The use of laser -induced fluorescence to characterize discharge cathode erosion in a 30 cm ring-cusp ion thruster.

Ion thruster technology has demonstrated the capability to provide exit velocities and efficiencies necessary to propel spacecraft on missions requiring very large changes in velocity including interplanetary missions. The NASA Space Technology Advancement Readiness (NSTAR) 30 cm ion thruster is being scaled to higher and lower powers to meet the needs of demanding missions. However, the discharge cathode, which ionizes the propellant enabling the high exit velocities, has experienced severe erosion in several thruster endurance tests. Without an understanding of the mechanisms leading to this erosion, the technology cannot be scaled with confidence which potentially precludes the near-term realization of these missions. A functional model ion thruster (FMT) was modified at NASA Glenn Research Center (GRC) to permit optical access to the discharge chamber, was operated over the entire NSTAR throttling range, and was loaned with a control system to the Plasmadynamics and Electric Propulsion Laboratory (PEPL) for extended operation and testing. A laser-induced fluorescence capability was developed at PEPL to permit interrogation of the plasma in the vicinity of the discharge cathode assembly (DCA) and of eroded material from the DCA. Xe I, Xe II, W I, and Mo I were interrogated with LIF in the FMT. The relative density of eroded species and the Xe II velocity field were measured as a function of thruster operating condition in both keepered and un-keepered DCA configurations. In both configurations, the erosion of the DCA orifice plates increased linearly with discharge current; erosion was observed at all operating conditions. However, increasing the cathode propellant flow rate and operating with a keepered electrode did significantly decrease the rate of erosion and may enable scaling of the NSTAR technology to much higher powers. All of the DCA erosion observed in the NSTAR ion thruster wear tests was explained in terms of the Xe II velocimetry, which showed that the ions (Xe II and potentially Xe III) were rapidly accelerated away from their regions of creation downstream of the DCA. In the un-keepered configuration, more ions impinged on the DCA because of the proximity of the region of primary ionization to the DCA. The fall through the sheath on the DCA imparted enough energy to the impinging ions to erode the Mo and W surfaces. Thus, this investigation provided an understanding of the erosion mechanisms.Ph.D.Aerospace engineeringApplied SciencesPlasma physicsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132919/2/9991014.pd