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Monte Carlo and Molecular Dynamics Simulations of Plasma-Material Interactions
No full textThe design of a suitable interface between plasma heated at millions of kelvin and the environment is one of the most critical areas within nuclear fusion research. The surface of interface components should sustain the energy delivered by plasma as electromagnetic radiation and transported through charge carriers such as ions and electrons.
The ratio between available plasma energy and heated surface increases with device dimension, so that for ITER and future fusion reactors the energy density striking the tokamak walls will be order of magnitude higher compared to what happens in current facilities.
Several plasma material interaction phenomena are relevant for PFCs design. The high energy heavy particles striking the chamber walls produce sputtering. The magnetic confinement suffer from frequent plasma instabilities, which result in a fast release of plasma energy on Plasma Facing Components. The ensuing thermal and mechanical actions produce material damage, erosion, thermal ablation and melting. Components lifetime is strongly limited by such extreme phenomena and require careful design and material selection to avoid unwanted maintenance stops and replacements.
The main aim of this research activity is to evaluate the PFMs erosion due to thermal ablation, to understand how material properties are affected by plasma interactions and how correctly select PFMs, without relying simply on empiric correlations with limited validity.
The main part of the performed work consist in the development of Molecular Dynamics and Monte Carlo Simulation aimed to evaluate the thermal properties of PFMs and to investigate how high energy electrons produced within the plasma carry energy towards tokamak chamber walls.
The analysis of Plasma Material Interactions has been historically carried out by putting together experimental activity and empiric relationships accounting for complex physical effects in an approximate fashion. The current research activity wants to investigate such phenomena starting from the underlying physical basic principles and through a comparison with experimental data with minimum employ of empiric relationship
Fenomeni di interazione ed erosione nei materiali a diretto contatto con il plasma nei reattori a fusione a confinamento magnetico
No full textIl soggetto della tesi è lo studio dei fenomeni di interazione del plasma con i materiali a diretto contatto con esso, chiamati anche Plasma Facing Components. In particolare viene analizzata l'erosione prodotta sulla prima parete (first Wall) e sul divertore provocata da eventi di instabilità del plasma, i Vertical Displacement Events (VDEs) e gli Edge Localized Modes
Non-modal stability analysis of magneto-hydrodynamic flow in a single pipe
Get PDFA complete understanding of the stability of fluid flows under varying magnetic field profiles is imperative for achieving control of plasma and operating fluids in the blankets of future fusion reactors. In this context, the primary objective of this study is to investigate the influence of varying magnetic profiles on the flow regime of a generic fluid, which is representative of both thermonuclear plasma and conductive fluids within a nuclear fusion reactor. To this aim in this work non-modal stability theory is adopted to perform stability analysis of a magneto-hydrodynamic (MHD) flow in an infinite circular pipe in order to study the effects of the magnetic field on the fluid dynamics of the pipe flow. In particular, the effects on the general stability of two magnetic field profiles are compared with the reference case of a pipe Poiseuille flow without magnetic field. Firstly, the classic modal stability technique is employed to study asymptotical stability. Then, non-modal stability analysis is applied to magneto-hydrodynamic pipe flow to study the system's response for a finite time immediately after a perturbation. Fourier–Chebyshev Petrov–Galerkin spectral method is used to compute the eigenvalues and pseudospectra of the weak formulation associated with the linearised system. Investigations on the dependence of spectra and transient growths on the specific magnetic profiles are conducted for different values of perturbation wave numbers. The obtained results show that in general the magnetic field has an effect of stabilization on the system, which depends on the specific magnetic profile considered. In addition, the non-modal stability analysis reveals that the inclusion of the magnetic field mitigates the effects of perturbations also in the short term, a phenomenon that cannot be seen using only modal stability analysis
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
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
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