Plasma and Fusion Research: Rapid Communications

Volume 11, 1205030 (2016)

Development of New Concept In-Vessel Cryo-Sorption Pump
for LHD Closed Helical DivertorTakanori MURASE, Gen MOTOJIMA, Hirohiko TANAKA, Tomohiro MORISAKIand Masahiro MITA1)
1)National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, JapanKYODO International Corporation, 2-10-9 Miyazaki, Miyamae-ku, Kawasaki 216-0033, Japan(Received 18 January 2016 / Accepted 23 February 2016)

The in-vessel cryo-sorption pump for the Closed Helical Divertor (CHD) in the Large Helical Device (LHD)
has been developed at the National Institute for Fusion Science (NIFS). An organic adhesive-free bonding technique for attaching activated carbon pellets to a copper cold panel was invented, which employs the indium
solder with intermediate materials. The prototype of the CHD with the newly developed cryo-sorption pump
was installed in the LHD. Performance of the cryo-sorption pump was estimated in the LHD vacuum vessel. A
satisfactory result of the maximum pumping speed up to 9 m3 /s was obtained with one divertor module in one
toroidal section (10% of the torus), which is equivalent to the required pumping speed of the CHD.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: in-vessel cryo-sorption pump, inorganic adhesion, closed helical divertor, LHD
DOI: 10.1585/pfr.11.1205030
In order to obtain the high-performance plasma intoroidal devices, the neutral particle control by divertor isindispensable. For the eﬀective divertor function, a strong
pump is required, together with an eﬀective closed configuration with baﬄes. In DIII-D [1] and EAST [2], invessel cryo-condensation pumps installed near the divertor plates are employed. The cryo-condensation pumpevacuates neutral particles, condensing them on the cryopanels cooled by liquid helium (∼4 K). Thus a huge cryogenic system consisting of the helium liquefier and supply lines is required. On the other hand, the cryo-sorption
pump evacuates neutrals, adsorbing them on a cryo-panel(∼20 K) with highly adsorbing materials, e.g., activatedcarbon or zeolite. Since the temperature of the cryo-panelof the sorption pump may be higher than that of the condensation pump, the required cryogenic system may besmall.
For bonding the activated carbon to the cryo-panel,the organic adhesive has been used so far, which, however, results in the unfavorable outgassing during the noncooled phase. Furthermore, deterioration of the organic
adhesion due to age sometimes causes the activated carbon to be detached from the cryo-panel. For these reasons, the cryo-sorption pump has not been used as an invessel pump for divertor pumping in fusion experimental
devices, although the high pumping speed due to the highconductance is expected in the directly connected system.
Therefore, the bonding technique without organic adhesiveis desired to develop the eﬃcient divertor pumping system.
At the NIFS, a new soldering technique between activatedauthor’s e-mail: murase.takanori@lhd.nifs.ac.jp

Fig. 1 Cross-section of the in-vessel cryo-sorption pump. The
pump is installed behind the dome structure in CHD.

carbon and copper has been developed [3] in the collaboration framework with KYODO International Corporation,and the prototype of the outgassing-free in-vessel cryosorption pump was installed in the vacuum vessel of theLHD. In this paper, a technical summary of the newly developed cryo-sorption pump and preliminary results fromthe performance test are presented.
Figure 1 shows the cross-section of the in-vessel cryosorption pump for the LHD divertor. The pump is installedbehind the dome structure of the CHD [4, 5]. The watercooled louver blind structure is installed between the liquid
nitrogen (LN2 ) cooled chevron-type shield and the divertor
striking point in order to shield the direct radiation fromthe divertor plasma and its striking point. The cryo-panel isbehind the LN2 -cooled shield (∼80 K) in order to shield thedirect radiation from the water-cooled shield. The cryo-1205030-1

c 2016 The Japan Society of Plasma
Science and Nuclear Fusion ResearchPlasma and Fusion Research: Rapid Communications

Volume 11, 1205030 (2016)

Fig. 2 Schematic view of the cryo-panel and its detail of the adhesive part.

Fig. 3 Pumping speed as a function of the vacuum vessel(plenum) pressure.

panel consists of the copper heat sink and the cooling tubes
for gas helium coolant. On the surface of the heat sink,
the activated carbon pellets are bonded with indium solder,which is described in detail in the following paragraph.
The activated carbon is the well-known porous material. However, its pores are too fine (0.4 - 50 nm in diameter) for normal tin solder to permeate. Therefore, it is difficult to bond the activated carbon to the copper heat sinkby soldering. A new adhesion technique with the indium
solder and intermediate materials is developed. Figure 2
shows the schematic view of the cryo-panel and its detail ofthe adhesive part. Both sides of the cryo-panel are covered
with activated carbon pellets of 4 mm in diameter. The pellets are precisely aligned along the grooves on the copper
heat sink to maximize the quantity to be bonded. Before
soldering the pellets to the copper heat sink, the bondingarea of each pellet is precoated with titanium and silver
by the sputtering technique. These intermediate materials
dramatically improve the solder wettability.
Six units of the newly developed cryo-sorption pumpwere installed in one of the ten CHD modules in LHD. The
pumping speed and capacity were measured, introducing
constant and continuous hydrogen flow into the LHD vacuum vessel. The eﬀective pumping area of the cryo-panel
is 0.21 m2 in one toroidal section. The pumping speed up
to 9 m3 /s was recorded, as shown in Fig. 3, which is almost
the same as the target pumping speed of one CHD module [4, 6]. The pumping speed slightly decreases as there isan increase in the plenum pressure. It is thought that theincrease of the plenum pressure causes deterioration of thethermal insulation of the cryo-panel, which results in theincrease of the panel temperature. The pumping capacityof 3,320 Pa m3 was also confirmed, which corresponds tothe total amount of the fueled particles in the high-density
experiment for one day.
The new bonding technique between activated carbonpellets and the copper heat sink without organic adhesive
was established. This outgassing-free cryo-sorption pump
has a potential not only for fusion plasma experiments but
also for various applications of the ultra-clean vacuum environment, e.g., process plasma instruments, semiconductor factories, etc.
Authors would like to thank the LHD ExperimentGroup members and the technical staﬀ for their supportand help in the test in the LHD. This work is funded by
NIFS14ULPP801, and performed under the collaboration
with KYODO International Corporation.
[1] K.M. Schaubel et al., Advances in Cryogenic Engineering
39, 1583 (1994).
[2] Q.S. Hu et al., Fusion Eng. Des. 85, 1508 (2010).
[3] Japanese patent pending number 2014-209086 (in Japanese).
[4] S. Masuzaki et al., Plasma Fusion Res. 6, 1202007 (2011).
[5] T. Morisaki et al., Nucl. Fusion 53, 063014 (2013).
[6] S. Masuzaki et al., Fusion Eng. Des. 85, 940 (2010).

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