Stimulated Cherenkov Effect

Abstract

We have clarified in Sect. 4.7 that a relativistic electron beam can support waves carrying negative energy. We can make negative-energy waves interact with positive-energy waves to generate or amplify high-power electromagnetic waves of short wavelengths. A typical example of generators or amplifiers based on this principle is a traveling wave tube in the microwave region. In the traveling wave tube, a slow space-charge wave propagated along an electron beam (negative-energy wave) is made to be coupled with an electromagnetic wave propagated along a helical conductor (positive-energy wave) to generate or amplify electromagnetic waves in the microwave frequency range. As described in Chap. 4, the negative-energy wave in an electron beam is propagated with a velocity nearly equal to its drift velocity. Hence, in order to get efficient coupling between the electron beam and the electromagnetic wave, we must reduce the phase velocity of the electromagnetic wave so that it becomes approximately equal to the drift velocity of the electron beam. Wave-guiding structures to decrease the phase velocity of the electromagnetic wave are called slow-wave structures. In the traveling wave tube, a helical conductor serves as a slow wave structure. Wave-guides with periodic structures or dielectric-loaded waveguides are other examples of slow wave structures.