Keeper Wear Mechanisms in the XIPS © 25-cm Neutralizer Cathode Assembly

Abstract

Abstract: The 25-cm Xenon Ion Propulsion System (XIPS © ) thruster has been life tested for over 16,000 hours for communication satellite station keeping applications. The neutralizer cathode assembly (NCA) was observed to experience a significant amount of erosion by the end of the life test. While the NCA competed the test successfully and the life exceeds the requirement for the Boeing 702 satellite orbit-raising and station-keeping mission, erosion of the NCA keeper is a concern for longer duration NASA missions. The performance of a 25-cm neutralizer cathode has been investigated in the JPL cathode test facilities to determine the mechanisms responsible for the observed erosion in the thruster life test. Experiments with fast scanning emissive probes showed that the thruster life test started in the 4.5 kW high power mode with the neutralizer cathode operating normally in the quiescent "spot mode" where low erosion rates are observed. After 2880 hours of operation in the high power mode, the thruster operation was changed to the 2 kW low power station-keeping mode and continued in that mode for remaining 13,370 hours of the test. The emissive probe measurements indicate that the neutralizer cathode started out in the low power mode with significant plasma oscillations in the near cathode region. This behavior is indicative of "plume-mode" operation, which produces energetic ions and is well correlated to high keeper and cathode electrode erosion rates. A reduction in the neutralizer cathode orifice diameter was effective in re-establishing the spot-mode operation and eliminating the oscillations responsible for energetic ion production. Additional wear reduction can be achieved using alternative materials with lower sputtering yields. A wear test is now underway of a modified version of this neutralizer cathode that incorporates the smaller orifice diameter and a replacement of the standard molybdenum keeper material by tantalum. The wear test, combined with JPL's validated neutralizer cathode life models, is intended to show that the erosion rate of the present keeper and of the smaller cathode-plate orifice is insignificant thereby demonstrating sufficient neutralizer life for deep space missions