Thermal content dynamics in magnetically confined plasmas

The present thesis is devoted to the physical investigation of magnetic reconnection events in the RFX-mod device operating in the Reversed Field Pinch (RFP) configuration for magnetic confinement. The analysis is carried out through a combined numerical and experimental approach. On the numerical side, thermal content (i.e. ions) dynamics is studied through simulations via the Hamiltonian guiding center code Orbit. On the experimental side, a preliminar activity, aimed at the refurbishment of the Neutral Particle Analyzer (NPA) data acquisition system, is presented. This is meant in view of the upgraded RFX-mod2 device, whose operations are expected to start by the end of 2026. Experiments conducted on the Madison Symmetric Torus (MST), a device similar to RFX-mod, have demonstrated that magnetic reconnection accelerates plasma charged particles. Similarly, in RFX-mod, significant ion heating has been observed through NPA measurements. Particle energization associated with magnetic reconnection is a fundamen tal topic also in space plasma physics; however a proper description of the phenomenon is still far to be achieved. The main objective of the thesis is therefore to investigate plasma particle dynamics during magnetic reconnection events, with particular emphasis on the case of the discharge #29324 at time t = 218 ms, when a strong magnetic reconnection event induced a Plasma-Wall Interaction (PWI) that was directly observed by a CCD camera imaging the first wall. The study considers a gradual approach of increasing complexity. A first simplified analysis, based on analytical methods, shows that phase-locking of m = 1 tearing modes represents the primary responsible for the observed PWI. This phase-locking involves a large number of tearing modes, actually more than those measured by the magnetic sen sors of RFX-mod. A more refined analysis is then performed through the calculation of the Connection Length to the wall metric using the Orbit code, confirming the role of the m =1 mode phase-locking as the leading loss mechanism toward the first wall, while also revailing an additional contribution of the m = 0, n = 7 mode. Furthermore, the cal culation of loss times for Maxwellian ions via Orbit highlights that particles lost through topologically different channels are also characterized by different energy and pitch. This result provides a means to discriminate loss channel topology based on the simulation of thermal content dynamics. Finally, the first implementation of a three-dimensional electrostatic potential associated with the magnetic reconnection in Orbit simulations is shown to be a fundamental key for understanding the thermal content behavior during magnetic reconnection, which had not been fully disclosed by numerical methods. The cal culation of highly-resolved, three-dimensional maps of Connection Length and Maxwellian ion loss times presented in this thesis relies on a new parallelized computing algorithm, implemented for the first time within the Orbit workflow.