On the relation between MHD turbulence and electron acceleration in magnetized plasmas

The thesis investigates the process of electron acceleration in magnetized plasmas and its relation with electrostatic and magnetic turbulence. Electron acceleration is a mechanism observed both in laboratory and astrophysical plasmas, of whom some examples are the solar wind and the magnetosphere. The thesis initially offers an overview about the most relevant theoretical concepts regarding plasmas, magnetohydrodynamics, magnetic reconnection and turbulence, in addition to information about the devices which collected the data for the analysis, which are two complementary Reversed Field Pinch toroidal devices: TPE-1RM20, which gathers data with insertable probes combining magnetic coils and Electron Energy Analyzer (EEA), and RFX-mod which provides the time evolution of the parallel currents. The electron acceleration phenomena consist in bursting events which emerge out of the time series of the signals, and they are likely to be associated with magnetic reconnection. First of all, the intermittent nature of these events has been confirmed, by verifying the non self-similar behaviour of the Probability Distribution Function of the magnetic fluctuations. The events responsible for the deviation from self-similarity have been selected using the Local Intermittency Measurement method. It is to be mentioned that TPE-1RM20 is particularly affected by an intense MHD activity at low frequency which affects the study of the spectrum in the higher frequency range, whereas RFX-mod is not limited from this point of view. The statistics have initially focused on the conditional average of single shots of signal which has provided the average fluctuating structure whose amplitude can be computed in relation to the scale. Later, the same analysis has been applied to multiple signals. The behaviour of the fluctuations of the EEA current and parallel current, of the magnetic and electric fields has been checked at the same instant of time, in order to detect any kind of possible correspondence. Successively, the waiting time distribution has been analyzed, in order to verify if it satisfied some particular relation with the frequency scale. The final step has been to prove the existence of a correlation between the minima present in the time trace of the reversal parameter indicating reconnection and the detected events. In the end, the proposed link between electron acceleration and magnetic reconnection is confirmed, as deduced by the associated magnetic fluctuations, whose shape is consistent with the typical structure of current sheets.