核融合マグネットの分割製造のためのインジウムを用いたREBCOラップ接合の曲げ捻り特性評価
Evaluation of Bending and Twisting Characteristics of REBCO Lap
Joint with Indium for Segment-fabrication of Fusion Magnets

7P07

張 啓帆、伊藤 悟、橋爪 秀利
Qifan Zhang, Satoshi Ito, Hidetoshi Hashizume
東北大学
Tohoku Univ.
1. Introduction
Bridge-type mechanical lap joint of simply stacked Rare-Earth
Barium Copper Oxide (REBCO) conductors with indium foil
insertion has been proposed for segment-fabrication of hightemperature superconducting (HTS) magnets especially for aheliotron-type fusion reactor FFHR-d1 [1]. Previous studies haveinvestigated straight mechanical joints of STARS conductors [2]. On
the other hand, segment-fabrication of helical shaped coil ideallyneeds bent and twisted joint parts, therefore, the evaluation ofbending and twisting characteristic of the lap joint is necessary.
There are two procedures to fabricate the joint parts, 1) joint, twist,and bend (JTB), and 2) twist, bend and joint (TBJ) procedures. In
this study, joint and bend (JB), and joint and twist (JT) testing were
performed to evaluate critical strain for the former as well as bend
and joint (BJ), and twist and joint (TJ) testing for the latter. In
previous research [3] the JB testing was performed but no picklingprocess [4] was applied to improve joint characteristic.
2. Sample preparation
For joint samples 4-mm-wide REBCO tapes were used, whosecritical current is 98 A at 77 K, self-field. As joint interface material,
100-μm-thick indium foil was used. We applied the pickling process[4] and heat treatment at 90C [5] to complete fabricating process ofthe joint to reduce joint resistance. Evaluated joint resistivity (=product of joint resistance and joint area) for the single lap joints was20-30 ncm2 at 77 K, self-field. In samples prepared for JB testing,Samples J1 with joint length, LJ, was 5, 8 and 20 mm, J2 and J3 with
LJ = 8 mm are lap joints of REBCO tapes showed in Fig. 1. In JTtesting, Sample J1 with LJ = 5 mm and used the twist jig with 5 MPapressure to apply torsion strain to the joint. In BJ and TJ testing, thetape was fixed on bend or twist jig then joining and LJ was 5 mm.
3. Result and Discussion
In JB testing, critical current deteriorated at a bending strain
larger than 0.6%, which is induced at REBCO layer inside theREBCO tape. Since the irreversible strain of REBCO tape wasreported to be 0.6% [6], the result is reasonable. For evaluating joint
resistance, the bending strain of interface of REBCO tape andindium foil, εb_int needs to be considered. εb_int is calculated by thefollowing equation,
𝜀b_int = 𝑖/(2𝑟 + 2𝑡 + 𝑖)(1)

where t is thickness of the sample, and r is bend radius of bend jigs,
i is thickness of indium foil. Fig. 2 shows results in JB testing. By
comparing with a previous study [3], the joint resistivity decreased
from 42 nΩcm2 to 20-30 nΩcm2 by using pickling process. The joint
resistivity increased when bending strain, εb_int = 0.2% with LJ = 5mm, εb_int = 0.4-0.6% with LJ = 8 mm and no increase until εb = 0.2%with LJ = 20 mm. In a previous study [3], the joint resistivity started
to increase at about εb_int = 0.17-0.26% with LJ = 5 mm and εb_int =0.52-0.61% with LJ = 10 mm. The result indicates critical bendingstrain for joint resistivity depends on the joint length but not on
whether the pickling process is performed. In a previous study [3],samples with higher joint resistivity showed lower critical bendingstrain, but in JB testing there is no significant impact when joint
resistivity is lower than 40 ncm2. In JT testing, no increase in joint
resistivity and no decrease on critical current were confirmed at
torsional strain lower than 0.26%. In the evaluation of the actual
FFHR-d1 conductor, it can be found that the bending and torsionalstrain of the conductor is much smaller than the critical strain. In theBJ, and TJ testings for TBJ procedure, the joint resistivity equivalent
to that of the straight joint sample was obtained. No increase in joint
resistivity or decrease in critical current was observed at 10-100 mm
bending radius and 50-350 mm twist pitch. The TBJ procedure canalso be applied not only to FFHR-d1 but also to smaller and more
complex coils.

Fig. 1 Test samples in JB testingFig. 2 Joint resistivityof bending strain for Samples J1, J2, J3 (LJ = 8 mm)
References[1] N. Yanagi, et al., Nucl. Fusion, 55, 2015, 053021.
[2] S. Ito et al., Plasma Fusion Res., 9, 2014, 3405086.
[3] S. Ito et al., IEEE Trans. Appl. Supercond. 27, 2017, 4600105.
[4] R. Hayasaka, et al, J. Phys. Conf. Ser., 1559, 2020, 012034.
[5] T. Nishio, et al, IEEE Trans. Appl. Supercond.,. 27, 2017,4603305.
[6] H. Song, et al, IEEE Trans. Appl. Supercond., 23, 2015, 4600806
Acknowledgement
This work was partially supported by JSPS KAKENHI Grant
Numbers 20H01884; and by NIFS Collaboration Research
Program under grant no. NIFS19KECF026 and NIFS22KIEF02;
and by JST, the establishment of university fellowships towards thecreation of science technology innovation, Grant Number
JPMJFS2102.