Effects of Sputtering, Re-deposition and Diffusion Processes for Helium Plasma Induced Metal-nanostructure with Multi-hybrid Simulation Analysis*

Fuzzy metal nanostructure, called “fuzz”, induced by low energy helium plasma irradiation was found in the experiment on a plasma-facing tungsten material used for the inner wall of plasma vacuum vessels. To form a fuzz, the helium irradiation energy should be less than about 100 eV and the total helium fluence should be greater than 10²⁴ m^-2s^-1. We have been investigating the formation mechanisms of the fuzz using molecular simulations. From those conditions of fuzz, we aimed for a simulation that achieves a long-term scale equivalent to an atomic experiment while keeping the spatial scale at the nanoscale.

We, therefore, developed the BCA-MD-KMC multi-hybrid simulation[2]. In this way, the irradiation process of helium ions is solved binary collision approximation (BCA), the thermal diffusion process of helium atoms in a tungsten material is solved by kinetic Monte-Carlo (KMC), and the deformation of the tungsten material due to the agglomerated helium nano-bubble is solved by molecular dynamics (MD). As a result, with the same irradiation flux as an experimental condition, the irradiation time of 100 seconds can be achieved in a simulation during about one month.

From the simulation, we propose the formation mechanisms as follows. By the low energy helium irradiation, which cannot sputter out the tungsten atom, many helium nano-bubbles generated in tungsten material. On rough surfaces after nano-bubbles burst, the binding energy of tungsten becomes low. As a result, tungsten atoms can be sputtered by the low energy helium ion. Simultaneously the sputtered tungsten atoms redeposit on the protrusions area created by the bursting. The transport of tungsten atoms by these sputtering and redeposition causes the fuzz to grow vertically from the surface.