Optical cavities for UV free electron lasers

Several Storage Ring Free Electron Lasers (SRFELs) now operate in the UV and new facilities in the UV and deep UV are near completion. The intense broadband UV-X ray emission from the undulator of an SRFEL leads to degradation and heating of the optics. When the beam energy gets higher and the undulator longer, this radiation becomes more critical, but the laser power and the beam lifetime also increase under these conditions leading to compromise values and specific choices concerning the cavity mirrors. Various schemes of laser extraction and coherent higher harmonics induced by FEL are considered.