Basic and applied studies of field emission at microwave frequencies

Experimental field-emission microwave devices ("femitrons") are described which locate a cold field-emission cathode in the gap of a cavity resonator to achieve direct emission bunching of the electron beam. These devices exploit the high emission density and strong nonlinearity of field cathodes to produce beams with high current density, strong bunching and high harmonic content. An analysis of beam bunching and harmonic content is presented, based on certain assumptions and particularly on the validity of the field-emission law at microwave frequencies. Several independent experiments are discussed which verify these assumptions and confirm the predicted bunching and harmonic content, thus establishing the feasibility of developing femitrons into high-efficiency amplifiers or oscillators, or for the performance of nonlinear operations such as frequency mixing, harmonic generation or rectification of microwave power. Recent developments and current performance of compact multiple-needle field cathodes with relatively high peak current and perveance capability are presented. Based on these, and on preliminary tests at intermediate power levels of microwave devices using such cathodes, a brief evaluation is made of the probable performance and possible applications of femitron devices.