Experimental Evaluation of Sub-scale Cbio Ion Optics Systems

An experimental study of a sub-scale carbon-based ion optics (CBIO) design proposed by the Jet Propulsion Laboratory (JPL) is described. Perveance and crossover beamlet current limits, above and below which accelerator grid direct impingement currents rise, are documented over wide throttling ranges. Test data reported for assemblies with 1, 7, 19, and 37 aperture pairs suggest that gridlets containing only a small fraction of the total number of apertures in a full-sized optics system can be used to investigate the behavior of larger systems. A study of the effects of grid spacing on beamlet current-limit data that was performed using different types of mica spacers is also presented. Comparisons to numerical simulations suggest that the mica spacer that contained organic binder material changed dimensions slightly during testing. The second type of iso-mica, which is intended for high temperature service, was much more stable and better suited to the hotter test conditions that occur during operation at high specific impulse. In addition, results are presented that examine the onset of the electron backstreaming condition that can develop at the end of life (EOL) in some mission scenarios. Backstreaming voltage limits were measured for grid spacings that simulate their transient behavior and for accel holes with progressively larger accel diameters that simulate the effects of accel barrel erosion over life.