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Particle transport in Reversed Field Pinch plasmas
Full text linkThis thesis is aimed at studying the transport of particles
in magnetically confined thermonuclear plasma.
The understanding of the transport properties in
devices for fusion plasmas is one of the key factor to keep the
correct operating conditions in a future fusion reactor.
Indeed one of the open issues in magnetic fusion studies,
which prevents the realization of an efficient thermonuclear
reactor, is the high level of energy and particle transport in
the direction perpendicular to the confining magnetic
field. This phenomenon reduces confinement properties and has to be solved in order to obtain energy from thermonuclear fusion processes.
The amount of particle and energy transport experimentally observed cannot be interpreted in the framework of the classical theory. Understanding the underlying physics of this anomalous transport remains the outstanding critical physical issue in fusion research.
Nowadays it is generally accepted that anomalous transport
is partially due to magnetic chaos owing to the magnetic
perturbations of the equilibrium magnetic fields. The Reversed
Field Pinch (RFP) configuration, with its wide spectrum of
magnetic perturbations, offers a suitable testbed to verify the
theory and to reveal the inner mechanism underlying the transport in fusion magnetic devices. The magnetic perturbations, also dubbed dynamo or MagnetoHydroDynamic (MHD) modes, sustain the RFP configuration against the resistive magnetic diffusion. Unfortunately they have global negative effects: as already stated they lead to the stochastization of the equilibrium magnetic field over a large part of the plasma core and moreover their phase locking generates an interference pattern that results in a global distortion of the plasma column: the so-called Locked Mode (LM) that has its maximum effect at a well defined toroidal position.
Many techniques have been tested with the aim of reducing the MHD modes. The most effective are the Pulsed Poloidal Current Drive (PPCD) that modifies the internal current profile and the active control of the radial field at the edge by means of a system of active coils, the so-called Virtual Shell (VS).
All the transport mechanisms acting inside the plasma modify the shape of the density profile. The density is measured by means of interferometer: a non-perturbative diagnostic that utilizes electromagnetic waves to probe the plasma.
A part of this thesis will be addressed to determine the global particle diffusion coefficients in relation to the magnetic perturbations amplitude. This analysis has been carried on TPE-RX device: a large RFP machine sited in Tsukuba (Jp). In order to study the global confinement properties, the transport analysis has been carried out analyzing data collected far from to the LM, where its local effect could be neglected.
A transport code (in our case TED, acronym of TEmperature and Density) computes the density profile according to transport parameters supplied by the user. The computed profile is compared to the experimental one, determining the correctness of the model assumed to provide the transport coefficients. With this analysis it has been confirmed that damping the MHD modes amplitude by means of the PPCD
the particle confinement globally improves and the diffusion coefficient is strongly reduced in the central zone of the plasma. This result has been further confirmed by the density behaviour during pellet injection experiments, where the particles released by the pellet in PPCD discharges are better confined inside the plasma than in plasmas with standard magnetic perturbations.
The dynamo modes, as already stated, generate a global distortion of the Last Close Flux Surface (LCFS) of the plasma: the LM. The plasma cross section results shrunk in a wide toroidal region of about 100° and bulging in another region of the similar toroidal range. Moreover an helical distortion of the column with magnetic lines that directly hit the wall is present. The VS system installed at RFX-mod (the largest RFP device in the world with design maximum plasma current of 2 MA, located in Padova) provides an important reduction
of the helical perturbation but is less effective on healing the shrinking of the LCFS, highlighting for the first time its effects on plasma confinement.
The two toroidal regions with different cross section have been characterized studying the density profile, the density fluctuations and the magnetic fluctuations: the shrunk region
shows an improved transport, providing the first experimental evidence of toroidal asymmetric confinement properties in an RFP plasma.
Moreover the RFX-mod pulses are affected by spontaneous reorganization of the internal current and magnetic profiles, the so-called Dynamo Relaxation Events (DREs). The density behaviour and the magnetic topology during the DREs have been analyzed, confirming the different nature of the shrunk and the bulging region of the plasma
Economic assessment of different operational reactor cycle structures in a pulsed DEMO-like power plant
No full textInternal Transport Barrier Broadening through Subdominant Mode Stabilization in Reversed Field Pinch Plasmas
No full textThe reversed field pinch (RFP) device RFX-mod features strong internal transport barriers when the plasma accesses states with a single dominant helicity. Such transport barriers enclose a hot helical region with high confinement whose amplitude may vary from a tiny one to an amplitude encompassing an appreciable fraction of the available volume. The transition from narrow to wide thermal structures has been ascribed so far to the transport reduction that occurs when the dominant mode separatrix, which is a preferred location for the onset of stochastic field lines, disappears. In this Letter we show instead that the contribution from the separatrix disappearance, by itself, is marginal and the main role is instead played by the progressive stabilization of secondary modes. The position and the width of the stochastic boundary encompassing the thermal structures have been estimated by applying the concept of a 3D quasiseparatrix layer, developed in solar physics to treat reconnection phenomena without true separatrices and novel to toroidal laboratory plasmas. Considering the favorable scaling of secondary modes with the Lundquist number, these results open promising scenarios for RFP plasmas at temperatures higher than the presently achieved ones, where lower secondary modes and, consequently, larger thermal structures are expected. Furthermore, this first application of the quasiseparatrix layer to a toroidal plasma indicates that such a concept is ubiquitous in magnetic reconnection, independent of the system geometry under investigation
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Full text linkWe 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
First measurements of the multichannel far-infrared polarimeter on RFX-mod
No full textA multichannel far-infrared (FIR) polarimeter has been recently installed and improved in RFX-mod to measure the Faraday rotation angle along vertical chords on a poloidal plasma section. Polarimetric data, associated with measurements of the electron density, permit the reconstruction of the poloidal magnetic field profile, Bp . The entire diagnostic is described and its main sections outlined. Emphasis is placed on the work performed on the polarimeter to reduce the fluctuations affecting the old diagnostic signals and to increase the S/N ratio. In the recent installation of the polarimeter the optical line was more carefully designed and the mirror holders have been made in insulating material to avoid any interaction with the variable magnetic fields. Moreover all the optics have been fixed on an inertial granite platform. Examples of the first Faraday angle measurements performed on five chords are presented and discussed. The measured Faraday rotation angles are compared to a theoretically calculated value, based on the mu-and-p model, showing a good agreement between experimental and predicted data in the central region of the plasma. The comparison between experimental and predicted data is reported and discussed in the present work
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