# Erosion Rate Measurements for DART Spacecraft Ion Propulsion System +
## 5 Conclusions  
The present study is the first to systematically vary flow conditions and other parameters while measuring real-time ion engine erosion rate across the grid face, with analysis of these results in the context of the overall erosion process. Erosion rate was found to be well behaved over the NEXT throttling envelope for DART, with no significant lifetime concerns. Results indicated pronounced rolloff of erosion rate from grid center to edge and strong variability at grid center. Four main sources of grid erosion were elucidated for DART operating points: (a) downstream CEX from background; (b) downstream CEX from source flow; (c) intergrid CEX from source flow; and (d) direct impingement. Additional contributors are ion-neutral intergrid scattering and ions from the neutralizer cathode. Evidence was found for elevated energy charge exchange ion impingement and beam ion direct impingement on accelerator barrels in the central grid region at low beam voltage. This increases as beam voltage is lowered at a fixed beam current and can dominate the local erosion process if the beamlet current is near the perveance limit. Raising the beam voltage modifies the intergrid ion trajectories and can substantially reduce beam ion and intergrid CEX collection current for beamlets that were near the extraction limit, as these ions are more effective agents of grid erosion than CEX ions formed in the downstream region. These ions typically impinge on the barrels at energies and angles of incidence that are more effective in generating sputter efflux per ion. Source gas in the intergrid region may collectively produce more sputtered atom density at grid center than downstream gas, despite much shorter collection length, due to elevated xenon density, more favorable angle of incidence, and higher average impingement energy. Variation of the relative magnitudes of these contributions underlies the molybdenum density profiles found across the grid face for DART throttle points. Facility effects from background gas and carbon deposition on the grids were further elucidated. Background gas increases the apparent thruster erosion rate, whereas carbon deposition reduces it. To effectively model observed grid current levels and their dependence on propellant utilization and background pressure, it was necessary to increase the ratio of facility background to source flow gas density. This increase can make sense if background gas has directional bias and elevated average velocity. Hall thruster work has previously suggested a similar phenomenon, where the apparent ingestion of background gas was several times higher than predicted from gas density measurements at the test chamber wall and the assumed thermal velocity distribution. Average neutralization length was estimated based on the data modeling, helping to pin down this historically elusive figure. Radial velocity bias of the sputtered molybdenum atoms was observed, increasing with the radial coordinate. Potential causes of this bias were discussed, but the ultimate origin is not certain. Implications with respect to site of origin and angular dependence of far-field flux were discussed. Barrel erosion rate is typically higher for new grids, slowing down as the geometry changes. For situations where the spacecraft is highly sensitive to molybdenum deposition, the initial rate and deposit depth could be reduced by using grids with smaller gap, long break-in period, or otherwise modified geometry. This must be balanced against any negative performance effects. Near-field molybdenum density and velocity measurements were calibrated with a single-point QCM flux measurement at 1-m distance, using an analytical model developed for this purpose. We then computed mob efflux rate on a 1-m arc from thrust axis, where the ion beam is far too intense for QCM measurements, to 36 degrees inside the backflow region.