J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Molecular Dynamics Study of Plasma Surface Interactions for Mixed
Materials
Kaoru OHYA1, Naohide MOHARA1, Kensuke INAI1, Atsushi ITO2, Hiroaki NAKAMURA2,
Yoshio UEDA3 and Tetsuo TANABE4
1

Institute of Technology and Science, The University of Tokushima, Tokushima 770-8506, Japan2

Department of Simulation Science, National Institute for Fusion Science, Toki 509-5292, Japan
3

Graduate School of Engineering, Osaka University, Suita 565-0871, Japan

4

Interdisciplinary Graduate School of Engineering Science, Kyushu University, Fukuoka 812-8581, Japan(Received: 30 October 2009 / Accepted: 26 February 2010)

The interaction of low-energy (1-100 eV) H atoms with co-deposited carbon layer on plasma facing materials isinvestigated by means of molecular dynamics. The atomic ratio of H and Be in the layer and the amount of W deposited on
the layer are independently varied. Although the interaction layer of H atoms becomes deep with increasing impact energy,
dominant interaction occurs in the H- and Be-implanted layers. W atoms on the layer tend to agglomerate so that H atoms
appear to have penetrated into the underlying layer. No H uptake to amorphous C leads to C sputtering as being physicalsputtering but no emission of hydrocarbon (CHy). The H uptake produces C sputtering at energies much less than thethreshold energy for physical sputtering, as well as dominant CHy sputtering. The Be, implanted with several % in the C layer,
clearly reduces C and CHy sputtering, whereas emission of Be is found at energies less than the sputtering threshold. Such
suppression effect for C and CHy sputtering is also found for the W deposition on hydrogenated amorphous C. However,rather weaker suppression of the sputtering is resulted from non-uniform deposition of W on the surface.

Keywords: Plasma wall interaction, material mixing, Be-C mixing, W deposition, chemical sputtering, physicalsputtering, hydrocarbon, molecular dynamics simulation.

chemical effects. Chemical sputtering can be reduced by
impurities on the surface and in the bulk [4]. Beryllium
used for the first wall of ITER is subjected to strong
physical sputtering and eroded Be atoms migrate towards
the divertor region. Therefore, in addition to the mutualcontamination between C and W in the divertor, apronounced Be deposition and formation of Be-C (andalso Be-W) mixed layers can strongly reduce thechemical sputtering of the divertor target (C), as observedin recent PISCES-B experiments for Be-C system [5].
This work has focused on the sputtering for Be-implanted
and W-deposited layers formed on an hydrogenatedamorphous C (a-C:H) surface due to the bombardmentwith 1-100 eV H atoms.

1. Introduction

Material mixing has attracted great interest since
ITER has beryllium (Be), carbon (C), and tungsten (W)as the first wall, the divertor target, and the divertor dome
and wing, respectively. It drastically changes plasma
surface interactions (PSI) of the underlying substratematerial. We extensively studied the PSIs of W-C mixed
layers formed on a W/C twin test limiter exposed to
TEXTOR edge plasmas by using a dynamic Monte Carlocode, EDDY, in which the binary collision approximation(BCA) is used [1,2]. It has been seen that the W-C mixed
layers on W suppressed physical sputtering of W, whereas
these layers on C strongly suppressed chemical sputteringof C. Characterization of the erosion and deposition of
the mixed layers requires more dedicated studies basedon physical and chemical properties of the materials. We
have developed a molecular dynamics (MD) code forcollisions of hydrogen isotopes and eroded impurities
with fusion related materials including hydrogenatedamorphous C and the W-C mixed layer on W [3].
A critical issue with C divertor targets is strong
sputtering due to the bombardment with high-fluxhydrogen (H) isotopes, which is dominated by the

2. Simulation models

2.1 Preparation of Be-C mixed and W-deposited layer
As found in the present fusion devices, the C surface
is modified as being amorphized and hydrogenated due tothe bombardment with high-flux H ions. An a-C:H
material with different atomic ratio of H to (C+H) is
prepared as initial surfaces by the following MD
technique. A crystal W (001) cell consisting 4000 atoms

author’s e-mail: ohya@ee.tokushima-u.ac.jp497©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research15
10

Be fluenceC(1014 cm-2)
0
1
2
5
10
20

5

Be fluence

64(1014 cm-2)
0
1
2
5
10
20

H

2
0
3

Be fluence(1014 cm-2)
1
2
5
10
20

2

Be
1

0
-2.0 -1.5 -1.0 -0.50Depth (nm)

0.5

1.0

15

1.5

between -0.43 nm and 0.14 nm in depth with increasing
number of both H and Be atoms up to 200, under thecondition that both C and H depth profiles are unchanged
as demonstrated in [11]; also shown in Fig. 1(a). The
original surface before the bombardment corresponds tozero-depth, whereas the positive and negative depths
correspond to net erosion and net deposition, respectively.
On the other hand, the simultaneous bombardment with H
and W atoms of the same energy (1 eV) as each otherforms a W deposition layer on the a-C:H materials. As
shown in Fig. 1(b), the average atomic ratio,
W/(C+H+W), is rather largely changed with increasing
number of W atoms. The W atomic ratio, W/(C+H+W),
is estimated to be 26% on average between -0.71 nm and-0.14 nm in depth for 200 bombardments with both H and
W atoms. At further less depth (<-1.0 nm), there isneither C nor H atom where the surface is partly coveredby agglomerated W atoms (not shown here).

(b)
W fluence10

H areal density (1014 cm-2)

0
8

20

W areal density (1014 cm-2)

Be areal density (1014 cm-2)(a)

C areal density (1014 cm-2)

20

H areal density (1014 cm-2)

C areal density (1014 cm-2)

K. Ohya et al., Molecular Dynamics Study of Plasma Surface Interactions for Mixed Materials

C(1014 cm-2)
0
2
5
10
20

5
0
8
6

W fluence(1014 cm-2)

4

0
2
5
10
20

H

2
0
3

2

W fluenceW(1014 cm-2)
2
5
10
20

1

0
-2.0 -1.5 -1.0 -0.50Depth (nm)

0.5

1.0

2.2 Interaction of H atoms with the mixed materials
Noncumulative bombardment of the above-preparedsurface with 1000 H atoms with the energies of 1 eV to
100 eV is performed where the same initial surface isused for each bombardment. The incident atoms hit the
top surface at random positions. The incident polar angle
is measured from the surface normal, typically 45o,
whereas the azimuthal angle is randomly selected from 0oand 180o. For each incidence, the trajectories of
constituent atoms are followed up to 1 ps by the same
MD technique used for the preparation of the mixed
surface. Figure 2 shows the probability distributions thata projectile H atom travels at a depth of amorphous
carbon, hydrogenated amorphous carbon, and Be-mixed
and W-deposited carbon layers. The interaction layer of H
atoms becomes deeper with increasing H energy.
Nevertheless, dominant interaction occurs in the H- andBe-implanted layer within 1~2 nm. For W-deposited layer,
due to the agglomeration, the surface is partly covered by
W atoms so that H atoms appear to have penetrated
across the layer. Anyway, the W-deposited layer generally
tends to prevent the projectile H from penetrating into
deeper region of the underlying layer.
Atoms and molecules, which pass through a virtualsurface at the distance of 2 nm apart above the initial
surface (i.e., -2 nm in depth), are counted as sputteredparticles, so that the total yield and the distribution of
sputtered species are determined by repetitive impacts.

1.5

Fig. 1. Depth profiles of C, H and Be (W) in ahydrogenated amorphous C layer bombarded with(a) Be and (b) W atoms at different fluences.
in 10×10×20 bcc unit cells is bombarded with 2000 C
atoms with the energy of 10 eV at random positions of
the top surface, so that an amorphized C layer with the
thickness of more than 2 nm is formed on W. The
hydrogen uptake in the amorphized C layer is followedby simultaneous bombardment with 1000 C atoms and1000 H atoms. The H impact energy is changed between
1 eV and 30 eV so as to change the H/(C+H) ratio,
whereas the C energy is kept to be 0.025 eV. Such impact
energies allow us to produce hydrogen-rich surfaces on
the amorphous C layer [6]. The H/(C+H) ratio varies
from 0.30 to 0.06 on average at 0.6 nm intervals with theimpact energies from 1 eV to 30 eV. In MD, theinteraction potential, the derivative of which is a force oneach atom, is a key parameter for the integration of
Newtonian equation of motion. For the ternary system
W-C-H, an analytic bond-order potential obtained by
Juslin et al [7] is combined with a Brenner’s hydrocarbon
potential. Detailed procedures of the MD calculation arepresented in [3].
For Be-C system, a modified Morse potential withfitted parameters of Patrick et al. [8] is used, in addition
to the Ueda potentials of Be-Be and Be-H systems [9,10].
Small amount of Be atoms is implanted in the a-C:H
materials by means of simultaneous bombardment with 1
eV H and 10 eV Be atoms. At such energies, the H atoms
are made up for the desorption (or sputtering) whenbombarding the surface, whereas the Be atoms are
implanted into the materials. The Be atomic ratio,
Be/(C+H+Be), is increased to be 11% on average

3. Results and discussion

3.1 C and CHy sputtering from a-C:H layers
In Fig. 3, no H uptake to amorphized C leads to noemission of hydrocarbons, as demonstrated in [10],except for the smallest one (CH) where the projectile H
bring a C atom away when escaping from the surface. C

498

EH

0.10
0.05

1eV
10eV
100eV

10

C

5

0
-2.0 -1.5 -1.0 -0.50Depth (nm)

0.20

(c)

1.0

0.5

0
1.5
4

Be-mixed in a-C:H

0.15

EH

0.10

1eV
10eV
100eV

3
2

Be

1

0.05
0
-2.0 -1.5 -1.0 -0.50Depth (nm)

0.5

1.0

0
1.5

(b)

0.15

a-C:H
EH
1eV
10eV
100eV

0.10

6
4

H
2

0.05
0
-2.0 -1.5 -1.0 -0.50Depth (nm)

0.20

1.0

W-depositedon a-C:H

(d)

0.15

0.5

EH

1eV
10eV
100eV

0.10

W

0.05
0
-2.0 -1.5 -1.0 -0.50Depth (nm)

0.5

0
1.5
4
3
2
1

1.0

H areal density (1014 cm-2)

15

8

0.20

0
1.5

W areal density (1014 cm-2)

a-C

Probability of traveling at a depth Probability of traveling at a depth

(a)

0.15

C areal density (1014 cm-2)

20

0.20

Be areal density (1014 cm-2)

Probability of traveling at a depth Probability of traveling at a depth

K. Ohya et al., Molecular Dynamics Study of Plasma Surface Interactions for Mixed Materials

Fig. 2. Probability distributions that a projectile (H) travels at a depth of (a)amorphous C layer, (b) hydrogenated amorphous C layer, (c) Be-implanted layer and(d) W-deposited layer.
0.1

(a)

H/(C+H)
0
0.06
0.12
0.30

0.01

0.001
1

H/(C+H)

CHy sputtering yieldC sputtering yield

0.1

0
0.06

0.12
0.30

(b)

0.01

10

Incident H energy (eV)

100

0.001
1

10

Incident H energy (eV)100Fig. 3. Energy dependence of (a) C and (b) CHy sputtering yields of hydrogenatedamorphous C layer with different H contents. Solid symbols are the corresponding C
sputtering yields calculated by EDDY.

Partial sputtering yield

0.1

(a) C

a-C

Incident angle

Incident angle

0deg.
45deg.
60deg.

0.01(c) CHy

a-C:H

(b) C

a-C:H

0deg.
45deg.
60deg.

Incident angle
0deg.
45deg.
60deg.

0.001
1

10

Incident H energy (eV)

100 1

10

Incident H energy (eV)

1001

10

Incident H energy (eV)Fig. 4. (a) C sputtering yield of amorphous C, and (b) C and (c) CHy sputtering yields of hydrogenatedamorphous C layer with H/(C+H)=0.3 for different angles of incidence. Solid symbols are thecorresponding C sputtering yields calculated EDDY.

499

100

K. Ohya et al., Molecular Dynamics Study of Plasma Surface Interactions for Mixed Materials

Partial sputtering yield

0.1

(a) C

Be/(C+H+Be)
0.01

Be/(C+H+Be)(b) BeBe/(C+H+Be)

0
0.01
0.02

0.01
0.02
0.04
0.06
0.11

0
0.01
0.02
0.04
0.06
0.11

0.04
0.06
0.11

(c)
CHy

0.001
1

10

Incident H energy (eV)

100 1

10

Incident H energy (eV)

100 1

10

Incident H energy (eV)100Fig. 5. Energy dependence of (a) C, (b) Be and (c) CHy sputtering yields of Be-implanted hydrogenatedamorphous C layer bombarded by H atoms. Solid symbols are the corresponding C and Be sputtering
yields calculated by EDDY.
threshold energy, due to scattering of calculated values
and small amount of sputtered atoms. At such low
energies, the increase in the sputtering yield with obliqueangles should be suppressed since the projectile H cannot
penetrate into the bulk, and then it interacts only withnear-surface atoms. Molecular sputtering may givesomewhat different dependence due to chemical effects,
such as the C-C bond breaking and C-H bonding.
Nevertheless, any systematic dependence is not found atpresent in Fig. 4(c). More dedicated studies including
different interaction potentials and simulation techniques
are under consideration.

sputtering is explained by the binary collision
approximation (BCA) code, such as EDDY. However, theH uptake to the amorphized C produces C sputtering at
the energies less than the threshold energy for physicalsputtering which the BCA codes expect. Also,hydrocarbon emission appears where larger molecules
tend to be emitted with increasing H content [11]. In the
figure, all hydrocarbons CHy (y=1-4) are counted. Theenergy dependence is rather weaker than the atomic C
sputtering, due to dominant chemical bonding (andbreaking) mechanisms, not so dependent on atomic
collisions such as collision cascade.
In BCA codes, dominant mechanism of the incident
angle dependence is originated from the reduction in thepenetration depth of the projectiles due to the oblique
incidence, which causes recoiling atoms to be easy toemit from the surface. In Fig. 4(a), the C sputtering from
amorphous C shows similar dependence to that obtainedby EDDY with the incident angles (but, not so grazing
angles) near the threshold energy for physical sputtering,
although the yield is somewhat larger than it. The
incident angle is measured from the normal to the planeat zero-depth (Fig. 2). One possible reason for thediscrepancy between the yields is the surface roughnessof the simulated surface, which is clearly observed in the
MD surface, but not taken into account in EDDY where aperfectly flat surface is assumed. In general, the surface
roughness increases the sputtering yield due to anincrease in the yield as a function of the angle ofincidence of projectiles (local inclination of the surface
varies). With grazing incidence to the surface, thesputtering yield is deceased again due to escaping justafter the incidence without interacting with many atoms.
Again, as shown in Fig. 4(b), the C sputtering yieldfrom a hydrogenated amorphous C with the H atomic
ratio, H/(C+H), of 0.3 increases with oblique angles ofincidences at energies of more than the physical thresholdenergy. However, it does not show clear dependence
on the incident angle at the energies of much less than the

3.2 C, Be and CHy sputtering from Be-C mixed layers
In EDDY calculations, the C sputtering yield of C
materials is not clearly influenced by the inclusion ofsmall amount of Be atoms, such as the Be atomic ratio of
11% or less. However, the low-energy sputtering of ahydrogenated amorphous C calculated by MD below thephysical sputtering threshold shows a clear suppression
effect of Be implantation, as shown in Fig. 5. With
increasing Be concentration, C sputtering disappears at
low energies, so that there is only physical sputtering. On
the other hand, by mixing of small amount of Be(Be/(C+H+Be)=0.1), additional Be sputtering appears at
rather high impact energy. With increasing Be
concentration, Be atoms are more sputtered in the sametendency as EDDY calculations. But MD allows Be to be
sputtered from the Be-C mixed surface even at the impact
energies of much less than the threshold energy forphysical sputtering; a small part of sputtered particles ismolecules including Be, C and H. Such low-energy (<10eV) sputtering of Be due to the bombardment withhydrogen isotope (i.e., D) was obtained very recently byother MD calculation and also experiments [12]. This isdue to the formation of BeD molecules in the vicinity ofthe surface. For our simulation, the mechanism is not soclear at present, but one explanation may be caused byweak bonding of Be atoms deposited on a hydrogenated500

K. Ohya et al., Molecular Dynamics Study of Plasma Surface Interactions for Mixed Materials

Partial sputtering yield

0.1

(a) C

(b) CHy

W/(C+H+W)
0.01

0.001
1

0
0.03
0.13
0.16
0.26

W/(C+H+W)
0
0.03
0.13
0.16
0.26

10

Incident H energy (eV)Partial sputtering yield

1

100 1

(c) C

10

Incident H energy (eV)

100

H-C
H - WCC-C
C - WC

0.1

0.01

0.001

1

10

Incident H energy (eV)100Fig. 6. Energy dependence of (a) C and (b) CHy sputtering yields of W-deposited hydrogenated amorphous C
layer bombarded by H atoms, and of (c) C sputtering yield of amorphous C and W-C mixed layer with the
C/W atomic ratio of ~0.4 bombarded by H and C atoms. Solid symbols are the corresponding C sputtering
yields calculated by EDDY.
amorphous carbon, which corresponds to the reduction in
the surface and bulk binding energies for BCA. The CHy
sputtering decreases as well due to the formation of the
Be-C mixed layer, but the decrease in the yield is rather
gradually and it does not disappear at low energy.
The reason for the reduction of CHy sputtering was
discussed in [11] with our modeled potentials by longer
attractive interaction range of Be-C system than that for
C-H system, as well as the longer dimmer bond distance.
However, the modified Morse potential used for
simulation contains some free parameters. Therefore,
additional simulation calculations have to be performed
to study the interplay of the parameters in the potentials.
Furthermore, very recently, the Brenner’s hydrocarbon
potentials was combined with new parameter sets of
Be-C and Be-Be interactions by Björjas et al. [13], in
addition to the W-W, W-C and W-H interactions [7].
More MD calculations are now being performed with thenew potentials, which will be presented elsewhere.

C atoms with deposited W enhances the C sputtering
because the collision cascade of C atoms is localized in
the shallow depth due to their large-angle scatterings
from W atoms. However, in MD, such effects are notfound at such W/(C+H+W) ratios, probably due toscattering of the calculated results.
In Fig. 6(b), the CHy sputtering is also suppressed
but more slowly with increasing the W atomic ratio,
W/(C+H+W), near the surface of the hydrogenatedamorphous C. For the prepared W-deposited surface, W
atoms on the surface were not distributed uniformly, butthey were mobile and tend to agglomerate. Therefore, therather weaker suppression of the CHy sputtering can be
resulted from the non-uniform deposition of W on thesurface, a part of which was not covered by W. In EDDY
calculation, W atoms are not sputtered from theW-deposited surface due to the much higher thresholdenergy for physical sputtering. Also in MD, very few W
particles are sputtered at such low-energy range, but some
W molecules including C and H can be seen at the case ofW/(C+H+W)=0.16.
Figure 6(c) compares C sputtering yields between
amorphous C and W-C mixed materials for the impacts of
H and C atoms. The W-C mixed material with the
thickness of more than 1 nm and the C/W atomic ratio of
about 0.4 was prepared by the cumulative bombardment

3.3 C and CHy sputtering from W-C mixed layers
Such suppression effect of impurities on C and CHy
sputtering of hydrogenated amorphous C is also found for
the W deposition in Figs. 6(a) and 6(b), respectively. The
W deposition suppresses C sputtering in the same manner
as for Be deposition (Fig. 5(a)). In EDDY, the mixing of501

K. Ohya et al., Molecular Dynamics Study of Plasma Surface Interactions for Mixed Materials

of a crystalline W cell with C atoms at the impact energyof 100 eV [3]. Although no H uptake to the amorphous C
causes only physical sputtering, the mixing of W
enhances C sputtering with a lowering of the thresholdenergy. The reason for the enhanced sputtering is that the
projectile H and C atoms are suppressed to penetrate into
the deep region of the material, by which they give theenergy enough to emit in the vacuum to more material
atoms collided near the surface. The mixing effect of C
with W was obtained as well by EDDY [2]. Nevertheless,the bombardment of W-C mixed layer with C atoms
causes the C sputtering at energies much less than thephysical sputtering threshold (~49 eV), where
non-negligible amount of the dimmers are sputtered fromthe surface.

4. Concluding remarks

No H uptake to amorphous C led to carbon sputtering
as being physical sputtering and no emission ofhydrocarbons. The H uptake produced C sputtering at
energies much less than the threshold energy for physicalsputtering, as well as dominant CHy sputtering. At such
low energies, both C and CHy sputtering show rather weak
dependence on the incident angle. The Be, implanted withseveral % in the hydrogenated amorphous C layer, clearly
reduced C and CHy sputtering, whereas emission of Be, as
well as C, was found at energies less than the sputtering
threshold. Such suppression effect for C and CHy
sputtering was also found for the W deposition onhydrogenated amorphous C. However, rather weakersuppression of the sputtering was resulted from
non-uniform deposition of W on the surface.

Acknowledgement

This work was supported by KAKENHI(190055005).

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