Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Insulating Materials for an Extreme Environment in a Supersonically Rotating Fusion Plasma(2024) Schwartz, Nick Raoul; Koeth, Timothy W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fusion energy has long been sought as the “holy grail” of energy sources. One of the most critical remaining challenges in fusion is that of plasma-facing materials, even denoted by the National Academies of Science. The materials challenge is particularly acute for centrifugal mirrors, an alternative concept to the industry-standard tokamak that may offer a more efficient scheme with a faster path to development. The centrifugal mirror incorporates supersonic rotation into a conventional magnetic mirror scheme, providing three primary benefits: (1) increased confinement, (2) suppression of instabilities, and (3) plasma heating through shear flow. However, this rotation, which is driven by an axial magnetic field and a radial electric field, requires the magnetic field lines to terminate on electrically insulating surfaces to avoid “shorting” the plasma. This unique requirement presents a novel materials challenge: the insulator must not only resist irradiation and thermal damage, but also be an excellent electrical insulator and thermal conductor that can be actively cooled. To address this materials challenge, the Centrifugal Mirror Fusion Experiment (CMFX) was developed at the University of Maryland. CMFX serves as a test bed for electrically insulating materials in a fusion environment, as well as a proof-of-concept for the centrifugal mirror scheme. To guide the design of future power plants and better understand the neutronand ion flux on the insulators, a zero-dimensional (0-D) scoping tool, called MCTrans++, was developed. This software, discussed in Chapter 2, demonstrates the ability to rapidly model experimental parameter sets in CMFX and predict the scaling to larger devices, informing material selection and design. Assuming the engineering challenges have been met, the centrifugal mirror has been demonstrated as a promising scheme for electricity production via fusion energy. One of the key aspects to the operation of CMFX is the high voltage system. This system, discussed in Chapter 3, was developed in incremental stages, beginning with a 20 kV, then 50 kV pulsed power configuration, and finally culminating in a 100 kV direct current power supply to drive rotation at much higher voltages, creating an extreme environment for materials testing. This work identified hexagonal boron nitride (hBN) as a promising insulator material. Computational modeling (Chapter 4) demonstrated hBN’s superior resistance to ion-irradiation damage compared to other plasma-facing materials. Additionally, fusion neutrons are crucial for assessing both material damage and power output. Chapter 5 details the neutronics for CMFX, including 3He proportional counters, which have been installed on CMFX to measure neutron production. In parallel, Monte Carlo computational methods were used to predict neutron transport and material damage in the experiment. Ultimately, a materials test stand was installed on CMFX to expose electrically insulating materials to high energy fusion plasmas (Chapter 6). Comparative analysis of hBN and silicon carbide after exposure revealed superior performance of hBN as a plasma-facing material. Two primary erosion mechanisms were identified by surface morphology and roughness measurements: grain ejection and sputtering, both more pronounced in silicon carbide. This work advances our understanding of insulating material behavior in fusion environments and paves the way for the development of the next-generation centrifugal mirror fusion reactors. Chapter 7 discusses conclusions and proposes future work. In particular this section suggests some changes that may allow CMFX to operate at much higher voltages, unlocking higher plasma density and temperature regimes for further material testing.Item Simulation and Optimization of the Continuous Electrode Inertial Electrostatic Confinement Fusor(2017) Chap, Andrew Mark; Sedwick, Raymond J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A concept for generating nuclear fusion power and converting the kinetic energy of aneutronic fusion products into electric energy is proposed, and simulations are developed to design and evaluate this concept. The presented concept is a spherical fusor consisting of linear ion acceleration channels that intersect in the sphere center, where the converging ions form a high-energy, high-density fusion core. The geometry is that of a truncated icosahedron, with each face corresponding to one end of an ion beam channel. Walls between the channels span radially from the outer fusion fuel ionization source to an inner radius delimiting the fusion core region. Voltage control is imposed along these walls to accelerate and focus the recirculating ions. The net acceleration on each side of the channel is in the direction of the center, so that the ions recirculate along the channel paths. Permanent magnets with radial polarization inside the walls help to further constrain the ion beams while also magnetizing electrons for the purpose of neutralizing the fusion core region. The natural modulation of the ion beams along with a proposed phase-locked active voltage control results in the coalescence of the ions into ``bunches'', and thus the device operates in a pulsed mode. The use of proton-boron-11 (p-11B) fuel is studied due to its terrestrial abundance and the high portion of its energy output that is in the form of charged particles. The direct energy converter section envelopes the entire fusion device, so that each fusion fuel channel extends outward into a fusion product deceleration region. Because the fusion device operates in a pulsed mode, the fusion products will enter the energy conversion region in a pulsed manner, which is ideal for deceleration using a standing-wave direct energy converter. The charged fusion products pass through a series of mostly-transparent electrodes that are connected to one another in an oscillating circuit, timed so that the charged fusion products continuously experience an electric field opposite to the direction of their velocity. In this way the kinetic energy of the fusion products is transferred into the resonant circuit, which may then be connected to a resistive load to provide alternating-current energy at the frequency of the pulsed ion beams. Preliminary calculations show that a one-meter fusor of the proposed design would not be able to achieve the density required for a competitive power output due to limits imposed by Coulomb collisions and space charge. Scaling laws suggest that a smaller fusor could circumvent these limitations and achieve a reasonable power output per unit volume. However, ion loss mechanisms, though mitigated by fusor design, scale unfavorably with decreasing size. Therefore, highly effective methods for mitigation of ion losses are necessary. This research seeks to evaluate the effectiveness of the proposed methods through simulation and optimization. A two-dimensional axisymmetric particle-in-cell ion-only simulation was developed and parallelized for execution on a graphics processing unit. With fast computation times, this simulation serves as a test bed for investigating long-timescale thermalization effects as well as providing a performance output as a cost function for optimization of the electrode positions and voltage control. An N-body ion-only simulation was developed for a fully 3D investigation of the ion dynamics in an purely electrostatic device. This simulation uses the individual time-step method, borrowed from astrophysical simulations, to accurately model close encounters between particles by slowing down the time-step only for those particles undergoing sudden high acceleration. A two-dimensional hybrid simulation that treats electrons as a fluid and ions as particles was developed to investigate the effect of ions on an electrostatically and magnetically confined electron population. Electrons are solved for at each time-step using a steady-state iterative solver. A one-dimensional semi-analytic simulation of the direct energy conversion section was developed to optimize electrode spacing to maximize energy conversion efficiency. A two-dimensional axisymmetric particle-in-cell simulation coupled with a resonant circuit simulation was developed for modeling the direct energy conversion of fusion products into electric energy. In addition to the aforementioned simulations, a significant contribution of this thesis is the creation of a new model for simulating Coulomb collisions in a non-thermal plasma that is necessary to account for both the low-angle scattering that leads to thermalization as well as high-angle scattering that leads to ion departure from beam paths, and includes the continuous transition between these two scattering modes. The current implementation has proven problematic with regard to achieving sufficiently high core densities for fusion power generation. Major modifications of the current approach to address the space charge issues, both with regard to the electron core population and the ion population outside of the core would be necessary.Item Microturbulent transport of non-Maxwellian alpha particles(2015) Wilkie, George John; Dorland, William; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A burning Deuterium-Tritium plasma is one which depends upon fusion-produced alpha particles for self-heating. Whether a plasma can reach a burning state requires knowledge of the transport of alpha particles, and turbulence is one such source of transport. The alpha particle distribution in collisional equilibrium forms a non-Maxwellian tail which spans orders of magnitude in energy, and it is this tail that is responsible for heating the plasma. Newly-born high-energy alpha particles are not expected to respond to turbulence as strongly as alpha particles that have slowed down to the bulk plasma temperature. This dissertation presents a low-collisionality derivation of gyrokinetics relevant for alpha particles and describes the upgrades made to the GS2 flux-tube code to handle general non-Maxwellian energy distributions. With the ability to run self-consistent simulations with a population of alpha particles, we can examine certain assumptions commonly made about alpha particles in the context of microturbulence. It is found that microturbulence can compete with collisional slowing-down, altering the distribution further. One assumption that holds well in electrostatic turbulence is the trace approximation, which is built upon to develop a model for efficiently calculating the coupled radial-energy turbulent transport of a non-Maxwellian species. A new code was written for this purpose and corrections to the global alpha particle heating profile due to microturbulence in an ITER-like scenario are presented along with sensitivity studies.