Design of a magnetized virtual cathode microwave generator

Microwave generation by electron beams in virtual cathode configurations can achieve significant power levels. However, most designs inherently have two competing mechanisms generating microwaves; namely the oscillating virtual cathode and the reflexing electrons. Further, they tend to interfere destructively with each other. Microwave production by oscillating virtual cathodes suffers from rapidly decreasing efficiency as the energy spread or angular scattering increases. In the configurations considered to date, the incoming electron beam can be adversely affected by the reflexing electrons. Specifically, the reflexing electrons subject the electron beams to two-stream instability, causing considerable heating of the electron beam. Further, the space-charge of the reflexing electrons can cause the diode impedance to fluctuate, resulting in oscillations of the electron beam energy. The authors have followed a novel idea to remove these undesirable effects in the design of a high-power, narrow-band, monochromatic microwave source based on oscillating virtual cathodes. The design uses a thick graphite anode with appropriate slit in combination with an axial magnetic field of suitable strength to essentially eliminate the reflexing electrons. The annular electron beam is guided through the anode slit by the magnetic field, and many of the reflected electrons are absorbed by the anode because of the transverse momentum induced by the self fields. The design has been numerically simulated using the two-dimensional particle-in-the-cell codes, CCUBE and ISIS