Ion Energy Measurements Near a Dormant Cathode in a Multiple-Cathode Gridded Ion Thruster

A rectangular ion thruster discharge chamber was investigated for operation with multiple discharge cathode assemblies (DCAs). the multiple cathode approach attempts to increase thruster throughput and lifetime by operating three DCAs sequentially, possibly providing a threefold increase in discharge life. Previous multiple-cathode electric propulsion devices, such as the SPT-100, have shown dormant cathode erosion to be a life-limiting phenomenon. Similar results in a multiple-cathode discharge chamber may decrease the anticipated gain in discharge lifetime. in order to assess possible dormant cathode sputtering erosion, a diagnostic canister (DC) was designed and utilized to measure bombarding ion energy at the dormant cathode locations. the DC appeared similar to the active DCA, but was outfitted with a retarding potential analyzer. Most probable ion energy measurements show ions with energy of 27-35 eV (±10%) with respect to cathode common and ion energy increases with increasing magnetic field strength. These results are consistent with an ion falling from the plasma potential to cathode common. a simple sputtering erosion model shows that, if doubly charged ions are present, these energies are enough to cause sputtering erosion of the dormant units