# Free Electron Laser

The Free Electron Laser (FEL) is a laser source obtained by the oscillation of relativistic electrons inside a periodical magnetic structure (called Undulator). The name, free electron, came from the fact that the laser medium has no bounded states like usual gas or solid-state lasers. The peculiar characteristics of this source is therefore its tunability in a very large bandwidth, as the wavelength produced is depending on the electron energy (trough the relativistic $\gamma$ factor), the Undulator magnetic period $\lambda_u$ and the magnetic strength parameter $K_u$ as $\large{\lambda_{FEL} = \frac{\lambda_u}{2 \gamma^2} \left(1+K_u^2/2\right)}$.

The SPARC_LAB FEL is a Single pass FEL that can be operated in both SASE and Seeded configuration. It is the result of a collaboration between INFN and the Free Electron Laser research group of the ENEA R.C. Frascati. In this test facility different operational schemes and configurations have been tested, spacing from pure SASE to HGHG, seeding with High Harmonics in Gas, two-color configuration in SASE and Seeded mode by using a comb electron beam.

Initially designed to produce radiation mainly in the visible region of the electromagnetic spectrum, in the facts our FEL is capable, in different configurations, to provide radiation between 800 nm and 40 nm with very short radiation pulse duration down to few tens of femtoseconds.