Thermal Flux Probe for Characterization of the Ion Source: Background Pressure Effects

This work presents the development and deployment of a thermal flux probe (calorimetric probe) for characterization of the plasma plume of a gridded ion source under varying background pressures. The background pressure can significantly alter plume characteristics including velocity distribution functions for ions and atoms/molecules, and plume angle for ion thrusters and Hall thrusters. The charge exchange (CEX) collisions between accelerated ions and slow background gas atoms/molecules results in the formation of a fast neutral flow and slow ion population. The latter also contributes to plume broadening. In this paper, we study the CEX process for the plume of the gridded ion source in the pressure range of 10^-5 torr to 10^-3 Torr. This background pressure range is relevant to environments at the thruster test facilities and industrial applications of these plasma sources. We also discuss the encountered challenges for reliable measurements of the fast neutral beam, such as thermal flux probe calibration and creation of slow ions due to the charge-exchange process.