NEXT Ion Engine Plume Deposition Rates: QCM Measurements

摘要

Gridded ion engines sputter grid material into the plume, potentially producing deposits that alter spacecraft surface properties. Flux levels depend on angle with respect to the thrust axis, throttle level, grid material, and facility background pressure. Net mass deposition rates are also a function of ion beam flux, due to simultaneous removal of depositing species. Ion beam flux varies strongly with angle. The first absolute measurements of real-time net mass deposition rate for surfaces exposed to the plume of a NASA NEXT ion engine were performed. The thruster was operated at numerous standard and extended throttle table conditions. A rotatable quartz crystal microbalance (QCM) with collimating system recorded the rate at 1-m distance from NEXT, during angular sweeps between 58 and 90 degrees, in 4 degree increments. A net deposition to net erosion transition was observed for most but not all operating points. Collimated witness plates were positioned at fixed locations and analyzed post-test using XPS. Carbon back-sputter rate was also measured as a function of throttle level. Net deposition measurements were performed for a total of 18 throttle levels. In addition, the effect of adding background xenon was studied for a limited subset. Some exploratory work was also done on the effect of changing accelerator grid voltage, and ion deflection away from the QCM detector. Data trends are discussed in detail, including throttle level and angular dependence, and mass erosion rates for the accelerator grid are estimated based on the results.