Electric Field Breakdown Characteristics of Carbon-based Ion Optics

Ion thrusters are being designed for long-term operation at power, specific impulse, and thrust levels of up to 29 kW, 7000 seconds, and 0.64 N, respectively. The total propellant throughput goal of these thrusters is nearly 100 kg/kW, which corresponds to ~100 khr of operation. Carbon-based grids have been chosen as the baseline design to achieve this propellant throughput and life time goal. One concern is the long term ability of an ion optics system comprised of advanced carbon-based material to withstand applied voltages in excess of 5 kV and electric fields up to 2.3 kV/mm. In recent testing, anomalous accelerator current behavior has occurred in full-sized, carbon-based ion optics systems. To address these concerns, sub-scale ion optics assemblies (i.e., gridlets) are used herein to evaluate the electric field breakdown and field emission characteristics of carbon-carbon (CC) composite and graphite materials. Arc characterization data have been collected over electric field values from 1.5 kV/mm to 11.4 kV/mm and for grid gaps ranging from 0.5 mm to 2.7 mm. Test results are presented for the mean period between arcing events for various operating voltages and applied electric fields for both beginning of life (BOL) and heavily worn gridlets. Wear of the gridlets was performed in an accelerated fashion that simulates the wear expected when ion optics systems are operated in space. Investigations to date correspond to simulated in-space operation of 3 to 6 years on some surfaces and 24 to 48 years on other surfaces. In addition to statistical data on arcing rates, measurements are presented of field emission and electric field-breakdown strength made with and without ion beamlet extraction. In general, the arcing characteristics of the CC gridlets were found to be very sensitive to the charge transferred in an arc and the number of conditioning arcs that were applied. A specially designed power supply system is discussed that was used to control the total coulomb transfer in an arc event from 0.01 mC to 1 mC. Gridlet test results indicate that electrode wear has a minimal effect on the field emission and voltage standoff capability of the gridlets once the electrode surfaces were conditioned with a series of controlled arcs. Conditioning to ~700 mC (at sub-millicoulomb increments) was found to be adequate for 7.5-cm x 7.5-cm gridlets to completely eliminate anomalous accelerator currents and return the field emission and electric breakdown-field strength to levels measured at BOL. A method to predict the maximum applied breakdown field for arbitrary grid geometry is described and validated that uses measurements of the localized electric field at breakdown and the field enhancement factor. Calculations of the maximum applied electric field using this method were found to be 10-15% of measured values. Properly conditioned NEXIS grids fabricated from carbon-carbon materials are predicted to operate at fields up to 4 kV/mm +15% at nominal spacing.