# Erosion Rate Measurements for DART Spacecraft Ion Propulsion System +
## 4 Discussion  
An ion optics set built to the DART standard was unavailable until after these LIF and QCM measurements were performed in late 2017-early 2018, therefore an engineering model grid set was utilized. This grid set had been operated for several hundreds of hours at the time, compared to several thousand for our previous LIF and QCM work [6,7]. The peak deposition rate of the newer grid set was similar, according to QCM results, but their behaviors differed in some other respects. Although NEXT design reduces the effect, barrel wear is still significantly elevated early in grid life [25]. Background neutrals approaching the screen grid from downstream have the advantage of not crossing a region with an intense, ionizing flux of electrons. Therefore, instead of becoming ionized and driven away by the electric field, most passing through an accel aperture will reach the screen grid. If the flux of these neutrals into the grid set is enhanced, as the analysis indicated, and directional, the strong influence of elevated background pressure on erosion rate that is typically observed becomes more understandable. In previous QCM work and for DTL28-H-H in the present study, it was found that the observed deposition rate was approximately proportional to background pressure for most operating points [6]. TL12 was a very notable exception. TL12 has over-focused beamlets at grid edge, resulting in onset of direct impingement there. Elevated background density was found to _reduce_ the deposition rate rather than increasing it. Evidently, the elevated density flattened the curvature of the plasma sheath at the screen grid, moving ion trajectories away from the over-focused condition and thereby reducing direct impingement and erosion rate at the grid edge [6]. In contrast, this effect on trajectories will _increase_ barrel impingement if beamlets are already under-focused, a condition associated with much higher ion throughput than pertains to TL12. Therefore, other operating points--for example DTL28-A-A--may receive a boost in barrel erosion rate from this effect in addition to the higher CEX production during the transition from DTL28-H-H.  
Based on past experimental and theoretical work, it is expected that all CEX ions formed in the region between accel grid and the downstream point at which ion beam neutralization is reached will be drawn upstream and impinge on the accel grid [25, 26, 27]. This distance is the neutralization length, which we equate to \(l_{d}\). impingement occurs on the downstream face and barrel and possibly the upstream face. Variation of beamlet current and beam voltage is predicted by simulations to moderately affect the value of \(l_{d}\)[20]. Theoretical treatments struggled to produce realistic neutralization length estimates, finding values that were too small [26]--much less than 1 cm. Later work was generally consistent with \(l_{d}\approx 2\) cm [25,27], which is close to our estimated value (see Table 3). The distance is nominally determined by the position where beamlets have merged to produce plasma of approximately uniform potential. This location will vary with beamlet current, beam and accel voltages, propellant utilization, background neutral density, and grid geometry; all these parameters influence ion trajectories. Some fraction of CEX ions formed further downstream of the neutralization point may also contribute to accel current. Extrapolation of DTL28 flow settings to 100% propellant mass utilization (zero neutral density from source flow) coincided with about 7 mA residual accel current (Ia). Extrapolation from DTL28 to zero background density coincided with about 5 mA residual Ia. In each case, a linear relationship was found. The accel current contribution was higher for background neutrals compared to source neutrals, on average. Grid-center only density varied quickly with Ia and more slowly with source neutral density (see slopes in Figure 8d).