Surface Damage Mechanism of Tungsten by Heat Flux and Deuterium Ion Irradiation in Nuclear Fusion

Abstract

Tungsten is a very promising material for use as a plasma-facing component in high-power fusion plasma devices, because of its high thermal conductivity, melting point, and plasma resistance. However, even tungsten is not fully capable of withstanding the energy transferred from the core plasma. In such a situation, the tungsten surface becomes damaged and develops both morphological and structural variations under specific plasma conditions. For plasma-facing material, energy transfer from the plasma to the material surface causes surface damage through thermal-dominant and particle-dominant effects. Each effect manifests on the tungsten surface in different ways, causing various types of damage. In addition, this damage manifests differently under the same plasma conditions, depending on the characteristics of the facing material. Therefore, an understanding of the damage sustained on the tungsten surface is required, considering both the correlation between thermal/particle effects from the plasma and the characteristics of the facing material. In this dissertation, this damage generation is studied using the heat flux and deuterium-ion irradiation of tungsten targets having vertically and horizontally elongated grains. The heat-flux irradiation is conducted with heat-flux conditions of 5 MW/m2 and a surface temperature of 1,700 °C, and the deuterium-ion irradiation is conducted with ion fluence conditions of 8.3 × 10231.9 × 1025 m-2 and an incident energy of 50200 eV. Heat-flux irradiation causes a tungsten target to recrystallize with increased grain size and saturated recrystallized-layer thickness. Differences in the initial grain structures of various tungsten targets lead to differences in the recrystallized targets, which initially have vertical or horizontal grain elongations. Only a recrystallized tungsten target that has vertically elongated initial grains demonstrates a large amount of sub-grains and internal cracks generated at the sub-grain boundaries. In contrast, a tungsten target with horizontally elongated grains exhibits no internal damage, e.g., crack and bubble structures. After deuterium-ion irradiation, a specific structure called a blister is generated on the tungsten target surface. Depending on their sizes and shapes, the blisters are classified as being small dome-shaped, medium terrace-shaped, and large irregular-shaped. The small dome-shaped blister has the thinnest roof and an inclined side wall, while the medium terrace-shaped and large irregular-shaped blisters originate from near-surface grain boundaries and have thicker roofs than those of the small blisters. On the surface of a small dome-shaped blister, the side wall is exposed to the deuterium-ion flux at an angle of 70°, which is the optimum value for maximum sputtering. The small dome-shaped blister is eradicated from the side wall by an increased sputtering-aided burst. The burst area is expanded along the blister edge, satisfying the required angle condition. In the case of a medium terrace-shaped blister, the blister is also destroyed through a local sputtering increment at the inclined side wall of the blister. However, an increase in the sputtering incident angle is not effective in the case of the large irregular-shaped blister, because of its moderately swollen surface. From these results, it is apparent that the small dome-shaped blister has the greatest susceptibility to the effects of high-energy deuterium ions. Thus, a vertically elongated grain structure that has a large number of small blisters only is strongly affected by deuterium-ion irradiation. The tungsten surfaces damaged by heat flux and deuterium-ion irradiation are aggravated using additional ion and heat-flux irradiation, respectively. The deuterium-ion irradiation causes sub-grain ejection with very little frequency from pre-heat-flux-irradiated tungsten with a vertically elongated grain. In contrast, hole structures are frequently generated on pre-recrystallized tungsten with a horizontally elongated grain. Additional heat-flux irradiation causes all three kinds of blisters to be aggravated by the thermal bursting due to this additional irradiation. The small dome-shaped blisters are fully destroyed, while the medium terrace-shaped and large irregular-shaped blisters increase in size and are partially destroyed but retain their size and shape.