Theses and Dissertations from UMD
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Item MEASURING AND MODELING ELECTROMAGNETIC FORCES THAT INFLUENCE GRANULAR BEHAVIOR(2024) Pett, Charles Thomas; Hartzell, Christine M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)On the surfaces of small, airless planetary bodies, forces other than gravity, such as cohesive, magnetic and electrostatic forces, may dominate the behavior of regolith. Yet, the magnitude of these forces remains uncertain, as well as the link between grain-scale and bulk-scale physics. In this work, techniques for measuring and modeling electromagnetic forces that influence granular behavior are developed. We discuss an experimental method for measuring interparticle cohesion by breaking cohesive bonds between grains with electrostatic forces. The centroid positions of the lofted grains at the moment of detachment are imaged in order to numerically calculate initial accelerations to solve for cohesion. We propose the design of a payload that would be deployed on the Moon or an asteroid and use an electrically biased plate to induce electrostatic dust lofting and measure interparticle cohesion in situ. We would call the system \textbf{Small--FORCES} because it would be able to image \textbf{Small} \textbf{F}orces \textbf{O}ptically \textbf{R}esolved for \textbf{C}ohesion \textbf{E}stimation via \textbf{E}lectrostatic \textbf{S}eparation. We numerically integrate Poisson's equation and develop a model for the potential distribution of a photoelectron sheath as a function of distance from surfaces. We use this model to gauge the extent to which the solar wind will perturb the Small-FORCES electric field that is used to loft charged regolith inside the sheath and obtain suitable trajectories for imaging lofted regolith that will be used to measure cohesion. We then derive a formula to quantify the maximum region of our system's electric field that we predict can be shielded from the ambient solar wind, which depends on system dimensions and applied voltage. In another experiment, we investigated the affect of magnetic cohesion on the avalanching behavior of magnetic grains. We will introduce an instrument and novel method for characterizing the bulk magnetic susceptibility of granular mixtures by submerging an inductor coil in a bed of metallic beads. In prior works, the magnetic force on grains was calculated based on the magnetic susceptibility of a single grain, but our coil uniquely quantifies effects from void spaces and demagnetization in the bulk. Compared to both a commercial Terraplus Inc. KT-10 meter and theoretical approximations, we report similar trends in susceptibility values measured as a function of mass of ferromagnetic material per volume. We conclude the talk with a discussion on a conductive model we developed to simulate surfaces other than dielectrics in the solar wind. We use a 2D grid-free treecode to enable complex surface geometries that would be computationally intensive for traditional PIC codes. Instead of using the capacitance matrix method to calculate the induced surface charge magnitudes, we discretized the conductor surface into point charges and allow them to have Coulomb interactions with the external plasma particles. The linear system used to explicitly solve for the induced surface charge magnitudes couples the interaction between surface charges and plasma particles self-consistently via the conductive boundary condition. The model has been validated thus far with image charge theory.Item Proton Energization during Magnetic Reconnection in Macroscale Systems(2024) Yin, Zhiyu; Drake, James; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Magnetic reconnection is a widespread process in plasma physics that is crucial for the rapid release of magnetic energy and is believed to be a key factor in generating non-thermal particles in space and various astrophysical systems. In this dissertation, a set of equations are developed that extend the macroscale magnetic reconnection simulation model kglobal to include particle ions. The extension from earlier versions of kglobal, which included only particle electrons, requires the inclusion of the inertia of particle ions in the fluid momentum equation. The new equations will facilitate the exploration of the simultaneous non-thermal energization of ions and electrons during magnetic reconnection in macroscale systems. Numerical tests of the propagation of Alfvén waves and the growth of firehose modes in a plasma with anisotropic electron and ion pressure are presented to benchmark the new model. The results of simulations of magnetic reconnection accompanied by electron and proton heating and energization in a macroscale system are presented. Both species form extended powerlaw distributions that extend nearly three decades in energy. The primary drive mechanism for the production of these nonthermal particles is Fermi reflection within evolving and coalescing magnetic flux ropes. While the powerlaw indices of the two species are comparable, the protons overall gain more energy than electrons and their powerlaw extends to higher energy. The power laws roll into a hot thermal distribution at low energy with the transition energy occurring at lower energy for electrons compared with protons. A strong guide field diminishes the production of non-thermal particles by reducing the Fermi drive mechanism. In solar flares, proton power laws should extend down to 10's of keV, far below the energies that can be directly probed via gamma-ray emission. Thus, protons should carry much more of the released magnetic energy than expected from direct observations. In Encounter 14 (E14), the Parker Solar Probe encountered a reconnection event in the heliospheric current sheet (HCS) that revealed strong ion energization with power law distributions of protons extending to 500keV. Because the energetic particles were streaming sunward from an x-line that was anti-sunward of PSP, the reconnection source of the energetic ions was unambiguous. Using upstream parameters based on the data observed by PSP, we simulate the dynamics of reconnection applying kglobal and analyze the resulting spectra of energetic electrons and protons. Power law distributions extending nearly three decades in energy develop with proton energies extending to 500keV, consistent with observations. The significance of these results for particle energization in the HCS will be discussed.Item Signal Processing and Forward Modeling of Space Debris Detection via Plasma Solitons(2024) DesJardin, Ian; Hartzell, Christine M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The nonlinear interaction of objects in low Earth orbit with the space plasma environment has been hypothesized to cause precursor soliton plasma waves. These plasma-object interactions may lead to unique engineering applications, especially the detection of hazardous sub-centimeter orbital debris that is undetectable by conventional methods. This nonlinear perturbation is currently modeled by the forced Korteweg - de Vries (fKdV) equation. This thesis aims to understand and characterize these waves through simulation beyond the fKdV model while progressing space-based and ground detection schemes. Ultimately this technique may play an important role in the problem of detecting small space debris. Three aspects of this detection scheme are developed. This includes two unconventional methods of detecting solitons. First the inverse scattering transform (IST), a mathematical spectral technique for decomposing a time series, is shown to automatically detect solitons from data. A numerical experiment using the fKdV model is performed to demonstrate this ability. The IST is suitable as an in situ detection method. It could be the basis of a debris collision early warning system for spacecraft. Second, the existing technique of ionospheric sensing using Global Navigation Satellite System (GNSS) is extended to detecting spacecraft plasma wakes. Traditionally, it is used for global scale space weather monitoring. An experiment is carried out using a known target, the International Space Station, on existing GNSS receivers that measure the ionospheric irregularity associated with the spacecraft. This experiment shows that there is a modification to the total electron content (TEC) when the ISS flies through the radio line-of-sight. Using models that are compared to the experiment, a multi-point sensor is proposed that would resolve the diffraction pattern from these plasma structures. This work uses multi-fluid plasma simulation to refine the fKdV model of soliton generation from debris. In particular, we find that the range of ion acoustic Mach numbers that are conducive to precursor soliton generation is larger than predicted by the fKdV equation. A new theory that matches the multi-fluid simulation results is derived using pressure balances to predict the supercritical Mach number. This new theoretical understanding of the critical Mach numbers predicts a wider range of orbits that will create precursor solitons than in previous studies. In addition, several new details of precursor solitons are discovered and characterized with multi-fluid simulation. This includes changes in the amplitude scaling of the periodicity of soliton generation (the "intersoliton interval"). Importantly, corrections to the first order results of the fKdV equation which couple fluid velocity, density, and electrostatic potential are identified. A theory that explains this in the small amplitude limit is derived. For debris detection, this effect impacts how the soliton is detected. The same soliton will manifest different amplitudes in each plasma species, contrary to the result of the fKdV equation. Thus, a model error in inferring debris properties from solitons has been discovered.Item Radiative Plasmas in Pulsar Magnetospheres(2024) Chernoglazov, Alexander; Philippov, Alexander; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Pulsars are highly magnetized rotating neutron stars known for their periodic bursts of radio emission. Decades of astronomical observations revealed that pulsars produce non-thermal radiation in all energy bands, from radio to gamma rays, covering more than 20 decades in photon energy. Modern theories consider strongly magnetized relativistic electron-positron plasmas to be the source of the observed emission. In my Thesis, I investigate physical processes that can be responsible for plasma production and the observed high-energy emission in the wide range of photon energies, from eV to TeV. In the first Chapter of my Thesis, I investigate relativistic magnetic reconnection with strong synchrotron cooling using three-dimensional particle-in-cell kinetic plasma simulations. I characterize the spectrum of accelerated particles and emitted synchrotron photons for varying strengths of synchrotron cooling. I show that the cutoff energy of the synchrotron spectrum can significantly exceed the theoretical limit of 16 MeV if the plasma magnetization parameter exceeds the radiation reaction limit. Additionally, I demonstrate that a small fraction of ions present in the current sheet can be accelerated to the highest energies, making relativistic radiative reconnection a promising mechanism for the acceleration of high-energy cosmic rays. In the second Chapter, I present the first multi-dimensional simulations of the QED pair production discharge that occurs in the polar region of the neutron star. This process is believed to be the primary source of the pair plasma in pulsar magnetospheres and also the source of the radio emission. In this work, I focus on the self-consistently emerging synchronization of the discharges in different parts of the polar region. I find that pair discharges on neighboring magnetic field lines synchronize on a scale comparable to the height of the pair production region. I also demonstrate that the popular “spark” model of pair discharges is incompatible with the universally adopted force-free magnetospheric model: intermittent discharges fill the entire polar region that allows pair production, leaving no space for discharge-free regions. My findings disprove the key assumption of the spark model about the existence of distinct discharge columns. In the third Chapter, I demonstrate how the key findings of two previous chapters can provide a self-consistent explanation of the recently discovered very-high-energy, reaching 20TeV, pulsed emission in Vela pulsar. Motivated by the results of recent global simulations of pulsar magnetospheres, I propose that this radiation is produced in the magnetospheric current sheet undergoing radiative relativistic reconnection. I show that high-energy synchrotron photons emitted by reconnection-accelerated particles efficiently produce electron-positron pairs. The density of secondary pairs exceeds the supply from the polar cap and results in a self-regulated plasma magnetization parameter of $\sim 10^7$. Electrons and positrons accelerate to Lorentz factors comparable to $\sim 10^7$ and emit the observed GeV radiation via the synchrotron process and ~10 TeV photons by Compton scattering of the soft synchrotron photons emitted by secondary pairs. My model self-consistently accounts for the ratio of the gamma-ray and TeV luminosities.Item LOW TEMPERATURE PLASMA-METAL INTERACTIONS: PLASMA-CATALYSIS AND ELECTRON BEAM-INDUCED METAL ETCHING(2024) Li, Yudong; Oehrlein, Gottlieb G; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Low-temperature plasma can generate different types of chemically reactive species at gas temperatures far below what is required to form such species from thermal excitation. Interactions between these reactive plasma-generated species and material surfaces have great potential for various applications, such as semiconductor etching or gas conversion. Synergistic effects, where the production rate with two inputs is greater than the sum of the consequences of each individually, have been demonstrated by combining the plasma with other energy inputs such as heat or kinetic energy from ions or electrons. Understanding the mechanisms by which these species interact with relevant surfaces is vital for the future development of plasma processing, chemistry and physics. In this work, we focus on the interaction of long-lived plasma species, particularly neutrals, with metal. A remote plasma-surface configuration was applied, where the plasma itself does not directly contact the surface. Two examples of plasma-metal interactions will be discussed, one taking place at atmospheric and the other at low pressure. The first case is plasma-assisted catalytic oxidation of methane (CH4) using a nickel (Ni) catalyst at atmospheric pressure, implemented by combining a remote plasma jet. The interrelation of real-time measurements of reaction products and surface adsorbates and plasma diagnostics allowed the identification of atomic oxygen as the key plasma-generated species that drives the synergistic plasma-catalytic reaction. The in-situ characterizations of the surface and gas phase reactions reveal the possible key reaction pathways for the plasma-catalysis reactions. We also observed the activation of the catalyst resulting from long-lasting catalyst surface modification induced by plasma species interaction. The second case is the damage-free etching of refractory metals, ruthenium (Ru) and tantalum (Ta), at low pressure. This was implemented by combining a remote plasma source (RPS) with an electron beam (EB) source. We investigated the effects of CF4 and Cl2 additions to Ar/O2 RPS effluents and we find that Ar/O2 with Cl2 addition induces the highest Ru etch rate (ER) and best removal selectivity over Ta. The surface chemistry characterization by spatially-resolved XPS reveals the possible mechanism of the electrons and neutrals induced materials etching. We also proposed a model that considers the fundamental aspects of the etching reaction and successfully predicts the major features of the electron and neutral induced etching reactions.Item Optimization of high-beta fusion devices against linear instabilities(2023) Gaur, Rahul; Dorland, William; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Magnetic confinement fusion is a technique in which a strong magnetic field is used tocontain a hot plasma, which enables nuclear fusion. In terms of overall energy efficiency, the two most promising magnetic confinement concepts are tokamaks (axisymmetric devices) and stel- larators (nonaxisymmetric devices). The power P produced by a magnetically confined nuclear fusion device is proportional to Vβ2B4, where V is the volume of the device, β is the plasma pressure - magnetic pressure ratio, and B is the magnetic field strength. Most tokamaks and stellarators currently in operation are low-β devices. In general, there are three ways to increase P , one may increase the operating β, the magnetic field or the volume of the device. The cost of these devices is proportional to V , making large enough devices expensive. Similarly, a large magnetic field (>10T) requires superconducting magnets that, even after the recent innovations in HTS (High-Temperature Superconductors), are expensive to manufacture. High-β devices are an attractive idea to efficiently produce fusion energy. However, a high-β generally also implies a large gradient in plasma pressure that can be a source of numerous instabilities. If fusion devices could be optimized against such instabilities, high-β operation would become an attractive approach compared to high field or large-volume reactors. Therefore, this thesis explores the optimization of high-β tokamak and stellarator equilibrium equilibria against linear instabilities. We will start by investigating the stability of high-β tokamaks and stellarator equilibria against the infinite-n ideal ballooning mode, an important pressure-driven MHD instability. We stabilize these equilibria against the ideal ballooning mode. To achieve this, we formulate a gradient-based adjoint technique and demonstrate its speed and effectiveness by stabilizing these equilibria. We also explain how this technique can be easily extended to low-n ideal-MHD modes in both tokamaks and stellarators. After demonstrating the adjoint technique for stabilizing against ideal MHD modes, wefirst analyze the kinetic stability of a sequence of axisymmetric equilibria. We study this by nu- merically solving the δf gyrokinetic model, a simplified version of the Vlasov-Maxwell model. Since these kinetic instabilities are driven by temperature and density gradients, we explore them by scanning multiple values of the plasma β, temperature and density gradients, and plasma boundary shapes, discovering interesting relationships between equilibrium-dependent quantities and growth rates of these instabilities. We then repeat the same process for two recently pub- lished stellarator equilibria with quasisymmetry — a favorable hidden symmetry in stellarators. With this study, we verify that our observations from high-β tokamaks can be generalized to quasisymmetric stellarators. From our microstability study, we find that electromagnetic effects are important for high-βdevices. Hence, using the numerical tools and knowledge derived from the previous chapters we build an optimization framework that searches for stable equilibria. Due to the similarity between axisymmetry and quasisymmetry, we then use the microstability optimizer to search for ideally and kinetically-stable, quasisymmetric, high-β stellarators.Item Increasing Helicity towards Dynamo Action with Rough Boundary Spherical Couette Flows(2022) Rojas, Ruben; Lathrop, Daniel P; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The dynamo action is the process through which a magnetic field is amplified and sustained by electrically conductive flows. Galaxies, stars and planets, all exhibit magnetic field amplification by their conductive constituents. For the Earth in particular, the magnetic field is generated due to flows of conductive material in its outer core. At the University of Maryland, our Three-meter diameter spherical Couette experiment uses liquid sodium between concentric spheres to mimic some of these dynamics, giving insight into these natural phenomena. Numerical studies of Finke and Tilgner (Phys. Rev. E, 86:016310, 2012) suggest a reduction in the threshold for dynamo action when a rough inner sphere was modeled by increasing the poloidal flows with respect to the zonal flows and hence increasing helicity. The baffles change the nature of the boundary layer from a shear dominated to a pressure dominated one, having effects on the angular momentum injection. We present results on a hydrodynamics model of 40-cm diameter spherical Couette flow filled with water, where torque and velocimetry measurements were performed to test the effects of different baffle configurations. The selected design was then installed in the 3-m experiment. In order to do that, the biggest liquid sodium draining operation in the history of the lab was executed. Twelve tons of liquid sodium were safely drained in a 2 hours operation. With the experiment assembled back and fully operational, we performed magnetic field amplification measurements as a function of the different experimental parameters including Reynolds and Rossby numbers. Thanks to recent studies in the hydrodynamic scale model, we can bring a better insight into these results. Torque limitations in the inner motor allowed us to inject only 4 times the available power; however, amplifications of more than 2 times the internal and external magnetic fields with respect to the no-baffle case was registered. These results, together with time-dependent analysis, suggest that a dynamo action is closer than before; showing the effect of the new baffles design in generating more efficient flows for magnetic field amplification. We are optimistic about new short-term measurement in new locations of the parameter space, and about the rich variety of unexplored dynamics that this novel experiment has the potential to reach. These setups constitute the first experimental explorations, in both hydrodynamics and magnetohydrodynamics, of rough boundary spherical Couette flows as laboratory candidates for successful Earth-like dynamo action.Item Mid-Infrared Laser Driven Avalanche Ionization and Low Frequency Radiation Generation(2022) Schwartz, Robert Max; Milchberg, Howard; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation, we discuss the applications of intense mid-infrared laser interactions in three main topics. First, we demonstrate and discuss the remote detection of radioactive materials using avalanche breakdowns driven by picosecond, mid-infrared laser pulses. In the presence of radioactive materials, an enhanced population of free electrons and weakly bound ions are created in air. Laser driven avalanche ionization is a powerful tool for amplifying and detecting this weak signature, allowing for detection at standoff distances beyond the stopping distance of the radioactive particles. This technique can be applied more generally to the detection of any low density plasma. In the second section, we apply a similar method to measure laser ionization yields in atmospheric pressure gas across an extremely wide range. Finally, we demonstrate and discuss the generation of THz and low harmonics from two-color mid-infrared laser pulses. This technique allows for the generation of highly efficient, ultra-broadband coherent radiation.Item Laser Wakefield Acceleration in Optical Field Ionized Plasma Waveguides(2021) Feder, Linus; Milchberg, Howard; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Laser wakefield accelerators (LWFA) can support acceleration gradients orders of magnitude higher than conventional radio frequency linear accelerators. This gives them the potential to drive the next generation of accelerators for high energy physics, as well as compact accelerators for many other applications. However, in order to reach higher energies and improve electron beam quality, LWFA requires the development of plasma waveguides. This thesis demonstrates two new all optical techniques for the creation of plasma waveguides. The first, “two-Bessel” technique uses a ?0 Bessel beam to form the core of the waveguide and a higher order ?? Bessel beam to form the cladding. In the second, “self-waveguiding” technique, the guided beam itself forms the cladding of the waveguide. Preliminary electron acceleration results using the self-guiding technique, as well electron acceleration simulations are also presented.Item PLASMA-BASED ATOMIC SCALE ETCHING APPROACHES USING EITHER ION OR ELECTRON BEAM ACTIVATION(2022) Lin, Kang-Yi; Oehrlein, Gottlieb; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Plasma dry etching has been extensively employed in semiconductor manufacturing processes for anisotropic pattern transfer. With device miniaturization, the conventional approach utilizing continuous wave plasma etching does not meet the requirement for sub-nanometer processing nodes, including profile control and atomic-scale etching selectivity. Additionally, the direct plasma exposure of a substrate raises the concern of plasma damage and undesired material removal. We describe improvements of plasma-based etching techniques and identified novel ways for enabling material removal. We have systematically studied different precursor chemistries for atomic layer etching on etching selectivity of SiO2 to Si3N4 and SiO2 to Si and obtained an understanding of the surface chemistry evolution. Compared to the conventional approach that mixes fluorocarbon and hydrogen precursors, selected hydrofluorocarbon can deliver optimal plasma chemistry that produces a reduced F/C film in the deposition step and realizes atomic-scale etching selectivity. We also report a new approach for establishing etching selectivity of HfO2 over Si by integrating substrate-selective deposition into an atomic layer etching sequence. The optimal precursor chemistry can selectively deposit on the Si surface as a passivation layer and convert HfO2 to metal-organic compounds for desorption. Finally, we designed and built a system that consists of an electron flood gun and a remote plasma source to demonstrate the concept of a new etching approach by exploiting electron-neutral synergistic effects. This configuration achieves precisely controlled SiO2 or Si3N4 etching by co-introducing an electron beam and Ar/CF4/O2 remote plasma. This approach also addresses the issue of limited precursor chemistries in electron beam-induced etching.