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

Dielectric multilayer mirrors, degraded through irradiation by high-en ergy undulator radiation, were successfully restored by surface treatm ent with RF-induced O-2 plasma. The mirror loss, which had been increa sed up to similar to 1000 parts in 10(6) (ppm) through the mirror degr adation, was drastically reduced to similar to 100 ppm during the trea tment. Such a mirror-restoration technique has been desired especially in short-wavelength free-electron lasers (FEL's), because the laser g ain is so small that even a mirror loss as small as similar to 1000 pp m interferes with the FEL oscillation. The mirror degradation is most likely caused by the deposition and doping of carbon atoms onto the di electric surface. The surface analysis by the x-ray photoelectron spec troscopy revealed that the plasma treatment effectively removed the ca rbon contamination covering the mirror surfaces without serious surfac e damage by high-energy particles from the plasma.