Electromagnetic mode characterization in a Hall thruster used as hydrogen ion source

ATHENIS is an electric Hall effect thruster, built to study the application of the said thrusters in H- ions production thanks to the interaction with Cesium-planted surfaces. The negative ions extraction mechanism is of crucial interest in the heating of nuclear fusion plasmas, generated in the ITER project. Thrusters, that exploit a magnetic field to confine electrons and accelerate ions, are subject to electrostatic oscillations that can affect performances depending on their propagation and frequency. This thesis task is addressed to study ATHENIS working conditions. In particular, I studied working regimes obtainable with different power supplies, the roles of thermoionic emission of a tungsten filament used as cathode and the propellant flux in the acceleration of the produced ions, and the characteristics of the plasmas generated in this way. Besides the general properties study, the purpose of this thesis is to obtain a characterization of the present oscillations. The work done covers varied experimental activities including probes building and wiring, acquisition system building, thruster managing, data recording and analysis. Collected data, analyzed through expressly written software in IDL environment, cover different working condition for Argon, Helium and Hydrogen used as propellant, in order to integrate previous studies on ATHENIS, limited to Hydrogen and Nitrogen. The results, obtained thanks to a Mach probe, mainly working with Argon, show the existence of optimal pressure and filament current conditions in ion acceleration, even if plasma parameters show weak bonds with working conditions. Moreover, the observation of electrostatic fluctuations, got by a circular electrostatic probes array set over the top of the thruster, showed the presence of three kinds of oscillation in three different frequency bands, strictly depending on the used gas. Their frequencies and propagation velocities rely on pressure conditions. In particular, oscillations known as rotating spoke and transit-time and one more curious oscillation near the MHz region have been observed.