• Plasma diagnostics
• Triple Langmuir probe
• Hall effect thruster
• Hollow cathode
• Emissive probe
• Electric propulsion

The applications of electric propulsion are widely increasing in commercial and scientific missions due to lower propellent consumption with respect to chemical propulsion, allowing for lower launch mass of a spacecraft or satellite and lower costs deriving from the use of smaller launch vehicles. Hall effect thrusters provide a trade-off between specific impulse and thrust, along with relatively low power-to-thrust ratios. Lanthanum hexaboride hollow cathodes providing discharge currents in the range 10-100 A are the primary choice for ionization and neutralization purposes of high-power Hall effect thrusters. The keeper erosion due to ion-induced sputtering is one of the main life-limiting mechanisms of the cathode and the overall electric propulsion system. Therefore, a deep understanding of the impact of the keeper erosion on the cathode performance is important to ensure a long-duration operation of the thruster unit. In this context, the spatial variation of plasma density, electron temperature and plasma potential in the plume of hollow cathodes have been measured using scanning probes, to correlate the plasma parameters with the cathode operation and performance. Two cathodes were investigated, namely HC20 and HC60, designed to be coupled with SITAEL’s HT5k and HT20k (5 kW- and 20 kW-class) Hall effect thrusters. Experimental campaigns were carried out at different operating conditions, and with two different orifice geometries for each cathode. The cathodes were mounted in stand-alone configuration with an auxiliary cylindrical anode. The experimental campaigns were performed using triple Langmuir probes to measure the plume parameters at various axial and radial distances from the keeper exit. General trends of electron temperature, plasma potential and plasma density are reported in terms of discharge current, mass flow rate and orifice geometry. The results are presented and commented regarding the better orifice geometry and operating conditions for extended life and improved performance. The plume characterization is shown in terms of plume, transition and spot mode for each experiment. Future developments will include the introduction of different diagnostic techniques as the emissive probe, of which a preliminary design is presented.