xvii, 189 p. ： ill. (some col.) / Low-dimensional electronic systems, systems that are restricted to single energy levels in at least one of the three spatial dimensions, have attracted considerable interest in the field of thermoelectric materials. At these scales, the ability to manipulate electronic energy levels offers a great deal of control over a device's thermopower, that is, its ability to generate a voltage due to a thermal gradient. In addition, low-dimensional devices offer increased control over phononic heat flow. Mesoscale geometry can also have a large impact on both electron and phonon dynamics. Effects such as ballistic transport in a two-dimensional electron gas structure can lead to the enhancement or attenuation of electron transmission probabilities in multi-terminal junctions. The first half of this dissertation investigates the transverse thermoelectric properties of a four-terminal ballistic junction containing a central symmetry-breaking scatterer. It is believed that the combined symmetry of the scatterer and junction is the key component to understanding non-linear and thermoelectric transport in these junctions. To this end, experimental investigations on this type of junction were carried out to demonstrate its ability to generate a transverse thermovoltage. To aid in interpreting the results, a multi-terminal scattering-matrix theory was developed that relates the junction's non-linear electronic properties to its thermoelectric properties. The possibility of a transverse thermoelectric device also motivated the first derivation of the transverse thermoelectric efficiency. This second half of this dissertation focuses on heat flow phenomena in InAs/InP heterostructure nanowires. In thermoelectric research, a phononic heat flow between thermal reservoirs is considered parasitic due to its minimal contribution to the electrical output. Recent experiments involving heterostructure nanowires have shown an unexpectedly large heat flow, which is attributed in this dissertation to an interplay between electron-phonon interaction and phononic heat flow. Using finite element modeling, the recent experimental findings have provided a means to probe the electron-phonon interaction in InAs nanowires. In the end, it is found that electron-phonon interaction is an important component in understanding heat flow at the nanoscale. This dissertation includes previously unpublished co-authored material. / Committee in charge: Dr. Richard Taylor， Chair； Dr. Heiner Linke， Advisor； Dr. David Cohen， Member； Dr. John Toner， Member； Dr. David Johnson， Outside Member

Aujourd’hui les propulseurs à effet Hall ont gagné une position dominante dans le marché des propulseurs électriques spatiales. Ce grand succès est du surtout à leur simplicité de réalisation (par rapport aux autres typologies des propulseurs) et à leur efficacité (par rapport aux propulseurs chimiques traditionnels). Les propulseurs à effet Hall sont aujourd’hui utilisés sur un très grand nombre des plateformes satellitaires (surtout pour les télécommunications). Les composants principales d’un propulseur à effet Hall sont : le circuit magnétique, le canal plasma, l’anode (placé au fond du canal plasma avec injecteur du gaz) et la cathode (placée à l’extérieur du canal plasma). Le fonctionnement d’un propulseur à effet Hall est basé sur la génération d’un champ électrique axial (généré entre l’anode et la cathode) et d’un champ magnétique radial (perpendiculaires entre eux). Le champ magnétique a le rôle de former une zone de très forte concentration électronique (il emprisonne les électrons générés par la cathode) pour permettre aux atomes neutres du gaz de se ioniser. Le champ électrique a le rôle d’accélérer les ions vers l’extérieur du canal. Cette accélération génère la poussée. Le champ magnétique joue un rôle crucial dans le fonctionnement d’un propulseur à effet Hall. La forme du champ magnétique impacte sur les performances propulsifs et sur l’érosion du propulseurs. La topologie magnétique classique des propulseurs à effet Hall n’a subi presque pas des changements depuis les années de développement de cette technologie parce qu’elle garanti des performances propulsifs assez satisfaisantes. Aujourd’hui, avec les nouvelles exigences propulsifs, il y a une très forte nécessité des moteurs avec une durée de vie plus longue. Des nouvelles topologie magnétique innovante sont proposés aujourd’hui comme par exemple le "Magnetique-Shielding" ou le "Wall-Less" . Ces topologies magnétique bouleverse complètement la topologie magnétique classique (en gardant des performances propulsif satisfaisantes) pour protéger le moteur de l’érosion du plasma. Dans cette thèse une autre approche a été adopté. Nous avons pensé d’utiliser une topologie magnétique classique et de déplacer les parties du circuit magnétique attaquées par l’érosion vers des zones moins dangereuses. Nous avons agit sur la forme du circuit magnétique et pas sur la forme de la topologie magnétique pour garder les même performances propulsifs de la topologie magnétique classique. L’objectif de la thèse était de créer des outils pour le design et l’optimisation des circuits magnétiques des propulseurs à effet Hall. Un algorithme nommé ATOP a été créé dans l’équipe de recherche GREM3 du laboratoire LAPLACE de Toulouse. Cette thèse a contribué à la création de la section d’optimisation paramétrique (ATOPPO) et d’une section d’optimisation topologique basée sur les algorithmes génétiques (ATOPTOga) de l’algorithme ATOP. Les algorithme conçues dans cette thèse permettent d’optimiser des propulseurs existants (en terme de forme, masse et courant) ou de concevoir des nouveaux propulseurs (nécessité de concevoir un nouveau propulseur capable de reproduire une topologie magnétique précise). Les algorithmes développées ont démontrés leur efficacité à travers leur application sur un propulseur réel, le PPS-1350-E® de SAFRAN. Ce propulseur a été optimisé en terme de masse et de courant bobines (minimisation de la masse et du courant bobines). Les algorithmes développés ont démontré donc leur efficacité comme instrument d’optimisation et de design. Ces deux algorithmes ont été utilisé pour le design d’un circuit magnétique innovant qui a comme objectif de réduire l’érosion du moteur. Les résultats de ce processus de design ont amené à la réalisation et à la construction d’un prototype qui possède la même topologie magnétique du propulseur PPS- 1350-E® commercialisé par SAFRAN mais avec une circuit magnétique de forme différente. / Nowadays, two types of space propulsion engines exist: the most common ones are the chemical propulsion engines which provide high thrust impulses allowing fast orbit transfers. But this technology requires a large amount of propellant and is not suitable for interplanetary displacements, whose propellant mass requirements are too high. The second type of propulsion engine is based on electrical propulsion that provide very low but continuous thrust, resulting in huge propellant savings at the cost of longer spacecraft transfer time. The main advantage of electric thrusters lies in their highly efficient utilization of propellant mass. The corresponding reduction in necessary propellant supply makes it possible to board a greater portion of useful payload possible. Hall thruster belongs to the electric spacecraft engines typology and it is constituted of a cylindrical plasma channel, an interior anode, an external cathode and a magnetic circuit that generates a primarily radial magnetic field across the plasma channel. The magnetic circuit of a Hall effect thruster must generate a specific electromagnetic distribution inside and near the outlet of the plasma channel. In a Hall Effect thruster the magnetic circuit constitutes more than half of the whole thruster. Consequently the design of this magnetic circuit must be optimized in order to minimize the embedded mass. The main components of this circuit are the coils which produce the magnetic and ferromagnetic parts which guide the and to shape the density. Usually the magnetic circuit includes four (or more) external coils located around the exterior radius of the plasma channel and one internal coil around the interior radius of the plasma channel. All the coils are supplied by the same DC. Two coils located at the rear of the plasma channel can be also used to perform the magnetic topography. The first objective of the design process of this type of structure is to obtain a specific magnetic topography in the thruster channel with given magnetic field radial component values and a certain inclination of the corresponding field lines. By considering nowadays the requirements in terms of lifetime new specifications concerning in particular the erosion of ceramic wall have to be taken into account.This weakness has its origins in plasma-surface interaction inside the discharge chamber. Thus, to solve this problem it has been proposed to move the ionization zone outside the thruster channel in order to avoid contact between the ions and ceramic material. Thanks to new studies carried on the impact of magnetic topology, new magnetic configurations have been highlighted to improve the efficiency and reduce the erosion of the ceramic walls. The aim of this work is to develop tools for solving this inverse magnetostatic problem and to find new magnetic structures that are able to produce these new magnetic cartographies. Methods based on topological optimization have already been developed for these structures. The algorithm ATOPTO (Algorithm To Optimize Propulsion with Topology Optimization) has already demonstrated its efficiency. In this work we try to extend the scope of the algorithm ATOP by adding a new parametric optimization section called ATOPPO. The ATOP algorithm becomes a very versatile optimization tool for Hall Effect thruster magnetic circuits.

Orientador: Prof. Dr. Francisco Eugenio Mendonça da Silveira / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Física, 2017.

The goal of this thesis was to both theoretically and experimentally show the effect of a plasma actuator for flow separation control.  In the theoretical part a solver was implemented in MATLAB code, to solve the governing equations describing the plasma actuator.  The experimental part included PIV (Particle Image Velocimetry) measurements of the velocity field induced by the plasma actuator, visualization of the effect in a wind tunnel and the development of a simple model of the plasma actuator based on the empirical result whose purpose is to be used in CFD (Computational Fluid Dynamics). The PIV measurements were performed with an acceptable result even though a lot of disturbance occurred in and near the plasma region.  The empirical result was used to develop the empirical plasma actuator model for CFD, which showed some interesting result.  The model implies that the induced force by the plasma actuator grows exponential with the applied peak-to-peak voltage.   The model was also used to predict airfoil performance with plasma actuators which showed an increase of the lift coefficient on a NACA0012 with a chord length of 0.1m.  Simulations were done for free-stream velocities up to 20m/s with three different configurations, without plasma actuator for comparison, with one actuator at the quarter-chord and one with three actuators on the airfoil.  With three actuators the increase of the lift coefficient was 108 percent at 5m/s and 14 percent at 20m/s. The simulations with one actuator were only performed up to 10m/s were the effect of the actuator still could be seen but for higher velocities the effect would probably be minor. The wind tunnel experiment clearly showed the effect and the advantages of utilizing plasma actuators for flow separation control.  The experiment showed that a single plasma actuator placed at the quarter chord of a fully stalled NACA0012 airfoil with a chord length of 0.1m, at approximately 20 degrees angle of attack and with a free-stream velocity of 1.5m/s, was able to reattach the flow behind the actuator. The result of the theoretical part was inconclusive, the code could not run with the appropriate voltage and frequency of the plasma actuator.  Some result was however obtained, implying that the time-average force induced by the plasma actuator was in the expected direction.  The theoretical model is however considered to have potential, the major problems concern the code which requires further development.

Efeitos da polarização eletrostática de eletrodos na periferia de tokamaks têm sido investigados em pequenos tokamaks e mesmo em alguns tokamaks de grande porte. Em geral as experiências são realizadas em condições em que bifurcação do campo elétrico radial é obtida, processo este identificado como modo H de polarização. No Tokamak TCABR, as experiências indicam que o confinamento aumenta para tensões aplicadas até +300 volts, atingindo um máximo de duas vezes o tempo de confinamento do modo L, mas sem bifurcação. Indícios de bifurcação foram notados com +400 V de polarização, mas a descarga termina devido à excitação da atividade MHD, ainda sob investigação. No presente trabalho, a pesquisa é aprofundada com a utilização de uma sonda de Langmuir com 18 pinos dispostos em duas fileiras sob a forma de um ancinho (rake probe) o que permite a medição da temperatura, densidade e flutuação de potencial ao longo do raio menor na periferia do Tokamak. A resolução temporal desse sistema é de cerca de 0,5 ms, para a temperatura, e 5 microssegundos para densidade e potencial flutuante do plasma. Outra sonda eletrostática com 5-pinos na mesma posição radial, mas em diferentes posições poloidal e toroidal foi usada para medições de turbulência e transporte de partículas. Os efeitos da polarização foram investigados e indicam que os níveis de turbulência e transporte começam a diminuir entre +150 e +200 V e para +300 V chegam a atingir uma quase supressão. Nesse mesmo intervalo de tensão a densidade começa a aumentar e para +300 V chega a ser um fator de aproximadamente 2. Quanto ao perfil de temperatura a variação é pouco significativa, mas as incertezas das medidas são maiores. Esses dados são compatíveis com a criação de uma barreira de transporte na região entre o eletrodo em r = 17 cm e o limitador em a = 18 cm. Além disso, o campo elétrico radial mostra forte cisalhamento nessa região. Tomando o início da subida do potencial flutuante como origem de uma escala de tempo, o atraso temporal do início da subida da densidade de elétrons e o atraso do início do decréscimo do transporte de partículas foram medidos. Os resultados são 50 microssegundos para a densidade de elétrons e 60 microssegundos para o transporte de partículas. A questão dos limiares de potência é discutida no texto. Os dados desta experiência indicam que o campo elétrico radial desempenha o papel principal para a melhoria do confinamento. / Electrode biasing effects in the periphery of tokamaks have been investigated in small tokamaks and in a few large tokamaks. Usually the experiments are performed in conditions were bifurcation of the radial electric field is achieved, identified as biased H mode. In the tokamak TCABR, the biasing experiments indicate that the confinement increases for applied voltages up to +300 Volts, reaching a maximum of twice the confinement time of the L mode, but without bifurcation. Indications of bifurcation were detected with +400 V biasing, but the discharge terminates due to excitation of MHD activity, still under investigation. In the present work, the research is improved with the use of a rake shaped 18-pins Langmuir probe, allowing the measurement of electron temperature, density and potential fluctuations along the minor radius near the edge of the tokamak with time resolution of about 0.5 ms, for the temperature and 5 microseconds for density and floating potential. Another electrostatic probe with 5-pins all in the same radial position, but in different poloidal and toroidal position was used for turbulence and particle transport measurements. The effects of biasing indicate that the levels of turbulence and transport begin to decrease between +150 and +200 V and for +300 V reaches an almost suppression. In that same voltage interval the density, begin to increase and for 300 V reach a factor of about 2. The temperature profile does not change significantly but the uncertainty of the measurements is larger. These data are compatible with the creation of a barrier between the position of the electrode at r = 17 cm and the limiter of the tokamak at a = 18 cm. In addition, the radial electric field data show strong shear in this region. Taking the start of the increasing of the fluctuation potential as the origin of a time scale, the temporal delay of the start of the edge electron density increase and transport decrease were measured. The results are 50 microseconds for the electron density and 60 microseconds for the particle transport. The power threshold for the confinement improvement is discussed in the text. The data obtained in this experiment confirm that the shear of the electric field has the stronger role for the confinement improvement with biasing in TCABR tokamak.

Através de um mapa de ondas de deriva, estudamos o transporte de partículas no Texas Helimak, considerando diversos perfis do campo elétrico radial. O Texas Helimak é um equipamento de confinamento magnético caracterizado por linhas de campo helicoidais e que fornece uma aproximação experimental de um plasma unidimensional. Ele possibilita a imposição de um potencial elétrico externo ao plasma, chamado bias, que altera o perfil radial do campo elétrico de equilíbrio e, consequentemente, possui influência sobre as características de transporte no plasma. Para estudar o efeito do bias sobre o transporte, utilizamos um modelo que considera flutuações eletrostáticas, associadas à deriva E x B, como mecanismo de turbulência. Com isso, introduzimos um mapa de ondas de deriva, cujos parâmetros estão relacionados a dados experimentais para diversos valores de bias. Assim, ao variar o bias, pudemos observar a formação e a destruição da curva sem shear, bem como seu efeito sobre o transporte das trajetórias no espaço de fase. / Using a drift wave map, we studied the particle transport in Texas Helimak considering various electric field radial profiles. Texas Helimak is a device for magnetic confinement characterized by helical field lines, and constitutes an experimental approximation to a one-dimensional plasma. It allows for the imposing of an external electric potential, known as bias, which changes the equilibrium electric field radial profile and hence the transport properties of the plasma. In order to study the effects of the bias potential on the particle transport, we used a model with electrostatic fluctuations associated to E x B drift as the turbulence mechanism. Thus, we introduced a drift wave map whose parameters are related to experimental data for various values of bias. Therefore, by varying the bias, we observed the formation and destruction of the shearless curve, as well as its effects on trajectories transport in the map\'s phase space.

Desenvolvemos uma aplicação das análises espectrais por Fourier e wavelet para o estudo de flutuações intermitentes e não estacionárias. Para isso, elaboramos algoritmos adequados, que revelam os modos principais presentes nas flutuações analisadas e as suas evoluções. Baseados nesses algoritmos, apresentamos um método para a seleção de \"bursts\" (irrupções) em meio ao background da série intermitente anali­sada. Inicialmente, aplicamos essa análise às flutuações intermitentes obtidas inte­grando numericamente as equações de Lorenz. Dessa forma, obtivemos a evolução do espectro das frequências dessas flutuações e selecionamos os seus bursts. Simi­larmente, obtivemos a evolução dos espectros de frequência de flutuações elétricas turbulentas medidas no tokamak TCABR. Confirmamos que a análise por wavelet é adequada para observar a modulação da turbulência pelas oscilações magnéticas. Para essas flutuações, selecionamos os bursts intermitentes presentes nas flutuações turbulentas e obtivemos as distribuições dos intervalos de tempo entre bursts sucessivos. Obtivemos, ainda, as distribuições das medidas da flutuação da turbulência no TCABR. Com a modulação mencionada, a distribuição obtida pode ser reproduzida como a convolução entre as distribuições senoidais (associadas às flutuações magnéticas) e uma distribuição (associada às flutuações turbulentas) como a obser­vada sem a modulação (como as observadas no plasma não perturbado). / We apply the Fourier and wavelet spectral analyses to study nonstationary inter­mittent fluctuations. For that we introduce algorithms that show the dominant fre­quency modes and their evolution. Moreover, applying these algorithms, we present a method to discriminate the burst from the background in the intermittent fluctuations. Initially, we apply this analysis to the intermittent fluctuations obtained integrating numerically the Lorenz equations. Thus, we obtain the frequency spectra evolution of these fluctuations as well as the selected bursts sequence. Similarly, we obtain the frequency spectra evolution for the electric plasma edge turbulence in the TCABR tokamak. We confirm that the wavelet analysis describes well the turbulence modulation by the magnetic fluctuations. For this turbulence, we selected the intermittent bursts and present the histograms of the time interval between two successive bursts. Furthermore, we also present the PDFs of the total turbulent fluctuations. When the turbulence is modulated by the magnetic fluctuation, the observed PDF is interpreted as the convolution between sine distributions (due to the magnetic fluctuations influence) and the distribution (due to the turbulence) observed without modulation.

Consideramos um modelo hamiltoniano do movimento eletrostático de deriva para investigar o trasnporte caótico de partículas na borda de plasmas confinados em Tokamaks. Este modelo leva em conta a turbulência eletrostática de deriva, responsável pelo transporte anômalo. O modelo Hamiltoniano provê as equações de movimento, que são dependentes de uma função para o potencial elétrico. Esta função é caracterizada por um potencial de equilíbrio mais um termo correspondente às ondas de deriva. Assumimos três diferentes perfis radiais para o campo elétrico radial de equilíbrio: um linear e outros dois não-monotônicos com extremos suaves. Para estes perfis, mostramos que o modelo pode ser reduzido a três mapas simpléticos bidimensionais e não integráveis: o mapa padrão, o mapa padrão não twist e um mapa modelo não twist introduzido neste trabalho. O mapa padrão não twist e o mapa modelo violam a condição twist, fundamental para os teoremas KAM e de Birkhoff. Para estes mapas não twist, estudaremos numericamente barreiras de transporte criadas próximas às curvas shearless. Mostramos que, para o mapa modelo, a barreira de transporte é robusta, isto é, persiste em um amplo intervalo de variação de um de seus parâmetros. Dentro da região da barreira, descrevemos o nascimento de cadeias de ilhas com períodos par e ímpar devido à variação do parâmetro de controle. Analisamos estes dois cenários calculando os números de rotação dentro da barreira e identificando as bifurcações que criam as ilhas. Finalmente, conjecturamos que todas as ilhas dentro da região da barreira são criadas por estes dois cenários. Além disso, se o número de rotação da curva shearless atinge um número racional, as cadeias de ilhas são criadas de acordo com os cenários descritos. / We consider a hamiltonian model of the electrostatic drift motion to investigate chaotic particle transport in the Tokamak plasma edge. This model takes into account the electrostatic drift turbulence, which is responsible for the anomalous transport. The Hamiltonian model provides the basic equations of motion, which are dependent on the form of an electric potential function. This function is characterized by the equilibrium potential and the term corresponding to the drift waves. We assume three diferent radial profiles for the equilibrium radial electric field: one linear and the other two non-monotonic with a smooth extremum. For these profiles, we show that the model can be reduced to three symplectic maps: the standard map, the nontwist standard map, and a nontwist model map introduced in this work. The nontwist standard map and the model map violate the twist condition, a property of fundamental importance for the applicability of the KAM and Birkhoff theorems. For these nontwist maps, we study numerically the transport barriers created around their shearless curves. We show for the model map that the transport barrier is robust,i.e., remains for a wide range of one of its parameters. Inside the barrier region, we describe the birth of island chains with even or odd periods due to the control parameter variation. We analyse these two scenarios by calculating the winding numbers inside the barrier region and identifying the bifurcations that create the islands. Finally, we conjecture that all the island chains inside the barrier are created by these two scenarios. Moreover, if the winding number of the shearless curve reachs a rational number, the island chains are created according to the described scenarios.

In this work we investigated the excitation of Alfvén eigenmodes in tokamaks using external antennas to the plasma column. The basic theory of Alfvén waves is revised, including non-ideal effects such as resistivity. Then the theoretical model for excitation Alfvén waves in a cylindrical plasma column, developed by Kurt Appert, is shown in detail, as an introduction to the more complex problem of Alfvén waves in toroidal plasmas. The cylindrical model is implemented in a numerical code, which is used to study the excitation of Global Alfvén Waves (GAWs), below to the so-called Continuum of Alfvén, in TCABR and JET tokamaks, using a realistic description of their antenna systems. In the sequel, it is given a brief description of Toroidal Alfvén eigenmodes (TAEs) that are excited in the gaps of the Continuum of Alfvén created by the periodicity condition of the toroidal configuration. The excitement of these modes in JET tokamak is studied using the codes HELENA, for reconstruction of magneto-hydrodynamic equilibrium, and CASTOR, which calculates the perturbed fields in this equilibrium, coupled with instability or modes excited within the magneto-resistive hydrodynamic model. This study was carried out in order to determine, consistently, the spectrum quality and the eigenmodes associated with TAEs, with different numbers toroidal n, excited by the new JET antenna system. In particular, it was investigated in detail the effect of the phases of the supply currents of the different modules (eight) of the antenna system in the quality of the excited spectrum, using an original method, implemented in this work, based on the CASTOR code. The results indicate that, although the excitation of a certain mode may be a privileged by an optimized choice of phases, satellite modes can also be excited with higher amplitude, so that the purity of the spectrum is not substantially improved. This is the main result obtained in this work. / Neste trabalho é investigada a excitação de modos própios de Alfvén em tokamaks, utilizando antenas externas à coluna de plasma. A teoria básica das ondas de Alfvén é revista, incluindo efeitos não ideais, como resistividade. A seguir, o modelo teórico para excitação de ondas de Alfvén numa coluna cilindrica de plasma, desenvolvido por Kurt Appert, é apresentado em detalhe, como introdução ao problema mais complexo de ondas de Alfvén em plasmas toroidais. O modelo cilindrico é implementado em um código numérico, que é utilizado para estudar a excitação de modos globais de Alfvén (GAWs - Global Alfvén Waves), abaixo do chamado Continuo de Alfvén, nos tokamaks TCABR e JET, utilizando uma descrição realista de seus sistemas de antenas. A seguir é feita uma breve descrição dos auto modos toroidais de Alfvén (TAEs - Toroidal Alfvén Eigenmodes) que são excitados nas brechas do Continuo de Alfvén criadas pela condição de periodicidade em configurações toroidais. A excitação desses modos no tokamak JET é estudada utilizando os códigos HELENA, para reconstrução do equilíbrio magneto-hidrodinâmico, e CASTOR, que calcula os campos perturbados nesse equilíbrio, associados a instabilidades ou modos excitados, dentro do modelo magneto-hidrodinâmico resistivo. Esse estudo foi feito com o objetivo de determinar, de forma consistente, a qualidade do espectro e as auto-funções associadas a TAEs, com diferentes números toroidais n, excitados pelo atual sistema de antenas do JET. Em particular, foi investigado em detalhe o efeito das fases das correntes de alimentação dos diferentes módulos (oito) do sistema de antenas na qualidade do espectro excitado, utilizando um método original, implementado neste trabalho, de utilizar o código CASTOR. Os resultados indicam que embora a excitação de um determinado modo possa ser privilegiado por uma escolha ótima das fases, modos satélites também podem ser excitados com maior amplitude, de modo que a pureza do espectro não é substancialmente melhorada. Este é o principal resultado obtido neste trabalho.

Neste trabalho foram estudadas instabilidades magnetohidrodinâmicas (MHD) utilizando um novo sistema bolométrico que foi instalado no tokamak TCABR para medidas da evolução temporal da potência irradiada. Este novo sistema conta com 24 cordas verticais, capazes de mapear toda uma secção poloidal da coluna de plasma com resolução espacial de aproximadamente 2 cm e uma resolução temporal de 20 µs. Como se sabe, as instabilidades MHD degradam o connamento do plasma e modicam a topologia das superfícies magnéticas, causando a perda da energia do plasma. Por conta disso, compreender essas instabilidades é fundamental para o sucesso dos futuros reatores de fusão nuclear. As perturbações (oscilações) causadas pelas instabilidades MHD modulam diversos parâmetros macroscópicos do plasma como a densidade, a temperatura e a potência irradiada. Então, utilizando o diagnóstico bolométrico, é possível medir as oscilações no perl de potência irradiada e, a partir deles, extrair informações importantes para determinar a origem e as características de tais instabilidades. No tokamak TCABR, as instabilidades foram caracterizadas através da análise espectral dos 24 sinais provenientes do novo sistema bolométrico. Para auxiliar a caracterização das instabilidades, um programa foi desenvolvido em Matlab para simular as medidas das perturbações no perl de potência irradiada. Através do mesmo procedimento de análise espectral, os resultados simulados foram comparados aos experimentais de forma que os parâmetros simulados, como largura e posição das ilhas magnéticas, fossem ajustados aos experimentais. Através dessa metodologia de análise, que combina simulação e experimento, foi possível caracterizar diversas instabilidades como o precursor dos dentes de serra e ilhas magnéticas de modos m = 2 e m = 3. / In this dissertation, magnetohydrodynamic (MHD) instabilities were investigated using a new bolometric system that was installed in the TCABR tokamak for radiation power measurements. This diagnostic is composed by 24 vertical chords that provide a full view of the poloidal cross section of the plasma column and provides spatial and temporal proles with approximately 2 cm space and 20 µs time resolution. As it is well known, the MHD instabilities degrade the plasma connement and modify the magnetic topology, leading to energy loss from the plasma. Therefore, the understanding of these instabilities is essential for the success of the controlled thermonuclear fusion reactors. The MHD instabilities also cause perturbations (oscillations) in various macroscopic parameters, such as plasma density, temperature, and radiated power. Therefore, the oscillations in the radiated power prole measured by the bolometric diagnostic system provide a possibility to investigate the origin and features of the instabilities. In the TCABR tokamak, the instabilities were characterized by spectral analysis of the 24 vertical chords of the bolometric signals. In addition, a Matlab program was developed to simulate the integral characteristic of the oscillations in the radiated power measured by the bolometric system. The spectral analysis of the simulated signals is then compared with the spectral analysis of the bolometric signals. The simulated parameters, island width and radial position, were then adjusted to t the experimental spectrum results. Using this method of analysis, which combines experiment and simulation, it was possible to characterize various instabilities, such as sawtooth precursor and m = 2 and m = 3 magnetic islands.