J. Plasma Fusion Res. SERIES, Vol.3 (2000) 297-302

carbon Transport in TExroR-94 Aspects of Long Term
Operation
WIENHOLD Peter', HILDEBRANDT DieteI, KIRSCHNER Andreasl,
MAYER Matejr and RUBEL Marek3
I

Institut filr Plasmaphy sik, Forschungszentrum Jiilich GmbH, As sociation EIJ RAT1M,

Trilateral Euregio Cluster (TEC), D-52425 liilich, Germany
2Max-Planck-Institut

fiir Plasmaphysik, EUMT)M Association, D-10il7 Berlin, Germany

3AyttAn Laboratory,

Royal Institute of Technology, S-10044 Stockholm, Sweden(Received: 18 January 2000 / Accepted: 19 May 2000)

Abstract
The paper describes an experiment carried out in TEXTOR-94 where l3C containing methane waspuffed into the discharge through a small hole in a graphite sample. The local deposition of r3C could be
distinguished from the global transport of 12C in the machine by SIMS depth profiling analysis. The r3C
travels several cm and becomes deposited on the sample surface with low efficiency (0.2Vo) but withratiosl3C/C =0.42. The co-deposited amounts of H and D were measured by NRA and ERDA. The
results can be simulated quantitatively by ERO-TEXTOR code calculations if very high re-erosion isassumed for the deposit formed initially by the CH^+ radicals. The majority of the puffed l3C travels tothe plasma facing graphite limiters and seems to create a stationary r3C/C ratio at the surface after the
total plasma exposure time of 108 s. If the value r3C/C = 0.27 found on a collector probe is representative
for the whole surface it would correspond to an interaction depth of about 27 nm which is involved into
the carbon interchange. Since plasma facing surfaces are net erosion areas the carbon transport finallyends on the side walls of the poloidal limiters and on other obstacles in TEXTOR.

Keywords:
carbon transport, l3CHo injection, deposition efficiency, simulation calculations, TEXTOR-94, long term
exposure

1. lntroductiondifficult to access. Deposited layers consist of mixtures

Steady state operation of nuclear fusion devices is
one of the next challenging goals [1], and machines like
LHD, W7X or ITER are already planned for long pulse
operation. Beside major concerns as e.g. the gas fuelling
cycle and the heat removal, the transport of impurities
will become a serious problem. They will continuouslyof all elements present in the machine [2]. These aremainly carbon, but also boron, tungsten, beryllium, steel
components and oxygen. Moreover, tritium will be
accumulated in these layers by co-deposition in
unacceptable amounts.

Erosion, transport and re-deposition of carbon havebeen studied in several machines, e.g. TEXTOR-94 t3l,

be eroded from plasma facing wall components byparticle impact and deposited at other locations. This
will reduce the lifetime of the components and lead tounwanted material accumulation and flaking at places

ASDEX-U [4] or JET [5]. Flakes with thicknesses in therange 10-100 pm are observed in the divertor,@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corresponding author's e-mail: p.wienhold@fz-juelich.de297

wienhold P. et al., carbon Transport in TEXTOR-94 Aspects of Long Term operation

heating (total exposure time 107.7 s). The surface of theblock was covered with an aluminum target plate (108 x
70 x 3 mm3;. Because of the inclination by 20" intoroidal direction the target reached a radial distance ofespecially in shadowed areas, but also on components inthe main chamber near the LCFS on plasma facing
surfaces of limiters, and on areas deep in the shadow
perpendicular to the magnetic field [6]. The balance

between carbon erosion and deposition is highly
sensitive to the circumstances of the exposure. The

50 cm from the plasma center (fig.1).

exposure time and operation conditions are as decisive
important as the transport length, the shape of thecomponents and possibly the surface morphology [7], orthe presence of other plasma impurities (metals,

gas puffing. In the following 18 discharges puffing
began at I s with a calibrated amount of 3.8 mbar'lr3CHr-gas (corresponding to 9.2 x lOre molecules)

oxygen).
Because plasma facing components made fromgraphite or CFC will still be used in the machines, theemphasis of this paper is put on the investigation of thetransport of carbon as the major impurity. In thein poloidal direction. Both intensities reached a

The first two discharges were carried out without

through a 1.7 mm wide hole, which ended at r = 47.3
cm. The CH and CII light emitted between the surfaceof the sample and the plasma edge region was observed
maximum at 1.26 s after discharge begin and levelled
off after 2 s. The CH light emission was brightest at thefollowing an experiment will be described where llC

LCFS (r = 46 cm), while the maximum of the CII
radiation appeared about lcm deeper in the plasma.
A part of the r3C penetrated into the plasma andcontaining methane (''CHn) has been puffed through ahole of a wall element in order to distinguish the local
transport and deposition of the r3C from the r2C

increased the CV signal by abottt33Vo. The rest became
12C

deposition out of the background. Of special interest
were the l3C deposition efficiency and the simulation
calculations by means of the ERO-TEXTOR code [8]taking into account the local plasma conditions. The
influence of the exposure time has been investigatedadditionally by means of long term exposed samples.

re-deposited on the target plate together with thecarried by the SOL plasma. Because hydrogen was codeposited, a transparent film was formed which showed
thickness dependent interference colours in illuminating

2. Puffing Experiment

pattern assigned by the colour fringes. The fringe shift
after each of the puffs yielded the incremental net

light.

This made in-situ colorimetry [0i possible, i.e.
the two-dimensional determination of the thickness

Because a part of the observations has already beenpresented elsewhere [9], only a brief description will bedeposition and, hence, the deposition rates. Nuclear

given. For the experiment, a graphite block (115 x 75 x
70 mm3) was moved from top of TEXTOR-94 andpositioned in the scrape-off layer (SOL) with its outerend at the last closed flux surface (LCFS, r = 46 cm). It
was kept there for 20 discharges (n" = 2.5 x l0r3 cm-3, 1oreaction analysis (NRA), secondary ion mass
spectroscopy (SIMS) and elastic recoil detection= 350 kA, Br = 2.25 T) in deuterium without auxiliary

deposited deuterium and hydrogen.

analysis (ERDA) were applied after dismounting of thesample in order to determine space and depth
distributions of the carbon isotopes and of the coAdditionally, an aluminium sample was mounted
perpendicular to the toroidal direction on the graphite
block on the ion drift side well in the shadow of thepuffed molecular l3CHo cloud. The deposits found here

lcml

LCFS

46.O

were formed therefore mainly by the impurities
travelling in the SOL. The long term samples were

47.3

I'c*+
graphite blockllSmm
toroidal dlrection

mounted on the Stockholm-TEXTOR collector probe
system and exposed there for several weeks up to about
6000 s of plasma exposure time.

50.o

3. Deposition Pattern and Efficiency

53.O

3.1 Colorimetric analysis
Fig. 2 gives an black and white representation of

3.8mbarlthe interference colours visible on the target plate after
the end of the puffing experiment. The different orders

Fig.1 Schematic of the puffing experiment.
298wienhold P. et al., carbon Transport in TEXToR-94 Aspects of Long Term operation
RAOIAL DISTAT{CE FFOM PLASMA C€NTER TcmI__r4,4t

Fig. 3 t'?C and 13C thicknesses as measured by SIMS
depth profiling along line 3 in ligure 2.

Fig. 2 Top view of the target after 18 puffs and the total

exposure time of 107.7 s.

ions in connection with SIMS analysis was appliedalong line 3 (fig. 2) at 13 equidistant locations. This
technique yields the total thickness of the deposit.
Moreover, the ratio t3clt2c within the layer can be
determined and hence, the contributions of the two
isotopes separately. For example, the l3C contributes

of interference which can be recognised as dark stripes
can be counted up to the 3'd, which corresponds to the
maximum thickness of about 300 nm (circle).
Note, that because of the inclination of the plateeach toroidal location corresponds to another radial
distance from the plasma center. In the left part ('plasmafar') the deposition is mainly due to the r2C in the SOL.
It causes contour lines of constant thickness which showwith a thickness of about 100 nm at a distance between
2 and 4 cm from the blowing hole, (black columns in
fig. 3), while the r2C deposition (hatched columns) is
about 200 nm. The sum of the two (open columns) fully
agrees with the total thicknesses determined

little variation in poloidal direction. The thickness
mainly increases in direction to the plasma near side(right) due to the increasing background carbon flux.

independently by colorimetry.
The r3C distribution shows, that r3C has been

Beyond the puffing hole the contours are bent back due
to the additional l3C deposition, resulting in a 'butterfly'
like pattern. This area of additional r3C deposition isshifted poloidally by about 6 mm (in direction to line 2)due to the E x B forces and is extended over about 8.7transported several cm across the declined surface intoroidal direction towards the plasma near end of ther3cl(r3c +r2C; istarget plate. The concentration r3C/C-

not constant, however. It reaches a value of O.42 near
the blowing hole, but decreases to = 0.25 withincreasing distance. This might indicate the limited local
transport of carbon. The slight decrease beyond thedeposition maximum at the radial distance r = 46.5 cm
is likely due to the increasing erosion by D* ions.
Deuterium is co-deposited with ratios D/C = 0.2
rather independent on distance as found by NRA.
Additionally, hydrogen from the injected molecules isco-deposited up to ratios HIC = 0.45 in the deposition
maximum as revealed by ERDA (not shown in the
figure). Both values drop to = 0.1 adjacent the end of
the target plate (r = 46 cm) due to the temperature

cm2. Other areas (e.g. along line l) are almost
unaffected by the r3C deposition, and the deposition ismainly due to the carbon flux in the SOL.

The differences of the thickness profiles along
these two lines yield a rough estimate of the l3C
contribution alone. This has been done by colorimetry
after each puff. The averaged incremental growth by l3C
deposition within the involved area is about 3.3 nm per
puff. This corresponds to the total number of 1.9 x l0l7
r3C atoms if a density of 6.5 x lO22 cm-3 is taken into
account. Compared to the 9.2 x lOre molecules injected
this is a deposition efficiency of only 0.2Vo.The scatter
was found tobe+O.l%o. Low efficiencies (few Vo)haveI l].

excursions. This value might therefore indicate an upper
limit for the possible hydrogen uptake from air.

3.2 SIMS, NRA and ERDA
Depth profiling by sputtering with 3 keV oxygen

3.3 Simulation calculations
The surprisingly low deposition efficiency of lessalso been observed earlier299wienhold P. et al., carbon Transport in TEXTOR-94 Aspects of Long Term Operation

than l%o seems to contradict to the spectroscopic
observation that the maximum of the CH light emission
occurs between the target and the LCFS. Since most of
the r3CH4 molecules have to break up and become
ionised within that small gap in order to reach the CH
stage, a high fraction of them must travel along themagnetic field lines towards the declined target plateand should become deposited there. This is obviously
not the case and points to the direction that the reerosion probability of the CH* radicals is very high. This
is not unlikely because of the high hydrogen content.

In order to prove this idea simulation calculations
with the ERO-TEXTOR Monte-Carlo code [12] havebeen performed. This code models three-dimensionallyFig. 4 13C deposition pattern as calculated with the EROTEXTOR code for the case of 'high re-erosion'.

the local transport and re-deposition of eroded

Obviously, the high sticking probability assumed is
responsible for the dramatic 13C deposition. Varying
other input parameters, such as density, puffing rate,
sputter yields, ionisation probabilities, did not much
lower the deposition efficiency or yielded results far off

impurities under the influence of a background plasma,taking magnetic and electric field forces, friction, crossfield diffusion, and thermal forces into account. Major
input parameters were the measured electron density andthe temperatures of electrons and ions at the LCFS andtheir decay lengths. These values result in an ion flux
(fo* = 1.33 1gte..-2t-l at the LCFS, io = 17 mm)"
which hits the declined surface and liberates CDa or C
by chemical and physical erosion with the according
yields. The fraction 0.0361p* = fs4* was assumed as l2C
impurity flux. The injection of r3CHo gas which lasted

the experimental findings. An optimum agreement was
found. however. if a direct erosion was assumed for thedeposited r3CH"* radicals. This can mean, that the initialradicals (mainly CHa* and CH3*) cany too little energy
to form a stable deposit when hitting the surface in a

first attempt. It can also mean that the surplus ofhydrogen causes the high re-erosion. Drasticallyroughly ls per plasma discharge was replaced by aincreased erosion by hydrogen is also observed when a-

constant inlet rate of 1.5 x lOre s-r of thermal molecules.

C:H films are formed in glow discharges with self-bias
voltages below 200 eV [3]. The case of 'high re-

All hydrocarbon molecules break up, become

erosion' was simulated in the code by setting thereflection coefficient to the extrem€ (Rro, = l). Thisionised and travel either to the surface of the target oraway. It is assumed that only ions can stick and change
the ratio t3C/t2C within a certain interaction depth of 40
nm. Neutral hydrocarbons do not contribute to theimmediately reduced the efficiency to the much lower
value of 2.5Vo, because the remaining deposition ismainly due to carbon ions. The r3C concentration islowered to 0.5. Moreover, the deposition maximum isshifted by about 3 cm towards the end of the target
plate. The calculated 13C pattern (fig. ) fully agrees
with the experimentally observed deposition area. In
addition, the calculated distributions for the CH and CII
light emission agree well with the measured ones within

deposition. Outputs of the code are e.g. the time
evolution of the twodimensional pattern of r3C and l2C,or the light emission of the radicals around the blowing

hole, which both can be compared directly withexperimental observations.
Usually, 507o of the ionised radicals hitting thesurface are assumed to stick. while the rest leaves it in
the form of r3CHo. This is described by a 'reflection
coefficient' Ri,n = 0.5 in the code. This assumption
leads, however, to insuperable disagreements with theexperimental results: The efficiency for l3C deposition
reaches 407o in this case and causes thicknesses of 80160 pm directly adjacent the blowing hole while in theexperiment a slight maximum of 300 nm was foundseveral cm away. The concentration r3ClC approaches
even unity in the calculated deposition maximum, again
in contradiction to the experiment (0.42).

the statistical errors.

According to the deposition efficiency of 2.5Vo the
maximum thickness is 1.4-2.3 pm. This is still 4-7
times too high. Possibly, erosion by oxygen impurity
ions or by hydrogen released from the hydrocarbons,
which were not taken into account, decrease the film
thickness further in the experiment.

4. Global Carbon Transport in TEXTOR-94
More than 99Vo (1.66 x l02r molecules) of the300

Wienhold P. et al., Carbon Transport in TEXTOR-94 Aspects of Long Term Operation

introduced r3C is deposited at other locations inside themachine, most likely on the graphite tiles of the toroidal
belt limiter ALTII (3.4 m2;. This amount corresponds toa thickness of 7.5 nm and should create a ratio l3ClC =0.2 provided the interaction depth is 40 nm as assumed
in the calculations.
Because of the co-deposition of l2C a mixed layer

will be formed within the interaction depth. Since these
are areas of net erosion the mixture is re-eroded andfeeds the carbon flux in the SOL with the according
isotopic ratio. The re-deposition will eventually end in
net deposition areas, as e.g. on the side walls of thepoloidal limiter ring or on other obstacles deep in the
shadow.

If such a perception is true, the carbon deposits in
the SOL should show an isotopic ratio independent of
the radial distance from the plasma center. This wasindeed found on the target mounted perpendicular to thetoroidal direction and outside the influence of the
puffing cloud. The value r3ClC = 0.27 was determinedby SIMS and NRA. It seems that the l3C concentrationin the limiter surface has reached quasi-stationarity
within the total exposure time (107.7 s) despite thecontinuous net erosion. Obviously, only a limited depthof the bulk is affected by the in- and outflowing r3C andr2C. If the observed ratio r3ClC= 0.27 is representative
for the whole plasma facing limiter surface (3.4 m2) itsuggests an interaction depth (= mixed layer thickness)of = 27 nm.
This is a lower limit. however. because the limiter
exhibits a complicated pattern of erosion and deposition

Fig. 5 Carbon flakes grown after long term exposure in
TEXTOR.

plasma discharges up to about 6000 s by means of theStockholm-TEXTOR collector probe system.

Two samples exposed 4 cm deep in SOL for 126Os and 5846 s are shown in fig.5. They yield net

regions, which is caused by the magnetic field ripple
and the complex shape of the plasma facing surface[4]. Only roughly one half of the area shows net
erosion, the other half net deposition. In depositionareas, carbon can accumulate after the erosion and the
local transport over short distances. Flakes of 45 1tm
thickness were observed in such areas after 14000
discharge seconds [5]. The life time of the component
will therefore not be determined by the average carbon
loss of the graphite tile, but by the local erosion rate indeposition rates of approximately 2-3 nmls. Growth
rates of l0 nm/s at the LCFS could be deduced from thesample exposed in the SOL along with the puffing

experiment. Provided these rates are not muchdepending on the poloidal location, they would
correspond to a permanent flux of eroded carbon in the
SOL of roughly 1.5 x l0re s-r or to an equivalent of 0.3
mg/s of carbon lost from the limiters. The deposits
permanently grow with time and reach e.g. thicknessesof 130 pm on the side walls of the poloidal limiters [6].
Flaking of the thick deposits as shown in fig. 5 might
limit the final thickness, but cause the formation of

the net erosion areas.

A long term exposure experiment was set up, inorder to estimate the average carbon loss from thelimiter surface. In TEXTOR, we can exclude themacroscopic particles ('dust').

boronized liner (r = 55 cm) as a sink for carbon because

it does not seem to contribute significantly to the carbon
interchange. After its erosion the carbon is travellingthrough the SOL. The transport eventually ends on

5. Summary and Gonclusions
The injection of r3C containing methane (ttCH+)
into TEXTOR-94 through a hole in a wall element

obstacles in the shadow. Here, samples were exposed toallowed to discriminate the local deposition and the301

Wienhold P. et al., Carbon Transport in TEXTOR-94 Aspects of Long Term Operation

global transport of carbon. In a surrounding of mainly
graphite (t2C), the isotope r3C behaves as a 'tracer
impurity'. It was transported several cm across the target
plate and deposited there with a low efficiency (0.2Vo)
r3C/C
together with r2C out of the background. The ratio
reached values up 0.42. These findings as well as the
CH and CII light emissions observed near the blowing
hole could be modelled by means of the ERO-TEXTOR
code if a high re-erosion probability for the hitting CH3*
and CHa* radicals is assumed. They, obviously, form afilm which is not very stable against erosion. Enhanced
re-erosion by the hydrogen out of the radicals can not be
excluded. Deuterium from the plasma and hydrogen are
co-deposited into the film.
The majority of the r3C is first transported to theplasma facing surfaces of the toroidal graphite limiter
ALTII. It seems to create an isotopic ratio in the r2C
bulk within an interaction depth. Since ALTII is the
major carbon source in TEXTOR the flux of the reeroded carbon should have the same isotopic ratio. It is

The ERO-TEXTOR code has been shown to be a very
helpful tool in order to quantitatively simulate thetransported through the SOL to obstacles. The value l3Cl

t5l P. Andrew et al., J. Nucl. Mat. 266-269, 153

experimental observations and in order to understand theprocesses of local transport of carbon.

Acknowledgement
The authors had great scientific support from H.G.
Esser, M. Lehnen, V. Philipps and F. Weschenfelder.
They are very thankfull for technical assistance by M.
Freisinger, W. Milller and H. Reimer.

Referenecestll

G. Federici et al., J. Nucl. Mat. 241-243, 260,L997\.

t2j M. Mayer et a/., J. Nucl. Mat.266'269,604 (1999)
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[3] P. Wienhold et al., Physica Scripta T81, 19 (1999).
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(1999).

C = 0.27 found independently on the radial distance on at6l J. von Seggern et al., Physica Scripta TEl, 3l

collector in the SOL suggests an interaction depth ofhigher than =27 nm and that quasi-stationarity has beenachieved despite the permanent erosion of the surface.

t7l D. Hildebrandt et al., Physica Scripta T8l, 25Long term exposed samples yielded a rough estimate ofabout 0.3 mg/s as the average carbon rate leaving

t8l A. Kirschner et a1.,26'h EPS Conf. on Contr. Fus.

(1e99).

0999\.

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Netherlands.

[9] P. Wienhold et al.,26tn EPS Conf. on Contr. Fus. in
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The local carbon transport depends not only on
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on the limiter surfaces despite a low average carbon
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Res.894,503 (1994).

lllj H.G. Esser et al., J. Nucl. Mat. 220-222, 457( l 995).

[2] A. Kirschner, PhD, University of Bochum, (1999),
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the components predictions can hardly be made as long[13] E. Vietzke et al., E-MRS Meeting, June 1987, Vol.

xvII.351.

as the processes are not better quantified. This suggests[4] T Denner, PhD, Univ. of Diisseldorf (1998), Ger-

further experiments in TEXTOR. Investigations in othermachines may benefit from the finding that r3C behaves
like a 'test impurity', the transport of which can nicely
be distinguished from the isotope r2C of the background.

many.

tl5l M. Rubel et al.,tobe published.

302