The evolution of hydrogen bubbles during ion irradiation and annealing in molybdenum for neutral beam injection inductively coupled plasma source

Molybdenum (Mo) is considered as a promising plasma-facing material, which is expected to be used in the manufacture of critical components for the neutral beam injection (NBI) inductively coupled plasma (ICP) source. Hydrogen isotope retention and bubble formation are regarded as the unavoidable issues that degrade the metal properties in the ion source. In this work, the evolution of hydrogen bubbles in Mo pre-irradiated by 35 eV hydrogen ion energy during different annealing processes is studied through conductive atomic force microscopy (CAFM) observation. Meanwhile, the corresponding numerical simulation is applied to analyze the mechanisms of hydrogen bubble growth. It is found that the average size and density of hydrogen bubbles are enhanced with the increase of hydrogen ion fluence at 400 K in experiments and simulations. In the subsequent annealing process, it is demonstrated that annealing temperature has a certain effect on bubble growth and evolution. No obvious changes in bubbles are observed at the annealing temperature below 573 K. The average radius of hydrogen bubbles is increased by similar to 1.0-2.5 nm in Mo annealed at 673-873 K. The bubble size increases with the rise of annealing temperature, indicating that the thermal annealing promotes the growth of bubbles. When the temperature rises above 873 K, the change in bubble size is not obvious. The simulation results indicate that temperature-induced changes in the internal pressure of hydrogen bubbles leads to the growth of bubbles, but this mechanism is not significant under high annealing temperatures due to the release of hydrogen from the surface.