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 SUPPRESSION OF ELECTRON THERMAL CONDUCTION IN THE HIGH β INTRACLUSTER MEDIUM OF GALAXY CLUSTERS(2019) Roberg-Clark, Gareth; Drake, James F; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding the thermodynamic state of the hot intracluster medium (ICM) in a galaxy cluster requires a knowledge of the plasma transport processes, especially thermal conduction. The basic physics of thermal conduction in plasmas with ICM-like conditions has yet to be elucidated, however. We use particle-in-cell simulations and analytic models to explore the dynamics of an ICM-like plasma (with small gyroradius, large mean-free-path, and strongly sub-dominant magnetic pressure) induced by the diffusive heat flux associated with thermal conduction. Linear theory reveals that whistler waves are driven unstable by electron heat flux, even when the heat flux is weak. The resonant interaction of electrons with these waves then plays a critical role in scattering electrons and suppressing the heat flux. In a 1D model where only whistler modes that propagate parallel to the magnetic field are captured, the only resonant electrons are moving in the opposite direction to the heat flux and the electron heat flux suppression is small. In 2D or more, oblique whistler modes also resonate with electrons moving in the direction of theheat flux. The overlap of resonances leads to effective symmetrization of the electron distribution function and a strong suppression of heat flux. The results suggest that thermal conduction in the ICM might be strongly suppressed. In a numerical model with continually supplied heat flux in the system, two thermal reservoirs at different temperatures drive an electron heat flux that destabilizes off-angle whistler-type modes. The whistlers grow to large amplitude, 𝛿B=B0, and resonantly scatter the electrons. A surprise is that the resulting steady state heat flux is largely independent of the thermal gradient. The rate of thermal conduction is instead controlled by the finite propagation speed of the whistlers, which act as mobile scattering centers that convect the thermal energy of the hot reservoir. The results are relevant to thermal transport in high β astrophysical plasmas such as hot accretion flows and the intracluster medium of galaxy clusters. When the plasma β is reduced in the numerical model, we find that a transition takes place between whistler-dominated (high-β) and double-layer-dominated (low-β) heat flux suppression. Whistlers saturate at small amplitude in the low β limit and are unable to effectively suppress the heat flux. Electrostatic double layers suppress the heat flux to a mostly constant factor of the free streaming value once this transition happens. The double layer physics is an example of ion-electron coupling and occurs on a scale of roughly the electron Debye length. The scaling of ion heating associated with the various heat flux driven instabilities is explored over the full range of β explored. The range of plasma-βs studied in this work makes it relevant to the dynamics of a large variety of astrophysical plasmas, including not just the intracluster medium but hot accretion flows, stellar and accretion disk coronae, and the solar wind.Item The Response of Molecular Gases and Modulated Plasmas to Short Intense Laser Pulses(2011) Pearson, Andrew; Antonsen, Thomas M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis we study the response of two systems to short, intense laser pulses. The first system is a gas of diatomic molecules whose ensemble-averaged alignment features rotational revivals. We analyze the effect of a background plasma on the revival peaks. Both the revivals and the plasma are the result of a laser pulse passing through the gas. The second system is a density-modulated plasma channel. We study the generation of electromagnetic radiation by a laser pulse passing through this structure. The molecules in the gas are modeled as rigid rotors that interact first with the cycle-averaged electric field of the laser pulse, and second with the fluctuating electric field of the background plasma. The laser pulse generates a broad superposition of angular momentum eigenstates, resulting in the transient alignment of the molecules. Because of the time evolution properties of the angular momentum states, the alignment re-occurs periodically in field-free conditions. The alignment is calculated using a density matrix, and the background plasma is modeled using dressed particles. The result is decoherence between the phases of the basis states of the wavefunction, which causes decay of subsequent alignment peaks. We find that field-induced decoherence is competitive with collisional decoherence for small ionization fractions. The corrugated plasma channel is modeled using linear plasma theory, and the laser pulse is non-evolving. Corrugated channels support EM modes that have a Floquet dispersion relation, and thus consist of many spatial harmonics with subluminal phase velocities. This allows phase matching between the pulse and the EM modes. Since the pulse bandwidth includes THz frequencies, significant THz generation is possible. Here we consider realistic density profiles to obtain predictions of the THz power output and mode structure. We then estimate pulse depletion effects. The fraction of laser energy converted to THz is independent of laser pulse energy in the linear regime, and we find it to be around one percent. Extrapolating to a pulse energy of 0.5 J gives a THz power output of 6 mJ, with a pulse depletion length of less than 20 cm.Item DEVELOPMENT OF AN ADAPTIVE MASKING METHOD TO IMAGE BEAM HALO(2011) Zhang, Hao; O'Shea, Patrick G; Fiorito, Ralph B; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Beam halo is a common phenomenon in most intense particle beams and is associated with many bad effects. Halo is very hard to characterize because of its low intensity, which requires a measurement system with high dynamic range (≥105). Here, we have developed a technique that employs a digital micro-mirror array to produce an image of the halo of an electron beam with an enhanced dynamic range. Light produced by the beam intercepting a phosphor screen is first imaged onto the array; an adaptive mask is created and applied to filter out the beam core; and the result is re-imaged onto a CCD camera. In this thesis, we describe the optics used, the masking operation and preliminary results of experiments we have performed to study beam halo at the University of Maryland Electron Ring.Item The ultrafast nonlinear response of air molecules and its effect on femtosecond laser plasma filaments in atmosphere(2011) Chen, Yu-hsin; Milchberg, Howard M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The nonlinear propagation of an intense ultrafast laser pulse in atmosphere or other gas media leads to filamentation, a phenomenon useful for applications such as remote sensing, spectral broadening and shaping of ultrashort laser pulses, terahertz generation, and guiding of electrical discharges. Axially extended optical filaments result from the dynamic balance between nonlinear self-focusing in the gas and refraction from the free electron distribution generated by laser ionization. In the air, self-focusing is caused by two nonlinear optical processes: (1) the nearly-instantaneous, electronic response owing to the distortion of electron orbitals, and (2) the delayed, orientational effect due to the torque applied by the laser field on the molecules with anisotropic polarizability. To study their roles in filamentary propagation as well as influences on plasma generation in atmosphere, these effects were experimentally examined by a sensitive, space- and time-resolved technique based on single-shot supercontinuum spectral interferometry (SSSI), which is capable of measuring ultrafast refractive index shift in the optical medium. A proof-of-principle experiment was first performed in optical glass and argon gas, showing good agreement between the laser pulse shape and the refractive index temporal evolution owing to pure instantaneous n2 effect. Then the delayed occurrence of the molecular alignment in the temporal vicinity of the femtosecond laser pulse, as well as the subsequent periodic “alignment revivals” due to the coherently excited rotational wavepacket were measured in various linear gas molecules, and the results agreed well with quantum perturbation theory. It was found that the magnitude of orientational response is much higher than the electronic response in N2 and O2, which implies that the molecular alignment is the dominant nonlinear effect in atmospheric propagation when the pulse duration is longer than ∼40 fs, the rotational response timescale of air molecules. Realizing the possibility of manipulating plasma generation by aligning air molecules, the molecular orientational effect was further investigated by a technique developed to directly measure, for the first time, the radial and axial plasma density in a meter-long filament. The experiment was performed using both ∼40 fs and ∼120 fs laser pulse durations while keeping the peak power fixed under various focusing conditions, and the alignment-assisted filamenation with ∼2–3 times plasma density and much longer axial length was consistently observed with the longer pulse, which experienced larger refractive index shift and thus stronger self-focusing. Simulations reproduced the axial electron density measurements well for both long and short pulse durations, when using a peak magnitude of instantaneous response as <15% of the rotational response.Item Gyrofluid Modeling of Turbulent, Kinetic Physics(2011) Despain, Kate Marie; Dorland, William; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Gyrofluid models to describe plasma turbulence combine the advantages of fluid models, such as lower dimensionality and well-developed intuition, with those of gyrokinetics models, such as finite Larmor radius (FLR) effects. This allows gyrofluid models to be more tractable computationally while still capturing much of the physics related to the FLR of the particles. We present a gyrofluid model derived to capture the behavior of slow solar wind turbulence and describe the computer code developed to implement the model. In addition, we describe the modifications we made to a gyrofluid model and code that simulate plasma turbulence in tokamak geometries. Specifically, we describe a nonlinear phase mixing phenomenon, part of the E∙B term, that was previously missing from the model. An inherently FLR effect, it plays an important role in predicting turbulent heat flux and diffusivity levels for the plasma. We demonstrate this importance by comparing results from the updated code to studies done previously by gyrofluid and gyrokinetic codes. We further explain what would be necessary to couple the updated gyrofluid code, gryffin, to a turbulent transport code, thus allowing gryffin to play a role in predicting profiles for fusion devices such as ITER and to explore novel fusion configurations. Such a coupling would require the use of Graphical Processing Units (GPUs) to make the modeling process fast enough to be viable. Consequently, we also describe our experience with GPU computing and demonstrate that we are poised to complete a gryffin port to this innovative architecture.Item Visualization of the Vortex Lattice Dynamics in Superfluid Helium(2010) Gaff, Kristina Teresa; Lathrop, Daniel P; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We study the lattice structure and dynamics of the quantized vortices in superfluid helium-4 using a new rotating experiment. This setup includes control of the entire apparatus from the rotating frame, installation of a new EMCCD camera that allows for imaging of nanoscale tracer particles, and the development and implementation of a new isolation cell, which permits investigation into new phenomena such as differential rotation in helium-II. We have observed the vortex lattice dynamics in the (r, &phi) plane (i.e. transverse to the vortices) and present here the first real-time visualization of Tkachenko waves in helium-II from this cross section. Additionally, we present evidence of differential rotation with distinct Stewartson layer boundaries, possible Kelvin-Helmholtz instabilities, and the formation and propagation of superfluid collective vortex eddies. We show that the angular velocity is a function of radius and may be driven by the geometry of the isolation cell. We also document the observation and analysis of gravity-capillary surface waves that demonstrate an interaction between the liquid helium free surface and the bulk of the fluid.Item Interaction of Intense Short Laser Pulses with Gases of Nanoscale Atomic and Molecular Clusters(2006-08-09) Gupta, Ayush; Antonsen, Thomas M.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We study the interaction of intense laser pulses with gases of van der Waals bound atomic aggregates called clusters in the range of laser-cluster parameters such that kinetic as well as hydrodynamic effects are active. The clustered gas absorbs the laser pulse energy efficiently producing x-rays, extreme ultraviolet radiation, energetic particles and fusion neutrons. First, we investigate the effect of pulse duration on the heating of a single cluster in a strong laser field using a 2-D electrostatic particle-in-cell (PIC) code. Heating is dominated by a collision-less resonant absorption process that involves energetic electrons transiting through the cluster. A size-dependent intensity threshold defines the onset of this resonance [Taguchi et al., Phys. Rev. Lett., v90(20), (2004)]. It is seen that increasing the laser pulse width lowers this intensity threshold and the energetic electrons take multiple laser periods to transit the cluster instead of one laser period as previously recorded [Taguchi et al., Phys. Rev. Lett.,v90(20), (2004)]. Results of our numerical simulations showing the effect of pulse duration on the heating rate and the evolution of the electron phase space are presented in this dissertation. Our simulations show that strong electron heating is accompanied by the generation of a quasi-monoenergetic high-energy peak in the ion kinetic energy distribution function. The energy at which the peak occurs is pulse duration dependent. Calculations of fusion neutron yield from exploding deuterium clusters using the PIC model with periodic boundary conditions are also presented. We also investigate the propagation of the laser pulse through a gas of clusters that is described by an effective dielectric constant determined by the single cluster polarizability. For computational advantage, we adopt a uniform density description of the exploding clusters, modified to yield experimentally consistent single cluster polarizability, and couple it to a Gaussian description of the laser pulse. This model is then used to study self-focusing, absorption, and spectral broadening of the laser pulse. The model is further extended to allow for a fraction of the gas to be present as unclustered monomers and to include the effect of unbound electrons produced in the laser-cluster interaction.