Formation and retention of surface pores in helium-implanted nano-grain tungsten for fusion reactor first-wall materials and divertor plates

Nano-grain W samples were implanted with 30 keV 3 He ions to fluences of 10 21 , 10 22 and 10 23 He m −2 , at temperatures ranging from approximately 1273 to 1423 K. One specimen was implanted with 30 keV 4 He ions at 1273 K to 10 24 He m −2 . Scanning electron microscopy showed the onset of visible surface pore formation occurred between approximately 10 21 and 10 22 He + m −2 and increased with higher implant fluences, eventually resulting in a 'coral-like' surface structure. Focused ion beam analysis revealed the depth of visible subsurface pores increased with higher doses, extending below the surface to a maximum value of approximately 730 nm. Using 3 He(d, p) 4 He nuclear reaction analysis on 3 He + implanted specimens retained helium fluences were found to range from 4.0 × 10 20 to 4.5 × 10 21 He m −2 . Each specimen sustained mass loss after implantation which was observed to increase with greater He + dose.