Deuterium trapping by carbon materials for tokamak plasma-facing components

The experiments on large tokamaks on measuring the hydrogen isotope pressure in the near-wall or divertor plasma are briefly described. Based on the analysis of exchange fluxes between the plasma and the wall it is stated that the plasma action, with respect to hydrogen sorption by graphite components of the tokamak first wall, is identical to materials exposure in the atmosphere of molecular hydrogen at pressures of 1-10(3) Pa and temperatures of 1200-1700 K. Within these ranges of pressures and temperatures RG-Ti-91 and USE-15 graphites are shown to be impermeable to molecular hydrogen in contrast to easily permeable POCO-AXF-5Q. Sorption capacity of well-treated dense graphites is about 10(2) appm. Sorption capacity of materials is increased to (1-3) x 10(4) appm with defects like unsaturated carbon bonds, introduced into the above materials by mechanical treatment, ion irradiation, graphite gasification (exposure at 1200 K, 133 Pa during 1 h). At the same time permeability of graphites is increased to molecular hydrogen. Graphite modification by boron retards the rise of sorption capacity. The hydrogen sorption process is described as the filling of two sets of traps, i.e. weak traps arranged over the boundaries of carbon materials and deep traps arranged in the volume of separate crystallises. Russian-made graphites RG-Ti-91 and USE-15 can be used in large tokamaks as divertor and first wall tiles, respectively. (C) 1998 Elsevier Science S.A. All rights reserved.