1,720,996 research outputs found
Electromagnetic Analysis of the Demo Magnet System: Electrical Behavior of a Toroidal Field Coil During an Electrical Transient
No full textThe toroidal magnet system of DEMO includes 16 toroidal field coils (TFCs) for vacuum vessel plasma confinement. Because all the coils are superconductive, during their operation, they could experience an unexpected quench (changing from a superconducting to a normal conducting state). Once the quench signals are validated, the quench protection system is activated to dump the magnetic energy into an external resistor to prevent potential damage. However, the electrical transient in the magnet system can lead to overvoltage or overcurrent across and inside each coil. To select the insulation coordination and the test voltage of the coils, the assessment of this electromagnetic load is necessary. Based on the international thermonuclear experimental reactor (ITER)-like topology and considering the winding pack geometry associated with the baseline scenario and the high-current scenario, a preliminary electromagnetic analysis is performed by the finite element method (FEM) model: the target is the knowledge of the inductance matrix of the TFC system in both the scenarios. Starting from this result, lumped networks have been implemented and used to investigate the transient behavior of the TFC system. In particular, the distribution of the voltage waveform, across and among adjacent layers of each TFC, during a fast discharge, has been estimated for different operative conditions: fast discharge unit synchronous and asynchronous activation. The simultaneous occurrence of a fault condition during the discharge has also been considered
Preliminary studies for the conceptual design of the quench detection system for the DTT TF superconducting magnets
No full textThe superconducting DTT magnetic system needs a quench detection systems (QDSs) fast enough to trigger the dumping of the magnetic energy in case of quench and avoid irreversible damage of the cable systems. With this aim, a primary system based on the detection of the resistive voltage associated with the quench offers the best quench detection guarantees. The tokamak environment is affected by several electromagnetic noises during plasma scenarios so that the resistive voltage detection, during normal operation, can be compromised by the presence of large voltages induced by self and mutual magnetic coupling among coils and with plasma or passive elements: the resulting inductive voltages across a TF coil, or part thereof, could be much higher than the quench voltage thresholds. Voltage compensations techniques, therefore, have necessarily to be foreseen in the QDSs conceptual design, to discriminate the resistive component associated with the quench. We present a reconnaissance of all known electromagnetic noises that could affect a TF coils QDS in DTT: this analysis is conducted by means of analytical calculations, made up with the aim to evaluate and have a prevision of the maximum extent of the voltages induced across TF coils and Double Pancakes (DPs) during, in particular, the Single Null (SN) scenario
Primary Quench Detection Analysis for the EU-DEMO Toroidal Field Coils
No full textTo protect the Toroidal Field Coils (TFC) of the EU DEMO tokamak, in case it will be realized with low temperature superconducting (LTS) materials, a reliable and fast quench detection system (QDS) is required. Although these TFC coils are operated in static mode, voltages across their winding pack (WP) will be generated during machine operation due to the presence of dynamic magnetic fields. Two sources of magnetic coupling, responsible for noisy signals on the quench detection circuitry in a Fast Plasma Disruption scenario, are here considered and analyzed. Due to the high electromagnetic noise environment, the co-wound (CW) technology must be adopted for the quench sensors and an aligned and twisted layout (CWA&T) is necessary due to the magnetic interaction with the superconductor (SC) strand helicity. The poloidal component of the plasma current is responsible for a noisy signal on the CW and a strategy for signal compensation will be suggested and the contribution evaluated numerically. The signal induced on a SC cable due to the helicity can be compensated adopting a CWA&T configuration. The twist pitch length (TP) of the CWA&T is a critical parameter for a proper noise compensation. Since it is challenging to obtain a reliable TP length from a numerical simulation, we here propose an experimental approach to assess the proper TP length. There is not a constraint on the TP length from the poloidal plasma current induced signal and we can conclude that a single TP, optimized for helicity compensation, will be sufficient
Primary Quench Detection Analysis for DTT Central Solenoid and Poloidal Field Coils
No full textThe Central Solenoid (CS) and the Poloidal Field (PF) coils of the DTT tokamak (Divertor Tokamak Test facility, currently being built in the site of ENEA C.R. Frascati - Italy) are operated in dynamic mode and, even in normal operation, the self and mutual voltages induced across each coil are of the order of magnitude of ≈100 V. In DTT, these coils will be realized with superconducting materials and, as a consequence, a reliable and fast quench detection system is required in order to protect the magnets. Being the quench signals 4 orders of magnitude smaller than the operating voltages, the cowound (CW) technology must be adopted for the quench sensors, the only degree of freedom being among a wiring configuration merely aligned with the coil winding (CWA) or aligned and twisted to the coil winding (CWA&TW). In this study, a model of the DTT poloidal magnetic system has been developed up to the details of the single windings (layer or pancake) of the various coils. CWA sensors have also been implemented in the model. Signals coming from the magnetic system and the CWA sensors during the Single Null plasma event have been simulated and compared. The analysis has been carried out up to the level of the elementary winding of each coil, using a lumped parameters circuital model. Finally, a simple theory has been developed aimed at the estimations of the improvements coming from the CWA&TW architecture
HTS Axial flux permanent magnets electrical machine prototype: Design and test results
No full textA prototype of high-temperature superconductors (HTS) Axial Flux Permanent Magnets, Synchronous Machine in a single-side configuration is presented. The stator and the rotor disc have the same outer diameter D out of 316 mm and its air gap is set at 5 mm. HTS coils placed on the stator disc and permanent magnets placed on the rotor disc have been used to assemble the Y-connected ironless armature concentrated winding and the field winding, respectively. Each phase of the armature winding is composed by two coils connected in series, although the ac power loss is the key problem: This might be less important for wind turbine applications interested by low armature frequency. Preliminary tests at the liquid nitrogen temperature on the prototype in generator and motor mode at different load conditions have been conducted in our laboratories: The characteristic curves (power, torque, efficiency, etc.) at different rotational speed are reported. A proper response at 77 K has been provided by the prototype so that HTS coils can be used to assemble armature windings for low-frequency applications. The test results show that the HTS permanent magnet machines in the axial flux configuration are more compact, efficient, and light weight than their conventional counterpart
Strain sensitivity and superconducting properties of Nb3Sn from first principles calculations
No full textUsing calculations from first principles based on density-functional theory we have studied the strain sensitivity of the A15 superconductor Nb 3Sn. The Nb3Sn lattice cell was deformed in the same way as observed experimentally on multifilamentary, technological wires subject to loads applied along their axes. The phonon dispersion curves and electronic band structures along different high-symmetry directions in the Brillouin zone were calculated, at different levels of applied strain, ε, on both the compressive and the tensile side. Starting from the calculated averaged phonon frequencies and electron-phonon coupling, the superconducting characteristic critical temperature of the material, Tc, has been calculated by means of the Allen-Dynes modification of the McMillan formula. As a result, the characteristic bell-shaped Tc versus ε curve, with a maximum at zero intrinsic strain, and with a slight asymmetry between the tensile and compressive sides, has been obtained. These first-principle calculations thus show that the strain sensitivity of Nb3Sn has a microscopic and intrinsic origin, originating from shifts in the Nb3Sn critical surface. In addition, our computations show that variations of the superconducting properties of this compound are correlated to stress-induced changes in both the phononic and electronic properties. Finally, the strain function describing the strain sensitivity of Nb3Sn has been extracted from the computed Tc(ε) curve, and compared to experimental data from multifilamentary, composite wires. Both curves show the expected bell-shaped behavior, but the strain sensitivity of the wire is enhanced with respect to the theoretical predictions for bulk, perfectly binary and stoichiometric Nb3Sn. An understanding of the origin of this difference might open potential pathways towards improvement of the strain tolerance in such systems. © 2013 IOP Publishing Ltd
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
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