Magnetic reconnection in high-temperature plasmas: from fluid to gyrofluid modeling

Magnetic 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.