Design and Performance Study of a Micro-Newton Thrust Stand for FEEP

An increasing number of future space missions and experiments are going to require propulsion systems capable of delivering accurate and finely controllable thrust down to the μN magnitude level, with minimal generation of dynamic noise. FEEP (Field Emission Electric Propulsion) thrusters are ideal candidates for these applications, but they need being accurately characterized in this low-thrust operational range. This article is concerned with assessing the feasibility of the direct laboratory measurement of the FEEP thrust with the required accuracy and with finding solutions to the related problems, namely: • the presence of environmental noise several orders of magnitude stronger than the signal to be measured; • the need to operate the FEEP in high vacuum, while providing the electrical connections to the high voltage supply system. After careful consideration of competing concepts, a homodynamic thrust measurement on a torsion balance with optical read-out has been selected for further analysis. An upper bound of both systematic and repeatability errors was determined and the performance degradation in the presence of environmental dynamic noise was evaluated in terms of attainable signal-to-noise-ratio (SNR) and r.m.s. error on the thrust measurement by means of a nonlinear random vibration analysis of the mechanical system. The results demonstrate the possibility of accurately measuring thrusts down to the μN range, while keeping the system complexity and costs within acceptable limits. Data on the influence of environmental noise from preliminary experiments on a mock-up of the thrust balance confirm the assumptions and predictions of the theoretical model.