# Erosion Rate Measurements for DART Spacecraft Ion Propulsion System +
###### Abstract  
The Double Asteroid Redirection Test (DART) spacecraft was developed to provide the first measurement for orbital deflection of an asteroid upon intentional impact. The NEXT ion engine is part of the mission, on its maiden voyage. As part of the pre-launch risk reduction, erosion characteristics of the extraction grid system were evaluated using laser measurements of sputtered molybdenum atoms over the envelope of potential throttle conditions for the mission. Erosion rate dependence on propellant flow rate as well as relative density and directionality of molybdenum sputter from grid center to edge were measured. Sputtered atoms were found to have average radial velocity directed toward the engine perimeter and increasing with radial distance. The relative contribution of source and facility background gas and other sources of accelerator grid current was examined as well as the influence of several engine operating parameters. Facility background gas was found to influence engine operation more than a wall-mounted pressure gauge and typical assumptions about ingestion would indicate. Far-field flux was estimated over the full angular range based on the near-field relative density and velocity results and relying on quartz crystal microbalance data at one location to fix absolute numbers everywhere. The results substantially deeper knowledge and understanding of the complex grid erosion process of the engine and its lifetime, as grid failure via erosion is the normal life limiter. Study results are also relevant to thruster-spacecraft integration issues such as molybdenum deposition rate on solar cells and other spacecraft surfaces. Laser; plasma; ion engine; accelerator grid; DART; asteroid; erosion; molybdenum; ion propulsion +
Footnote †: This paper is an extended version of paper published in AIAA SciTech Forum, held in San Diego, CA, USA, 7-11 January 2019.  
designs, the primary failure mechanism is still expected to result from degradation of the accelerator grid structure by ion sputtering effects. Plume sputter flux depends on distance and angle, thruster operating point, grid material, and facility background pressure. Net mass deposition rates are also a function of ion beam flux profile, due to competition between deposition and erosion rates. Ion beam flux varies much more strongly with plume angle than molybdenum flux. Absolute measurements of real-time net mass deposition rate for surfaces exposed to the plume of a NASA NEXT ion engine with molybdenum grids have been previously performed [6]. Although NEXT had been characterized in considerable detail previously, data were lacking for engine operation over the range of potential flow rate conditions for the DART mission. Flow conditions deviate from standard throttle table settings established in ground tests due to reduced flow system accuracy for the flight configuration. The flow of interest is the total passing through the discharge chamber, which is the sum of discharge cathode (DCA) and main chamber rates. For the present study, the flow rate dependence of thruster extraction grid erosion was investigated, varying the flow rate about the standard operating condition targeted for DART. In addition to effects on thruster lifetime, erosion and deposition effects lead directly to integration issues for the spacecraft that use them. Metal and insulator atoms and ions produced via sputter erosion may find their way into the plume and deposit on spacecraft surfaces, modifying their properties. Due to the inherent divergence of the ion beam, there may be intercepted spacecraft surfaces that undergo sputter erosion, modifying roughness, composition, and thickness, and other surfaces with net deposition that leads to modified thermal, optical, and electrical properties. Erosion and deposition processes inside thrusters may modify lifetime potential of the device, and facility effects can create a difference between ground test and in-space operation and lifetime. Ion engine grids are typically fabricated from molybdenum.