# Erosion Rate Measurements for DART Spacecraft Ion Propulsion System +
## 4 Discussion
Considering all these factors together, the 8\(\times\) erosion rate reduction for DTL30 MOD relative to DETL2.7B-H-O seems understandable. The measured behavior of NEXT with nominal and reduced grid gaps indicated constant Ia over a wide total voltage range for the reduced gap but steadily declining Ia with increasing voltage over a similar range for the nominal gap [32]. DETL2.7B-H-O is close to the perveance limit, defined as the beam or total voltage for which \(\Delta\)Ia/\(\Delta\)V = \(-\)0.02 mA/V. This occurs around the knee of the Ia vs. \(V_{b}\) plot, and the limit is always preceded by nonzero slope over a significant range of voltage due to beam ion direct impingement. Both primary beamlet ions and high energy CEX and scattered ions can contribute to the barrel erosion, and behavioral similarities with respect to change of beam voltage are expected. We conclude that ion impingement at energies much higher than characteristic of accel voltage contributes to the LIF signal over at least a portion of the grid area and can be greatly reduced by raising the beam voltage. Theoretical simulations provide significant support for the hypothesis we have outlined. In one instance, similar average erosion rates were found for accel barrel and downstream face, early in grid life [20]. In a second study [16] (see Figures 10 and 11), CEX ions for each termination surface--barrel, downstream, and upstream faces, etc.--areshown according to their region of formation for several total voltages. Here it is apparent that the lowest beam current case has much greater barrel impingement by high energy CEX ions. Possible explanations for the radial velocity bias of sputtered atoms include preferential carbon deposition masking of accel grid erosion on one side of the beamlet aperture, beamlet asymmetry, and off-normal impingement bias of backstreaming CEX ions on the downstream accel face. Inspection of the apertures revealed substantial, progressively asymmetric wear from grid center to edge, and significant carbon deposition. High back-flux of carbon atoms is a feature of the test facility, due to internal cryopump surfaces that see direct ion impingement. Back-sputter rate estimated via a witness sample measurement was approximately 2.3 \(\upmu\)m/kh for DTL28-H-H. Carbon masking on one side of the barrels, due to differing downstream view factors, potentially leads to a peak shift for the Doppler profile, growing progressively with radial distance as carbon deposition increases. This was illustrated by Figure 11, but quantification of carbon deposition was not performed, and its distribution is likely complex. The peak position moves mainly by elimination of a portion of the Doppler profile, rather than an overall shift. Evidence is mostly lacking that ions impinging on the downstream accel face with the necessary radial direction bias, although we do observe radial bias at the grid edge, as illustrated by Figure 17. However, this does not appear to be consistent with regular progression of the Doppler profile peak position. Instead, it is a phenomenon associated with the final few apertures at the edge, presumably due to beamlet space charge, which becomes radially unbalanced near the perimeter. The erosion pattern superimposed on areas of net carbon deposition suggests that downstream CEX ions acquire radial velocity of the same order of magnitude as axial and therefore impinge radially outward from apertures along the perimeter. Their motion does not always follow a straight line, as indicated by arrows in Figure 17, because the hole pattern is not azimuthally symmetric. The long tail observed in the Doppler profile at grid edge, which degrades the quality of Gaussian fits, likely results from this radial bias and strongly off-normal AOI of the charge exchange ions. The aperture (barrel) wear trend is progressively more asymmetric with the radial coordinate, which may be driven by radial potential gradient, aperture misalignment, nonuniform carbon deposition in the barrel, or other factors.