Hydrogen trapping and re-emission in TiB 2 coatings for tokamaks upon thermal, pulsed electron and laser annealing

Abstract

The trapping and retention of hydrogen ( 1H) implanted into thin coatings of TiB 2 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/cm 2) 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 TiB 2 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/cm 2 pulse from a 1 μm wavelength laser causes a 40% H loss while an 0.8 sec, 2 kW/cm 2 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.