25pC01
革新的核融合炉実現に向けた分割型高温超伝導マグネットの研究開発～ 高温超伝導導体の着脱可能な接合法の開発状況 ～R&D on a segmented high-temperature superconducting magnet towardrealization of an innovative fusion reactor:
Progress in development of a demountable joint of high-temperature
superconducting conductors
伊藤 悟1，橋爪 秀利1，遊佐 訓孝1，柳 長門2，田村 仁2，相良 明男2
Satoshi Ito1, Hidetoshi Hashizume1, Noritaka Yusa1,
Nagato Yanagi2, Hitoshi Tamura2, Akio Sagara2
1

東北大学，2核融合科学研究所
1
Tohoku University, 2National Institute for Fusion Science
1. Introduction
Fabrication of huge and complex superconducting
coils and magnets is one of the important technicalissues for fusion DEMO and commercial reactors.
The segmented fabrication of superconducting coils
using high-temperature superconducting (HTS)
conductors [1]–[4] is now being considered as asolution for the above technical issue. The use ofHTS conductors can allow the coils to be operated
at elevated temperatures (> 20 K), where the coils
can have higher heat capacity and lower
refrigerationenergythanlow-temperature
superconducting (LTS) coils. These features could
make it possible to fabricate the coils by assembling
short conductors or segments with resistive joints.
The joint of HTS conductors is the key technology
for the concept, and R&D for the joints has been
conducted since the year of 2000 in Tohoku
University. Since 2012, Tohoku University andNational Institute for Fusion Science (NIFS) havecollaborated to develop the Stacked Tape
Assembled in Rigid Structure (STARS) conductor
using REBCO (REBa2Cu3O7−δ) HTS tapes and its
joint to be applied to the helical fusion reactor,
FFHR-d1 [4]–[6]. During these 15 years, we haveincreased 104 times in achieved current anddecreased 10-5 times in joint resistance withdeveloped joints [2], [7].
In this presentation, we firstly introduce thedesign concept of the segmented fabrication of theHTS magnets especially for the helical reactor, andthen, address recent progress in R&D on joints ofHTS conductors.
2. Design concepts
Fig. 1 shows schematic illustrations of segmented
fabrication of HTS coils proposed for the helicalFig. 1. Schematic illustration
fabrication of HTS helical coils.

ofsegmentedreactor. The initial proposal was the remountable(demountable) HTS helical coil, whose half-pitch
coil segments are assembled with demountablejoints [1], [2]. The design gives attractive features:
easy fabrication of huge and complex helical coils
at a construction phase and high accessibility toinner reactor components such as a blanket and adivertor at a maintenance phase. As an extension of
the concept of the remountable coil, joint-windingof the helical coils was also proposed: the helicalcoil is wound by connecting half-pitch or one-pitchof conductor segments with permanent joints [3],[4]. Although the conductor segments cannot bedemounted after the construction, the concept still
has an advantage of easy fabrication of huge andcomplex helical coils, which is the most important
technical issue in the helical reactor.
3. R&D on joints of HTS conductors
3-1. Proposed joints and achieved resistanceMechanical joints have been investigated as
demountable or permanent joint, in which joint
surfaces of the STARS conductors are just pressed
together with joint force. Some mechanical jointshave been developed [2]: butt joint (scarf joint),edge joint and lap joint (normal-type andbridge-type). Among the above joints, thebridge-type mechanical lap joint was actuallyapplied to 100 kA class prototype STARS
conductor as shown in Fig. 2 and we achieved ajoint resistance of 1.8 n at 100 kA, 4.2 K [4], [5].
The achieved joint resistance is sufficiently small
from the viewpoint of refrigeration energy.
3-2. Resistance dependency on operating conditions
In order to design and optimize the joint with
sufficiently low resistance, it is necessary to predict
joint resistance dependency on operating conditions
and conductor joint structure. For that purpose, wefirstly evaluated joint resistance as functions oftemperature, magnetic field orientation andconductor structure, using samples of themechanical lap joint of REBCO tapes [7]–[9]. Then
we proposed a model to predict contact resistance,
which is one of the factors of the joint resistance.
The model so far can predict joint resistance asfunctions of temperature and magnetic field except
in the case at temperatures of <10 K and magnetic
field parallel to the joint surface [9].
3-3. Mechanical strength
To discuss mechanical strength of the joint,
structural analysis for the FFHR magnet systemwith joint-winding of the helical coil has been
conducted using the multi-scale analysis [7], [10],[11]. The analysis showed that the maximum shear
stress of 32 MPa is induced in a plane of joint
surface. We also conducted tensile shear tests ofmechanical lap joints with inserted indium foils andsoldered lap joint of REBCO tapes [7], [10], [12].
The mechanical joint has shear strength of > 32
MPa with joint pressure of > 50 MPa as shown inFig. 3. On the other hand, soldered lap joint
fractured due to stress concentration at the edge ofsolder before reaching the required load. From theresults, the mechanical joint is preferable even inFig. 2. Bridge-type mechanical lap joint applied to
the 100 kA-class prototype STARS conductor.

Fig. 3. Shear strength of the mechanical lap joint as afunction of joint pressure.
the joint-winding of the helical coil with permanentjoints.
3-4. Quality assurance
In order to reduce joint resistance and its variation,
we proposed to apply heat treatment of joint during
the fabrication [13]. The joint with the heattreatment showed the joint resistance, reduced to60% of the value obtained without heat treatment,
when we controlled the heating temperature to be
75–90 C. We have also tried to develop
nondestructive testing to inspect the joints:
radiation,electromagneticandultrasonicinspections. So far, we confirmed X-ray CT candetect macroscopically non-contact region [7], andthe other candidates are now under development.
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5 (2002) 532.
[2] H. Hashziume and S. Ito, Fusion Eng. Des., 89(2014) 2241.
[3] N. Yanagi et al., Fusion Sci. Technol., 60(2011) 648.
[4] N. Yanagi et al., Nucl. Fusion, 55 (2015)053021.
[5] S. Ito et al., Plasma Fusion Res. 9 (2014)
3405086.
[6] A. Sagara et al., Fusion Eng. Des., 89 (2014)
2114.
[7] S. Ito et al., 3rd HTS4Fusion Conductor
Workshop, Sep. 2015, Pieve S. Stefano, Italy.
[8] Y. Seino et al., IEEE Trans. Appl. Supercond.,
25 (2015) 6603405.
[9] S. Ito et al., MT24, Oct. 2015, Seoul, Korea.
[10] S. Ito et al., IEEE Trans. Appl. Supercond., 25(2015) 4201205.
[11] H. Tamura et al., EUCAS 2015, Sep. 2015,Lyon, France.
[12] L. Aparicio et al., ITC-25, Nov. 2015, Toki,Japan.
[13] T. Nishio et al., MT24, Oct. 2015, Seoul,
Korea.