Molecular dynamics simulation analysis of the stress around an up-migrating helium bubble under tungsten surface and its bursting

Abstract

Tungsten has been considered as the most qualified candidate for the plasma-facing material of modern fusion devices up to date. However, in the future tokamaks such as ITER and CFETR, the helium species will be trapped inevitably in tungsten, and these alien species gradually agglomerate and coalesce into bubbles under appropriate conditions. The building-up of internal pressure in a bubble near the tungsten surface finally engenders the bubble bursting. The bubble rupture not only produces helium and tungsten impurities but also changes the topography of the tungsten surface; the latter further affects the sputtering. This paper uses molecular dynamics simulations to show how a helium bubble behaves in the course of its migration in tungsten towards the surface at different temperatures, and correspondingly, estimates the critical bursting pressure of bubbles when they come close to the surface. In the meantime, the paper also studies how the helium bubble at different depths affects its surrounding stress in tungsten at the atomic level. The study indicates that the helium bubble makes the stress of its close neighboring part of tungsten increase and that the internal pressure of the bubble is higher when it is deeper below the surface. The study also simulates how the surface morphology evolves after the bubble bursts. It is found that the bubble can burst easily when the temperature is over 1200 K and a considerably deep, cone-shaped pit forms after the bursting. During the bubble bursting, tungsten atoms are ejected out of the bulk. The velocity of these emitted atoms increases with temperature. The emitted atoms fall around the pit more locally during the bursting at 1200 K than at 2300 K. Further discussion is also made over the implications towards the formation of fuzz.