Prediction of Near Surface Gas Bubble Evolution in the ITER Divertor with Cluster Dynamics*

Plasma surface interactions in fusion tokamak reactors involve an inherently multiscale set of phenomena, for which current models are inadequate to predict the divertor response to and feedback on the plasma. In this presentation, we describe the latest code developments of Xolotl, a spatially-dependent reaction diffusion cluster dynamics code. Xolotl is part of a code-coupling effort to model both plasma and material simultaneously, including SOLPS for simulations of the edge plasma in steady-state conditions; the effect of the sheath at shallow magnetic angles, evaluated by hPIC; GITR calculations of migration and redeposition of impurities eroded from the surface; and the response of the wall surface to these plasma conditions modeled by coupling F-TRIDYN and Xolotl. The latter has been extended from helium only to mixed hydrogen-helium plasma, increasing the computational complexity due to the large number of clusters to model and requiring optimization. A simplified helium bubble bursting model is included to take into account the gas release happening when a bubble is near the surface in order to predict more realistic surface evolution and sub-surface composition under ITER conditions. Results from a range of locations in the divertor will be presented.