Enhanced Hydrogen Pumping with Sputter-Ion Pumps

Abstract

The physical processes which give rise to enhanced hydrogen pumping and anomalous differences between steady-state and dynamic pumping speeds are investigated. Observations suggest that two factors of dominant importance in the pumping of hydrogen are cathode surface conditions, which affect the rate at which incident molecular hydrogen is adsorbed, and permeability of the bulk material to hydrogen diffusion. Surface conditions are strongly affected by oxide films and layers of adsorbed gas, while permeability is influenced by temperature, crystal sizes and orientations, impurities and defects, and stresses in the metal. Results also show that a dramatic enhancement of pumping speed is observed after the pump cathodes are cleaned in vacuum by sputtering with argon ions. Thus, the initial value of the residual pumping speed suggests that with sufficiently clean cathode surfaces most of the hydrogen pumping can occur by direct molecular contact. (N.W.R.)