Academic literature on the topic 'Plasma confinement Tomography'

Three-dimensional images of the undercoat structure on the cytoplasmic surface of the upper cell membrane of normal rat kidney fibroblast (NRK) cells and fetal rat skin keratinocytes were reconstructed by electron tomography, with 0.85-nm–thick consecutive sections made ∼100 nm from the cytoplasmic surface using rapidly frozen, deeply etched, platinum-replicated plasma membranes. The membrane skeleton (MSK) primarily consists of actin filaments and associated proteins. The MSK covers the entire cytoplasmic surface and is closely linked to clathrin-coated pits and caveolae. The actin filaments that are closely apposed to the cytoplasmic surface of the plasma membrane (within 10.2 nm) are likely to form the boundaries of the membrane compartments responsible for the temporary confinement of membrane molecules, thus partitioning the plasma membrane with regard to their lateral diffusion. The distribution of the MSK mesh size as determined by electron tomography and that of the compartment size as determined from high speed single-particle tracking of phospholipid diffusion agree well in both cell types, supporting the MSK fence and MSK-anchored protein picket models.

AbstractOptical interferometry techniques allow phase changes in a medium to be accurately determined. Since plasmas introduce a phase modification proportional to electron density, interferometry has been used routinely to determine the time development of electron density profiles in magnetic confinement fusion plasmas. The present paper describes the basic principles of interferometry of plasmas and establishes criteria regarding wavelength selection, vibrational phase noise, phase averaging etc. In addition, recent advances at UCLA in interferometry of fusion plasmas will be described. For example, high spatial sampling has allowed the detailed study of MHD phenomena such as sawteeth and Mimov oscillations, and 2-D imaging interferometry has permitted tomographic reconstruction of electron density contours. Difficulties associated with interferometry on future fusion devices, such as CIT, will also be discussed together with possible solutions.

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.

Cette these porte sur l'etude experimentale de l'activite m. H. D. Dans le tokamak tore supra, associee a la presence de la surface rationnelle q = 1 dans le plasma. L'existence de cette surface est correlee a l'apparition du phenomene de dents de scies, qui correspond a la relaxation periodique de la temperature du cur de la decharge. La partie rapide de celle-ci se compose d'une premiere phase de deplacement rapide du centre chaud du plasma, suivie de la perte brutale de son contenu energetique. Quelle que soit la direction poloidale du deplacement, le centre chaud se deforme (haricot) et vient butter contre la surface q = 1. La temperature chute alors violemment et le contenu energetique central est evacue vers l'exterieur de la decharge. L'analyse de stabilite (m. H. D. Ideale) du mode kink interne montre que l'effondrement intervient dans la zone stable du diagramme de stabilite ; ce qui suggere de prendre en compte d'autres parametres tels que le cisaillement magnetique local pres de la resonance afin d'expliquer ce desaccord. L'etude experimentale du serpent obtenu apres injection de glacon a constitue le deuxieme phenomene aborde au cours de cette these. La forme du profil du facteur de securite reste une des cles pour la comprehension des deux principaux problemes souleves par le serpent : sa creation est sa survie. La localisation du serpent sur une surface q = 1 differente de celle liee a l'inversion des dents de scies, montre l'existence simultanee de deux surfaces q = 1 dans le plasma. Sa composition chimique indique que les impuretes ont tendance a s'accumuler a l'interieur du serpent. Nous avons montre que la contribution du courant de bootstrap et les impuretes dans le serpent ont un effet destabilisant a cisaillement magnetique positif. Cependant, l'evolution precise de la taille de l'ilot est difficile a decrire completement dans la mesure ou l'on ne connait pas les courants perturbes qui interviennent dans la stabilite. L'analyse des phenomenes complexes tels que le transport dans le serpent, sa rotation, ou l'effet des dents de scies sur le serpent justifient la poursuite de ces etudes.

La consommation mondiale d'énergie a fortement augmenté durant le siècle dernier et va continuer de croître au cours des prochaines décennies. Le développement d'énergies durables et alternatives aux énergies fossiles constitue un enjeu crucial pour les générations futures. Dans ce contexte, la fusion thermonucléaire contrôlée serait un candidat de premier choix pour assurer la transition énergétique. Le tokamak, basé sur la fusion par confinement magnétique, est actuellement la solution la plus en vue pour contrôler la réaction de fusion et utiliser cette énergie à des fins civiles.Dans les plasmas de tokamak, les impuretés lourdes comme le tungstène présent dans les éléments de la paroi face au plasma, peuvent migrer vers le plasma de cœur et fortement dégrader les performances fusion par rayonnement. L’objectif de cette thèse est d’utiliser ce rayonnement dans la gamme des X-mous afin d’en déduire des informations sur le transport du tungstène dans le plasma de cœur. Le but est de contrôler à terme cette concentration en impuretés et d’identifier les actuateurs pouvant agir sur cette distribution
Global energy consumption has increased significantly during the last century and will continue to grow in the coming decades. The development of sustainable energies alternative to fossil fuels is a crucial issue for the future generations. In this context, controlled thermonuclear fusion is a good candidate for the energy transition. Magnetic confinement fusion and tokamaks are currently the most promising solution to control the fusion reaction and use it for civil purposes.In tokamak plasmas, heavy impurities such as tungsten sputtered from plasma-facing components can migrate to the core plasma and strongly degrade fusion performance by radiation. The goal of this PhD thesis is to use this radiation in the soft X-ray range in order to obtain valuable information on tungsten transport in the core plasma. The final perspective is to control the impurity concentration and identify actuators that can act on this distribution