Periodic Structures and the Coupling of Modes

Abstract

During the operations of traveling-wave amplifiers or particle accelerators, the electron or ion beam is to interact with an electromagnetic wave quite effectively, for this purpose, the charged particles (electrons or ions) need to be kept in phase with a retarding field in the amplifiers’ case or an accelerating field in the accelerators’ case over a long distance, e.g., over a distance of the order of several wavelengths. This means that the velocities of the charged particles need to be roughly equal to the phase velocity of the wave, for the phase velocity is the velocity with which an observer would have to move so as to always be able to remain in the same phase of the wave. Since electrons and ions can be accelerated only to velocities less than the velocity of light, we need to look for electromagnetic structures capable of sustaining waves propagating with phase velocities less than that of a plane wave in free space, i.e., the speed of light. Such waves are called slow waves and the structures capable of having slow waves propagating along them are called slow-wave structures or slow-wave systems.