ION ENERGY-DISTRIBUTION EFFECTS ON DEGRADATION OF OPTICAL-PROPERTIES OF ION-BOMBARDED COPPER MIRRORS

The cathodic sputtering of copper mirrors in a hydrogen plasma was rea lized by means of alternating negative voltage. The time behavior of v oltage was chosen to provide the broad ion energy spectrum, in qualita tive agreement with the results of measurements of energy spectrum of the charge exchange atoms in fusion devices. The comparison of the deg radation rates of mirror reflectivity due to the long-term sputtering was made in a multi-step way in the wavelength range lambda = 253-650 nn. After several steps of sputtering, the microtopography of the mirr or surface was analyzed by means of a scanning electron microscope. It was shown that sputtering with energy-distributed ions leads to diffe rent degradation rates of mirror reflectivity compared to the case of monoenergetic ions. This difference is surmised to be linked with pecu liarities of defect creation in the near surface layer under condition s of mirror bombardment with variable ion energy in the vicinity of a displacement-induced threshold energy. (C) 1998 Elsevier Science S.A.