S9-3
The progress of study of steady-state discharge for helical plasma
and its international collaboration
ヘリカルプラズマにおける高性能定常放電の進展と国際協力
Hiroshi Kasahara, Yasuo Yoshimura, Kazunobu Nagaskai1, Massyuki Tokittani, Yoshio Ueda2,
Naoko Ashikawa, Testsuo Seki, Kenji Saito, Ryohsuke Seki, Shuji Kamio, Goro Nomura, ShinKubo, Takashi Shimozuma, Hiroe Igami, Hiromi Takahashi, Toru Ii, Sakuji Kobayashi, Satoshi Ito,
Yoshinori Mizuno, Kouta Okada, Mamoru Shoji, Kenji Tanaka, Hirohiko Tanaka, Hayato Tsuchiya,Takihio Tokuzawa, Suguru Masuzaki, Gen Motojima, Junichi Miyazawa, Ichihiro Yamada, Takashi
Mutoh, Hiroshi Yamada, Yasuhiko Takeiri, Akio Komori and LHD experiment group
笠原寛史, 吉村泰夫，長崎百伸1, 時谷政行, 上田良夫2, 芦川直子, 関哲夫, 斉藤健二, 關良輔, 神尾修治, 野村吾郎, 久保伸, 下妻隆, 伊神弘恵, 高橋裕巳, 伊井亨, 小林策治, 伊藤哲,水野嘉識, 岡田広太, 庄司守, 田中謙治, 田中宏彦, 土屋隼人, 徳沢季彦, 増崎貴, 本島厳,宮沢順一, 山田一博, 武藤敬, 山田弘司, 竹入康彦, 小森彰夫, LHD実験グループ
National Institute for Fusion Science, 322-6, Oroshi-cho, Toki-city, Gifu, 509-5292, Japan核融合科学研究所 〒番号509-5292 岐阜研土岐市下石町322-61) Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan2) Graduate School of Engineering, Osaka University, 2-1, Yamadaoka, Suita, Osaka, 565-0871, JapanAn ultra-long-pulse plasma with a duration time of 48 min, a line-averaged electron density of 1.2x1019 m-3, andelectron and ion temperatures (Te0, Ti) of 2 keV has been achieved by the averaged heating power (PICH+ECH) of 1.2 MW.
The ultra-long-pulse plasmas are often terminated by a sudden increase in carbon impurity, which might be caused byentrance of mixed-material deposition layers on the plasma facing surface (PFS). A large amount of mixed-material
layers, consisting mainly of carbon (> 90%) and iron impurities, are formed over a wide surface area of the PFS.
Carbon impurity originally from the divertor region and iron impurity from the first wall by physical sputtering aredeposited on the PFS during the steady-state operation (SSO). On the PFS, helium radiation damage is also created justunder the mixed-material deposition layers, and they increase retention of helium. Such a long deposition process plays
an important role of sudden impurity source causing the particle source and plasma termination in the SSO.

1. Introduction
Long-pule plasma duration is one of the mostimportant issues for the realizing commercial
fusion reactor, and long-pulse discharges withplasma duration time over a couple ten minutes
have been investigated in the Large HelicalDevice (LHD) using helical plasmas. In LHD,
only superconducting coils without current drive
will fulfill magnetic configuration, and it is greatadvantage to maintain plasma duration.
2. Extending the operation region of long-pulse
plasma duration
The high-performance plasma experiments with aline-averaged electron density (ne) > 1x1019 m-3 andTe > a few keV have been investigated in order tounderstand PWI effect on a long time scale (> a few
thousand seconds) in LHD. Figure 1 shows anultra-long-pulse plasma discharge of a helium
plasma with ne ~ 1.2 x1019 m-3, Te ~ Ti ~ 2keV, PRF~ 1.2 MW (PICH ~ 0.9MW, PECH ~ 0.3MW), andduration time ~ 2,859 sec. The newly achieved
heating energy of 3.36GJ is twice as the previous
world record, 1.6GJ, with PRF ~ 0.5MW and ne ~Fig. 1 Helium dischargehydrogen minority regime.

waveform

in0.4x1019 m-3 in LHD [1]. Three types of ICRF
antennas were installed at different toroidal sections
in order to avoid local hot spots around each ICRF
antenna. Real-time feedback control for impedance
matching, antenna phasing and heating power
boosting for unexpected impurity influx were also
developed. These heating techniques enabled stable
plasma heating. In early long-pulse discharge withthe duration time ~ 19 min with similar plasma
performance, electron density rose due tooverestimate
gas-fueling
rate
[2].
The
overestimation of gas-fueling rate, which wasstrongly related to time evolution of wall-pumping
rate, was observed around duration tiem > 1000 sec.
In
thelastcampaign,aproportional-integral-derivative control method wasused to maintain a constant level of the plasma
density, and plasma duration time reached 48 min.
Radiation power (Prad) was less than approximately
17 % of PRF, where the carbon and iron impurity
accumulation was negligible in the discharge. The
spike frequency of the line intensity for the carbon
spectrum began to increase after 600 sec. At thattime, the divertor temperature was almost saturated
at 460 ºC. Many flashes are observed with amonitor TV camera which views mostly outside of
LCFS and the divertor region. These flashes are notalways synchronized to spikes in total radiation andline intensities in impurity spectra. It may be causedby exfoliation of the mixed-material layers andinitiation of the arcs. Both the frequency andintensity are increased as the discharge time goeson, which suggest that a large size of carbon
mixed-material layers on dome plates near divertors
are ejected into the plasma. The ultra-long-pulse
plasma was terminated within 0.1 ~ 0.2 sec, where
the radiation power fraction reached 38 % of PRF.
This suggests that it is very important to analyze
mixed-material layers and understand the growthmechanism in the ultra-long-pulse plasma
operation.
3. Growing mixed-material layer by the Title
The Figure 2 shows the cross-sectionaltransmission electron microscope (TEM) image ofthe stainless steel specimen (SUS316L) andcorresponding
schematicviewof
themicrostructural modifications such as bubbles anddeposition layer. The mixed-material deposition
layer with the thickness of ~ 5 nm was formed on
the top surface. In addition, fine He bubbles withsize of 2-3 nm were densely formed on SUS316L
matrix just under the deposition layer at the sametime. This specimen was located on the equivalent
position of the first wall surface, and it was exposed

Fig. 2 Cross-sectional TEM image of thestainless steal specimen after only exposer
to SSO discharge (1000 sec) at the first wall
equivalent position.

to SSO discharges up to duration time ~ 1000 sec.
The thickness of deposition layer was increased
under continuous heat and particle fluxes with anincrease in steady-state plasma duration. Majority
composition of the deposition layer was carbon(>90%), and a few % of Fe element was observed.
In the previous studies of the impurity transport andformation of the mixed-material, these thickdeposition layers were observed in particular dome
plates near the divertor [3]. Thus, the short-range
transport of the carbon impurity from divertor
plates plays an important role for the formation ofcarbon rich mixed-material deposition layer. Sincesuch layers are hard and brittle, deposition layers
are easily removed as a flake. In theultra-long-pulse discharge for helium plasma, theplasma termination process was followed by large
amount of mixed-material ejection from near dome
plates, locally. During strong flashes, the line
intensity of carbon spectrum was increased largely
30 ~ 40 times, and finally became 100 times large.
The frequency of Carbon III emission was clearly
increased just after long-pulse discharge with theduration time of 30 min for helium plasma rather
than after repeated high-power short pulse
operation for hydrogen plasma using NBI. Throughthe international collaborations in steady-state
devices, we have to assess the mixed-material effectto each other, and it will be more important issues
for fusion reactors.
Acknowledgments
This work was supported by LHD projects(ULRR022-023, 701, 703, 801 – 806, KLPR016
and KUGM051).
References[1] T. Mutoh et al.: Nucl Fusion 53 (2013) 063017.
[2] H. Kasahara te al,: Phys. Plasmas 21 (2014) 57.
[3] M. Tokitani et al.: J. Nucl. Mater. 438 (2013) S818.