Simultaneously measured direct and indirect thrust of a FEEP thruster using novel thrust balance and beam diagnostics

For the characterisation of an electric propulsion system, the determination of the thrust has a crucial role. The thrust of an Indium FEEP Multiemitter (IFM) Nano Thruster laboratory model is measured directly with a thrust balance and indirectly via beam diagnostics. Both measurements are carried out simultaneously to enable mutual verification with high accuracy. The novel mN-torsion thrust balance by FOTEC resolves six different thrust magnitudes, ranging from 10μN to 1N. It is based on the so-called force-feedback method using a voice coil actuator to determine the thrust, which leads to an accuracy of better than 2%. The indirect thrust measurements were performed with a high-precision beam diagnostics system by FOTEC. A semi-circular diagnostics arm equipped with 23 digital Faraday cups is used to measure the ion current density distribution of the entire beam. The thrust results of both systems show an agreement with a deviation less than 5% for thruster operation points ranging from 50 to 450μN. This proves that the performance of a FEEP thruster can be characterised very precisely by indirect trust measurements.