The response of polycrystalline tungsten to 30Â keV helium ion implantation at normal incidence and high temperatures

Polycrystalline tungsten samples were irradiated with 30 keV helium ions in the Materials Irradiation Experiment (MITE-E). Samples were held at temperatures of 500-900 °C while irradiated to fluences of ∼6 × 1016 to ∼4 × 1019 He/cm2. Drastic changes to the surface morphology were observed at implant doses exceeding ∼1 × 1018 He/cm2 and classified as blisters, pitting, or a highly directional surface structure dubbed “grass.” Morphological details of the grass structure depended on the crystal orientation of the grains at the sample surface. A sub-surface semi-porous layer was formed during irradiation. Post-implantation analysis revealed that this layer extended up to ∼1000 nm below the surface in the highest fluence cases. Specimens irradiated to fluences in excess of ∼1 × 1018 He/cm2 experienced statistically significant mass loss which increased with higher implant dose. Implications of this mass loss are discussed for the plasma facing components of both magnetic and inertial fusion reactors.