Saturation of cavity-mirror degradation in the UV FEL

Behavior of surface and volume degradation in dielectric multilayer mirrors, manufactured with ion beam sputtering (IBS) and electron beam evaporation (EBE), was investigated in the ultraviolet (UV) wavelength range. It was found in the IBS mirrors that the saturation in volume degradation enabled us to use the mirrors for a long period, because the surface degradation, causing only the narrowing of the mirror bandwidth, proved not to contribute seriously to the mirror loss as far as the mirrors are used at their optimized wavelength. This will reduce the cavity-mirror degradation problem in relatively high-gain free-electron laser (FEL) system by carefully selecting the laser wavelength, although the wavelength tunability in FEL is limited to a certain extent. In the EBE mirrors, only the surface-type degradation was found and the evolution of the mirror degradation was different from that in IBS mirrors. The detail on the mirror degradation in IBS and EBE mirrors is shown in the text.