Plasma characterization of a Hall Effect Thruster for a Negative Ion Source concept

Negative ion sources are a key component of neutral beam injection systems. A novel concept for a negative ion source based on existing well tested Hall Thrusters (HT) is presented. The thruster has been designed in order to maximize the hydrogen dissociation process so that the atom flux which exits the thruster has an optimized energy to generate negative ions when impinging on a low work function surface. The novel concept can in principle offer several advantages, for instance a limited amount of co-extracted electrons, a more uniform generation of negative ions and a lower rate of destruction of negative ions. The thruster has been completed and is now in operation. In this contribution, the behavior of the thruster with different mass flow rate and voltage is presented. In particular, the first measurements of the plume plasma parameters by a Langmuir probe are shown, both with nitrogen and with hydrogen as injected gas. The spatial profile of the plasma electronic temperature and density have been obtained by scanning the region outside the exit plane of the thruster. The measurements are aimed to determine the best position of a caesiated sample in order to maximize the generation of the negative hydrogen ions.