Topic/Type: 1.2 Fusion Plasmas (magnetic & inertial confinement), Poster

### Correlation of Surface Temperature and Hydrogen Coverage in Chemical Sputtering Processes on Graphite by Molecular Dynamics

Atsushi Ito, Hiroaki Nakamura

National Institute for Fusion Science, 322-6 Oroshi-cho, Toki-shi 509-5292, Japan.

1)National Institute for Fusion Science, 322-6 Oroshi-cho, Toki-shi 509-5292, Japan.
*Corresponding author: ito.atsushi@nifs.ac.jp

We have investigated chemical sputtering due to hydrogen injection onto plasma facing graphite materials of inside wall of nuclear fusion devises by the use of molecular dynamics (MD) simulation. Important problem is difference between experiments and theoretical simulation in surface temperature. In this paper, the influence of surface hydrogen atoms on surface temperature variation is researched.

MD simulation is performed by using modified REBO potential [1] based on Brenner\'s original potential in 2002 [2]. Moreover, to treat layered structures of graphite, we employ interlayer intermolecular potential with cone cutoff function proposed in the previous (20 th) ICNSP with US-Japan workshop in 2007 [3].
Graphite surfaces of 2.0 x 2.0 nm$\small {}^2$ facing z direction is set on center of simulation box under periodic boundary condition for x and y direction. Prepared three kinds of surfaces, flat (0 0 0 1), armchair (1 1 $\small \bar{2}$ 0) and zigzag (1 0 $\small \bar{1}$ 0) surfaces, consist of 1280, 1260 and 1248 carbon atoms, respectively. Hydrogen atoms are injected into surface at a regular time interval of 0.1 ps. To support these graphite surfaces, 2-6 carbon atoms in the deepest position are fixed during MD simulations.

The present work shows the following fact. Surface temperature increased with time. However, the surface temperature saturates in the case of hydrogen atom injection of less than 0.5 eV, though the hydrogen atom injection continued. Since heat reservoir system is not employed in this simulation, raised kinetic energy cannot be dissipated from the surface. Namely, in the hydrogen atom injection of less than 0.5 eV, there is mechanism to suppress the transport of the kinetic energy into the surface. We find that hydrogen atom coverage on the surface increases and saturates along with the surface temperature. This correlation indicates that dominant source of the raised surface kinetic energy is the binding energy between adsorbed hydrogen atom and the surface in low energy injection. Since the surface coverage depends on lattice structures of the surfaces, the different surfaces reach different surface temperatures.

[1] A. Ito, H. Nakamura and A. Takayama, J. Phys. Soc. Jpn. 77, 114602 (2008).

[2] D. W. Brenner, O. A. Shenderova, J. A. Harrison, S. J. Stuart, B. Ni, and S. B. Sinnott, J. Phys.: Condens. Matter 14, 783 (2002).

[3] A. Ito and H. Nakamura, Commun. Comput. Phys. 4, 592 (2008)