Improving the Bubble Bursting Model in Cluster Dynamics Simulations of Low-Energy Helium Plasma-Exposed Tungsten*

In this presentation, we describe a modified bubble bursting model which is implemented in Xolotl, a spatially-dependent reaction-diffusion cluster dynamics code to simulate the divertor surface response to fusion-relevant plasma exposure. In the base model, the helium release from over-pressurized bubbles is accounted as a stochastic event, which is dependent on the size of a given helium bubble and its distance from the free surface (Xolotl does not explicitly compute the pressure within a bubble). The predicted helium retention behavior agrees with observations from high flux molecular dynamics (MD) simulations, although the size and depth distributions of the bursting bubbles vary significantly.

We investigate the fidelity of the modified model in which the partially random bursting events are replaced with a set of equivalent reactions with rates formulated to depend on size and distance in the same way as in the base model. This new implementation provides a performance gain because, unlike the base model, the time step of the solver does not need to be refined after each disruptive bursting event. We also propose improvements to the bursting model based on more extensive comparisons against MD data to provide a proxy for the bubble pressure in Xolotl.