We have used laser-induced fluorescence (LIF) to monitor the charge-ex
change ion erosion of the molybdenum accelerator electrode in ion thru
sters. This real-time, nonintrusive method was implemented by operatin
g a 30-cm-diam ring-cusp thruster using xenon propellant. With the thr
uster operating at a total power of 5 kW, laser radiation at a wavelen
gth of 390 nm (corresponding to a ground state atomic transition of mo
lybdenum) was directed through the extracted ion beam adjacent to the
downstream surface of the molybdenum accelerator electrode. Molybdenum
atoms, sputtered from this surface as a result of charge-exchange ion
erosion, were excited by the laser radiation. The intensity of the la
ser-induced fluorescence radiation, which is proportional to the sputt
er rate of the molybdenum atoms, was measured and correlated with vari
ations in thruster operating conditions such as accelerator electrode
voltage, accelerator electrode current, and test facility background p
ressure. We also demonstrated that the LIF technique has sufficient se
nsitivity and spatial resolution to evaluate accelerator electrode lif
etime in ground-based test facilities.