Dynamics and stabilization of the Elettra storage-ring free-electron laser

The ultimate performance of a storage-ring free-electron laser in terms of light stability and extracted power depends on the possibility of simultaneously controlling the electron-beam and laser dynamics. As a preliminary requirement, the level of longitudinal and transverse electron-beam stability must be high enough to guarantee the laser start-up and growth. This is usually obtained by means of dedicated feedback systems. Once such a requirement is satisfied, the possibility of establishing and maintaining a continuous-wave operation mode finally resides in a deep understanding of the strongly coupled laser-electrons dynamics. For this purpose, we have developed a simple theoretical model which has been proved to be able to provide insight into the evolution of the laser intensity. In this framework, we have also proposed the possibility of utilizing a derivative closed-loop feedback to create or enlarge the region of stable signal. A feedback of this type has been implemented on the Elettra storage-ring free-electron laser. The obtained results, which fully confirm our predictions, are discussed in this paper.