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Full-Text Articles in Physics
On-Line System Identification For Control System Applications In Particle Accelerators, Mahesh Chowdhary
On-Line System Identification For Control System Applications In Particle Accelerators, Mahesh Chowdhary
Dissertations, Theses, and Masters Projects
Particle accelerators require a number of feedback systems in order to stabilize a variety of parameters. The Continuous Electron Beam Accelerator at Thomas Jefferson National Accelerator Facility presents a unique set of control and identification problems. This accelerator produces a continuous electron beam with energies between 0.5 and 4.0 GeV to be delivered to the experimental halls. In order to meet stringent beam quality requirements specified by the experimental halls, the position and the energy of the electron beam needs to stabilized at various locations in the accelerator.;A number of noise measurement tests were conducted at various locations in the …
Collision-Induced Secondary Electron And Negative Ion Emission From Metallic Surfaces, John Christopher Tucek
Collision-Induced Secondary Electron And Negative Ion Emission From Metallic Surfaces, John Christopher Tucek
Dissertations, Theses, and Masters Projects
Absolute yields of negative ions and secondary electrons resulting from collisions of positive sodium ions (Na{dollar}\sp{lcub}+{rcub}){dollar} with polycrystalline aluminum (Al) and molybdenum (Mo) surfaces and the Mo (100) surface have been measured as a function of the oxygen (O) coverage, from none up to several monolayers, for impact energies, E {dollar}<{dollar} 500 eV. Negative oxygen ions (O{dollar}\sp{lcub}-{rcub}){dollar} are found to be the dominant sputtered negative ions and for the three surfaces at all O coverages and Na{dollar}\sp{lcub}+{rcub}{dollar} impact energies. The O{dollar}\sp{lcub}-{rcub}{dollar} and secondary electron yields share a common impact energy threshold at {dollar}\rm E\sb{lcub}th{rcub}\approx 50{dollar} eV, and both have a strong dependence on the oxygen coverage of the surface.;The kinetic energy distributions of the secondary electrons and sputtered O{dollar}\sp{lcub}-{rcub}{dollar} were determined as functions of O coverages and impact energies. The O{dollar}\sp{lcub}-{rcub}{dollar} distributions are characterized by a narrow, low energy peak (at {dollar}\sim{dollar}1-2 eV) followed by a low level, high energy tail. The secondary electrons have a narrow (FWHM {dollar}\sim{dollar} 1-2 eV) kinetic energy distribution, centered essentially at the same most probable kinetic energy as the ions. The shapes of the respective distributions and the most probable kinetic energies are essentially invariant with the impact energy, O coverage and the metal surface.;The results were analyzed in terms of conventional collision cascade model, but the calculation could not be fitted to the experimental results. An electronic excitation mechanism is proposed to augment the collision cascade and to provide a mechanism for secondary electron emission. In the model, adsorbed O, which resides on the surface essentially as O{dollar}\sp{lcub}-{rcub},{dollar} is collisionally excited into an (MO{dollar}\sp{lcub}-{rcub})\sp*{dollar} repulsive state, and as the O{dollar}\sp{lcub}-{rcub}{dollar} exits the surface along the surface potential energy curve, it can decay by emission of an electron to the metal or to the vacuum, or it can survive as an ion. The parameters of this model can be adjusted such that the calculated kinetic energy distribution, together with that of the collision cascade, can reasonably reproduce the experimental observations for the ions and provide a reasonable fit to the corresponding electron kinetic energy distributions as well.
Thermal Lattice Boltzmann Simulations Of Variable Prandtl Number Turbulent Flow, Min Soe
Thermal Lattice Boltzmann Simulations Of Variable Prandtl Number Turbulent Flow, Min Soe
Dissertations, Theses, and Masters Projects
With the advent of massively parallel processor machines, thermal lattice Boltzmann equation (TLBE) techniques offer an attractive way of handling turbulence simulations. TLBE is new form of DNS (direct numerical simulation method)--with the important advantages of being ideal for multi-parallel processors as well as being able to handle complicated geometries. Since there are many kinetic models that will reproduce the macroscopic nonlinear (compressible) transport equations, TLBE chooses that subset which can be readily solved on a discrete spatial lattice. The lattice geometry must be so chosen that the discrete phase representation of TLBE will not taint the rotational symmetric continuum …
Cp Violation In K-Long Decays, Gregory Dominick Dorata
Cp Violation In K-Long Decays, Gregory Dominick Dorata
Dissertations, Theses, and Masters Projects
The main topic of this dissertation concerns alternative models of CP violation and the rates predicted by these models for the particular decay {dollar}K\sb{lcub}L{rcub}\to\pi\sp0\nu\bar\nu .{dollar} This decay is promising as a probe of the origin of CP violation in the kaon system. It is virtually free of hadronic uncertainties and its branching ratio would (along with a measurement of the CP asymmetry in {dollar}B\sbsp{lcub}d{rcub}{lcub}0{rcub}\to\psi K\sb{lcub}S{rcub}){dollar} allow a determination of all the elements of the CKM matrix (assuming all CP violation arises from the CKM matrix).;The first group of models of CP violation considered consists of three extensions of the Higgs …
Propagation Of Rayleigh Waves In Thin Films, Agus A. Ananda
Propagation Of Rayleigh Waves In Thin Films, Agus A. Ananda
Dissertations, Theses, and Masters Projects
With the advent of thin film technology and more recently its applications in microelectronics and control of surface properties, the interest in mechanical properties of thin films has grown tremendously. Mechanical defects such as creep, fracture and adhesion loss, play a very important role in physical instabilities of thin film materials. An acoustic microscope has been built to study mechanical properties of thin-films. The microscope operates at a nominal frequency of 50 MHz. Rayleigh surface waves velocities on the surface of film-substrate systems were measured from V(z) curves generated by the acoustic microscope. V(z) curves are produced from interference between …
Radiation-Induced Luminescence In Terbium-Doped Silicate Glasses, Michael Stuart West
Radiation-Induced Luminescence In Terbium-Doped Silicate Glasses, Michael Stuart West
Dissertations, Theses, and Masters Projects
The purpose of this study is to characterize radiation-induced luminescence of terbium-doped silicate glasses. Experiments performed investigated the optical properties, isothermal time-evolution, and temperature dependence of the radiation-induced luminescence of two commercially available terbium-doped glasses. A problem common to this type of glass is the persistent luminescence, or afterglow, that occurs following the end of excitation from an external source of radiation. While the processes that govern characteristic luminescence of rare earth ions, including terbium, are well understood, the processes that give rise to afterglow in doped glasses are not. Identifying the source of long-term luminescence is essential for controlling …
Microstructural Characterization Of An Ultra-High-Performance Polyimide, April Heather Baugher
Microstructural Characterization Of An Ultra-High-Performance Polyimide, April Heather Baugher
Dissertations, Theses, and Masters Projects
The motivation for this study was to further characterize a relatively new high-temperature polymer for application as a matrix resin for carbon fiber composites. The two primary questions addressed in this study dealt with the structural and chemical changes occurring in these polymers on exposure to high temperature.;To investigate the structural changes in the heat-treated samples, a positron annihilation lifetime spectrometer was designed, built and optimized. Because the lifetime of a positron in a material reflects the electronic structure of the material in which it annihilates, measurements by positron annihilation lifetime spectroscopy can be used to investigate changes in a …