CHANGE OF OPTICAL PROPERTIES OF BERYLLIUM MIRRORS UNDER DEUTERIUM ION BOMBARDMENT

In this paper the results of experimental investigations of beryllium mirrors behaviour under bombardment by ions of deuterium plasma are presented. In the experiments small Be mirror samples were exposed to D+ ions of fixed energy in the range from 0.05 to 1.35 keV at deuterium pressure of (3-5) x 10(-2) Pa, and step by step the normal incident reflectivity of the samples was measured after every exposure. The observed dependence of the reflectivity on the thickness of the sputtered layer agrees with an assumption that under D+ ion bombardment there occurs the transformation of the BeO surface layer into the layer of hydroxide, Be(OD)2. The mass loss at early stage of ion bombardment, when such a transformation takes place, is negligible, and only after much longer exposures the reflectivity is slowly diminishing due to changing the surface layer morphology. The exposed mirrors heating up to >300degreesC during one hour leads to restoration of their reflectance, probably due to the backward process of BeO layer restoration.