Superpermeable membrane for particle control in divertor: the effect of impurity deposition

The effect of impurity (stainless steel (SS) components, carbon) deposition onto niobium membrane surface on the membrane permeability to hydrogen particles is investigated with a plasma device. The deposition of SS components onto the upstream surface of the membrane at the membrane temperature (TM)<800°C decreases plasma driven permeation (PDP) rate but increases molecule gas driven permeation (GDP) rate. In the case of carbon deposition at TM<800°C, PDP rate is reduced while GDP rate remains almost unchanged. It is argued that deposited SS components layer enhances the absorption/release of hydrogen molecules, on the other hand, deposited carbon layer lets hydrogen molecules freely pass through but traps suprathermal hydrogen particles. The impurities deposition effects on PDP rate disappear at TM≥800°C. It appears to be due to the dissolution of the impurities deposited onto the upstream surface into the membrane bulk within the measurements.