IRIS Università degli Studi di Padovahttps://www.research.unipd.itIl sistema di repository digitale IRIS acquisisce, archivia, indicizza, conserva e rende accessibili prodotti digitali della ricerca.Mon, 01 Mar 2021 20:27:29 GMT2021-03-01T20:27:29Z101981Electromagnetic analysis of superconducting cables and joints in transient regimehttp://hdl.handle.net/11577/117317Titolo: Electromagnetic analysis of superconducting cables and joints in transient regime
Abstract: The paper deals with the electromagnetic analysis of the superconducting multistrand cables used to manufacture magnets for fusion research. An analysis method is described, in which the cable is represented as an equivalent lumped network, whose parameters are automatically computed starting from the cable geometrical data. The network is then solved in transient regime.
Thu, 01 Jan 2004 00:00:00 GMThttp://hdl.handle.net/11577/1173172004-01-01T00:00:00ZOverview of RFX-mod results with active MHD controlhttp://hdl.handle.net/11577/2431908Titolo: Overview of RFX-mod results with active MHD control
Abstract: Plasma experiments resumed in December 2004 on RFX-mod. The machine now has a thin (3mm) Cu shell with one overlapped poloidal gap and one toroidal gap. Shell penetration time for Bv has been lowered from 450 to 50 ms and shell/plasma proximity from b/a=1.24 to 1.1. Toroidal equilibrium is feedback-controlled and new power supplies provide a better control of the toroidal field. Newly designed graphite tiles protect the vessel from highly localized power deposition. The MHD Control System, MHD-CS, a set of 192 external saddle coils controlled by a digital feedback system, is used to control radial fields due to field errors, MHD modes and Resistive Wall Modes (RWMs). A dramatic improvement of plasma performance was obtained by using the MHD-CS to cancel all of the radial field components, an operational mode dubbed Virtual Shell (VS). The toroidal loop voltage was lowered by more than 40% and the plasma pulse duration tripled. In practice, steady state RFP pulses are now limited only by the applied volt-seconds. Hence RFX-mod initial operation demonstrated the possibility to operate a large RFP without a thick conducting shell, and opened enhanced RFP scenarios. Indeed the improved magnetic boundary in VS mode, which mimics an ideal closely fitting shell, has an effect on the tearing modes underlying the sustainment of the RFP configuration, the so-called dynamo modes, which are also responsible for field line stochastization in the plasma core and confinement limitation. With the VS the amplitude of such modes in the plasma centre was nearly halved. As expected, this led to improved particle and energy confinement. For instance, peak electron temperature in reference pulses at 600 kA was increased from 200 to 300 eV with more peaked profiles, which corresponds to a reduction of the thermal conductivity by a factor 2 in the region r/a < 0.9. The MHD-CS is extremely flexible and can be used for a variety of mode control experiments. The most important result already obtained was the demonstration of the active control of RWMs. We found that full VS control completely inhibits the growth of RWMs, whereas such modes are indeed seen to grow in agreement with the theoretical prediction if the MHD-CS operated in Selective VS mode, i.e. leaving one or more mode helicity uncontrolled.
Sun, 01 Jan 2006 00:00:00 GMThttp://hdl.handle.net/11577/24319082006-01-01T00:00:00ZControl of the magnetic field configuration in the MITICA Neutral beam injectorhttp://hdl.handle.net/11577/2683758Titolo: Control of the magnetic field configuration in the MITICA Neutral beam injector
Abstract: MITICA is a prototype of the Heating Neutral Beam (HNB) Injectors for ITER [1]. The MITICA scientific objective is to validate the design and optimize the operating performances of the injector before installation on ITER. The magnetic field configuration in the injector is critical for obtaining the required efficiency and beam optics. In order to quickly achieve the optimal operating condition, it is highly advisable to provide a flexible control of the most critical features of the magnetic field, without opening the vacuum vessel. The possibility of introducing an independent variation of the magnetic field in the plasma source and in the accelerator has been analyzed and a feasible solution proposed, based on small lateral coils. The possibility of recreating the expected stray magnetic field conditions of the ITER HNB was already foreseen in the past by using a further set of external coils, whose design has been revised and updated. The paper describes the rationale and the magnetic design optimization of these systems.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/11577/26837582013-01-01T00:00:00ZPlasma Contour Identification using Multipole Series Expansionhttp://hdl.handle.net/11577/117271Titolo: Plasma Contour Identification using Multipole Series Expansion
Abstract: The paper presents a method for the reconstruction of an axisymmetric configuration of magnetic field. The reconstruction procedure is based on a multipolar series expansion of the magnetic field and requires a number of experimental measurements of magnetic field and flux. The accuracy and efficiency of the procedure is analysed and compared with another procedure based on an integral approach
Mon, 01 Jan 1996 00:00:00 GMThttp://hdl.handle.net/11577/1172711996-01-01T00:00:00ZA novel approach for solving three dimensional eddy current problems in fusion deviceshttp://hdl.handle.net/11577/3169470Titolo: A novel approach for solving three dimensional eddy current problems in fusion devices
Abstract: We present a technique to efficiently solve 3D eddy current problems in fusion devices, whose structures exhibit a geometrical symmetry. The proposed approach is based on the exploitation of symmetry via harmonic analysis and it is suitable to treat also problems where the field sources (plasma or external coils) do not share the domain symmetry. A simple test case is presented to describe the methodology. Then, a more complicated geometry is considered, which represents a realistic vacuum vessel of an ITER like fusion device (double layer structure with several portholes). The results are discussed for both axisymmetric and non-axisymmetric excitations in frequency domain
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/11577/31694702015-01-01T00:00:00ZA boundary integral method for computing eddy currents in non-manifold thin conductorshttp://hdl.handle.net/11577/3169480Titolo: A boundary integral method for computing eddy currents in non-manifold thin conductors
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/11577/31694802015-01-01T00:00:00ZThe Greenwald density limit in the Reversed Field Pinchhttp://hdl.handle.net/11577/2437908Titolo: The Greenwald density limit in the Reversed Field Pinch
Abstract: In the Reversed Field Pinch RFX the density operating space exhibits an upper boundary that is well described by the Greenwald limit. The basic findings are described and similarities and differences with the Tokamak case are analysed. The high-density limit occurs as a non-disruptive limit. Fast terminations have been observed but in a wide area of the operating space and therefore are unlikely to be related to the density limit. Radiation induced thermal collapse can also be ruled out being radiation losses always a relatively small fraction of the ohmic input power. The occurrence of plasma detachment while approaching the density limit can be excluded. The absence of additional heating makes it difficult to distinguish with certainty between a power balance effect and an intrinsic physical limit but the overlapping of the Greenwald limit with the upper boundary of the density operating space is however remarkable, especially in the case of He plasmas . While in Tokamaks one of the plausible causes for the limit is a transport induced edge thermal instability, in RFX, as density increases the density profile becomes hollow, inside the last closed magnetic surface particle diffusion decreases and the global energy confinement time improves. The normalised density fluctuations measured by Langmuir probes and by the outermost chords of an interferometer do not increase. Simulations of the hydrogen discharges with the RITM code confirm the importance of recycling in determining the edge density gradient and the minor role of radiation losses. Differences between H and He cases are analysed in terms of particle penetration capability and in terms of edge ExB shear. A statistical analysis of the edge density fluctuations is presented, looking for differences arising as density is increased. Finally, the possible role of pressure driven modes is discussed.
Thu, 01 Jan 2004 00:00:00 GMThttp://hdl.handle.net/11577/24379082004-01-01T00:00:00ZA Novel Tool for Breakdown Probability Predictions on Multi-Electrode Multi-Voltage Systemshttp://hdl.handle.net/11577/2806718Titolo: A Novel Tool for Breakdown Probability Predictions on Multi-Electrode Multi-Voltage Systems
Abstract: An innovative approach for the voltage breakdown prediction in high-voltage systems, insulated by large vacuum gaps, is presented.
It is based on the correlation between the clump mechanism and a statistical approach to the breakdown probability. The aim
of this paper is twofold. First, the numerical solution of 3-D electrostatic problems by a couple of complementary formulations is
presented. Second, an efficient post-processing tool is introduced, based on the analytical solution of the equations of motion in a
domain covered by a tetrahedral mesh, to estimate the breakdown probability associated to the electrically charged microparticles
leaving one electrode and clashing to the electrode with opposite polarity with sufficient energy to get vaporization. This approach
has been benchmarked on a reference configuration (sphere/plane) problem and applied to calculate the particle trajectories in a
very complex multi-electrode multi-voltage system.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/11577/28067182014-01-01T00:00:00ZSparsification of BEM matrices for large-scale eddy current problemshttp://hdl.handle.net/11577/3169479Titolo: Sparsification of BEM matrices for large-scale eddy current problems
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11577/31694792016-01-01T00:00:00ZOptimal strategies for real-time sparse actuator compensation in RFX-mod MHD control operationshttp://hdl.handle.net/11577/3169474Titolo: Optimal strategies for real-time sparse actuator compensation in RFX-mod MHD control operations
Abstract: In many devices aiming at magnetic confinement of fusion relevant plasmas, feedback control of MHD instabilities by means of active coils is nowadays mandatory to ensure the robustness of high performance operational scenarios. Actuators involved in the control loop are often coupled in the sensor measurements and an optimal strategy for decoupling can be limited by the need of reducing as much as possible the cycle time of the control loop itself. It is also important to stress the fact that the problem is intrinsically 3D, involving different non-axisymmetric contributions. The baseline situation in RFX-mod is documented, where the identity matrix is chosen to represent the simplest case of mutual coupling matrix. The problem of missing or broken actuators is introduced and tackled with dedicated compensation strategies. A detailed description is given for a possible compensation concept which can be applied in real-time operation thanks to its implementation strategy, yielding very promising results in terms of local field reconstruction.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/11577/31694742015-01-01T00:00:00Z