Tungsten material is one of promising candidates for divertor plates in nuclear fusion reactors. Bubble formation is observed on the surface of the tungsten material which is irradiated by helium plasma. Furthermore, a number of experiments show that helium plasma constructs filament (fuzz) structures whose diameter is in nanometer-scale on the tungsten material under the suitable condition (i.e., material temperature of 1,000-2,000~K and incident energy of 20-100~eV). The formation of these structures does not only decrease the thermal conductivity of the material, but also enhances the tolerance in cracking and the radiation efficiency.　It also causes reduction of sputtering yield.　Some of theoretical models related to the mechanism of fuzz formation were advocated.　For example, the theoretical using fluid model describing viscose effects in incompressible material flow under impact of large stress was proposed. However, the formation mechanism has not been well understood yet. Therefore, we, PWI group in NSRP, are challenging to reveal the formation mechanism of the nano structure. As the first step, we investigated the penetration depth for helium, neon, and argon gases by binary-collision-approximation-based (BCA) simulation to reveal the possibility of fuzz formation under noble gas irradiation. The penetration depth strongly depends on the incident gases. Accoring to our BCA simulation, only helium can invade the tungsten (Fig.1). Next, we examine the binding energy of helium trapped in a tungsten monovacancy using first-principles calculation based on density functional theory (DFT) and investigate the trapping of multiple helium atoms within a tungsten Monovacancy (Fig. 2). Calculation shows that a tungsten monovacancy can contain at least nine helium atoms. We find that six monovacancy-trapped helium atoms form a kind of a cluster structure with an octahedral configuration, and the cluster structure is tightly bound around a monovacancy located at the center of a W cube.

Fig.1 Incident energy dependence of mean depth of He, Ne, and Ar injection into tungsten material [S. Saito, et al., Journal of Nuclear Materials 438 (2013) S895].

Fig. 2 Binding energy as a function of the number of He atoms trapped within a W monovacancy [A Takayama, et al., Japanese Journal of Applied Physics 52 (2013) 01AL03].

1. Seiki Saito, Atsushi M. Ito, Arimichi Takayama, Hiroaki Nakamura: 　"Anisotropic Bond Orientation of Amorphous Carbon by Deposition", 　Japanese Journal of Applied Physics 51 (2012) 01AC05.
2. Seiki Saito, Atsushi M. Ito, Arimichi Takayama, Hiroaki Nakamura: 　"Structural Change of Single-Crystalline Graphite under Plasma Irradiation", 　Japanese Journal of Applied Physics 52 (2013) 01AL02.
3. Arimichi Takayama, Atsushi M. Ito, Seiki Saito, Noriyasu Ohno, Hiroaki Nakamura: 　"First-Principles Investigation on Trapping of Multiple Helium Atoms within a Tungsten Monovacancy", 　Japanese Journal of Applied Physics 52 (2013) 01AL03.
4. Atsushi M. Ito, Arimichi Takayama, Seiki Saito, H. Nakamura: 　"Formation and Classification of Amorphous Carbon by Molecular Dynamics Simulation", 　Japanese Journal of Applied Physics 52 (2013) 01AL04.
5. Miyuki Yajima, Masato Yamagiwa, Shin Kajita, Noriyasu Ohno, Masayuki Tokitani, Arimichi Takayama, 　Seiki Saito, Atsushi M. Ito, Hiroaki Nakamura, Naoaki Yoshida: 　"Comparison of Damages on Tungsten Surface Exposed to Noble Gas Plasmas", 　Plasma Science and Technology 15 (2013) 282.
6. Seiki Saito, Arimichi Takayama, Atsushi M. Ito, Hiroaki Nakamura: 　"Binary-collision-approximation-based simulation of noble gas irradiation to tungsten materials" 　Journal of Nuclear Materials 438 (2013) S895. [to be published in July 2013]