THERMODESORPTION OF DEUTERIUM FROM A POROUS GRAPHITIC CARBON

Thermodesorption of hydrogen isotopes from graphite is of interest for current thermonuclear fusion experiments since the plasma properties are strongly affected by the recycling of gases between the plasma and the graphite tiles used to protect the limiters and the wall of the tokamak. Graphitic Matrix A3-3 specimens were exposed at 900-degrees-C to deuterium partial pressures of 7.6 or 74 Pa until the adsorption equilibrium was established. Subsequently, the thermodesorption of deuterium was measured at temperatures up to 1700-degrees-C. D2 desorption commenced at congruent-to 900-degrees-C. The release spectrum consists of a broad peak with a weak shoulder near 1180-degrees-C for an initial surface concentration of n1(t = 0) = 2.2 x 10(14) D/cm2 and a shoulder near 1120-degrees-C for n1(t = 0) = 6.6 x 10(14) D/cm2. The maximum of the release rate is observed at T(p) = 1500-degrees-C. The results are explained by assuming associative desorption of deuterium with an activation energy of desorption E(d) = 3.8 eV/D2 around 1150-degrees-C and release of a large fraction of the adsorbed deuterium from pores by a short range diffusion process at higher temperatures.