24C02
Revival of Arcing Issue in Fusion Reactors
核融合炉内でのアーキング問題の再来
Shin Kajita, Noriyasu Ohno, Shuichi Takamura, Masayuki Tokitani, Suguru Masuzaki,Naoaki Yoshida, Takaki Hatae, Masakatsu Fukumoto, Kiyoshi Itami,
Tomohide Nakano, Yoshio Ueda
1
梶田信 ，大野哲靖 2，高村秀一 3，時谷政行 4，増崎貴 4，吉田直亮 5，
波多江仰紀 6，福本正勝 6，伊丹潔 6，仲野友英 6，上田良夫 7
1

EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan
Graduate School of Engineering, Nagoya University, Nagoya 464-8603,
3
Aichi Institute of Technology, Toyota 470-0392, Japan4National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu 509-5292, Japan
5
Research Institute for Applied Mechanics, Kyushu University, Fukuoka 816-8580, Japan
6
Japan Atomic Energy Agency, Ibaraki 801-1, Japan
7
Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan1
名古屋大学エコトピア科学研究所 〒464-8603 名古屋市千種区不老町2
名古屋大学工学研究科エネルギー理工学専攻 〒464-8603 名古屋市千種区不老町3
愛知工業大学 〒470-0392 豊田市八草八千草1247
4
自然科学研究機構 核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
5
九州大学応用力学研究所 〒816-8580 福岡県春日市春日公園 6-1
6日本原子力研究開発機構 〒311-0193 茨城県那珂市向山 801-17
大阪大学大学院工学研究科 〒565-0871 大阪府吹田市山田丘２－１
2Arcing is a long standing plasma-surface interaction issue in nuclear fusion research, and the issue is being
revived recently from new points of view. In the linear plasma device NAGDIS-II, the demonstration
experiments of arcing/unipolar arcing have been conducted by mimicking the transient heat load using apulsed laser. The ignition condition of arcing and erosion rate of tungsten accompanied with the arcing areinvestigated. It is shown that the helium irradiation significantly makes it easier to initiate arcing by thesurface morphology change. Actually, in LHD, arcing was initiated on helium irradiated tungsten surface
even without transients. From JT-60U, arcing phenomena occurred on baffle plate inside the vacuum
vessel and diagnostic mirror outside the vessel are presented.

1. Introduction
After divertor configuration was regarded as aplausible option, arcing phenomena have not been
paid attention in fusion research so much, because
they were thought to be avoidable and sputtering ofmaterial was regarded as the major impurity source.
However, recently, arcing issue has been revived
from new different points of view: the effects oftransient heat loads such as ELMs and disruptions
and surface morphology changes caused by plasma
irradiation [1]. Indeed, arcs in several tokamaks
including ASDEX-U [2] and D-IIID [3] have beenreported recently. These observations suggested
that arcing could be an important topic in ITER or
future fusion devices like DEMO, because thetransient heat load will be greater than those inpresent devices when ELMs occur and irradiationof helium, which will be produced by the nuclear
fusion process, may contribute to morphology
changes as suggested in many literatures [4-6].
In this paper, we present the Japanese arcing
activities in fusion research from a linear divertor

simulator, LHD and JT-60U. First, arc tracks
observed in the buffle tiles and a diagnostic mirror
in JT-60U are presented. Then, the arcing
exponents conducted in the linear plasma device
NAGDIS-II and LHD are shown in detail.
2. Arc trails observed in JT-60U
Figure 1(a) shows a laser scanning microscope
micrograph of the baffle time of JT-60U. It was
found that there are many trails that run verticaldirection, which is vertical to the magnetic field.
From the depth analysis of the surface using thelaser microscope, it was found that eroded depth
was several tens of micrometers in some parts. It
was suspected that massive amount of impurities
has been released when the arc was initiated.
Another arcing issue in JT-60U is discharge trails
recorded on the diagnostics mirror for laserThomson scattering. Because the damages were sofatal that the mirror had to be replaced after thebreakdown. Details of the characteristics of thedamage are investigated from the observation of thetrails. It is thought that surface discharge wasinitiated on the mirror. The phenomena issue
warning on the components to be installed in ITER
and future fusion devices both inside and outside
the vacuum vessel.

1 mm

Figure 1: A laser scanning microscope micrographof carbon baffle plate of JT-60U.
3. Arc demonstration experiments
3.1 In LHD
A tungsten sample was exposed to helium plasmain NAGDIS and the nanostructured sample exposedto the Large Helical Device (LHD) divertor plasma
for 2 s. After the exposure to the LHD plasma, arc
trails were observed on the surface, as shown in Fig.
2 [7]. Interestingly, the arcing was initiated even
without any transients. Because the magnetic field
direction was almost normal to the surface, thefractality measured using a box-counting method
was almost two, indicating that the motion wassimilar to the Brownian motion. This results
support the fact that the arcing is easily initiated on
the nanostructured tungsten surface.

Figure 2: A laser microscope image exposed tothe LHD plasma. From ref. [7].
3.2 In linear device NAGDIS-II
The initiation condition and behavior of arc spot
was demonstrated in detail in the linear divertor
simulator NAGIS-II. To simulate the transient heatload, a pulsed laser with the pulse width of ~0.6 ms
irradiates the sample. A W sample was used for theexperiments.

Ignition condition of arcing was investigatedsystematically by changing the laser power, plasma
conditions, surface nature, and so on.
Arcingwas never ignited unless the target was sufficiently
biased negatively, say less than -50 V, and one
could say that the target voltage is one of the key
parameters to control arcing. The helium irradiation
changes the surface morphology and makes theignition of arcing much easier. When thenanostructure was formed on the surface, necessary
laser pulse energy to ignite arcing wasapproximately 0.01 MJm-2, which was several
orders of magnitude lower than the heat loadaccompanied with type-I ELMs in ITER. Thus, it
raises a concern that the arcings are also triggeredin response to type-II or type-III ELMs in ITER if
the nanostructures are formed on the surface.
The cross section of unipolar arc trail recordedon the surface was analyzed with transmission
electron microscopy (TEM) using focused ion beam(FIB) milling.
From the cross sectional image, it was found that33-46% of nanostructured tungsten layer was still
remained on the surface for single spot. Roughly,with some assumptions for the spot velocity andmass density of nanostructured tungsten, the eroded
tungsten by arcing per second is estimated to be 10mg. The value will provide a benchmark to assess
the influence of arcing in fusion devices. When thespots form a group, the damages were moresignificant, indicating that the erosion rate would beincreased. Detailed investigation of erosion ofgrouped spots will be a issue for future study.
Acknowledgments
This work was performed as a part of thecollaborative research with RIAM Kyushu
University. This work was supported in part by aGrant-in-Aid for Young Scientists (A) 23686133.
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