Direct comparison of different probe positioning and pointing schemes for plume measurements using an Advanced Cusp Field Thruster

We investigate how probe positioning and pointing schemes affect ion current density measurements, specifically total beam current and divergence efficiency. Experiments were conducted using a five-axis gantry and a nude Faraday probe in the plume of an Advanced Cusp Field Thruster (ACFT). By mimicking Cartesian and spherical diagnostic geometries, we compare typical measurement strategies within a single setup. Additionally, maximizing local current via probe rotation reconstructs the ion plume origin, revealing a volumetric source shaped by the discharge chamber. While mathematical corrections for rigid probe misalignment exist, uncertainties from collector angle and secondary electron emission grow significantly with incidence angle. Results show that optimizing probe alignment to within reduces divergence and beam current measurement uncertainty to below 2.5%. These findings highlight diagnostic geometry as a key factor in high-precision plume characterization.