# Erosion Rate Measurements for DART Spacecraft Ion Propulsion System +
## 2 Materials and Methods  
A tunable ring laser was pumped by a Millenia eV 15 Watt diode-pumped solid-state system, providing multi-line output at 532 nm. The unit is sealed and requires no laser head adjustments in normal operation. Although the pump laser does not operate with single frequency output, its amplitude stability and beam quality are good. The input power level selected for ring laser pumping was approximately 11 Watts. A Thermo Neslab RTE7 chiller provided temperature-controlled cooling water at low flow rate to the system, which is sensitive to temperature variations. A Coherent 899-21 ring laser operated in Ti:Sa configuration generated tunable single frequency output that was locked to a scanning Fabry Perot reference cavity. A mix of short wave and medium wave optical elements installed in the ring cavity enabled usable power output over the 780-860 nm wavelength range without changing any optics. Typical power output at 855 nm was 0.8 Watt. The Ti:Sa crystal was temperature controlled either by a dedicated RTE7 chiller or by sharing a single unit that cooled Millenia diodes and Ti:Sa crystal in parallel. A small portion of the ring laser output was split off to be monitored by a Burleigh WA-1500 wavemeter, with accuracy of about 0.002 wavenumber (specified accuracy of \(+/-0.2\) ppm at the current wavelength) and a 1500 MHz optical spectrum analyzer. An optical telescope expanded the beam prior to admission to an MBD200 external frequency doubler. An LBO crystal optimized for 780 nm operation was used for frequency doubling. Output power at the 390 nm doubled wavelength, used for molybdenum LIF, was approximately 5 mW. Figure 1 shows an overall layout diagram for the laser system. After converting the dye laser fundamental to doubled output, the resulting ultraviolet beam was chopped and sent up a periscope to a Brewster window port on the vacuum facility. The beam was sent through a collimator before passing through the thruster plume approximately 1 cm downstream of grid center, transverse to the thrust axis (see Figure 2). A photodiode near the far wall of the vacuum facility was used to verify the beam alignment or adjust as needed. At each LIF probe location across the 18-cm span from center to edge of the extraction zone, the laser beam remained tangent to the nearest point on the accelerator grid surface. The interrogation point was fixed in space, and the thruster was moved to permit measurements along a linear path from grid center to the perimeter on one side. To accomplish all this, the thruster had single-axis rotation (\(RS\) stage) and two-axis (\(X\) and \(Y\) stages) linear translation degrees of freedom. In Figure 3, the outer dashed circle and vector \(R_{c}\) defines the radius of curvature of the ion engine grids, \(X_{0}\) is the distance between interrogation point and nearest grid surface, \(\theta\) is the rotation angle from interrogation point to grid center, and \(R\) is the difference between \(R_{c}\) and the vector \(R_{a}\), which extends from the \(RS\) stage axis of rotation to grid center. The positioning equations \(RS=\theta\), \(R=R_{c}-R_{a}\), \(Y=Rsin\theta\), and \(X=R(1-cos\theta)+X_{0}\) were used. \(X\) and \(Y\) equal \(X_{0}\) and 0, respectively, when \(\theta=0\). The grid angle displayed in data plots is referenced with respect to the grid radius of curvature. Axisymmetry was assumed for data analysis and interpretation to  
Figure 1: Diagram of tunable laser system used for the LIF measurements. remain within the scope of the study. Small deviations from axisymmetry are known to exist, based on more extensive current probe measurements [11]. Improved results could be obtained by collecting and analyzing data for a series of azimuthal angles. Quartz crystal microbalance (QCM) measurements were obtained separately on the same side of the grid, with a 15 MHz MK10 sun wise sensor made by QCM Research. This detector, its collimation system, typical scanning methodology, and other details have been documented previously [6].