Title: Stable Sputter Ion Pump Design Abstract: The cause of unstable leakage currents in diode sputter ion pumps (SIP) was investigated. The discharge current of diode SIP's of various designs was monitored through a testing protocol of alternate gas exposures. Discharge current instabilities were detected after gas exposures. Specific patterns of current instability were found characteristic to the gas exposure sequence and pump design parameters. Post-mortem examination revealed fields of dendritic protrusions on the cathode plates localized upon the highly ion bombarded cathode craters. Dendritic protrusion density was greatest in cathode craters adjacent to either virtual anode cells or standard cylindrical anode cells optimized for maximum current with close anode to cathode spacing. The dendritic protrusions were found to be responsible for field emission based unstable leakage currents. Various high stability diode SIP's were built and characterized. High stability was achieved through the implementation of the optimal combination of anode cell diameter, anode to cathode spacing, operating voltage and magnetic field as was found necessary to minimize the growth rate of dendritic protrusions. An ultra-high stability diode SIP was built and tested incorporating a muffin tin cathode shape in conjunction with the elimination of virtual anode cells. Single Penning cell optimization results are presented. J. B. McGinn FEI Company 7425 NW Evergreen Pkwy Hillsboro OR 97124 jbm(at)feico.com (503) 640-7580.