Kinetic and structural aspects of trapping and recycling of hydrogen on metallic surfaces exposed to low-pressure hydrogen plasma

Studies of hydrogen trapping and recycling were performed on cleaned and on contaminated metallic surfaces exposed to a hydrogen discharge with parameters typical of the scrape-off layer in devices for controlled nuclear fusion research. Detailed analysis of the time- and temperature- dependent trapping and release of hydrogen upon start and termination of the discharge, respectively, allows one to draw conclusions regarding the nature of the trapping sites. These data were complemented by transmission electron microscopic investigation of the structural damage in the near-surface region and by the measurement of the amount of hydrogen trapped in the damaged area. Formation of dislocation loops at small doses and their agglomeration into an extended dislocation network at a fluence of ≤1017 cm-2 were found. The data indicate that the defects are located within a depth significantly larger than the projected range of the impinging ions. The concentration of hydrogen within this area reaches up to several tens of at . %, and its stability with respect to thermal release suggests that hydrogen is strongly trapped at the defect sites. Only a small fraction (10%--30%) of the implanted hydrogen takes part in the recycling during a 10--30 s long discharge pulse at a wall temperature of ≤300 0C. Implications of these results for the current models of hydrogen recycling are discussed