The H2020 MINOTOR project focused on the study and the optimization of the ECRA thruster, an electric propulsion system for satellites. First, a 30 W prototype of this Electron Cyclotron Resonance Thruster (ECRT) is optimized. Then, a 200 W prototype is design based on these findings. The performances of both prototypes were assessed at two different facilities: at ONERA in France and at Justus Liebig University in Germany. Significant improvement of performances of both thrusters with the decrease of background pressure is presented. Total thrust efficiencies as high as 50% are measured at JLU, as well as erosion and lifetime assessment. The stability of the thruster for a constant point of operation was measured for one hundred hours of operation.

Even after half a century of development, many phenomena in Hall Effect Thrusters are still not well-understood. While numerical studies are now widely used to study this highly non-linear system, experimental diagnostics are needed to validate their results and identify specific oscillations. By varying the cathode heating current, its emissivity is efficiently controlled and a transition between two functioning regimes of a low power thruster is observed. This transition implies a modification of the axial electric field and of the plasma plume shape. High-speed camera imaging is performed and the data are analysed using a Proper Orthogonal Decomposition method to isolate the different types of plasma fluctuations occurring simultaneously. The low-frequency breathing mode is observed, along with higher frequency rotating modes that can be associated to rotating spokes or gradient-induced instabilities. These rotating modes are observed while propagating outside the thruster channel. The reduction of the cathode emissivity beyond the transition comes along with a disappearance of the breathing mode, which could improve the thruster performance and stability.

The evolution of the radial profile of the rotation of a weakly magnetized plasma column is investigated experimentally in a radio-frequency argon plasma at low pressure when a strong electron current is emitted by large emissive cathodes. Current injection from large emissive cathodes over a background plasma column (with a plasma density of a few $10^{18}\ \textrm {m}^{-3}$ ) is characterized. Radial scans of the ion velocity show that a continuous control of the rotation profile may be obtained using two spatial configurations for the locations of the emissive cathodes (either in the centre or at the edge of the plasma column). The rotation profile results from the electric drift velocity, damped by the drag exerted on the ions. We demonstrate that the evolution of the rotation profile with the injected current is then controlled by the modification of the plasma potential profile in the presence of strongly emissive cathodes.

Electrostatic probe and thrust balance measurements of a coaxial electron-cyclotron-resonance plasma thruster with magnetic nozzle are compared against numerical simulations of the device that solve self-consistently the plasma transport problem with a hybrid particle-in-cell/fluid approach and the microwave electromagnetic fields using mixed finite elements. A simple phenomenological anomalous transport model similar to those used in Hall thruster modeling is applied. Reasonable average relative errors are reported on the ion current density (8.7%) and plasma density (12.8%) profiles along the plume. Good agreement is found in terms of relative errors on thruster performance parameters as the 90%-current divergence angle (0%–3%), utilization efficiency (3%–10%), peak ion energy (9%–15%), and energy efficiency (2%–17%). The comparison suggests that enhanced cross-field diffusion is present in the plasma. Differences in the experimental and numerical behavior of electron temperature point to the areas of the model that could be improved. These include the electron heat flux closure relation, which must correctly account for the axial electron cooling observed.

The measurement of electron density is a key issue in understanding and controlling plasma applications. To date, plasma density in electric thrusters has been mainly evaluated with electrostatic techniques, such as the Langmuir probe, which could be quite invasive. In this paper, we propose the application of a microwave resonant probe, the curling probe, to the diagnostic of an electrodeless plasma thruster. The measurable electron density range and the probe accuracy are found to be limited by the probe natural frequency. We present the numerical study and the experimental characterization of three curling probes with different natural frequencies (700, 1400, and 3000 MHz, approximately). First, an analytical equation of the natural frequency as a function of geometrical parameters is drawn from 2D numerical simulations. Then, a procedure based on solid dielectric etalons is proposed for the absolute calibration of the probe. Finally, measurements are performed in the plume of an electron cyclotron resonance plasma thruster. Electron densities from 10⁸ to 10¹¹ cm⁻³ have been obtained in agreement with hairpin and Langmuir probes results. A wall-embedded probe has enabled measurements inside the thruster source with minimal plasma perturbation. A possible curling probe configuration, embedded in a reactor wall, is proposed as a fully non-invasive diagnostic for plasma sources.

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.