Back-gated Diamond Field Tip Array Cathodes for 220 GHz TWT

Abstract

Abstract : High current density field emission (FE) cathodes were studied as a long sought alternative to thermionic cathodes which must operate above 1100 deg C in order to meet the performance required for the HiFive program. A back-gate design with the gate electrode positioned behind the array of FE tips was studied as a way to minimize the chance of arching. The use of diamond as the emitter material provided a robust, low effective work function material coupled with a tip radius of approx. 5nm to provide high field enhancement. Modeling predicted field enhancement as a result of voltage applied to the back-gate to be between 102-103. Diamond tip arrays laid out in the geometry suitable for the HiFive program were fabricated and tested. A total emission current of 600 mA at 6.1A/cm2 was produced matching the best know reported current from a single cathode. The best current density, 15 A/cm2, at a high emission current 15 mA is among the highest reported for FE cathodes in light of achieving simultaneously both high current and current density. Both sets of data were obtained in pulse mode. The modeling predicts the HiFive program goal of 750 A/cm2 can be met with present design.