The Study Of Nucleation And Diffusion Of Noble Gas Elements In Tungsten

Tungsten (W) is considered to be the most promising plasma facing materials due to its high melting point, high thermal conductivity and low sputtering characteristics. He is one of the fusion produces and Ne, Ar are introduced as the coolant in tokamak. He atoms have extremely low solubility and the accumulation of He atoms also lead to the formation of bubbles and holes which have significant effect on the macroscopic properties of W. Under the low-flux He plasma irradiation, the nano structured tungsten (or "fuzz") was formed on W surface experimentally, which severely impact the operation of fusion devices. Ne and Ar are noble gas atoms and have extremely low solubility in W. Many properties of Ne and Ar are in common with He, however, the fiber-formed nano structures were not formed on the W surface by exposure to Ar or Ne plasmas. The different influence on W surface microstructure caused by He, Ne and Ar plasma irradiation is receiving more attention recently. Therefore, it is of great significance to compare the behaviors of He, Ne and Ar in W.To study the behavior of noble gas atoms (He, Ne and Ar) in bulk tungsten, new DFT-based potentials for W-He, W-Ne and W-Ar interactions were developed by fitting the results obtained from density functional theory (DFT) calculations. The new potentials adopt the embedded atom method (EAM) formalism, and the "s-band model" is used to describe the many-body interactions between each of the noble gas atoms and its neighboring W atoms. These potentials reproduce the formation energies of point defects in the substitutional site, tetrahedral interstitial site, octahedron tetrahedral interstitial site and the migration barriers of single noble gas atoms. The simulations using these potentials successfully predict that the tetrahedral interstitial site is more stable than the octahedral interstitial site for X (= He, Ne or Ar) interstitials. Based on these new potentials, the binding energies of a single X atom with the Xn and Xn-Vacancy clusters, the diffusion properties of Xn clusters, the nucleation of X atoms and the dislocation loop punching caused by the over pressurized bubbles in bulk W were studied using molecular dynamics simulations. The present results indicate that the binding energies of clusters obtained using the new potentials are good in agreement with the results of DFT calculations. The migration energies of the clusters increase with both the increase in the atomic radius of noble gases and the increase in the size of the clusters. The results also showed that the formation of bubbles and the dislocation loop punching was observed in the growth of all the three noble gas bubbles with different levels of difficulty.