L'objectif de la fusion par confinement magnétique, et notamment du tokamak, est de produire de l'énergie à partir des réactions de fusion nucléaire, dans un plasma à faible densité et haute température. Expérimentalement, une amélioration de la performance des tokamaks a été observée en présence de rotation toroïdale. Or, les sources extérieurs de quantité de mouvement seront très limitées dans les futurs tokamaks, et notamment ITER. Une compréhension de la physique de la génération intrinsèque de rotation toroïdale permettrait donc de prédire les profils de rotation dans les expériences futures. Parmi les mécanismes envisagés, on s'intéresse ici à la génération de rotation par la turbulence, qui domine le transport de la chaleur dans les tokamaks. Les plasmas de fusion étant faiblement collisionnels, la modélisation de cette turbulence suppose un modèle cinétique décrivant la fonction de distribution des particules dans l'espace des phases à six dimensions (position et vitesse). Cependant, ce modèle peut être réduit à cinq dimensions pour des fréquences inférieures à la fréquence cyclotronique des particules. Le modèle gyrocinétique qui découle de cette approximation est alors accessible avec les ressources numériques actuelles. Les travaux présentés portent sur l'étude du transport de quantité de mouvement toroïdale dans les plasmas de tokamak, dans le cadre du modèle gyrocinétique. Dans un premier temps, nous montrons que ce modèle réduit permet une description précise du transport de quantité de mouvement en dérivant une équation locale de conservation. Cette équation est vérifiée numériquement à l'aide du code gyrocinétique GYSELA. Ensuite, nous montrons comment la turbulence électrostatique peut briser l'axisymétrie du système, générant ainsi de la rotation toroïdale. Un lien fort entre transport de chaleur et transport de quantité de mouvement est mis en évidence, les deux présentant des avalanches à grande échelle. La dynamique du transport turbulent est analysée en détail et, bien que l'estimation standard gyro-Bohm soit vérifiée en moyenne, des phénomènes non-diffusifs sont observés. L'effet des écoulements de bord du plasma sur la rotation toroïdale dans le coeur est étudié en modifiant les conditions aux bords dans le code GYSELA. Enfin, le champ magnétique d'équilibre, qui n'est pas rigoureusement axisymétrique, peut également participer à la génération de rotation toroïdale, via des mécanismes purement collisionnels. Dans un tokamak, cet effet est suffisamment important pour entrer en compétition avec la rotation générée par la turbulence électrostatique.

Thesis (doctoral)--Rijksuniversiteit te Utrecht, 1987. / Vita: p. 179. eContent provider-neutral record in process. Description based on print version record. Includes bibliographies.

This thesis presents the magnetic fields of the COMPASS tokamak and work done during the COMPASS reinstallation in the Czech Republic. The geometry, vacuum magnetic fields and Power Supplies for the poloidal field coils circuits are described in the technical part of the thesis. The design of Power Supplies filters and improvements in the controller algorithm are also introduced. The MHD equilibrium reconstruction code EFIT++ and implementation of the induced currents model are described in the physical part of the thesis. The EFIT++ code was adapted for COMPASS. The utilization of the EFIT++ code for the COMPASS operation is shown. The global power balance in the non-stationary phases of the tokamak discharge is explained and examples are shown. The attached articles describe Power Supplies and tokamak feedback system developed for the COMPASS plasma control. Powered by TCPDF (www.tcpdf.org)

The need to make maximum use of renewable resources to the detriment of fossil fuels to achieve environmental goals with an increasing energy demand is driving research into the development of technologies to obtain energy from sources that are not currently being exploited, one of them being fusion energy. The aim of this report is to provide a general overview of fusion and to provide a critical opinion on whether fusion will become a commercial energy source in the future, and if so when. The followed methodology has been a literature review complemented by an interview to B Henric M Bergsåker, teacher and researcher at the KTH on fusion plasma physics and information person for the Swedish fusion research.In the results section the fusion physics and different technological approaches have been presented. Among the studied different projects, the ITER Tokamak magnetic reactor has been selected as the most promising of these projects, as a product of international collaboration, and it has been analyzed in more detail. The obtained results have been that fusion can be an inexhaustible, environmentally friendly and safe energy source. The first-generation fusion commercial reactors are expected to be part of the energy mix before 2100.

Syftet med det här arbetet har varit att välja ut och integrera en lämplig fysikmotor med öppen källkod i simuleringsprogramvaran EWSim på Totalförsvarets Forskningsinstitut (FOI). EWSim är ett ramverk för duellsimulering i olika telekrigsscenarier. Teorin bakom fysikmotorer och några vanliga tekniker för fysiksimulering har beskrivits kortfattat. Tre fysikmotorer har presenterats och utvärderats med avseende på lämplighet för integrering i EWSim. Fysikmotorn Bullet valdes ut och integrerades i simuleringsprogramvaran. En av de huvudsakliga slutsatserna av arbetet är att fysikmotorer som Bullet mest kan bidra med ökad visuell realism i EWSim. För att återskapa verkliga egenskaper hos exempelvis simulerade fordon med hjälp av fysikmotorn skulle förhållandevis mycket ytterligare arbete krävas.

Neste trabalho foi feito um estudo do perfil tomográfico da emissão da linha do átomo de hidrogênio, H-alfa (?=656,28 nm) no plasma do TCABR, um tokamak de porte médio em operação no Laboratório de Física de Plasmas do Instituto de Física da Universidade de São Paulo. Nosso trabalho centrou-se no estudo dos efeitos da introdução de um eletrodo polarizado na borda do plasma no tokamak TCABR. O eletrodo pode ser introduzido até 1,5 cm para dentro da coluna do plasma, sem causar disrupturas para polarização positiva de 0 até +350V, e situado no plano equatorial do tokamak. Perfis tomográficos de H-alfa com e sem polarização foram medidos. A comparação dos perfis mostra um aumento da densidade de linha na posição central, quando a emissividade H-alfa diminui. A análise dos perfis tomográficos de H-alfa, tempo de confinamento das partículas e também do estudo de reciclagem das partículas neutras, indica que o plasma entra no regime de alto confinamento (modo-H). Cálculos de turbulência e de transporte na borda do plasma (SOL), feitos medindo o potencial flutuante e a corrente de saturação de íons, mostram uma diminuição forte no espectro de potência e de transporte. Também foram feitos estudos do novo regime de descargas com elétrons fugitivos (\"runaway electron\"), descoberto no tokamak TCABR. As características distintivas deste regime são um plasma de baixa temperatura fracamente ionizado, destacado do limitador devido a processos de recombinação, e instabilidade de relaxação com fortes picos de emissão H-alfa correlacionados com instabilidade dente de serra da densidade eletrônica de linha. No presente trabalho fazemos a descrição das condições experimentais para a geração destas descargas. A produção dos elétrons fugitivos é analisada; mostrando que a geração de elétrons fugitivos somente pode ser explicada pelo mecanismo de avalanche. A confirmação de baixa temperatura do plasma é obtida de uma análise do perfil tomográfico da emissão H-alfa. Esta emissão não pode ser explicada por excitação de elétrons no plasma. A recombinação, de outro lado, dá uma explicação plausível para a dependência temporal da emissão, em particular para alta densidade de partículas neutras. / A study of the tomography profile of the emission of the line of Hydrogen, atomic H-alpha line (?=656.28 nm), was carried out in TCABR, a medium-size tokamak in operation at the Laboratory of Plasma Physics of the Institute of Physics of the University of São Paulo. Our work focuses on the study of the effects of due to the introduction of a biased electrode in the plasma edge of the TCABR tokamak. The electrode could be introduced up to 1.5 cm inside the plasma, without plasma disruptions for positive voltages from 0 to +350V, and was located on the equatorial plane of the plasma column. Tomography profiles of H-alpha with and without bias were measured. Comparison of the profiles shows an increase of the central line-averaged density, while the emissivity of the line H-alpha decreases. The analysis of the tomography profiles of H-alpha, time of confinement of particles and also the study of recycling of the neutral particles, indicate that the confined plasma enters the H-mode regime. Calculations of turbulence and transport at the Scrape-Off-Layer, using measured floating potentials and ion saturation currents, show a strong decrease in the power spectra and transport. The H-alpha tomography was also employed to study the new regime of runaway discharges that has been discovered in the TCABR tokamak. The distinctive features of this regime are weakly ionized low-temperature plasma detached from the limiter due to the recombination process, and a relaxation instability with strong spikes of H-alpha emission correlated with sawtooth relaxation of the line density. In the present thesis we report experimental data on conditions for generation of these discharges. The runaway electron production is analyzed; show that generation of runaway electrons can only be explained by the runaway avalanche mechanism. The confirmation of low plasma temperature is a obtained from an analysis of the tomography profile of H-alpha emission. This emission cannot be explained by excitation by plasma electrons. Recombination, on the other hand, gives a rather plausible explanation for the time dependency of the emission, in particular at high neutral densities.

Um injetor de pastilhas (pellets) de impurezas foi projetado, construído e instalado junto ao tokamak TCABR do Instituto de Física da Universidade de São Paulo - USP. O injetor é basicamente constituído por uma câmara com gás a alta pressão cuja vazão é controlada por uma bobina de acionamento rápido. Uma fonte de alta tensão (7kV) e alta corrente (6kA) foi construída para alimentar a bobina. Ao ser disparada, o gás propulsor (N2 ou He) acelera a pastilha para o interior do vaso do tokamak, sem que gás penetre no mesmo. Pastilhas de grafite cilíndricas com diâmetro de 0,4mm a 0,9mm, e comprimento de 0,5mm a 1mm, foram utilizadas para investigar a possibilidade de se mitigar os e*feitos de uma disrupção maior O processo de ablação da pastilha no TCABR foi investigado através de simulações que consideraram, de uma maneira simplificada, o resfriamento sofrido pelo plasma devido à propagação da pastilha no seu interior. O modelo, ao ser aplicado aos dados experimentais do tokamak T-10, por exemplo, apresentou resultados bastante encorajadores. Na maioria dos casos em que houve a injeção das pastilhas no TCABR, o plasma terminou devido ao surgimento de uma disrupção maior. Analisando a taxa de queda da corrente de plasma em dois instantes próximos ao fim da descarga, tanto para pulsos que sofreram uma disrupção natural quanto aos que sofreram um disrupção induzida (devido à injeção de pastilhas de grafite), constatou-se que a corrente de plasma decresce mais lentamente nos casos em que houve o disparo do injetor. Isto significa uma menor sobrecarga nos sistemas eletro-mecânicos do tokamak, em resultado à ocorrência da disrupção. A análise da atividade MHD foi realizada no entorno da disrupção causada pela injeção de pastilha. Observou-se, em conseqüência, uma redução da velocidade de rotação das ilhas magnéticas após o início da interação da pastilha com o plasma. Também constatou-se, em todos os disparos analisados, um aumento significativo da atividade MHD. Um outro resultado interessante foi a constatação de que a emissão de raios-X de altas energias decresce significativamente no final das descargas nas quais ocorre a injeção de pastilhas. / An impurity pellet injector has been projected, built and installed in the TCABR tokamak, at the Physics Institute of Physics of the University of São Paulo - USP. Basically, the injector is composed of a high pressure gas chamber, in which the gas flow (N2 or He) is controlled by a fast switch valve. An high voltage (7kA) and high electric current (6kA) power supply has been built to provide energy for the valve. When fired, the propellant gas move the pellet into the interior of the tokamak vessel. During this process, the gas is properly collected before reaching the tokamak vacuum chamber. For this work, cylindrical carbon pellets with 0,4mm to 0,9mm in diameter and 0,5mm to 1mm in length were chosen as to investigate how the hazardous effects of a major disruption could be mitigated. The pellet ablation process in TCABR was studied trough running simulations that take into account, in a simplified way, the cooling of the plasma by the propagating pellet. The model, when applied to the T-10 tokamak experimental data, for example, exhibited very encouraging results. For the TCABR tokamak, in most of the cases in which pellets were injected, the plasma was terminated because of the advent of a major disruption. By analyzing the plasma current decay rate in two time intervals - within the end of plasma discharges, with and without the injection of pellets, it was observed that the plasma current decays significantly slower when pellets are injected. Consequently the load on the tokamak\'s electromechanical systems is reduced. Fourier analysis has been carried out to investigate the MHD activity near the disruption time, caused by the pellet injection. It could be noticed a reduction on the magnetic island\'s velocity rotation, after the pellet-plasma interaction initiates. Also, for all discharges analyzed, the MHD activities increased in amplitude after the pellet-plasma interaction. Another interesting result refers to the fact that the hard X-ray emission was observed to decrease significantly within the end of discharges in which pellets were injected

No description available.

An important part of the fusion research program is to understand and control the large number of plasma instabilities that a fusion plasma can exhibit. One such instability is known as the “sawtooth” instability, which is a perturbation in the plasma electric and magnetic fields that manifests itself as periodic relaxations of the temperature and density in the plasma center. The aim of this project was to investigate how the fuel ions in a fusion plasma react to the sawtooth instability. We were able to implement a model of the plasma electromagnetic field during a sawtooth relaxation into an existing code that computes the orbits of the fuel ions in the tokamak magnetic field. To this end, it was necessary to modify the orbit code to allow for non-zero electric fields, and for time-varying fields. In order to validate the new additions to the code, we compared simulated results to analytical ones. The model of the sawtooth electromagnetic fields required for our simulations was set up within a different student project. However, due to unforeseen complications, only the magnetic (not the electric) field contribution was available to us during our project, but once the electric field is available it is straightforward to include in our code. Our simulations did not exhibit any noticeable perturbation to the particle orbit during a sawtooth crash. However, before the electric field contribution is included it is not possible to draw any physics conclusions from these results. Our code could also be used as a foundation for future projects since it is possible (with further implementations to the existing code) to simulate how the spatial profile of the neutron emission is expected to vary during the sawtooth. These simulations can be compared against experimental measurements of the neutron emission profile in order to investigate the accuracy of the sawtooth model under consideration.

Edge localized modes (ELMs) are a concern for future magnetic fusion devices, such as ITER, due to the large transient heat loads they generate on the plasma facing components. A very promising method of ELM suppression is an application of resonant magnetic perturbations (RMP); however, such application leads to localized places of higher heat fluxes called footprints. Both ELMs and RMP could limit the operational lifetime of the device. In this thesis, we analyze the temporal and spatial distribution of footprints using the tangle distance method in the aim to prevent a transient overheating. We also analyze quasi-double-null configuration of the ITER plasma which can be expected to be the most susceptible to overheating of the upper wall. Based on the modelling, the potentially dangerous configurations of the RMP have been shown. Using the ELM filament model included in the LOCUST GPU code, we study temporal and spatial distribution of the heat fluxes caused by ELMs in the axially symmetric and the asymmetric magnetic field. The results are compared with published experimental observations. Powered by TCPDF (www.tcpdf.org)