Operation of a low-voltage high-transconductance field emitter array TWT

Summary form only given. L-3 Communications continues development and testing of a moderate power helix TWT incorporating a new low-voltage high-transconductance field emitter array cold cathode developed by SRI International. The TWT employs a custom electron gun required to control the high beam space charge density created in the acceleration region between the HV cathode surface and the grounded input TWT helix. A custom FEA mounting header was also developed to correctly position the emitter within the electron gun and to allow for electrical connection to the base and gate. The TWT itself uses periodic permanent magnet focusing employing a single sever and a high power collector. The collector was redesigned to maximize pumping speed and improve overall TWT vacuum. The program goals are 100 mA operation at 100 W and 5 GHz. Additional goals include evaluating emitter operation in a TWT vacuum environment, determining lifetime issues and limitations, and evaluating the impact on FEA-generated beams on beam focus, circuit interaction, and RF noise characteristics. The Spindt-type field emitter arrays employed in this program demonstrate significantly improved emission characteristics over those used in previous FEA-TWT experiments using SRI cathodes. These cathodes are comprised of 50,000 Mo tips in a 1 mm diameter area with Fowler Nordheim coefficients of a_FN = 0.01 A/V² and b_FN = 470 V. Emission characteristics for currents up to 50 mA and cathode current densities up to 6.3 A/cm² have been measured and show a 63% reduction in operating voltage and a 9X increase in transconductance, dl/dV, from the previous generation of SRI emitters. Program development details and FEA TWT RF data for beam currents up to 50 mA including RF power, efficiency, small signal gain, noise power density, and noise figure will be presented.