# Erosion Rate Measurements for DART Spacecraft Ion Propulsion System +
## 4 Discussion
It extrapolated approximately to zero as \(\eta\) approached unity, whereas LIF signal was completely lost with much more modest Ia percentage change. Here it is important to remember that only relative density was a local measurement, whereas Ia and \((1-\eta)/\eta\) are \"global\" parameters averaged over the entire grid area. Beamlet current is higher near grid center compared to grid edge, and the same may be true for source density, yet the periphery is weighted more strongly due to area factor. NEXT was designed for improved beam flatness [28], but the variation that exists still limits the beam extraction capability. The neutral density profile is unknown, and although the design was intended to avoid the severe neutral depletion observed with NSTAR [29], the dual peaks in the Xe\({}^{2+}\) beam profile [11] suggest a double dip neutral profile. Background gas density, in contrast, is nearly constant over the grid face, so dependence of only density on source flow compared to background gas is enhanced at grid center. The rapid variation of grid-center only density with Ia is consistent with greater Ia change at grid center compared to grid average. A similar statement applies for source flow xenon neutral density as reflected by \((1-\eta)/\eta\), although the contribution of background gas reduces the percentage change for overall xenon neutral density as \(\eta\) is varied. In any case, if 7 mA of accel current remains after excluding the source density, and 5 mA of that is related to background gas, additional factors may contribute on the order of 2 mA (see Table 4). One of these potential sources is the ion beam neutralizer [30], another is direct impingement by primary beamlet ions. Dependence of QCM deposition rate on background pressure, as plotted in Figure 8b, indicates that background pressure is the dominant factor producing high angle only flux in the far field. The inherent QCM view factor to various grid elements and radial bias of only flux from those elements enhances the deposition rate profile at high angles from the thrust axis. Plots in Figure 12 suggest that the relative importance of source to background neutral density decreases from grid center to edge, helping to ensure the deposition rate registered by the QCM is dominated by background gas. A similar pressure dependence study was not performed for LIF signal across the grid, but two-point measurements at both grid center and edge found DTL28-H-H moly density increased quickly with background pressure. However, here the situation is clearly complicated with multiple factors contributing to the observed erosion. Figure 7 data show that molybdenum density is strongly influenced by accel collection of CEX ions with kinetic energy that tracks \(V_{a}\) magnitude. Moly density change from DTL30 MOD to DTL28-H-H throttle condition was particularly dramatic, as documented in Figure 6. The only difference is \(V_{b}=1021\) V for the latter condition and 1396 V for the former; background pressure and utilization nominally did not change. The value of Ia increased from 8.85 to 9.4 mA during the transition to lower voltage. For comparison, Ia increased from 8.7 to 9.4 mA for the transition DTL28-O-O to DTL28-H-H, with 35% moly density enhancement, and the density ratio for the DTL30 MOD to DTL28-H-H transition was 5.3. These numbers appear to require that ions associated with the accel current increase for DTL30 MOD to DTL28-H-H produce probe region only much more efficiently than those associated with the DTL28-O-O to DTL28-H-H increase. The efficiency ratio could be estimated as \((0.7\) mA\(/0.55\) mA\()\)\((430\%/35\%)=15.6\), except that we expect the accel current increase when beam voltage is lowered to go preferentially to grid center and other regions with high beamlet current. Utilization, like beamlet current, is not uniform over the grid area, and source neutral density across the grid face is unknown, as it has not been measured during thruster operation. It is reasonable to assume the