Graphite as a plasma-facing material in fusion experiments

The four major processes involved in hydrogen retention in graphite are reviewed. These processes are saturation of the near-surface region, diffusion on surface-connected porosity, intragranular diffusion, and erosion of carbon with concomitant formation of codeposited hydrogen-carbon films. It is suggested that in the retention of tritium in a fusion experiment the fourth process will dominate; therefore, from that point of view, it is the most important. To address the question of plasma fuel purity, more data are needed on ion-induced release of hydrogen isotopes by hydrogen isotopes and by heavier ions (e.g. carbon and oxygen) as a function of temperature