DEGRADATION AND RESTORATION OF DIELECTRIC-COATED CAVITY MIRRORS IN THE NIJI-IV FEL

In the NIJI-IV free-electron laser (FEL) system, the 6.3-m-long optical klystron generates very intense undulator radiation and its higher harmonics. As a result, the rapid degradation of dielectric multilayer cavity mirrors terminates the laser oscillation in a little while. We reported previously that the mirror degradation was mainly caused by the deposition and/or doping of carbon onto the mirror surface by the undulator radiation in case of the visible FEL and the degraded mirrors were restored to a considerable extent by surface treatment with RF-induced oxygen plasma. However, recent experiments have revealed that the volume degradation inside the dielectric cannot be ignored in the NIJI-IV FEL. Moreover, surface analysis by XPS (X-ray Photoelectron Spectroscopy) indicated that, in the UV FEL around 350 nm, the irradiation of the mirrors by undulator radiation can give rise to the ablation of the top dielectric layer for surface protection. We investigated the relation between the mirror degradation and the condition of both surface and inside of the dielectric for two types of mirrors (HfO2/SiO2 and Ta2O5/SiO2), by using XPS and slow positrons. The degradation mechanism is discussed. The method of restoring the surface- and volume-degraded mirrors are shown.