A new model has been developed for hydrogen retention and trapping in and release from graphite. Two different regions in the graphite with different hydrogen transport and trapping behaviors are distinguished, the bulk region within, and the surface region on graphite crystallites. The model incorporates new experimental results related to atom diffusion and recombination on inner surfaces. Recombination is explained from a fundamental viewpoint by linking it to diffusion using a classical expression. The model is applied to a number of reemission and thermal desorption experiments, in particular, the reemission of hydrogen atoms during irradiation with energetic hydrogen ions and the formation of HD during irradiation with H+ and D+ or during thermal desorption of graphite that was preimplanted with H+ and D+ ions with different energies.

Chemical sputtering measurements in Tore Supra by aftershot mass spectrometry outgassing studies J. Vac. Sci. Technol. A 15, 2597 (1997 Fabrication of cross-linked polymer shells for inertial confinement fusion experiments J. Vac. Sci. Technol. A 15, 683 (1997) Tritium permeation and inventory in an international thermonuclear experimental reactor divertor J. Vac. Sci. Technol. A 15, 169 (1997) Measurements of tritium retention and removal on the Tokamak Fusion Test ReactorAmorphous C:D ͑or C:H͒ films were deposited on the inner surface of a spherical ultrahigh vacuum chamber by means of a direct current glow discharge in a ϳ75% D 2 ͑or H 2 ͒ and ϳ25% CD 4 ͑CH 4 ͒ gas mixture. Laser Raman spectroscopy analysis of the film exhibited the absence of a diamond peak at 1334.3 cm Ϫ1 . The general appearance of the spectra was indicative of a polymerlike film. Upon exposure of the C:D film to 18 O 2 , almost all of the released deuterium was seen to be in the D 2 18O chemical form, with less than 1% released as D 2 . C 18 O 2 was observed to be the main C-containing reaction product of the 18 O 2 exposure. No methane release was detected. x-ray photoelectron spectroscopy and secondary ion mass spectroscopy analyses of the films indicated the appearance of carbonyl groups ͑ϾCvO͒ and an increase in the concentration of hydroxyl groups ͑-COD͒ due to surface oxidation. The generally accepted scheme for simple thermal oxidation of hydrocarbon polymers was used to provide a plausible reaction mechanism leading to emissions of the reaction products D 2 O, CO 2 , and CO.

Graphite is extensively being used in present-day fusion devices and has been selected as the reference material for plasma-facing components in the international thermonuclear experimental reactor, ITER. Graphite’s major advantages are associated with its low Z and excellent thermomechanical properties. Its drawbacks include susceptibility to erosion and large capacity for H retention under hydrogen plasma exposure. H-retention affects fuel recycling and also presents a potential environmental risk when operating with tritium, during vacuum vessel maintenance and accidental vacuum loss. The objective of the present study was to investigate the removal of energetically implanted deuterium from pyrolytic graphite via chemical reaction with atmospheric gases (N2, O2, and H2O) for different graphite temperatures. N2 exposure at 300–570 K appeared to be ineffective for the removal of implanted D. Oxygen exposure, on the other hand, was most effective in removing the implanted D from graphite, most likely by the erosion of carbon through O2+C reactions. For example, 10 h O2 or air exposure at 570 K resulted in an order of magnitude reduction in D content. At ∼770 K, three orders of magnitude reductions were observed in less than an hour of O2 or air exposure. While H2O was also effective in reducing the implanted D concentration, the corresponding D-removal rates were observed to be lower than those for the case with O2. Comparisons of D-removal rates for ion-implanted D and chemisorbed thermal D0 atoms on graphite surfaces are also discussed.

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