Structural characterization and deuterium ion irradiation effects of tantalum as an absorbing first wall material

Tantalum’s hydrogen absorbing properties make it an interesting first wall candidate for many fusion research devices. It will be first tested in WHAM, a compact high-field mirror with high auxiliary heating power, where cold-sprayed Ta powder is adhered on stainless steel walls of the vacuum chamber. The cold sprayed technique created dense Ta coatings with enhanced hardness compared to bulk Ta. ​​The cold-spray process enhances the hardness compared to bulk Ta and remained adhered to the substrate during multiple annealing cycles up to 1200 K and during D ion bombardment at 95 eV with 3*10²⁵ D/m² fluence. The microstructure of the coatings have been characterized at the surface and in the bulk before and after plasma exposure. Measurements with x-ray diffraction (XRD) revealed expansion of the crystal lattice following ion irradiation at a surface temperature of 523 K as well as an increase in residual stress by a factor of 2. The increase in residual stress after ion irradiation signifies probable D embedding in the lattice. Thermal desorption spectroscopy (TDS) performed on pristine cold sprayed Ta implanted with D ions showed significant outgassing of D, with the most substantial release of D above 900 K.