Effect of Thermal Cycling on Morphology Changes and Deuterium Retention in Helium Exposed Tungsten

Near-surface morphology modification of tungsten (W) plasma-facing material (PFM) under intense particle and heat fluxes raises concerns about material property conservation and tritium retention in the ITER divertor. The accumulation of helium (He) in W induces formation of cavities, so-called helium bubbles, which may alter the retention property of the metal [1].

In this work, the shape and density evolution of helium bubbles in W is investigated for temperature and He flux conditions relevant to those expected in ITER. Sets of thermal cycling experiments up to 1350 K with a temperature ramp of 1 K/s followed by an extensive surface and near-surface characterization of He-exposed W revealed shaping of surface holes and helium bubbles and their removal in the 5 nm subsurface layer. Cross-sectional observations of the bubbles under different zone axes indicate that the shape of the bubbles is most likely rhombic dodecahedron consisting of 12 congruent rhombic faces of (110) orientation delimited by <110> edges. This is in agreement with theoretical considerations since the shape of the bubbles is determined by the surface energy minima, thus the voids in the W bcc metal should form faces of (110) orientation. Furthermore, the observed structural changes in He-exposed W correlate with a modification of deuterium release mechanisms [2].

Electron energy loss spectroscopy measurements confirmed the presence of He in the bubbles even after annealing up to 1350 K. In conclusion, our work gives additional knowledge on the fundamental physics of the gas-metal interactions and confirms the importance to pursue investigations on the effects of high fluence He exposure on W material properties for its successful application as a PFM in the next generation fusion devices.