Development of Hollow Cathodes for 5 to 20 kW Hall Thrusters

The use of electric propulsion systems is growing wider for military and commercial applications, as well as scientific missions. The benefits provided by electric propulsion are tied to a lower propellant mass with respect to chemical propulsion, allowing for a higher payload mass, a lower total mass, therefore less expensive launch vehicles. Hall effect thrusters provide an optimal trade-off between specific impulse and thrust, along with a lower power-to-thrust ratio with respect to other electric propulsion technologies. For the high-power class of thrusters, the cathodes must be able to sustain operating currents up to several tens of amperes. In this context, two lanthanum hexaboride (LaB6) hollow cathodes have been developed and tested at Sitael, conceived for Hall thrusters in the range 5 – 20 kW. The cathodes for 5 kW-class Hall thrusters, HC20 and HC20h, are required to provide discharge currents in the 8 – 20 A range, with a lifetime in excess of 104 hours. The hollow cathode to be coupled with 20 kW-class Hall thrusters operates at discharge currents up to 60 A, hence the name HC60. The cathodes were characterized during stand-alone test campaigns, as well as with the respective Hall thrusters. The experimental results will be presented and compared with the predictions from a previously developed model describing the operation of thermionic hollow cathodes, showing a good agreement between empirical and theoretical data. The model was expanded to consider the cathode plume exiting from the keeper. As a matter of fact, a two-fluid description of the plasma between the keeper and an anode plate was implemented in a numerical model, which will be described and whose results will be discussed and compared with the available experimental data.