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Magnetic reconnection in high-temperature plasmas: from fluid to gyrofluid modeling
No full textMagnetic reconnection is a fundamental process in highly conducting fluids and plasmas whereby the magnetic field line topology is rearranged and magnetic energy is converted into thermal energy, bulk kinetic energy and fast particle energy. It has been widely recognized to play an essential role in many events occurring in laboratory plasmas, classical examples of which are sawtooth crashes and disruptions in fusion devices, as well as in space and astrophysical plasmas, with magnetospheric substorms and solar flares being the most prominent examples. For this reason magnetic reconnection has attracted increasing consideration in recent years. Indeed, to deepen the knowledge of the microscopic scale reconnection physics, a Magnetospheric MultiScale (MMS) mission has been planned by NASA. Furthermore, several dedicated laboratory experiments have been designed in the last decade with the aim to advance the understanding of reconnection phenomena in regimes of interest for fusion plasmas. The modeling of magnetic reconnection in these regimes, characterized by low particle collisionality and high temperature, should require a kinetic description. On the other hand, important effects occurring at kinetic scales as the gyro-radii and particle inertial lengths can be described within a generalized fluid description, which is particularly desirable because of the physical intuition, the analytical tractability and the computational gain. For these reasons, in this thesis we have modeled magnetic reconnection adopting the latter approach. The analytical analysis and numerical simulations performed in this work have allowed us to obtain new results on the behaviour of magnetic reconnection in high-temperature plasmas. In particular, we have found new dispersion relations for the growth rate of the reconnecting instability in the presence of an equilibrium flow velocity, and we have also shown the relevance of the ion gyration on the growth rate and the field structures characterizing fast reconnection phenomena. The most remarkable result consists in having found that the gyro-motion of hot ions causes a novel acceleration phase of the reconnection process, which may help with the interpretation of experimental observations
Fast gyrofluid reconnection in high temperature plasmas
No full textThe nonlinear evolution of collisionless magnetic reconnection in the presence of a strong guide field is analyzed on the basis of a gyrofluid model for compressible plasmas. It is found that, in a certain regime of plasma parameters, ion gyration contributes to generate two distinctive nonlinear acceleration phases of the growth rate. Furthermore, in the advanced nonlinear phase, finite values of the ion Larmor radius are identified to be responsible for a splitting of the narrow layer structures of ion guiding-center parallel velocity and density perturbations around the magnetic equilibrium null line
Two-dimensional effects in the problem of tearing modes control by electron cyclotron current drive
No full textThe design of means to counteract robustly the classical and neoclassical tearing modes in a tokamak by localized injection of an external control current requires an ever growing understanding of the physical process, beyond the Rutherford-type zero-dimensional models. Here a set of extended magnetohydrodynamic nonlinear equations for four continuum fields is used to investigate the two-dimensional effects in the response of the reconnecting modes to specific inputs of the localized external current. New information is gained on the space- and time-dependent effects of the external action on the two-dimensional structure of magnetic islands, which is very important to formulate applicable control strategies.</jats:p
Formation of plasmoid chains in fusion relevant plasmas
No full textThe formation of plasmoid chains is explored for the first time within the context of the Taylor problem, in which magnetic reconnection is driven by a small amplitude boundary perturbation in a tearing-stable slab plasma equilibrium. Numerical simulations of a magnetohydrodynamical model of the plasma show that for very small plasma resistivity and viscosity, the linear inertial phase is followed by a nonlinear Sweet-Parker evolution, which gives way to a faster reconnection regime characterized by a chain of plasmoids instead of a slower Rutherford phas
Nonlinear response of magnetic islands to localized electron cyclotron current injection
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