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Full-Text Articles in Physical Sciences and Mathematics
Experimental Studies Of Neutral Particles And The Isotope Effect In The Edge Of Tokamak Plasmas, Ryan Chaban
Experimental Studies Of Neutral Particles And The Isotope Effect In The Edge Of Tokamak Plasmas, Ryan Chaban
Dissertations, Theses, and Masters Projects
The H-mode plasma edge is a region of steep gradients in density and temperature known as the “pedestal” which greatly increases energy confinement. The complex links between neutral-plasma interactions and both diffusive and convective transport in the pedestal must be understood to model, predict, and achieve the high performance required for a fusion power plant. This dissertation explores the effects of different hydrogenic isotope neutral particles and plasma transport from the edge pedestal region into the Scrape-Off Layer. Current experiments typically use deuterium (H with amu=2 or D), however future fusion power plants may startup with hydrogen (H), and eventually …
Edge Fueling And Neutral Density Studies Of The Alcator C-Mod Tokamak Using The Solps-Iter Code, Richard M. Reksoatmodjo
Edge Fueling And Neutral Density Studies Of The Alcator C-Mod Tokamak Using The Solps-Iter Code, Richard M. Reksoatmodjo
Dissertations, Theses, and Masters Projects
Understanding edge neutral dynamics in high-field tokamaks has strong consequencesfor both fueling and plasma profile predictions. We validate the ability of SOLPS-ITER, a 2D fluid plasma/kinetic Monte Carlo neutral code, to accurately model the upstream neutral density profiles of L-mode, I-mode, and H-mode discharges in the Alcator CMod tokamak, for which Lyman-alpha emission measurements were available. We achieve simulated Lyman-alpha emission and neutral density profiles that are within one standard deviation of empirically inferred profiles for all three discharges, via iterative tuning of the perpendicular transport coefficient profiles alone, providing confidence in the conclusion that while further physics (drifts, impurities, …
Comparison Of Full-Wave And Ray-Tracing Analysis Of Mode Conversion In Plasmas, Yanli Xiao
Comparison Of Full-Wave And Ray-Tracing Analysis Of Mode Conversion In Plasmas, Yanli Xiao
Dissertations, Theses, and Masters Projects
This dissertation reports on the first direct comparison between the results of ray-based and full-wave calculations for mode conversion in plasma. This study was motivated by the modular method originally developed by Ye and Kaufman to treat a magnetosonic wave crossing a cold minority-ion gyroresonance layer. We start with the cold plasma fluid model and introduce a system of evolution equations for electrons and two ion species: deuterium and hydrogen. We first study this system of equations for uniform plasma by Fourier methods, which gives the dispersion relations. We discuss how the traditional approach---which eliminates all other dynamical variables in …
Topics In Mode Conversion Theory And The Group Theoretical Foundations Of Path Integrals, Andrew Stephen Richardson
Topics In Mode Conversion Theory And The Group Theoretical Foundations Of Path Integrals, Andrew Stephen Richardson
Dissertations, Theses, and Masters Projects
This dissertation reports research about the phase space perspective for solving wave problems, with particular emphasis on the phenomenon of mode conversion in multicomponent wave systems, and the mathematics which underlie the phase space perspective. Part I of this dissertation gives a review of the phase space theory of resonant mode conversion. We describe how the WKB approximation is related to geometrical structures in phase space, and how in particular ray-tracing algorithms can be used to construct the WKB solution. We further review how to analyze the phenomena of mode conversion from the phase space perspective. By making an expansion …
Methods For Stabilizing High Reynolds Number Lattice Boltzmann Simulations, Brian Robert Keating
Methods For Stabilizing High Reynolds Number Lattice Boltzmann Simulations, Brian Robert Keating
Dissertations, Theses, and Masters Projects
The Lattice Boltzmann Method (LBM) is a simple and highly efficient method for computing nearly incompressible fluid flow. However, it is well known to suffer from numerical instabilities for low values of the transport coefficients. This dissertation examines a number of methods for increasing the stability of the LBM over a wide range of parameters. First, we consider a simple transformation that renders the standard LB equation implicit. It is found that the stability is largely unchanged. Next, we consider a stabilization method based on introducing a Lyapunov function which is essentially a discrete-time H-function. The uniqueness of an H-function …
Fluorescence Imaging Study Of Free And Impinging Supersonic Jets: Jet Structure And Turbulent Transition, Jennifer Ann Inman
Fluorescence Imaging Study Of Free And Impinging Supersonic Jets: Jet Structure And Turbulent Transition, Jennifer Ann Inman
Dissertations, Theses, and Masters Projects
A series of experiments into the behavior of underexpanded jet flows has been conducted at NASA Langley Research Center. This work was conducted in support of the Return to Flight effort following the loss of the Columbia. The tests involved simulating flow through a hypothetical breach in the leading edge of the Space Shuttle Orbiter along its reentry trajectory, with the goal of generating a data set with which other researchers can test and validate computational modeling tools. Two nozzles supplied with high-pressure gas were used to generate axisymmetric underexpanded jets exhausting into a low-pressure chamber. These nozzles had exit …
Ultraviolet Sources For Advanced Applications In The Vacuum Uv And Near Uv, Sheng Peng
Ultraviolet Sources For Advanced Applications In The Vacuum Uv And Near Uv, Sheng Peng
Dissertations, Theses, and Masters Projects
This dissertation documents a systematic study consisting of experimental investigations and theoretical analyses of intense ultraviolet sources in VUV and near-UV. Some engineering issues regarding two prototypes of electrodeless lamps using rf and microwave are discussed.;Various excimers that produce intense UV light are investigated, including: (1) A benchmark Xe2 excimer which has been proven to be very efficient in our novel rf capacitively coupled discharge lamp; (2) A rarely studied excimer, KrI, which suffers from predissociation and was reported to be very weak or invisible by most of other studies; (3) XeI excimer whose emission dominates around 253 nm and …
Control Of Integrable Hamiltonian Systems And Degenerate Bifurcations, Christopher W. Kulp
Control Of Integrable Hamiltonian Systems And Degenerate Bifurcations, Christopher W. Kulp
Dissertations, Theses, and Masters Projects
In this dissertation, we study the control of near-integrable systems. A near-integrable system is one whose phase space has a similar structure to an integrable system during short time periods and for some parameter regime. We begin by studying the control of integrable Hamiltonian systems. The controller targets an exact solution to the integrable system using dissipative and conservative terms. We find that a Takens-Bogdanov bifurcation occurs in the limit of no dissipative control. The presence of a Takens-Bogdanov bifurcation implies that the control is highly susceptible to noise. We illustrate our results using a two- and four-dimensional integrable systems …
Magnetohydrodynamic Turbulence: The Development Of Lattice Boltzmann Methods For Dissipative Systems, Angus Ian Duncan Macnab
Magnetohydrodynamic Turbulence: The Development Of Lattice Boltzmann Methods For Dissipative Systems, Angus Ian Duncan Macnab
Dissertations, Theses, and Masters Projects
Computer simulations of complex phenomena have become an invaluable tool for scientists in all disciplines. These simulations serve as a tool both for theorists attempting to test the validity of new theories and for experimentalists wishing to obtain a framework for the design of new experiments. Lattice Boltzmann Methods (LBM) provide a kinetic simulation technique for solving systems governed by non-linear conservation equations. Direct LBMs use the linearized single time relaxation form of the Boltzmann equation to temporally evolve particle distribution functions on a discrete spatial lattice. We will begin with a development of LBMs from basic kinetic theory and …
Low Damage Processing And Process Characterization, Xianmin Tang
Low Damage Processing And Process Characterization, Xianmin Tang
Dissertations, Theses, and Masters Projects
Two novel plasma sources (one neutral source and one pulsed inductively coupled plasma source) and ashing process characterization were investigated. The primary goal was to characterize these source properties and develop corresponding applications. The study includes process damage assessment with these two sources and another continuous wave (13.56MHz) plasma source. A global average simulation of the pulsed discharges was also included.;The transient plasma density and electron temperature from the double probe analysis were compared with single Langmuir probe results with sheath displacement corrections in pulsed discharges (200Hz--10kHz). The equivalent resistance method can be used effectively to analyze these double probe …
Surface Processing By Rfi Pecvd And Rfi Psii, Lingling Wu
Surface Processing By Rfi Pecvd And Rfi Psii, Lingling Wu
Dissertations, Theses, and Masters Projects
An RFI plasma enhanced chemical vapor deposition (PECVD) system and a large-scale RF plasma source immersion ion implantation (PSII) system were designed and built to study two forms of 3-D surface processing, PECVD and PSII. Using the RFI PECVD system, Ti-6Al-4V substrates were coated with diamond-like carbon films with excellent tribological and optical properties. as an innovation, variable angle spectroscopic ellipsometry (VASE) was successfully applied for non-destructive, 3-D, large-area tribological coatings quality investigation.;Based on the experience with the RFI PECVD system, a large-scale RFICP source was designed and built for the PSIL Langmuir probe and optical emission spectroscopy studies indicated …
Turbulence In Binary Fluid Flow Systems: A Lattice Boltzmann Approach, Darren M. Wah
Turbulence In Binary Fluid Flow Systems: A Lattice Boltzmann Approach, Darren M. Wah
Dissertations, Theses, and Masters Projects
A method for simulating a turbulent binary fluid flow system based on the Lattice Boltzmann Method (LBM) is presented. The fluid equations up to the Navier-Stokes transport level are derived for this two fluid system, and results from numerical simulations using this method are shown. Finally, grid resolution is performed in a single fluid (LBM) simulation which determines the largest valid mesh size for a simulation that seeks to resolve physical structures of all scales.
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 …
Transport In Chaotic Systems, Xian Zhu Tang
Transport In Chaotic Systems, Xian Zhu Tang
Dissertations, Theses, and Masters Projects
This dissertation addresses the general problem of transport in chaotic systems. Typical fluid problem of the kind is the advection and diffusion of a passive scalar. The magnetic field evolution in a chaotic conducting media is an example of the chaotic transport of a vector field. In kinetic theory, the collisional relaxation of a distribution function in phase space is also an advection-diffusion problem, but in a higher dimensional space.;In a chaotic flow neighboring points tend to separate exponentially in time, exp({dollar}\omega t{dollar}) with {dollar}\omega{dollar} the Liapunov exponent. The characteristic parameter for the transport of a scalar in a chaotic …
Surface Reflection Hyperthermal Neutral Stream Source, Christopher A. Nichols
Surface Reflection Hyperthermal Neutral Stream Source, Christopher A. Nichols
Dissertations, Theses, and Masters Projects
A novel source of hyperthermal (1-30 eV) reactive neutrals based on the surface-reflection-neutralization technique is described. This source is potentially capable of minimizing the charge-induced damage associated with plasma based semiconductor processing steps. The goal of this thesis is to investigate the issues involved in scale-up of this technology for processing of 8{dollar}\sp{lcub}\prime\prime{rcub}{dollar} diameter wafers used today in the semiconductor industry. This includes modeling the plasma ion source and trajectory simulations of the reflected neutral flux. A prototype source was constructed for experimental verification of the plasma model.;An inductively coupled plasma (ICP) source is used to provide a source of …
A Delta-F Monte Carlo Method To Calculate Parameters In Plasmas, Maciek Sasinowski
A Delta-F Monte Carlo Method To Calculate Parameters In Plasmas, Maciek Sasinowski
Dissertations, Theses, and Masters Projects
A Monte Carlo code has been developed which very efficiently calculates plasma parameters, such as currents, potentials and transport coefficients for a fully three dimensional magnetic field configuration. The code computes the deviation, f, of the exact distribution function, f, from the Maxwellian, {dollar}F\sb{lcub}M{rcub},{dollar} with {dollar}\psi{dollar} the toroidal magnetic flux enclosed by a pressure surface and H the Hamiltonian. The particles in the simulation are followed with a traditional Monte Carlo scheme consisting of an orbit step in which new values for the positions and momenta are obtained and a collision step in which a Monte Carlo equivalent of the …
A Study Of The Guiding Center Approximation, Qun Yao
A Study Of The Guiding Center Approximation, Qun Yao
Dissertations, Theses, and Masters Projects
A Hamiltonian treatment for the motion of a charged particle in a toroidal magnetic field is given. Assuming the plasma equilibrium, Boozer coordinates are used. The Hamiltonian of the exact trajectory is given in the guiding center coordinates. The higher order corrections to the standard drift Hamiltonian are derived. It is shown that the exact Hamiltonian depends on both the field strength and the shape of the magnetic surfaces (the metric of Boozer coordinates) while the standard drift Hamiltonian depends only on the field strength. The first order correction to the standard drift Hamiltonian, in gyroradius to system size, depends …
Quasilinear Theory Of Laser-Plasma Interactions, Alastair John Neil
Quasilinear Theory Of Laser-Plasma Interactions, Alastair John Neil
Dissertations, Theses, and Masters Projects
The interaction of a high intensity laser beam with a plasma is generally susceptible to the filamentation instability due to nonuniformities in the laser profile. In ponderomotive filamentation high intensity spots in the beam expell plasma by ponderomotive force, lowering the local density, causing even more light to be focused into the already high intensity region. The result--the beam is broken up into a filamentary structure.;Several optical smoothing techniques have been proposed to eliminate this problem. In the Random Phase Plates (RPS) approach, the beam is split into a very fine scale, time-stationary interference pattern. The irregularities in this pattern …
Painleve Singularity Analysis Applied To Charged Particle Dynamics During Reconnection, Jay Walter Larson
Painleve Singularity Analysis Applied To Charged Particle Dynamics During Reconnection, Jay Walter Larson
Dissertations, Theses, and Masters Projects
For a plasma in the collisionless regime, test-particle modelling can lend some insight into the macroscopic behavior of the plasma, e.g conductivity and heating. A common example for which this technique is used is a system with electric and magnetic fields given by B = {dollar}\delta y{dollar}cx x + xcx y + {dollar}\gamma{dollar}cx z and E = {dollar}\epsilon{dollar}cx z, where {dollar}\delta{dollar}, {dollar}\gamma{dollar}, and {dollar}\epsilon{dollar} are constant parameters. This model can be used to model plasma behavior near neutral lines, ({dollar}\gamma{dollar} = 0), as well as current sheets ({dollar}\gamma{dollar} = 0, {dollar}\delta{dollar} = 0). The integrability properties of the particle motion …
Magnetic Field Strength Of Toroidal Plasma Equilibria, David Alan Garren
Magnetic Field Strength Of Toroidal Plasma Equilibria, David Alan Garren
Dissertations, Theses, and Masters Projects
The goal of nuclear fusion research is to confine a deuterium-tritium plasma at a sufficiently high temperature (15 keV) and density (3 $\times$ 10$\sp{20}$ m$\sp{-3}$) for a sufficient length of time (1 sec) to produce net fusion power. One means to attain the required plasma confinement is to embed the plasma within a magnetic field. The global structure of this magnetic field determines the variation of magnetic field strength within the surfaces of constant plasma pressure. This field strength variation in turn determines many of the stability and confinement properties of the plasma. This dissertation gives the first detailed exposition …
Completely Bootstrapped Tokamak, Richard Henry Weening
Completely Bootstrapped Tokamak, Richard Henry Weening
Dissertations, Theses, and Masters Projects
A fundamental requirement for the successful operation of a tokamak is the maintenance of a toroidal electric current within the tokamak plasma itself. Maintaining this internal plasma current can be a very difficult technological problem. In this work, a well-known but non-standard method for maintaining the tokamak current called the bootstrap effect is discussed. The bootstrap effect occurs when a fusion plasma is near thermonuclear conditions, and allows the tokamak to greatly amplify its electric current.;Because the bootstrap effect amplifies but does not create a plasma current, it has long been argued that a completely bootstrapped tokamak is not possible. …
Statistically Constrained Decimation Of A Turbulence Model, Timothy Joe Williams
Statistically Constrained Decimation Of A Turbulence Model, Timothy Joe Williams
Dissertations, Theses, and Masters Projects
The constrained decimation scheme (CDS) is applied to a turbulence model. The CDS is a statistical turbulence theory formulated in 1985 by Robert Kraichnan; it seeks to correctly describe the statistical behavior of a system using only a small sample of the actual dynamics. The full set of dynamical quantities is partitioned into groups, within each of which the statistical properties must be uniform. Each statistical symmetry group is then decimated down to a small sample set of explicit dynamics. The statistical effects of the implicit dynamics outside the sample set are modelled by stochastic forces.;These forces are not totally …
Renormalization Group Theory Technique And Subgrid Scale Closure For Fluid And Plasma Turbulence, Ye Zhou
Renormalization Group Theory Technique And Subgrid Scale Closure For Fluid And Plasma Turbulence, Ye Zhou
Dissertations, Theses, and Masters Projects
Renormalization group theory is applied to incompressible three-dimension Navier-Stokes turbulence so as to eliminate unresolvable small scales. The renormalized Navier-Stokes equation includes a triple nonlinearity with the eddy viscosity exhibiting a mild cusp behavior, in qualitative agreement with the test-field model results of Kraichnan. For the cusp behavior to arise, not only is the triple nonlinearity necessary but the effects of pressure must be incorporated in the triple term.;Renormalization group theory is also applied to a model Alfven wave turbulence equation. In particular, the effect of small unresolvable subgrid scales on the large scales is computed. It is found that …
Turbulent Disruptions From The Strauss Equations, Jill Potkalitsky Dahlburg
Turbulent Disruptions From The Strauss Equations, Jill Potkalitsky Dahlburg
Dissertations, Theses, and Masters Projects
The subject of this thesis is an analysis of results from pseudospectral simulation of the Strauss equations of reduced three-dimensional magnetohydrodynamics. We have solved these equations in a rigid cylinder of square cross section, a cylinder with perfectly conducting side walls, and periodic ends. We assume that the uniform-density magnetofluid which fills the cylinder is resistive, but inviscid. Situations which we are considering are in several essential ways similar to a tokamak-like plasma; an external magnetic field is imposed, and the plasma carries a net current which produces a poloidal magnetic field of sufficient strength to induce current disruptions. These …
Stability And Transition Of The Driven Magnetohydrodynamic Sheet Pinch, Russell B. Dahlburg
Stability And Transition Of The Driven Magnetohydrodynamic Sheet Pinch, Russell B. Dahlburg
Dissertations, Theses, and Masters Projects
The stability and transition properties of a bounded, current carrying magnetofluid are explored, using the hydrodynamic theory developed for plane shear flows as a guide. A driven magnetohydrodynamic sheet pinch equilibrium is employed. A sixth order, complex eigenvalue equation which governs the normal modes of small oscillations is derived, and solved numerically by the Chebyshev tau method. Eigenfunctions are shown, as well as the curve of neutral stability. The locus of critical Lundquist numbers has the form of a hyperbola. The nonlinear stability of a primary disturbance of the system is considered. For regions in parameter space close to criticality, …
Catastrophes In The Elmo Bumpy Torus, Alkesh R. Punjabi
Catastrophes In The Elmo Bumpy Torus, Alkesh R. Punjabi
Dissertations, Theses, and Masters Projects
Experimentally it is observed that the plasma in Elmo Bumpy Torus (EBT) shows discontinuous changes in the electron line density, electron and ion temperatures and fluctuation levels as the ambient gas pressure or electron cyclotron heating is varied continuously. We use the Point Model of Hedrick et al. for the toroidal core plasma in EBT. The Point Model is not a gradient dynamic system. Hence the Elementary Catastrophe Theory is not directly applicable to the Point Model. Nonetheless, the Point Model equilibria will be shown to exhibit properties which are quite akin to the canonical cusp catastrophe. The ambipolar electric …
Long-Time States Of Inverse Cascades In The Presence Of A Maximum Length Scale, Murshed Hossain
Long-Time States Of Inverse Cascades In The Presence Of A Maximum Length Scale, Murshed Hossain
Dissertations, Theses, and Masters Projects
It is shown numerically, both for the two-dimensional Navier-Stokes equations and for two-dimensional magnetohydrodynamics, that the long-time asymptotic state in a forced inverse-cascade situation is one in which the spectrum is completely dominated by its own fundamental. The growth continues until the fundamental is dissipatively limited by its own dissipation rate. An algebraic model is proposed for the dynamics of such a final state.
Anisotrophy In Mhd Turbulence Due To A Mean Magnetic Field, John V. Shebalin
Anisotrophy In Mhd Turbulence Due To A Mean Magnetic Field, John V. Shebalin
Dissertations, Theses, and Masters Projects
The development of anisotropy in an initially isotropic spectrum is studied numerically for two-dimensional magnetohydrodynamic (MHD) turbulence. The anisotropy develops due to the combined effects of an externally imposed dc magnetic field and viscous and resistive dissipation at high wave numbers. The effect is most pronounced at high mechanical and magnetic Reynolds numbers. The anisotropy is greater at the higher wave numbers.;The statistical structure of two-dimensional MHD turbulence is also considered. It is shown that the three known rugged invariants of the isotropic case reduce to two for the anisotropic case. Randomness and ergodicity are also briefly discussed.
The Structure Of Axisymmetric Turbulence, Charles W. Smith
The Structure Of Axisymmetric Turbulence, Charles W. Smith
Dissertations, Theses, and Masters Projects
A wide range of laboratory and naturally occurring plasmas are frequently attributed a fluid description and as such, demonstrate turbulent flows. We will investigate a variety of forms which may be taken by the correlation functions of these turbulent flows. The most commonly discussed isotropic symmetry is not generally applicable since most systems of interest have been shown to be strongly anisotropic. This thesis will develop an axi-symmetric description from which the magnetic helicity may be extracted together with its spectrum. This description will be compared to the form taken by axi-symmetric, helical Navier Stokes turbulence which will also be …
Most Probable Magnetohydrostatic Equilibria For Tokamaks And Reversed Field Pinches, John Joseph Ambrosiano
Most Probable Magnetohydrostatic Equilibria For Tokamaks And Reversed Field Pinches, John Joseph Ambrosiano
Dissertations, Theses, and Masters Projects
The determination of magnetohydrostatic equilibria usually requires that two of the equilibrium functions be given. as there is usually no a priori basis for specifying the form of these two functions, the functions and the equilibria they determine may be considered random.;In this dissertation, the author reviews a recent statistical method for determining the equilibrium of an axially symmetric cylindrical plasma which is most probable (in the maximum entropy sense) given four global constraints (i.e., energy, magnetic helicity, longitudinal magnetic flux, and longitudinal current flux). Previous results from this model have been limited to non-negative random equilibrium functions (B(,z), J(,z), …