Evanescent oscillations in bunching systems

At the initial stage of acceleration, it is often necessary to create an increasing distribution of accelerating electric field for improving the bunching process. Such distributions can be created by gradually decreasing the coupling coefficients between the resonant volumes. In such bunching structures charged particles are bunched and accelerated by the travelling wave with increasing amplitude. But there is another possibility of creating such increasing distributions. It is well known that in periodic structures there are two different in basis electromagnetic eigen-oscillations supported by metal walls without external currents and charges-propagating and evanescent. In forbidden bands (stopbands) electromagnetic oscillations transfer no energy in the direction of periodicity and have decreasing (increasing) dependence on the longitudinal coordinate. Results of our investigations of the properties of the electromagnetic oscillations in the stopbands of the periodic structures are represented. We discuss the usage of such oscillations in accelerating systems for the bunching and accelerating of electron beams