Application of optical reflectivity measurements to diagnostics for plasma facing materials

Abstract

Optical reflectivity measurement is proposed as a convenient diagnostics method of surface modifications in plasma facing materials (PFMs) and its applicability to a plasma confinement device is evaluated. The optical reflectivity for mirror polished stainless steel (SUS316L) exposed to the Large Helical Device (LHD) plasma was examined with a spectroscopic ellipsometer. In-situ measurement of the reflectivity change under the glow discharge was also performed using a super continuum white laser. The detectable change of the reflectivity was observed and the level of the degradation depended on the location, exposure period and employing wavelength. Microstructure observation of the mirror surface revealed that the behavior of the reflectivity change correlated closely with the ion-induced damages and the depositions thickness in the vicinity of the sample surface. Thus, the optical reflectivity measurement is considered to be a possible method for convenient in-situ diagnostics for PFMs.