Hydrogen trapping and re-emission in TiB2 coatings for tokamaks upon thermal, pulsed electron and laser annealing

The trapping and retention of hydrogen (1H) implanted into thin coatings of TiB2 on POCO graphite substrates has been investigated by isochronal and isothermal annealing as well as pulsed electron and laser annealing. After room temperature implantation, almost all the H implanted at 60 keV is retained in traps with activation energies ranging from 2.04 to 2.63 eV; whereas, low-energy (250 eV), high-flux (0.5 mA/cm2) H implants result in very small retentions (~0.2%). One-half of the 60 keV room temperature implanted H is released after a 20-minute 400°C anneal. Predamaging the TiB2 by 30 to 140 keV He-implantation produces traps with higher H trapping energies and release temperatures. Depth profile measurements directly show H retrapping at He-produced damage that is nearer the surface than the implanted H, but H trapping at deeper damage or buildup of H at the coating-substrate interface is not observed. Annealing with a 20 ns, 1.5 J/cm2 pulse from a 1 μm wavelength laser causes a 40% H loss while an 0.8 sec, 2 kW/cm2 pulse of 8 keV electrons completely purges the H from the coating. These pulsed annealing results are important because such conditions occur in the use of coatings for limiter and armor applications and because pulsed annealing provides a promising tool for surface conditioning and hydrogen release for tritium inventory control.