EROSION RATE DIAGNOSTICS IN ION THRUSTERS USING LASER-INDUCED FLUORESCENCE

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.