S4−3

Production and Transport of Carbon Impuritiesin Fusion Plasma Devices
核融合プラズマ装置における炭素不純物の発生と輸送
H. Kubo
久保博孝
Naka Fusion Research Establishment, Japan Atomic Energy Research Institute,801-01 Mukoyama, Naka-machi, Naka-gun, Ibaraki-ken, 311-0193 Japan
日本原子力研究所、那珂研究所、〒311-0193茨城県那珂郡那珂町向山

For high-heat load divertor plates, carbon materials are used in present and next generation tokamaks (suchas ITER) because of their good thermal properties and low atomic number. However, erosion andredeposition of carbon materials is a concern for long operation of next generation tokamaks. In this talk,recent studies on production of carbon impurities due to chemical sputtering and transport and redepositionof carbon are reviewed, and future direction of this field is discussed.

1. Introduction
In ITER, operation with cold divertor plasmas
such as partially detached divertor plasmas isconsidered for mitigating the severe problem ofconcentrated power loading of the divertor plates.
Carbon material is considered to be used for thehigh-heat-load divertor plates: strike-point tiles,
because of its high thermal shock resistance andtolerance to off-normal events (ELMs anddisruptions) without melting. However, carbon has
properties of strong chemical reactions withhydrogen. While the physical sputtering yield,
which has been well investigated, decreases
drastically in cold divertor plasmas, the chemical
sputtering yield does not decrease and the chemical
sputtering can reduce the lifetime of the divertor
plates. Carbon impurities produced by physical orchemicalsputtering
aretransported,andredeposition of the transported carbon impurities
with tritium gives rise to operational and safetyconcerns for tritium inventory. Therefore, the studyof production and transport of the carbon impurities
is important for ITER [1-3].
In this paper, recent studies on production of thecarbon impurities due to the chemical sputteringand transport and redeposition of carbon isreviewed, and future direction of this field isdiscussed.
2. Production of Carbon Impurity
The chemical sputtering of carbon has beenstudied in detail in ion beam experiments, and thesputtering yields are available as functions ofincident ion energy and surface temperature.
However, the high-ion-flux condition (~1024 m-2s-1)
in ITER divertor plasmas cannot be simulated by

the ion beam experiments. Therefore, it is necessary
to measure ion flux dependence of the chemical
sputtering yield in real divertors under high-ionflux conditions. Using data from PSI-1, PSI-2,PISCES-B, JET, Tore Supra, TEXTOR, ASDEX
Upgrade and JT-60U, the dependence of thechemical sputtering yield on the ion flux, Φ, hasbeen determined to be Φ-0.54 [4]. With this flux
dependence in addition to the ion energy andsurface temperature dependence, the chemical
erosion rate of the ITER divertor plates has beencalculated. The calculated erosion rate was an orderof magnitude lower than previous estimates using a
constant chemical sputtering yield of 1.5 %. Here,
only production of methane (CH4/CD4) was
considered, and production of heavier hydrocarbons(C2Hn/C2Dn) was not investigated quantitatively. InJT-60U, the chemical sputtering yield due toC2Hn/C2Dn production in addition to CH4/CD4
production has been measured [5]. The
measurement showed that the contribution ofC2Hn/C2Dn production to the total number of thesputtered carbon atoms was ~80%. Therefore, thecontribution of C2Hn/C2Dn production should beinvestigated in addition to CH4/CD4 production.
3. Transport and Redeposition of Carbon
In JET, D III-D, ASDEX Upgrade and JT-60U, it
has been found that erosion was dominant or almost
balanced with deposition at the outer divertor plates
and deposition was dominant at the inner divertor
plates as shown in Fig. 1 [2,6]. One of the reasonsof this in/out asymmetry is considered temperatureasymmetry between the in/out divertor plasmas; thetemperature in the outer divertor plasma is usuallyhigher than that in the inner divertor. The energy

that hydrocarbon molecules might be released bythermal decomposition of soft hydrocarbon films
formed on hot surfaces in the inner divertor andthey might attach preferentially to the water cooled
louver at the end of the pump ducts. On the other
hand, in JT-60U, careful inspection and dust
collection has shown that little deposition wasappreciable in remote areas and the amount ofcarbon dust was very small; the total dust amount
was estimated to be ~7 g [3,8]. The difference
between JET and JT-60U might be attributed todifference of their divertor structures and surface
temperatures of the divertor plates and walls [9].
Further study of hydrocarbon chemistry is required
to understand the remote deposition.

Fig. 1. Poloidal cross-section of JT-60U anddeposition thickness and erosion depth in thedivertor [6]. The arrows indicate the directions of
the plasma flow in the SOL and the Er x B drift flowin the private flux region [7].

dependence of the sputtering yield means that thetemperature asymmetry can shift the equilibriumfrom net erosion to net deposition. Some of thecarbon deposition in the divertor is considered to beoriginated from the erosion of the main chamber
wall with preferential deposition in the innerdivertor. In JET, 13CH4 has been injected at the topof the machine, and it has been found that the mostof the 13C was deposited in the inner divertor [2].
The preferential deposition in the inner divertor
may be consequence of the SOL flow towards theinner divertor plates that has been observed when
the ion grad B drift was directed towards thedivertor target (Fig. 1) [2,7]. In the JT-60U divertor,Er x B drift in the private flux region has beenevaluated, and the evaluation showed that the drift
flow contributed to in/out asymmetry in the divertor
ion flux (Fig. 1) [7]. The drift flow from the outer
divertor to the inner divertor via the private region
might provide a path for carbon impurity to reachthe inner divertor. Quantitative investigation ofcarbon impurity production in the main chamberand carbon transport due to the plasma flow in theSOL and private region is required.
In JET, significant deposition has been observed
on areas with no direct plasma ion impact (remoteareas) in the inner divertor [2]. It was considered

4. Summary
For carbon impurity production, the contributionof C2Hn/C2Dn production should be studied inaddition to CH4/CD4 production. For understandingof carbon transport and redeposition, quantitativeinvestigation of carbon transport due to the plasma
flow in the SOL and private region is required. Inaddition, carbon impurity production in the mainchamber should be investigated quantitatively.
Study of effects of the divertor structure and thesurface temperature of the divertor plates and walls
on deposition of carbon in the remote area isimportant.
Acknowledgments
The author would thank Profs. T. Tanabe and N.
Ohno of Nagoya Univ. and Drs. Y. Gotoh, N.
Asakura, T. Nakano, K. Masaki and K. Tsuzuki ofJapan Atomic Energy Research Institute for their
help to prepare this talk.
References[1] G. Federici, et al., Nucl. Fusion 41 (2001) 1967.
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[3] T. Tanabe, et al., to be published in Proc. 20thIAEA Fusion Energy Conf. (IAEA, Vilamoura,2004) EX/P5-32.
[4] J. Roth, et al., to be published in J. Nucl. Mater.
[5] T. Nakano et al., Nucl. Fusion 42 (2002) 689.
[6] Y. Gotoh, et al., J. Nucl. Mater. 313-316 (2003)
370.
[7] N. Asakura, et al., J. Nucl. Mater. 313-316 (2003)
820.
[8] K. Masaki, et al., to be published in J. Nucl. Mater.
[9] T. Tanabe, et al., to be published in J. Nucl. Mater.