Les éléments de parois des réacteurs de fusion nucléaire sont soumis à de forts flux de chaleur, des disruptions du plasma et des forts flux des particules. Cette interaction donne lieu à la dégradation de la performance globale des matériaux, diminue la durée de vie des composants et a une forte influence sur la performance du plasma. Un des gros problèmes des interactions plasma-surface est la formation de bulles dans les matériaux, et de cloques en surface. En effet, la formation des bulles et des cloques modifie les propriétés du matériaux et favorise la rétention d’hydrogène. Les expériences ont montré que ce phénomène est influencé par plusieurs paramètres tels que l’énergie des ions impactant la surface, la fluence du plasma, la microstructure et la direction cristallographique des matériaux. Ce travail se concentre sur l’analyse de la dynamique de croissance des bulles et des cloques due à l’exposition à un plasma d’hydrogène dans des matériaux avec un système cristallin cubique et une faible solubilité de l’hydrogène, i.e. l’aluminium. Cela fournit un contexte approprié pour comprendre les phénomènes liés à la structure cristallographique dans des systèmes hexagonaux plus complexes tels que le béryllium. Afin d’effectuer les expériences, les échantillons sont polis et soumis à un traitement thermique pour obtenir un matériau de base de basse rugosité sans contraintes. Par la suite, les échantillons ont été exposés à un plasma d’hydrogène entièrement caractérisé dans lequel plusieurs paramètres ont été variés, tels quel’énergie ionique incidente, la fluence du plasma et la nature de l’exposition. Ce dernier point a été réalisé afin d’étudier les effets de la contrainte, de la relaxation et du refroidissement sur la formation de bulles et de cloques, étant donné que les réacteurs plasma de fusion actuels fonctionnent en cycles au lieu de l’exposition continue au plasma. De plus, la microstructure et l’orientation cristallographique des matériaux ont été investiguées au cours des expériences.L’analyse de l’orientation cristallographique a été effectuée en utilisant des monocristaux{100}, {110} and {111}. Cela permet d’étudier la morphologie des cloques sans l’effet des joints des grains et de poser les bases pour comprendre les systèmes de cristaux hexagonaux.Finalement, afin de comprendre la dynamique de l’hydrogène dans les matériaux, un modèle d’équations macroscopiques 1D avec un code appelé Hydrogen Isotope Inventory Processes Code (HIIPC) a été utilisé. Ce modèle permet de prédire la quantité des isotopes d’hydrogène(HI) retenue dans les matériaux et les processus physiques impliqués dans cette interaction, / Plasma facing components in fusion reactors are exposed to intense thermal loads, plasma disruptions and high-flux particle bombardment. This leads to a plasma wall interactionthat degrades the overall performance of the materials, limits the lifetime of the components and has a strong influence on the plasma performance. One problem derived from plasmawall interactions is bubble and blister formation in materials. This poses a great concern since it changes the material properties and favors hydrogen isotope (HI) retention. Since tritium, a HI, is radioactive, its inventory is quite limited. Experiments have shown that surface modifications are highly influenced by several parameters such incident ion energy,fluence and crystallographic orientation. This work focuses on analyzing blister and bubble dynamics due to hydrogen plasma exposure in materials with a cubic crystal system and alow hydrogen solubility (i.e. Al and W). This provides a suitable background to understand phenomena related to crystallographic structure in hexagonal systems such as beryllium. In order to perform the experiments, the samples were polished and submitted to a heat treat mentto obtain a well-defined low-roughness base material. Afterwards, they were exposedto a fully characterized hydrogen plasma in which several parameters were varied, such asincident ion energy, fluence and discharge regime. The latter was performed in order to studythe effects stress, relaxation and cooling have on bubble and blister formation given tha tcurrent plasma reactors work in cycles instead of continuous plasma exposure. In addition,the microstructure and crystallographic orientation of the materials was varied during the experiments. The analysis of crystallographic orientation were performed by using {100},{110} and {111} single crystals. This allows studying blister morphology without the effect of grain boundaries and setting the basis to understand hexagonal crystal systems. Finally, in order to understand hydrogen dynamics in materials a 1D macroscopic rate equations model with a code named Hydrogen Isotope Inventory Processes Code (HIIPC) was used. This model allows predicting the amount of retained HI’s in materials and the physical processes involved in this interaction such as HI implantation, migration, depth distribution and their release. The results obtained with HIIPC support the results obtained in the experimental section and contribute in the understanding of hydrogen dynamics in material.

Dans la conception des futurs réacteurs de fusion, l’impact des interactions plasma – paroi pèse grandement sur le choix des matériaux à utiliser en première interface. L’utilisation du tritium en tant que combustible impose de plus des limites de sécurité quant à la quantité totale contenue dans le réacteur. L’analyse d’échantillons de parois de Tokamaks a montré une pénétration et une rétention du deutérium (utilisé à la place du tritium) au sein des matériaux carbonés; cette rétention est problématique car contrairement à la rétention dans les couches co-déposées, on ne peut espérer l’éliminer facilement. De part l’accès difficile aux échantillons réels, l’étude de ce phénomène se limite souvent à des analyses post-mortem.Afin d’accéder à la dynamique du phénomène et de s’affranchir de potentielles redistributions des éléments lors du stockage, un dispositif couplant micro analyse nucléaire (µNRA) et implantation basse énergie simultanée, visant à reproduire l’interaction entre le deutérium et les matériaux de la première paroi, a été mis en place. L’analyse µNRA permet de caractériser les profils de répartition en trois dimensions du deutérium en temps réel, à des échelles micrométriques. Des tests ont permis de confirmer le caractère non-perturbateur du faisceau d’analyse.On observe sur l’ensemble des données obtenues que la surface de l’échantillon (0-1 µm) présente une teneur en deutérium élevée et quasi constante ; la répartition du deutérium y est uniforme. A contrario, le deutérium piégé en profondeur (1-11 µm) se concentre dans des sites préférentiels liés à la microstructure du matériau. L’inventaire deutérium en profondeur semble augmenter avec la fluence incidente, malgré une grande dispersion des données attribuée à la variation de structure des zones étudiées. La saturation surfacique comme la migration en profondeur sont instantanées ; le stockage sous vide entraine une légère désorption du deutérium.Les observations faites par µNRA ont été croisées avec celles obtenues via d’autres techniques expérimentales. La µtomographie X a permis d’identifier clairement les porosités comme sites de localisation préférentielle du deutérium en profondeur. La micro-spectrométrie Raman a révélé la formation d’une couche amorphe fine (~30 nm) et saturée en deutérium à la surface du CFC suite à l’exposition au faisceau de deutérium. Enfin, la caractérisation expérimentale de la migration du deutérium dans les CFC obtenue est confrontée aux modèles existants, et un modèle simplifié original est proposé. Considérant que le dépôt en profondeur se produit par le biais de l’implantation et de la diffusion coulombienne du deutérium à la surface des porosités, il permet de reproduire qualitativement les profils de migration observés. / Plasma-wall interactions play an important part while choosing materials for the first wall in future fusion reactors. Moreover, the use of tritium as a fuel will impose safety limits regarding the total amount present in the tokamak. Previous analyses of first-wall samples exposed to fusion plasma highlighted an in-bulk migration of deuterium (used as an analog to tritium) in carbon materials. Despite its limited value, this retention is problematic: contrary to co-deposited layers, it seems very unlikely to recover easily the deuterium retained in such a way. Because of the difficult access to in situ samples, most published studies on the subject were carried out using post-mortem sample analysis.In order to access to the dynamic of the phenomenon and come apart potential element redistribution during storage, we set up a bench intended for simultaneous low energy ion implantation, reproducing the deuterium interaction with first-wall materials, and high energy microbeam analysis. Nuclear reaction analysis performed at the micrometric scale (µNRA) allows characterizing deuterium repartition profiles in situ. This analysis technique was checked to be non-perturbative.We observed from the experimental data set that the material surface (depth 0-1 µm) displays a high and nearly constant deuterium content, with a uniform distribution. On the contrary, in-bulk deuterium (1-11 µm) localizes in preferential trapping sites related to the material microstructure. In-bulk deuterium inventory seems to increase with the incident fluence, in spite of the wide data scattering attributed to the structure variation of studied regions. Deuterium saturation at the surface as well as in-depth migration is instantaneous; in-vacuum storage leads only to a small deuterium global desorption.Observations made via µNRA were combined with results from other characterization techniques. X-ray µtomography allowed identifying porosities as the preferential trapping sites for in-depth deuterium retention. Raman µspectrometry disclosed the formation of an amorphous layer at the surface, very thin (~30 nm) and deuterium saturated, following deuterium irradiation.At last, we confronted the experimental characterization obtained with existing models for deuterium behaviour in carbon materials and proposed a simple and original one. Considering that in-depth retention is due to deuterium implantation and Coulombian diffusion at the open porosity surfaces, it allows reproducing qualitatively the observed experimental profiles.

Trois raisons poussent à caractériser et contrôler l´interaction plasma-surface dans les machines de fusion thermonucléaire : 1/ la limitation du temps de vie des éléments de première paroi du fait de leur érosion par le plasma, 2/ la pollution de celui-ci par les particules érodées et la diminution des performances qui en découle et 3/ la rétention du gaz de travail (D, T) dans les parois ou dans les couches redéposée constituées de produits d´érosion. Dans les machines à mur carbone, les points 1/ et 3/ sont étroitement couplés, jouant un rôle capital dans le bilan de matière du fait de la forte affinité chimique du carbone avec l´hydrogène ou ses isotopes. Si le bilan érosion/redéposition est souvent obtenu à partir d'analyses post-mortem d´echantillons prélevés dans la machine, deux méthodes sont utilisées pour établir le bilan de rétention en quantifiant la quantité d'hydrogène piégée dans la paroi : les analyses post-mortem, et le bilan de gaz décharge par décharge. Ces deux méthodes conduisent à des estimations en fort désaccord, la quantité d´hydrogène piégée obtenue à partir des analyses post-mortem étant inférieure d'un facteur quatre à celle obtenue à partir du bilan de gaz. La raison en est que la première est résolue dans l´espace et intégrée sur le temps, alors que la seconde st une valeur globale pour toute la machine mais résolue choc à choc. Résoudre le désaccord entre les eux estimations précitées implique alors d´étendre les mesures post-mortem à l´ensemble des éléments de première paroi et le bilan de gaz à toute la période pendant laquelle ces éléments ont été utilisés (périodes sans plasma, ou pendant lesquelles la machine était ouverte). / Three reasons can be invoked to characterize and control plasma-surface interaction in thermonuclear fusion devices : 1/ the plasma erosion limits the lifetime of the first wall components, 2/ the penetration of eroded particles in the plasma is the cause of fuel dilution and loss of performance and 3/ part of the fuel (D/T) is trapped in the wall or layers resulting from the redeposition of eroded particles. In carbon wall devices, points 1/ and 3/ are strongly coupled due to the chemical affinity of carbon with hydrogen or its isotopes. If the erosion/redeposition balance is often obtained from post-mortem analyses of samples extracted from the vacuum chamber, two methods are currently used to build the fuel balance, and particularly to quantify the amount of which trapped in the vessel : the post-mortem analyses cited above, and discharge per discharge gas balance. Estimations by these two methods exhibit a significant discrepancy, the amount of trapped hydrogen estimated by post-mortem being typically four times lower than that obtained from gas balance. The main reason is that the former value is resolved in space (it depends of the location of the sample in the vacuum vessel) but integrated in time (it concerns the whole period during which the sample was in the device), when the latter is a global value for the whole machine but is resolved discharge per discharge. For solving the discrepancy, one must perform post-mortemanalyses on a number of samples large enough for covering the whole vessel and extend gas balance measurements to the whole period during which the considered first wall elements were used, including the period in between plasmas and vents.

Eines der größten Probleme bei der kontrollierten Kernfusion ist die immense thermische Belastung der mit dem Plasma in Berührung kommenden Materialien. Um den Energiefluß aus dem Plasma auf eine möglichst große Fläche zu verteilen, werden die betroffenen Komponenten in der Regel so angebracht, da"s das magnetische Feld annähernd parallel zur Oberfläche verläuft. Im Rahmen dieser Arbeit wurden ein spezieller drehbarer Meßkopf zur winkelabhängigen Messung des Strom- und Energieflusses entwickelt und ausführliche experimentelle Untersuchungen zur Winkelabhängigkeit der Teilchen- und Energieflüsse auf eine Fläche durchgeführt. Zum Verständnis der zu Grunde liegenden Mechanismen wird basierend auf den Gyrationsbahnen der Teilchen ein analytisches Modell entwickelt und dessen qualitative Übereinstimmung mit den experimentellen Befunden festgestellt. Die Durchführung der Experimente erfolgte am Plasmagenerator PSI-2, einem linearen Divertor-Simulator mit einem moderaten magnetischen Feld. Der Aufbau des Meßkopfes als ebene Sonde in einer isolierten Fläche enspricht dabei in etwa der einer sogenannten ,,flush-mounted probe''''. Die äußeren Maße der Sonde sind dabei vergleichbar mit dem Ionengyroradius ri}. Während die Elektronen bei den Experimenten stark magnetisiert sind, variieren die Bedingungen für die Ionen zwischen unmagnetisiert und magnetisiert je nach Ionenmasse und Magnetfeldstärke. Bei den Experimenten wurden verschiedene Größen der Plasmarandschicht als Funktionen des Winkels zwischen der Oberflächennormale der Sonde und dem Vektor des magnetischen Feldes bestimmt. / In fusion experiments, the energy flux to the target plates is an important issue. In order to spread the heat load, surfaces are usually designed to intersect magnetic field lines at very shallow angles. In the course of this work, a sensitive probe allowing simultaneous measurements of energy flux and current density as functions of a bias voltage was developed. Extensive experimental data on the particle and energy flux densities as functions of the angle between a surface and the confining magnetic field are provided. An analytical model is developed in order to reveal the physics involved; it is in good qualitative agreement with the experimental results. The experiments were conducted at the PSI-2 facility, a linear divertor simulator with moderate magnetic field strength. The probe was rotated in a spatially homogeneous plasma. The active area, a tungsten covered Peltier module, was immersed in a ceramic surface, closely resembling the geometry of a flush mounted probe. Its dimensions were comparable to the ion gyro radius ri. While the electrons were strongly magnetized, the ion conditions varied between unmagnetized and magnetized depending on the ion species. Sheath parameters were determined as functions of the angle alpha between the probe surface normal and the magnetic field.

Der Einsatz von Kohlenstoffmaterialien z.B. in ITER ist damit verbunden, dass es durch physikalische und chemische Zerstäubung zur Bildung von Kohlenwasserstoffen kommt, die im Randschichtplasma zersetzt werden und sich in Form tritiumreicher amorpher Kohlenwasserstoffschichten auf den Wänden ablagern. Deshalb ist ein besseres Verständnis der Bildung, der Zersetzung, des Transports und der Haftung von Kohlenwasserstoffen infolge der Plasma-Wand-Wechselwirkung von großem Interesse. Die genannten Prozesse wurden am linearen Plasmagenerator PSI-2 mit Hilfe verschiedener Diagnostiken für unterschiedliche Plasmen systematisch untersucht. Die Analyse der ablaufenden Reaktionen mittels einfacher Bilanzgleichungen machte es möglich, wichtige Zerfalls- und Bildungskanäle für die verschiedenen Kohlenwasserstoffe und deren Abhängigkeiten von bestimmten Parametern zu ermitteln. Es zeigte sich, dass die starke Zersetzung und Umwandlung von Methan in Wasserstoffplasmen auf die dominierenden Ladungsaustauschreaktionen im Niedertemperaturbereich zurückzuführen ist. Weiterhin wurden Zersetzungslängen für Methan und Ethen gefunden, die im Bereich einiger Zentimeter liegen. Die Untersuchungen der CH-Band-Emission und der Wachstumsprozesse von a-C:H-Schichten ermöglichten die Detektion von globalen und lokalen Zersetzungsprozessen in unterschiedlichen Plasmen, die je nach Plasmadichte (und Gasart) erklärt werden können. Bisher fehlte in allen Modellierungen die atomare Wasserstoffdichte, die hier mit zwei unterschiedlichen Methoden bestimmt wurde. Durch Depositionsexperimente mit und ohne direkten Plasmaeinfluss konnte der durch Stickstoffeinlass verursachte Scavenger Effekt und die damit verbundene Reduktion der Depositionsrate eindeutig nachgewiesen werden. Die Auswertung der QMS-Daten mit Hilfe der Bayesschen Statistik ermöglichte erstmals die Spezifikation der beteiligten Volumenreaktionen. / The materials envisaged for the thermally heavily burdened divertor plates of the international fusion device ITER are CFC materials. As a result of physical and chemical sputtering of these materials many different hydrocarbons are formed which are decomposed at the plasma edge and lead to the deposition of tritium- rich amorphous hydrocarbon films on the vessel walls. Consequently a better understanding of hydrocarbon formation, fragmentation, transport and sticking is an important issue in fusion research. The aforementioned processes are studied systematically at the linear plasma generator PSI-2 using various diagnostics in a range of plasmas. By means of simple balance equations the ongoing reactions could be analysed making it possible to identify important decomposition and formation channels for the various hydrocarbons and their dependence on certain parameters. The strong decomposition and transformation of methane in hydrogen plasmas can be traced back to dominant charge exchange reactions in the low temperature range. In addition, decomposition lengths for methane and ethylene in the range of a few centimetres were found. Spatially resolved measurements of CH band emission and investigations of the growth processes of a-C:H layers enabled the detection of global and local decomposition processes of injected hydrocarbons in different plasmas which can be explained according to the plasma density (and gas type). So far all simulations have lacked information on the density of atomic hydrogen, which in this work has been determined using two different methods. In addition, by means of deposition experiments with and without direct plasma influence, the scavenger effect induced by nitrogen injection and the associated reduction in the deposition rate has been clearly demonstrated. The analysis of the QMS data using Bayesian statistics enabled verification of the volume reactions involved for the first time.

One major step needed for fusion to become a reliable energy source is the development of materials for the extreme conditions (high temperature, radioactivity and erosion) caused by hot plasmas. The main goal of the present study is to use and optimise ion beam methods (lateral resolution and sensitivity) to characterise the distribution of hydrogen isotopes that act as fuel. Materials from the test reactors JET (Joint European Torus), TEXTOR (Tokamak Experiment for Technology Oriented Research) and Tore Supra have been investigated. Deuterium, beryllium and carbon were measured by elastic recoil detection analysis (ERDA) and nuclear reaction analysis (NRA). To ensure high 3D spatial resolution a nuclear microbeam (spot size <10 µm) was used with 3He and 28Si beams. The release of hydrogen caused by the primary ion beam was monitored and accounted for. Large variations in surface (top 10 µm) deuterium concentrations in carbon fibre composites (CFC) from Tore Supra and TEXTOR was found, pointing out the importance of small pits and local fibre structure in understanding fuel retention. At deeper depths into the CFC limiter tiles from Tore Supra, deuterium rich bands were observed confirming the correlation between the internal material structure and fuel storage in the bulk. Sample cross sections from thick deposits on the JET divertor showed elemental distributions that were dominantly laminar although more complex structures also were observed. Depth profiles of this kind elucidate the plasma-wall interaction and material erosion/deposition processes in the reactor vessel. The information gained in this thesis will improve the knowledge of first wall material for the next generation fusion reactors, concerning the fuel retention and the lifetime of the plasma facing materials which is important for safety as well as economical reasons.

In der vorliegenden Arbeit wurde das Strömungsverhalten eines magnetisierten Argonplasmas beim Auftreffen auf eine neutralisierende Oberfläche untersucht. Mit Hilfe der Laserinduzierten Fluoreszenz wurde dazu nicht-invasiv die Geschwindigkeitsverteilung der Ionen mit einer Ortsauflösung von standardmäßig dz=0.5 mm als Funktion des Abstandes zur Oberfläche gemessen. Zwei Situationen wurden untersucht (a): praktisch das ganze Plasma strömt auf ein großes Target (Durchmesser 100 mm) und (b) die Größe des Targets ist wesentlich kleiner (Durchmesser 15 mm) als der Durchmesser der Plasmasäule. Unmittelbar vor der Oberfläche war in beiden Fällen die Strömungsgeschwindigkeit u mindestens so groß wie die Ionenschallgeschwindigkeit cs, genau wie von Bohm bereits 1949 vorhergesagt[]. Unter fusionsrelevanten Bedingungen ist dies die erste direkte Beobachtung des Bohmkriteriums. Bei Annäherung an die Oberfläche steigt die Machzahl M=u/cs von 0.5 auf 1 auf typischen Skalenlängen lambda_a=30 mm bzw. lambda_b=5 mm an. Um diese kurzen Längen erklären zu können wurden die Messdaten in (a) mit einem Stoß-Diffusionsmodell und im Falle von (b) mit dem Modell von Hutchinson[] verglichen. Eine gute Übereinstimmung in (a) wurde erzielt, wenn eine sehr niedrige Neutralgastemperatur von etwa 400 K angenommen wird. Die Messdaten in (b) werden sehr gut durch das Modell wiedergegeben, wenn ein Transportkoeffizient von D=20 m²/s angenommen wird. Ein derartig hoher Transport kann unmöglich allein durch Diffusion verursacht werden. Teilweise kann dieser Transport anhand der endlichen Gyroradien erklärt werden, vermutlich aber spielen auch zeitabhängige Phänomene, wie z.B. Driftwellen eine wichtige Rolle. Weiterhin wurde die Abhängigkeit von dem Winkel zwischen Flächennormalen und B-Feld untersucht. Die unmittelbar vor der Oberfläche auftretenden Überschallströmungen werden verhältnismäßig gut von dem Modell von Chodura[] beschrieben. Im Gegensatz dazu ist die Größe der Zone in der Machzahlen größer eins auftreten deutlich kleiner, als vom Modell vorhergesagt. / In the present work the streaming behavior of a magnetized argon plasma impinging on a neutralizing surface was investigated. For that purpose the ion velocity distribution was measured non-invasively as a function of the distance to the surface by means of Laser Induced Fluorescence. The spatial resolution was typically dz=0.5 mm. Two situations are investigated, (a): when practically the whole plasma streams onto a large target (diameter 100 mm), and (b): when the size of the target (diameter 15 mm) is significantly smaller than the diameter of the plasma column. In both cases the streaming velocity u was at least as high as the ion acoustic sound speed, as already predicted by Bohm in 1949. Under fusion relevant conditions this is the first direct observation of the Bohm criterion. Approaching the target surface the Mach number M=u/c_s increases from values of around 0.5 to 1 on typical scales of lambda_a=30 mm and lambda_b=5 mm, respectively. In order to explain these very short scale lengths the measured data were compared with a collisional-diffusive model in the case of (a) and with Hutchinson''s model[] in the case of (b). A good agreement was achieved in (a) by assuming a very low neutral gas temperature of about 400 K. In (b) the model fits the data excellently when the transport coefficient is chosen as high as D=20 m²/s. Such a high transport cannot be caused solely by diffusion. Partly it is explained by finite gyro-radii effects, but presumably time dependent phenomena, like drift waves, play an important role. In addition the dependence on the angle between surface normal and B-field was investigated. The supersonic fluxes found in the immediate vicinity of the surface are described fairly well by the model developed by Chodura[]. By contrast the size of the region, where Mach numbers greater one appear is significantly smaller than predicted.

Es wurden die Transportphänomene magnetisierter Plasmen in der Umgebung materieller Limiter am linearen Plasmagenerator PSI-2 unter Verwendung von elektrischen und optischen Diagnostiken untersucht. Der Senkrechtdiffusionskoeffizient wurde mit D= 5 m^2/s und einer 1/B-Abhängigkeit bestimmt. Der dominierende Prozess des Radialtransportes ist turbulenter Natur. Unter Berücksichtigung der Volumenionisation kann das radiale Dichteprofil konsistent beschrieben werden. Der Schaft einer Langmuirsonde reduziert in einem magnetisierten Plasma die Elektronendichte. Dieses wurde mit zwei Sonden untersucht. Es wird eine globale Teilchenbilanz vorgestellt, die diese Reduktion beschreibt. Bringt man einen Limiter in ein strömendes magnetisiertes Plasma ein, so bildet sich hinter diesem ein Schatten aus. Vor dem Limiter zeigt sich für einige Plasmaregime ein inverser Schatten. Beide sind durch die starke Abhängigkeit der Emissivität von der Elektronentemperatur zu erklären. Ortsaufgelöste Messungen mit Langmuirsonden und optischer Spektroskopie bestätigen dieses experimentell und zeigen kurze Skalenlängen der Dichte für den Abfall vor und den Anstieg hinter einem Limiter. Die Längen zeigen keine klare Skalierung mit dem Ionengyrationsradius. Es werden ortsaufgelöste Messungen der Plasmaparameter mit einer Langmuirsonde vor einem Limiter unter schrägem Einfall vorgestellt. In Wasserstoffplasmen lässt sich der Dichteverlauf mit dem Modell von Chodura [Cho:82] gut beschreiben. In einer stationären Bogenentladung sind Fluktuationen in der Entladungsspannung zu finden. Diese Fluktuationen erzeugen suprathermische Elektronen, die von Limitern geblockt werden. Durch das Einbringen von Limitern werden turbulente räumliche Strukturen erzeugt, die in das Schattengebiet eindringen. Diese sind sowohl optisch mit Photomultipliern als auch als Fluktuation des Ionensättigungsstromes einer Langmuirsonde nachweisbar. Die Strukturen können den Quertransport in den Plasmaschatten verstärken. / The transport phenomena of magnetized plasmas in the vicinity of a material limiter have been investigated. The investigations were carried out at the linear plasma generator PSI-2 with electrical and optical diagnostics. The perpendicular diffusion coefficient was determined as D= 5 m^2/s with a magnetic field dependence of 1/B. The dominant process of the radial transport is therefore anomalous. By consideration of volume ionization the radial density profile can be consistently described. The shaft of a Langmuir probe acts as a particle sink and reduces electron density. This was investigated using two probes. The reduction can be explained in terms of a global particle model. On immersing a limiter into a streaming, magnetized plasma a distinct shadow region is observed downstream of the target. In addition, for some plasma conditions the region upstream of the target forms an inverse shadow. Both observations can be explained by the strong dependence of the emissivity on electron temperature. This is confirmed experimentally by Langmuir probes and optical spectroscopy. Spatially resolved measurements reveal short scale lengths for a decrease of density in front of and an increase behind a limiter. These lengths do not clearly scale with the ion gyroradius. Spatially resolved Langmuir probe measurements taken in front of a limiter at oblique incidence are presented. For hydrogen plasmas the model of Chodura [Cho:82] describes the density profile quite well. In a stationary arc discharge fluctuations in the discharge voltage are found. The fluctuations produce suprathermal electrons. These electrons are blocked by limiters. A limited plasma produces turbulent spatial structures which penetrate into the shadow region. These structures can be detected with photomultipliers and also as a fluctuation in the ion saturation current of a Langmuir probe. They can enhance perpendicular transport into the plasma shadow.

In this work we study the magnetic field modeling of realistic non-axisymmetric plasma equilibrium configurations and the heat flux patterns on the plasma facing components of tokamak divertor discharges. We start by establishing the relation between generic magnetic configurations and Hamiltonian dynamical systems. We apply the concept of magnetic helicity, used to establish topological bounds for the magnetic field lines in ideal plasmas, and to understand the self-consistency of reconnected magnetic surfaces in non-axisymmetric configurations. After this theoretical discussion, we present some results on magnetohydrodynamic equilibrium and the use of analytical solutions to the Grad-Shafranov equation for describing real tokamak discharges based on the experimental diagnostics and realistic boundary conditions. We also compare the equilibrium reconstruction of a DIII-D discharge obtained with a numerical reconstruction routine, developed as part of this research, and the EFIT code used by several tokamak laboratories around the world. The magnetic topology and plasma profiles obtained with our method are in considerable agreement with the numerical reconstruction performed with the other code. Then, we introduce a simplified description of the generic non-axisymmetric magnetic field created by known sources and implement it numerically for describing the magnetic field due to external coils in tokamak devices. After that, we use this routines to develop a numerical procedure to adjust a suitable set of non-linear parameters of internal filamentary currents, which are intended to model the plasma response based on the magnetic field measurements outside the plasma. Finally, these methods are used to model the magnetic field created by a slowly rotating plasma instability in a real DIII-D discharge. The plasma response modeling is based on the magnetic probe measurements and allow us to calculate the magnetic field in arbitrary locations near the plasma edge. Using this information we determine the non-axisymmetric plasma edge through the magnetic invariant manifolds routine developed during this work. The intersection of the calculated invariant manifold with the tokamak chamber agrees considerably well with the heat flux measurements for the same discharge at the divertor plates, indicating the development of a rotating manifold due to the internal asymmetric plasma currents, giving quantitative support to our simplified description of the magnetic field and the plasma edge definition through the invariant manifolds. / Neste trabalho estuda-se a modelagem do campo magnético em configurações realistas de plasmas em equilíbrio não-axissimétrico e o fluxo de calor nos componentes em contato com o plasma em descargas de tokamaks com desviadores poloidais. Começa-se estabelecendo a relação entre configurações magnéticas arbitrárias e sistemas dinâmicos Hamiltonianos. Então aplicamos o conceito de helicidade magnética, que é usado para estabelecer limitações topológicas sobre as linhas de campo magnético em plasmas ideais, assim como para compreender a auto-consistência das superfícies magnéticas reconectadas em configurações não-axissimétricas. Após esta discussão teórica, apresentam-se alguns resultados sobre o equilíbrio magnetohidrodinâmico e o uso de soluções analíticas à equação de Grad-Shafranov para descrever descargas reais em tokamaks, com base em diagnósticos experimentais e condições de contorno realistas. Também realiza-se uma comparação entre a reconstrução do equilíbrio de uma descarga do DIII-D, obtida mediante uma rotina numérica desenvolvida para esta pesquisa, com a obtida mediante o código EFIT, usado amplamente em diversos tokamaks. Após isso, apresenta-se uma descrição simplificada do campo magnético não-axissimétrico, criado por fontes determinadas, e a sua implementação para descrever o campo magnético devido às correntes externas em tokamaks. Então, usam-se estas rotinas para desenvolver um procedimento numérico que ajusta um conjunto adequado de parâmetros não-lineares de correntes filamentares internas, com as quais pretende-se modelar a resposta do plasma com base nas medidas de campo magnético fora do plasma. Finalmente, estes métodos são utilizados para modelar o campo magnético criado por uma instabilidade com rotação lenta numa descarga do DIII-D. Com base nas medidas das sondas magnéticas é possível modelar os campos criados em regiões arbitrárias próximas da borda do plasma. Usando esta informação é possível determinar a borda não-axissimétrica do plasma mediante as invariantes magnéticas calculadas com a utilização de uma rotina desenvolvida durante este trabalho. A intersecção da superfície invariante com a câmara do tokamak coincide satisfatoriamente com as medidas de fluxo de calor nas placas do divertor para a mesma descarga, indicando o desenvolvimento de uma variedade giratória criada pelas correntes de plasma não-axissimétricas, e sustentando quantitativamente a nossa descrição simplificada do campo magnético, assim como a definição da borda do plasma mediante as invariantes magnéticas.

In this work we study the magnetic field modeling of realistic non-axisymmetric plasma equilibrium configurations and the heat flux patterns on the plasma facing components of tokamak divertor discharges. We start by establishing the relation between generic magnetic configurations and Hamiltonian dynamical systems. We apply the concept of magnetic helicity, used to establish topological bounds for the magnetic field lines in ideal plasmas, and to understand the self-consistency of reconnected magnetic surfaces in non-axisymmetric configurations. After this theoretical discussion, we present some results on magnetohydrodynamic equilibrium and the use of analytical solutions to the Grad-Shafranov equation for describing real tokamak discharges based on the experimental diagnostics and realistic boundary conditions. We also compare the equilibrium reconstruction of a DIII-D discharge obtained with a numerical reconstruction routine, developed as part of this research, and the EFIT code used by several tokamak laboratories around the world. The magnetic topology and plasma profiles obtained with our method are in considerable agreement with the numerical reconstruction performed with the other code. Then, we introduce a simplified description of the generic non-axisymmetric magnetic field created by known sources and implement it numerically for describing the magnetic field due to external coils in tokamak devices. After that, we use this routines to develop a numerical procedure to adjust a suitable set of non-linear parameters of internal filamentary currents, which are intended to model the plasma response based on the magnetic field measurements outside the plasma. Finally, these methods are used to model the magnetic field created by a slowly rotating plasma instability in a real DIII-D discharge. The plasma response modeling is based on the magnetic probe measurements and allow us to calculate the magnetic field in arbitrary locations near the plasma edge. Using this information we determine the non-axisymmetric plasma edge through the magnetic invariant manifolds routine developed during this work. The intersection of the calculated invariant manifold with the tokamak chamber agrees considerably well with the heat flux measurements for the same discharge at the divertor plates, indicating the development of a rotating manifold due to the internal asymmetric plasma currents, giving quantitative support to our simplified description of the magnetic field and the plasma edge definition through the invariant manifolds. / Neste trabalho estuda-se a modelagem do campo magnético em configurações realistas de plasmas em equilíbrio não-axissimétrico e o fluxo de calor nos componentes em contato com o plasma em descargas de tokamaks com desviadores poloidais. Começa-se estabelecendo a relação entre configurações magnéticas arbitrárias e sistemas dinâmicos Hamiltonianos. Então aplicamos o conceito de helicidade magnética, que é usado para estabelecer limitações topológicas sobre as linhas de campo magnético em plasmas ideais, assim como para compreender a auto-consistência das superfícies magnéticas reconectadas em configurações não-axissimétricas. Após esta discussão teórica, apresentam-se alguns resultados sobre o equilíbrio magnetohidrodinâmico e o uso de soluções analíticas à equação de Grad-Shafranov para descrever descargas reais em tokamaks, com base em diagnósticos experimentais e condições de contorno realistas. Também realiza-se uma comparação entre a reconstrução do equilíbrio de uma descarga do DIII-D, obtida mediante uma rotina numérica desenvolvida para esta pesquisa, com a obtida mediante o código EFIT, usado amplamente em diversos tokamaks. Após isso, apresenta-se uma descrição simplificada do campo magnético não-axissimétrico, criado por fontes determinadas, e a sua implementação para descrever o campo magnético devido às correntes externas em tokamaks. Então, usam-se estas rotinas para desenvolver um procedimento numérico que ajusta um conjunto adequado de parâmetros não-lineares de correntes filamentares internas, com as quais pretende-se modelar a resposta do plasma com base nas medidas de campo magnético fora do plasma. Finalmente, estes métodos são utilizados para modelar o campo magnético criado por uma instabilidade com rotação lenta numa descarga do DIII-D. Com base nas medidas das sondas magnéticas é possível modelar os campos criados em regiões arbitrárias próximas da borda do plasma. Usando esta informação é possível determinar a borda não-axissimétrica do plasma mediante as invariantes magnéticas calculadas com a utilização de uma rotina desenvolvida durante este trabalho. A intersecção da superfície invariante com a câmara do tokamak coincide satisfatoriamente com as medidas de fluxo de calor nas placas do divertor para a mesma descarga, indicando o desenvolvimento de uma variedade giratória criada pelas correntes de plasma não-axissimétricas, e sustentando quantitativamente a nossa descrição simplificada do campo magnético, assim como a definição da borda do plasma mediante as invariantes magnéticas.