STABILIZATION SYSTEM FOR THE OPTICAL CAVITY OF THE DARMSTADT FREE-ELECTRON LASER

The optical cavity of the FEL at the superconducting Darmstadt linear accelerator, S-DALINAC, has been designed in a nearly concentric configuration and has a total length of 15 m. In order to achieve maximum gain and maximum power extraction it was established that the mirror position and tilt have to be kept accurate to 1-mu-m and 30-mu-rad, respectively. To fulfil these requirements, and to reduce deviations of the optimum adjustment caused by vibrations and to prevent these from having an influence on the cavity detuning, a mechanical low pass system for the mirror mounts has been designed and set up. Furthermore, an active length and angular stabilization system has been developed which will ensure that an active feedback loop will keep the FEL cavity stable.