Plasma and Fusion Research: Regular Articles

Volume 19, 1405013 (2024)

Electric Potential Profile with Distributed Degradation in the
Terminal Joint for ITER TF Coil System
Shin HASEGAWA, Hideki KAJITANI, Yasuhiro UNO, Tsutomu KAWASAKI, Mio NAKAMOTO
and Masataka NAKAHIRA
National Institutes for Quantum Science and Technology, Naka 311-0193, Japan(Received 3 August 2023 / Accepted 24 January 2024)
ITER toroidal field coils are electrically connected to 68-kA main busbars (terminal joints). We propose themeasurement of the electric potential distribution in a terminal joint using electrical probes (e-probe method) toinspect the contact resistance in the joint. In this study, we experimented with a mockup of a terminal joint. The
test current was 20 A, and the electric potential was measured using the e-probe method at room temperature and77 K. Nine diﬀerent degradation patterns were prepared by distributing polyimide films in the joint interface.
Next, we performed finite element analysis to investigate the detailed relationship between the electric potential
distribution and contact resistance at 300 K and 77 K. In the numerical analysis, the same degradation patterns andtest current as in the experiment were assumed. The analysis results agree with the experimental results. Diﬀerent
degradation patterns exhibit diﬀerent electric potential profiles with 10-µV-scale diﬀerences. The analysis results
also indicate that the e-probe method works when the contact resistance in the degraded area is larger than
1.0 × 10−5 Ωmm2 at 77 K and 300 K.
c 2024 The Japan Society of Plasma Science and Nuclear Fusion Research⃝

Keywords: ITER, fusion magnet, terminal joint, low-temperature superconductor, contact resistance
DOI: 10.1585/pfr.19.1405013

1. Introduction
An ITER toroidal coil (TFC) is electrically connectedto the 68-kA main busbar of a feeder (terminal joint), as
shown in Fig. 1 [1, 2]. A large electrical contact resistanceleads to severe consequences, such as magnet quenching
and arc discharge. Thus, confirmation of the local increase

Fig. 1 Terminal joint for the ITER TF coil system.

in the contact resistance due to joint degradation is necessary, and it is desirable to perform it at room temperature(RT) during the TFC-feeder assembly.
In addition, the contact resistance in the TFC double pancake joint (DPJ) was inspected during winding
pack (WP) manufacturing [3, 4]. The electric potential
distribution along the DPJ was measured using electricalFig. 2 Diﬀerent current path in (a) DPJ [3] and (b) terminal
joint.

author’s e-mail: hasegawa.shin2@qst.go.jp

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Fig. 3 E-probe method for the terminal joint where electrical probes are distributed on the copper sleeve of the terminal joint.

probes (e-probes) when a test current was applied (e-probe
method). This method was implemented for all WPs ofJapanese TFCs. Although the structure of the DPJ diﬀers
from that of the terminal joint, the materials and electricalcharacteristics of the jointed superconducting conductors
are almost the same. Thus, we propose the use of the eprobe method for the inspection of the terminal joint.
The diﬀerence between the structures of the terminal
joint and the DPJ may lead to a diﬀerence in the current
path and electric potential distribution, as shown in Fig. 2.
Thus, in this study, we first experimentally investigated theelectric potential distribution in the terminal joint using the
e-probe method with a mockup of the joint. As a first stepof this investigation, we focus on the degradation (large
contact resistance), which is easily detected to understandthe characteristics of the electric potential distribution andlimitations of the e-probe method. The degradation assumed in this study is uniform in the width direction anddistributed in the longitudinal direction. This degradation
possibly occurs because of the imbalanced compression of
the joint during the TFC-feeder assembly. The experiment
was conducted at RT and 77 K to improve the sensitivityof the electric potential distribution to the degradation distribution. The relationship between the value of contact
resistance in the degraded area and the electric potential
distribution is important for understanding the limitationsof the e-probe method. However, it is diﬃcult to precisely
control the value of the contact resistance in the experiment. Thus, a numerical analysis was performed assuming
a wide-range contact resistance, its distribution, and diﬀerent operating temperatures (RT and 77 K).
Section 2 explains the e-probe method for the terminal joint. Sections 3 and 4 present the methodology of theexperiment and numerical analysis, respectively. Section 5
presents the results and discussion, followed by a conclusion in Sec. 6.

2. E-Probe Method for Terminal Joint
Figure 3 shows the structure of the terminal joint anda conceptual diagram of the e-probe method. Two superconducting conductors are joined by a copper shim andan indium foil. The contact resistance in superconducting
conductors is already known during their manufacturing.
Therefore, the resistance between the two copper sleevesof the superconducting conductors should be measured. In
the e-probe method, electrical probes are attached to theside surface of the copper sleeve of each conductor along
the longitudinal direction, and a test current is applied.
Then, the electric potential diﬀerence between the two copper sleeves is measured along the longitudinal direction.
First, we should obtain the measurement or simulated data
for the terminal joint without any degradation. Using thesedata as a reference, an increase in the contact resistancecan be identified.

3. Experiment
An experiment was conducted by preparing a mockupof the actual size of the terminal joint. Figure 4 showsa mockup of the terminal joint. The two joint box samples [3–5], which primarily comprise superconducting cables, stainless steel covers, and copper sleeves, were joinedwith indium wires and a copper shim. The superconductor used for the TFC winding and joint box sample was
Nb3 Sn, whereas that for the main busbar was made of NbTi. This study aimed to confirm the characteristics of theelectric potential distribution in the joint. The influential
parameter that diﬀers between Nb-Ti and Nb3 Sn is electrical resistivity. This diﬀerence is only 6% - 9% at 77 K
- 300 K [6, 7]. Thus, this is considered to be a negligible
diﬀerence in this study. The assembly procedure of themockup was as follows: 1) indium wires with a diameter of 1.2 mm were wound on the copper shim; 2) one joint
box sample was placed; 3) the copper shim with the indium
wires was placed on the copper sleeve of the superconducting conductor; 4) stainless steel shims with a thickness of
0.2 mm were placed on the copper shim to make the thickness of indium 0.2 mm) another joint box was placed on thecopper shim. Polyimide sheets were inserted between thecopper shim and sleeve instead of the indium wire to represent an increase in the contact resistance (degradation).
The region of the joint interface with a length of 588 mm
was divided into two (Zones A1 and A2) or three (Zones

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Fig. 4 Assembly procedure of the mockup corresponding to case 3 in which the polyimide sheet covers 33% of the interface (t: thickness).

Table 1 Degradation patterns for experiment and numerical simulaiton. Rd indicates the contact resisntace at the degraded area in theterminal joint.

B1, B2, and B3) regions to assume some degradation patterns. Because the purpose of this study was to observe
the tendency of electric potential distribution in the terminal joint with significant degradation, the smallest area ofdegradation was set to be 33% of the entire area of the joint
interface. Table 1 shows the distribution of the polyimide
sheets for nine cases (cases 0 - 8). This mockup was placed
between two stainless steel plates, as shown in Fig. 5, andwas compressed by tightening the bolts at 80 N/m. Copper
e-probes with a diameter of 1.5 mm were then attached tothe copper sleeve surface with a support jig, as shown in
Fig. 5. The interval of the probes in the longitudinal direction (the x-direction) was 68 mm. The distance between
two e-probes paired in the same x-direction was 14.2 mm.
A test current of 20 A was applied to the mockup to
avoid significant heating in the conductor. The electric
potential diﬀerence between two copper sleeves (V0 (x))
was then measured using pairs of two e-probes along thex-direction. V0 (x) was measured at both RT and 77 K.
The mockup was cooled by immersion in liquid nitrogenat 77 K. The DC power supply PAN35-20A (Kikusui Electronics Corp.) and a nanovolt meter 2182A (Keithley) wereused for the current supply and V(x) measurements, respectively. V0 (x) was constantly measured for 30 s, and its
mean value at each location (V0m (x)) was recorded. The
output voltage from the nanovolt meter without the test
current was also measured as the background voltage before (VBGb ) and after (VBGa ) each measurement. Finally,V(x) ≡ V0m (x) − (VBGb + VBGa )/2 was evaluated for discussion.

4. Numerical Analysis
It is diﬃcult to experimentally investigate the influence of a wide-range contact resistance on the electric
potential distribution. Thus, numerical analysis was performed to assess the contact resistance dependence ofthe electric potential distribution. The analysis was performed using the commercial finite element analysis code
Ansys Mechanical 2022 [8]. Figure 6 shows the three-1405013-3

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Fig. 5 Mockup of terminal joint for experiment; (a) location of e-probes on copper sleeve, (b) support jig for e-probes, (c) structure of
e-probe, and (d) wiring of measurement leads for e-probes.

Fig. 6 Three-dimensional analytical model of the terminal joint and boundary conditions for the finite element analysis.

dimensional analytical model. The model comprises two
superconducting conductors and several interface materials. The superconducting conductor comprises a stainlesssteel jacket, copper sleeve, and superconducting cables.
There are two types of interface materials: copper shim
and indium. The thicknesses of the copper shim and indium are 4 and 0.2 mm, respectively. Figure 6 also shows
the boundary conditions. A test current of 20 A was applied to the cross-section of the superconducting conductor
at one edge, and the electric potential was fixed to 0 V on
the cross-section of the superconducting conductor at another edge. A total of 196,216 hexahedron elements wereused, with the maximum element size being 5 mm × 5 mm
× 5 mm and the minimum element size being 3.9 mm ×
0.1 mm × 5 mm.
Similar to the experiment, nine degradation patterns in
the terminal joint were assumed by locally increasing theTable 2 Electrical resistivity used for numerical analysis.

interface resistance between the indium and copper shim(Rd ), as shown in Table 1. Rd was set to 5.0 × 10−9 109 Ωmm2 . 300 K (RT) and 77 K were assumed as the two
operating temperatures. The influence of temperature was

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considered in the temperature dependence of the electricalresistivity.
Table 2 shows the resistivity values of the materials assumed in the analysis, which were obtained from [5,9–11].
Notably, the influence of the contact resistance among thesuperconducting wires and between the superconducting
wires and stainless steel jacket is considered in the analysis of the resistivity of the superconducting cable.

5. Results and Discussion
5.1 Electric potential distribution for diﬀerent degradation patterns
The electric potential distributions between the eprobes on the upper and lower copper sleeves (V(x)) in theexperiment and numerical analysis at Rd = 109 Ωmm2 arecompared. Figures 7 and 8 show examples of V(x) in (a)case 0 (no degradation is considered), (b) case 4 (degradation is in the 33% central area), and (c) case 7 (degradation is in the 66% downstream area) at RT and 77 K.
Note that the “downstream area” represents the area near
the grounded surface. The degraded area is highlighted inorange in the figures.
The location of the peak of V(x) corresponds to thecenter of the degraded area. The peak value of V(x), whichis the maximum |V(x)|(Vmax ), varies in accordance with thedegradation conditions. Table 3 shows Vmax with diﬀerent
degradation patterns at RT and 77 K. Here, the nanovolt
meter outputs voltages up to 10 nV. The table also shows
the background noise (= |VBGb − VBGa |) in brackets. The
background noise at 77 K shows the same level or a larger
value than that at RT, except for case 0. This is becauseof the boiling of liquid nitrogen during the measurement.
When there is degradation in the contact resistance (cases
1 - 8), Vmax becomes 4 - 80 µV and 2 - 70 µV in the experiment and numerical analysis, respectively, and theseresults agree well.
Although the degraded area in this study (33% - 66%of the joint interface) is smaller than that in the case of theDPJ (80% of the joint interface) [3], the largest (smallest)
Vmax in this study is almost the same as (larger than) the
Vmax in the DPJ. This is because of the diﬀerence in thecurrent path between the DPJ and terminal joint, as illustrated in Fig. 2. The current decreases in the area near theend of the joint in the DPJ [3], whereas it flows over theentire area in the terminal joint. Thus, V(x) is more sensitive to nonuniform degradation in the terminal joint than in
the DPJ.

Fig. 7 The values of V(x) obtained from experiment (open circle), numerical analysis (solid curve); (a) case 0, (b) case 4, and (c) case 7 at
RT. V(x) denotes the voltage diﬀerence between copper sleeves, and x represents the distance from the edge of the joint interface.
The values of V(x) estimated with simplified circuit in (a) case 0 and (c) case 7 are also plotted (dotted curve).

Fig. 8 The values of V(x) obtained from experiment (open circle), numerical analysis (solid curve); (a) case 0, (b) case 4, and (c) case
7 at 77 K. V(x) denotes the voltage diﬀerence between copper sleeves, and x represents the distance from the edge of the joint
interface. The values of V(x) estimated with simplified circuit in (a) case 0 and (c) case 7 are also plotted (dotted curve).

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Table 3 Vmax in the experiment and numerical analysis, wherethe values in brackets are the background noise.

Fig. 9 Simplified circuit model of terminal joint in case 0; (a)major current paths, and (b) distribution of current at eachbranch (green color) and electric potential at each node
(blue color).

In the cases of the terminal joint with degradation,
both experimental and analysis results at RT and 77 K show
the smallest Vmax in case 4. The largest Vmax was observedin cases 6 and 7 in the experiment and numerical analysis,
respectively. T his diﬀerence is caused by the backgroundnoise of the measurement at 77 K in the experiment, which
is 0.39 - 1.75 µV, as shown in Table 3. Although the Vmax
in case 7 is larger than that in case 6 in the numerical analysis, the diﬀerence is only ∼0.2 µV. Thus, this diﬀerence
is practically negligible considering the background noise
at 77 K.
The causes of the variation of Vmax values can bequantitatively understood from the current distribution in
the terminal joint with a simplified circuit analysis. Figures 9 and 10 show the one-dimensional simplified electrical circuit of the terminal joint in cases 0 and 7, respectively. Note that continuous current distribution inside each
196-mm region is not considered in this circuit. Only theresistace of the joint box (bulk resisatnce) in the horizontal
direction are shown since it would be larger than that in thevertical direction and the resistance fo the inserted materials, such as copper and indium. R [Ω] denotes the equivalent resistance of the joint box. Figures 9 and 10 show thecurrent and electric potential distribution in cases 0 and 7,
respectively. In Fig. 10, two major current paths, A and B,
are illustrated for the upper and lower joint boxes, respectively. The current and electric potential were estimatedon the basis of the simplified circuit. In case 4, the current distribution showing the small |V(x)| would be similar

Fig. 10 Simplified circuit model of terminal joint in case 7; (a)major current paths, and (b) distribution of current at
each branch (green color) and electric potential at eachnode (blue color).

to that in case 0, except for the local current distribution
around the degradation. Conversely, in case 7, the equivalent resistances of current paths A and B are almost infinite and 3R, respectively. Thus, the smaller current flows
into the upper copper sleeve, resulting in a large |V(x)| of
40R [V] (= |0 − 40R|) at x = 588 mm. V(x) estimated using the simplified circuit model (Vs (x)) is also plotted in
Figs. 7 and 8. R was assumed to be 1.14 µΩ at RT and
0.094 µΩ at 77 K. |Vs (x)| is lower than |V(x)| in the experiment (|Vexp (x)|) and numerical analysis (|Vnum (x)|) in case
7 because the resistance between the superconducting filaments and the copper sleeve is not considered in the simplified circuit model. However, Vs (x) quantitively agrees
with Vexp (x) and Vnum (x). Thus, the discussion above isreasonable, and the bulk resistance through the major current path is considered a key parameter for the relationship1405013-6

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between V(x) and the degraded location at >77 K.

5.2 Dependence of contact resistance on theelectric potential profile
Considering the sensitivity of signals to degradation, it
is important to focus on not only the value of Vmax but also
on the extent to which Vmax changes with diﬀerent degradation patterns. Thus, the parameter α is defined as α ≡the largest Vmax (case 7) / the smallest Vmax (case 4) as
an indicator of the eﬀect of temperature on the proposed
method. Notably, cases 7 and 4 exhibited large and small
Vmax values, as shown in Table 3. Thus, α represents how
much the temperature changes the diﬀerence in V(x) with
diﬀerent degradation patterns. The α values obtained in
the numerical analysis at 300 K and 77 K are compared.
Consequently, α is 13.9 at both 300 K and 77 K in the
numerical analysis. Thus, there appears to be no advantage in cooling the joint in the case of Rd = 109 Ωmm2 .
However, the α value at 77 K may be larger than thatat 300 K with a smaller Rd if Rd is suﬃciently smaller
than the bulk resistance in the terminal joint. Figure 11
summarizes α and the diﬀerence in Vmax between cases 4
and 7 (∆Vmax ) when Rd is 5.0 × 10−9 - 109 Ωmm2 . The
α value at 77 K is larger than that at 300 K when Rd is< 5.0 × 10−4 Ωmm2 . This larger α value at 77 K helps
to distinguish the degradation patterns. Given the levelof background noise, 1 µV would be the limit, and thismethod would fail when Rd is < 1.0 × 10−5 Ωmm2 . Notably, detectable Rd can be increased by increasing the test
current as much as possible. Considering the minimum detectable Rd , cooling of the terminal joint is eﬀective only
when 1.0 × 10−5 Ωmm2 < Rd < 5.0 × 10−4 Ωmm2 in termsof α. Moreover, ∆Vmax at 300 K is several times larger thanthat at 77 K. Thus, the proposed method works at RT without cooling the terminal joint when the degraded area ismore than 33% of the joint interface. Degradation of 33%
of the joint interface with Rd of 1.0 × 10−5 Ωmm2 corresponds to 0.73 nΩ (= 1.0 × 10−5 Ωmm2 /(588 mm × 71 mm

× 33%)), which is less than the criteria for an overall resistance of 3 nΩ at 4 K [1]. Thus, this method is useful tocheck major distributed degradation, such as the air gap
due to the misalignment of the joint boxes at any step before cooling or even at the training of the joint assembly.

6. Conclusion
In this study, we proposed the use of the e-probe
method for the inspection of an ITER toroidal field terminal joint, and its applicable conditions and characteristics
of voltage signals were numerically and experimentally investigated. The assumed degradation is uniform in thewidth direction and distributed in the longitudinal direction, which could occur because of the imbalanced compression of the joint during the TFC-feeder assembly. The
highlights are as follows:• The experimental and analysis results indicate that 2.1- 81.4 µV and 0.7 - 82.9 µV are detectable when Rd in
more than 33% of the joint interface is 109 Ωmm2 .
The diﬀerence in the voltage signal above 77 K is primarily caused by the bulk resistance of the major current paths.
• The parameter α is defined to evaluate the sensitivity of the voltage signal to degradation. The valueof α at 77 K is larger than that at RT when Rd is< 5.0 × 10−4 Ωmm2 . Considering the limitation of detectable contact resistance in the degraded area, where
Rd is < 1.0 × 10−5 Ωmm2 , cooling of the terminal
joint is eﬀective when 1.0 × 10−5 Ωmm2 < Rd <5.0 × 10−4 Ωmm2 . However, the range of Rd in which
cooling is eﬀective is small, and ∆Vmax at RT is stillseveral times larger than that at 77 K. Thus, the proposed method works at RT without cooling to detecta degraded area exceeding 33% of the joint interface.
These findings show the limitations and applicabilityof the proposed method and will be useful information forthe design of an inspection procedure of the ITER toroidal
field terminal joint during the assembly phase.

Acknowledgments
The authors would like to extend our appreciation toEnglsh proofreading service by enago (https://www.enago.
jp/) for their expert English language editing.

Fig. 11 Relationship between α and Rd (solid line), and between
Vmax and Rd (dotted line) at 300 K and 77 K where α isdefined as the largest Vmax (case 7) / the smallest Vmax
(case 4).

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Volume 19, 1405013 (2024)[4] H. Kajitani et al., “Results of All ITER TF Full-Size Joint
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