J. Plasma Fusion Res. SERIES, Vol.3 (2000) 255-259

Morphology of Thick Co-deposited Layers on PlasmaFacing Components
RUBEL MareK*, PHILIPPS VoIKerI, TANABE Tetsuo2 and WIENHOLD PeterI
Alfvdn Laboratory, Royal Institute of Technology, Teknikringen 31, SE-100 44 Stockholm, Sweden
tlnstitutePlasma Physics, Trilateral Euregio Cluster, Forschungslentrum Jiilich, D-52425 Jillich, Germany
2Centerforfor Integrated Research in Science and Engineering, Nagoya IJniversity, 464-8601 Nagoya, Japan(Received: 18 January 2000 / Accepted: 17 May 2000)

AbstractLocal and global transport phenomena modify plasma facing surfaces and cause the formation of codeposited layers which accumulate fuel atoms and, therefore, create a problem of tritium inventory in D-T
operated devices. This contribution reports on the distribution and content ofdeuterium plasma impurity species(boron, silicon, carbon, medium-Z and high-Z metals) on various plasma facing components: tiles of the main

poloidal and toroidal limiters, test limiters, long term probes exposed to the plasma edge. The investigation wasperformed by means of accelerator based ion beam analysis techniques, microscopy methods and X-ray
spectroscopy. It was found that in some deposition zones the accumulation of deuterium was above 3 x l0reatoms cm-2 in the layers exceeding 40 microns in thickness.

Keywords:
hydrogen inventory, co-deposition, plasma facing components

1. lntroduction

the accumulation of fuel species leading, in the case ofPlasma - surface interactions in controlled fusion
devices occur in a closed system implying that all nonvolatile particles eroded from plasma facing components(PFC) must eventually return to the surfaces. Particles
not returning to their place of origin are engaged in thetritium, to unacceptable radioactivity level (tritiuminventory problem) [5-17]. The layer growth,accompanied by the fuel accumulation, is the mostpronounced in carbon wall machines [5-24], Similareffects have also been observed with the beryllium wall

local and global material transport. The net effect of[21,22] whereas the least pronounced accumulation

those erosion and deposition processes is the formationof co-deposited / co-implanted layers being a mixture ofoccurs in high-Z metals 125-271. Therefore, theformation of co-deposits is one of major concerns whendevices. several different elements (various PFC

the steady-state operation of fusion reactors isconsidered, especially if carbon-based materials wouldmaterials, wall protective coatings [5-9] and edge

be used for the first wall.

plasma impurity and fuel atoms [1-4]. In most ofHandling and examination of tritium-containing

cooling media [10]) are simultaneously in contact withthe plasma and the formation of co-deposits leads to theso-called material mixing Il-14]. The most serious
consequences of this phenomenon are: i) the change ofsamples create significant technical problems [5-17].
Under these circumstances there is an urgent need forintensive and broad studies of co-deposits formed innowadays devices operated with deuterium fuel. The

properties of PFC (thermo-mechanical, particle
recycling, chemical, etc.) influencing their life time; ii)@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corresponding author's e-mail: rubel@ msi.se255

Rubel M. et al., Morphology of Thick Co-deposited Layers on Plasma Facing Components

range of issues to be studied includes: the total
inventory of hydrogen isotopes, their depth and areal
distribution on PFC, the influence of wall temperature
on fuel accumulation, the assessment of the co-deuterium (including depth distribution) and boron,
Rutherford backscattering spectroscopy (RBS), energy
dispersive X-ray spectroscopy (EDS) and electron
microscopy [31].

deposits'growth rate and the determination of conditions
under which the layers crack, exfoliate, flake and peel-

3. Results and Discussion
3.1 Poloidal limiters and rf antenna protection
tiles
Figure I a exemplifies the structure of a flaking codeposit on the rf antenna protection tile. The layer

off forming dust particles [8,23,28]. It is equallyimportant is recognize co-deposits' chemical reactivity
and the development of methods leading to the fuelremoval accompanied by the decomposition of thelayers [28,291. All these issues have also been addressed

thickness, as previously reported [18], reaches 170
microns. There is a variety of co-deposits' structures
observed on PFC under examination, but the common
feature of those layers is their brittleness. Therefore,
peeling-off and disintegration of flakes is a very
probable pathway for dust formation in fusion devices.
Figure I b shows a flake which peeled-off and wascollected as a dust particle from the liner floor. It isin studies carried out at the TEXTOR tokamak [30] - amedium size device operated at the Institute of Plasma
Physics of the Forschungszentrum Jiilich - whose
scientific mission is focused on plasma edge engineeringand plasma - surface interactions. Recently, a large
number of PFC and long term samples have beenstudied in order to map the deuterium content anddistribution in the machine. In this paper we report on adetailed characterization of co-deposits and dust
particles formed on major plasma facing components.

2. Experimental
2.1 Specimens
The study was carried out with co-deposits formedon the main toroidal and poloidal limiters. Some limiter
tiles were dismounted from the machine following a few
months operation period (approx. 14 000 s exposure tothe plasma). Also dust particles were collected from theliner floor using a vacuum cleaner equipped with a
specially designed filter. In order to determine the codeposits' growth rate and, to recognize if there is a
critical thickness at which the deposit starts flaking andpeeling-off, several graphite and silicon targets wereexposed to the scrape-off layer (SOL) plasma. The
exposure lasted totally over 5400 s and there were bothtargets exposed continuously and samples exposed
during shorter periods: from 500 s to 2100 s.

2.2 Methods

Detailed examination of PFC morphology(composition and structure) is a mosaic work involving
the application of some tens of analytical methods, bothfor studies of the co-deposits' growth in-situ, i.e. in thetorus, and ex-situ after the materials have been removed

from the machine. Among the latter ones, thermal
desorption, various types of microscopy and ion beam

analysis methods are the most frequently used. The
study reported below were performed by means of
nuclear reaction analysis (NRA) for the determination ofFig. 1 Structure of a thick co-deposit (a) and dust
particles collected from the liner floor (b).
256

Rubel M. et al., Morphology of Thick Co-deposited Layers on Plasma Facing Components

IE3goslEog=l2

Gr FeNIFig. 3 A co-deposit on the toroidal limiter tiles.The layer

structure reflects the topography of the graphite

ENERGY (kev)substrate.

Fig. 2 X-ray spectrum showing the composition of thenear surface region (1 micron) of the codeposit.

I

Eoo
E10o

worthwhile to mention that the amount of dust collected
after a six months operation was in the milligram range.
Deuterium content on the plasma facing side of thetiles does not exceed the level of 0.6 x l0l7 cm-2 and thecorresponding concentration ratio of deuterium-tocarbon (CD/CC) is below 0.06. Greater D contents (up to4.7 x l0t7 cm-2) and CD/CC (0.16) values are measured
on side surfaces of the tiles, i.e. on surfaces hidden in
the gaps between the tiles. These values are considered
to be low in comparison to the ones observed in otherdevices [32]. Besides C and D, other constituents of thelayers are: boron and silicon originating from regularboronization and siliconization of the machine andinconel components eroded from the liner. Certain
quantities of tungsten are also present as a consequenceof the tests performed with high-Z limiters of various
types [3,14,25,33]. However, the local distribution ofimpurity species is non-uniform. There are areas richeither in carbon, boron, silicon or metal atoms, asillustrated by the X-ray spectrum in Fig. 2. Significant
amounts of the detected oxygen originate from thesorption of atmospheric oxygen and adsoprtion of water
vapour when that layers are exposed to air following
their removal from the torus.

|EoN60trEF
fr ''oozoof=Eo (3He*) = 1.80 tleVECGDEFOSIT THICKI{ES{I ovcr 0.2 umFOJUJoCo = 6.0G r'10[ atorm cm-2ul0123456

DEPTH (microns)

Fig.4 NRA based depth profiles of deuterium recorded
at two energies of the 3He analysingbeam.

discussed limiters, the co-deposit on the ALT II tiles has

fairly smooth appearance as shown in Fig. 3. The
3.2 Belt toroidal limiter
A toroidal belt limiter (ALT II) is the main PFC

structure seen on the deposit is a replica of the substrate
structure. The co-deposit does not peel-off itself during
the plasma operation but, on some tiles, it starts to flakein air and. therefore. it can be detached from thegraphite substrate. This layer, of a stratified structure,reaches 50 pm in thickness. NRA measurements
13tte1O,p;4Ue1 were performed at two different energies

with the area of 3.4 m2 corresponding to 97o of the total
first wall area of TEXTOR. Mapping of the deuterium
distribution by means of NRA reveals the presence of
deposition zones covering about 357o of the tiles'
surface area. In contrary to the layers on the previously257

Rubel M. et al., Morphology of Thick Co-deposited Layers on Plasma Facing Components

of a rHe beam in order to recognize the depth

4. Summary and Conclusions

distribution in the region accessible for the analysis: 4.5

Various plasma facing components and long term
graphite targets exposed to the plasma at the TEXTOR
tokamak were studied. The most important observationis related to the fact that the deuterium content isrelatively low, Cp/C6 is about 0.1. This value isconsidered to be low in comparison to much greaterpm at 1.5 MeV and 6.2 1tm at 1.8 MeV. The results
shown in Fig. 4 reflect the stratified layer structure andthey also indicate fairly uniform in-depth content ofdeuterium.

Deuterium depth profiling on both sides of thevalues observed in other devices. This is related to thehigh temperature, 300oC - 350"C, at which theTEXTOR liner and PFC are maintained between thedetached flakes and in the underlying graphite substrateenables the estimation of the D content in the deposition

zones to be on the level of 3.5 x l0re cm-2. The fuelcontent in the erosion zone (3 - 7 x lOtT cm-2) and on
the back of the tile (0.9 - l.S x l0r7 cm-2) was also
determined. As a result, a total amount of D atoms
trapped in all the limiter tiles is assessed to be about 2 x1023 atoms. The value is a good agreement with a gasbalance measurement at the machine and, therefore, itdischarges. The results strongly indicates that, withrespect to the inventory of hydrogen isotopes, elevatedwall temperatures (above 350"C) would be beneficial
when the steady-state devices with carbon walls are
considered as the next step machines.

indicates that the deuterium is mostly trapped on thetoroidal limiter [34]. Another important information isthat the deuterium content in the co-deposits accounted

Acknowledgements

for approximately l0 at. Vo (CDlCg - 0.1).

the NFR Contracts A/AC-FF-065571-307 and3l2, anda Grant-in-aid of the Ministry of Education, Culture andScience of Japan. The Wallenberg Foundation is highly
acknowledged for funding the SEM/EDS equipment.

The authors are grateful to the TEXTOR team forkind co-operation. This work was partly supported by3.3 Long term graphite samples
Table I summarizes the results obtained following
the exposure of four graphite targets to the SOL plasma,i.e. approximately 35 mm from the plasma edge. In mostReferencest1l H. Bergsiker et al.,J. Nucl. Mater. 145-147,727

of the cases the layer grows at the rate of approximately
1.5 nm/s. Only in the first experiment the rate is threetimes higher (4.9 nm/s) and this result is associated withthe fact that plasma position was shifted by 1 cm
towards the outer wall. Flaking and/or peeling-off thelayer could be observed on those targets. Such effectsoccurred, however, on the graphite plates exposed forover 5400 s; layers of the thickness exceeding l0
microns were formed [35]. For all the samples exposed,
the deuterium content in the layer is around lo7o,i.e. on
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