J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Procurement of Nb3Sn superconducting conductors in ITER
Yoshihiro NABARA, Takaaki ISONO, Yoshihiko NUNOYA, Norikiyo KOIZUMI,Kazuya HAMADA, Kunihiro MATSUI, Tsutomu HEMMI, Katsumi KAWANO, Yasuhiro UNO,
Syuichi SEKI, Sachito ITO, Masayuki OSHIKIRI, Masatoshi YOSHIKAWA, Katsutoshi TAKANO,
Humiaki TSUTSUMI, Yoshikazu TAKAHASHI, Hideo NAKAJIMA and Kiyoshi OKUNOJapan Atomic Energy Agency, Naka, Ibaraki, Japan(Received: 18 November 2009 / Accepted: 24 February 2010)

Superconducting strands are applied to Toroidal Field (TF) coil, Poroidal Field (PF) coil and Centre Solenoid(CS) in ITER. Japanese share of TF conductor is 25% and that of CS conductor is 100%. The conductor of TF coilcontains 900 Nb3Sn superconducting strands and 522 copper wires. As described in the length of superconducting
strand, the length of Japanese share is 23,000km. In order to generate the magnetic field of which maximum valueis 11.8T, 68kA of current is sent through the conductor under the rated operation. Although the critical current mustbe high, the high critical current tends to make the hysteresis loss rise at the same time. The hysteresis loss must below because that is the heat generation under a fluctuating magnetic field. The strands which satisfy these
performances compatibly had been developed. In advance of the other parties, the production of strands for TF coilstarted in 2008. To date, 3,400km long strands have been fabricated. Some of them are going to be cabled soon. Thejacketing facility of TF conductor is being newly built. The building is almost completed. The procurement ofstrands for TF coil is underway.
Keywords: ITER, toroidal field coil, centre solenoid, Nb3Sn superconducting strand, jacket

1. Introduction

Superconducting strands are applied to Toroidal
Field (TF) coil, Poroidal Field (PF) coil and Centre
Solenoid (CS) in ITER. Figure 1 shows the schematic
view of these coils and Japanese share of thesecomponents. Japanese share of TF conductor is 25% andthat of CS conductor is 100%. The procurement
arrangement of TF conductor was concluded at the end of2007 and the procurement of superconducting strands ofTF coil started in 2008 in advance of the other parties [1].
The procurement arrangement of CS conductor will beconcluded soon.
The configuration of TF conductor is illustrated in
Fig.2. Main specifications of the conductor are shown in
Table 1. The conductor contains 900 superconducting
strands and 522 copper wires. The spiral channel is set atthe centre of the cable to promote the coolant flow.
Flowing in the spiral channel and between strands, thecoolant of supercritical helium keeps the working
temperature of 5.0K. In order to generate the magnetic
field of which maximum value is 11.8T, 68kA of current
is sent through the conductor under the rated operation.
The current sharing temperature, which is the limitationof the temperature to maintain a superconducting state, is5.8K. As the coolant temperature is 5.0K, the temperature
margin is 0.8K. In one TF coil, 4,630m long TF
conductors are wound with 134 turns into D-shape. As

Fig.1

Superconducting coils in ITER and Japaneseshare of these components

Fig.2

Configuration of TF conductor

author’s e-mail: nabara.yoshihiro@jaea.go.jp

270©2010 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchY. Nabara et al., Procurement of Nb3Sn Superconducting Conductors in ITER

described in the length of conductor, the length ofJapanese share is 22km. As described in the length of
Nb3Sn strand, the length of Japanese share is 23,000km.
The manufacture of the TF conductor has three steps.
First, large amount of superconducting strands areproduced. Second, 900 superconducting strands and 522
copper wires are cabled. Finally, the cable is covered withthe stainless steel jacket. This paper describes the present
condition of the procurement of TF conductor on eachstep in Japan.

0.820±0.005mm

Cr plate : Reducing the AC loss across strands
Cu : Stabilizing by circumventing electric current fromNb3Sn to Cu in case of breaking of superconducting state

Nb or Ta sheet : Barrier to prevent Sn from diffusing to Cu
Nb3Sn superconducting filaments: There are about 10,000 filaments.
Each filament diameter is a few Pm.

Fig.3

Table 1. Main specifications of superconducting
conductor for TF coil
Item
Specification
Current
68.0 [kA]
Magnetic field
11.8 [T]Working temperature
5.0 [K]
Current sharing temperature
5.8 [K]
Outer diameter of conductor43.7±0.2 [mm]Cross sectional view of the Nb3Sn
superconducting strand

Table 2. Main specifications of superconducting
strand for TF coil
ItemSpecification
Strand diameter
0.820±0.005 [mm]Critical current> 202.2 [A] (for supplier A)> 212.5 [A] (for supplier B)
Hysteresis loss
< 500 [mJ/cm3]
Residual Resistivity Rratio> 100 [-]

2. Procurement of Nb3Sn superconducting strand

25

relative frequency [%]The cross sectional view of the Nb3Sn
superconducting strand is illustrated in Fig.3. One strand
has several km long. Main specifications onsuperconducting strand are shown in Table 2. The
allowable margin of error of the strand diameter is only
0.01mm. The critical current Ic is the maximum current
through the core region in Fig.3 as the superconducting
state is maintained. Although the joule heating of asuperconducting strand is zero under a steady state, theheating arises under varying magnetic field or current.
The hysteresis loss is an index of this joule heating.
Although the critical current must be high, the highcritical current tends to make the hysteresis loss rise at
the same time. The hysteresis loss must be low becausethat is the heat generation under a fluctuating magneticfield. It is difficult that the strand must satisfy these
performances compatibly. The Cu in Fig.3 works as thestabilizer by circumventing current from Nb3Sn
superconducting filaments to Cu in case of breaking of
the superconducting state. Thus the electrical resistivityof Cu must be low. Impurities in Cu markedly increase
the electrical resistivity at the low temperature. In orderto prevent Sn from diffusing to Cu, the sheet of Nb or Tain Fig.3 works as the barrier. The residual resistivity ratio(RRR) of Cu is the ratio of the electrical resistance at273K to that at 20K. In the mass production, these
performances of superconducting strands vary each inspite of the same production way, as shown later.
The specifications of hysteresis loss and RRR aredefined by the requirement of stability. The specificationof Ic is defined as follows. At the beginning of the mass
production of strand, the average critical current Ic_ave andthat of standard deviation V are calculated using the data20Spec.
> 202.2A

Supplier A151050210 215 220 225 230 235 240 245 250 255 260

critical current [A] at 12T, 4.2K

relative frequency [%]3025Spec.
> 212.5ASupplier B

20151050210 215 220 225 230 235 240 245 250 255 260

critical current [A] at 12T, 4.2K
Fig.4 Critical current dispersion of fabricatedsuperconducting strands on each supplier271

Y. Nabara et al., Procurement of Nb3Sn Superconducting Conductors in ITER
1445Supplier ASpec.
< 500mJ/cm3353025201510Supplier ASpec.
> 100

12

relative frequency [%]relative frequency [%]4010864250300

3253503754004254504750500100 110 120 130 140 150 160 170 180 190 200 210 220 230

hysteresis loss [mJ/cm3] for ±3T40

residual resistivity ratio [-]30Supplier B

Supplier B

25Spec.
< 500mJ/cm3

3025

relative frequency [%]relative frequency [%]35

201510Spec.
> 100

2015105500350

375

400

425

450

475

500

100

125150175200225250275

300

residual resistivity ratio [-]Fig.6 Residual resistivity ratio (RRR) dispersion of
fabricated superconducting strands on each supplierhysteresis loss [mJ/cm3] for ±3T
Fig.5 Hysteresis loss dispersion of fabricatedsuperconducting strands on each supplierof the first production of 100km strand. The specificationof critical current Ic_spc is defined as Ic_spc = Ic_ave±3V.
Thus Ic_spc depends on a supplier. The values of Ic_ave andV are revised after the production of 760km strand.
Two suppliers have been producing superconducting
strands for TF coil so far. Figure 4 shows the criticalcurrent dispersion of fabricated superconducting strands
each supplier. These data are higher than eachspecification. These histograms indicate that bothsuppliers’ Ic have the peek at around 235A. Figure 5shows the hysteresis loss dispersion of fabricatedsuperconducting strands each supplier. These histograms
point out that all data are lower than the specification of500mJ/cm3 and the supplier A tends to have moremargins. Figure 6 shows the RRR dispersion of fabricatedsuperconducting strands each supplier. All data are higher
than the specification of 100. In the histogram of supplierA, two peaks appear at around 125 and 170, of which
reason is not certain yet.
To date, 3,400km long strands have been fabricatedand are ready to be cabled.

3. Cabling of Nb3Sn superconducting strands

Geometric specifications of the superconducting cablefor TF coil (TF cable) are shown in Table 3. The TF cablecontains 900 Nb3Sn superconducting strands and 522
cupper wires. The cabling process has five steps. Figure 7
illustrates the process. Two Nb3Sn strands and one Cu
wire are cabled into the 1st cable with a twist pitch of80mm. Three 1st cables are cabled into the 2nd cable with atwist pitch of 140mm. Five 2nd cables are cabled into the3rd cable with a twist pitch of 190mm. Five 3rd cables andone Cu sub-cable are cabled into the 4th cable with a twist
pitch of 300mm. Six 4th cables are cabled into the 5th cablewith a twist pitch of 420mm. The 5th cable also contains acentre spiral tube. These twist pitches are optimized
through R&D. The allowable margin of error of the 5thcable diameter is only 0.5mm as shown in Table 3.
The technical difficulty is the large deformation of thethsix 4 cables on the last step to make six 4th cables to one
5th cable. Figure 8 shows the comparison between thecables for the CS model coil [2] assembled in 2000 and thecables for TF coil. The six solid circles are the cross272

Y. Nabara et al., Procurement of Nb3Sn Superconducting Conductors in ITER

section of the 4th cable before the last cabling process. The
dotted circle is the diameter of the 5th cable. The circle inthe centre is the centre channel. The diameter of centre
channel for CS model coil is 12mm and that for TF coil is9mm. The void fraction for CS model coil is 36% and thatfor TF coil is 33%. Thus, comparing the cable for TF coilwith that for CS model coil, the density is thicker and thedeformation is much larger. In the R&D of the cabling,some strands were kinked or crushed and some stainlesssteel wraps on the 4th cable of which thickness is 0.1mm
were broken. Through the optimization of dies and rollers,
these troubles did not occur throughout the cablingprocess.
Figure 9 shows the manufacturing facility of the 5thcable using the six 4th cables. Six 4th cables are supplied
from the left side and are cabled with a twist pitch of420mm. Through three dies and four rollers, the cable
comes to be circle 41.8mm in diameter in precise with0.5mm. The 5th cable is wrapped with the stain less steel
sheet which has thickness of 1mm and wound on the roundbobbin which has 1.9m diameter.
The 10m long conductor for the performance
examination was manufactured without any problem.
Using 1422 copper wires (with no superconducting strand),the trial manufacture of the dummy cable having 760m
long has been successfully completed. The production of760m long cables to be fabricated in TF coil will begin
soon.

Table 3. Geometric specifications of thesuperconducting cable for TF coil
Item
Specification
Twist pitch of
1st cable
80±5 [mm]140±10 [mm]2nd cable190±10 [mm]3rd cable
th300±15 [mm]4 cable420±20 [mm]5th cable
Diameter of 5th cable
41.8+0.2-0.3 [mm]5th cable diameter
38.5mm5th cable diameter
41.8mmCentre channel diameterof TF coil : 9mm
Centre channel diameterof CS model coil : 12mm
4th cable for CS model coil
4th cable for TF coil

(a)
Fig.8

Cabling process(b)Comparison of six 4th cables before 5th cabling(a) for CS model coil and (b) for TF coilPhotographs of 4th and 5th cable
41.8

Central spiral

4th cable

5th cable before the wrapping
Fig.7

5th cable after the wrapping

Constitution of the TF cable273

Y. Nabara et al., Procurement of Nb3Sn Superconducting Conductors in ITER

suppliedsix 4th cables

Diameter
1.9m

manufactured
5th cableFig.9

winding the 5th cable on the round bobbin

Cabling facility for cabling 5th cable

4. Jacketing factory of TF conductor

5. Summary

The jacketing factory is being newly constructed.
The aerial photo is shown in Fig.10. The superconducting
cable wound on the round bobbin is shipped from thecabling manufacturer to the building A in Fig.10. The
superconducting cable is straightened in the line B.
Welding 13m long jackets in series in the building C, thejacket comes to be 760m or 415m long. After the helium
leak test of the welded jacket, the cable is inserted intothe jacket. The cable and the jacket are combined into theconductor by the compaction machine shown at thebottom of Fig.10. Geometric specifications of the TF
conductor are shown in Table 4. In the building C, theconductor is wound on the round spool which has 4m
diameter. After the radio-transparency test and the helium
leak test, the TF conductor is shipped to a TF coilmanufacturer and wound into the D-shape of TF coil.
Then the conductor is heat-treated at around 650℃ forabout 10 days to produce the superconducting material of
Nb3Sn. This heat treatment also relieves the residualstrain which is applied to the cable through the cabling,
jacketing and winding processes.
The building is almost completed. Facilities arebeing set up steadily. Using the 760m long dummy cablemade of 1,422 copper wires, the trial jacketing will beconducted soon. This facility is designed to be applicableto the jacketing of CS conductor as well.

The present condition of the procurement ofsuperconducting strands for TF coil is described asfollows.
(1) Japanese share of Nb3Sn strands for TF coil is
23,000km long. The production of superconducting
strands has been started since 2008 by two suppliers.
To date, those of 3,400km long have been
manufactured. Although the performance of themvaries in the mass production, all specifications aresatisfied.
(2) The 10m long conductor for the performance
examination was manufactured without any problem.
Using 1,422 copper wires, the manufacture of thedummy cable which has 760m long has beensuccessfully completed. The production of 760m
long cables to be fabricated in TF coil will beginsoon.
(3) The jacketing factory is being newly constructed.
The building is almost completed. Facilities arebeing set up and will work soon. Using the 760m
long dummy cable made of 1,422 copper wires, thetrial jacketing will be conducted soon.

6. References[1][2]

Table 4. Geometric specifications of the TF
conductor
Item
Specification
Thickness of jacket
1.9±0.1 [mm]before compaction
Outer diameter of jacket
48.0±0.2 [mm]before compaction
Outer diameter of conductor43.7±0.2 [mm]after compaction

Y. Takahashi et al., Abstracts of CSJ Conference 76,1B-a06 (2008).
T. Ando et al., Journal of the Cryogenic Society of Japan36, 6 (2001).

Acknowledgements

The authors are grateful to thank Hitachi cable andJapan superconductor technology for manufacturing
Nb3Sn superconducting strand. We also wish toacknowledge Hitachi cable and J-Power systems oncabling strands. We are deeply indebted to Nippon steel
engineering and Kobe steel for jacketing cables.

274

Y. Nabara et al., Procurement of Nb3Sn Superconducting Conductors in ITER

In the line B

Aerial photo of jacketing factoryline B (850m length)building A

building C

Jacket pulling tables are set up.

In the building CJackets (SUS316LN)

thickness : 1.9mm
outer diameter : 48.0mm
length : 13m

Automated
welding machineSuperconducting cableradius : 41.8mm, length : 760m or 415m

Welding 13m long jackets in series,
the jacket comes to be 760m or 415m long.
Insertion of the cable for TF coil

Conductor for TF coilCompaction machine(Outer diameter : 43.7mm±0.2mm)

Combined by
the compaction
machineFig.10Jacketing factory of TF conductor

275