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Three dimensional magnetohydrodynamics of fusion plasmas
Get PDFThe primary aim of the research on nuclear fusion is to obtain a new energy source to help satisfying a growing and sustainable consumption. This objective has to be reached through scientific research, both from the physics point of view and through the demonstration of the technological feasibility of a nuclear fusion reactor. The option on which the major efforts of the international community are focused is to obtain controlled nuclear fusion using a magnetic field to confine a plasma formed by
deuterium and tritium, in a vacuum chamber of toroidal shape. The most promising magnetic configuration is the so called tokamak configuration. The scientific community aims at addressing the remaining problems connected with physics performing the experiment ITER (International Thermonuclear Experimental Reactor) and to verify the technological feasibility of a nuclear fusion reactor with the DEMO experiment.
An important part of the scientific efforts is addressed to the study of configurations alternative to the tokamak, like the stellarator and the reversed-field pinch (RFP). These configurations achieve three dimensional helical states: in the RFP a global helical state is obtained spontaneously, due to the presence of a strong current flowing in the plasma, while currents flowing in external helically shaped coils generate a global helical state in the stellarator. Helical states can be obtained also in the tokamak configuration, for instance due to the presence of external magnetic field perturbations. The research activity of my PhD focuses on the study of the 3D nonlinear magnetohydrodynamics model applied to the numerical study of the RFP and tokamak helical configurations.
The main aim of my research is the characterization, under three different aspects later described, of the three dimensional helical states. These states are presently believed to provide possible scenarios for reducing dangerous MHD activity for both RFP (magnetic chaos transport reduction) and tokamak (sawtooth mitigation, disruption avoidance).
The research activity included the development and the exploitation of advanced numerical tools to deal with the numerical solution of the 3D nonlinear MHD model, while the interaction with the experimental environment provided the opportunity to develop tools for model-experiment comparison (validation) and benchmarking of numerical tools (verification).
The results obtained during my PhD provide a further step towards a predictive capability of the employed modelling tools. In fact, the boundary conditions are proved to be a key ingredient in bringing the comparison of MHD simulations with the experiment at a quantitative level. Moreover it recently inspired a successful and promising experimental activity in RFX-mod, the biggest RFP experiment in the world, located in Padova.
My PhD research activity and results can be divided into three main areas. The first is the dynamical simulation of a magnetically confined plasma through numerical solution of the 3D nonlinear visco-resistive MHD model. The second area of research consists in the topological study of the magnetic field configurations obtained from MHD simulations. The third area is the study of transport due to magnetic stochasticity in both tokamak and RFP states, with data coming from MHD simulations, gyrokinetics simulations and experimental results.
The first area of research deals with the simulation of the dynamical properties of a magnetically confined plasma, performed using the 3D nonlinear MHD codes SPECYL and PIXIE3D. The most important achievement is represented by the level of agreement between MHD simulation and experimental dynamics of the RFP, a degree of agreement obtained in simulations where, for the first time, a helical boundary condition is applied. It is also demonstrated that by imposing a finite helical radial magnetic field at the edge it is possible to induce a global helical regime with the chosen helicity. As for the tokamak configuration the study of helical boundary conditions shows that they can favour a steady helical equilibrium, thus mitigating the sawtooth dynamics typically detrimental for the confinement. This area of research leads to a unifying vision for the RFP and the tokamak, as the use
of helical boundary condition for the magnetic field seems to allow the easier establishment of a helical equilibrium in both configurations, with interesting properties for the configurations.
The second area of research is centred on the topological study of the magnetic configurations obtained from the MHD simulations of the RFP. The separatrix expulsion of the dominant helical mode has been studied analyzing the magnetic field topology with the field line tracing code N EMATO. Two so called paradigmatic cases, characterized by a simplified MHD dynamics, have been analyzed. In the first one it was shown that the dominant mode separatrix expulsion can reduce the level of magnetic field lines stochasticity remarkably, in the second case an “exotic” (before these studies) dynamics was considered, i.e. the development of a helical equilibrium from a non-resonant mode. These results confirmed older studies that placed separatrix expulsion in direct connection with helical RFP states obtained in RFX-mod, which develop internal transport barriers observed as electronic temperature steep gradients. Furthermore it showed that the helical equilibrium based on a non-resonant mode can result in particularly strong magnetic order. The favourable properties found led to the proposal to
experimentally drive QSH states built upon non-resonant MHD modes in the RFX-mod experiment: these states were successfully produced in the experiment, and the study of thermal properties is presently ongoing. Topological studies on more realistic cases coming from MHD simulations that show a quantitative agreements with the standard operation of the RFX-mod experiment are also tackled in this thesis. The results obtained underline the importance of the spectrum of secondary perturbations to the helical equilibrium.
The third area of research focuses on the consequences of transport produced by the presence of magnetic stochasticity. Two specific cases relevant for the RFP and the tokamak are considered: the magnetic chaos produced by microtearing activity at the electron internal transport barrier in the RFP, and the case of edge magnetic stochasticity due to the action of edge helical magnetic perturbations in the tokamak. The tools to study transport were developed and used to calculate the energy diffusion coefficient and other meaningful quantities. Such tools are now available for further and more general
applications.
On a numerical ground two important activities were performed during the PhD. The parallelization of the field line tracing code NEMATO, during one month mobility at Oak Ridge National Laboratory, was fundamental for the speeding up of the research activity. The numerical verification of NEMATO and ORBIT was also performed. The verification gave a positive result, showing a satisfactory agreement, both qualitative and quantitative, on the features of the magnetic field topology in the RFP configuration
Detection of magnetic barriers in a chaotic domain: First application of finite time Lyapunov exponent method to a magnetic confinement configuration
No full textMagnetic field lines embedded in a plasma confinement system are often characterized by
a chaotic motion. This weakens the confinement properties of any magnetic configuration.
However, even in case of chaotic domains, magnetic barriers can emerge and limit the field
line motion itself. In the context of the numerical simulation of a Reversed-Field Pinch
configuration a new magnetic topology analysis, borrowed from previous fluid dynamic
studies, is discussed. This methodology relies on the behavior of the Finite Time Lyapunov
Exponent (FTLE) associated with the magnetic field. By referring to a previous work in which
the magnetic field is given in terms of analytical function the FTLE field shows the presence of ridges, special gradient lines normal to the
direction of minimum curvature, forming magnetic barriers. These ridges can be recognized
as Lagrangian Coherent Structures (LCSs) for the system, actually opposing the penetration
of magnetic field lines across them. In this article a more general numerical scheme for the
detection of the LCSs has been adopted that allows analysis of realistic cases in which the
magnetic fields are numerically known on a discrete mesh. After a validation test performed
on the analytical case, a first application to a numerical magnetohydrodynamics simulation
of the RFP, characterized by a broad chaotic region, has been performed. A strong magnetic
barrier has been observed that effectively limits the field lines motion inside the chaotic sea
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Kinematic viscosity estimates in reversed-field pinch fusion plasmas
Get PDFThis paper concerns the kinematic viscosity in reversed-field pinch fusion plasmas, including both the study of numerical magneto-hydrodynamics (MHD) simulations and the analysis of RFX-mod experimental data. In the first part, we study the role of non-uniform time-constant radial viscosity profiles in 3D non-linear visco-resistive MHD simulations. The new profiles induce a moderate damp (for the velocity field) and a correspondent enhancement (for the magnetic field) of the spectral components resonating in the regions where the viscosity is higher. In the second part, we evaluate the kinematic viscosity coefficient on a wide database of RFX-mod shots according to the transport theories of Braginskii (considering parallel, perpendicular and gyro viscosity coefficients), considering the action on viscosity of ITG modes (ion temperature gradient) and according to the transport theory of Finn. We then exploit the comparison with the visco-resistive MHD simulations (where the visco-resist..
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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