Application of field emitter arrays to microwave power amplifiers

Abstract

Field emitter arrays have potential for use as an electron source in a wide variety of applications including microwave power amplifiers, flat panel displays, electron microscopy, electron beam lithography, and space propulsion systems. Use of an FEA as a cold cathode source for a microwave power amplifier such as the traveling wave tube (TWT), presents significant technical challenges when one considers the high current and high current density requirements coupled with the necessity for excellent beam control. With the recent advances in FEA technology, FEA current densities as well as total array currents have reached values meeting or exceeding many moderate power TWT beam requirements and thus have become an attractive alternative to the thermionic emitters currently used almost exclusively in such devices. One of the most attractive characteristics of FEA emitters is that, by the nature of their emission process, modulation of the electron beam at rf frequencies becomes possible. Interesting and previously unattainable performance appears within reach with the advent of emission gating. Not only does modulated operation offer exciting performance enhancements, many advantages exist for unmodulated operation as well, an aspect of FEA cathodes sometimes overlooked. This work describes the operation of a field emitter array as the electron source of a traveling wave tube amplifier. Issues of beam control and focus at high current density and low magnetic field are addressed as well as issues relating to the inherent high emittance of the FEA beam and cathode protection from ion bombardment.