Ion Thruster Performance Impacts due to Cathode Wear

It has been observed in ion thruster wear tests that the discharge loss increases with time and that the thruster performance correspondingly degrades. This behavior is usually attributed to an enlargement of the accelerator grid apertures during the tests due to ion erosion, which increases the grid transparency and thereby reduces the neutral gas pressure inside the thruster and decreases the ionization efficiency of the plasma generator. An analysis of thruster life test data using a discharge plasma model shows that this mechanism is insufficient to explain the observed results. Tests at Jet Propulsion Laboratory in an ion thruster simulator used in a 16,000-h discharge cathode wear test show similar increases in discharge loss with time, in spite of the fact that there are no ion accelerator or grid apertures eroding and that the average pressure in the discharge chamber is essentially constant. Experiments show that increases in keeper orifice diameter cause increases in discharge loss, and this trend is reproduced by two-dimensional numerical simulations that show a reduced ion generation rate in the near-keeper plume region as the electrode eroded. This cathode-electrode erosion mechanism is likely responsible for roughly half of the total discharge loss increases observed in ion thruster life tests. [PUBLISHER ABSTRACT]