Investigating the temperature dependence of helium bubble dynamics in plasma exposed tungsten via in-situ TEM annealing

Abstract

In-situ TEM annealing and imaging were performed on tungsten samples pre-exposed at either 573 K or 1013 K in a pure helium plasma to a fluence of 1025 m−2. To investigate the thermal evolution of helium bubbles, the samples were imaged for annealing temperatures up to 998 K in steps of 100 K. It is found that annealing temperatures had little effect on the bubble dynamics for the lower temperature exposure of 573 K. Elongated bubble structures remained in the sample throughout the annealing process and there is minimal change in both the average bubble size and cross-sectional number density. However, annealing had a significant effect on the higher temperature exposure of 1013 K. Bubbles were found to be trapped in large clusters and increasing annealing temperature caused a significant increase in the average bubble size accompanied by a decrease in the cross-sectional bubble number density. The experimental observations are supported by theoretical calculations that show a simultaneous increase in the average number of He atoms per bubble in the higher exposure temperature sample but an overall decrease in He atoms retained within bubbles as the annealing temperature increased. These trends in bubble dynamics with annealing temperature suggest that Ostwald ripening dominates, whereby bubble growth is determined by the different pressures of different bubble sizes, rather than through bubble migration and coalescence.