Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references (leaves 84-91). Available also in a digital version from Dissertation Abstracts.

In recent years, many alternative models of the substorm process have been proposed to explain different aspects of this magnetospheric phenomenon. Some features in these competing models are compatible while others, such as the nature and location of substorm onset, remain controversial. The objective of this thesis is to assess the viability of the ballooning instability as a mechanism for initiating substorms. A review of the history and development of magnetospheric substorm research as well as a review of substorm models is presented. In these models, the crosstail current disruption responsible for the onset of the expansion phase is usually ascribed to the onset of some microinstability. An alternative triggering mechanism is a macroscopic magnetohydrodynamic instability such as the ballooning instability. To derive a threshold condition for the ballooning instability, a simplified magnetotail geometry with cylindrical symmetry near the equatorial plane is assumed. In such circumstances, the torsion of the magnetic field lines is zero and they can be characterised by their curvature. The hydromagnetic equations with isotropic pressure are linearised to find the dispersion relation. This leads to a threshold condition which depends on the pressure and magnetic field intensity gradients. In order to obtain realistic numerical results for the threshold condition, a quasistatic, self-consistent, two-dimensional numerical model of the magnetotail during conditions typical of substorm growth phase is used. The model involves solving the Grad-Shafranov equation with appropriate boundary conditions. It provides time-dependent magnetospheric magnetic field configurations that are characterised by the development of a minimum in Bz in the equatorial plane. Calculations of the detailed configuration of the magnetotail during onset allow an estimate of the instability criterion. In a model which does not allow an increase of pressure with radius, it is found that the magnetotail is not unstable to ballooning. Part of this work has been presented at a conference, viz.: Dormer, L.A. and A.D.M. Walker, Investigation of local MHD instabilities in the magnetotail using a two-dimensional magnetospheric convection model. Poster presented at the 39th annual South African Institute of Physics conference, University of Bophuthatswana, 1994. / Thesis (M.Sc.)-University of Natal, 1995.

It has been recognized for some time that the very high fluence of fast (14.1MeV) neutrons produced by deuterium-tritium fusion will represent a major materials challenge for the development of next-generation fusion energy projects such as a fusion component test facility and demonstration fusion power reactor. The best-understood and most promising solutions presently available are a family of low-activation steels originally developed for use in fission reactors, but the ferromagnetic properties of these steels represent a danger to plasma confinement through enhancement of magnetohydrodynamic instabilities and increased susceptibility to error fields. At present, experimental research into the effects of ferromagnetic materials on MHD stability in toroidal geometry has been confined to demonstrating that it is still possible to operate an advanced tokamak in the presence of ferromagnetic components. In order to better quantify the effects of ferromagnetic materials on tokamak plasma stability, a new ferritic wall has been installated in the High Beta Tokamak—Extended Pulse (HBT-EP) device. The development, assembly, installation, and testing of this wall as a modular upgrade is described, and the effect of the wall on machine performance is characterized. Comparative studies of plasma dynamics with the ferritic wall close-fitting against similar plasmas with the ferritic wall retracted demonstrate substantial effects on plasma stability. Resonant magnetic perturbations (RMPs) are applied, demonstrating a 50% increase in n = 1 plasma response amplitude when the ferritic wall is near the plasma. Susceptibility of plasmas to disruption events increases by a factor of 2 or more with the ferritic wall inserted, as disruptions are observed earlier with greater frequency. Growth rates of external kink instabilities are observed to be twice as large in the presence of a close-fitting ferritic wall. Initial studies are made of the influence of mode rotation frequency on the ferritic effect, as well as observations of the effect of the ferritic wall on disruption halo currents.

Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (p. 116-121).

This thesis presents an improved feedback system on HBT-EP and suppression of the fast rotating kink mode using this system. HBT-EP is an experimental tokamak at Columbia University designed to study the magnetohydrodynamic (MHD) instabilities in confined fusion. The most damaging instabilities are global long wavelength kink modes, which break the toroidal symmetry of the magnetic structure and lead to plasma disruption and termination. When a tokamak is surrounded by a close fitting conducting wall, then the single helicity linear dispersion relation of the kink instability has two ominating branches: one is the "slow mode", rotating at the time scale of wall time, known as resistive wall mode (RWM), the other is the fast mode, that becomes unstable near the ideal wall stability limit. Both instabilities are required to be controlled by the feedback system in HBT-EP. In this thesis, improvements have been made upon the previous GPU-based system to enhance the feedback performance and obtain clear evidence of the feedback suppression effect. Specifically, a new algorithm is implemented that maintains an accurate phase shift between the applied perturbation and the unstable mode. This prevents the excitation of the slow kink mode observed in previous studies and results in high gain suppression for fast mode control at all frequency for the first time. When the system is turned off, suppression is lost and the fast mode is observed to grow back. The feedback performance is tested with several wall configurations including the presence of ferritic material. This provides the first comparison of feedback control between the ferritic and stainless wall. The effect of plasma rotation on feedback control is tested by applying a static voltage on a bias probe. As the mode rotation being slowed by the radial current flow, a higher gain on the kink mode is required to achieve feedback suppression. The change in plasma rotation also modifies the plasma response to the external perturbation. The optimal phase shift for suppression changes with the modified response and these observations are consistent with the predictions of the single helicity model.

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 114-120).

Thermonuclear fusion has potential to offer an economically, environmentally and socially acceptable supply of energy. A promising reactor design to execute thermonuclear fusion is the toroidal magnetic confinement device, tokamak. The tokamak still faces challenges in the major areas which can be categorised into confinement, heating and fusion technology. This thesis addresses the problem of confinement, in particular the role of transport along magnetic field lines perturbed by diverse MHD instabilities.<p><p>Unstable modes such as ideal ballooning-peeling, tearing etc. break closed magnetic surfaces and destroy the axisymmetry of the magnetic configuration in a tokamak, providing deviation of magnetic field lines from unperturbed magnetic surfaces. Radial gradients of plasma parameters have nonzero projections along such lines and drive parallel particle and heat flows which contribute to the radial transport. Such transport can significantly affect confinement as this takes place by the development of neoclassical tearing modes (NTMs) in the core and edge localised modes (ELMs) at the plasma periphery.<p><p>In this thesis, transport of heat through non-overlapped magnetic island chains is first investigated using the 'Optimal path' approach, which is based on the principal of minimum entropy production. This model shows how the effective heat conduction through islands increases with parallel heat conduction and with the perturbation level. A more standard analytical approach for the limit cases of "small" and "large" islands is also presented. Transport of heat through internally heated magnetic islands is next investigated by further development of the 'Optimal path' method. In addition the approach by R. Fitzpatrick, has been extended for this investigation. By application of these approaches to experimental observations made at TEXTOR tokamak, heat flux limit, limiting parallel heat conduction in low collisional plasmas, is elucidated.<p><p>Models to study transport of heat and particles due to ELMs have also been developed. Energy losses during ELMs have been estimated considering contribution from parallel conduction due to electrons and parallel convection of ions, with constant level of the magnetic field perturbation, steady profiles for density and temperature, and by accounting for the heat flux limit. The estimate shows good agreement with experimental observations. The model is developed further by accounting for the time evolution of the perturbation level due to ballooning mode, and of density and temperature profiles. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Dans les réacteurs industriels ou dans la nature, l'écoulement de fluides peut être couplé à des réactions chimiques. Dans de nombreux cas, il en résulte l'apparition de structures complexes dont les propriétés dépendent entre autres de la géométrie du système.<p><p>Dans ce contexte, le but de notre thèse a été d'étudier de manière théorique et sur des modèles réaction-diffusion-convection simples les propriétés de dynamiques spatio-temporelles résultant du couplage chimie-hydrodynamique. <p>Nous nous sommes focalisés sur les instabilités hydrodynamiques de digitation visqueuse et de densité qui apparaissent respectivement lorsqu'un fluide dense est placé au-dessus d'un fluide moins dense dans le champ de gravité et lorsqu'un fluide visqueux est déplacé par un fluide moins visqueux dans un milieu poreux.<p><p>En particulier, nous avons étudié les problèmes suivants:<p>- L'influence d'une réaction chimique de type A + B → C sur la digitation visqueuse. Nous avons montré que les structures formées lors de cette instabilité varient selon que le réactif A est injecté dans le réactif B ou vice-versa si ces réactifs n'ont pas un coefficient de diffusion ou une concentration initiale identiques.<p>- Le rôle de pertes de chaleur par les parois du réacteur dans le cadre de la digitation de densité de fronts autocatalytiques exothermiques. Nous avons caractérisé les conditions de stabilité de fronts en fonction des pertes de chaleur et expliqué l'apparition de zones anormalement chaudes lors de cette instabilité.<p>- L'influence de l'inhomogénéité du milieu sur la digitation de densité de solutions réactives ou non. Nous avons montré que les variations spatiales de perméabilité d'un milieu poreux peuvent figer ou faire osciller la structure de digitation dans certaines conditions.<p>- L'influence d'un champ électrique transverse sur l'instabilité diffusive et la digitation de densité de fronts autocatalytiques. Il a été montré que cette interaction peut donner lieu à des nouvelles structures et changer les propriétés du front.<p><p>En conclusion, nous avons montré que le couplage entre réactions chimiques et mouvements hydrodynamiques est capable de générer de nouvelles structures spatio-temporelles dont les propriétés dépendent entre autres des conditions imposées au système.<p>/<p>In industrial reactors or in nature, fluid flows can be coupled to chemical reactions. In many cases, the result is the emergence of complex structures whose properties depend among others on the geometry of the system.<p>In this context, the purpose of our thesis was to study theoretically using simple models of reaction-diffusion-convection, the properties of dynamics resulting from the coupling between chemistry and hydrodynamics.<p><p>We focused on the hydrodynamic instabilities of viscous and density fingering that occur respectively when a dense fluid is placed above a less dense one in the gravity field and when a viscous fluid is displaced by a less viscous fluid in a porous medium.<p><p>In particular, we studied the following issues:<p>- The influence of a chemical reaction type A + B → C on viscous fingering. We have shown that the fingering patterns observed during this instability depends on whether the reactant A is injected into the reactant B or vice versa if they do not have identical diffusion coefficients or initial concentrations.<p>- The role of heat losses through the reactor walls on the density fingering of exothermic autocatalytic fronts. We have characterized the conditions of stability of fronts depending on heat losses and explained the appearance of unusually hot areas during this instability.<p>- The influence of the inhomogeneity of the medium on the density fingering of reactive solutions or not. We have shown that spatial variations of permeability of a porous medium may freeze or generate oscillating fingering pattern under certain conditions.<p>- The influence of a transverse electric field on the Rayleigh-Taylor and diffusive instabilities of autocatalytic fronts. It was shown that this interaction may lead to new structures and may change the properties of the front.<p><p>In conclusion, we showed that the coupling between chemical reactions and hydrodynamic motions can generate new space-time structures whose properties depend among others, on the conditions imposed on the system. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished