The surface chemistry of stainless steel and evaporated titanium layers in tokamaks

Abstract

The combined use of X-ray photoelectron spectroscopy and secondary ion mass spectroscopy is shown to be a powerful tool in the investigation of the chemical modifications of a surface in interaction with a plasma. The reported investigations are performed in-situ in the surface analysis station of the PDX tokamak. The evolution of stainless steel surfaces is followed after various treatments such as hydrogen glow discharge conditioning (GDC), exposure to tokamak discharges, and ion sputtering. Iron oxides are shown to be partially reduced by exposure to GDC: the metal-oxide binding is converted to metal-hydroxide binding from which water can be desorbed. The oxygen behaviour on evaporated titanium films is substantially different: the oxide layer evolves to higher oxidation states during exposure to GDC. In contrast, the behaviour of carbon is identical on both stainless steel and titanium surfaces: carbon is removed during exposure to GDC via the formation of methane which can spontaneously desorb from the surface.