Study of mirror degradation in short-wavelength FELs and recent FEL application experiments on the storage ring NIJI-IV

Cavity-mirror degradation is one of the serious problems especially in short-wavelength storage ring free-electron lasers (FELs). To reduce the degradation problem, we tried to treat the Al2O3/SiO2 mirror, degraded through irradiation by undulator radiation, with both oxygen plasma and thermal annealing. As a result, the optical loss of the degraded mirrors optimized around 200 nm was successfully restored. With such efforts, FEL application experiments are being performed. The recent progress on surface observation using a photoelectron emission microscopy (PEEM) at NIJI-IV is also reported.