24D09
Design Window Analysis for the Helical DEMO Reactor FFHR-d1
ヘリカル原型炉FFHR-d1の設計領域解析
Takuya Goto, Junichi Miyazawa, Hitoshi Tamura, Teruya Tanaka, Nagato Yanagi, Akio Sagara,
the FFHR Design Group後藤 拓也，宮澤 順一，田村 仁，田中 照也，柳 長門，相良 明男，FFHR設計グループ
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 conceptual design for the LHD-type heliotron DEMO reactor FFHR-d1 has started from the last fiscalyear under the Fusion Engineering Research Project in National Institute for Fusion Science. As the firststep, design window analysis is carried out using the system design code HELIOSCOPE. Density andtemperature profiles which directly extrapolated from the LHD experimental results are adopted in theanalysis to enhance the reliability. One of the key issues is the radial-build design at the inboard side of thetorus. Therefore, further detailed analysis for the optimization of blanket thickness and the cross-sectional
shape of the helical coils are being carried out.

1. Introduction
According to the steady progress in the plasma
experiments and related engineering R&Ds of theLarge Helical Device (LHD)[1], conceptual
design studies for an LHD-type heliotron fusion
reactor FFHR[2] has been advanced from the1990’s. The latest design FFHR-2m2 proposed acommercially-attractive design concept whichenables a 30 full power years operation. Since thelast fiscal year, a new conceptual design, named
FFHR-d1, has started under the Fusion
Engineering Research Project in National
Institute for Fusion Science (NIFS). By utilizing
design bases established so far on the conceptual
designs of the FFHR series [2], this newconceptual design focuses on a demonstration ofmaintainability, tritium self-sufficiency, inherent
safety and a net electric power generation as thenext step DEMO reactor.
The conceptual design activity and related
engineering R&Ds for FFHR-d1 has been
conducted by 13 task groups in the Fusion
Engineering Research Project. As the first step of
the conceptual design, design window analysis iscarried out using the system design code forheliotron reactors HELIOSCOPE [3]. In thispresentation, the initial result of the design
window analysis is shown and the direction of thedesign and the issues in the design which clarified
through the analysis are discussed.
2. Prerequisite in the design
As described in the previous section, FFHR-d1 isa DEMO reactor, which will be constructed as thenext device after LHD to demonstrate a steady-state

control of a burning plasma. Therefore, highreliability in the core plasma design is required.
Here we adopted a new approach to determine thecore plasma profile named DPE (Direct Profile
Extrapolation) method, which directly extrapolates
the core plasma profile obtained in the experiment
to a reactor condition [4]. This method eliminatesuncertainties arose from the assumption in theplasma profile and the plasma volume. The
existence of a stable equilibrium is also secured.
As shown in Fig. 1, the space for blankets of an
LHD-type heliotron device is limited especially at
the inboard side on vertically elongated poloidal
cross-section. In the design study of FFHR-2m2,
the design point with this minimum blanket spaceof >1.0m were considered to secure a sufficient
tritium breeding ratio and suppression of fastneutron flux on the superconducting coil.
Consequently, the major radius of Rc=17m isselected for FFHR-2m2. Such enlargement ofreactor size leads to a reduction of thermal andneutron wall load and a flexibility in the design ofmaintenance ports. However, the magnetic fieldstrength is restricted to suppress the stored magnetic
energy Wmag below 160GJ, which is considered as
the maximum allowable value to wind a large
helical coil with the ITER-relevant technology. On
the other hand, higher magnetic field strength isfavorable from the standpoint of the core plasma
design. Since a plant-scale electricity generation
and a long-term operation are not necessaryconditions of a DEMO reactor, the thickness ofblanket could be reduced and the increase in thewall load is acceptable. Thus design window
analysis including the region which has a blanketthickness of <1.0m are carried out. Since it isfavorable to leave the possibility of a flexible
selection in the magnetic configuration from theviewpoint of the design robustness, design window
analysis are carried out with the configuration ofhelical pitch parameter c=1.25 (corresponding tosmaller plasma aspect ratio compared with the caseof FFHR-2m2) and the inward-shifted magnetic
axis with the ratio between the magnetic axis
position and the major radius of Rax/Rc=3.6/3.9,
which shows a large plasma volume and good
confinement property in the LHD experiment.

region around Rc=15-16m and Bt,c=4.5-5.0T can bea candidate for FFHR-d1. Since the space between
the helical coil and the plasma is limited, a detailed
optimization of the radial build design including
blanket thickness and the cross-sectional shape ofthe helical coil is one of the key issues for toadvance the design of FFHR-d1. Therefore, detailedanalysis is being carried out by the related task
groups (blanket, in-vessel components andsuperconducting magnets).

Fig.1. Cross-sectional view of the plasma, blankets,vacuum vessel and coils of an LHD-type heliotron
reactor.

3. Design window analysis and design issues
Figure 2 shows the contours of several physics
and engineering design parameters on the plane of
the magnetic field strength (averaged toroidal fieldon the winding center of the helical coil Bt,c) and themajor radius. All design points in Fig. 2 satisfy theself-ignition condition and have the electron density
and temperature profiles determined by the DPE
method with the confinement improvement factor
DPE=1.3, which is expected by an improvement inplasma heating profile (note this confinement
improvement factor is relative to the experimental
result used for the DPE).
Here we consider three design constraints. One is
the stored magnetic energy, which is the same as in
the design study of FFHR-2m2; Wmag≤160GJ.
Second is the density limit at the plasma edge. Theedge density nea should be lower than the Sudo
density limit nSudo [5]. Third is the peak beta value0. Although the achievable maximum peak betavalue needs to be carefully examined throughtheoretical and experimental analysis, here 0=10%,
which is the maximum value obtained in the past
LHD experiment, is selected as a design limit (notethat we didn’t consider a loss of alpha particles andthe existence of impurity ions). Consequently, theregion with a wedge shape that is not shadowed inFig. 2 is a possible design window for FFHR-d1.
Since a larger blanket space is favorable, the design

Fig.2. Example of design window analysis forFFHR-d1. Contours of the stored magnetic energy
Wmag, the minimum gap width between the plasma andthe helical coil c-p, enhancement factor of the beta
value f, fusion power Pfus and average neutron wallload <nw> are shown.

5. Summary
Design window analysis for the LHD-type
heliotron DEMO reactor FFHR-d1 is carried out.
According to the core plasma design based on thedirect profile extrapolation from the LHD
experiment, the design direction and the key issues
in the design are clarified. Further detailed analysis
including 3D structure of in-vessel components areneeded to enhance the feasibility and reliability ofthe design.
Acknowledgments
This research is supported by the Fusion
Engineering Research Project in National Institutefor Fusion Science (NIFS).
References[1] A. Komori et al.: Fusion Sci. Technol. 58 (2010) 1.
[2] A. Sagara et al.: Fusion Eng. Des. 83 (2008) 1690.
[3] T. Goto et al.: Nucl. Fusion 51 (2011) 083045.
[4] J. Miyazawa et al.: Fusion Eng. Des. (2011) in press.
[5] J. Miyazawa et al.: Fusion Sci. Technol. 58 (2010)
200.