24P027-P
Baffle-structured Divertor Experiments in LHD
LHDにおけるバッフル構造ダイバータ実験
Tomohiro Morisaki1, Suguru Masuzaki1, Masahiro Kobayashi1, Mamoru Shoji1,Junichi Miyazawa1, Ryuichi Sakamoto1, Gen Motojima1, Motoshi Goto1, Hisamichi Funaba1,
Hirohiko Tanaka1, Kenji Tanaka1, Ichihiro Yamada1, Satoshi Ohdachi1, Yukio Nakamura1,
Hiroshi Yamada1 and LHD Experiment Group1
森崎友宏1，増崎 貴1，小林政弘1，庄司 主1，宮澤順一1，坂本隆一1，本島 厳1，
後藤基志1，舟場久芳1， 田中宏彦1，田中謙治1， 山田一博1，大舘 暁1，中村幸男1，山田弘司1，LHD実験グループ1
National Institute for Fusion Science, Oroshi, Toki, Gifu 509-5292, Japan
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
The baffle-structured helical divertor has been constructed in two helical sections in LHD to control edge
plasma via neutral particles. The divertor consists of vertically installed divertor plates and domes, which
spontaneously form the baffle-structure. Before installing the pumping system, effect of the baffle-structure
on neutral gas compression was investigated. In the experiment, it was demonstrated that the neutralpressure in the baffle-structured divertor was more than 10 times higher than that in the open region.

1. Introduction
The neutral particle control with a divertor is a
crucial issue for fusion research. Especially inreactors, hydrogen isotopes, helium ash andimpurities have to be pumped out to sustain theburning plasma in steady-state. In tokamaks,
particle control experiments with closed divertors
have been performed. On the other hand, in helicaldevices, closed divertor experiments have also been
conducted using magnetic islands, i.e. island
divertor. In LHD the super dense core plasma withthe formation of the internal diffusion barrier wasobtained by the highly pumped local island divertor(LID) [1]. However, due to the small wetted area of
LID, it cannot be utilized for the long pulse or
steady-state operation. Hence helical divertor (HD)which is intrinsically equipped with the heliotron
configuration shall be utilized for the future devices.
Recently the high density operation with HD has
been performed to investigate the prospect of HD.
According to the experimental results, however, theneutral pressure in HD was ~ 0.01 Pa at the highest,in spite of the high line averaged density more than
1020 m-3, although ten times higher neutral pressure
is necessary for the effective pumping. Therefore
the closed configuration with baffles is urgently
required to increase the neutral pressure in HD.
2. Baffle-structured divertor
After the numerical study for the optimization of
the configuration, baffles combined with target
plates and domes have been installed in LHD,
which are made of graphite to withstand the highheat load more than 1.5 MW/m2 (steady-state). In

2010, baffles for two helical sections (20 %) wereinstalled, although the pumping system has notbeen installed yet. Fig. 1 shows thebaffles-structured helical divertor installed on theinboard side of the torus for one helical section inthe LHD vacuum vessel. The schematic view of its
cross section is also presented in Fig. 2. It is foundthat divertor plates combined with baffles face theprivate region to concentrate neutrals in the closedregion. In the private region the “dome” structure isinstalled. To measure the neutral pressure in thebaffle-structured HD, three ASDEX-type fast ion
gauges are installed under the dome. Another fast
ion gauge is also installed in HD without
baffle-structure, to compare the neutral pressure
each other.

Fig. 1. Baffle-structured divertor installed in LHD.

Refrences[1] T. Morisaki, et. al., Phys. Plasmas 14 056113(2007).

[2] S. Masuzaki, et.al., Plasma Fusion Res. 6,1202007 (2011).
[3] S. Sudo, et al., Nucl. Fusion 30 (1990) 11.

Fig. 2. Schematic of its cross section.

3. Experimental results
Experiments were carried out in density
ramp-up discharges where the magnetic axis was atR = 3.6 m [2]. Fig. 3 shows the time evolution ofline averaged electron density ne at the midplane
and the neutral pressure P0 in HD with and without
baffles. Hydrogen gas puffing was performed
during the discharge as shown in Fig. 3 (b), thus theline averaged density increased up to 7×1019 m-3.
The neutral pressures in both configurations
increased with the line averaged density. It isfavorable that the neutral pressure in thebaffle-structured HD was more than 10 times higher
than that in HD without baffles, which is called“neutral particle compression”. No negative effecton the core plasma was observed during thedischarge. Furthermore, in the detailed inspection
after vent, no serious damage on tiles was found.
In the long pulse operation of the SDC
discharge in the open HD, it was usual that the edge
electron density ne increases gradually during thedischarge. The increased edge ne enhances the edge
radiation, and finally brings about the radiation
collapse when it exceeds the density limit [3]. In
this experimental campaign, however, with thebaffle-structured divertor of only two sections, theenhanced recycling is not observed, i.e., edge ne iskept low during the discharge for ~ 6 sec. The low
recycling regime is achieved even though the active
pumping in the baffle-structured divertor has notbeen installed. The reason for this phenomenon is
not clear up to now.
Summarizing the initial experimental results, it
can be concluded that the baffle-structured HD
works well and is expected the efficient neutral
particle control when the pumping system isinstalled in the dome.

Fig. 3. Time evolution of (a) line averaged
density, NB injection scheme, and (b) neutralpressure, together with gas puff injection scheme.

Fig. 4. Temporal evolutions of neutral pressurein open divertor configuration (blue) andbaffle-structured divertor configuration (red).