Rizzoni, Marco (2016) Eccitazione di onde di AlfvÃ©n in plasmi magnetizzati: il caso della configurazione Reversed Field Pinch. [Magistrali biennali]
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The AlfvÃ©n weaves are fluctuations that propagate in the inside of the magnetized plasmas. These weaves can be observed both in astrophysic plasmas, like the solar crown and the Earthâ€™s magnetosphere, and in laboratory plasmas, like the magnetically confined plasmas used in the study of the controlled thermonuclear fusion. The AlfvÃ©n waves supply means of transportation of heat in plasmas, that is believed to carry out an important role in the heating of the solar crown, but also in the propagation of the heat from the center to the border of the fusion plasmas with the consequent degradation of their confinement. Therefore results to be extremely important to characterize and understand the effects of such weaves. Purpose of the present thesis is to study the effect of spontaneous excitation of AlfvÃ©n weaves in the magnetic configuration reversed-field pinch (RFP). In this configuration can be observed intermittently strong events of magnetic reconnection with conversion of magnetic energy in kinetic energy. With this thesis I aim to determine if such events can excite AlfvÃ©n weaves and, once proved that it is true, to characterize their properties. These studies have a direct repercussion on the interpretations of the observations on the magnetized plasmas produced with the experimental device RFX-mod operant in Padova. The thesis work was managed principally by the experimental analysis of the data collected with the above-mentioned RFX-mod device, and in particular have been observed two different kind of mode with the characteristics of the AlfvÃ©n modes: The first kind consists in two distinct modes, both characterized by low poloidale and toroidal periodicities, enduring all the discharge time, whose experimental observations suggest an interpretation of these modes in term of Global AlfvÃ©n Eigenmodes (GAE). The second kind appears only when the plasma undergoes a transition to states of helical equilibrium, called Single Helical Axis state (SHAx), and it is characterized by lower frequencies than the first kind. The study of the second kind of modes has been conducted principally by the study of the temporal evolution of the frequencies associated to them by the verify of their dependence from many plasma parameters, including the electronic density. The study of the first kind of modes, characterized for their being border modes and so easier to study, has been conducted instead by the comparison between the AlfvÃ©nic frequencies theoretically expected, calculated starting by the characteristics of the plasma, and the frequencies experimentally observed, by the identification of the poloidal and toroidal weave numbers of the observed modes, by the study of their eventual temporal evolution and by the searching of eventual dependences from elements like the phase and the amplitude of the so-called dynamo modes that determine the balance of magnetic configuration.
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