Theses and Dissertations from UMD
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Item WAVES IN PLASMAS GENERATED BY A ROTATING MAGNETIC FIELD AND IMPLICATIONS TO RADIATION BELTS(2010) Karavaev, Alexey V.; Papadopoulos,, Konstantinos; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The interaction of rotating magnetic fields (RMF) with magnetized plasmas is a fundamental plasma physics problem with implications to a wide range of areas, including laboratory and space plasma physics. Despite the importance of the topic the basic physics of the phenomenon remains unexplored. An important application of a RMF is its potential use as an efficient radiation source of low frequency waves in space plasmas, including whistler and shear Alfven waves (SAW) for controlled remediation of energetic particles in the Earth's radiation belts. In this dissertation the RMF waves generated in magnetized plasma are studied using numerical simulations with a semi-analytical three-dimensional magnetohydrodynamic (MHD) model and experiments on the generation of whistler and magnetohydrodynamic waves conducted in UCLA's Large Plasma Device. Comparisons of the simulation results with the experimental measurements, namely, measured spatiotemporal wave structures, dispersion relation with finite transverse wave number, wave amplitude dependence on plasma and RMF source parameters, show good agreement in both the whistler and MHD wave regimes. In both the experiments and the 3D MHD simulations a RMF source was found to be very efficient in the generation of MHD and whistler waves with arbitrary polarizations. The RMF source drives significant field aligned plasma currents confined by the ambient magnetic field for both the whistler and MHD wave regimes, resulting in efficient transport of wave energy along the ambient magnetic field. The efficient transfer of the wave energy results in slow decay rates of the wave amplitude along the ambient magnetic field. The circular polarization of the waves generated by the RMF source, slow amplitude decay rate along the ambient magnetic field and nonzero transverse wave number determined by the RMF source size lead to nonlocal gradients of the wave magnetic field in the direction perpendicular to the ambient magnetic field. A RMF can be generated by a system of polyphase alternating currents or by a rotating permanent or superconducting magnet. For the magnetospheric plasma rotating permanent or superconducting magnets are suitable for injection of very low frequency (VLF) shear Alfven and magnetosonic waves. The generation of whistler waves in the magnetosphere plasma requires frequencies of the order of kHz, so in order to inject whistler waves generated by a RMF it is necessary to use an antenna with polyphase alternating currents. The interactions of the waves generated by a RMF source with highly energetic electron population were investigated in LAPD experiment and by test-particle simulations of non-resonant pitch angle scattering of trapped energetic electrons using the electromagnetic fields calculated using the 3D model. It was found in both the experiment and test-particle simulations that waves generated by a RMF source are, indeed, very efficient in pitch angle scattering of trapped hot electrons due to the creation of magnetic field gradients in the direction perpendicular to the ambient magnetic field. Different scenarios for the applications to the precipitation of highly energetic electrons in the magnetosphere are presented.Item Experimental Characterization of Turbulent Superfluid Helium(2010) Paoletti, Matthew S.; Lathrop, Daniel P; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fundamental processes in turbulent superfluid 4He are experimentally characterized by refining a visualization technique recently introduced by Bewley et al. A mixture of hydrogen and helium gas is injected into the bulk fluid, which produces a distribution of micron-sized hydrogen tracer particles that are visualized and individually tracked allowing for local velocity measurements. Tracer trajectories are complex since some become trapped on the quantized vortices while others flow with the normal fluid. This technique is first applied to study the dynamics of a thermal counterflow. The resulting observations constitute the first direct confirmation of two-fluid motions in He II and provide a quantitative test of the expression for the dependence of the normal fluid velocity, vn, on the applied heat flux, q, derived by L. D. Landau in 1941. Nearly 20,000 individual reconnection events are identified for the first time and used to characterize the dynamics by the minimum separation distance, $delta(t)$, between two reconnecting vortices. Dimensional arguments predict that this separation behaves asymptotically as $delta(t) approx A left ( kappa vert t-t_0 vert right ) ^{1/2}$, where $kappa=h/m$ is the quantum of circulation. The major finding of the experiments is strong support for this asymptotic form with $kappa$ as the dominant controlling quantity. Nevertheless there are significant event-to-event fluctuations that are equally well fit by two modified expressions: (a) an arbitrary power-law expression $delta(t)=B vert t-t_0 vert ^{alpha}$ and (b) a correction-factor expression $delta(t)=Aleft (kappa vert t-t_0 vert right ) ^{1/2}(1+c vert t-t_0 vert )$. In light of various physical interpretations we regard the correction-factor expression (b), which attributes the observed deviations from the predicted asymptotic form to fluctuations in the local environment and boundary conditions, as best describing the experimental data. The observed dynamics appear statistically time-reversible, suggesting that an effective equilibrium has been established in quantum turbulence on the time scales investigated. The hydrogen tracers allow for the first measurements of the local velocity statistics of a turbulent quantum fluid. The distributions of velocity in the decaying turbulence are strongly non-Gaussian with 1/v3 power-law tails in contrast to the near-Gaussian statistics of homogenous and isotropic turbulence of classical fluids. The dynamics of many vortex reconnection events are examined and simple scaling arguments show that they yield the observed power-law tails.Item Experimental Investigations of Capillary Effects on Nonlinear Free-Surface Waves(2010) Diorio, James Daniel; Duncan, James H; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents the results of three experiments on various aspects of the effects of surface tension on nonlinear free-surface waves. The first two experiments focus on capillary effects on the breaking of short-wavelength gravity waves, a problem of interest in areas of physical oceanography and remote sensing. The third experiment is concerned with the bifurcation of solitary capillary-gravity waves, a problem that is relevant in the study of nonlinear, dispersive wave systems. In the first set of experiments, streamwise profile measurements were made of spilling breakers at the point of incipient breaking. Both wind-waves and mechanically generated waves were investigated in this study, with gravity wavelengths in the range of 10--120 cm. Although it has been previously argued that the crest shape is dependent only on the surface tension, the results reported herein are to the contrary as several geometrical parameters used to describe the crest change significantly with the wavelength. However, the non-dimensional crest shape is self-similar, with two-shape parameters that depend on a measure of the local wave slope. This self-similarity persists over the entire range of wavelengths and breaker conditions measured, indicating a universal behavior in the near-crest dynamics that is independent of the method used to generate the wave. The measured wave slope is found to be related to the wave growth rate and phase-speed prior to breaking, a result that contributes towards the development of a breaking criterion for unsteady capillary-gravity waves. The second set of experiments examines the cross-stream surface structure in the turbulent breaking zone generated by short-wavelength breakers. Waves in this study were generated using a mechanical wedge and ranged in wavelength from 80--120 cm. To isolate the effects of surface tension on the flow, the important experimental parameters were adjusted to produce Froude-scaled, dispersively-focused wave packets. The results show the development of ``quasi''-2D streamwise ripples along with smaller cross-stream ripples that grow as breaking develops and can become comparable in amplitude to the streamwise ripples for larger breakers. It is found that the amplitude of the cross-stream surface ripples scale as $\bar{&lambda}^3$, where $\bar{&lambda}$ is the average wavelength of the wave packet. The cross-stream ripple activity appears to be highest in the ``troughs'' of the larger streamwise ripples, with the appearance of persistent ``scar''-like features. Based on these observations, a simple model for the coupling between the vorticity and capillary structure in the breaking zone is conjectured. The third set of experiments focuses on the generation of capillary-gravity waves by a pressure source moving near the minimum phase speed cmin. Near this minimum, nonlinear capillary-gravity solitary waves, or ``lumps'', have been shown to exist theoretically. We identify an abrupt transition to a wave-like state that features a localized solitary wave that trails the pressure forcing. This trailing wave is steady, fully localized in 3D, elongated in the cross-stream relative to the streamwise direction, and has a one-to-one relationship between height and phase speed. All of these characterisitics are commensurate with the freely propogating ``lumps'' computed by previous authors, and a quantitative comparison between these previous numerical calculations and the current experiments is presented. At speeds closer to cmin, a new time-dependent state is observed that can qualitatively be described by the shedding of solitary depressions from the tips of a ``V''-shaped pattern. These results are discussed in conjunction with a new theoretical model for these waves that employs nonlinear and viscous effects, both of which are crucial in capturing the salient features of the surface response. While discussed in the context of water waves, these results have applicaiton to other wave systems where nonlinear and dispersive effects are important.Item A Continuum Model for Flocking: Obstacle Avoidance, Equilibrium, and Stability(2010) Mecholsky, Nicholas Alexander; Ott, Edward; Antonsen, Jr., Thomas M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The modeling and investigation of the dynamics and configurations of animal groups is a subject of growing attention. In this dissertation, we present a partial-differential-equation based continuum model of flocking and use it to investigate several properties of group dynamics and equilibrium. We analyze the reaction of a flock to an obstacle or an attacking predator. We show that the flock response is in the form of density disturbances that resemble Mach cones whose configuration is determined by the anisotropic propagation of waves through the flock. We investigate the effect of a flock `pressure' and pairwise repulsion on an equilibrium density distribution. We investigate both linear and nonlinear pressures, look at the convergence to a ‘cold’ (T → 0) equilibrium solution, and find regions of parameter space where different models produce the same equilibrium. Finally, we analyze the stability of an equilibrium density distribution to long-wavelength perturbations. Analytic results for the stability of a constant density solution as well as stability regimes for constant density solutions to the equilibrium equations are presented.Item Dispersion of ion gyrocenters in models of anisotropic plasma turbulence(2009) Gustafson, Kyle Bergin; Dorland, William D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Turbulent dispersion of ion gyrocenters in a magnetized plasma is studied in the context of a stochastic Hamiltonian transport model and nonlinear, self-consistent gyrokinetic simulations. The Hamiltonian model consists of a superposition of drift waves derived from the linearized Hasegawa-Mima equation and a zonal shear flow perpendicular to the density gradient. Finite Larmor radius (FLR) effects are included. Because there is no particle transport in the direction of the density gradient, the focus is on transport parallel to the shear flow. The prescribed flow produces strongly asymmetric non-Gaussian probability distribution functions (PDFs) of particle displacements, as was previously known. For kρ=0, where k is the characteristic wavelength of the flow and ρ is the thermal Larmor radius, a transition is observed in the scaling of the second moment of particle displacements. The transition separates nearly ballistic superdiffusive dispersion from weaker superdiffusion at later times. FLR effects eliminate this transition. Important features of the PDFs of displacements are reproduced accurately with a fractional diffusion model. The gyroaveraged ExB drift dispersion of a sample of tracer ions is also examined in a two-dimensional, nonlinear, self-consistent gyrokinetic particle-in-cell (PIC) simulation. Turbulence in the simulation is driven by a density gradient and magnetic curvature, resulting in the unstable ρ scale kinetic entropy mode. The dependence of dispersion in both the axial and radial directions is characterized by displacement and velocity increment distributions. The strength of the density gradient is varied, using the local approximation, in three separate trials. A filtering procedure is used to separate trajectories according to whether they were caught in an eddy during a set observation time. Axial displacements are compared to results from the Hasegawa-Mima model. Superdiffusion and ballistic transport are found, depending on filtering and strength of the gradient. The radial dispersion of particles, as measured by the variance of tracer displacements, is diffusive. The dependence of the running diffusion coefficient on ρ for each value of the density gradient is considered.Item Numerical Investigations of Gaseous Spherical Diffusion Flames(2009) Lecoustre, Vivien Renaud Francis; Sunderland, Peter B.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Spherical diffusion flames have several unique characteristics that make them attractive from experimental and theoretical perspectives. They can be modeled with one spatial dimension, which frees computational resources for detailed chemistry, transport, and radiative loss models. This dissertation is a numerical study of two classes of spherical diffusion flames: hydrogen micro-diffusion flames, emphasizing kinetic extinction, and ethylene diffusion flames, emphasizing sooting limits. The flames were modeled using a one-dimensional, time-accurate diffusion flame code with detailed chemistry and transport. Radiative losses from products were modeled using a detailed absorption/emission statistical narrow band model and the discrete ordinates method. During this work the code has been enhanced by the implementation of a soot formation/oxidation model using the method of moments. Hydrogen micro-diffusion flames were studied experimentally and numerically. The experiments involved gas jets of hydrogen. At their quenching limits, these flames had heat release rates of 0.46 and 0.25 W in air and in oxygen, respectively. These are the weakest flames ever observed. The modeling results confirmed the quenching limits and revealed high rates of reactant leakage near the limits. The effects of the burner size and mass flow rate were predicted to have a significant impact on the flame chemistry and species distribution profiles, favoring kinetic extinction. Spherical ethylene diffusion flames at their sooting limits were also examined. Seventeen normal and inverse spherical flames were considered. Initially sooty, these flames were experimentally observed to reach their sooting limits 2 s after ignition. Structure of the flames at 2 s was considered, with an emphasis on the relationships among local temperature, carbon to oxygen atom ratio (C/O), and scalar dissipation rate. A critical C/O ratio was identified, along with two different sooting limit regimes. Diffusion flames with local scalar dissipation rates below 2 1/s were found to have temperatures near 1410 K at the location of the critical C/O ratio, whereas flames with greater local scalar dissipation rate exhibited increased temperatures. The present work sheds light on important combustion phenomenon related to flame extinction and soot formation. Applications to energy efficiency, pollutant reduction, and fire safety are expected.Item H (sub) alpha & Neutral Density Scaling in the Maryland Centrifugal eXperiment(2009) Clary, Ryan; Ellis, Richard; Hassam, Adil; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Maryland Centrifugal eXperiment (MCX) is a hydrogen plasma confinement experiment with a rotating mirror magnetic configuration. This experiment was designed to test the concepts of centrifugal confinement and velocity shear stabilization which may allow scaleability to a fusion reactor. These two concepts, however, rely on supersonic plasma fluid velocities, which, apart from possible plasma instabilities, could be greatly reduced by fluid drag with neutral hydrogen, leading to decreased confinement. Resonant charge exchange between a hydrogen ion and a hydrogen atom is believed to be the dominant drag mechanism on the rotating plasma. Neutral hydrogen emission lines (particularly the Balmer-alpha line, H (sub) alpha) are therefore of primary interest in diagnosing how neutral hydrogen affects plasma confinement. For this purpose, a multi-chord H (sub) alpha emission detector (multi-chord HED) was designed and constructed by the author in order to measure emissivity profiles. These profiles, together with an atomic collisional-radiative model, provide estimates of neutral hydrogen density and local charge-exchange times. Varied experimental parameters were applied to MCX discharges and the resulting variations in neutral density are compared to theoretical scaling laws. The charge-exchange times are compared to the measured momentum confinement time. We find that the inner and outer-most flux surfaces are not distinctly identified by the emissivity profile and the emissivity is dominant at the vacuum chamber wall. We also find that, while the overall emissivity profile does not match theoretical prediction, neutral density scaling is approximately described by the models. In addition, charge-exchange times are found to be much smaller than the momentum confinement time as well as to scale differently than the momentum confinement time. This dissertation includes a detailed description of the multi-chord HED system and its calibration, both spectrally and absolutely. We also present models based on neutral and plasma interaction which provide the scaling laws used to compare to experimental results.Item EFFICIENT SIMULATION OF ELECTRON TRAPPING IN LASER AND PLASMA WAKEFIELD ACCELERATION(2009) Morshed, Sepehr; Antonsen, Thomas M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Plasma based laser Wakefield accelerators (LWFA) have been a subject of interest in the plasma community for many years. In LWFA schemes the laser pulse must propagate several centimeters and maintain its coherence over this distance, which corresponds to many Rayleigh lengths. These Wakefields and their effect on the laser can be simulated in the quasistatic approximation. The 2D, cylindrically symmetric, quasistatic simulation code, WAKE is an efficient tool for the modeling of short-pulse laser propagation in under dense plasmas [P. Mora & T.M. Antonsen Phys. Plasmas 4, 1997]. The quasistatic approximation, which assumes that the driver and its wakefields are undisturbed during the transit time of plasma electrons, through the pulse, cannot, however, treat electron trapping and beam loading. Here we modify WAKE to include the effects of electron trapping and beam loading by introducing a population of beam electrons. Background plasma electrons that are beginning to start their oscillation around the radial axis and have energy above some threshold are removed from the background plasma and promoted to "beam" electrons. The population of beam electrons which are no longer subject to the quasistatic approximation, are treated without approximation and provide their own electromagnetic field that acts upon the background plasma. The algorithm is benchmarked to OSIRIS (a standard particle in cell code) simulations which makes no quasistatic approximation. We also have done simulation and comparison of results for centimeter scale GeV electron accelerator experiments from LBNL. These modifications to WAKE provide a tool for simulating GeV laser or plasma wakefield acceleration on desktop computers.Item Beam halo creation and propagation in the University of Maryland Electron Ring(2009) Papadopoulos, Christos F.; O'Shea, Patrick G; Kishek, Rami A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis we discuss the phenomenon of halo creation in charged particle beams. For this, we combine analytical, numerical and experimental work, which focuses on the University of Maryland Electron Ring, but is applicable to a wide range of accelerators in the same intensity regime. We find that the details of the beam distribution do not affect the structure of the halo, but are nonetheless important as they determine the number of particles in the halo and whether the latter can be regenerated. Furthermore, we show that the halo in configuration and velocity space comprises of the same particles, a prediction that has great importance for halo removal and diagnostics. In particular, we show that even in the case of ideal halo removal in phase space, the complicated internal dynamics of the beam core lead to halo regeneration. Following on previous work, we also construct a theoretical particle-core model for a skew quadrupole focusing channel, and compare the results to PIC simulations as well as measurements on UMER. The agreement between these three approaches is satisfactory, within the constraints of each case.Item Transport in Poygonal Billiard Systems(2009) Reames, Matthew Lee; Dorfman, J. R.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The aim of this work is to explore the connections between chaos and diffusion by examining the properties of particle motion in non-chaotic systems. To this end, particle transport and diffusion are studied for point particles moving in systems with fixed polygonal scatterers of four types: (i) a periodic lattice containing many-sided polygonal scatterers; (ii) a periodic lattice containing few-sided polygonal scatterers; (iii) a periodic lattice containing randomly oriented polygonal scatterers; and (iv) a periodic lattice containing polygonal scatterers with irrational angles. The motion of a point particle in each of these system is non-chaotic, with Lyapunov exponents strictly equal to zero.