27E23P
ヘリウム照射によるタングステン表面構造変化の照射量依存性
Simulation Study on Time Evolution of Tungsten Surface under Helium ion
Irradiation
剣持貴弘1， 和田 元2
KENMOTSU Takahiro1, WADA. Motoi2
1

同志社大学 生命医，2同志社大学 理工1Doshisha University, 1Faculty of Life and Medical Sciences,2
Doshisha University, Gradicate School of Science and EngineeringTungsten has been selected as a candidate materialof the ITER divertor plates because of its high melting
temperature, good thermal conductivity and highly
resistive nature against erosion due to high thresholdenergy for sputtering. Divertor plates in a fusion device
are exposed to high intensity heat fluxes of energetic
particles.
The formation of nanostructure “fuzz” at tungsten
surface due to helium plasma irradiation has been
observed in recent experiments [1,2]. The formation of
the fuzz at tungsten surface leads to the change in
effective thermal conductivity and effective
particle/energy reflection coefficients of tungsten.
These properties relevant to hydrogen isotope retention
on tungsten are also expected to change by the fuzzy
nanostructure formation. The fundamental processes ofthe fuzzy nanostructure formation, however, have notbeen clarified yet.
A Monte Carlo simulation code, ACAT [3] (AtomicCollision in Amorphous Target), have been used tocalculate the modification of surface structure of atarget material. So far, the ACAT code has treatedthat the surface of the target material forms acompletely flat boundary, and the modification of thematerial proceeds homogeneously. As the surface ofa plasma facing material should be modified due toenergetic particle bombardments, the ACAT code has
been being revised so as to calculate theinhomogeneous development of the modification of thesurface structure due to energetic ion bombardment.
The target material is divided into unit cells forcalculation of collision cascades, and the positions of
the vacant cells formed by ejection process areintegrated to calculate the surface geometrical structure.
The modifications of the surface structure of tungsten
have been calculated with the revised ACAT code.
Figures 1 (a), (b), (c), (d) show the change in surface
structure of a tungsten material depending upon
fluences of 100 eV helium ion bombardments at
normal incidence. The results indicate inhomogeneousdevelopment of pits forming a pattern on the surface ataround 4.96  1020 m-2 .

(a) Fluence = 4.96×1019 m-2(b) Fluence = 9.92×1019 m-2(c) Fluence = 4.96×1020 m-2(d) Fluence = 9.92×1020 m-2

Figure 1. Developments of the modification of tungsten
surface structure due to 100 eV helium ion bombardments atnormal incidence. Heilum ion fluence of (a); 4.96×1019 m-2,(b); 9.92×1019 m-2, (c); 4.96×1020 m-2, and (d); 9.92×1020
m-2.

We have also performed the case where theinitial positions of target atoms in unit cells arechanged for the above mentioned calculations by using
the different set of random numbers in the ACAT
calculations. Even though the distributions of target
atoms are different, the surface structures have shown
similar patterns. The results have indicated that thechange of the surface structure does not depend
strongly upon the initial condition of a target material.
However, the erosion pattern may become sensitive tothe initial conditions if one assumes a large difference
like the non-uniform distribution of defects initiallypresent in the target material.

References[1] Y. Ueda et al., J. Nucl. Mater. 386-388 (2009) 725.
[2] M.J. Baldwin, R.P. Doerner, J. Nucl. Mater. 404(2010) 165.
[3] Y. Yamamura, Y. Mizuno, IIPJ-AM-40, Inst. Plasma
Physics, Nagoya Univ., 1985.