Study of low-z coatings for JET under exposure to electrons, laser radiation and atomic hydrogen

In an attempt to lower radiation losses due to metal plasma impurities in the JET fusion device, the use of low-Z wall coatings has been proposed (Si, TiC, SiC, TiO2, Al2O3 and MgAl2O4 on Inconel 600). This report presents experimental results obtained by exposing these samples to electron, laser radiation and atomic hydrogen impact. The studies performed include measurements of (i) gases evolved due to low energy (300 eV) electron bombardment, (ii) inherent gas content in the near-surface region, and (iii) retained deuterium subsequent to exposure sub-eV D0. Also described is the Laser Release Analysis technique which has been developed to enable us to perform the last two sets of measurements. Electron impact desorption rates for hydrogen and methane due to electron bombardment span the range 10-1 to 10-3 H2/e- and 10-2 to <10-4 CH4/e-. Following normal system bakeout at 500 K for 24h, the major species released by laser heating were found to be H2 and CO, with levels up to ∼7x1016 H/cm2 and ∼4x1016 CO/cm2. A similar concentration of argon was found for the TiC coating produced by sputter ion plating. Further heating of the samples to 800-900K for 1h resulted in a reduction of hydrogen and CO release levels by about an order of magnitude. Subsequent to the 800-900 K heating procedure, the samples were exposed to sub-eV D0 atoms to fluences of ∼2x1019 D0/cm2, and deuterium retention levels were measured to be of the order of 1014 - 1016 D/cm2 for the various coatings. Implications of these results for JET's first-wall tritium inventory are discussed. 23 refs