Mirrors issues for FELs

Most of the FELs operating in the world are optical resonator-based devices. According to the covered spectral range and the purposes of the FEL, the mirrors composing the optical cavity must gather different major characteristics, some of them revealing to be at the edge of the most advanced technology. Thus, for Storage Ring driven FELs, in small signal small gain regime, minimization of the losses is searched in the UV–VUV range. On the other hand, High Power Linac-based FELs are submitted to the laser thermal load, calling for an accurate modeling and mirror design and control for preventing considerable losses on the extracted power and the laser instabilities. Some of these aspects are here reviewed.