Dormant Cathode Plasma Properties and Erosion Analysis in a Multiple-Cathode, High-Power, Rectangular Discharge Chamber

A high-power, rectangular discharge chamber is being investigated by the University of Michigan for operation with multiple discharge cathode assemblies (DCAs). The multiple cathode approach attempts to increase thruster lifetime by operating three DCAs sequentially, possibly providing a threefold increase in discharge life. Previous multiplecathode 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 (MCDC) may decrease the anticipated discharge lifetime. For the experiments presented here a MCDC test article (TA) is operated in the University of Michigan Large Vacuum Test Facility. In order to assess potential dormant cathode erosion, diagnostic cylinders (DCs) are designed and utilized to measure plasma properties at the dormant cathode locations. Each DC appears similar to the active DCA, but is outfitted with plasma diagnostics, such as Langmuir probes and a retarding potential analyzer (RPA). Langmuir probe results indicate that typical dormant cathode number densities, electron temperatures, and plasma potentials are 5.0x10 cm, 5 eV, and 27 V, respectively. Most probable ion energy measurement results are consistent with the plasma potential measurements. Pre-operation erosion rates for molybdenum (Mo) and graphite dormant cathode keepers are estimated based on sputter threshold, yield, ion incidence angle, and ion flux equations. Results suggest little to no graphite erosion and a Mo erosion rate between 040 mm/khr.