1,721,070 research outputs found
1D Josephson quantum interference grids: Diffraction patterns and dynamics
No full textWe investigate the magnetic response of transmission lines with embedded Josephson junctions and thus generating a 1D underdamped array. The measured multi-junction interference patterns are compared with the theoretical predictions for Josephson supercurrent modulations when an external magnetic field couples both to the inter-junction loops and to the junctions themselves. The results provide a striking example of the analogy between Josephson phase modulation and 1D optical diffraction grid. The Fiske resonances in the current-voltage characteristics with voltage spacing Φ0 (C ̄/2L), where L is the total physical length of the array, Φ0 the magnetic flux quantum and C ̄ the speed of light in the transmission line, demonstrate that the discrete line supports stable dynamic patterns generated by the ac Josephson effect interacting with the cavity modes of the line. © 2016 IOP Publishing Ltd
Experimental and numerical analysis of interfilament resistances in Nb 3Sn and NbTi strands
No full textThe transverse resistivity among filaments is a crucial parameter of composite superconducting wires, since it affects the coupling losses in ac regime, the current transfer length of the wire subjected to bending strain, and the current redistribution among filaments in the case of a quench. A direct measurement of the transverse interfilament resistance as a function of temperature in several Nb{3} \hbox{Sn} multifilamentary wires was performed by means of a 4-probe method. This paper deals with the numerical analysis of the measurements performed on a OCSI wire manufactured by Luvata, containing 82 superconducting subelements each made of 150 filaments. The measurements are analyzed by applying a 2-D FEM of the wire cross-section and a 3-D electrical circuit model of the wire sample. The results obtained have been compared with those found on two other Nb{3}\hbox{Sn} wires with different architecture and a NbTi wire taken as a reference. © 2013 IEEE
Experimental and theoretical analysis of transverse resistances in a Nb3Sn LMI-EM strand
No full textIn Nb3Sn strands, hundreds or thousands of fine superconducting filaments are embedded in a metallic matrix for thermal and electrical stabilization. The transverse electrical resistivity between filaments plays a fundamental role in determining the ac losses, the thermal stability, and the current transfer length of the wire. The direct measurements of the transverse electrical resistances give useful information both for stability computations and to analyze the mechanical performance of the wire. In this paper, the interfilament resistances measured with a four-probe technique on a Nb 3Sn wire produced by Europa Metalli have been interpreted through a simulation code. A 2-D finite element method model of the wire cross-section and of a 3-D electrical circuit model of the wire sample have been applied to derive qualitative and quantitative information about the transverse electrical resistance matrix. A comparison with measurements performed on a Nb 3Sn wire with a different configuration shows the relevance of the wire layout in determining the interfilament resistance between filament bundles. © 2002-2011 IEEE
Thermal-Hydraulic Test and Analysis of the ENEA TF Conductor Sample for the EU DEMO Fusion Reactor
No full textThe ENEA conductor for the EU DEMO Nb3Sn toroidal field (TF) magnets, cooled by supercritical He, features a rectangular cross section with two small pressure relief channels ('holes'), separated from the cable bundle by means of a flat spiral, twisted together with the last cabling stage. A well-instrumented short sample of the ENEA TF conductor has been tested in SULTAN at SPC, Villigen (Switzerland) in 2016, aimed at its thermal-hydraulic characterization, and the test results are presented here. A correlation for the friction factor in the small holes is derived, best fitting the results of a set of computational fluid dynamics simulations. The new correlation (combined with existing correlations for the He friction factor in the bundle region) is shown to allow a proper reproduction of the measured hydraulic characteristic of the conductor. The heat slug propagation tests are used to calibrate the hole-to-bundle heat transfer coefficient in the 4C thermal-hydraulic code and to estimate the characteristic length for the homogenization of the He temperature on the conductor cross section, following a localized thermal perturbation. © 2002-2011 IEEE
Coupled thermal and electromagnetic analysis of the NAFASSY magnet
No full textThe paper presents an analysis of the current distribution and electromagnetic losses in the NAFASSY magnet carried out with the THELMA code, thanks to a brand-new thermal module coupled with the pre-existing electromagnetic module. The non-linear thermal and electrical properties of both superconducting and copper strands, depending on the local temperature, current density and magnetic field, are taken into account. The model analyses a single turn of the magnet, located in the highest field zone, focusing on the current distribution in the cable, the coupling AC and DC losses during ramped waveforms. The results are then extrapolated to estimate the behaviour of the overall magnet. A description of the models is given, together with a parametric analysis of different boundary conditions and cable discretizations. The analysis shows that, in nominal working conditions, no thermal instability should take place. However, local current redistribution among the strands may occur, mainly driven by the interstrand contact pattern, the local magnetic field and the strand current density. © 2002-2011 IEEE
A Combined Electromagnetic and Mechanical Approach for EU-DEMO Toroidal Field Coils
Full text linkThe roadmap to fusion electricity of the European scientific program aims at the realization of the future DEMOnstration (DEMO) fusion power plant. In 2020, the pre-concept design phase of DEMO was completed, defining the concept and characteristics of the main magnets and structures of the machine. Sixteen toroidal D-shaped magnets, six poloidal annular coils and a central solenoid constitute the functioning system core. The reactor is subjected to huge mechanical loads, mainly due to the Lorentz force produced by the combination of the high magnetic fields and operative currents. As a consequence, the loading conditions are extremely demanding for the structural components, and it is crucial to complete a comprehensive static and fatigue assessment before proceeding with the next design iteration. This work focuses on the electromagnetic and structural analyses performed on the toroidal field coil system and its support structures to present the methodological approach developed. Exploiting the finite element method, a three-dimensional model has been defined to obtain the electromagnetic loads on the main time points of the reference plasma scenario and then transfer them to a related 3D structural model, corresponding to the discretization of the electromagnetic one. The structural model was used to obtain the displacement and stress fields at the various time points to perform the mechanical evaluation as well as the fatigue assessment
A top-down modeling approach for DEMO magnetic system
No full textThe DEMO magnet system embodies a set of mechanical structures to join together the different Toroidal Field coils and to provide a suitable support to thermal, magnetic and gravitational loads. Over the years, different baselines have been produced which have required revisions and updates of the studies of the magnetic system and its supporting structures. This work presents a 3-level modeling approach which is fast and therefore suitable to evaluate different requirements. Following a top-down sequence, at the macro-level, a global 3D model of the entire tokamak is developed, which is very efficient (around 2 sec CPU time on a common laptop) and allows sensitivity studies and inverse design. At the intermediate level one sector of the system is considered, to evaluate the possible critical points. At the local level, detailed models of particular structures or parts of the system can be exploited, without compromising the computational efficiency. The consistency across the scale is guaranteed by imposing the conditions at the boundary taken from the preceding level
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
Comparison of direct inter-filament resistance measurement on Nb 3Sn strands between university of twente and ENEA
No full textExperimental results of interfilament resistance measurements obtained with different facilities are compared. Two internal tin Nb3Sn} strand types are tested at the University of Twente (UT) and ENEA Frascati. The direct interfilament resistance is measured with a standard four-point voltage-current (V-I) method. At the UT, a probe-station is used with micropoint-contact needles as voltage taps and current leads. At ENEA, the results are attained by a setup with microbonded contacts through thin aluminum (Al) wires. To extract values for the filament-to-matrix contact resistance and for the effective transverse resistivity from these experiments, finite element method simulations are required. The results of the experiments are in good agreement. In addition, we correlate the effective transverse resistivity, derived from the direct interfilament resistance measurement, to values measured and calculated from ac coupling loss. © 2011 IEEE
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