Retention and Surface Pore Formation in Helium Implanted Tungsten as a Fusion First Wall Material

Single- and polycrystalline tungsten samples were implanted with 30 keV³He ions to fluences of 5e16, 4e17 and 5e18 He/cm² at temperatures ranging from ~850 - 1000 °C. After implantation tungsten's retention characteristics were studied using ³He(d,p)⁴He nuclear reaction analysis (NRA) and ³He(n,p)T neutron depth profiling (NDP). Morphological analyses included scanning electron microscopy (SEM), focused ion beam (FIB) milling, and X-ray diffraction on the single crystalline W samples (XRD).

SEM analysis showed that the threshold forsurface pore formation occurs in both single-crystalline tungsten (SCW) and polycrystalline tungsten (PCW) between ~5e16 - 4e17 He⁺/cm². Both surface and sub-surface pore formation is observed to increase with higher implant fluences. Focused ion beam (FIB) milling revealed a sub-surface porous layer in both SCW and PCW, which increased in depth with implanted fluences. NRA measured the retained He fluence in SCW between 1.1e16 - 1.1e17 He/cm² and in PCW between 1.3e17 - 1.5e17 He/cm². NDP analysis measured the retained He fluence in SCW between 2.0e16 - 2.7e17 He/cm² and in PCW between 4.1e16 - 3.2e17 He/cm². Both of these analysis techniques reveal that the retained helium saturates in both single and polycrystalline W at ~4e17 cm^-2. The NDP analysis showed that the peak helium concentration shifted deeper into the specimens as the dose was increased, indicating a decrease in the effective density of the surface layer with an increased dose. Average retained helium concentrations were found to range from 0.7 - 8.6 at% in SCW and from 1.3 - 11.4 at% in PCW.