PLASMA TREATMENT FOR RESTORATION OF DIELECTRIC MULTILAYER MIRRORS IN SHORT-WAVELENGTH FREE-ELECTRON LASERS

Dielectric multilayer mirrors, degraded through irradiation by high-energy undulator radiation, were successfully restored by surface treatment with RF-induced O-2 plasma. The mirror loss, which had been increased up to similar to 1000 parts in 10(6) (ppm) through the mirror degradation, was drastically reduced to similar to 100 ppm during the treatment. Such a mirror-restoration technique has been desired especially in short-wavelength free-electron lasers (FEL's), because the laser gain is so small that even a mirror loss as small as similar to 1000 ppm interferes with the FEL oscillation. The mirror degradation is most likely caused by the deposition and doping of carbon atoms onto the dielectric surface. The surface analysis by the x-ray photoelectron spectroscopy revealed that the plasma treatment effectively removed the carbon contamination covering the mirror surfaces without serious surface damage by high-energy particles from the plasma.