Performance Evaluation of 8-cm Diameter Ion Optics Assemblies Fabricated from Carbon-Carbon Composites

Performance data collected on two sets of 8-cm diameter carbon-carbon (C-C) ion optics assemblies are presented in this paper. Each C-C grid was designed and fabricated to grid feature geometries that were identical to the NSTAR ion engine and their performance was characterized by mounting them to an 8-cm diameter Structurally Integrated Thruster (SIT-8). Electron backstreaming and impingement-limited total voltage data were measured over wide beam current ranges for both flat and dished ion optics assemblies. Electron backstreaming performance was observed to be in good agreement with NSTAR data for the flat grids while the dished grids performed slightly better. Impingement-limited total voltage behavior for the flat grids was observed to be similar to an 8-cm ion optics system fabricated from pyrolytic graphite that was reported by Haag and Soulas. The dished grids displayed slightly less total voltage margin due to slightly larger grid-to-grid spacing near the central regions of the assembly. In addition, random vibration tests were successfully performed up to 29 grms, demonstrating the structural integrity of both the flat and dished grid assemblies. Test data are also presented for sub-scale ion optics assemblies (gridlets) that were fabricated from carbon-carbon composites using the same procedures used to fabricate full grid assemblies. The gridlets were used to characterize impingement and backstreaming limits on beamlet current over electric field conditions ranging from 2.3 to 3.4 kV/mm, which correspond to NSTAR and NEXT thruster operating conditions. The test results obtained to date using carbon-carbon composites support the feasibility of directly replacing grid assemblies fabricated from molybdenum and subsequently increasing the propellant throughput of an ion engine. A discussion of the anticipated lifetime enhancement afforded by carbon-based ion optics systems is also presented.