Erosion Measurements by Cavity Ring-Down Spectroscopy for the VHITAL Program

We report the use of cavity ring-down spectroscopy (CRDS) as a diagnostic tool to study sputter erosion of the VHITAL (Very High Isp Thruster with Anode Layer) thruster. The lifetime of the VHITAL thruster is largely governed by sputter erosion of guard-rings and other critical components. The CRDS technique will be used to measure sputtered molybdenum in the plume of the thruster for various thruster operating conditions. Because the sputter rates of interest are relatively low (material erosion rates are ~0.1-10 μm/hour), a sensitive measurement technique such as CRDS is required. Our group has recently demonstrated the use of CRDS for sputter erosion studies. Here, we describe our bench-top sputtering setup consisting of an ion beam and target which we use for diagnostic development. We present CRDS measurements of sputtered molybdenum number density and velocity, including the dependence on number density on beam current, and comparison with a numerical sputter model. We present cavity hardware designs and bench-top testing for CRDS implementation on the VHITAL thruster. Challenges for the thruster implementation include maintaining cavity alignment and stability for the relatively long (≈3m) optical-axis, and minimizing the reduction of effective cavity finesse (and CRDS sensitivity) due to mirror contamination from condensed bismuth or sputtered products. Nomenclature Aki = Einstein A coefficient, 1/s AbsEff = Effective absorbance c = Speed of light, 2.998 x 10 m/s gi = Degeneracy of state i Isp = Specific impulse, s k(ν) = Absorption coefficient, m keff(ν) = Effective absorption coefficient, m L(ν) = Laser lineshape, Hz l = Length of the ring-down cavity, m Ni = Lower state concentration, m R = Mirror reflectivity ROC = Mirror radius-of-curvature, m S(t,ν) = Ring-down signal ν = Laser frequency, Hz νki = Transition frequency, 1/s τ = Ring-down time, s τ0 = Empty cavity ring-down time, s * Assistant Professor, Mechanical Engineering, [email protected]. † Graduate Student, Mechanical Engineering, [email protected]. ‡ Assistant Professor, Mechanical Engineering, [email protected]. The 29 International Electric Propulsion Conference, Princeton University, October 31November 4, 2005 1