The realization of fusion as a practical, 21st Century energy source requires improved knowledge of plasma surface interactions and the materials engineering design of component systems to survive the incredibly extreme heat and particle flux exposure conditions of a fusion power plant. The objective of this proposal is to further advance understanding of the response of tungsten, the proposed ITER divertor, to low energy, mixed H-He plasma exposure. In particular, two tasks are envisioned that investigate helium behavior and gas bubble aggregation kinetics in tungsten, and begin to address hydrogen retention in sub-surface gas bubbles: Task 1: Continued development of an atomistic database of helium-hydrogen bubble aggregation kinetics and evaluation of hydrogen retention in sub- urface gas bubbles We will continue MD simulations using the LAMMPS code to study the exposure of tungsten surfaces to mixed He-H implementation characteristic of plasma exposure anticipated in ITER operation. ALCC computing resources are necessary to study a range of gas implantation rates, and to develop sufficiently long simulations to identify the mechanisms controlling bubble populations, surface topology changes, overall gas inventory and release. These simulations will provide a database to verify continuum simulations, and provide necessary knowledge to identify materials design strategies to effectively manage the high gas exposures in the fusion energy environment. Task 2: Hydrogen diffusion behavior in the presence of sub-surface gas bubbles or dislocation microstructure, utilizing different interatomic potentials to evaluate He-H synergies in W-He-H. The presence of nanometer-sized gas bubbles, in addition to other microstructural features including dislocations, below plasma-exposed tungsten surfaces raise substantial questions to the extent with which these features will act as traps, or modify tritium permeation and diffusion behavior. Previous ALCC efforts have characterized the trapping characteristics, and we now propose a more significant study to identify modifications to tritium diffusivity. In this task, we will perform MD simulations of hydrogen diffusion and trapping interactions in near surface tungsten with representative helium bubble or dislocation populations using a range of potentials from semi-empirical embedded atom method, or Tersoff based potentials, and newly created Spectral Neighbor Analysis Potential (SNAP) potentials. This will identify and validate the common physics resulting from the different models, as well as reveal the underlying physics to assess tritium retention in tungsten plasma facing components in a fusion reactor.