Surface conditioning of fusion devices by carbonization: Hydrogen recycling and wall pumping

Abstract

Measurements made in the tokamak Torus Experiment for Technology Oriented Research (TEXTOR) with carbonized graphite limiters and carbonized Inconel liner (‘‘all carbon machine’’) and from a carbonized simulation vessel are presented and discussed with particular emphasis on hydrogen recycling phenomena. The hydrogen recycling during tokamak discharges is strongly affected by the wall temperature. Slow loading or deloading of the wall with hydrogen occurs at low (150 °C) wall temperatures, whereas stationary conditions are reached rapidly at 350 °C. At 350 °C, recycling coefficients smaller than 1 can be achieved. Active density control during tokamak discharges is demonstrated by using the wall pumping effect in combination with shifts of the plasma position. No tendency for saturation of the wall pumping as a function of the discharge number is found when the wall temperature is 350 °C. A model is presented for the explanation of the observed effects. It involves particle induced desorption, thermal outgassing, and implantation of hydrogen. Particle induced desorption dominates at low temperatures and high hydrogen concentrations in the carbon films, thermal outgassing becomes increasingly important at higher temperatures. Hydrocarbon formation and release at wall areas exposed to high hydrogen fluxes and the codeposition of C:H at areas of low fluxes is a possible additional hydrogen sink, which could be effective during the wall pumping phenomenon.