Applications of the virtual cathode in relativistic electron beams

Computer simulations have been performed to study the injection of monoenergetic. electron beams with uniform number density into a evacuated cylindrical drift tube. An oscillating virtual cathode is produced by having, the injected current exceed the space-charge limiting current of the tube. The results of oscillation are readily noted downstream from the virtual cathode in two competing processes. The first is the propagation of high power microwaves. They take the form of coherent traveling EM waves in a TN waveguide mode. The microwave power exiting the drift tube was on the order of 10¹⁰ watts. The competing process is the autoacceleration of transmitted electrons to 3.0 times their initial kinetic energy. This acceleration is optimized in the absence of a strong external axial magnetic field. The efficiency, however, is low - on the order of a few percent.