Plasma and Fusion Research: Regular Articles

Volume 14, 3405097 (2019)

Engineering Design of the Chinese First Quasi-Axisymmetric
Stellarator (CFQS)∗)
Shigeyoshi KINOSHITA1) , Akihiro SHIMIZU1) , Shoichi OKAMURA1) , Mitsutaka ISOBE1,2) ,Guozhen XIONG3) , Haifeng LIU3) , Yuhong XU3) and The CQFS Team1,2,3,4)
1)

National Institute for Fusion Science (NIFS), Toki 509-5292, JapanThe Graduate University for Advanced Studies (SOKENDAI), Toki 509-5292, Japan
3)
Institute of Fusion Science, School of Physical Science and Technology, Southwest Jiaotong University (SWJTU),
Chengdu 610031, China4)
Hefei Keye Electro Physical Equipment Manufacturing Co., Ltd, Hefei 230000, China
2)(Received 8 January 2019 / Accepted 24 March 2019)
The Chinese First Quasi-Axisymmetric Stellarator (CFQS) is being constructed as an international joint
project between the National Institute for Fusion Science in Japan and Southwest Jiaotong University in China.
It is being designed as a low-aspect-ratio quasi-axisymmetric stellarator with a modular coil system in a CHS-qa
configuration. The physics design is almost finished, and we are now proceeding with the engineering design. Its
physical properties are as follows: toroidal periodic number m = 2, aspect ratio Ap = 4, magnetic field strength
Bt = 1 T, major radius R0 = 1 m, and typical rotational transform iota-bar = 0.4 in the all-radial region. We are
currently evaluating the design’s technical feasibility and manufacturability using a 3D CAD system, together
with structural and electromagnetic analysis software. This paper reports on the CFQS’s feasibility from anengineering design viewpoint. We are currently targeting 16 normal conducting modular coils, divided into fourtypes, and a total current of 5 MAT. Each coil has a rectangular cross section of 132×69 mm2 , and is composed of72 hollow copper conductors with water-cooling holes. These conductors carry currents of 4.3 kA, with a current
density of 74 A/mm2 .
c 2019 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: CFQS, quasi-axisymmetric configuration, engineering, normal conducting coil, stellarator
DOI: 10.1585/pfr.14.3405097

1. Introduction
The Chinese First Quasi-Axisymmetric Stellarator(CFQS) project began in July 2017 as a joint international nuclear fusion research project on advanced stellarators, involving the National Institute for Fusion Science inJapan (NIFS) and Southwest Jiaotong University in China(SWJTU) [1].
The CFQS is a low-aspect-ratio quasi-axisymmetricstellarator, based on a CHS-qa configuration, which aims
to improve neoclassical transport and explore high-β configurations by combining the best features of advanced
tokamaks and optimized stellarators [2]. It is expected tobe stable in magneto-hydrodynamic (MHD) without a conducting structure, require no current drive at high β, give
good orbit confinement, and provide neoclassical confinement equivalent to that of a tokamak. In addition, achieving a low-aspect-ratio in a quasi-axisymmetric configuration is an attractive way to minimize the costs of near-term
experiments and the capital costs of possible future power
plants. The physics design is almost finished and has preauthor’s e-mail: kinoshita.shigeyoshi@nifs.ac.jp∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

viously been reported [1, 3]. In this paper, we discuss thefeasibility of the CFQS’s design from an engineering viewpoint.

2. Physical Specifications
Figure 1 shows three poloidal cross sections of theequilibrium magnetic surfaces without the plasma pressure eﬀect. The CFQS design reduces the major radius,increases the aspect ratio, and reduces the magnetic fieldslightly in order to reduce costs and make it easier to manufacture by using the 2b32 CHS-qa configuration [2].
The CFQS’s main parameters are as follows: toroidal

Fig. 1 Shape of the equilibrium magnetic flux surfaces for β =0.

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Volume 14, 3405097 (2019)

Fig. 3 Schematic of the CFQS. CSS: Cage-type support structure. CPS: Central pillars. DBS: Diagonal beams. VV:
Vacuum vessel. LRP: Large rectangular port.

Fig. 2 Equilibrium parameter profile, showing (a) the rotationaltransform and well depth and (b) the Fourier spectrum ofthe magnetic field strength.

periodic number m = 2, aspect ratio Ap = 4, magnetic
field strength Bt = 1 T, and major radius R0 = 1 m. Figure 2 (a) shows the typical rotational transform and magnetic well profile. We are able to achieve low magnetic
shear and a shallow well structure throughout the entire
region. Figure 2 (b) shows Fourier spectra of the magnetic field strength in Boozer coordinates. Here, the dominant ripple is the toroidal mode B1,0 , which indicates a
tokamak-like or quasi-axisymmetric configuration. TheCFQS project aims to confirm the eﬀectiveness of thisequilibrium.

3. Overall Structure
Figure 3 shows a schematic of the CFQS, which is approximately 2790 mm high and has an outer diameter of4300 mm. The electromagnetic force applied to the modular coils is supported by a cage-type support structure,with diagonal beams to absorb the overturning force andtwo central pillars to absorb the centripetal force.
These devices are installed in a 15 × 45 m torus hallwith power supplies for plasma heating, as shown inFig. 4. The plasma heating apparatus is intended to provide for tangential neutral beam injection (NBI) of 40 kV
and 1 MW and electron cyclotron resonance heating of
54.5 GHz and 450 kW. For plasma diagnostics, we will use

Fig. 4 Torus hall for the CFQS.

a Thomson scattering system, a microwave interferometer,a charge exchange system, a heavy ion beam probe, andmagnetic measurements, among others.

4. Coil and Support Structure
Table 1 gives technical data for the coil system, which
is illustrated in Fig. 5. A basic quasi-axisymmetric configuration can be produced by four types of modular coils (fora total of 16), whose shapes are shown in Fig. 6. However,in order to allow for flexible configuration, we have also installed four poloidal field coils (to move the magnetic axishorizontally) and eight toroidal field coils (to change therotational transform). The coil currents are driven by four
power supplies for the modular coils, two for the poloidal
field coils, and one for the toroidal field coils.

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Table 1 Technical data for the coil system.

Fig. 5 Schematic of the coil system, showing 16 modular coils(brown), 4 poloidal field coils (yellow), and 8 toroidal
coils (pink). The outermost magnetic flux surface isshown in beige.

Fig. 6 Modular coils, showing the legs and supports.

Fig. 7 Cross section of a modular coil, which is composed of 72
hollow conductors.

Figure 7 shows the cross section of a modular coil.
Since the current density is very high, the conductor temperature rises by about 20◦ per second of operation. In
order to absorb this heat before the next shot, the coils arecooled by pure water.
Figure 8 shows the electric magnetic force profiles applied to the four types of modular coil. The forces’ magnitudes and directions are very complicated, and their characteristics are not easy to describe. They are similar toFig. 8 Electromagnetic force profiles. Here, vector diagrams
show the forces applied to the four types of modular coils.

those of a tokamak’s toroidal field coil (TFC) but, since it
is necessary to reveal even the components that are negligible for tokamaks, analyzing the modular coils is morediﬃcult. Their characteristics include vertical forces andvertically asymmetric overturning moments, and the generation of forces in the toroidal direction that cause attraction between adjacent coils. Naturally, as with tokamaks,
we also need to consider large centripetal and expansion
force components, and we need a support structure that canwithstand such complex forces.

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Fig. 9 (a) Cage-type structure. Outer pillars and diagonal beams
prevent the overturning forces rolling the coils over.
Due to the use of tangential NBI, it is not possible toinstall pillars around the large rectangular port, so weinstead use diagonal beams.

Fig. 10 Preliminary structural analysis based on the applied electromagnetic forces. The estimated maximum deformation is 1.4 mm.

Fig. 9 (b) Absorbing centripetal forces. Since it is not possible
to realize the wedge structure used in tokamaks, partial reinforcing plates and two central pillars absorb thecentral forces.

Fig. 11 Schematic of the vacuum vessel, which will be manufactured by welding together four sections in the toroidal direction and supported by eight leaf-type legs. The flanges
for welding are shown in purple. The vessel has eight
winding frames for the TFC (shown in blue).

Fig. 9 (c) Absorbing expansion and toroidal forces. Beams between the coils are used to cancel out the toroidal force
imbalance and the outer pillars absorb the coils’ expansion forces.

Figure 9 shows the support structures used. Figure 9 (a) shows a cage-type structure with diagonal beams
that prevents deformation by the overturning moments orvertical forces and handles the entire load. It also has,among other components, a vacuum vessel, other coils,
and a diagnostic system attached to it. For use in future
NBI experiments, we adopted a design that does not havepillars around the large rectangular port, instead surrounding it with diagonal beams.
Large tokamaks are typically designed with TFC
wedges to support the centripetal forces. Since the CFQS
cannot take this approach due to shape constraints, it instead absorbs the centripetal forces with two central pillars,
as shown in Fig. 9 (b). The attractive forces between adjacent coils are canceled out by beams sandwiched between
the coils, as shown in Fig. 9 (c). For parts that experience
substantial coil deformation due to expansion forces, theseforces are absorbed by the outer pillars.
Figure 10 shows the results of a preliminary structuralanalysis in terms of the electromagnetic forces, which indicate that the estimated maximum displacement is 1.4 mm.

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Fig. 13 Preliminary structural analysis in terms of the applied atmospheric pressure. The estimated maximum deformation will be 0.4 mm.

Fig. 12 Vacuum vessel cross section, showing the shapes of theplasmas for the (a) standard 2b40 equilibrium configuration (aspect ratio: 4.0) and (b) 2b32 island bundle diverter
configuration (aspect ratio: 3.2). VVI: Inner vacuum vessel surface. CCS: Current carrying surface, on which thecenter of the modular coil is placed. ISC: Innermost peripheral surface of the modular coil. IBD: Island bundle
diverter.

5. Vacuum Vessel
Figure 11 shows a schematic of the vacuum vessel,
which will be manufactured by welding together four sections in the toroidal direction. The flanges used for welding
are shown in purple. Since the electromagnetic forces on
the vacuum vessel are expected to be small, it will be fabricated from SUS316L with relatively thin walls (6 mm). Asa result, the one-turn resistance is roughly 0.3 mΩ which
we believe to be suﬃciently large while not producing significant Joule heating. The walls will be conditioned bybaking them at 130 - 150◦ via induction heating at about
500 Hz.
The vacuum vessel is fixed to the cage-type support
structure shown in Fig. 3 by eight leaf-type legs, added toallow for the vacuum vessel’s thermal expansion during
baking. Winding frames have also been included on thevessel in order to wind the TFCs. More than 40 large ports,
with a diameter of at least 114 mm, will be provided forheating and measurement.
The vacuum vessel will be assembled as follows.

(1) Manufacture the four sections with port flanges.
(2) Perform a vacuum leak test by temporarily connecting
the sections with rubber O rings.
(3) Insert the vacuum vessel sections into the modularcoils.
(4) Weld the four sections together from the inside.
We have drawn up a design plan in order to carry out thisprocess and conduct various equilibrium experiments.
Figure 12 shows a cross section of the vacuum vessel. The inner surface cross section (VVI) is much larger
than that of the 2b32 reference surface defined for the island bundle diverter experiment [4], with the minimum gap
between the 2b32 plane and the VVI surface being 30 mm.
The outermost magnetic surface of the standard 2b40 equilibrium configuration is narrower than the 2b32 surface.
The width and height of the VVI are both at least 450 mm,
which is suﬃcient for people to work inside. Using sucha large vacuum vessel also helps to make the experiment
more flexible. Two large rectangular ports are also included, both for workers to enter and for use in NBI andThomson scattering.
Figure 13 shows the results of a structural analysis
based on the applied atmospheric pressure. Here, the estimated maximum displacement is 0.4 mm, which is within
an acceptable range.

6. Next, Steps
The engineering designs for the modular coils andvacuum vessel are complete, and the detailed design processes have begun. We are also producing a mockup coil toconfirm the modular coil design’s eﬀectiveness and manufacturability.
Work has also begun on the conceptual design of the3405097-5

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power supplies and water-cooling systems. Shortly, wewill begin the detailed design process for the torus hall andexamination of the diagnostic system.

7. Summary
The CFQS is being constructed as an international
joint project between the NIFS and SWJTU, and will produce several quasi-axisymmetric stellarator configurations.
The physics design is almost finished, and now we are discussing the project’s feasibility from an engineering viewpoint. An overall structure and layout has also been proposed for the CFQS’s torus hall.
In addition, the engineering design processes havebeen completed for the modular coils (and their supportstructures) and the vacuum vessel, and analyses conductedto preliminarily confirm their eﬀectiveness.
The vacuum vessel design takes both the assembly
procedure and the need to conduct experiments with varying plasma configurations into account.
[1] H. Liu, A. Shimizu, M. Isobe et al., Plasma Fusion Res. 13,3405067 (2018).
[2] S. Okamura, K. Matsuoka et al., Nucl. Fusion 41, 1865
(2001).
[3] Y. Xu, H. Liu et al., 27th IAEA Fusion Energy Conference(FEC 2018), Ahmedabad, India, (2018) EX/P5-23 (Poster).
[4] S. Okamura, H. Liu et al., 45th EPS Conference on Plasma
Physics, (2018), Prague, Czech Republic, Vol. 42A, P5.1034(Poster).

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