This dissertation addresses the general problem of transport in chaotic systems. Typical fluid problem of the kind is the advection and diffusion of a passive scalar. The magnetic field evolution in a chaotic conducting media is an example of the chaotic transport of a vector field. In kinetic theory, the collisional relaxation of a distribution function in phase space is also an advection-diffusion problem, but in a higher dimensional space.;In a chaotic flow neighboring points tend to separate exponentially in time, exp({dollar}\omega t{dollar}) with {dollar}\omega{dollar} the Liapunov exponent. The characteristic parameter for the transport of a scalar in a chaotic flow is {dollar}\Omega\ \equiv\ \omega L\sp2/D{dollar} where L is the spatial scale and D is the diffusivity. For {dollar}\Omega\ \gg\ 1{dollar}, the scalar is advected with the flow for a time {dollar}t\sb{lcub}a{rcub}\ \equiv{dollar} ln(2{dollar}\Omega{dollar})/2{dollar}\omega{dollar} and then diffuses during the relatively short period 1/{dollar}\omega{dollar} centered on the time {dollar}t\sb{lcub}a{rcub}{dollar}. This rapid diffusion occurs only along the field line of the {dollar}\rm \ s\sb\infty{dollar} vector, which defines the stable direction for neighboring streamlines to converge. Diffusion is impeded at the sharp bends of an {dollar}\rm \ s{dollar} line because of a peculiarly small finite time Lyapunov exponent, hence a class of diffusion barriers is created inside a chaotic sea. This result comes from a fundamental relationship between the finite time Lyapunov exponent and the geometry of the {dollar}\rm \ s{dollar} lines, which we rigorously show in 2D and numerically validated for 3D flows.;The evolution of a general 3D magnetic field in a highly conducting chaotic media is also related to the spatial-temporal dependence of the finite time Lyapunov exponent. The Ohmic dissipation in a chaotic plasma will become a dominate process despite a small plasma resistivity. We show that the Ohmic heating in a chaotic plasma occurs in current filaments or current sheets. The particular form is determined by the time dependence of spatial gradient of the finite time Lyapunov exponent along a direction in which neighboring point neither diverge nor converge.

Quantum Cascade Lasers are a novel semiconductor light source with the unique property of wavelength tunability over the mid-infrared and terahertz range of frequencies. Advances since their first demonstration in 1994 have led to highly efficient designs capable of continuous room temperature operation. In lieu of increased advances in laser core efficiency, power scaling with broad area quantum cascade lasers has demonstrated enhanced continuous power. This initial work is used as a starting point for continuing advances in average brightness of quantum cascade lasers. A figure of merit calculation reliably predicts to within parts in thousands the qualitative beam profile of continuously driven and high duty cycle devices. Further, a model is developed to project performance not only in continuously driven conditions, but also in variable duty cycles. This is combined with the figure of merit calculation to guide designs for optimized average brightness.

A Hamiltonian treatment for the motion of a charged particle in a toroidal magnetic field is given. Assuming the plasma equilibrium, Boozer coordinates are used. The Hamiltonian of the exact trajectory is given in the guiding center coordinates. The higher order corrections to the standard drift Hamiltonian are derived. It is shown that the exact Hamiltonian depends on both the field strength and the shape of the magnetic surfaces (the metric of Boozer coordinates) while the standard drift Hamiltonian depends only on the field strength. The first order correction to the standard drift Hamiltonian, in gyroradius to system size, depends in a generic way on the shape of the magnetic surface, as does the exact Hamiltonian. Numerical calculations are done to compare the trajectories of the exact Hamiltonian, the standard drift Hamiltonian and the higher order drift Hamiltonian for a quasihelical symmetric vacuum field. The numerical results show that the difference in phase space structure between the exact and standard drift Hamiltonian can be predicted by the drift Hamiltonian with the first order correction.

Thesis advisor: Ilija Zeljkovic / The discovery of topological phases has ushered in an era of new materials with exotic electronicproperties; one particular area of excitement is realizing and studying topologically superconducting systems. These topological superconductors are theorized to host exotic excitations that can be applied towards making fault tolerant quantum computations. One way to achieve this is depositing thin films of topological insulators onto superconducting substrates. Molecular beam epitaxy offers precise control for fabricating thin film heterostructures down to the single layer limits. In this thesis I will present my work on the synthesis of thin film topological insulators grown epitaxially on both an iron based superconductor FeT e0.55Se0.45 as well as a cuprate superconductor Bi2Sr2CaCu2Ox+8. Additionally I will cover the scanning tunneling microscopy/spectroscopy characterization of the emergent phenomena on the surface as well as at the interface of these heterostructures. This work presents a viable platform for exploring the emergence of superconductivity in topologically insulating materials, as well as demonstrates the importance of a clean interface. / Thesis (PhD) — Boston College, 2022. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

The H-mode plasma edge is a region of steep gradients in density and temperature known as the “pedestal” which greatly increases energy confinement. The complex links between neutral-plasma interactions and both diffusive and convective transport in the pedestal must be understood to model, predict, and achieve the high performance required for a fusion power plant. This dissertation explores the effects of different hydrogenic isotope neutral particles and plasma transport from the edge pedestal region into the Scrape-Off Layer. Current experiments typically use deuterium (H with amu=2 or D), however future fusion power plants may startup with hydrogen (H), and eventually burn a mixture of deuterium and tritium (H with amu=3 or T). As isotope mass increases, energy confinement also increases in direct contradiction to the predicted scaling from diffusive theory, a phenomenon known as the “isotope effect”. Using a database of H and D pedestals on the DIII-D tokamak, we show indications that both species pedestals are in electron-dominant turbulence regimes, where increased outwards convection for hydrogen contributes to lower electron density pedestals consistent with gyrokinetic theory. However, this is complicated by indications of fueling differences between the isotopes. To isolate the effect of fueling, we conducted an experiment on DIII-D to match electron density (ne) and temperature (Te) profiles between hydrogen and deuterium, where neutral particle ionization profiles were measured with the newly available Lyman-α LLAMA diagnostic. On the High Field Side (HFS), neutral penetration lengths (λn0 ) across the pedestal are 40% longer for hydrogen compared to deuterium, consistent with the thermal velocity ratio between the isotopes (v_{H_{th}}/v_{D_{th}} = √2), while measurements on the Low Field Side (LFS) suggest a transport effect. The influence of neutral penetration on electron density pedestal width (∆ne) diminishes with increasing opaqueness (η ≡ ∆ne /λn0 ), shown by low opaqueness pedestals (η ∼ 0.9) in hydrogen being ∼ 40% wider than their deuterium counterparts. Higher opaqueness η ∼ 2 more effectively shields the pedestal from neutrals, as indicated by hydrogen and deuterium having nearly identical ∆ne . Moving across the separatrix into the Scrape-Off Layer, we conducted a diagnostic validation exercise using the ELZAR analysis method with stereoscopic fast cameras on MAST to detect field-aligned turbulent transport events known as “filaments”. Comparisons between the cameras show similar data in individual frames, inversion maps, and statistical distributions of located blobs. The two most important parameters for the framework connecting the statistics to SOL steady-state profiles are the lognormal distributions for filament radial position and major axis width which are within error between the cameras. The remaining distributions are unimportant to the statistical framework and we systematically attribute their differences to the experiment setup.

Understanding edge neutral dynamics in high-field tokamaks has strong consequencesfor both fueling and plasma profile predictions. We validate the ability of SOLPS-ITER, a 2D fluid plasma/kinetic Monte Carlo neutral code, to accurately model the upstream neutral density profiles of L-mode, I-mode, and H-mode discharges in the Alcator CMod tokamak, for which Lyman-alpha emission measurements were available. We achieve simulated Lyman-alpha emission and neutral density profiles that are within one standard deviation of empirically inferred profiles for all three discharges, via iterative tuning of the perpendicular transport coefficient profiles alone, providing confidence in the conclusion that while further physics (drifts, impurities, etc.) may be important in certain cases, the baseline SOLPS neutral model sufficiently simulates neutral distributions in the upstream edge region. We then use SOLPS to show that electron density pedestal structure is mostly unaffected by increased edge fueling when approaching fusion-relevant opaqueness conditions. Simulation results also suggest that upstream fueling dominates over X-point fueling, an important consideration for future fusion devices operating in high density, high neutral opacity regimes. In order to assess the relative role of fueling versus transport at the plasma edge, an experimental basis is formed from a set of similar high density (ne,PED 2x1020 m-3) EDA H-mode discharges from Alcator C-Mod. Gas puffs of varying magnitude are applied to these discharges to probe the response of the pedestal to increased edge fueling. Analysis of the simulated neutral e-folding length around the plasma reveals that in high opacity conditions, neutrals tend to become trapped in the PFR, confirming our assertion that edge fueling mainly occurs through the upstream midplane.

The work presented in this dissertation is in the subject area of radiation chemistry of polymeric materials and begins with a literature review of that subject. Along with the general literature review, three smaller and more specific reviews are presented for background information relevant to the three systematic studies addressed in the authors research. Following the literature review, these three studies involving the electron beam processing and subsequent characterization of various polymeric materials are presented. The first study involves the generation of cellular materials by electron beam irradiation. A technique was developed for generating a cellular structure in a radiation curable composition prior to irradiation, extruding the composition, and then curing the froth in the electron beam accelerator, thus locking into place the cellular structure. A systematic study of different monomer/oligomer systems was conducted with mono-, di-, and triacrylate model systems utilized for comparative purposes. The second study is a follow up investigation of previous work in the area of physical aging of polymers. A unique phenomena was discovered in which a physically aged polymer displaying a typical excess enthalpic aging peak by DSC was observed to deage when exposed to electron beam irradiation. The goal of the presented work was to investigate what, if any, volumetric changes were occurring in these materials and to further explain the observed phenomena. The third study involves the irradiation of high density polyethylene films having simple and well-defined stacked lamellar morphology without a distinct presence of row-nucleated fibril structures. These materials have been utilized to carry out investigations into the effects of radiation on their solid state structure-property relationships. These materials were characterized (as a function of radiation dose) with respect to the following areas with the utilized characterization technique(s) following: tensile behavior (Instron), dynamic mechanical behavior (DMS), melting behavior (DSC), lamella thickness (SAXS, TEM, and HSEM), orientation (WAXS) and microporous membrane formation (HSEM). / Ph. D.

With the development of lightweight concrete and design optimizations, floor vibration problems are becoming a serious serviceability problem. The castellated beam is a prime example and was the focus of this study. The vibration and flexural strength performance were verified in this paper. The vibration characteristics of castellated beams were examined using experimental and analytical test methods. The effective moment of inertia is essential to accurately predict the frequency and deflection of a floor system due to human occupancy. Since castellated beams have non-prismatic cross-sections, their effective moment of inertia is an uncertainty and was verified in this study. This paper confirmed the accuracy of the AISC Design Guide procedures used in for prismatic beam, when applied to castellated beams. The flexural strength of various composite castellated beam were studied. Three full-scale specimens were tested to failure to evaluate their yield and maximum applied load. Each specimen's moment strength was verified based on span, beam properties, concrete slab, and amount of shear connection. / Master of Science

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ITC/USA 2010 Conference Proceedings / The Forty-Sixth Annual International Telemetering Conference and Technical Exhibition / October 25-28, 2010 / Town and Country Resort & Convention Center, San Diego, California / Direct spatial antenna modulation (DSAM) is a new approach to phased array control that opens up new "smart antenna" architecture possibilities. The DSAM technique leverages the inherent spatial differences of excitation in an antenna in a novel way to achieve the equivalent of conventional modulation and beam control effects. Smart antenna techniques are of potentially increasing importance to test range operations given a trend toward more flexible, internetworked, and autonomous test activities. The DSAM technique has been demonstrated through several generations of analysis, simulation, and prototyping, but has previously only been applied to narrowband antenna designs. Furthermore, the IQ DSAM approach in particular has not been previously implemented in hardware. This paper details the application of IQ DSAM to achieve wideband phase control using a commercial off the shelf (COTS) antenna. The phase control performance of IQ DSAM over a range of 1.5 GHz to 4 GHz is measured across relative field control angles of +/- 45 degrees. The measured IQ DSAM performance is compared to what could be expected from a conventional phased array element control architecture.