Impurity Deposition on a First Mirror Surface during Hydrogen Discharges in LHD

First mirrors in fusion devices have to retain the reflectivity and the degradation mechanism and dominant plasma operations that affect the reflectivity have to be understood. The reflectivity of visible laser light (635 nm) of a corner cube mirror is almost entirely determined by the initial hydrogen glow discharges for wall conditioning before conducting a main plasma experiment in the Large Helical Devices (LHD). The hydrogen glow discharge forms a carbon deposition layer on the mirror surface, which degrades its reflectivity. A neon glow discharge and subsequent main discharges had less effect.