrical and laser plug showed similar (improved) performance with the prechamber laser plug having a slightly higher degree of combustion variability. Computational fluid dynamics (CFD) was performed to examine the turbulent flow inside the optical prechamber. The CFD found that to optimize the use of the optical prechamber the spark should be located in the bottom half of the prechamber to reduce the quenching due to turbulence. Bench top experiments were also performed to examine the possibility of increasing the energy in the laser spark by employing dual laser pulses. The first pulse would initiate the spark while the second pulse deposits additional energy into the spark. It was shown that initial spark can absorb 80 to 90% energy of the second pulse if the inter-pulse separation (Δt) is about 20 to 40 nanoseconds. Overall, better understanding of the use of optical prechamber sparkplugs, as well as the use of dual laser pulses to increase the amount of energy deposited into the spark, will aid in the progression of a practical laser ignition system.Item Open Access Optical lattice deceleration of a cryogenic metastable atomic hydrogen beam(Colorado State University. Libraries, 2023) Cooper, Samuel F., author; Yost, Dylan C., advisor; Roberts, Jacob L., committee member; Gelfand, Martin, committee member; Van Orden, Alan, committee memberHydrogen is the most abundant and simple naturally occurring element in existence, making it an ideal platform for study of fundamental atomic physics. Theoretical physics has the capacity of making extraordinarily precise predictions of atomic hydrogen's energy levels, owing to hydrogen's innate simplicity. To provide valuable new information to the theoretical models, such as definitions of fundamental constants, requires pushing experimental measurement of these energy levels to extreme precision, and obtaining experimental values that agree or disagree with theory provide a rigorous test of fundamental physics. Unfortunately, hydrogen has yet to benefit from the advent of laser cooling and trapping techniques pioneered in other species due to the prohibitive ultraviolet wavelengths required. As a consequence, modern best measurements to-date are limited by uncertainties due to thermal energies of atomic hydrogen samples. The next generation of ultra-high precision experiments will require new ways to obtain slow and or cold atomic hydrogen. This work contains progress made towards this goal, where advent of a novel high power UV radiation source on this experiment opened the viability for exploring new horizons. Specifically detailed in this dissertation are the efforts toward generating a cryogenic helium temperature beam of metastable (2S) atomic hydrogen with velocity characterization and a first ever demonstration of a novel, all-optical deceleration method which utilizes an electro-optically controlled far detuned optical lattice. In the proof-of-principle experiment a velocity selected portion of the atomic hydrogen beam was decelerated from 300 ms–1 to 280 ms–1 in a single 30 ns optical pulse.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 Quantum dot clusters as single-molecules: deciphering collective fluorescence and energy transfer signatures(Colorado State University. Libraries, 2016) Ryan, Duncan P., author; Gelfand, Martin P., advisor; Van Orden, Alan, advisor; Roberts, Jacob L., committee member; Prieto, Amy L., committee memberApplications of quantum dot nanocrystals span from the individual single-molecule use to large, densely-packed bulk solids. Already, the fluorescence behavior of individual particles is complex and nuanced, particularly involving the blinking phenomenon. When particles are combined into higher-order structures where interaction may occur, a complete description becomes intractable. However, clusters---between two and ten particles---can be effective model systems to explore the local behaviors that occur in larger networks. A benefit of small clusters is the viability of using single-molecule spectroscopic techniques, which are often more informative than bulk measurements. In this work we combine fluorescence microscopy with structure-probing electron microscopy to elucidate the fluorescence dynamics clusters of semiconductor nanocrystals. The spectral characteristics of clusters are explored in the context of an energy transfer model showing low-intensity emission is blue-shifted, corresponding to the weaker emission from donor particles with a larger band gap. Because energy transfer depends intimately on the specific topographical structure of the cluster, the inter-particle spacing, and relative alignment, characterization of specific cluster behavior is better informed by correlated measurements. Next, we present the mapping results from super-resolution microscopy where the spatial distributions of fluorescence in the sub-10 nanometer realm is clearly correlated with scanning electron microscopy imaging of the same clusters. Stochastic blinking events enable such observations. The enhanced blinking associated with energy transfer has practical implications for donor and acceptor roles in clusters. Finally, the dynamic evolution of the emission dipole orientation for single nanocrystals and nanocrystal clusters is measured. The orientation signature suggests coupling strengths and constitutes a first-step towards determining corrections to Förster resonant energy transfer theory involving nanocrystals.Item Open Access Real-time erosion measurements of the HiVHAc and SPT-70 Hall thrusters via cavity ring-down spectroscopy(Colorado State University. Libraries, 2014) Lee, Brian Christopher, author; Lundeen, Stephen R., advisor; Yalin, Azer P., advisor; Roberts, Jacob L., committee member; Krueger, David A., committee member; Williams, John D., committee memberElectric propulsion has moved to the forefront of in-space propulsion in recent years. By making exceptionally efficient use of propellant, electric propulsion devices have significantly reduced the cost of some missions and enabled others, which had not previously been possible. Among these devices, Hall thrusters have shown particular promise. However, for many thrusters of interest, sputter erosion of the insulating channel remains a problem and continues to limit the thruster lifetime. Diagnostic tools to assess the absolute channel erosion rate rapidly remain limited. This thesis describes the use of ultraviolet cavity ring-down spectroscopy (CRDS) as a real-time diagnostic of sputtered boron atoms in the thruster plume. Cavity ring-down spectroscopy is an ultra-sensitive laser-absorption technique which is particularly apt at measuring trace species number densities in the gas phase. In this work, ground-state atomic boron, which was sputtered from the thruster channel, was measured near 250 nm. The interrogating laser was swept across the exit plane of a Hall thruster, providing spatially-resolved boron number density measurements. Additionally, laser-induced fluorescence was used to measure the velocity of sputtered boron along the thruster axis, which were the first measurements of its kind. The measured boron number density and velocity component together provided a total boron flux from the thruster, and therefore, a channel erosion rate. Channel erosion rates of the NASA HiVHAc and the SPT-70 Hall thrusters were measured using CRDS. Absolute erosion rates and trends with operating condition were investigated. Both thrusters were found to erode at rates proportional to the discharge power, which is consistent with the available literature. Profilometry was also used to measure the channel erosion rate of the SPT-70 thruster and revealed a factor of ~5 disagreement with estimates made by CRDS. Calcium fluoride (CaF2) prism retroreflectors were developed, for the first time, as a means to improve both the bandwidth and finesse of optical cavities in the ultraviolet region. The CRDS technique used in thruster erosion measurements employed multilayer dielectric mirrors, which have relatively poor performance in the ultraviolet region. Calcium fluoride prism retroreflectors show promise to outperform the best available dieletric mirrors at 250 nm as well as provide broadband cavity operation. The design, construction, and characterization of the CaF2 prisms is presented.Item Open Access Studies of U6+ with the RESIS method: difficulties and future directions(Colorado State University. Libraries, 2015) Smith, Christopher Scott, author; Lundeen, Stephen R., advisor; Bartels, Randy A., committee member; Roberts, Jacob L., committee member; Wu, Mingzhong, committee memberThis dissertation analyzes the details of previous resonant excitation Stark ionization spectroscopy (RESIS) measurements carried out on U5+ Rydberg states, aiming to determine properties of the ground state of U6+. These measurements were unsuccessful for apparently two reasons, a large background, and unexpectedly small signal sizes. It has been concluded that the background is a result of a prodigious amount of metastable states within the initial U6+ ion beam. Detailed simulations of the metastable population within the beamline showed the metastable hypothesis is plausible. Some reduction of the background within the RESIS technique was achieved with a redesign of the detection region in the apparatus. Detailed simulations of the RESIS signal size showed that the experimental resolved signals of the U5+ Rydberg states were no more than 1/4 the size expected from just ground state signal ions. A satisfactory explanation was proposed that metastable Rydberg ions bound to the lowest metastable level (J = 1) are forbidden to autoionize and can contribute to the measured signal. These metastable states compose 75% of the measured signal, diluting the ground state signal and preventing identification of the resolved signals. A possible future approach to the U5+ Rydberg state experiment is proposed that probes the Rydberg electron energies with a 1.5 μm laser, and looks like it could successfully measure the properties of the U6+ ion.Item Open Access Volumetric creation of ultra-high-energy-density plasma by irradiation of ordered nanowire arrays(Colorado State University. Libraries, 2016) Bargsten, Clayton, author; Rocca, Jorge J., advisor; Marconi, Mario C., committee member; Roberts, Jacob L., committee memberCreating appreciable volumes of Ultra-High-Energy Density (UHED) matter in the laboratory is a challenge. Recent developments in the fabrication of vertically aligned nanowire array targets, in coordination with ultra-high-contrast femtosecond laser pulses focused to relativistic intensity, have opened the door to creating UHED matter using compact laser facilities with laser pulses of ~ 0.6 J. These high aspect ratio, vertically aligned nanostructure targets allow the laser energy to penetrate deep into the near-solid density material and heat plasmas to keV temperatures, generating Gbar pressures that are only surpassed in the laboratory by the central hot-spot of highly compressed thermonuclear fusion plasmas. The depth of the heated volume is key in governing the properties of these new UHED plasmas, and is reported here for the first time in vertically aligned nanowire arrays using a buried-tracer technique. In this study, arrays of 55 nm diameter nanowires, manufactured with a variable length segment of nickel on top of a buried cobalt segment, were irradiated with relativistic femtosecond laser pulses of (4±1) x 1019 W cm-2 intensity. Buried Co atoms are observed to ionize to the He-like state for depths greater than 4 μm, in good agreement with particle-in-cell simulations. The measured heat penetration demonstrates that the UHED plasma regime can be accessed with small high repetition rate lasers.