High-precision digital Faraday cups for FEEP thrusters

Abstract

The number of scientific missions that require an extremely precise alignment of spacecraft achieved by electric thrusters has increased significantly in recent years. This requires a highly accurate knowledge of the thrust vector misalignment and the thrust intensity variation over time. In order to be able to resolve this high accuracy of the thruster beam, a high-precision plasma diagnostics system is required. For example, for upcoming missions like NGGM, it is required to get a spatial resolution of the ion beam current measurements < 0.5" in polar and azimuthal direction. FOTEC’s plasma diagnostics system based on Faraday cup analysis was optimised in order to be able to analyse the thrust vector misalignment with high spatial resolution. On the one hand, the geometry was adapted to keep the reduction of the signal due to secondary electron emission at a minimum. On the other hand, the entire measuring electronics were built into the head of each Faraday cup. This completely eliminates signal interference due to the cable length and movement of the diagnostics arm. Measurements of the digital Faraday cup diagnostics system will be presented, which were carried out at an IFM Nano Thruster. In particular, the signal-to-noise ratio could be significantly improved.