Plasma probe measurements for electric propulsion device ion beam: optimization and standardization

Abstract

This dissertation focuses on the optimization and the design standardization of electrostatic probes to measure ion current and energy within the plume of differ- ent low-power electric thruster technologies: a Field-emission electric propulsion thruster, a Hall thruster and a Radio frequency ion thruster. The work relies on experimental methods to iterate several architectures of Faraday cup to converge towards a common design to be used for these three thruster categories.A large fraction of the work was dedicated to the development of a Faraday cup to accurately measure the ion current of a FEEP thruster. The advantage of these electric propulsion devices is that they can be operated with controlled and stabilized emission parameters (e.g. current and voltage). Hence, these reference values can be used for comparison with the ion current or energy measured with a Faraday Cup (Iiint ) or a RPA (Ei). We showed that the firing needles distribution from a crown can lead to a decrease in the FC accuracy. This is mainly due to the mathematical method used to integrate the current density profiles to retrieve the ion current. There, we make the assumption that the ion beam has a symmetry around the thruster axis. This becomes false when the ion beam largely deviates from the thruster firing axis as ion signal measured by the probe is off-set from the beam centre. In one case study we showed that a crown firing with only 32% of its capacity without homogeneous distribution can lead to an error of -20%. Errors are maximized when the extractor voltage magnitude is further increased because it leads to increased beam deviation from the thruster centre axis. Nonetheless, our integration method works for firing crowns with 60% or more of its capacity and with homogeneous needle distribution.