# Erosion Rate Measurements for DART Spacecraft Ion Propulsion System +
## 2 Materials and Methods
Molybdenum atoms were excited from the ground state to an upper state that strongly fluoresces, using a transition at 390 nm. Resonance fluorescence was collected with a 15-cm diameter concave aluminum mirror, and the converging beam was guided vertically upward through a window port and focused into a 0.3-m McPherson 218 monochromator. The detector was a photomultiplier tube (PMT). The PMT output was fed into a lock-in  
Figure 3: Diagram of thruster positioning relative to the fixed laser beam interrogation point, using rotation of the thruster and translation along \(X\) and \(Y\) axes to keep the laser tangent with a constant separation distance for each measurement location. \(R_{a}\) is not shown. Figure 2: Photo of set-up inside the vacuum chamber (**left**), with magnified view of collection optic assembly (**right**). amplifier and demodulated via phase-sensitive detection at the chopping frequency. The laser frequency was scanned to record the Doppler profiles of sputtered molybdenum atoms at various grid locations. The relative density of molybdenum at each location could then be obtained by integrating over the Doppler profile. For one subset of LIF measurements at grid center, the laser frequency was parked at the peak of the Doppler profile. Laser scatter from the grid surface produced background signal that had to be subtracted from the molybdenum LIF signal. This was particularly an issue near the edge of the grid, due to scatter from the grid mask. At times \(X_{0}\) was increased by as much as 1.3-cm to greatly reduce scatter from the mask. When this was done, a modest correction factor was applied to the molybdenum signal. Because of only sputter deposition around the engine, an acrylic sheet was mounted to a rotatable stage and positioned above the collection optic. The acrylic was transmissive at the fluorescence wavelength. The sheet was protected everywhere except above the mirror. By rotating the sheet in 90-degree steps under vacuum, an unexposed region could be placed above the mirror in case transmission of the previously exposed area was degraded. This could be done three times without overlapping a previously exposed surface. The collection optic was below the thruster, displaced approximately 7 cm downstream from the interrogation point to avoid occlusion of the collected fluorescence by the thruster. NEXT was operated in the Aerospace EP2 facility, which is 9.8-m long and cylindrical with a 2.4-m diameter. Up to 10 PHIPK cryopumps of re-entrant and standard 1.2-m designs can be operated simultaneously. The re-entrant pumps provide improved xenon pumping speed performance, up to 35,000 L/s each, with the drawbacks that the chamber internal volume is reduced and carbon backscatter rate is elevated. Data were obtained for various potential DART throttle conditions, with additional measurements on effects of elevating background xenon gas density and varying beam and accel voltage. To elevate background pressure, xenon gas was bled into the vacuum chamber through a sidewall port outlet about 1.8 m from the thruster, at a plume angle of approximately 45 degrees. The principal pressure gauge (Stabil-ion type ionization gauge) for background measurements was 1.3 m away, mounted off the chamber wall approximately 5 cm downstream from the grid apex. It did not have a line-of-sight view to the plasma inside the vacuum chamber. NEXT has three distinct flow rates going through the neutralizer, the discharge cathode (DCA), and the discharge chamber. Direct flow into the discharge chamber (main flow) dominates the total and is the sum of the discharge cathode and the discharge chamber; this is what is plotted in Figure 4. For DART flight operations, the neutralizer flow rate is elevated substantially beyond the minimum needed. Flow rate variations were evaluated to \(\pm\)6% for the cathode (five settings) and the neutralizer (three settings), and to \(\pm\)3% for the main (three settings). Throttle level TL28 is included in the standard NEXT throttle table; TT10 and TT11 are the latest versions [4].