Erosion Rate Diagnostics in Ion Thrusters Using Laser-induced Fluorescence*

fio = atomic transition oscillator strength g = linewidth distribution function, sec We have demonstrated the use of laser-induced flug = energy-averaged Gaunt factor orescence (LIF) to monitor the charge-exchange ion 9i = degeneracy for atomic level i erosion of the molybdenum accelerator electrode in h = Planck's constant, J sec ion thrusters. This real-time, nonintrusive method h = Planck's constant/2r, J-sec was implemented by operating a 30-cm-diameter I = intensity of optical radiation, W/m 2 ring-cusp thruster using xenon propellant. With the JA = accelerator electrode current, A thruster operating at a total power of 5 kW, laser raJb = extracted beam ion current, A diation at a wavelength of 390 nm (corresponding to JE = cathode emission current, A a ground state atomic transition of molybdenum) was k = Boltzmann's constant, J/°K directed through the extracted ion beam adjacent to M = mass of molybdenum atom, kg the downstream surface of the molybdenum accelermass of the electron, kg ator electrode. Molybdenum atoms, sputtered from = Maxwellian electron density in the exthis surface as a result of charge-exchange ion erotracted beam, sion, were excited by the laser radiation. The intenb sity of the laser-induced fluoresence radiation, which P = beam power, W is proportional to the sputter rate of the molybdenum Po = vacuum chamber pressure, Pa atoms, was measured and correlated with variations PT = total thruster power, W in thruster operating conditions such as accelerator Rp = electron-impact excitation rate, m /sec electrode voltage, accelerator electrode current, and R, = atomic stimulated excitation test facility background pressure, rate, m 3 /sec R, = Rydberg energy, eV NOMENCLATURE kTA = temperature of molybdenum atom, eV kTe = electron temperature, eV 4 = Einstein A coefficient between levels i VA = accelerator electrode voltage, V and j, sec-' VB = beam voltage, V ao = Bohr radius, m VD = discharge voltage, V c = speed of light in free space, m/sec Y = sputter yield, atoms/ion e = charge of the electron, C ( = beam-ion-production cost, W/A *This work was supported by NASA under contract number (o = permittivity of free space, F/m NAS 3-25553 A = wavelength of optical radiation, m *Member of the Technical Staff, Optical Physics 7le = electrical efficiency, % Department tMember of the Technical Staff, Plasma Physics Depart(. d)unC = discharge propellant utilization effiment, Member AIAA ciency (uncorrected for doubly charged *Project Manager, Plasma Physics Department, Senior ions), % Member AIAA = energy difference between levelF i t ISenior Staff Physicist, Plasma Physics Department, MemJ ber AIAA and j, eV