Thesis (M.S.)--University of Nevada, Reno, 2005. / "August 2005." Includes bibliographical references (leaves 118-127). Online version available on the World Wide Web.

Dissertation (Ph.D.)--Auburn University, 2008. / Abstract.

We investigate the properties of two distinct disordered systems: the two-species predator-prey Lotka-Volterra model with rate variability, and an elastic line model to simulate vortex lines in type-II superconductors. We study the effects of intrinsic demographic variability with inheritance in the reaction rates of the Lotka-Volterra model via zero-dimensional Monte Carlo simulations as well as two-dimensional lattice simulations. Individuals of each species are assigned inheritable predation efficiencies during their creation, leading to evolutionary dynamics and thus population-level optimization. We derive an effective subspecies mean-field theory and compare its results to our numerical data. Furthermore, we introduce environmental variability via quenched spatial reaction-rate randomness. We investigate the competing effects and relative importance of the two types of variability, and find that both lead to a remarkable enhancement of the species densities, while the aforementioned optimization effects are essentially neutral in the densities. Additionally, we collected extinction time histograms for small systems and find a marked increase in the stability of the populations against extinction due to the presence of variability. We employ an elastic line model to investigate the steady-state properties and non-equilibrium relaxation kinetics of magnetic vortex lines in disordered type-II superconductors. To this end, we developed a versatile and efficient Langevin molecular dynamics simulation code, allowing us to do a careful study of samples with or without vortex-vortex interactions or disorder allows us to disentangle the various complex relaxational features present in this system and investigate their origin. In particular, we compare disordered samples with randomly distributed point defects versus correlated columnar defects. We extract two-time quantities such as the mean-square displacement, the height and density correlations, to investigate the relaxation kinetics of the system of flux lines. Additionally, we compare the steady-state mean velocity and gyration radius as a function of an external driving current in the presence of point-like and columnar disorder. We validate our simulation algorithm by matching our results against a previously-used Monte Carlo algorithm, verifying that these microscopically quite distinct methods yield similar results even in out-of-equilibrium settings. / Ph. D.

This thesis presents a comprehensive investigation into the observations and suppression of sawtooth instabilities on the High Beta Tokamak-Extended Pulse (HBT-EP) device. The principle and design of a new tangential multi-energy extreme ultraviolet and soft x-ray (ME-EUV/SXR) diagnostic system is presented. This system enables the clear detection of sawtooth events for the first time on HBT-EP. It is the first multi-energy tangential-view system designed to work in a temperature range below 200 eV in a tokamak, which enables measurements of the electron temperature and the examination of mode dynamics. By employing a combination of 0.1 um aluminum and 0.2 um titanium filters, the system allows measurements of electron temperature profiles through reconstruction of the emission profile using the standard ``double-foil'' technique. Using the tangential ME-EUV/SXR diagnostic system, the thesis reports on the first detailed observations of sawtooth events on HBT-EP, analyzing their features and comparing the findings with results from other devices. It investigates the phenomenon of discharge scenario bifurcation, where plasma exhibits distinct behaviors under similar parameters. The study examines the correlation between the amplitude of the edge mode and the strength of sawtooth events, along with the role of the conducting wall system in this context. It was found that when the normalized wall radius 𝒃/𝒂 is within a critical value, the edge mode can be stabilized and strong sawtooth events occur. In-depth analysis is performed on the modes present during sawtooth-suppressed stages, with a particular focus on the coupling between the 1/1 helical core (HC), 2/1 tearing mode (TM) and the 3/1 external kink mode (𝐗𝐊). Evidence is provided to support the effectiveness of magnetic flux pumping in suppressing sawtooth instabilities when the 3/1 𝐗𝐊 exhibits a significant amplitude. Conversely, the suppression of the 3/1 𝐗𝐊 due to stabilization by the conducting wall leads to the weakening of magnetic flux pumping, resulting in the occurrence of strong sawtooth events. In conclusion, this thesis contributes to the understanding of sawtooth instabilities on the HBT-EP tokamak and highlights the role of magnetic flux pumping as a mechanism for sawtooth suppression. It broadens the understanding of flux pumping across various tokamak operational regimes and demonstrates the potential of sawtooth suppression through external mode manipulation. This contributes to the future development of sawtooth control strategies, improving plasma stability and advancing fusion energy research.

Experiments on the DIII-D device in the negative triangularity (NT) regime of tokamak operation demonstrate core conditions that offer advantageous stability properties. Long duration, stationary discharges in this scenario maintain performance metrics that scale to viable reactor gain. Deleterious global modes of toroidal mode number n=1 are infrequent in these plasmas, which operate free of core instability cycles that can kick off global instabilities. These plasmas operate free of edge instability cycles that would damage reactor components, as do all strongly shaped NT plasmas. Reproducible access to high-power stationary states was developed at two values of q95, the edge magnetic winding number or “safety factor”. Core MHD instabilities manifest in one form of internal ideal mode, the quasi-interchange mode (QI), found to be consistent with modeling of the profiles and parameter space in which NT operates. The GATO and DCON ideal MHD codes are used to characterize the limits to normalized pressure in NT, finding global kink modes with strong poloidal harmonic m=1,2 components at normalized plasma pressure βN=3-3.5. Limits to β_N are predicted to be mostly insensitive to plasma boundary shape in NT and similar at both q95 values obtained in experiments. Average triangularity is shown to affect ideal limits, when modified at the outer midplane. A similar result is obtained with the RDCON resistive MHD code, which is used to characterize the stability to resistive “tearing modes”. Experimental NT equilibria and equilibria across shape scans were investigated. Only outer midplane modifications affected tearing calculations. Ideal kink modeling and experimental observations of sporadic QI mode provide an explanation for current diffusion not predicted by neoclassical theory. This effect is found in experiments at q95=3, analyzed with the ONETWO transport code’s facility to evolve magnetic flux over a discharge consistently with measured profiles and reconstructed magnetic flux surfaces. This result is compared with GATO calculations and ONETWO flux diffusion analysis of a conventional shape, ITER baseline demonstration discharge that is shown to have an intrinsically 3D core. Radiation from accumulated plasma impurities seems to alter the core q profile. This makes unstable a QI mode that spurs formation of a helical core, sustained by anomalous magnetic flux diffusion. NT experiments at q95=4 are limited in energy confinement by poor fast ion confinement, as a result of nondisruptive core 3/2 tearing modes. Analysis with ONETWO shows agreement with neoclassical flux diffusion predictions in these cases, corresponding to a removal of core instabilities and elevation of minimum safety factor values qmin to unity. This understanding of the core MHD, performance, and operational limits of NT scenarios in DIII-D advances the development of negative triangularity scenarios and informs the core phenomena observed in experiments spanning the regime.

No description available.

Thesis (PhD (Physics))--University of Stellenbosch, 2002. / Includes bibliography. / ENGLISH ABSTRACT: High temperature superconductivity is an important topic in contemporary solid state physics, and an area of very active research. Due to it’s potential for application in low temperature electronic devices, the material has attracted the attention of researchers in the electronic engineering and material science fields alike. Moreover, from a fundamental point of view, several questions remain unanswered, related to the origin of superconductivity of this class of materials and the nature of quantised magnetic flux present in magnetised samples. In this work, flux creep phenomena in a thin superconducting YBa₂Cu₃O₇ film deposited by pulsed laser deposition, is investigated near the critical temperature 0 ≤ Tc – T ≤ 10 K. Creep activation energy U0 and critical current density jc were determined as a function of temperature close to Tc, providing important data to a problem of high-Tc superconductivity which is still a matter of debate. In particular it is still an open question whether restoring the temperature in a creep freezing experiment in fact restores the film to it's original state before the freezing. The most important novel results concern the regime of critical fluctuations in the vicinity Tc - T < 1 K. We studied the isothermal relaxation of trapped magnetic flux, and determined that the long time decay follows a power law, where the exponent is inversely proportional to the creep activation energy. The temperature dependence of the critical current density jc(T) of the YBa₂Cu₃O₇ film close to Tc was obtained during warming runs. It was determined that jc(T) follows a square root dependence on T to high accuracy in the range 0.2 ≤ Tc – T ≤ 1.5 K. During flux creep experiments an interesting phenomenon called creep freezing related to the strong temperature dependence of the relaxation rate was observed. A pronounced slowing of relaxation with only a small drop in temperature from a starting temperature close to Tc was detected. Experiments were conducted by initiating an isothermal flux decay run. At a certain point the temperature was slightly lowered, and the flux decay stopped within experimental accuracy. When the temperature was restored to the initial value, the flux decay resumed at the previous rate before cooling. An argument based on vortex drift velocity was employed to explain the phenomenon qualitatively. During the course of this investigation, a pulsed laser deposition (PLD) system was designed and built from scratch. PLD involves the interaction of a focussed laser pulse with a multielemental solid target material. Material ablated from the target forms a fast moving plume consisting of atomic and molecular particles, directed away from the target, and towards a usually heated substrate on which the particles condense layer by layer to form a thin film. The substrate temperature and background gas are carefully controlled to be conductive to the growth of a desired phase of the multi-elemental compound. The PLD system proved to be quite versatile in the range of materials that could be deposited. It was used to deposit thin films of different materials, most notable were good quality superconducting YBa₂Cu₃O₇, thermochromic VO2, and magnetoresistive LaxCa1-xMnO3. Metallic Au and Ag layers were also successfully deposited on YBa2Cu3O7 thin films, to serve as protective coatings. The critical temperatures of the best superconducting films were 90 K as determined by resistivity measurement. The optimal deposition conditions to deposit high quality superconducting YBa₂Cu₃O₇ thin films was found to be: deposition temperature 780°C, laser energy density 2-3 J/cm2, oxygen partial pressure 0.2 mbar, and target-substrate distance 35 mm. This yields film with Tc ~ 90 K. It was found that deposition temperature plays the predominant role in determining the quality of YBa₂Cu₃O₇ thin films deposited by PLD. / AFRIKAANSE OPSOMMING: Hoë temperatuur supergeleiding is tans ’n aktuele onderwerp van vastetoestandfisika en dit is ’n gebied van baie aktiewe navorsing. Weens die potensiaal vir toepassings van hoë temperatuur supergeleiers in elektronika, het dié klas materiale die aandag van fisici and elektronici getrek. Verskeie fundamentele vraagstukke bly steeds onbeantwoord, veral met betrekking tot die oorsprong van supergeleiding in hierdie materiale en die gedrag van gekwantiseerde magnetiese vloed (“vortekse”) in gemagnetiseerde monsters. In hierdie werk word diffusie van vortekse in dun supergeleidende YBa₂Cu₃O₇ films ondersoek naby die kritieke temperatuur (0 ≤ Tc - T ≤ 10 K). Die temperatuur afhanklikheid van die diffusie aktiveringsenergie U0 en die kritieke stroomdigtheid jc word bepaal naby Tc. Dit verskaf belangrike inligting tot probleme in hoë temperatuur supergeleiding wat tans nog onbeantwoord bly. In die besonder is dit steeds ’n ope vraag of die herstel van die aanvanklike temperatuur in ’n vloedstollings eksperiment waarlik die film tot die oorspronklike toestand herstel. Die belangrikste nuwe resultate hou verband met die gebied van kritieke fluktuasies van die orde parameter in die omgewing 0 < Tc - T < 1 K. Ons het die isotermiese ontspanning van vortekse verstrik in die kristalstruktuur bestudeer, en bepaal dat die lang tydsverval ’n magsverwantskap handhaaf, waar die eksponent omgekeerd eweredig is aan U0. Die temperatuur afhanklikheid van die kritieke stroomdigtheid jc(T) van die YBa₂Cu₃O₇ film naby Tc is bepaal tydens verhittingslopies. Daar is bevind dat naby Tc, jc ’n vierkantswortel verband met T volg, tot hoë noukeurigheid in die gebied 0.2 ≤ Tc – T ≤ 1.5 K. Gedurende vorteksdiffusie eksperimente is ’n interessante verskynsel naamlik vloedstolling (“flux freezing”) waargeneem. Dit hou verband met die sterk temperatuur afhanklikheid van die vervaltempo van die magnetiese moment van ’n gemagnetiseerde film. ’n Skerp daling van die vervaltempo, weens slegs ’n klein temperatuurdaling vanaf die begin temperatuur naby Tc, is waargeneem. Gedurende eksperimente is daar aanvanklik ’n isotermiese vloedontspanning teweeg gebring. Op ’n sekere tydstip is die temperatuur effens verlaag, waarby die vloedontspanning tot stilstand gekom het binne grense van waarneming. Wanneer die temperatuur weer herstel is na die oorspronklike, het die vloedontspanning voortgegaan teen die tempo voor die temperatuurverlaging. ’n Verklaring wat gebaseer is op vorteks dryfsnelheid was aan die hand gedoen om hierdie gedrag te verklaar. ’n Groot komponent van die projek was om die dun YBa₂Cu₃O₇ films self te vervaardig. Tydens hierdie ondersoek, is ’n gepulseerde laser deposisie (“PLD”) sisteem eiehandig ontwerp en gebou. PLD behels die interaksie van ’n gefokuseerde laser puls met ’n teiken bestaande uit ’n multi-element vastestofverbinding. Materiaal wat verdamp (“ablate”) word, vorm ’n snelbewegende pluim bestaande uit atomiese en molekulêre deeltjies. Dit beweeg vanaf die teiken na ’n verhitte substraat, waarop die deeltjies kondenseer om laag vir laag ’n dun film te vorm. Die substraat temperatuur en agtergrond gas word sorgvuldig beheer om die groei van die verlangde fase van die multi-element verbinding teweeg te bring. Die PLD sisteem is baie veeldoelig ten opsigte van die verskeidenheid materiale wat suksesvol neergeslaan kan word. Dit was aangewend om verskillende materiale neer te slaan, onder andere supergeleidende YBa₂Cu₃O₇, termochromiese VO2, en magnetoresistiewe LaxCa1-xMnO3. Geleidende Au en Ag lagies is ook suksesvol neergeslaan op YBa₂Cu₃O₇ dun films, om te dien as beskermingslagies. Die kritieke temperatuur van die beste supergeleidende films was 90 K soos bepaal deur weerstandsmetings. Die optimale neerslaan toestand vir hoë kwaliteit YBa₂Cu₃O₇ dun films was: substraat temperatuur 780°C, laser energiedigtheid 2 - 3 J/cm2, suurstofdruk 0.2 mbar, en teiken-substraat afstand 35 mm. Daar is bevind dat die substraat temperatuur die deurslaggewende rol speel tydens die neerslaan proses om die kwaliteit van die supergeleidende films te bepaal.

Flux is continually emerging on the Sun, making its way from the solar interior up into the atmosphere. Emergence occurs on small-scales in the quiet Sun where magnetic fragments emerge, interact and cancel and on large-scales in active regions where magnetic fields emerge and concentrate to form sunspots. This thesis has been concerned with the large-scale emergence process and in particular the results from previous solar flux emergence modelling endeavours. Modelling uses numerical methods to evolve a domain representing simplified layers of the Sun’s atmosphere, within which the subsurface layer contains magnetic flux. The flux is initialised such that it will rises towards the surface at the start of the simulation. Once the flux reaches the solar surface, it can only emerge into the atmosphere if a magnetic buoyancy instability occurs, after which it expands rapidly both vertically and horizontally. The aim of this thesis is to test the robustness of these general findings from simulations to date upon the seed magnetic field. More explicitly, we have used three-dimensional numerical simulations to investigate how variations in the subsurface magnetic field modify the emergence process and the resulting atmospheric field. We initially consider a simple constant twist flux tube for the seed field and vary the tube’s magnetic field strength and degree of twist. Additionally, we have examined the effects of using non-constant twist flux tubes as the seed field by choosing two different profiles for the twist that are functions of the tube’s radius. Finally, we have investigated the effects of increasing the complexity of the seed field by positioning two flux tubes below the solar surface and testing two different configurations for the tubes. In both cases, the magnetic fields of the two tubes are such that, once the tubes come into contact with each other, reconnection occurs and a combined flux system is formed. From our investigations, we conclude that the general emergence results given by previous simulations are robust. However, for constant twist tubes with low field strength and twist, the buoyancy instability fails to be launched when the tubes reach the photosphere and they remain trapped in the low atmosphere. Similarly, when the non-constant twist profile results in a low tension force throughout the tube, we find that the buoyancy instability is not initialised.

No description available.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A manipulação e o controle das propriedades magnéticas de materiais com pequenas dimensões tem atraído interesse crescente nos últimos anos. Para sistemas magnéticos micrométricos ou submicrométricos, diferentes configurações magnéticas são energeticamente acessíveis. Vórtices magnéticos merecem destaque entre essas configurações e figuram em um grande número de pesquisas tecnológicas que vão desde o armazenamento magnético (VMRAM) até a biofuncionalização de estruturas para o tratamento do câncer. Em uma configuração de vórtice magnético, a energia magnetostática é minimizada por uma configuração de caminho fechado no plano do filme e uma região central com magnetização perpendicular à superfície. A quiralidade (sentido de rotação da magnetização no plano) e a polarização (direção da magnetização na região central) são os dois principais parâmetros que caracterizam um vórtice magnético. Apesar do esforço recente, ainda não se alcançou um entendimento detalhado que permita a manipulação controlada dessas características. Um aspecto importante para a aplicação tecnológica das estruturas de vórtice magnético é a uniformidade e a reprodutibilidade do comportamento de inversão de magnetização da partícula. O tamanho do núcleo do vórtice e o valor da magnetização, fatores que dependem fortemente da anisotropia do sistema, são aspectos relevantes a serem considerados para que as aplicações destas estruturas magnéticas se tornem realidade. Neste trabalho, arranjos regulares de discos multicamadas Co/Pt com diâmetro de 1 e 2 &#956;m e pemalloy com diâmetro na faixa de 5 a 17 &#956;m, ambos com espessura nanométrica, foram investigados por Microscopia de força magnética (MFM) e magnetometria (VSM e PPMS). Um dos objetivos foi investigar a correlação entre a anisotropia magnética nas multicamadas e o tamanho do núcleo do vórtice magnético. Os resultados obtidos demonstraram a presença de estados de vórtice magnético em algumas das amostras estudadas, em função do diâmetro do disco. Além disso, foram estudadas propriedades magnéticas da configuração de vórtices magnéticos desde a nucleação à aniquilação e efeitos de variação de dimensões de disco (diâmetro e espessura) e anisotropia magnética (multicamadas Co/Pt). / The manipulation and control of magnetic properties in size reduced materials have attracted a great interest in the last years. For micrometric or submicron magnetic structures different magnetic configurations are energetically accessible. Magnetic vortex noteworthy belongs to those configurations, and often represents the lowest energy configuration. Nowadays, it appears in a number of technological research ranging from the magnetic storage (VRAM) to the biofunctionalized microdisks for cancer treatment. In a magnetic vortex configuration, magnetostatic energy is minimized by in-plane closed flux domain structure and this curling magnetization turns out of the plane at the centre of the vortex structure. The chirality (direction of rotation of the in-plane magnetization) and polarization (up or down direction of the vortex core) are two topological features that characterize a magnetic vortex. In spite of the great effort on this matter, a controlled manipulation of magnetic vortex features was not reached. A critical aspect for the technological application of magnetic vortex structures is the uniformity and reproducibility of the reversal behavior of the particle magnetization. The vortex core size and the related value of its overall magnetization are also very relevant for the use of such magnetic structures. It is usually considered that the size of the vortex core depends on parameters such as anisotropy, thickness and diameter of the magnetic disk. In this work, regular arrays of Co/Pt multilayers disks with diameter of 1 and 2 &#956;m and pemalloy disks with diameter in the range 5 -17 &#956;m, both nanometer-thick, were investigated by Magnetic Force Microscopy (MFM) and magnetization measurements (VSM and PPMS). The results show the existence of magnetic vortex states for the samples, depending on the disk diameter. Furthermore, it was investigated the magnetic properties of the magnetic vortex, since the nucleation to annihilation, and the effect of variation of disk dimensions (diameter and thickness) and magnetic anisotropy (Co/Pt multilayers).