Molecular dynamics simulation for hydrogen recycling on tungsten divertor for neutral transport analysis

In magnetic field confinement plasma devices such as the International Thermonuclear Experimental Reactor (ITER), hydrogen atoms and molecules are emitted from plasma-facing materials by hydrogen recycling. The effect of the recycled hydrogen in edge plasma can be investigated by neutral transport analysis. In this paper, in order to obtain the boundary condition at the tungsten divertor for neutral transport simulation, a molecular dynamics simulation is performed to investigate the emission ratio of hydrogen atoms and molecules, the emission angle, the translational energy of emitted hydrogen atoms and molecules, and the vibrational and rotational energies of emitted hydrogen molecules. Moreover, from the simulation results, it is also found that the number of emitted hydrogen molecules increases as the ratio of the number of hydrogen and tungsten atoms (H/W) in the material increases, although the H/W dependence of the number of emitted hydrogen atoms is not significant in the range of our simulation, 0.49 <= H/W <= 1.37. The elementary process of the emission of hydrogen is also investigated by trajectory analysis. The analysis reveals that the hydrogen atom travels on the surface of the tungsten material and remains there for a while because it is trapped on the surface of the tungsten material.