Hollow Cathodes for Low-Power Hall Effect Thrusters

The operation of hollow cathodes greatly impacts the overall performance of low-power Hall effect thrusters (HETs), belonging to a class of electric thrusters with an operating power lower than 500 W. This class of HETs can be installed on-board the satellites for telecommunications and Earth observation missions, being particularly suited for small satellites where power and mass budgets are inherently limited. A performance improvement of the cathode is particularly needed in such applications, since a reduction in power and propellant consumption, typically within about 10% of the respective values for the entire system, has a significant impact on the overall thruster performance. In this context, two thermionic hollow cathodes, HC1 and HC3, have been designed and tested at Sitael, to be coupled with the Sitael low-power Hall thrusters. Both cathodes feature a lanthanum hexaboride (LaB6) emitter, and an orifice designed to extend the spot-mode operation to low values of mass flow rate. An in-house numerical model was used for the cathode design to define the geometry, in accordance with the thruster unit specifications in terms of discharge current, mass flow rate, and lifetime. HC1 is a cathode designed to provide a discharge current in the 0.3 – 1 A range, operating in steady-state conditions at mass flow rates between 0.08 and 0.5 mg/s of xenon. HC3 was designed for the range 1 - 3 A of discharge current, with 0.08 – 0.5 mg/s of mass flow rate. Both HC1 and HC3 have an expected lifetime higher than 104 hours, estimated on the basis of the emitter evaporation at the operating surface temperature, computed with the aid of the numerical model. Experiments were carried out, including preliminary characterization campaigns of each of the two cathodes as well as coupling tests. The collected data are presented and discussed with reference to the model predictions, showing a good agreement between theoretical and experimental results.