Summary:
Tungsten material is attracting many researchers because of the use of a plasma facing material in nuclear fusion device because of its excellent features such as high-melting point, high threshold energy of physical spattering, and high reflection rate of hydrogen isotope and impurities. For example, tungsten material is planning to employ for divertor plates in ITER (International Thermonuclear Experimental Reactor). Because the divertor plates directly contacts with energetic plasmas, the erosion of the surface of the divertor plates is inevitable.
Many experimental studies reported bubble formation on the surface of tungsten materials under helium plasma irradiation in wide range of incident energy. Helium ash is generated when nuclear fusion between deuterium and tritium is caused in nuclear fusion device. Therefore, the bubbles are possibly generated on the divertor plates of the fusion device. The existence of the bubbles may affect the tritium inventory of divertor plates. In this study, therefore, molecular simulation is performed for the investigation of the effects of the existence of helium bubbles on the quantity related to plasma-material interaction such as sputtering yield, penetration depth, and reflection rate.
The figure shows the time evolution of surface of tungsten material irradiated by helium plasma calculated by molecular simulation with tungsten-helium potential function. The red and black dots denote the positions of tungsten and helium atoms, respectively. The growth of helium bubbles is successfully observed. As a result of the simulation, it is found that the sputtering yield, penetration depth, and the reflection rate decreases when helium bubbles exist.

Simulation result of helium plasma irradiation to tungsten material
1. Saito, et. al., Jpn. J. Appl. Phys. 55 (2015) 01AH07
Copyright (2015) The Japan Society of Applied Physics