Physics Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2800

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    NONLINEAR PROPAGATION OF ORBITAL ANGULAR MOMENTUM LIGHT IN TURBULENCE AND FIBER
    (2024) Elder, Henry; Sprangle, Phillip; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Light that carries orbital angular momentum (OAM), also referred to as optical vortices or twisted light, is characterized by a helical or twisted wavefront ∝exp[imφ]. In contrast to spin angular momentum (SAM), where photons are limited to two states, OAM allows for, in principle, an infinite set of spatially orthogonal states. OAM-carrying light has found applications ranging from quantum key distribution in free space and guided-wave communication systems, particle trapping and optical tweezers, nanoscopy, and remote sensing. Understanding how OAM light propagates through complex environments, and how to efficiently generate particular OAM states, is critical for any such application. In the first part of this dissertation, we describe how OAM light propagates through a turbulent atmosphere. We build analytic models which describe (1) the OAM mode mixing caused by turbulence, (2) the evolution of short, high-power OAM pulses undergoing the effects of self-phase modulation (SPM) and group velocity dispersion (GVD), and (3) the evolution of high-power Gaussian pulses including SPM, GVD, and turbulence. The models are validated against both experimental data and nonlinear, turbulent pulse propagation simulation programs, the latter of which we have made freely available. We also explore how self-focusing can minimize certain deleterious effects of turbulence for OAM light. The second part of this dissertation considers nonlinear effects of OAM light propagating in azimuthally symmetric waveguides. Such waveguides have so-called spin-orbit (SO) modes, which are quantized based on their total angular momentum (TAM). We develop a generalized theory of four wave mixing-based parametric amplification of SO modes and show that these processes conserve TAM, but under certain circumstances can be taken to conserve SAM and OAM independently. Our theory is validated against a nonlinear multimode beam propagation simulation program which we developed and, again, have made freely available.
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    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.
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    Ionospheric Turbulence Near the Upper Hybrid Layer: Theory and Experiment
    (2016) Najmi, Amir Christopher; Papadopoulos, Konstantinos; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The thesis presents experimental results, simulations, and theory on turbulence excited in magnetized plasmas near the ionosphere’s upper hybrid layer. The results include: The first experimental observations of super small striations (SSS) excited by the High-Frequency Auroral Research Project (HAARP) The first detection of high-frequency (HF) waves from the HAARP transmitter over a distance of 16x10^3 km The first simulations indicating that upper hybrid (UH) turbulence excites electron Bernstein waves associated with all nearby gyroharmonics Simulation results that indicate that the resulting bulk electron heating near the upper hybrid (UH) resonance is caused primarily by electron Bernstein waves parametrically excited near the first gyroharmonic. On the experimental side we present two sets of experiments performed at the HAARP heating facility in Alaska. In the first set of experiments, we present the first detection of super-small (cm scale) striations (SSS) at the HAARP facility. We detected density structures smaller than 30 cm for the first time through a combination of satellite and ground based measurements. In the second set of experiments, we present the results of a novel diagnostic implemented by the Ukrainian Antarctic Station (UAS) in Verdansky. The technique allowed the detection of the HAARP signal at a distance of nearly 16 Mm, and established that the HAARP signal was injected into the ionospheric waveguide by direct scattering off of dekameter-scale density structures induced by the heater. On the theoretical side, we present results of Vlasov simulations near the upper hybrid layer. These results are consistent with the bulk heating required by previous work on the theory of the formation of descending artificial ionospheric layers (DIALs), and with the new observations of DIALs at HAARP’s upgraded effective radiated power (ERP). The simulations that frequency sweeps, and demonstrate that the heating changes from a bulk heating between gyroharmonics, to a tail acceleration as the pump frequency is swept through the fourth gyroharmonic. These simulations are in good agreement with experiments. We also incorporate test particle simulations that isolate the effects of specific wave modes on heating, and we find important contributions from both electron Bernstein waves and upper hybrid waves, the former of which have not yet been detected by experiments, and have not been previously explored as a driver of heating. In presenting these results, we analyzed data from HAARP diagnostics and assisted in planning the second round of experiments. We integrated the data into a picture of experiments that demonstrated the detection of SSS, hysteresis effects in simulated electromagnetic emission (SEE) features, and the direct scattering of the HF pump into the ionospheric waveguide. We performed simulations and analyzed simulation data to build the understanding of collisionless heating near the upper hybrid layer, and we used these simulations to show that bulk electron heating at the upper hybrid layer is possible, which is required by current theories of DAIL formation. We wrote a test particle simulation to isolate the effects of electron Bernstein waves and upper hybrid layers on collisionless heating, and integrated this code to work with both the output of Vlasov simulations and the input for simulations of DAIL formation.
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    Phase Mixing in Turbulent Magnetized Plasmas
    (2014) Kanekar, Anjor Vishwas; Dorland, William; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Landau damping (phase mixing) is perhaps the most salient feature of weakly collisional plasmas. Phase mixing plays a crucial role in kinetic plasma turbulence-- it transfers energy to small velocity space scales, and provides a route to dissipation to the turbulent cascade. Phase mixing has been well understood in the linear limit for nearly seventy years, however, we do not yet fully understand the behavior of phase mixing in presence of a fluid-like turbulent cascade--a common scenario in weakly collisional systems. In this thesis, we consider simple models for kinetic passive scalar turbulence that simultaneously incorporate phase mixing and turbulent cascade, in order to study the effects of turbulence on phase mixing. We show that the nonlinear cascade scatters energy in the phase space so as to generate a turbulent version of the plasma echo. We find that this stochastic plasma echo suppresses phase mixing by reducing the net flux to small velocity space scales. Further, we study the problem of compressive fluctuations in the solar wind at scales larger than the ion Larmor radius (the so-called inertial range). The compressive perturbations at these scales are passively mixed by the Alfvenic turbulence. Hence, the general results regarding kinetic passive scalar turbulence are directly applicable to this problem. We find that the suppression of phase mixing by the stochastic plasma echo is key to the persistence of the turbulent cascade of compressive fluctuations at scales where these fluctuations are expected to be strongly damped. A new code, Gandalf was developed for the GPU architecture using the CUDA platform in order to study these systems, in particular to study solar wind turbulence in the inertial range.
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    A NEW OPTICAL TECHNIQUE FOR PROBING CLEAR AIR TURBULENCE
    (2010) Harris, Joseph Daniel; Davis, Christopher C.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A new optical technique for probing the small scales turbulence has been developed. When light is transmitted through the atmosphere, it can scatter off vortex filaments in the air that are at different densities from the surrounding air, and hence, have different indices of refraction. These filaments, or eddies are distributed through a turbulent flow. Our experiment illuminated a turbulent flow with an expanded Gaussian laser beam. Two detectors, capable of translation perpendicular to the beam path, observed intensity fluctuations at different points. By analysis of two point spatial transmission correlation functions, the smallest length scales of clear air turbulence can be determined in real time without disturbing the flow. By changing the type of air flow, different length scales associated with different conditions have been measured optically. The measured scales agree with measurements done by hotwire techniques and correspond to the Kolmogorov microscale.
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    Non-linear Development of Streaming Instabilities in Magnetic Reconnection with a Strong Guide Field
    (2009) Che, Haihong; Drake, James F.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Magnetic reconnection is recognized as a dominant mechanism for converting magnetic energy into the convective and thermal energy of particles, and the driver of explosive events in nature and laboratory. Magnetic reconnection is often modeled using resistive magnetohydrodynamics, in which collisions play the key role in facilitating the release of energy in the explosive events. However, in space plasma the collisional resistivity is far below the required resistivity to explain the observed energy release rate. Turbulence is common in plasmas and the anomalous resistivity induced by the turbulence has been proposed as a mechanism for breaking the frozen-in condition in magnetic reconnection. Turbulence-driven resistivity has remained a poorly understood, but widely invoked mechanism for nearly 50 years. The goal of this project is to understand what role anomalous resistivity plays in fast magnetic reconnection. Turbulence has been observed in the intense current layers that develop during magnetic reconnection in the Earth's magnetosphere. Electron streaming is believed to be the source of this turbulence. Using kinetic theory and 3D particle-in-cell simulations, we study the nonlinear development of streaming instabilities in 3D magnetic reconnection with a strong guide field. Early in time an intense current sheet develops around the x-line and drives the Buneman instability. Electron holes, which are bipolar spatial localized electric field structures, form and then self-destruct creating a region of strong turbulence around the x-line. At late time turbulence with a characteristic frequency in the lower hybrid range also develops, leading to a very complex mix of interactions. The difficulty we face in this project is how to address a long-standing problem in nonlinear kinetic theory: how to treat large amplitude perturbations and the associated strong wave-particle interactions. In my thesis, I address this long-standing problem using particle-in-cell simulations and linear kinetic theory.Some important physics have been revealed. 1: The lower hybrid instability (LHI) dominates the dynamics in low $beta$ plasma in combination with either the electron-electron two-stream instability (ETS) or the Buneman instability (BI), depending on the parallel phase speed of the LHI. 2: An instability with a high phase speed is required to tap the energy of the high velocity electrons. The BI with its low phase speed, can not do this. The ETS and the LHI both have high phase speed. 3: The condition for the formation of stable electron holes requires $|v_p -v_g|< sqrt{2e|phi|/m_e}$, where $|phi|$ is the amplitude of the electric potential, and $v_p$ and $v_g$ are the phase and group velocity of the relevant waves. Like ETS and BI, LHI all can form electron holes. 4: The overlapping resonance in phase space is the dominant mechanism for transporting the momentum and energy from high velocity electrons to low velocity electrons, which then couple to the ions.
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    Advanced Lagrangian Simulation Algorithms for Magnetized Plasmas Turbulence
    (2008-08-05) Broemstrup, Ingmar; Dorland, William; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Nonlinear processes in hot, magnetized plasma are notoriously difficult to understand without the use of numerical simulations. In recent decades, first principles, kinetic simulations have been widely and successfully used to study plasma turbulence and reconnection in weakly collisional systems. In this thesis, extensions of well-known, Lagrangian, particle-in-cell (PIC) simulation algorithms for problems such as these are derived and implemented. The algorithms are tested for multiple species (electrons and ions, with the physical mass ratio) in non-trivial magnetic geometry (cylindrical/toroidal). The advances presented here address two major shortcomings of conventional gyrokinetic PIC algorithms, with demonstrated excellent performance on large, parallel supercomputers. Although the gyrokinetic formalism rigorously describes the evolution of fluctuations which are small compared to a typical Larmor radius, most existing algorithms use low-order approximations of the gyroaveraging operator, and cannot accurately describe small scale fluctuations. The gyroaveraging algorithm presented here accurately and uniquely treats a wide range of fluctuation scales, above and below the thermal gyroradius. The second shortcoming of traditional algorithms relates to the slow loss of accuracy that is associated with the build-up of noise. In this thesis, a PIC pitch-angle scattering collision operator is developed. This collision operator is physically motivated and controls the growth of noise without introducing non-physical dissipation. Basic tests of the new algorithms are presented in linear and nonlinear regimes, using one to thousands of processors simultaneously.