• Onset
• Applied-Field magneto-plasma-dynamic thruster
• Electric propulsion

An investigation of operative instabilities affecting the SX3 thruster has been performed by mean of two different criteria suitable for Applied-Field Magneto Plasmadynamic Thrusters (AF-MPDTs). The analysis has been related to experimental campaigns in 2012 and 2013 and has been extended to planned future configurations of the SX3. The outcome has been summarized in a graphic representation of admissible operative regions in which the thruster can be operated
with no occurrence of instabilities. Experimental data related to the SX3 have been analyzed in order the verify the presence of instabilities in early phases of the thruster operation. It was found that, within the available frequency range
considered, peaks in the frequency power spectrum estimate are related to the frequency of the plume oscillations spotted during the thruster operation. The analysis of operated conditions of the SX3 also suggested that different applied
magnetic field topologies can strongly affect the thruster behavior at subcritical and onset related regimes. Therefore, a detailed characterization of the applied magnetic field configuration for the present configuration has been performed, determining a number of parameters able to describe the applied magnetic field for the given coil arrangement.
In order to relate the behaviors of the thruster at the onset with operational parameters a simplified voltage model has been used to reproduce the voltage trend of experimental results. A simplified model useful to compute the ionization current as lower limit of operation for the SX3 has been also considered.
The analysis with instability criteria suggest the use of larger cathode configurations
in order to prevent the onset occurrence. The characterization of the applied magnetic field highlighted the advantage to operate with smaller anode configurations rather than with larger cathodes in reducing the magnetic flux lines
divergence.