Sapphire surface layer structure and transmission in visible after sputtering in H-=SUB=-2-=/SUB=--N-=SUB=-2-=/SUB=- RF discharge

Abstract

Leucosapphire (c-LS) structure and transmission in visible after surface treatment in 90%H2-10%N2 RF discharge are studied. According to AFM, the number of scratches of the mechanically polished surface decreased significantly after removal of about a 300 nm layer (exposure time of 12 h) under unchanged rms of roughness. According to TEM, a two-layer structure formed in the near-surface region consists of an outer 10 nm amorphous layer followed by a crystalline layer of 40-50 nm with a high defect density. The c-LS transmission in the angle of 400-1000 nm either slightly increased or remained unchanged. The demonstrated transmission stability during exposure in 90%H2-10%N2 RF discharge allows us to consider the plasma sputtering as a promising technique for cleaning contaminated windows protecting first mirror of divertor Thomson scattering being developed for ITER divertor Keywords: Leucosapphire, RF discharge, hydrogen, nitrogen, AFM, TEM, surface layer structure, visible transmission.