Influence of Carbonization on Hydrogen Recycling Phenomena

Abstract

This paper describes experimental results on the recycling of hydrogen on the carbonized wall of a vacuum vessel and the interpretation based on Desorbable phase Model for the recycling mechanism, in order to provide a rational view of the wall pumping phenomena observed in tokamaks. The recycling outflux φ_out of hydrogen from the amorphous hydrogenated carbon layers during glow discharge with hydrogen or helium gas in the vessel is determined from the pressure measurement of hydrogen. Even after the steady state of the hydrogen surface concentration in the film is reached, the recycling behavior of hydrogen as desorption or absorption is responsible for the energy of hydrogen ions im-planted into the film. The typical change of hydrogen concentration for increase or decrease of the ion energy by e.g. 50 eV from a fixed value is, reproducibly, about 10¹⁵ atoms/cm² within discharge time of 20 s.