Rational magnetic flux surfaces in fusion (toroidal plasma confinement) devices can break the magnetic field lines and reconnect them in the form of magnetic islands. Formation of these magnetic islands can have a serious impact on the plasma confinement properties of the device. Islands can in general degrade the confinement by mixing up different regions of the plasma. However there has been experimental evidence of confinement improvement by island induced transport barriers, under certain conditions. Even though there are a large number of theoretical and experimental works on magnetic islands to date, there is clearly a paucity of convincing experimental understanding on the nature of behaviour of islands in plasma. This thesis reports detailed experimental studies conducted on the H-1NF heliac stellarator, to gain an in-depth understanding of magnetic islands and their influence in plasma confinement.¶ Work reported in this thesis can be mainly divided into three parts: (a) high resolution imaging of vacuum magnetic islands and flux surfaces of H-1NF, (b) accurate computer modeling of H-1NF magnetic geometry and (c) detailed experiments on magnetic islands in plasma configurations.¶ Electron-beam wire-tomography in the H-1NF has been used for the high resolution mapping of vacuum magnetic flux surfaces and islands. Point-to-point comparison of the mapping results with computer tracing, in conjunction with an image warping technique, has enabled systematic exploration of magnetic islands and surfaces of interest. A fast mapping technique has been developed, which significantly reduced the mapping time and made this technique suitable for mapping at higher magnetic fields.¶ Flux surface mapping has been carried out at various magnetic configurations and field strengths. The extreme accuracy of this technique has been exploited to understand the nature of error fields, by point-by-point matching with computer tracing results. This has helped in developing a best-fit computer model for H-1NF magnetic configurations, which can predict rotational transform correct to three decimal places. Results from plasma experiments on magnetic configuration studies are best explained by the new model.¶ Experiments with low order magnetic islands in plasma configurations yielded some new results. It has been observed that the low order magnetic islands (m = 2) near the core of the plasma serve as pockets of improved confinement region under favourable conditions. This results in significant profile modifications including enhancement of the radial electric field near the core to a large positive value. The characteristics of islands are found to be dependent on the plasma collisionality and the island width.¶ Experiments with a magnetic configuration which exhibits no vacuum islands, but the core rotational transform very close to low order rational value, show a spontaneous transition of the radial electric field near the core to a large positive value (nearly 5 kV/m), with a strong electric field shear (nearly 700 kV/m2) and localised improvement in confinement, during the discharge. Evidence indicates that the transition is driven by the excitation of low order magnetic islands near the axis during the plasma discharge, due to the modification of rotational transform profile by toroidal plasma currents. The situation is similar to the Core Electron-Root Confinement (CERC) observed during high temperature ECH plasma discharges on other helical devices. This result provides an experimental evidence for the hypothesis that the threshold conditions for observing CERC can be reduced by exciting magnetic islands near the core of the plasma.

The application of a theoretical framework for calculating the radial electric field in the DIII-D tokamak edge plasma is discussed. Changes in the radial electric field are correlated with changes in many important edge plasma phenomena, including rotation, the L-H transition, and ELM suppression. A self-consistent model for the radial electric field may therefore suggest a means of controlling other important parameters in the edge plasma. Implementing a methodology for calculating the radial electric field can be difficult due to its complex interrelationships with ion losses, rotation, radial ion fluxes, and momentum transport. The radial electric field enters the calculations for ion orbit loss. This ion orbit loss, in turn, affects the radial ion flux both directly and indirectly through return currents, which have been shown theoretically to torque the edge plasma causing rotation. The edge rotation generates a motional radial electric field, which can influence both the edge pedestal structure and additional ion orbit losses. In conjunction with validating the analytical modified Ohm’s Law model for calculating the radial electric field, modeling efforts presented in this dissertation focus on improving calculations of ion orbit losses and x-loss into the divertor region, as well as the formulation of models for fast beam ion orbit losses and the fraction of lost particles that return to the confined plasma. After rigorous implementation of the ion orbit loss model and related mechanisms into fluid equations, efforts are shifted to calculate effects from rotation on the radial electric field calculation and compared to DIII-D experimental measurements and computationally simulated plasmas. This calculation of the radial electric field will provide a basis for future modeling of a fast, predictive calculation to characterize future tokamaks like ITER.

A presente dissertação trata da implementação, em um equipamento de eletroforese capilar (CE) com detecção condutométrica sem contato (C4D), de um sistema de controle externo do fluxo eletrosmótico (EOF) via tensão radial externa (Vrad). Através do potencial externo, aplicado diretamente ao capilar, é possível ter o controle do fluxo eletrosmótico de CE, pois, de forma simplificada, esta prática acopla vetorialmente um potencial externo aplicado com o potencial através da solução tampão dentro do capilar. O emprego da técnica possibilitou o aumento de resolução de 2 aminoácidos - Leucina e Isoleucina, cujas mobilidades diferem apenas de 0,12 cm2.V-1.s-1 entre si, em ácido acético 500 mmol.L-1 com pH = 2,55. A estratégia empregada aqui foi a que denominamos de \"coluna capilar infinita\", na qual, com as sucessivas inversões na direção do EOF, conseguimos aprisionar, dentro da coluna capilar, espécies com mobilidade eletroforética menor que a mobilidade do EOF. A literatura descreve que a inversão do EOF se torna mais difícil com o aumento do pH. Foram realizados testes em eletrólitos contendo agentes inversores de fluxo como o CTAB, o CaCl2 e o BaCl2. Ambos os aditivos foram usados em concentrações muito baixas, nas quais foi mantida a direção normal do EOF, sendo que a utilizaçãode tais agentes teve a finalidade apenas de reduzir os grupos silanolatos em soluções de pH acima de 6,0. Tal estratégia proporcionou a reversão do EOF no sistema tampão MES/HIS, cujo pH estava em torno de 6,1. Por fim, a pesquisa gerou uma perspectiva interessante que é a possibilidade de se encontrar combinações de eletrólitos de corrida e surfactantes com o intuito de se estender a faixa de alcance do Vrad para valores altos de pH. / This work presents the implementation, in an equipment for capillary electrophoresis (CE) with contactless conductivity detection (C4D), of a system for external control of the electroosmotic flow (EOF) via external radial voltage (Vrad). Through external potential, directly applied to the capillary, the electroosmotic flow can be controlled, because this practice couples the applied external potential to the zeta potential through the buffer solution within the capillary. The use of the technique allowed the baseline resolution of two amino acids (Leucine and Isoleucine), whose mobilities differ only by 0,12 cm2.V-1.s-1, using acetic acid 500 mmol.L-1 at pH = 2,55 as the running electrolyte. The approach, called \"infinite capillary column\", consists in successive reversals in the direction of the EOF, trapping species within the capillary column with electrophoretic mobility smaller than the EOF mobility. Thus, the two amino acids were retained by a period of approximately 120 minutes in the capillary that was enough to promote the baseline resolution. Previous works describe that the reversion of the EOF becoming more difficult as pH increases. In order to achieve a more effective control of EOF at high pH values (limiting the technique to a narrow performance band), tests were carried out in electrolytes containing flow reversing agents such as CTAB, CaCI2 and BaCI2. These additives were used at very low concentrations, which kept the normal direction of EOF, and the use of such agents had only the purpuse of reducing the density of silanolate groups in solutions of pH above 6,0. This approach allowed the reversion of the EOF using MES/HIS buffer, which pH was 6,1. Finally, this research has generated an interesting perspective about the possibility of finding combinations of electrolytes and surfactants aiming the Vrad range´s extension at high pH values.

A presente dissertação trata da implementação, em um equipamento de eletroforese capilar (CE) com detecção condutométrica sem contato (C4D), de um sistema de controle externo do fluxo eletrosmótico (EOF) via tensão radial externa (Vrad). Através do potencial externo, aplicado diretamente ao capilar, é possível ter o controle do fluxo eletrosmótico de CE, pois, de forma simplificada, esta prática acopla vetorialmente um potencial externo aplicado com o potencial através da solução tampão dentro do capilar. O emprego da técnica possibilitou o aumento de resolução de 2 aminoácidos - Leucina e Isoleucina, cujas mobilidades diferem apenas de 0,12 cm2.V-1.s-1 entre si, em ácido acético 500 mmol.L-1 com pH = 2,55. A estratégia empregada aqui foi a que denominamos de \"coluna capilar infinita\", na qual, com as sucessivas inversões na direção do EOF, conseguimos aprisionar, dentro da coluna capilar, espécies com mobilidade eletroforética menor que a mobilidade do EOF. A literatura descreve que a inversão do EOF se torna mais difícil com o aumento do pH. Foram realizados testes em eletrólitos contendo agentes inversores de fluxo como o CTAB, o CaCl2 e o BaCl2. Ambos os aditivos foram usados em concentrações muito baixas, nas quais foi mantida a direção normal do EOF, sendo que a utilizaçãode tais agentes teve a finalidade apenas de reduzir os grupos silanolatos em soluções de pH acima de 6,0. Tal estratégia proporcionou a reversão do EOF no sistema tampão MES/HIS, cujo pH estava em torno de 6,1. Por fim, a pesquisa gerou uma perspectiva interessante que é a possibilidade de se encontrar combinações de eletrólitos de corrida e surfactantes com o intuito de se estender a faixa de alcance do Vrad para valores altos de pH. / This work presents the implementation, in an equipment for capillary electrophoresis (CE) with contactless conductivity detection (C4D), of a system for external control of the electroosmotic flow (EOF) via external radial voltage (Vrad). Through external potential, directly applied to the capillary, the electroosmotic flow can be controlled, because this practice couples the applied external potential to the zeta potential through the buffer solution within the capillary. The use of the technique allowed the baseline resolution of two amino acids (Leucine and Isoleucine), whose mobilities differ only by 0,12 cm2.V-1.s-1, using acetic acid 500 mmol.L-1 at pH = 2,55 as the running electrolyte. The approach, called \"infinite capillary column\", consists in successive reversals in the direction of the EOF, trapping species within the capillary column with electrophoretic mobility smaller than the EOF mobility. Thus, the two amino acids were retained by a period of approximately 120 minutes in the capillary that was enough to promote the baseline resolution. Previous works describe that the reversion of the EOF becoming more difficult as pH increases. In order to achieve a more effective control of EOF at high pH values (limiting the technique to a narrow performance band), tests were carried out in electrolytes containing flow reversing agents such as CTAB, CaCI2 and BaCI2. These additives were used at very low concentrations, which kept the normal direction of EOF, and the use of such agents had only the purpuse of reducing the density of silanolate groups in solutions of pH above 6,0. This approach allowed the reversion of the EOF using MES/HIS buffer, which pH was 6,1. Finally, this research has generated an interesting perspective about the possibility of finding combinations of electrolytes and surfactants aiming the Vrad range´s extension at high pH values.