Lasing at 300 nm and below: optical challenges and perspectives

The FEL experiment in the visible and near UV on the Super ACO storage ring has given, since 1989, important informations on the SRFEL dynamics and, furthermore, a very good beam stability has been achieved. This allowed us to perform first application experiments. al challenge is now given by lasing at shorter wavelengths. 1992 the efforts have been concentrating to look for every kind of solution allowing us to overcome the problem of having a very low gain. Much interest is given by the problems concerning the optical cavity geometry and its behaviour under the thermal load of the undulator emission. ning mirrors, in the range of the explored wavelengths there is a lack of transparent dielectric materials for substrates and coatings. In addition, the extreme environmental conditions given by the presence of the broad spectrum and powerful undulator emission reveal a relatively fast degradation of the initial losses of the optical cavity. This work presents some recent results on degradation and losses recovery and, by some new results, gives some openings for explaining degradation.