*Article* **Hybrid (Oscillator-Amplifier) Free Electron Laser and New Proposals**

**Andrea Doria**

> ENEA, Fusion Physics Division, C.R. Frascati, via E. Fermi 45, I-00044 Frascati (Rome), Italy; andrea.doria@enea.it

> **Abstract:** The present work analyses a hybrid free electron laser (FEL) scheme where the oscillator is based on a radiation source operating with a slow-wave guiding structure as, for instance, a Cerenkov FEL or a Smith–Purcell FEL. Such devices, often running in transverse magnetic (TM) modes, present a longitudinal electric field which can easily affect the longitudinal electrons' velocities, inducing an energy modulation on the beam. Such a modulation, properly controlled, can induce a strong radiation emission in a magnetic undulator properly designed to operate as a radiator. General considerations will be exposed together with a practical numerical example in the far infrared region of the spectrum.

**Keywords:** free electron laser; Oscillator-Amplifier; TeraHertz
