(28e) Large-Scale Simulation of Plasma-Facing Materials for Tokamaks and Linear Devices

Helium from linear plasma devices and tokamak plasmas is known to cause formation of nanometer-sized features on the surface of plasma-exposed materials after only a few hours of plasma exposure.  The precise details of such surface modifications are as yet uncertain.  This study examines the initial and intermediate stages of tungsten surface evolution by large-length-scale molecular dynamics (MD) simulations of helium-implanted tungsten over time scales of up to microseconds using single-crystal and polycrystalline supercell models of tungsten.  The large-scale MD simulations employ state-of-the-art many-body interatomic potentials and implantation depth distributions for the insertion of helium atoms into the tungsten matrix constructed based on MD simulations of helium-atom impingement onto tungsten surfaces under prescribed thermal and implantation conditions.  The large-scale MD simulations reveal features both above and below the surface, indicating that (a) the crystallographic orientation of the surface has a significant effect on retention and surface deformation, (b) the grain microstructure of the tungsten matrix (i.e., the presence or absence of grain boundaries) has strong effects on the resulting agglomeration behavior and resultant surface features, and (c) grain boundaries and surfaces are both "sinks" for helium, resulting in immobilization and/or enhanced bubble formation near such defects.  Our findings have significant implications for the surface morphological evolution and the near-surface structural evolution of plasma-facing components in nuclear fusion reactors.