Reduced D trapping by plasma-implanted He nanobubbles in radiation damaged tungsten

The presence of a plasma-implanted helium nanobubble layer significantly reduces deuterium (D) retention in undamaged commercial ITER grade tungsten (W). In this paper, we show evidence that this phenomenon can survive displacement damage. A He plasma exposure (sample temperature 643 K, ion flux 10²² m⁻² s⁻¹ at 100 eV, fluence 10²⁵ m⁻²) pre-treatment was performed to create a thin He nanobubble layer in the first ∼15 nm of ITER grade W samples. Samples were then irradiated by 5 MeV Cu ions at room temperature to create 0.001 to 0.1 dpa with peak damage rates occurring about 860 nm below the sample surface. Samples without He plasma exposure pre-treatment were also irradiated by 5 MeV Cu ions to provide a controlled baseline. All samples were subsequently exposed to D plasma at 373 K to a fluence of 10²⁴ m⁻². Nuclear reaction analysis results show that across a range of peak dpa ranging from 0.001 to 0.1 dpa, D retention inventory in the samples with He plasma exposure pre-treatment is reduced by a factor of two compared to samples without He plasma exposure pre-treatment. Transmission electron microscopy directly showed a surviving nanobubble layer after 0.1 dpa damage. However, the thickness of the bubble layer appears to have been reduced. The results suggest that the plasma-implanted He nanobubble layer can survive radiation damage and still function to reduce D diffusion and retention in tungsten-based plasma facing components. (paper)