J. Plasma Fusion Res. SERIES, Vol.3 (2000) 288-292

Time Evolution of Surface Topography of Plasma Facing
Materials with Non-Uniform lmpurity Deposition
OHYA Kaoru*, KAWAKAMI Retsuo and AIZAWA Hiroshi
Faculty of Engineering, The University of Tokushima, Tokushima 770-8506, Japan(Received: 18 January 2000 / Accepted: 6 June 2000)

Abstract
The development of the surface topography and its influence on the erosion rate of plasma facing
materials are simulated. The calculations start with slight irregularities (10 nm range) of the surface andchange with time due to the dependence of the sputtering yield on ion and target species as well as on theangle of incidence. The topography of a boron surface with an initial non-uniform carbon deposition
drastically changes due to high-fluence deuterium ion irradiation. Sharp triangular cone like structuresdevelop at the beginning of the irradiation. The calculations simulate the experimental observation in
TEXTOR-94 where the initial slow erosion of a boron film was followed by an accelerated erosion andthe eventual decrease of the rate. Deposition of implanted carbon occurs more at the grooves between
cones than the top. This tends to fill the grooves.

Keywords:
plasma facing component, surface topography, boron erosion, carbon deposition, computer simulation,
simultaneous erosion/deposition

1. Introduction
For long-pulse or steady-state operation ofmagnetic fusion devices, sputter erosion leads to a

In this study, the development of the surface
topography is simulated with the initial condition of aslight non-uniform carbon deposition on boron. The aimof this paper is to discuss the changes in surface
topography and its influence on the erosion rate, due tomeasured ion fluxes of deuterium (D) and carbon (C)shortened lifetime of plasma facing components (PFC).

Deposition of impurities, which are eroded at the otherlocations and transported in a plasma, may reduce thenet erosion rate of the PFCs, but the erosion/deposition
will also modify the surface morphology, which alters
the lifetime of the PFCs. Recently, the erosion of a thinboron film (a-B:D) of -100 nm on a graphite block

impurity.

2. Model for Surface Topography
Development
A two-dimensional sinusoidal change in thicknessof carbon deposition of y = asin(x/b) on a flat boron
surface (y = -a) is assumed and the surface isunder simultaneous carbon deposition has been observedin TEXTOR-94 []. The erosion rate was not constantwith time at a fixed location; the initial thickness began
to changes with accelerating erosion rates andbombarded in the negative y-direction by an ion flux,l,as shown in Fig. 1. The incremental value, Ay, ofdisplacement of y-value at constant r on the surface
during the lapse of the time At, is evaluated to be (l/eventually with dropping rates. Furthermore, the nonuniform carbon deposition, in the range of 10-30 pm,
on a boron film was discovered in another experiment
I2l.
*C o rre sp ondin

I autho r' s e mail : ohy a@ e. tokush i ma u. a c.j p-e-288@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchOhya K' et al., Time Evolution of Surface Topography of Plasma Facing Materials with Non-Uniform

D* or Ca* ions

Impurity DeDosition

assumed to be exposed to the edge plasma in TEXTOR94. In the experiment [1], the boron surface (6 cm x 6.3cm) was declined by 20.8' to the toroidal magnetic-E0field, so that the different locations in the boron surface
corresponded to different radial distances from the lastclosed flux surface (LCFS: r = 46 cm), which ranged-10

from 47.2 cm to 49.5 cm. The total exposure time ofl2l s during 22 discharges was shared to an ohmic
heated (OH) phase of95 s and an auxiliary heated phaseof 26 s (neutral beam injection, NBI). In the simulation,
the electron temperature, 7", and the ion fluxes, f, and-20 -10010203040
x (nm)f6,, at the boron surface are average values weighted by
the exposure times in the OH and NBI phases; Ti = T" isFig. 1 Two-dimensional model for surface topography ofboron with sinusoidal deposition of carbon: y =asin(></bl, a= b= 10 nm.

assumed, where Z, is the ion temperature. The energy of
the ion bombarding the surface, C,, is approximately theN)Y(EbqLt, and the new y-value is obtained bysum of the thermal energy and the acceleration due tothe sheath potential: Ei= 2Ti + 3QT", where Q is thecharge state of the ion [6]. The values of E;, fp and f6.
decrease exponentially with increasing radial distance, r.
At each point of the surface from r = 47.3 cm to r =49.7 cm, E; for D* (Ca') ions ranges from 76 eY (213eV) to 3l eV (86 eV), whereas lo* (fcat) from 1.0 x
l0l8 cm-2s-r (8.4 x l0'5 cm-2s-') to 2.0 x lgtr
(3.4"--zs-r

subtracting the value from the original one. Here, N is

the atomic density of the target and y(E,, 0) thesputtering yield as functions of incident ion energy, d,and the angle of incidence, 0, with respect to the normal.

The angle 0 is simply evaluated from the slope of thesurface by taking sampling points along.r-axis. To avoid
non-correctable computer errors due to finite Ar and Ax(the interval of sampling points), the present simulation

x l0r5 cm-2s-r).

takes into account two correction procedures aspresented by Ishitani et al. I3l. One is a removal of any

3. Results of Calculation

overestimate of Ay, which reverses the slope of thesurface, with At. Another correction is an averaging
technique in the calculation of Ay, so that (Ly,4 + 2Ly^
+ Ly,,)14 is used instead of Ay, at the nth sampling
point. These corrections are based upon the practicalsituation that the sputtering process is dictated by acollision cascade of finite dimensions, so that it is

The physical sputtering yield of boron due to theimpact of D ions increases with the angle of incidence
4 reaches a maximum of about 2 to 3 times larger thanthe value for normal incidence (9 = 0) at about 0 = 70oand returns to an apparent zero at 0 = 85" - 90'. Points
between the extreme y values on the sinusoidal curve of
the surface, therefore. are more eroded than near themaximum and minimum points of the curve. Even if
carbon is not deposited (Fig. 2(a)), the maximum of thecurve becomes sharp in the early stage of exposure; butwith further exposure the resulting wave-like structureshrinks gradually. This development of the surface
profile results in slight non-linear change in the erosion
rate at the maximum point of the profile in the earlystage (Fig. 2(b)). The rate decreases with increasingradial distance, due to the decrease of both the ion
temperature and the ion flux.

unrealistic to consider that the apical point can remain to
be unaffected by neighboring sputtering. As a result, themotion of an initially 0 = 0 point is also dictated byneighboring points.
The angular dependence of the sputtering yield is
described according to Yamamura et al. l4l by Y(Eu 0)= f(Ei,0Xcos0)rexplf tl - (cosO)-rlsin4l with fitting
parameters/ and 4. For the impact of D ions f = 2.24and 2.53 for boron and carbon, respectively, whereas 4= 0.161 rad and 0.151 rad. For normal incidence, theIf an initial condition of a sinusoidal carbon

sputtering yield Y(E;,0) is calculated by using theBohdansky formula [5]. For simplicity, instead ofdeposition is assumed on the boron surface, the film
behavior drastically changes not only the developmentof the surface profile but also the erosion rate. This isshown in Fig. 3. At the energies of less than I keV, thesputtering yield of carbon is smaller than the yield ofboron. therefore. the surface location where thick carbon

experimentally observed a-B:D (l x 1023 B/cm3) and a-C/B:D (0.65 x 1023 C/cm3; films, a boron with aninfinite thickness and the density of crystalline boron(1.3 x 1023 B/cm3) under a very small size of depositionof pure carbon (l.l x l02r C/cm3) with a = b = l0 nm is289

Ohya K. et al., Time Evolution of Surface Topography of Plasma Facing Materials with Non-Uniform Impurity Deposition

20(a)

20:

^r=47.9cmFFE -20

E -20-40-40-60-80 -60 -40 -20-60-80 -60 -40 -20

o20406080

x (nm)

0 20 40 60 80x (nm)202000^tr^tr -20-20

E -40

E -40

I -ooi -uo

-80

-80
-1 00

(a)

r=47.9cm-100

0 20 40 60 80 100 120
Time of exposure (s)o2040 60 80 100 120

Time of exposure (s)140Fig. 2 (a) Development of sinusoidal surface of boron at
the radial distance of r = 47.9 cm from the Dlasma
center at 6 s intervals under D* ion sputtering. (b)Time evolution of its erosion at the maximum andE12of,roo
P80

minimum points of the surface at different radial

E60distances.

Fqo20deposition occurs is less eroded, whereas the carbon-free

40 60 80 100 120

Time of exposure (s)areas are more eroded. The non-uniform erosion
enhances the initial surface imperfections, like as cones,

Fig. 3 (a) Development of a boron surface with an initialat the early stage (Fig. 3(a)). As far as new carbons arecondition of a sinusoidal carbon deposit at theradial distance of r = 47.9 cm from the plasma

not deposited, with further exposure, the cones shrink inthe same way as the wave-like profile in Fig. 2(a).
Although the carbon deposition suppresses the erosion
rate of boron at the early stage of exposure, theenhancement of the surface imperfections accelerates
the rate at the side areas of the cones due to increased
angles of bombarding ions. As shown in Fig. 3(b) theinitially reduced erosion is followed by a steep increasein the rate, which is two or more times larger than thatfor the initially deposited carbon. The erosion lasts until
the rate is reduced to that at the minimum point of thesurface structure. These erosion features of boron with a
non-uniform carbon coverage are very similar to thetime evolution of a boron (a-B:D) film exposed in ancenter at 6 s intervals under D. ion sputtering. (b)Time evolution of its erosion at the maximum andminimum points of the surface at different radialdistances. (c) Observed changes in the thicknessof boron film at different radial distances inTEXTOR-g4 [11.

for boron and carbon on the angle of incidence (0 <60"), in spite of an about 10 times larger sputter yieldfor C4* ions compared to D* ions. The reflection
coefficient of C ions from carbon and boron is verysmall for normal incidence, but increases largely withoblique incidence. This may cause that the implanted Cedge plasma of TEXTOR-94 (Fig. 3(c)).

ions, which are not reflected, are deposited non-The morphological change and erosion of carbondeposited boron are little influenced by the impact of C
impurity ions out of the plasma (Fig. 4). This is due totheir small fraction (0.84-l.1Vo) of total ion flux to thesurface and the much less dependent sputtering yieldsuniformly on the sinusoidal surface. The morphologicalchange of the surface is calculated in Fig. 5(a), andtends to fill the valleys. The deposition of implanted
carbons can explain the experimentally observed net
deposition of carbon at large radial distances (r > 48.7290

Ohya K. et al., Time Evolution of Surface Topography of Plasma Facing Materials with Non-Uniform Impurity Depositioncm) in Fig. 3(c), whereas the increasing thickness at
small distance (r < 48.3 cm) cancels the sputter erosion
due to the impact of D ions (Fig. 3(b)).

Ec

4. Discussion
The dynamic change of composition due to theimplantation of carbon impurities in the surface layer
causes a strong decrease in the boron sputtering yield
and a formation of the carbon-boron mixed layer. Thiscan explain the experimental observations of a loweringof the surface erosion at the early stage of the plasma
exposure near the LCFS and a carbon deposition at thesurface location far from it. However, the non-linear
erosion rates, discussed here, were not reproduced byEc

the previous TRIDYN dynamic simulation [7].

The simulation calculation are based on an

20 40 60 80 100 't20

assumption of sinusoidal carbon coverage with the range

Time of exposure (s)of l0 nm on a boron film, which is clearly different

Fig. 4 (a) Development of a boron surface with an initialfrom the experimentally observed ranges (10-30 pm)

condition of a sinusoidal carbon deposit at theradial distance of r = 47.9 cm from the plasma[2]. Nevertheless, plasma-irradiated surfaces will feature
an atomic-scale imperfection with the macroscopic
structure superimposed. Furthermore, the assumed initialcondition leads to an simulation of the observed changeof the erosion rate with time, therefore, the validity of
our model can not be excluded. Although the physicalsputtering yield of boron due to the impacts of D ions islarger than that of carbon, the observed erosion rate ofa-B:H films in hydrogen plasma is in general smaller

center at 6 s intervals under C+ ion sputtering. (b)Time evolution of its erosion at the maximum andminimum points of the surface at different radial
distances.

than that of a-ClB:H films [8]. This indicates thedominance of chemical erosion of carbon in a-ClB:Hfilms. The use of the experimental sputter yield of theEchydrogenate films had led to an explanation of theexperimental results by using time-dependent C
concentration of the boron film [].

The cones produced by non-uniform carbon
deposition on the boron surface are unstable, i.e., they(b)develop but finally shrink after long-time irradiation
with D ions, due to the correction procedures for thecalculation mentioned above. If the angle of incidence
will be set to zero for the maximum and minimum
points of the sinusoidal surface (Fig. 1), the cones neverdisappear as calculated for an molybdenum-seeded spoton copper by Hirooka et al. [91. However, it is not clear
for real surfaces whether individual cones survive oruof
E 40fc20fof201
020 40 60 80 100 't20
Time of exposure (s)new cones appear to replace disappearing cones duringcontinuous carbon deposition.
If the deposited impurities are sufficiently mobile,
they tend to agglomerate u0l. The surface mobility andcarbon agglomeration is perhaps one process to form
growing islands on the surface, although theFig. 5 (a) Development of a boron surface with an initialcondition of a sinusoidal carbon deposit at theradial distance of r = 49.1 cm from the plasma
center at 6 s intervals under C deoosition. (b)Time evolution of its deposition at the maximumand minimum points of the surface at different
radial distances.

29r

Ohya K. et al., "fime Evolution of Surface Topography of Plasma Facing Materials with Non-Uniform Impurity Deposition

agglomerates in the range of 10-30 nm were notReferencesP. Wienhold et al., J. Nucl. Mater.24l-243,8O4observed after long-time irradiation. A secondlll

possibility for carbon transport across the surface might
be the emission and direct transport in the edge plasma,followed by the re-deposition.

[2] P. Wienhold et al., J. Nucl. Mater. 266-169,986(1997).
(1999).

[3] T. Ishitanr et al., J. Mater. Sci. 9, 505 (1974).
t4l Y. Yamamura et al., Rep. Inst. Plasma Phys.,
Nagoya Univ., IPPJ-AM-26 ( 1983).
[5] J. Bohdansky, Nucl. Instrum. Meth. Phys. Res. B 2,

5. Gonclusions
Surface topography of a boron surface with nonuniform carbon deposition drastically changes due tohigh-fluence D ion irradiation. A sharp triangularstructure like as a cone is developed at the early stage.

s87 (1994).

[6] G.M. McCracken et al., Fusion Technology 1988

As long as new carbons are not deposited, however, it
finally shrinks after long-time irradiation. The
calculations reproduce the experimental observation in
TEXTOR-94 that the initial slow decreases of boron
were followed by large drop and the eventual decreasein the erosion rate. Erosion by C ion impact causesmuch less change of the topography, whereas thedeposition of implanted carbon occurs more in the(Elsevier, Amsterdam, 1989) p. 15l.

t7l K. Ohya and N. Saji, J. Plasma and Fusion Res. 75,416 (1999).

t8l A. Annen et a/., J. Nucl. Mater., 231, l5l (1996).
t9l Y. Hirooka et a/., Nucl. Instrum. Meth. Phys. Res.
B 23,4s8 (1987).
[10] O. Auciello, Radiat. Eff. 60, | (1982).

grooves between cones than the top.

292