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We consider a free-electron laser (FEL) operating in the high-gain regime including the mechanism of self-induced harmonic generation (SIHG). We discuss the mechanism leading to saturation at these higher harmonics and analyze the effect of beam quality variations. Finally, we study an undulator configuration that allows the saturated harmonic power to become comparable with that of the fundamental.

Experimental observations on FEL Storage Rings (Aco, Super-Aco, VEPP3, TERAS) have shown that in a storage ring with an operating FEL there is a mutual effect between the FEL operational mode and the beam longitudinal distribution. The main effects are the birth of a macro-temporal structure of the FEL radiation and a suppression of the synchrotron sidebands, evidence of beam stabilisation against the microwave instabilities. In this paper we discuss the main features of the beam dynamics analysed with a simulation code recently developed, which includes the FEL-beam interaction. Furtherly, we propose an heuristic model which enable to describe in a simple way the overall system.

In this note we review the essential features of the Cerenkov Free Electron Laser (CFEL) from both the theoretical and experimental point of view. In particular we discuss the CFEL microtron joint ENEA Frascati‐Dartmouth experiment.