The aim of the thesis is to examine plasma-wall interaction using computer modeling. Tokamak- relevant plasma conditions are simulated using the particle-in-cell model family SPICE working in three or two dimensions. SPICE model was upgraded with a parallel Poisson equation solver and a heat equation solver module. Plasma simulation aimed at synthetic Langmuir probe measurements were performed. First set considered a flush-mounted probe and the effect of variable magnetic field angle was studied with aim to compare existing probe data evaluation techniques and assess their operational space, in which the plasma parameters estimation via fit to the current-voltage characteristic is accurate. Second simulation set studied a protruding probe pin. Effective collecting area of such probe was investigated with intentions of density measurement collection. This area was found to be influenced by a combination of two factors. First, the density dampening inside the magnetic pre-sheath of the probe head, and the second, the extension of the area caused by Larmor rotation. A comparison with experimental results obtained at COMPASS tokamak was was performed, confirming these results. Keywords Langmuir probe, simulation, particle-in-cell, tokamak, Poisson equation, COMPASS 1

Title: Development and application of methods for measurement of magnetic field on tokamaks Author: Ing. Karel Kovařík Department: Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University in Prague Institute of Plasma Physics of the Czech Academy of Sciences, v. v. i. Supervisor: Ing. Ivan Duran, PhD. (Institute of Plasma Physics of the Czech Academy of Sciences, v. v. i.) Abstract: Thesis describes two parts of my work in frame of magnetic confined fusion research. First part describes irradiation testing of the Hall sensors based on InSb semiconductor. It evaluates radiation effects of fast and thermal neutrons on the Hall sensors sensitivity. Tests indicate low level of changes for fluence lower only by order of magnitude than expected in the whole lifetime of ITER tokamak. Second part describes measurement of turbulent plasma structures parameters in scrape--off layer of the COMPASS tokamak, particularly the parallel electric current flowing along them. Design of novel combined probe allowing measurement of not only electrostatic parameters but also the parallel electric current is presented. Evaluation of parallel electric current is discussed and compared with experiment. Statistical analysis of the plasma structures is presented. Empirical formulas of the...

The thesis summarizes the current state of research of thermonuclear fusion with magnetic confinement and decribes the possible role of stochastization of magnetic field lines and magnetic perturbations in solving some of the problems that are encountered on the road to the exploitation of fusion. It presents a theoretical introduction to deterministic chaos and explains the connection of this theory to magnetic perturbations in tokamak. The results are presented mainly in the form of publications in journals and conference proceedings. Among them are: the comparison of chaotic diffusion of particles and field lines, where significant differences were found; the application of chaotic diffusion of particles to the problem of runaway electrons originating in disruptions, where our simulations contributed to explaining the experimental results from the JET tokamak; the calculation of spectra of perturbations for the COMPASS tokamak, done as a preparation for the upcoming experiments; and modelling of screening of perturbations by plasma, where the observations of divertor footprints show as a promising method to detect the screening.

Future magnetic fusion reactors such as tokamaks will need innovative, fast, deep-fueling systems to inject frozen deuterium-tritium pellets at high speeds and high repetition rates into the hot plasma core. There have been several studies and concepts for pellet injectors generated, and different devices have been proposed. In addition to fueling, recent studies show that it may be possible to disrupt edge localized mode (ELM) formation by injecting pellets or gas into the fusion plasma. The system studied is capable of doing either at a variety of plasma and pellet velocities, volumes, and repetition rates that can be controlled through the formation conditions of the plasma. In magnetic or inertial fusion reactors, hydrogen, its isotopes, and lithium are used as fusion fueling materials. Lithium is considered a fusion fuel and not an impurity in fusion reactors as it can be used to produce fusion energy and breed fusion products. Lithium hydride and lithium deuteride may serve as good ablating sleeves for plasma formation in an ablation-dominated electrothermal plasma source to propel fusion pellets. Previous studies have shown that pellet exit velocities, greater 3 km/s, are possible using low-z propellant materials. In this work, a comprehensive study of solid lithium hydride and deuteride as a pellet propellant is conducted using the ETFLOW code, and relationships between propellants, source and barrel geometry, pellet volume and aspect ratio, and pellet velocity are determined for pellets ranging in volume from 1 to 100 mm3. / Master of Science

In a fusion power plant the plasma must be confined to prevent instabilities and energy loss. The confinement machine studied is a tokamak. The focus of the project is to optimize ion resonance heating in WEST. The code called FEMIC was used to simulate different scenarios where parameters such as ion concentrations, antenna frequency are analysed for the case of minority hydrogen in deuterium plasma. The optimal Hconcentration in D-plasma was found to be 10 5%, which allows for a wide concentration interval. However, the strongest coupled power was found to be at the H-concentration of 6%. Thus, the optimal concentration for strongest coupled power and H-minority absorption is 6% in WEST for the case of Hydrogen minority in Deuterium plasma.

The historical success of friction stir welding (FSW) on materials such as aluminum and magnesium alloys is associated with the absence of melting and solidification during the solid-state process. However, commercial adoption of FSW on steels and other non-ferrous high-strength, high-temperature materials such as nickel-base and titanium-base alloys is limited due to the high costs associated with the process. In this dissertation, the feasibility of using an FSW approach to fabricate certain structural components made of nitrogen containing austenitic stainless steels that go into the vacuum vessel and magnetic systems of tokamak devices was demonstrated. The FSW weldments possessed superior application-specific mechanical and functional properties when compared to fusion weldments reported in the technical literature. However, as stated earlier, the industrial adoption of FSW on high temperature materials such as the ferrous alloys used in the present study is greatly limited due to the high costs associated with the process. The cost is mainly dictated by the high temperature FSW tools used to accomplish the weldments. Commercially available high temperature FSW tools are exorbitantly priced and often have short lifetimes. To overcome the high-cost barrier, we have explored the use of integrated computational materials engineering (ICME) combined with experimental prototyping validation to design next-generation tool materials with high performance and relatively low cost. Cermet compositions with either tungsten carbide or niobium carbide as the hard phase bonded by high entropy alloy binders were processed via mechanical alloying and spark plasma sintering. The feasibility and effectiveness of the newly developed cermet tool materials as potential next generation high temperature FSW tool materials was evaluated.

京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第24004号 / エネ博第440号 / 新制||エネ||83(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 田中 仁, 教授 中村 祐司, 教授 長﨑 百伸 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

Fusion reactors need methods to couple externalpower to confined plasmas. Ion cyclotron resonance heating(ICRH) is a method to radiate electromagnetic waves to couplepower to the kinetic motion of gyrating ions in a plasma. In thisreport we study ICRH with regard to the dispersive effects ofthe confined plasma in the JET tokamak. We study this witha division of the electric field with regard to toroidal modenumbers. Specifically we examine where dispersive effects arelocated in the plasma and if they have any importance. We alsostudy the ion-ion hybrid layer, where according to the dispersionrelation a singularity can occur. To do the analysis, we use thecode FEMIC to simulate fusion scenarios.The results show that ion absorption is stronger and morelocalized for low toroidal mode numbers. This is true for toroidalmode numbers n 10, and for n < 6 the resolution ofthe solutions does not suffice for a meaningful analysis. Anexamination of the effects of the singularity in the dispersionrelation at the ion-ion hybrid layer shows that the amount ofabsorbed power in the ion-ion hybrid layer is significantly smallerthan for the central region of absorption. This means that thesingularity does not affect ICRH heating in our scenarios in anymajor sense. / Fusionsreaktorer behöver metoder för attöverföra extern effekt till ett inneslutet plasma. Joncycklotronresonansuppvärmning (ICRH) är en uppvärmningsmetod därman strålar elektromagnetiska vågor in i ett plasma, vars effektöverförs till roterande joner. I denna rapport studerar vi ICRHmed avseende på de dispersiva effekterna i ett inneslutet plasma itokamaken JET. Detta undersöks med en uppdelning av det totalaelektriska fältet med avseende på toroidala moder. Vi undersökerspecifikt var i plasmat de dispersiva effekterna uppstår ochanalyserar deras påverkan på effektöverföringen. Vi studeraräven en singularitet som uppstår i dispersionsrelationen vid jonjon-hybridlagret. För att göra en analys används koden FEMICför att simulera ICRH-scenarion.Resultaten visar att jonernas absorption är starkare och merlokaliserad för låga toroidala modtal. Detta är sant för modtaln 10. För n < 6 är lösningarnas upplösning för dålig fören betydelsefull analys. En undersökning av jon-jon-hybridlagretvisar att mängden absorberad effekt inom detta område är avsevärt mindre än för de centrala områdena för absorption. Dettamedför att singulariteten i dispersionsrelationen inte påverkarICRH-uppvärmningen i våra simuleringar till en betydelsefullgrad. / Kandidatexjobb i elektroteknik 2021, KTH, Stockholm

Ces travaux abordent la problématique des mesures de sonde de Langmuir dans un environnement RF. Les mesures expérimentales ont montré que des courants DC négatifs (électroniques) étaient collectés sur la structure d'une antenne ICRF sous tension, pendant que des courants DC positifs (ioniques) sont recueillis par une sonde de Langmuir à l'autre bout du tube de flux magnétique ouvert connecté à l'antenne, la sonde étant au potentiel de la machine. Un modèle de tube de flux asymétrique, de type de sonde double, est présenté. Celui-ci modélise un plasma, confiné le long des lignes de champ magnétique, ayant à chaque extrémité une électrode dont l'une est polarisée à un potentiel RF et l'autre à la masse. L'électrode polarisée modélise le potentiel RF résultant de l'intégration, le long d'une ligne champ magnétique, du champ électrique rayonné par les straps d'une antenne ICRF, tandis que l'autre électrode modélise la sonde au potentiel de la machine. Ce modèle permet d'expliquer l'apparition de courants DC en émettant simplement l'hypothèse qu'il faut à la fois une asymétrie de la source RF par rapport à une masse fixe, une conductivité RF transverse non nulle autorisant des courants RF transverses ainsi qu'une caractéristique courant-tension non linéaire due aux gaines pour favoriser des courants négatifs du côté RF et des courants positifs côté sonde. Ce modèle permet également de modéliser les caractéristiques Courant DC - Tension DC d'une sonde en présence de RF et ainsi d'évaluer les propriétés du plasma. Dans ce cas l'électrode modélisant la sonde n'est plus à la masse, mais à un potentiel donné. Des résultats analytiques sont trouvés dans certaines limites / This work investigates the problematic of probe measurements in RF environment. DC currents flowing along magnetic field lines connected to powered ICRF antennas have been observed experimentally. Negative (i.e. net electron) current is collected on the powered ICRF antenna structure, while positive (i.e. net ion) current is collected by magnetically connected Langmuir probes. An asymmetric model based upon a double probe configuration was developed. The ICRF near field effect is mimicked by a ?driven? RF electrode at one extremity of an "active" open magnetic flux tube, where a purely sinusoidal potential is imposed. The other connection point is maintained at ground potential to model a collecting probe. This "active" flux tube can exchange transverse RF currents with surrounding "passive" tubes, whose extremities are grounded. With simple assumptions, an analytical solution is obtained. We can thus explain how DC currents are produced from RF sheaths. This model also makes it possible to model the characteristics DC Current' DC Voltage of a probe in the presence of RF and thus to evaluate some plasma properties. In this case the electrode at ground potential (probe) is polarized at a given potential. Analytical results are found within certain limits

Afin de générer des plasmas performants sur de longues durées, un tokamak nécessite des dispositifs de chauffage et de génération de courant additionnels. Des antennes haute-fréquences, délivrant des puissances de plusieurs mégawatts au plasma, sont actuellement utilisées dans de nombreux tokamaks. Pour optimiser les performances de chauffage et de génération de courant obtenues avec le système LH (fréquence de quelques gigahertz), une bonne maitrise du couplage de l'onde émise par l'antenne au plasma de bord est nécessaire. Or, des effets non-linéaires dépendant du niveau de puissance HF injectée dans le plasma perturbent fortement le couplage de l'onde LH pour certains paramètres de bord (densité et température en particulier). Les travaux présentés dans ce manuscrit portent sur l'étude du couplage linéaire et non-linéaire de l'onde LH au plasma de bord. Dans le cadre de l'installation d'une antenne dite « Passive Active Multijunction » en 2009 sur le tokamak Tore Supra visant à tester le système LH proposé pour ITER, la caractérisation du couplage obtenu avec cette antenne a été réalisée à partir d'expériences menées à basse puissance sur Tore Supra. Les résultats, analysés conjointement avec l'utilisation d'un code de couplage (ALOHA), ont ainsi validé les prédictions théoriques prévoyant de bonnes propriétés de couplage à des densités de plasma de bord faibles. Par ailleurs, l'effet pondéromoteur a été clairement identifié comme responsable de la forte détérioration du couplage de l'onde mesurée sous certaines conditions de plasma de bord. / In order to achieve long pulse operation with a tokamak, additional heating and current drive systems are necessary. High frequency antennas, which deliver several megawatts of power to the plasma, are currently used in several tokamaks. Moreover, a good control of the coupling of the wave launched by the antenna to the edge plasma is required to optimize the efficiency of heating and current drive LH systems. However, non-linear effects which depend on the level of injected power in the plasma strongly damage the coupling of the LH wave at particular edge parameters (density and temperature profiles). Results presented in the manuscript deal with the study of the linear and non-linear coupling of the LH wave to the plasma. In the framework of the commissioning of the Passive Active Multijunction antenna in 2009 on the Tore Supra tokamak aiming at validating the LH system suggested for ITER, the characterisation of its coupling properties was realized from low power experiments. The experimental results, which are compared with the linear coupling code ALOHA, have valided the theoretical predictions of good coupling at edge plasma density around the cut-off density. Besides, the ponderomotive effect is clearly identified as responsible for the deterioration in the coupling of the wave, which is measured under particular edge plasma conditions. A theoretical model combining the coupling of the LH wave with the ponderomotive force is suggested to explain the experimental observations.