Because of the accuracy and ease of implementation, Monte Carlo methodology is widely used in analysis of nuclear systems. The obtained estimate of the multiplication factor (keff) or flux distribution is statistical by its nature. In criticality simulation of a nuclear critical system, whose basis is the power iteration method, the guessed source distribution initially is generally away from the converged fundamental one. Therefore, it is necessary to ensure that the convergence is achieved before data are accumulated. Discarding a larger amount of initial histories could reduce the risk of contaminating the results by non-converged data but increases the computational expense. This issue is amplified for large loosely coupled nuclear systems with low convergence rate. Since keff is a generation-based global value, frequently no explicit criterion is applied to the diagnostic of keff directly. As an alternative, a flux-based entropy check available in MCNP5 works well in many cases. However, when applied to a difficult storage fuel pool benchmark problem, it could not always detect the non-convergence of flux distribution. Preliminary evaluation indicates that it is due to collapsing local information into a single number. This thesis addresses this problem by two new developments. First, it aims to find a more reliable way to assess convergence by analyzing the local flux change. Second, it introduces an approach to simultaneously compute both the first and second eigenmodes. At the same time, by computing these eigenmodes, this approach could increase the convergence rate. Improvement in these two areas could have a significant impact on practicality of Monte Carlo criticality simulations.

Results from models that describe frequency chirps of toroidal Alfvén eigenmode excited by energetic particles are presented here. This structure forms in TAE gap and may or may not chirp into the continuum. Initial work described the particle wave interaction in terms of a generic Hamiltonian for the particle wave interaction, whose spatial dependence was xed in time. In addition, we have developed an improved adiabatic TAE model that takes into account the spatial prole variation of the mode and the nite orbit excursion from the resonant ux surfaces, for a wide range of toroidal mode numbers. We have shown for the generic xed prole model that the results from the adiabatic model agree very well with simulation result except when the adiabatic condition breaks down due to the rapid variations of the wave amplitude and chirping frequency. We have been able to solve the adiabatic problem in the case when the spatial prole is allowed to vary in time, in accord with the structure of the response functions, as a function of frequency. All the models predict that up-chirping holes do not penetrate into the continuum. On the other hand clump structures, which down chirp in frequency may, depending on detailed parameters, penetrate the continuum. The systematic theory is more restrictive than the generic theory, for the conditions that enable clump to penetrate into the continuum. In addition, the systematic theory predicts an important nite drift orbit width eect, which eventually limits and suppresses a down-chirping response in the lower continuum. This interruption of the chirping occurs when the trapped particles make a transition from intersecting both resonant points of the continuum to just one resonant point. / text

The cavity supports the orthogonal reference beam families as its eigenmodes while enhancing the reference beam power. Such orthogonal eigenmodes are used as additional degree of freedom to multiplex data pages, consequently increase storage densities for volume Holographic Data Storage Systems (HDSS) when the maximum number of multiplexed data page is limited by geometrical factor. Image bearing holograms are multiplexed by orthogonal phase code multiplexing via Hermite- Gaussian eigenmodes in a Fe: LiNbO3 medium with a 532 nm laser at multiple Bragg angles by using Liquid Crystal on Silicon (LCOS) spatial light modulators (SLMs) in reference arms. Total of nine holograms are recorded with three angular and three eigenmode.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20174号 / 理博第4259号 / 新制||理||1612(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 菅沼 秀夫, 教授 國廣 悌二, 教授 川合 光 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第23538号 / エネ博第429号 / 新制||エネ||81(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)准教授 門 信一郎, 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

The increased effects of global warming have been a driving force to further research and develop sustainable energy sources, such as fusion. In this study, two different fusion devices are compared in terms of ion cyclotron resonance heating (ICRH)of plasma. The two devices are the tokamaks ASDEX Upgrade and not yet built ITER. To make the comparison, ICRH was simulated in the two tokamaks using the FEMIC code. ASDEXUpgrade was simulated with a deuterium plasma and ITER was simulated both with a deuterium and a deuterium-tritium plasma. In all scenarios a 3% minority species concentration,consisting of helium-3, was introduced. The obtained results showa higher and more centered wave absorption in ITER, compared to ASDEX Upgrade. This is mainly due to the size difference of the tokamaks. The smaller plasma radius of ASDEX Upgrade allowed for more wave reflection in the plasma, resulting in standing waves that formed eigenmode patterns. For simulationsin ITER, the waves were absorbed before they could be reflected in the plasma. Instead of standing waves and eigenmode patterns,the waves behaved as beams, propagating in a narrow region of the plasma. This indicates that ITER is more effective in terms of ICRH, as the absorption is greater and more focused to the center, minimizing power losses to the surroundings. / De ökade konsekvenserna av den globala uppvärmningen har varit en drivkraft för fortsatt forskning och utveckling av hållbara energikällor, bland annat fusion. I den här studien jämförs två olika fusionsanläggningar med avseende på joncyklotronresonansuppvärmning (ICRH) av plasma. De två anläggningarna är tokamakerna ASDEX Upgrade och ännu inte byggda ITER. För att göra jämförelser simulerades ICRH i de två tokamakerna med hjälp av FEMIC-koden. ASDEX Upgrade simulerades med ett deuteriumplasma och ITER simulerades med både ett deuteriumoch ett deuterium-tritiumplasma. För alla scenarier introducerades en 3% minoritetskoncentration av helium-3. Resultaten visar en högre och mer centrerad vågabsorption i ITER jämfört med ASDEX Upgrade. Detta beror framför allt på storleksskillnaden mellan tokamakerna. Den kortare plasmaradien av ASDEX Upgrade tillät mer reflektion i plasmat, vilket resulterade i stående vågor som bildade egenmodsmönster. För simuleringar i ITER absorberades vågorna innan de kunde reflekteras i plasmat. I stället för stående vågor och egenmodsmönster uppförde vågorna sig som strålar som propagerade över en smal region i plasmat. Det här indikerar att ITER är mer effektiv med avseende på ICRH, då absorptionen re och mer centrerad, vilket minimerar effektförluster till omgivningen. / Kandidatexjobb i elektroteknik 2022, KTH, Stockholm

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No description available.

Nous avons étudié le mélange de fluides visqueux dans des écoulements 2-D ouverts et fermés où des agitateurs créent de l'advection chaotique, i.e. des trajectoires lagrangiennes complexes. Notre étude, expérimentale, numérique et théorique, s'appuie sur deux types d'expériences de mélange chaotique, en domaine fermé et dans un canal ouvert. En système fermé, nous avons d'abord proposé une caractérisation topologique du mélange reposant sur l'enchevêtrement des trajectoires de points périodiques -- les "tiges fantômes". D'autre part, l'étude expérimentale du champ de concentration d'un colorant nous a permis de décrire le rôle des murs du domaine où se fait le mélange, pour les écoulements fermés comme ouverts. En fermé, la nature chaotique ou régulière des trajectoires initialisées près des bords détermine l'évolution du champ de concentration, même loin des bords. Nous avons ainsi observé une dynamique lente (algébrique) de l'homogénéisation quand la région chaotique s'étend jusqu'à des murs non-glissants. En ouvert, nous avons décrit l'évolution du champ de concentration dans, et en aval de la région de mélange, résultant de l'injection d'un blob de colorant. Nous avons décrit les éléments mal mélangés qui s'échappent aux temps courts, et l'apparition d'un motif permanent (auto-similaire) aux temps longs, déterminé par les orbites périodiques de la région de mélange. Des modifications de ce scénario apparaissent quand la région de mélange va jusqu'aux murs. Enfin, une modélisation à base de transformation du boulanger généralisée nous a permis de comprendre l'essentiel des mécanismes rencontrés.

Ce mémoire traite de l’harmonicitié d’instruments de musique à travers la géométrie spectrale. Nous y présentons, en premier lieu, les résultats connus concernant la corde de guitare, le tambour circulaire et puis le tabla ; le premier est harmonique, le deuxième ne l’est pas et puis le dernier s’en approche. Le cas de la timbale est ce qui constitue la majeure partie de notre travail. L’ingénieur-physicien Robert E. Davis en avait déjà étudié la quasi-harmonicité et nous faisons ici une relecture mathématique de sa démarche. En alliant les méthodes analytiques et numériques, nous montrons que la caisse de résonance de la timbale permet à la fois d’ajuster les fréquences de vibration de la forme ω_(i1) , avec 1 ≤ i ≤ 5, afin qu’elles s’approchent du rapport idéal 2 : 3 : 4 : 5 : 6, et elle permet aussi d’étouffer certains autres modes dissonants. Pour ce faire, nous élaborons un modèle simplifié de timbale cylindrique basé sur la physique et sur ce que propose Davis dans sa thèse. Ce modèle nous fournit un système d’équations divisé en trois parties : la vibration de la peau et la pression à l’intérieur et à l’extérieur de la timbale. Nous utilisons la méthode des fonctions de Green pour trouver les expressions des deux pressions. Nous nous servons de celles-ci ainsi que d’un développement en série de Fourier-Bessel modifiée pour résoudre les équations de la vibration de la peau. La résolution de ces équations se ramène finalement à celle d’un système matriciel infini dont nous faisons l’analyse numériquement. À l’aide de Mathématica et de ce système matriciel, nous trouvons les fréquences de vibration de la timbale, ce qui nous permet d’analyser l’harmonicité de l’instrument. Grâce à une mesure de dissonance, nous optimisons l’harmonicité de la timbale en fonction du rayon du cylindre, de sa hauteur et de la tension. / This thesis deals with the harmonicity of musical instruments through spectral geometry. First, we present the known results concerning the guitar string, the circular drum and the tabla ; the first is harmonic, the second is not, and the last is somewhere in between. The case of the timpani constitutes the major part of our work. The physicist-engineer Robert E. Davis had already studied its quasi-harmonicity and here we undergo a mathematical proofreading of his approach. By combining analytical and numerical methods, we show that the sound box of the timpani allows an adjustement of the vibration frequencies of the form ω_(i1) , with 1 ≤ i ≤ 5, so that they get close to the ideal 2 : 3 : 4 : 5 : 6 ratio, while it also stifles some other dissonant modes. To do so, we develop a simplified model of a cylindrical timpani based on physics and on what Davis suggests in his thesis. This model provides a system of equations divided into three parts : the vibration of the skin and the pressure inside and outside the timpani. We use the method of Green’s functions to find the expressions of the pressures. We use these together with a modified Fourier-Bessel series development to solve the equations of the vibration of the skin. In the end, the solving of these equations is reduced to an infinite matrix system that we analyze numerically. Using Mathematica and this matrix system, we find the vibrational frequencies of the timpani, which allows us to analyze the harmonicity of the instrument. Thanks to a measure of dissonance, we optimize the harmonicity of different timpani models with different cylinder radii, heights and tensions.