S6-1
Requirements for divertor (ITPA Reports)
ダイバータ要件(ITPA報告など)
Suguru Masuzaki
増崎 貴
National Institute for Fusion ScienceOroshi 322-6, Toki 509-5292, Japan
核融合科学研究所 〒509-5292 土岐市下石町322-6
The leading candidate of the material for the divertor plates in a fusion reactor is tungsten because it is a
refractory metal, and its erosion rate and the hydrogen isotopes retention in bulk material are much lessthan those of carbon. It was decided in the last year, 2013, that full tungsten divertor is installed from theinitial experiment phases in the ITER. However there are some issues to be solved to use tungsten as aplasma facing material in the ITER and a fusion reactor.

The roles of a divertor in a fusion reactor are(i) impurities removal including helium ash, and(ii) pumping of the residual fuel particles. For
these roles, the separatrix and the neighbormagnetic field lines connect on the divertor plates,
and the plasma heat load is very high on theplates. Therefore carbon and/or refractory metals
are used for the divertor plates material.
Carbon was used in various fusion devices as
the plasma facing material in 1990’s to 2000’s.
The reason why the refractory metals had notbeen used in fusion devices was the degradationof the plasma performance with large radiationfrom the high-Z impurities, which were generated
by the erosion of the refractory metals. The
plasma performances were improved in carbon
devices, and carbon was planned to be used as adivertor plates material in the ITER [1]. On theother hand, carbon was considered not to be usedas the divertor plates material in a fusion reactor
because of the large erosion rate of carbon. Inaddition
tophysicalsputteringandevaporation/sublimation, which are common
processes for carbon and the refractory metals,
there is another erosion process for the carbon inhydrogen isotope plasma environment, that is,chemical sputtering. If carbon is used as thedivertor plates material in a fusion reactor, thelifetime of the plates become so short. The otherproblem of using carbon in fusion reactor is theretention of hydrogen isotopes. It is well knowthat hydrogen isotopes can be retained in a bulk
carbon up to around 0.4 in H/C ratio at relatively
low temperature [2]. In addition to that, there-deposition layer of carbon can retain hydrogenisotope continuously with the co-deposition
process with H/C ratio exceeding 1 [3]. The
tritium inventory in vacuum vessel should besmall in a fusion reactor, and there is the in-vessel
tritium inventory limit for operation in the ITER
[3].
In 2000’s, the experiments with the metallic
plasma facing component were conducted in theTEXTOR [4] and the ASDEX-Upgrade [5],respectively.
In
particular,
intheASDEX-Upgrade, the experiments with the fulltungsten plasma facing components were startedfrom 2007 [6]. As the results of the experimentswith metallic plasma facing components, it has
been revealed that the effects of the high-Z
impurities can be avoided with operational
scenarios, such as the central heating and ELM
pacing [7]. As a result, the material of the ITER
divertor plates for the initial experiment phases
was changed to be all tungsten in 2013, though
the main reason of the change was budget
restrictions [8]. In designs of fusion reactors,
tungsten is also the leading candidate of theplasma-facing material.
The tungsten issues for ITER have been
discussed in the International Tokamak PhysicsActivity (ITPA) Scrape-Off-Layer (SOL) andDivertor topical group for years. The ITPA
provides a framework for internationally
coordinated fusion research activities. The ITPA
continues the tokamak physics R&D activities
that have been conducted on an international level
for many years. This has resulted in theachievement of a broad physics basis essential for
the ITER design and useful for all fusion
programs and for progress toward fusion energy
generally [9]. In the ITPA SOL & Divertor group,
the discussion has been focused on theplasma-surface interactions and the phenomena atnear the surface region of the material. The topics

related to tungsten in the group meetings in therecent two years were as below:(1) Migration, including measurement of thetungsten source mainly with spectroscopy,
calculation of tungsten transport, and comparison
to surface analysis(2) Fuel retention, including fuel retention
machine database, isotope exchange, wallconditioning and effects of neutron damage.
(3) Dust, including characterization of ejection
velocities, sizes of molten droplets and size
distributions of collected dust, dust monitors andmeasurements.
(4) Melting, including melt tungsten in tokamak,measurement of the melt layer dynamics andpenetration to core plasma, modelling validation(melt layer behavior and plasma transport),
predictions for ITER.
(5) Morphology, including the impacts of themelting and solidification (cracking), roughening,
helium induced morphology and recrystallization
on erosion and plasma operation.
(6) Tungsten technology, including high heat
flux testing and developments of the plasma
facing components with tungsten

In the presentation, some topics from thediscussions in the ITPA SOL and Divertor groupmeetings will be reported.
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