24P052-P
Optimization of the Vacuum Pumping System for the Closed Helical Divertor inthe Large Helical Device III
LHDにおける閉ダイバータ用真空排気装置の最適化検討Ⅲ
Mamoru Shoji, Suguru Masuzaki, Tomohiro Morisaki, Masahiro Kobayashi, Masayuki Tokitani
and Yasuhiko Takeiri
庄司 主, 増崎 貴，森崎友宏，小林政弘，時谷政行，竹入康彦
National Institute for Fusion Science322-6, Oroshi-cho, 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
A vacuum pumping system for a Closed Helical diverotr (CHD) in the Large Helical Device (LHD) has
been designed for active control of the periperal plasma denstiy. The distance between the puming sytem
and divertor plates is short (~0.1m) in the CHD configuration. One of the major concerns for designing thepumping system is reduction of the heat load by the heated divertor plates with good puming efficiency.
The heat load and the pumping efficiency are investigated using a neutral particle transport simulation code,
which proposes an improved design of the pumping system for the CHD configuration.

1. Introduction
Physical issues required for a vacuum pumpingsystem installed in the divertor region in futurefusion reactors are helium ash removal, impurity
reduction in the core plasma, neutral particle
pumping in the peripheral plasma.
Two test modules of a closed helical divertor(CHD) have been installed in the inboard side of
the torus in the Large Helical Device (LHD) fromthe last experiment campaign (2010y) [1]. The
CHD consists of three instrumental components:
slanted divertor plates, a roof shaped dome andtarget plates. It successfully enhanced the neutral
particle density behind the dome by more than
one-order of magnitude compared to that in theopen divertor [2].
In the near future, a vacuum pumping systembehind the dome will be installed for active
control of the peripheral plasma density. Becauseof the short distance (~0.1m) between the divertor
plates and the pumping system, heat loads byradiation from the heated divertor plates and by
thermal conduction due to neutral particles
released from the plates should be carefully
considered for designing the pumping system.
Using a neutral particle transport simulation code,
an improved design of the pumping system
optimized to the CHD configuration is proposed.
2. Vacuum Pumping System for the Closed
Helical Divertor
The vacuum pumping system consists of thethree components: water cooled (WC) blinds, liquid
nitrogen (LN2) cooled chevrons and gas/liquid

helium (LHe) cooled panel. Figure 1 shows athree-dimensional model of the pumping systeminstalled behind the dome (a half part). There are
two heat transfer mechanisms to the LHe cooled
panel: radiation from heated divertor plates, thermal
conduction due to neutral particles. The LN2 cooled
chevrons are covered with the WC blinds forprotecting from the heat load. The LHe cooled
panel is enclosed with the LN2 cooled chevrons forminimizing the heat load to the panel.
A previous analysis using a finite element
method based software for multi-physics analysis(ANSYS) and a neutral particle transport simulation
code (EIRENE) proposed the above configuration
for reducing the heat load with good pumping
efficiency. It showed that the heat load on the LHecooled panel by thermal conduction due to neutral
particles is dominant over that by radiation. It also
proved that the installation of buffer plates, on
which many deep grooves are scratched, is very
effective to minimize the heat load [3].
3. Improvement of the Configuration of thePumping System
The configuration of the WC component is
modified in order to improve the pumping
efficiency. It is defined as the current of neutral
particles reaching to the LHe cooled panel to thetotal plasma current flowing to the divertor plates.
The neutral current is counted by tracking test
particles (representatives of neutral hydrogenatoms/molecules) in the neutral particle transport
simulation code. Examples of the track of the test
particles released from the divertor plates are

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illustrated as gray thin lines in Fig. 1.
The pumping efficiency and the heat loads arecalculated in the case of four different WC
components which cross-sections (a half part) areillustrated in Fig. 2. Type I is the original
configuration which is previously proposed [3].
Type II is a configuration with expanded inlets of
the WC blinds with a small sized dome toefficiently take in neutral particles. Type III is thatwith a slit at the bottom of the WC component inorder to introduce neutral particles beneath thepumping system. Type IV is that with a buffer plateon the vacuum vessel in the inboard side for
reducing the energy of neutral particles impacting
the LN2 cooled chevrons through the bottom slits.
In the all configurations, the buffer plates are
installed on the WC components and the LN2cooled chevrons. A 50% transparent mesh ismounted between the WC and LN2 cooled
components for microwave protection.
The calculations of the pumping efficiency andthe heat load on the three components for the four
different WC components are shown in Fig. 3. The
current flowing to the divertor plate (IDiv) is fixed tobe 1A. The pumping efficiency is improved bymodifying the configuration of the WC
components: the expanded inlet of the WC blind,the bottom slit. The heat load on the LN2 chevrons
increases by these two modifications. The
calculations show that the buffer plate in theinboard side of the torus is effective for reducing
heat load on the LN2 cooled chevrons.
4. Summary
The neutral particle transport simulation showsthat the improved configuration of the pumping
system with the expanded inlet of the WC blinds
and with the bottom slit can enhance the pumping
efficiency by about 60% compared to that in theoriginal configuration. It also indicates that the heatload by conduction due to neutral particles on theLN2 cooled chevrons rises by about factor 20 for
the improved configuration compared to that in theoriginal configuration. Suppression of the heat load
by about factor 3 is expected by installing the buffer
plate in the inboard side of the torus with keeping
the enhanced pumping efficiency.
References[1] A. Komori, H. Yamada, S. Imagawa et al.: Fusion
Sci. Technol. 58 (2010) 1.
[2] S. Masuzaki, M. Kobayashi, M. Shoji et al.: Plasma
Fusion Res. 6 (2011) 1202007.
[3] M. Shoji, S. Masuzaki, T. Morisaki et al.: Plasma
Fusion Res. 6 (2011) 2401035.

Fig.1. A three-dimensional model of the pumping
system for the closed helical divertor

Fig.2. Cross sections (a half part) of the four different
configurations of the WC components in the pumping
system

Fig.3. Heat load and pumping efficiency in the four
different configurations of the WC components

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