Lithium pellet injection into high pressure magnetically confined plasmas

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 195-201).The ablation of solid pellets injected into high temperature magnetically confined plasmas is characterized by rapid oscillations in the ablation rate, and the formation of field aligned filaments in the ablatant. High speed imaging of the ablation (> 250, 000 frames/second) during the 2003-2004 campaign revealed that these filament move away from the pellet primarily in the poloidal direction with a characteristic speeds of ~ 5km/s. Significant differences appeared in the filament drifts in RF heated H-mode plasmas compared to ohmic L-mode plasmas. Filaments in ohmic L-mode plasmas moved in both the electron and ion diamagnetic directions while filaments in H-mode move only in the electron diamagnetic direction. Furthermore, the motion of the filaments in L-mode plasmas appeared to be semi-random, with the direction changing randomly from shot to shot, but with a distinct preferred direction during each shot. The susceptibility of the filament's motion to variations in the background plasma conditions indicate that the drift is a result of interactions with the background plasma, and not a result of the internal dynamics of the ablation cloud. Furthermore, the chaotic, or semi-random, nature of the filament drift suggests that the drift could be due to ExB flows resulting from plasma turbulence. A stereoscopic imaging system was installed on Alcator C-Mod to make a detailed study of three dimensional evolution of the filaments. By examining a large number of pellet injections into ohmically heated L-mode plasmas, we were able to demonstrate that filaments do indeed move primarily along flux surfaces, and that the filament flow direction is correlated for sequential filaments. Additionally, a statistical examination of the trajectory data revealed that filaments have a wider distribution of speeds at lower values of the local safety factor, q. The measurements of the stereo-imaging system were compared with the implied turbulent ExB drifts determined by the gyrokinetic solver GYRO. Simulations conducted using profiles consistent with both pre-pellet and post-pellet conditions demonstrate that the filament drifts are more consistent with the turbulent conditions prevalent after the injection, indicating the filament drifts are most likely the result of turbulence generated by the modified plasma profiles from injection process itself.by Brock Böse.Ph.D

Reasoning strategies for semantic Web rule languages

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references (p. 101-104).Dealing with data in open, distributed environments is an increasingly important problem today. The processing of heterogeneous data in formats such as RDF is still being researched. Using rules and rule engines is one technique that is being used. In doing so, the problem of handling heterogeneous rules from multiple sources becomes important. Over the course of this thesis, I wrote several kinds of reasoners including backward, forward, and hybrid reasoners for RDF rule languages. These were used for a variety of problems and data in a wide range of settings for solving real world problems. During my investigations, I learned several interesting problems of RDF. First, simply making the term space big and well names paced and the language low enough expressivity did not make computation necessarily easier. Next, checking proofs in an RDF environment proved to be hard because the basic features of RDF that make it possible for it to represent heterogeneous data effectively make proofs difficult. Further work is needed to see if some of these problems can be mitigated. Though rules are useful, using rules correctly and efficiently for processing RDF data proved to be difficult.by Joseph Scharf.M.Eng

An experimental and theoretical investigation of shot cloud ballistics.

This thesis is concerned with an investigation of the evolution in time and space of shot clouds. The shot cloud is produced by launching a number of spherical projectiles from a shotgun barrel. With limited detailed information on the distribution of pellets within a shot cloud generated from previous studies, a considerable part of the investigation is dedicated to the design and implementation of a measurement facility to acquire accurate experimental data. Opto- and acousto-electronics were employed to meet contractual and research requirements in generating timing and positional information on the distribution of pellets in a shot cloud travelling, typically, at transonic velocities. The nature of the measurement facility also allows three-dimensional graphical reconstruction of shot cloud outlines. From the experimental data statistical analysis on the distribution of pellets within shot cloud was performed. Theoretical models are introduced which describe the dispersion of pellets within a shot cloud as it develops in flight. The preliminary work involved the motion of a single sphere in free flight and with the experimental data the deceleration characteristics of pellets were determined. U sing this information a model was developed which predicted the development of the shot clouds from the point where the pellets become independent of one another. Finally, a stochastic model was developed to describe the behaviour of a shot cloud. The equation of motion of a single sphere was perturbed by the addition of a random force term, and the width and length of the cloud were determined by performing ensemble averages. The prediction of these theoretical models were then compared to the experimental data to assess their validity

On Flows, Paths, Roots, and Zeros

This thesis has two parts; in the first of which we give new results for various network flow problems. (1) We present a novel dual ascent algorithm for min-cost flow and show that an implementation of it is very efficient on certain instance classes. (2) We approach the problem of numerical stability of interior point network flow algorithms by giving a path following method that works with integer arithmetic solely and is thus guaranteed to be free of any nu-merical instabilities. (3) We present a gradient descent approach for the undirected transship-ment problem and its special case, the single source shortest path problem (SSSP). For distrib-uted computation models this yields the first SSSP-algorithm with near-optimal number of communication rounds. The second part deals with fundamental topics from algebraic computation. (1) We give an algorithm for computing the complex roots of a complex polynomial. While achieving a com-parable bit complexity as previous best results, our algorithm is simple and promising to be of practical impact. It uses a test for counting the roots of a polynomial in a region that is based on Pellet's theorem. (2) We extend this test to polynomial systems, i.e., we develop an algorithm that can certify the existence of a k-fold zero of a zero-dimensional polynomial system within a given region. For bivariate systems, we show experimentally that this approach yields signifi-cant improvements when used as inclusion predicate in an elimination method.Im ersten Teil dieser Dissertation präsentieren wir neue Resultate für verschiedene Netzwerkflussprobleme. (1)Wir geben eine neue Duale-Aufstiegsmethode für das Min-Cost-Flow- Problem an und zeigen, dass eine Implementierung dieser Methode sehr effizient auf gewissen Instanzklassen ist. (2)Wir behandeln numerische Stabilität von Innere-Punkte-Methoden fürNetwerkflüsse, indem wir eine solche Methode angeben die mit ganzzahliger Arithmetik arbeitet und daher garantiert frei von numerischen Instabilitäten ist. (3) Wir präsentieren ein Gradienten-Abstiegsverfahren für das ungerichtete Transshipment-Problem, und seinen Spezialfall, das Single-Source-Shortest-Problem (SSSP), die für SSSP in verteilten Rechenmodellen die erste mit nahe-optimaler Anzahl von Kommunikationsrunden ist. Der zweite Teil handelt von fundamentalen Problemen der Computeralgebra. (1) Wir geben einen Algorithmus zum Berechnen der komplexen Nullstellen eines komplexen Polynoms an, der eine vergleichbare Bitkomplexität zu vorherigen besten Resultaten hat, aber vergleichsweise einfach und daher vielversprechend für die Praxis ist. (2)Wir erweitern den darin verwendeten Pellet-Test zum Zählen der Nullstellen eines Polynoms auf Polynomsysteme, sodass wir die Existenz einer k-fachen Nullstelle eines Systems in einer gegebenen Region zertifizieren können. Für bivariate Systeme zeigen wir experimentell, dass eine Integration dieses Ansatzes in eine Eliminationsmethode zu einer signifikanten Verbesserung führt

Geospatial phrase grounding and disambiguation

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references (p. 101-107).GeoCoder is a spatial reasoning system that converts natural language inputs into a set of precise spatial coordinates to display on a map. GeoCoder's spatial knowledge is represented in a set of ontologies. GeoCoder parses input phrases and adds location reference individuals to its ontology model. Relationships between location references are recognized based on mid-level structural patterns in the parsed phrase. GeoCoder grounds (or finds possible geometries for) location references in an iterative process, in which locations are grounded based on their relationships to previously grounded locations. GeoCoder improves upon previous systems by grounding and disambiguating at the phrase level, interpreting parses with rules that match mid level structure patterns, expressing disambiguation heuristics in ontologies, and improving scalability by separating grounding from reasoning about relationships.by Amy Michelle Slagle.M.Eng

Mining software repositories to support software evolution

Software evolution represents a major phase in the development life cycle of software systems. In recent years, software evolution has been recognized as one of the most important and challenging areas in the field of software engineering. Studies even show that 65-80% of the system lifetime will be spent on maintenance and evolution activities. Software repositories, such as versioning and bug tracking systems are essential parts of various software maintenance activities. Given the often large amounts of information stored in these repositories, researchers have proposed to mine and analyze these large knowledge bases in order to study and support various aspects of the evolution of a software system. In this thesis, we introduce a common ontological representation to support the mining and analysis of software repositories. In addition to this common representation, we introduce the SVN-Ontologizer and Bugzilla-Ontologizer tools that provide automation for both data extraction from remote repositories and ontology populations. A case study is presented to illustrate the applicability of the present approach in supporting software maintainers during the analysis and mining of these software repositorie

Mathematical modelling of gas flow networks in pellet induratlon systems

The objective of this research is to develop a simulation software tool, GASFLO, which should evaluate pressure, flow and temperature distributions of process gas in pellet induration system networks. Pellet induration systems are complex industrial systems composed of heterogenous components. The magnitude of gas through leaks i.e. the air entering or leaving the system from the points other than the known exits, is substantial and it adversely effects the performance of induration process. These leaks are very difficult to measure because of the hostile environment in the plant. The modelling of such industrial systems requires a notable amount of experimentation so the tool has been designed to enable the user modeller to change the component models and solution algorithms easily. The conventional methods for flow network simulation are based on process centred approach, mostly composed of homogeneous components. For ease of computation, the non-pipe elements are modelled with an approximate linear or non-linear generic equation, whose coefficients can simulate different states of the element. The resulting set of non-linear equations is linearised and solved simultaneously using some iterative method. By contrast, GASFLO is based on device centred or unit based approach, and uses a two level hierarchical solution algorithm. The pellet induration system network is first idealised into a connected graph of streams (sets of serially connected components) and nodes. At the top or coordination level the flow and pressure distributions satisfying the Kirchhoff's laws are evaluated for the connected graph. At the lower or component level the exact mathematical models of components ale computed, in order of their occurrence in respective streams, using coordination variables as parameters. The converged flows are used for the temperature computation. The solution algorithm requires partitioning of the connected graph into forest and coforest structures, for which secondary algorithms have been developed using specific heuristics relevant to the pellet induration systems. The rigorous application of software engineering techniques for the design and implementation of software, enabled the resolution of the complexity of the modelled system, embedded the characteristics of 'quality software' into the resulting code and benefits from object orientation, even though it is implemented in standard FORTRAN 77. GASFLO predicted results are in a good agreement with the measured results, it has been validated for a real life pellet induration system. It has been applied to simulate several practical scenarios, like addition of extra wind boxes to the zones and to determine how the plant production can be increased by certain ratio, such simulations were not feasible otherwise. GASFLO takes less than a minute to simulate a real-life pellet induration system on a 486 PC. The combined simulation with an other software tool, INDSYS, which evaluates the heat distribution in the solids, is also feasible

Cylindrically convergent implosions of metal liners for quasi-isentropic compression of deuterium

To date our understanding of strongly coupled, degenerate plasmas is incomplete. In particular considerable disagreement exists between theories of hydrogenic matter (HM) at pressures greater than 100 gigapascals (GPa) and temperatures below 3 electronvolts (eV). The predicted transition of fluid molecular hydrogen to a metallic atomic liquid has large implications for models of the interior structure and evolution of gas giants. Experimental confirmation of this transition is still pending. Furthermore, the properties of deuterium-tritium in this regime are of great interest to inertial confinement fusion. In this thesis we propose a scheme that can create strongly coupled, degenerate hydrogenic plasmas at terapascal (TPa) pressures on pulsed power machines. Our results show that this can be achieved by initiating cylindrically converging isentropic flow of sample material inside a metal liner. The liner acts as a pusher that drives a predefined compression of the fill, which is obtained by constructing an asymptotically self-similar implosion of cryogenically condensed HM. An empirical model that gives the required pulse shape for recreating this implosion inside the liner is introduced. Results of magnetohydrodynamic simulations demonstrate that a peak current of 10.8 megaamperes (MA) is sufficient for assembling nearly uniform HM at a stagnation pressure of 13 TPa and at temperatures of approximately 1.5 eV. A study of the stability of the implosion to imperfections of the liner's surface finds that liner-driven isentropic compression of hydrogen is robust to magneto-Rayleigh-Taylor growth for sufficiently thick liners. Since the methods in this work are readily adapted to a range of materials, an experimental realization could significantly extend our knowledge of degenerate, strongly coupled plasmas in general. Finally, we broaden our focus to the compression of the metal liner itself. Potential advantages of reducing shock heating and thereby increasing the degeneracy of liner material during a magnetized liner inertial fusion implosion are discussed.Open Acces