NON-LINEAR EVOLUTION OF SHORT PULSES IN FEL CASCADED UNDULATORS AND THE FEL HARMONIC CASCADE

Abstract

Free Electron Laser (FEL) devices based on sequence of amplifiers with an harmonic relation between the resonant frequencies of each section, have been proposed to extend to shorter wavelengths the FEL operating range. When this “cascade” of undulators is operated at intensities above saturation, a short optical pulse propagates in a solitary wave-like regime. The pulse is up-converted in frequency at the transitions between the cascade sections, according the resonance central frequency of each undulator. In this paper we briefly review the main dynamical aspects of the FEL cascade operating in this regime and we study the case where the undulators of the cascade are not resonant at the fundamental frequency but have one of the higher order harmonics at a common frequency. In these conditions high harmonic multiplication factors in the final stage may be obtained with a relatively low energy electron beam.