S4Ѹ1

Subjects of Plasma-Carbon Material Interactionsin Fusion Devices

ᰶ⼝ྙࣈࣚࢫ࣏⿞⨠ࡡࣈࣚࢫ࣏࣬⅛⣪⣌∸㈻┞பష⏕࡞㛭ࡌࡾㅎㄚ㢗
N. Ohnoኬ㔕ူ㟱
EcoTopia Science Institute, Nagoya University464-8603, Furo-cho, Chikusa-ku, Nagoya, JapanྞཿᒁኬᏕ࢙ࢤࢹࣅ࢓◂✪ᶭᵋ
ࠚ464-8603 ྞཿᒁᕰ༐⛸༇୘⩹⏣The scope of this symposium it to review the recent investigation on plasma-wall interaction infusion plasma devices. In this presentation, important subjects in recent studies of theplasma-material interaction will be briefly reviewed in order to make the scope of thissymposium clear.
Plasma-wall interaction in magnetically confined
fusion devices regulates performance of coreplasma and lifetime of the plasma-facing
components. Carbon materials are widely used as
plasma-facing material in present fusion devices
because of its good material properties. In the nextfusion
devices
likeITER(InternationalThermonuclear Experimental Reactor) intended tohave D-T burning plasma in long or steady state
operation, it is one of the critical issues to make theprecise estimation of the lifetime of plasma-facing
components and tritium inventory. For this purpose,comprehensive study of erosion process of theplasma-facing materials, transport of the erodedmaterials and re-deposition process should berequired because the net erosion of theplasma-facing material is determined by the balance

betweenerosion and deposition processes.
Moreover, hydrogen isotopes (D, T) could be also
co-deposited with the deposited material, which isrelated to tritium inventory and fueling/pumping
efficiency. Figure 1 shows the schematic diagram
representing important topics of plasma-wallinteraction in fusion devices reviewed anddiscussed in this symposium.
(a) Erosion process of carbon
Plasma-facing materials are usually eroded byphysical sputtering. However, the sputtering
process due to chemical reaction between hydrogenand carbon, so called, chemical sputtering is
considered to be a dominating erosion process ofcarbon wall in the present fusion devices. Precise
evaluation of the chemical erosion yield is very

Fig.1 Schematic diagram of plasma-wall interaction in fusion devices reviewed anddiscussed in this symposium.

Fig. 2 Dependence of chemical erosion yieldon carbon surface temperature as
parameter of ion flux in several plasma
devices in Ref. [1].

important to estimate the lifetime of the carbon wall.
Then, there are many studies of the chemical
sputtering in edge plasma condition of fusion
devices. However, the physical mechanism of thechemical sputtering has not been fully understoodyet. Figure 2 show dependence of chemical erosion
yield on the surface temperature in several devices.
These data are so scattered depending on theincident ion flux. Understanding of the fundamental
physical mechanism of the chemical sputtering is
one of the most important subjects, which will be
discussed based on molecular dynamic simulationin S4-2.
(b) Transport of carbon impurity
Hydrocarbon species, CxHy, which could begenerated as a result of erosion of carbon materials,
are transported as impurity in the fusion device. The
impurity transport in fusion devices is very
complicated strongly depending on plasma
parameters such as electron density and temperatureand plasma flow, and geometry of the devices.
There could be two different type of impurity
transport (local and long range transport) as shownin Fig. 1. In the local transport, the hydrocarbon
species are deposited near the eroded area becausehydrocarbon species ionized in plasma aretransported to the surface near the eroded area due
to strong friction force by streaming plasma or
Lorenz force in strongly inclined magnetic field (socalled prompt re-deposition). On the other hand,hydrocarbon species can be transported to remotearea. Carbon dust, which is a key issue for safetyconcern, is considered to be generated fromre-deposited carbon layers formed on cold remote

Fig .3 Re-deposited carbon layer observed in
the Tore Supra tokamak[3].

material surfaces. In the JET tokamak, thick
re-deposited carbon layers were observed at thecold remote region near the pumping area [2].
Similar re-deposited carbon layers were also
observed at the target plate located below thetoroidal pump limiter in the Tore Supra tokamak [3]as shown in Fig. 3. Carbon dust particles or flakes
could be generated by erosion of these re-deposited
carbon layers. The production, transport anddeposition processes of carbon impurities in theJT-60U tokamak are reviewed in S4-3.
(c) Co-deposition process with hydrogen isotopes
Hydrogen isotopes (D, T) could be co-deposited
with the deposited material, shown in Fig. 1
Hydrogen retention in co-deposited layer has agreat influence on fueling and pumping in long
and/or steady state operation, which is also one of
the critical issues in the next generation fusion
devices. In the TRIAM-1M, the wall role forsink(pumping) and source(fueling) is changed intime, critically depending on the wall temperature,
which is discussed in S4-4. Moreover, Co-depositedlayer with hydrogen isotopes like C-H layer isconsidered to have different material property frompure one, such as for chemical and physicalsputtering yield. It is also necessary to make aquantitative characterization of the co-depositedlayer.
References[1] J. Roth, et al. Nucl. Fusion, 44(2001) L21.
[2] M. Rubel et al. Nucl. Fusion, 41(2001) 1087.
[3] E. Delchambre et al., Proc of 30th EPS Conf.
on Contr. Fusion and Plasma Phys., St.
Petersburg, 7-11 July 2003 ECA Vol. 27A,
P-3.169