Physics
Permanent URI for this communityhttp://hdl.handle.net/1903/2269
Browse
10 results
Search Results
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 Characterization of Quantum Efficiency and Robustness of Cesium-Based Photocathodes(2010) Montgomery, Eric J.; O'Shea, Patrick G.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)High quantum efficiency, robust photocathodes produce picosecond-pulsed, high-current electron beams for photoinjection applications like free electron lasers. In photoinjectors, a pulsed drive laser incident on the photocathode causes photoemission of short, dense bunches of electrons, which are then accelerated into a relativistic, high quality beam. Future free electron lasers demand reliable photocathodes with long-lived quantum efficiency at suitable drive laser wavelengths to maintain high current density. But faced with contamination, heating, and ion back-bombardment, the highest efficiency photocathodes find their delicate cesium-based coatings inexorably lost. In answer, the work herein presents careful, focused studies on cesium-based photocathodes, particularly motivated by the cesium dispenser photocathode. This is a novel device comprised of an efficiently photoemissive, cesium-based coating deposited onto a porous sintered tungsten substrate, beneath which is a reservoir of elemental cesium. Under controlled heating cesium diffuses from the reservoir through the porous substrate and across the surface to replace cesium lost to harsh conditions -- recently shown to significantly extend the lifetime of cesium-coated metal cathodes. This work first reports experiments on coated metals to validate and refine an advanced theory of photoemission already finding application in beam simulation codes. Second, it describes a new theory of photoemission from much higher quantum efficiency cesium-based semiconductors and verifies its predictions with independent experiment. Third, it investigates causes of cesium loss from both coated metal and semiconductor photocathodes and reports remarkable rejuvenation of full quantum efficiency for contaminated cesium-coated surfaces, affirming the dispenser prescription of cesium resupply. And fourth, it details continued advances in cesium dispenser design with much-improved operating characteristics: lower temperature and cleaner operation. Motivated by dispenser integration with semiconductor coatings, initial fabrication of those coatings are reported on dispenser-type substrates with measurement of quantum efficiency and analysis of thermal stability. Detailed investigations are performed on dispenser substrate preparation by ion beam cleaning and on dispenser pore structure by electron microscopy and focused ion beam milling. The dissertation concludes by discussing implications of all results for the demonstration and optimization of the future high quantum efficiency cesium dispenser photocathode.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 Scattering from chaotic cavities: Exploring the random coupling model in the time and frequency domains(2009) Hart, James Aamodt; Ott, Edward; Antonsen, Thomas M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Scattering waves off resonant structures, with the waves coupling into and out of the structure at a finite number of locations (`ports'), is an extremely common problem both in theory and in real-world applications. In practice, solving for the scattering properties of a particular complex structure is extremely difficult and, in real-world applications, often impractical. In particular, if the wavelength of the incident wave is short compared to the structure size, and the dynamics of the ray trajectories within the scattering region are chaotic, the scattering properties of the cavity will be extremely sensitive to small perturbations. Thus, mathematical models have been developed which attempt to determine the statistical, rather than specific, properties of such systems. One such model is the Random Coupling Model. The Random Coupling Model was developed primarily in the frequency domain. In the first part of this dissertation, we explore the implications of the Random Coupling Model in the time domain, with emphasis on the time-domain behavior of the power radiated from a single-port lossless cavity after the cavity has been excited by a short initial external pulse. In particular, we find that for times much larger than the cavity's Heisenberg time (the inverse of the average spacing between cavity resonant frequencies), the power from a single cavity decays as a power law in time, following the decay rate of the ensemble average, but eventually transitions into an exponential decay as a single mode in the cavity dominates the decay. We find that this transition from power-law to exponential decay depends only on the shape of the incident pulse and a normalized time. In the second part of this dissertation, we extend the Random Coupling Model to include a broader range of situations. Previously, the Random Coupling Model applied only to ensembles of scattering data obtained over a sufficiently large spread in frequency or sufficiently different ensemble of configurations. We find that by using the Poisson Kernel, it is possible to obtain meaningful results applicable to situations which vary much less radically in configuration and frequency. We find that it is possible to obtain universal statistics by redefining the radiation impedance parameter of the previously developed Random Coupling Model to include the average effects of certain classical trajectories within the resonant structure. We test these results numerically and find good agreement between theory and simulation.Item Calculation of Realistic Charged-Particle Transfer Maps(2007-10-28) Mitchell, Chad Eugene; Dragt, Alex; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The study and computation of nonlinear charged-particle transfer maps is fundamental to understanding single-particle beam dynamics in accelerator devices. Transfer maps for individual elements of the beamline can in general depend sensitively on nonlinear fringe-field and high-multipole effects. The inclusion of these effects requires a detailed and realistic model of the interior and fringe magnetic fields, including knowledge of high spatial derivatives. Current methods for computing such maps often rely on idealized models of beamline elements. This Dissertation describes the development and implementation of a collection of techniques for computing realistic (as opposed to idealized) charged-particle transfer maps for general beamline elements, together with corresponding estimates of numerical error. Each of these techniques makes use of 3-dimensional measured or numerical field data on a grid as provided, for example, by various 3-dimensional finite element field codes. The required high derivatives of the corresponding vector potential A, required to compute transfer maps, cannot be reliably computed directly from this data by numerical differentiation due to numerical noise whose effect becomes progressively worse with the order of derivative desired. The effect of this noise, and its amplification by numerical differentiation, can be overcome by fitting on a bounding surface far from the axis and then interpolating inward using the Maxwell equations. The key ingredients are the use of surface data and the smoothing property of the inverse Laplacian operator. We explore the advantages of map computation using realistic field data on surfaces of various geometry. Maps obtained using these techniques can then be used to compute realistically all derived linear and nonlinear properties of both single pass and circular machines. Although the methods of this Dissertation have been applied primarily to magnetic beamline elements, they can also be applied to electric and radio-frequency beamline elements.Item Self-consistent simulation of radiation and space-charge in high-brightness relativistic electron beams(2007-06-25) Gillingham, David; O'Shea, Patrick G; Antonsen, Thomas M.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ability to preserve the quality of relativistic electron beams through transport bend elements such as a bunch compressor chicane is increasingly difficult as the current increases because of effects such as coherent synchrotron radiation (CSR) and space-charge. Theoretical CSR models and simulations, in their current state, often make unrealistic assumptions about the beam dynamics and/or structures. Therefore, we have developed a model and simulation that contains as many of these elements as possible for the purpose of making high-fidelity end-to-end simulations. Specifically, we are able to model, in a completely self-consistent, three-dimensional manner, the sustained interaction of radiation and space-charge from a relativistic electron beam in a toroidal waveguide with rectangular cross-section. We have accomplished this by combining a time-domain field solver that integrates a paraxial wave equation valid in a waveguide when the dimensions are small compared to the bending radius with a particle-in-cell dynamics code. The result is shown to agree with theory under a set of constraints, namely thin rigid beams, showing the stimulation resonant modes and including comparisons for waveguides approximating vacuum, and parallel plate shielding. Using a rigid beam, we also develop a scaling for the effect of beam width, comparing both our simulation and numerical integration of the retarded potentials. We further demonstrate the simulation calculates the correct longitudinal space-charge forces to produce the appropriate potential depression for a converging beam in a straight waveguide with constant dimensions. We then run fully three-dimensional, self-consistent end-to-end simulations of two types of bunch compressor designs, illustrating some of the basic scaling properties and perform a detailed analysis of the output phase-space distribution. Lastly, we show the unique ability of our simulation to model the evolution of charge/energy perturbations on a relativistic bunch in a toroidal waveguide.Item SUPERCONDUCTING ARTIFICIAL MATERIALS WITH A NEGATIVE PERMITTIVITY, A NEGATIVE PERMEABILITY, OR A NEGATIVE INDEX OF REFRACTION(2007-05-30) Ricci, Michael Christopher; Anlage, Steven M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Artificial materials are media made of inclusions such that the sizes and spacing of the inclusions is much smaller than the incident electromagnetic radiation. This allows a medium to act as an effective bulk medium to electromagnetic radiation. Artificial materials can be tailored to produce desired values of the permittivity, permeability, and index of refraction at specific frequencies. The applications of this tailoring include electromagnetic cloaking, and, theoretically, subwavelength imaging resolution. However, the success of these applications depends on their sensitivity to loss. This research uses superconducting niobium (Nb) metals to create arrays of wires, split-ring resonators, and a combination of wires and split-ring resonators, with very low loss. These arrays are used to investigate properties of a medium with an index of refraction that contains a bandwidth of frequency where the real part is negative. The Nb wire arrays produce a frequency bandwidth with a negative real part of the permittivity, while the Nb split-ring resonators produce a frequency bandwidth with a negative real part of the permeability. The combination of Nb wires and Nb split-ring resonators creates an artificial medium with a negative real part of the index of refraction. The electromagnetic transmission of the wires, split-ring resonators, and combination medium is measured in a waveguide as a function of frequency, and models of the permittivity and permeability are used to fit this data. For a single Nb split-ring resonator, the change in the resonant frequency and quality factor with temperature is measured and fit with a two-fluid model of superconductivity. The change in the resonant frequency and quality factor with an applied dc H field and applied power is also measured and compared to, respectively, magneto-optical imaging and laser scanning photoresponse measurements. Bianisotropy and perturbations in the resonant frequency are investigated, and simulated with commercial electromagnetic modeling software. The electromagnetic transmission of a single Nb split-ring resonator is compared to resonators made of YBCO, Copper, and a Nb closed-ring resonator. Similar measurements are made with the single resonators embedded in a metallic wire array.Item statistics of impedance and scattering matrices in microwave chaotic cavities: the random coupling model(2005-08-01) Zheng, Xing; Ott, Edward; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A model is proposed for the study of the statistical properties of the impedance (Z) and scattering (S)matrices of open electromagnetic cavities with several transmission lines or waveguides connected to the cavity. The model is based on assumed properties of the eigenfunctions for the closed cavity. Analysis of the model successfully reproduces features of the random matrix model believed to be universal, while at the same time incorporating features which are specific to individual systems. Universal statistical properties of the cavity impedance Z are obtained in terms of the radiation impedance. These universal properties are independent of system-specific details and shared by the members of the general class of systems whose corresponding ray trajectories are chaotic. In the single channel case, I obtained the normalized impedance and scattering coefficients whose probability density functions (PDF) are predicted to be universal. In the multiple-channel case, I focused on correlations in the phases of the eigenvalues of the S-matrix, and derived a formula for the averaged reflection coefficients in terms of the port radiation impedance. Effects of time-reversal symmetry and wall absorption are discussed. urthermore, I study the characterization of statistical fluctuations of the scattering matrix S and the impedance matrix Z, through their variance ratios. The variance ratio for the impedance matrix is shown to be a universal function of distributed losses within the scatterer, which contrasts with variance ratio of the scattering matrix for which universality applies only in the large loss limit. Theoretical predictions are tested by direct comparison with numerical solutions for a specific system, and also agree with experimental results obtained from scattering measurements on a chaotic microwave cavity.Item Quantitative Materials Contrast at High Spatial Resolution With a Novel Near-Field Scanning Microwave Microscope(2005-04-21) Imtiaz, Atif; Anlage, Steven M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A novel Near-Field Scanning Microwave Microscope (NSMM) has been developed where a Scanning Tunneling Microscope (STM) is used for tip-to-sample distance control. The technique is non-contact and non-destructive. The same tip is used for both STM and NSMM, and STM helps maintain the tip-to-sample distance at a nominal height of 1 nm. Due to this very small tip-to-sample separation, the contribution to the microwave signals due to evanescent (non-propagating) waves cannot be ignored. I describe different evanescent wave models developed so far to understand the complex tip-to-sample interaction at microwave frequencies. Propagating wave models are also discussed, since they are still required to understand some aspects of the tip-to-sample interaction. Numerical modeling is also discussed for these problems. I demonstrate the sensitivity of this novel microscope to materials property contrast. The materials contrast is shown in spatial variations on the surface of metal thin films, Boron-doped Semiconductor and Colossal Magneto-Resistive (CMR) thin films. The height dependence of the contrast shows sensitivity to nano-meter sized features when the tip-to-sample separation is below 100 nm. By adding a cone of height 4 nm to the tip, I am able to explain a 300 kHz deviation observed in the frequency shift signal, when tip-to-sample separation is less than 10 nm. In the absence of the cone, the frequency shift signal should continue to show the logarithmic behavior as a function of height. I demonstrate sub-micron spatial resolution with this novel microscope, both in tip-to-sample capacitance Cx and materials contrast in sheet resistance Rx. The spatial resolution in Cx is demonstrated to be at-least 2.5 nm on CMR thin films. The spatial resolution in Rx is shown to be sub-micron by measuring a variably Boron-doped Silicon sample which was prepared using the Focus Ion Beam (FIB) technique.Item The Self-Assembly of Particles with Multipolar Interactions(2004-12-17) Stambaugh, Justin John; Losert, Wolfgang; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we describe results from investigations of the self-assembly of anisotropically interacting particles. In particular, we focus upon the roles of dipolar and higher order multipolar interactions on the patterns of self-organization. Using an experimental model system of vertically vibrated magnetic spheres, we investigate the e®ects of octopolar and higher order interactions on the pattern of self-assembly. We show that simple theoretical point charge models can be used to provide insight into the underlying causes of the observed phenomena. We also show that such models can be used to better understand the pattern formation in several related physical systems, including biological macromolecular self-assembly and cohesive granular materials.