Cleaning and conditioning of the walls of plasma devices by glow discharges in hydrogen

Abstract

Surface conditioning by rf-assisted dc glow (RG) discharges has been studied in a stainless steel vessel in order to optimize the rate of surface deoxidization via water formation. This is the simplest way to decontaminate the almost carbon-free surfaces of plasma devices which have been exposed to air after a prolonged operation in hydrogen. This rate depends strongly on the wall temperature TW, the pump speed Sp, the discharge current IRG, and the hydrogen pressure P2. It is small at ambient temperature; the conditioning should be made at TW>100 °C. Surface oxygen is removed, even at room temperature, when 1% to 2% of methane is added to the hydrogen. Carbon monoxide is formed and evacuated. The addition of He or Ne does not accelerate the oxygen removal from reoxidized surfaces. An equation derived from a simplified model describes the parametric dependences of the water release well. The occurrence of arcs during RG discharges depends on wall conditioning; they are not observed after a prolonged bakeout at 200 °C. The problems which arise when a residual gas analyzer is used to measure the partial pressures of water, carbon oxides, and hydrocarbons in hydrogen are addressed, and a conditioning technique which reduces the resulting errors is described.