T. Kawamura

T. Kenmotsu

Y. Hirooka

Y. Yamamura

National Institute for Fusion Science (NIFS-Repository)

§18. Simulation Studies of Chemical Sputtering on Carbon Material under Hydrogen Bombardment Kenmotsu, T. Kawamura, T., Hirooka, Y. Yamamura, Y. (Okayama Univ. Sci.) We had simulated the incident ion fluence dependence of methane emission rate from carbon under hydrogen bombardment using the ACAT-DIFFUSE code [1], a Monte Cario code with a binary coUision approximation with diffusion effects take into account. The chemical sputtering was studied by Roth [2]. But the Roth's model is only suitable for steady state methane reaction. Then, \ve have proposed an empirical formula, modifying Roth's formula. In this empirical formula, we assumed the reaction region where chemical sputtering takes place. The reaction region was defined with respect to the implantation range of hydrogen as shown in Fig.I. Empirical iormuia ior ehemicai reaction Bulk Surface Fig. 1 Chemical reaction model In Fig. 1 , R (E) is the peak range of hydrogen distribution. E is the incident energy. The empirical formula for methane reaction yield is given by the following equation : YCH /E,I,1)= ( QI"\ A exp - kBT) R(E) ( o)L nH(x,t,T)dx B+Cexp - k:} (1). where Y CR4 is the methane reaction yield (CH4 · molecules/ion), nR is the surface concentration of hydrogen (cm-2), Ql is the activation energy (e V) of methane formation, Q2 is the activation energy (e V) of 332 recombination, T is the temperature, t is the irradiation time. x is the depth from surface, kB is Bolztmann constant and A, Band C are fitting parameters. Furthermore, we assumed that D/C ratio in graphite is 0.4. Results and disseusion The ACAT-DIFFUSE code, which incorporates the above empirical formula on chemical sputtering, was used to fit Yamada's data [3] Figure 2 shows depth distribution of deuterium in graphite at several fluence. The hydrogen in graphite indicate the saturation concentration at the fluence of 1.0 X 1017 (I-I Icm2/s) because we assume the HlC ration is 0.4. iii i o 50 J i 1 I· .... 4.0x10 15 i~ns/em 211 ~ I· - 1.2X101: Ions/em: ~ --10)(10 1, j"n<:drm< -j , 100ev~ ~~::~~~ '1 T .... , .. , gs_ f.k .. l ~ .emperamre _ -, -1 1 1 i j 100 150 Depth (A) Fjg.~ 2 Depth disrri.b!!.tiol"! .of hydrogel"! (:o!"!(~entration in graphite Figure 3 shows the result indicating that generally there is good agreement between the model and data although further adjustment may be necessary. 0.035 ~ I . , I ' , , I ' , , ~ ~ .+ ... ~~·.l r::: .~ ... CII c.. 0: ~ · +/:/ 1 0:: ~ .1 100 eV H +-----.. Carbon l / z.. 0.Q1 1_ ! Temperatu re 798 [k] () t / 0.005 ~ / ~')--. experirr.ent (100 eV) tl .. Presentwork(100eV) oJ" . I 1-1 o 21016 410 16 610 16 810 16 11017 Fluence (atoms/cm 2) Fig.3 Fluence dependence of methane formation [1] YYamamura, Nuc!. Instrum. Methods B28 (1987) 17 [2] J. Roth and C. Garcia-Rosales, Nuclear Fusion voi. 36 no. 12 (1996) 1647 [3] R. Yamada, K. Nakamura, K. Sone and M. Saido, 1. NucI. ~v'rater. 95 (1980) 278 

Simulation Studies of Chemical Sputtering on Carbon Material under Hydrogen Bombardment

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