Special Issue

Superconductivity
Contribution to the achievement of SDGs
by Furukawa Electric Group

Development of Cu-Nb Reinforced Nb3Sn Wires
Masahiro Sugimoto*, Ryo Taniguchi*, Koichi Ebisawa*, Ka Kei Mun*, Hiroyuki Fukushima*,
Kengo Nakao*, Hidekazu Hara*, Kiyoshige Hirose*, Hirokazu Tsubouchi*

We have developed Nb-rod method Cu-Nb reinforced Nb3Sn wires (Cu-Nb/
Nb3Sn wires), which have significantly improved superconducting characteristics under mechanical stress compared to conventional Nb3Sn wires. Cu-Nb/Nb3Sn wires can be
manufactured into coils not only with the conventional wind-and-react (W&R) method but alsowith the react-and-wind (R&W) method, and the superconducting performance and the mechanical characteristics are both improved by a strain control through pre-bending treatment.
Furthermore, elemental technology developments for a higher strength, a higher critical current
density, a rectangularization, an enamel coating, a thinner wire for small radius bending, and astranded wire have made them practical wires with an excellent manufacturability. In the future,
Cu-Nb/Nb3Sn wires will be expected to be applied in advanced superconducting equipment
such as a high magnetic field NMR/MRI, a particle accelerator, and a large fusion reactor.

ABSTRACT1.

INTRODUCTION

Alloy-based NbTi wires and A15-based intermetallic compound Nb3Sn wires are in practical use in many superconducting devices. In particular, Nb3Sn wires are used insuperconducting magnets that generate high magnetic
fields of 10 T or more, which exceed the critical magnetic
field of NbTi wires. However, Nb3Sn wires require elemental technologies different from Nb-Ti superconducting
wires, such as an Nb 3Sn generation heat treatment
according to the structural design after processing theminto wires made by combining Nb filaments with a Cu-Sn
alloy (unreacted Nb3Sn wires). Also, wires after Nb3Sn
generation heat treatment (reacted Nb3Sn wires) are not
only mechanically fragile, but also have superconducting
characteristics that change due to external strain1), so,
the wind and react (W&R) method, which applies Nb3Sn
generation heat treatment after winding unreacted Nb3Sn
wires into coils, are commonly used. Furthermore, as
improvements in current-carrying characteristics underlarge electromagnetic stress are required in order toimprove the performance of magnets for higher magnetic
fields and larger sizes, it is necessary to increase thestrength of the Nb3Sn wire itself and to develop a conductor made by twisting Nb3Sn wires together.
Through joint research with Tohoku University,
Furukawa Electric Co., Ltd. has developed Nb3Sn wires
reinforced with a Cu-Nb composite material (Cu-Nb/
Nb3Sn wires)2) produced using a new method (Nb-rod*

Superconducting Products Department, R&D Division.

method), which is different from the conventional In-situ
method for Cu-Nb alloys 3)-5). The Cu-Nb/Nb 3Sn wires
have improved superconducting characteristics underhigh stress by applying the pre-bending strain application
treatment (pre-bending treatment)6)-8), which is describedlater, in the react-and-wind (R&W) method of winding areacted Nb3Sn wire into a coil, which has been thought tobe difficult to put into practical use until now.
In order to achieve the required characteristics for various Nb3Sn superconducting coils in which not only the
R&W method but also the W&R method is used, we carried out elemental technology developments for increasing critical current density, increasing strength, rectangularization, enamel insulation, small diameter bending, andconductorization of Cu-Nb/Nb3Sn wires9)-18). This paper
describes the development results of the Cu-Nb/Nb3Sn
wires to date and their future prospects.

2.

CHARACTERISTICS OF THE Cu-Nb
REINFORCED Nb 3Sn WIRES

2.1 Structure of the Cu-Nb Reinforced Nb3Sn Wire5), 10)
The structure of the Nb-rod method Cu-Nb/Nb3Sn wire
(diameter 0.80 mm) is shown in Figure 1 (a). A Cu-Nb
composite material with a structure in which multiple
Cu-sheathed Nb mono-core wires are embedded in a Cu
matrix is placed around the Nb3Sn filament group. The
submicron-sized or less Nb filaments in the Cu-Nb composite material have a fiber-reinforcement function, andthe Cu matrix functions as a stabilizing material. The
Cu-Nb reinforcement material part in the Cu-Nb/Nb3Sn
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Development of Cu-Nb Reinforced Nb3Sn Wires

wire shares the external stress (tensile stress, compressive stress, bending stress), which reduces the strain that
the Nb3Sn filament receives and suppresses the occurrence of damage. This Nb-rod method Cu-Nb reinforcement material is superior to conventional In-situ method
Cu-Nb alloys in terms of mechanical characteristics,
residual resistance characteristics, and manufacturability 2). Figure 1 (b) shows the R&W type Cu-Nb/Nb 3Sn
Rutherford cables (width 6.45 mm x thickness 1.53 mm)
used in the Tohoku University 25 T-cryogen-free superconducting magnet (25 T-CSM). The superconducting
characteristics under tensile stress and compressive
stress has been improved by applying pre-bending strain
as described below after Nb3Sn generation heat treatment 4), 5).

Nb-rodCu-Nb reinforcer
Cu
Nb

Mono-coreCu-Nb rod
Cu
Cu
CuNb

Multiﬁlamentary
Cu-Nb rod
Cu-Nb

Cu

Cu

Tin-diﬀusion-barrier
Nb3Sn ﬁlament-bundle10 µm

Nb3Sn ﬁlament-bundleTin-diﬀusion-barrier(a) Cross-sectional structure of Nb-rod method Cu-Nb
reinforced Nb3Sn wires

stress strain consisting of axial tensile stress, lateral compressive stress, and bending stress due to the electromagnetic force during coil operation.
Figure 2 shows a comparison of the process flow diagrams for manufacturing Nb 3Sn coils using the W&R
method and the R&W method. In the W&R method, a gapof the thermal contraction coefficient with the coil components is created by the temperature difference (900°C
or more) that occurs when cooling from the Nb3Sn generation heat treatment temperature (up to 700°C) throughroom temperature to an extremely low temperature (up to–269°C). Due to the difference in the thermal contraction
coefficients, the Nb3Sn filament undergoes large residualcompressive strain. On the other hand, in the R&W method, the compressive strain applied to the wire can bereleased by separating the Nb3Sn wire from the structuralmembers such as a winding frame, which is used during
heat treatment at room temperature, after Nb3Sn generation heat treatment. After that treatment, the coil is woundat room temperature and cooled to an extremely low temperature (up to –269°C), so the residual compressive
strain generated in the Nb3Sn filament due to the difference in thermal contraction with the component parts
becomes significantly smaller than in the W&R method,which improves the superconducting characteristics.
Furthermore, by applying pre-bending strain 6)-8), which will
be described later, in order to optimize the strain that is
applied to the Nb3Sn filament during cooling operation
before and during coil winding, to bring out current carrying performance that is higher than that of the coils manufactured with the W&R method becomes possible. On theother hand, the R&W method Nb3Sn wire has the advantage that polyimide tape, PVF, polyester braid, etc., can beused in addition to heat resistant materials such as glass
braid which is commonly used in the R&W method Nb3Sn
wires, and that materials other than metal and ceramic
can be selected for coil manufacturing components.
Un-reacted Nb3Sn Bare Wire or Cable(b) Nb-rod method Cu-Nb reinforced Nb3Sn Rutherford
cable for 25 T-CSM
Figure 1

2.2

Structure of the Nb3Sn wire reinforced with Cu-Nb
manufactured by the Nb-rod method and the
Rutherford cables for the 25 T-CSM.

Processing Methods of the Nb3Sn Wires(W&R Method and R&W Method)
In both the W&R and the R&W methods described above,
the Nb3Sn superconductor in the Nb3Sn wire is subject tocomplex strains due to at least the following three typesof stress: (1) Cooling compressive residual strain thatremains due to the difference in thermal contraction
between the Nb3Sn superconductor and the composite
material inside the wire when cooling from the Nb3Sn
generation heat treatment temperature through room
temperature to critical temperature and less, (2) Residualstrain after winding due to tensile stress and bending
stress when winding the wire into a coil, (3) External

W&R method

R&W method

Insulation

Heat-treatment for
Nb₃Sn-reaction
Pre-bendingtreatment

Un-reacted Nb₃Sn
Conductor
Insulation
Coil Winding

Reacted Nb₃Sn
Conductor

Heat-treatment for
Nb3Sn-reaction

Coil Winding

W&R-processed
Nb3Sn CoilR&W-processed
Nb3Sn CoilFigure 2

Comparison of the Nb3Sn coil manufacturing flow
between the W&R method and the R&W method.

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Development of Cu-Nb Reinforced Nb3Sn Wires

2.3 Pre-bending Strain Application Treatment
The pre-bending strain application treatment 6), 7) in the
R&W method coil windings has industrially enabled theimprovement of the superconducting characteristics 8)-17)
brought by the pre-strain effect19). In the R&W method, it
is necessary to control the pulley diameter, the coil winding diameter and the bending direction that make up the
pass line so that the bending strain applied to the wire is
below the allowable value. Figure 3 shows a typicalexample of a pass line configuration for applying prebending strain5). When the wire is heat-treated with abobbin of diameter D h and then bent by a pulley of diameter D b, a bending strain ε b given by equation (1) isapplied 20).

Figure 4 shows the effect of the pre-bending strain
application treatment on the axial tensile strain dependence of the critical current (Ic) in the Cu-Nb/Nb3Sn wirefor 25 T-CSM 5). In this report, the wire immediately after
the Nb3Sn generation heat treatment is referred to as
As-reacted wire (-AR), and the wire that has been subjected to the pre-bending strain application treatment isreferred to as Pre-bent wire (-PB). Due to the pre-bendingstrain application treatment, the axial strain (ε m) at which
the Ic is maximized shifted from approximately 0.3% toapproximately 0.1% to the low strain side, and the I cvalue itself increased. This is the effect of improving
superconducting characteristics due to three-dimensional
relaxation of the compressive residual strain applied to
the Nb3Sn filaments 21).

 (1)

Db+

Drw

Dh
Paying-oﬀ bobbin(Heat-treatment)

Pass-line pulleys

Taking-up bobbin(Winding coil bobbin)

Critical current Ic / A

120

（a）

60

60

εirr

40
20

Figure 4

Db-

Paying-oﬀ bobbin(Heat-treatment)
Figure 3

80

40
20

n -value
0

0.2

0.4

0.6

0.8

1

0

Axial tensile strain / %
Db+

Dh

100

14.5 T, 4.2 K

Ic

80

0

120

As-reacted（AR）
Pre-bent
（PB）

100

（b）

140

Cu-Nb/Nb3Sn wire for 25 T-CSM

n -value

140

Transverse compressive stress dependence of the Ic
and the n-values of the Cu-Nb/Nb3Sn in the5)
25 T-CSM at 14.5 T and 4.2 K .

Drw
Pass-line pulleys

Taking-up bobbin(Winding coil bobbin)

Schematic diagram of the pass line for applying prebending strain; (a) forward bending, (b) bidirectional
5)
bending (forward and backward directions) .

The sign ± represents the winding directions of the heat
treatment bobbin, with the forward direction shown as +and the backward direction shown as –. That is, D b+means the pulley diameter of the forward bending, andD b– means the pulley diameter of the backward bending.
The maximum value of d (y) is the outer diameter of thewire d (y = d/2), with the center of the cross section of thewire as the reference point (y = 0) (Figure 6 (a) below). In acoil where a wire is wound around a bobbin with a diameter (Drw) different from that of the heat treatment bobbin,pure-bending strain (strain when only a bending moment
occurs) is applied to the wire, and the Nb3Sn filaments
located at the diameter d fb of the filament bundle in thewire experiences the maximum bending strain12). When
comparing the bending strain characteristics of the wires
with different cross-sectional structures, a diameter d non-Cuof the Non-Cu part including the Sn diffusion barrier may
be used16).

A similar effect can be obtained with a conventional
Cu/Nb3Sn wire without reinforcement, but the mechanicalstrength of a Cu/Nb3Sn wire at room temperature is considerably lower than that of the Cu-Nb/Nb3Sn wires13),
and it causes a greater risk of damage on Nb3Sn filaments during handling. Therefore, it is difficult to proactively apply the pre-bending strain application treatment
on it. In addition, when the pre-bending strain application
treatment is applied to a Rutherford cable using Cu-Nb/
Nb3Sn wires (Figure 1 (b)) 5), the adhesive parts between
the wire surfaces that occur during heat treatment peeloff, causing individual wires to move separately. As aresult, the allowable bending strain of the stranded wire
becomes equivalent to that of a single wire, and it can bebent to a small diameter.
2.4 Current Carrying Characteristics Under PureBending Strain
The Relationship between the critical current when purebending strain is applied to the Cu-Nb/Nb3Sn wires for 25
T-CSM and the bending strain (peak pure-bending strain)
generated in the filament placed in the outermost layer of
the Nb3Sn filament bundle is shown in Figure 5 (a)12). In
the range of ±0.5% pure-bending strain, the Pre-bent(-PB) wire had a higher Ic than that of the As-reacted (-AR)
wire in the unstrained state.
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Development of Cu-Nb Reinforced Nb3Sn Wires

50
0

4.2 K, 1 µV/cm
-1

-0.5

0

0.5

1

Peak pure-bending strain / %Figure 5

17 T

4.2 K, 1 µV/cm
-1

-0.5

0

0.5

1

Peak pure-bending strain / %wfb

Figure 6

Ic characteristics of the Cu-Nb/Nb3Sn wire for the
12)
25 T-CSM against the peak pure-bending strain .

3.

Center of wire for bending direction
Bending neutral axis

Cross-sectional structure model of the wire for the
analysis of the pure-bending strain effect on the Ic of
17)
the Cu-Nb/Nb3Sn wire . (a) round wire, (b) tape wire

PERFORMANCE IMPROVEMENT IN
THE Cu-Nb/Nb 3Sn WIRES

3.1 Improvement in the Mechanical Characteristics
When a superconducting coil with a winding radius R (m)is operated in an external magnetic field B (T) and an
average current density J e (A/m2), the tensile stress σ (Pa)given by BJ e R is applied to the winding axial direction,
and a transverse compressive stress determined by thecoil structure is applied to the wire. In order to obtainstructural design guidelines for Cu-Nb/Nb3Sn wires that
can achieve both the Ic and the strength required under
tensile and transverse compressive stress during coil
operation, we made prototype wires with different crosssectional structures shown in Table 1 5), 15), 16).
By using the high Sn bronze Cu-15.7wt%Sn-0.3wt%Ti 9),
the critical current density (non-Cu-Jc) was increased byabout 10% compared to the 25 T-CSM wire using the
Cu-14wt%Sn-0.2wt%Ti. In three types of structures with
Nb filament volume fractions in Cu-Nb of 20 vol%(LK199), 25 vol% (LK200), and 30 vol% (LK201), and inother three types of structures with Cu-25 vol%Nb reinforcement occupancy per wire cross section of 38%(LK200), 55% (LK202), and 0% (LK206), the current carrying characteristics under external stress were compared

Cu-Nb/Nb3Sn wire rods with various cross-sectional structure designs
Wire IDx

Compression side

Filament
bundleCenter of bending radiusThis indicates that by applying appropriate pre-bendingstrain to the Cu-Nb/Nb3Sn wire and controlling the bending diameter below the allowable bending diameter, the Icof the R&W method can be improved more than that of
the W&R method. However, as shown in Figure 5 (b), consideration must be given to the n-value, which decreases
in different degrees depending on the applied magnetic
field and the applied bending strain.
Figure 6 shows a cross-sectional structural diagram of
Ekin etc.’s analytical model for critical current in a roundwire under pure-bending strain 22), 23) applied to a rectangular wire17). The permissible bending diameter of the Nb3Sn
wire is limited by the maximum bending strain experienced by the Nb3Sn filament located at the outer edge ofthe filament region. Therefore, in order to bend the Nb3Sn
wire to a smaller diameter, it is effective to reduce thearea of the Nb3Sn filament, but there is a problem that
reducing the wire diameter reduces the critical current per
wire. In addition to a cable with thin strands of 0.4 mm indiameter15), we have confirmed that a forming a Cu-Nb/
Nb3Sn rectangular wire (tape wire) with a large aspectratio is an effective method for increasing the critical current value when bent to a small diameter in order to makethe Nb3Sn filament region smaller to the bending direction17).
Table 1

Tensile side

0
δ

20

0

（b）

wt

t

30

10

17 T

Cu-Nb reinforcer
Sn barrierCu

14.5 Td

14.5 T

100

y

12 T

40

150

（a）

Rb_pure12 T

25 T-CSM-PB
25 T-CSM-AR

（b）

tfb

50

dfb

25 T-CSM-PB
25 T-CSM-AR（a）n -value / -Critical current Ic / A

2005), 15), 16).

25 T-CSM

LK199

LK200

LK201

LK202

LK206

LK224

Cross-sectional structure

Cu/Cu-Nb/non-Cu (%)

20/35/45

19/39/42

21/38/41

19/39/42

20/55/25

57/0/43

20/45/35

Nb occupancy in Cu-Nb
reinforcement material (vol%)

20

20

25

30

25

―

30

Diameter of the
Nb3Sn filament region (mm)

0.52

0.49

0.49

0.49

0.37

0.5

0.45

Wire diameter (mm)

0.80

0.80

Nb3Sn filament diameter (μm)

3.3

3.2

3.2

Sn diffusion barrier

Ta

Nb

Ta/Nb

Bronze composition

Cu-14wt%Sn-0.2wt%Ti

Twist pitch (mm), Twist direction

0.80

Cu-15.7wt%Sn-0.3wt%Ti
24，S direction

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Development of Cu-Nb Reinforced Nb3Sn Wires

Normalized Critical Current Ic /Icm / A

respectively. In the LK224, the Nb ratio in the Cu-Nb wasincreased to 30 vol% and the Cu-Nb cross-sectional area
ratio in the wire was increased to 45% in order to improvethe current-carrying characteristics under tensile stressexceeding 300 MPa16).
Figure 7 (a) shows the Ic values (4.2 K, 14.5 T) normalized by the peak Ic value of each bare wire and the Ic
value at zero stress against axial tensile stress, respectively. The strain value of the pre-bending strain application treatment was defined per non-Cu area. The tensilestress at which the I c of each Cu-Nb/Nb 3 Sn wiredecreased was twice or more than twice that of the Cu/
Nb3Sn wire without reinforcement (LK206-PB±0.3%). The
LK201-PB±0.3% and the LK202-PB±0.3% had smaller
Ic decrease than in the 25T-CSM-PB±0.3% in the regionof tensile stress of 300 MPa or higher, and the
LK201-B±0.5% had a significantly restrained Ic decreaseat 400 MPa or more. In the LK224-PB±0.38%-D, evenwith multi-stage heat treatment (575°C x 100 hrs + 670°C
x 50 hrs), which increases the critical current density, it
maintained the same high mechanical characteristics aswith heat treatment at 670°C x 96 hrs.
In the I c characteristics of each Cu-Nb/Nb 3Sn wire
shown in Figure 7 (b) under transverse compressive stress,
LK199-PB±0.3%
LK200-PB±0.3%
LK201-PB±0.3%
LK201-PB±0.5%
LK202-PB±0.3%
LK206-PB±0.3%
25 T-CSM-PB±0.3%
LK224-PB±0.38%-D

1.2
1
0.8
0.6
0.4
0.2
0

（a）14.5 T, 4.2 K
0

100

200

300

400

500

600

700

800

Normalized Critical Current Ic /Icm / A

Axial tensile stress / MPa
LK199-PB±0.3%
LK200-PB±0.3%
LK201-PB±0.3%
LK201-PB±0.5%
LK202-PB±0.3%
LK206-PB±0.3%
25 T-CSM-PB±0.3%
LK224-PB±0.38%

1.2
1
0.8

the transverse compressive stress value at which the Ic
decreases by 10% or more was 50 MPa or more larger
than that of the Cu/Nb3Sn wire without a reinforcement
material (the LK206-PB±0.3%). This confirmed that
Cu-Nb reinforcement is also effective in improving lateralcompression characteristics. A comparison of the characteristics of the LK201-PB±0.3% and the LK201-PB±0.5%with different applied pre-bending strain values showedthat an increase in pre-bending strain value improved the
Ic characteristics under tensile stress, but the Ic characteristics under transverse compressive stress decreased.
This indicates that the pre-bending strain application conditions must be selected by taking into consideration the
cross-sectional structure of the Cu-Nb/Nb3Sn wire andthe distribution of external stress applied to the wire during actual superconducting magnet operation.
3.2 Improvement in the Conductor Current Density
There are various specifications for Nb3Sn wires used inhigh-field Nuclear Magnetic Resonance (NMR), accelerators, large-capacity conductors for nuclear fusion, etc.,
and as their magnetic field becomes higher and they
become larger, achieving both the improvement of the
conductor current density (current density Je per wirecross-sectional area: engineering Jc ) and high mechanicalstrength becomes necessary. As shown in Table 2, theLK144 (REC) was manufactured into a rectangular crosssection shape to increase the coil winding density for theapplication to high-field NMRs 13). As the LK145 and the
LK288 are considered for application in accelerators andnuclear fusion magnets, they were manufactured to around wire shape to improve the current density bystranding multiple wires together and applying compression processing13), 18). With respect to the cross-sectional
composition ratio, the Cu-Nb reinforcement material was
reduced to 20 to 23% and non-Cu part was set to 50% in
order to increase the Je, but the stabilized Cu amount,
including Cu in the Cu-Nb reinforcement material, wassecured at 45 to 46%.
Table 2High critical current density type Cu-Nb/Nb3Sn
13),18).
wire

Wire ID

LK144（REC）

LK145

LK288

0.80

0.827

Cross sectional
structure

0.6
0.4
0.2
0

（b）14.5 T, 4.2 K
0

50

100

150

200

250

300

Transverse compression stress / MPa
Figure 7tw

R

Dimension (mm)1.13 x 1.70 - 0.3

Nb3Sn filament
diameter (µm)

3.2

3

2.3

Twist pitch (mm)

50

24

15

Direction

S direction

Z direction

Sn diffusion barrier

Nb

Ta

Cu/Cu-Nb/non-Cu(%)

30/20/50

27/23/50

Ic values normalized at the respective peak Ic valuesof the Cu-Nb reinforced Nb3Sn wire and theunreinforced Cu/Nb3Sn wire at 4.2 K and 14.5 T under
tensile stress. (a) Under axial tensile stress. (b) Undertransverse compressive stress.

Bronze composition

Cu-15.7wt%Sn-0.3wt%Ti

Note: H eat treatment condition was 670°C x 96 hours. However,data of the LK224 under axial tensile stress was obtained by5), 15), 16)
heat treatment at 575°C x 100 hours + 670°C x 50 hours.

Cu-Nb reinforcement
material

Nb-rod method Cu-20vol%Nb

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Development of Cu-Nb Reinforced Nb3Sn Wires

Critical current density non-Cu Jc / A/mm2

LK145-AR
700
LK145-PB
25 T-CSM-AR 600
25 T-CSM-PB
LK288-AR500
LK288-PB

1400
1200
1000
800

400

4.14 K

600

300

400

200

200
0

100

（a）
11

Figure 8

12

13

14

B/T

15

16

17

0

LK144（REC）-PB
LK144（REC）-PB-e
LK144（REC）-PB-f
LK144（REC）-AR

4.14 K

4.2 K（b）14 15 16 17 18 19 20 21 22 23 24 25

B/T

Magnetic field dependence of the critical current5), 13), 18).
density in the non-Copper part(a) round wire, (b) rectangular wire

Figure 9 shows the non-Cu-Jc characteristics (14.5 T,
4.2 K) of the LK145 and the LK288 (heat treated at 575°C
x 175 hrs + 650°C x 125 hrs), and the 25 T-CSM wire
(heat treated at 670°C x 96 hrs) with respect to the axial
tensile strain and the axial tensile stress. The non-Cu-Jc
values of the LK145-PB was higher than that of the
LK145-AR at 600 A/mm2 or more up to an axial strain of
0.3% (stress of 250 MPa), and higher than that of the
25 T-CSM-PB up to approximately 300 MPa. The nonCu-Jc values of the LK288-PB was higher than that of the
25 T-CSM-PB up to approximately 400 MPa. As the
LK288 is expected to have both the improvement in the
manufacturability of short-pitch, low-void multi-stranded
wires and the maintenance of the superconducting characteristics under high electromagnetic stress due to theexcellent stress resistance characteristics of the Ic, it isexpected to be applied to cable-in-conduit conductors
for large nuclear fusion reactor magnets18).
Critical current density non-Cu Jc
A/mm2

Figure 8 (a) shows the magnetic field (B) dependence ofthe non-Cu-Jc when subjected to the Nb3Sn generation
heat treatment at 575°C x 175 hrs + 650°C x 125 hrs. The
non-Cu-Jc value increased by applying the pre-bendingstrain application treatment (wire surface definition±0.5%). The LK145-PB had 1115 A/mm2 at 12 T and
400 A/mm2 at 17 T, which was about 30% increase to thecharacteristics of the 25 T-CSM-PB. The rectangular wire
LK144 (REC) shown in Figure 8 (b) was evaluated bychanging the direction in which the pre-repetitive bendingstrain of ±0.5% was applied. The non-Cu-J c of the
LK144-PB, which was applied the pre-bending strain
alternately from both the flatwise direction and the edgewise direction for five times each, was 355 A/mm2 at 17 T,
which was about 1.6 times that of the LK144-AR. It wasapproximately 1.1 times when it was applied 10 times
from the flatwise direction (PB-f) and from the edgewise
direction (PB-e). Using the Kramer model 24), the effective
upper critical magnetic field (B*c2) of the LK144-AR, theLK144-PB-f, and the LK144-PB, calculated from theextrapolation of the Jc values from 18 T to 25 T, were24.4 T, 24.8 T and 25.0 T respectively. Similar to the
Cu-Nb/Nb3Sn rectangular wire, even when the conductor
is made into a stranded wire, pre-bending strain application from two directions is considered to be effective.

LK145-AR
LK145-PB
25 T-CSM-AR
25 T-CSM-PB
LK288-AR
LK288-PB

800
700
600
500
400
300
200

100 （a）14.5 T, 4.2 K
00

Figure 9

0.2

0.4

0.6

（b）14.5 T, 4.2 K
0.8

Axial tensile strain / %0

100 200 300 400 500 600 700

Axial tensile stress / MPa

Comparison of the critical current density
characteristics in the non-Copper part at 14.5 T
between the high critical current density round wires(LK145, LK288) and the 25 T-CSM wire under axial5), 13), 18).
strain and axial stress(a) Axial tensile strain, (b) Axial tensile stress

3.3 Application of Enamel-coated Insulation
As shown in Figure 1, in the W&R method Nb3Sn coils,
the unreacted Nb3Sn wire is generally insulated with glass
braid and heat treated after winding, but it has the following disadvantages: (1) Glass braiding takes long production time and high cost, (2) Breakdown voltage decreases
due to carbonization of the binder in glass braiding,(3) Wire fill factors in the coil decreases due to thick insulation coating and variations in dimensions, etc. On theother hand, in the case of the R&W method, various
materials can be used for the insulation of the wire after
Nb3Sn generation heat treatment, including polyimide
tapes, polyester braids, and enamel coating such as
Polyvinyl Formal (PVF). With respect to enamel-coated
Nb3Sn wires, there has been a report on the developmentof an R&W method Nb3Sn coil using a PVF-coated internal tin-diffusion Nb3Sn wire with a strict control of tension
and bending strain 25), but at present, there has been nopractical application yet.
We coated the Cu-Nb/Nb3Sn wires shown in Table 3
with PVF and evaluated their critical current characteristics under tensile stress14). The applied bending strain
was defined as the peak strain at the outer diameterdfb (mm) of the Nb3Sn filament bundle part, and the bending strain εpb+ in the forward direction, which is same as
the bending direction during heat treatment, and thebending strain ε pb− in the backward direction were calculated using the formula (1) mentioned above. The LK165
was wound on a bobbin with a diameter (D h) of 500 mm
and subjected to Nb3Sn generation heat treatment at
670°C for 96 hrs, after which 10 sets of D pb+ = 125 mm
and D pb− = 250 mm pulleys were alternately connected
and bidirectional bending strain ε pb± = ±0.31% wasapplied repeatedly (Figure 3 (b)). For the LK179, only theforward bending pulley of D pb+ = 250 mm was used after
heat treated with a bobbin of D h = 150 mm, and only
–0.34% of backward strain that would be experienced
when returning to the straight position was applied(Figure 3 (a)). For the LK183, only a forward bending pulFurukawa Electric Review, No. 55 202415

Development of Cu-Nb Reinforced Nb3Sn Wires

Table 3

14)

Cu-Nb/Nb3Sn wire with PVF insulation coating .
Wire ID

LK165

LK179

PVF

Cu

Nb

Cu-Nb

Ta

Cu

PVF

LK183

Nb3Sn ﬁlament bundle
PVF

Cu

Cu-Nb

Nb

Cross sectional
structure
Cu-Nb

Wire diameter after PVF
coating (mm)

0.84

0.88

1.39

PVF coating thickness
(mm)

20

40

40

Wire diameter before
PVF coating (mm)

0.80

1.31

Nb3Sn filament bundle
diameter (mm)

0.51

0.79

Nb3Sn filament diameter
(µm)

3.3

3.3

Twist pitch (mm),direction

24，
S direction

36，
S direction

Bronze structure

Cu-14wt%Sn-0.2wt%Ti

Cu-15.7wt%Sn-0.3wt%Ti

Sn diffusion barrier

Nb

Ta

Cu/Cu-Nb/non-Cu (%)

20/35/45

20/40/40

Cu-Nb reinforcement
material

Nb-rod method Cu-20vol%Nb

ley with D pb+ = 270 mm was used after heat treated with abobbin of D h = 700 mm, and a forward bending strain
εpb+ = +0.19% and a backward bending strain εpb− = –0.11%
that would be applied when returning to the straight position were applied.
Figure 10 (a) shows the critical current characteristics of
the LK165-PVF and the LK179-PVF with PVF coating
under tensile stress. With respect to the tensile stress
LK165_PVF-insulated
LK179_PVF-Insulated
25 T-CSM_Pre-bent
25 T-CSM_As-reacted

Critical current / A

150
125
100
75
50
25

（a）14.5 T, 4.2 K

Critical current / A

0

LK183-PVF

250

LK183-AR

200
150
100
50
0

（b）14.5 T, 4.2 K
0

100

200

300

400

500

600

Axial tensile stress / MPa
Figure 10 Comparison of critical current characteristics underaxial tensile stress of the Cu-Nb/Nb3Sn wire with
PVF insulation coating and the wire without PVF
14)coating .
(a) Comparison of the PVF-coated wire K165, theLK179 and the 25 T-CSM wire, (b) Comparison of the
PVF-coated wire LK183 and the uncoated wire LK183

dependence of the critical current value of the LK165PVF, the critical current in the tensile stress region of
260 MPa or less improved compared to that of the
25 T-CSM-AR immediately after heat treatment, and thecritical current in the tensile stress region of 150 MPa or
less improved compared to that of the 25 T-CSM-PB.
Figure 10 (b) shows the critical current characteristics of
the LK183-PVF coated with PVF under tensile stress. The
critical current value of the LK183-PVF improved compared to that of the LK183-AR immediately after heat
treatment in the tensile stress region of 250 MPa or less.
As a result, the Cu-Nb/Nb3Sn wires can be insulatedwith enamel such as PVF, and by applying appropriate
pre-bending strain, the current carrying characteristics
under practical tensile strain and tensile stress can be
improved after coating PVF.

4.

SUMMARY AND FUTURE OUTLOOK 11)

The effectiveness of the Nb-rod method Cu-Nb/Nb3Sn
wires have been demonstrated in the Tohoku University
25 T-CSM Cu-Nb/Nb3Sn Rutherford cable, and subsequent development of various elemental technologies has
clarified the excellent manufacturability and the highsuperconducting characteristics in a wide stress range
could be achieved not only by the R&W method but also
by the W&R method.
In the application for the Nb3Sn magnets for nuclear
fusion, the JA-DEMO-TF conductor (TF conductor for a
Japanese demo reactor) 26) utilizes the developmental
achievement of the ITER-CS conductor 27)-29), and in orderto maintain excellent superconducting characteristics
under high electromagnetic stress, a short-pitch and lowvoid W&R type cable-in-conduit Nb3Sn conductor structure is promising. To apply the above, it is necessary to
improve the stress resistance characteristics and the
manufacturability of stranded wires of Nb 3Sn wires.
Therefore, the application of Cu-Nb/Nb3Sn wires is effective18). In addition, for EU-DEMO-TF conductors, as an
R&W type Nb3Sn conductor option 30) is being considered,a method of forming a conduit after applying pre-bendingstrain to the Cu-Nb reinforced Nb3Sn conductor is an
option 10). On the other hand, for the Future Circular
Collider designed under the FCC project of the European
Organization for Nuclear Research (CERN) 31), development of manufacturing technologies for a robust high current density type Nb3Sn magnet that reliably generates a
specified magnetic field is necessary. Therefore, in addition to improving the basic coil manufacturing technology,
consideration has been given to increasing the strengthof the Nb3Sn wire itself. Based on the characteristics of
Nb3Sn superconducting wires32), both a high J c and a highstrength can be achieved at the same time by replacing amember of the Nb3Sn filament bundle part made by thebronze method to the one using an advanced internal tindiffusion method that achieves high current density.
Furthermore, in the fields such as high-field NMRs,
Furukawa Electric Review, No. 55 2024

16

Development of Cu-Nb Reinforced Nb3Sn Wires

Magnetic Resonance Imaging (MRI), and medical accelerators, it is expected that the right material will be used in
the right place for coils that cover a wide range of magnetic fields and temperature regions between NbTi wires
and high-temperature superconducting wires.
In the future, we will steadily advance the developmentof elemental technology for application to a variety of
Nb3Sn magnet application devices, taking advantage ofthe fact that the Cu-Nb/Nb3Sn wires are practical wires
that can be mass-produced at a reasonable cost.

ACKNOWLEDGMENT

The development of the Cu-Nb/Nb3Sn wires has beenpromoted through joint research with Institute forMaterials Research, Tohoku University and Tokai University,and currently many researchers and engineers in Japanand abroad are working on their application to various
superconducting devices. We would like to thank everyone involved for their guidance and support.

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