Oxygen removal of codeposited a-C:D layers from tokamak tiles

To investigate the thermo-oxidative removal of hydrogen from codeposited layers formed in tokamaks, codeposited specimens from a TFTR bumper limiter tile and from JET and DIII-D divertor tiles have been exposed to O-2 gas at 523 and 623 K. From D-content measurements, derived erosion rates were found to be several micrometers per hour, with faster erosion rates obtained for thicker codeposits. Erosion rates at 523 K could be increased by increasing the O-2 pressure above the nominal pressure of 2.1 kPa; at 21 kPa, the initial erosion rate was similar to 5 times as fast. To study the effect of water on the film erosion and D removal rates, specimens were exposed to pure water vapour. Although water vapour alone was found to decrease the D-content of the films after 'short' exposures, 'long-term' exposures to H2O had no additional effect, consistent with an isotope exchange mechanism. Exposure to air led to similar erosion rates as observed with O-2, for an equivalent O-2 partial pressure, indicating that the other constituents in air had no significant effect on the chemistry. (C) 1999 Elsevier Science B.V. All rights reserved.