Investigation of the directional Faraday cup and improvement of the comparison between direct and indirect thrust measurements of a magnetic nozzle ECR thruster

The thrust of an electric propulsion device estimated from electrostatic probe measurements may be different from direct thrust measurements. In order to reduce this discrepancy for electron cyclotron resonance thrusters (ECRT), a new diagnostic is presented: the directional Faraday cup (FCDi). Thanks to a peculiar design, it assesses the angular distribution of ion current density in the plume of plasma thrusters. First, a theoretical model of the angular selectivity of the FCDi is introduced. It is validated with both simulations and experiments. Guidelines are given to use the FCDi in the plume of ECRT. Second, the observed discrepancies between direct thrust measurements made on a thrust stand and indirect ones are reduced when using the FCDi instead of a planar Faraday probe with a guard ring (FPGR). Relative errors come down to 10% with the FCDi, which are compared to 20%-30% observed with the FPGR. An analysis demonstrates that it is not due to the effect of local ion trajectories but comes from the magnitude of the ion current measured. A large sheath in front of the negatively biased FPGR seems to be the cause of this phenomenon. The grounded opening of the FCDi reduces this phenomenon and improves the ion flux measurement accuracy. This new probe, with angular selectivity characteristics, allows for the comparison of the ion flux ejection direction with the magnetic field line and contributes to a better description of ion population dynamic in the magnetic nozzle of the thruster.