As leading candidate material for the PFM of future fusion reactors, tungsten has been identified as the plasma-facing material at the divertor baffles in the ITER design. The choice of the full tungsten wall makes it necessary to seed impurities into the edge plasma for radiative cooling. The energy confinement improvement with the inert-gas impurity (Ne and Ar) seeding in many tokamaks devices has been reported. While these results are encouraging, questions remain as to the irradiation effects of these inert-gases on the tungsten. Experiments show that there is a large difference in the irradiation effects on tungsten between the He and Ne/Ar plasmas, despite the fact that the physical properties of Ne and Ar are in common with He. The reasons for this difference are unclear, and while the detailed physics by themselves are of considerable interest, a more pressing matter is the inert-gas atom clustering processes and the corresponding growth mechanism, which play an important role in the formation of bubbles and nanostructures.

Recently, a research team, led by Prof. LIU Changsong in Institute of Solid State Physics, Hefei Institutes of Physical Sciences investigated the clustering behaviors of He, Ne and Ar in tungsten by first-principles calculations, which reveals the physical mechanism of the He, Ne, and Ar clustering in metals and provides an explanation for the differences in irradiation effects of He, Ne and Ar plasma. Their study was published in Nuclear Fusion entitled Towards understanding the differences in irradiation effects of He, Ne and Ar plasma by investigating the physical origin of their clustering in tungsten.

The researchers find that the small interstitial clusters, Hem, Nem, and Arm, can form as a result of the attractive interaction between the interstitial inert-gas atoms, and tend to expand along the (110) planes.

This clustering behavior can be well explained by the intrinsic repulsive interaction between the inert gas atoms and the attractive interaction coming from the reduced valence-electron density by interstitial inert gas atoms. Once the formation of interstitial cluster occurs, it seems to create a “snowball effect", i.e., the cluster will increase rapidly due to its increasing ability to trap an additional isolate atom and the low diffusion barrier of the interstitial atom. As the cluster grows up, it can mutate into a vacancy-inert-gas-atom complex (VacHem, VacNem, and VacArm) by emitting a self-interstitial atom (SIA).

This series of process is named by "self-trapping process". They predicted that the emission of a SIA can occur spontaneously when m>6, 3, 3 for Hem, Nem, and Arm, respectively.

Compared to He, Ne/Ar has a much greater attraction and a lower trigger condition of ‘self-trapping process’. Therefore, the VacNem and VacArm complexes are easier to form compared to the VacHem complex. So it is reasonable to expect that a large amount of VacNem/VacArm complexes are formed near the surface layer under the irradiation (Fig. 1).

These complexes are immobile and can capture the interstitial Ne/Ar atom and the small clusters nearby, impeding their diffusion to the bulk. In contrast, the density of the VacHem complex is much lower than that of the VacNem/VacArm near the surface region.

Thus, more He atoms can diffuse deeper into the bulk compared to Ne/Ar. Consequently, the penetration depths for Ar and Ne are much shorter than that for He under the irradiation when the incident energy is low.

The research was financially supported by the National Magnetic Confinement Fusion Energy Research Program, the National Natural Science Foundation of China and the Youth Innovation Promotion Association of Chinese Academy of Sciences.

The research was financially supported by the National Magnetic Confinement Fusion Energy Research Program, the National Natural Science Foundation of China and the Youth Innovation Promotion Association of Chinese Academy of Sciences.

Fig.1. Schematics of the irradiation effects on tungsten between Ne/Ar and He plasmas when the incident energy is low. The big black empty and small dark cyan filled circles represent tungsten and inert gas atoms respectively. (Image by KONG Xiangshan)

Contact:

LIU Changsong

Institute of Solid State Physics (http://english.issp.ac.cn/)

Hefei, Anhui 230031, China.

Tel: 0086-551-65591062

E-mail: xskong@issp.ac.cn, csliu@issp.ac.cn