Oxygen adsorption on stainless steel

Oxygen is one of the impurities always present in toroidal devices during plasma discharges. Materials and surface treatment for the first wall of these machines have been studied. Stainless steels have good general characteristics but to reduce impurity release induced by plasma particle bombardment, the walls are normally cleaned and passivated. It would be desirable to eliminate rapidly the particles in passivating layers and in residual impurity components, when the apparatus is under vacuum conditions. There seems to be particular difficulty in eliminating oxygen completely from stainless steel; it is probable that oxygen ions present in the plasma come from the wall and, after the discharge, they are easily readsorbed on the wall due to a great affinity for oxygen of bombarded heated stainless steel. To verify this fact quantitatively, the authors have simulated this wall behaviour by using samples of 304 and 316 L stainless steel in an experiment of etching, excitation and heating of the sample surfaces, bombarding them with noble gas ions in conditions of variable oxygen pressure in the experimental chamber. (Auth.)