Fusion Engineering and Design 58–59 (2001) 13–16

Design of the toroidal field coil for A-SSTR2 using high Tcsuperconductor

T. Ando a,*, T. Kato a, K. Ushigusa a, T. Nishio a, R. Kurihara a, I. Aoki a,K. Hamada a, H. Tsuji a, M. Hasegawa b, H. Naito b

a Naka Fusion Research Establishment, Japan Atomic Energy Research Institute (JAERI), 801-1 Mukouyama, Naka-machi,Naka-gun, Ibaraki-Ken 311-0193, Japan

b Mitsubishi Electric Corporation, 7-10-4 Nishigotanda, Shinagawa-ku, Tokyo 141-8537, Japan

Abstract

Advanced Steady State Tokamak Reactor2 (A-SSTR2) which meets both economical and environmental require-ments, has been designed with the thermal fusion power of 2 GW. The toroidal field (TF) coil has a maximummagnetic field of 23 T at conductor and a magnetic stored energy of 181 GJ. For the realization of this coil, largein size and with high magnetic field, the application of high Tc superconductor has been considered. © 2001 ElsevierScience B.V. All rights reserved.

Keywords: Toroidal field (TF) coil; A-SSTR2; High Tc superconductor; Tokamak reactor

www.elsevier.com/locate/fusengdes

1. Introduction

The new tokamak reactor, Advanced SteadyState Tokamak Reactor2 (A-SSTR2), has beenproposed with simple concept by JAERI [1]. Theoutline of the reactor and the principal parame-ters are shown in Fig. 1 and Table 1. The coilsystem of A-SSTR2 consists of toroidal field (TF)coils and poroidal field (PF) coils without a cen-tral solenoid because of adoption of a non-induc-tive current ramp scenario. The magnetic field at

the plasma center is designed to drive be 11 T.Therefore, the maximum field at the winding ofthe TF coils is required to be 23 T. On the otherhand, since high Tc superconductor was discov-ered, the development for its practical use israpidly progressed. Recently, the generation of23.4 T, which is a new world record as supercon-ducting coil, has been achieved on a small coilusing high Tc superconductor [2]. Furthermore, asapplication of high Tc superconductors to fusionreactors current lead has been already developedup to 10 kA and a current lead of 60 kA is beingdeveloped [3]. With such a background, the TFcoil of A-SSTR2 has been designed using high Tcsuperconductor. In this paper, the conceptual de-sign of the TF coil in A-SSTR2 is presented.

* Corresponding author. Tel.: +81-29-270-7541; fax: +81-29-270-7579.

E-mail address: ando@naka.jaeri.go.jp (T. Ando).

0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S0920 -3796 (01 )00467 -7

T. Ando et al. / Fusion Engineering and Design 58–59 (2001) 13–1614

2. TF coil design

The TF coils are composed of 12 D-shapedcoils which have a height of 15 m and a width of12.4 m. The centering force per coil is 2400 MNand is supported with the wedge between coilsand the backing cylinder installed into the centerof the coil system. Each TF coil consists of sixdisk plates stored into the case. The conductorsare installed into spiral grooves of both sides ofthe disk plates to support the conductors againstlarge electromagnetic forces (Fmas=3.1 MN/m).The conductors are cooled by cooling channelsco-wound with conductors. The cross-section ofthe TF coil winding and the parameters areshown in Fig. 2 and Table 2. In order to reducestress in disk plates, the distance between groovesfor the outer side area in the winding is largerthan that for the inner side area. The maximumtresca stress is 1260 MPa in disk plate and 1192MPa in the case, respectively. JN1 whose yieldstress is 1300 MPa, is used for materials of thedisk plate and the case in order to withstand thesehigh tresca stresses [4].

3. Conductor design

The conductor of the TF coil in A-SSTR2 isrequired to be operated in a magnetic field of 23 Twith the nominal current of 134 kA. This highfield requirement is difficult to achieve with lowtemperature superconductors. Therefore, high Tcsuperconductors which are expected to be stillmore developed in the near future, are consideredto be applied to the TF coils [2].

3.1. Design base

On the coil design using high Tc conductor, theoperating temperature of the coil is the mostimportant issue. As the operating temperature ishigher from 4.5 to 20 K, the capacity of cryogenicsystem is expected to be reduced by around 40%system. So, the fabrication and operating cost offusion reactors is cheaper. On the other hand, asthe operating temperature is higher, the supercon-ducting performance of the conductor is de-

Fig. 1. Outline of A-SSTR2.

creased. Also the mechanical properties ofstructural materials are decreased if operating atmore than 30 K [5]. In this design, 20 K is chosenas the operating temperature.

3.2. Selection of high Tc superconductor and itsperformance

At present, high Tc superconductor which ismostly developed for coil application, is Bi2212.Recently, fine multi-filamentary Bi2212 roundshaped strands similar to low Tc superconductingstrands such as NbTi, Nb3Sn are rapidly devel-oped. The round strands have not the directioneffect within transverse magnetic field on criticalcurrent density being observed in tape shapedstrands and it is also easy for coil fabrication. It isuseful for conductors for the TF coil which aresubjected to magnetic fields with various direc-tions. At present, Jc in Bi2212 round strand isaround 2000 A/mm2 at 10 T, 4.2 K and 1 �V/cm[6]. From this value Jc at 23 T and 20 K isestimated as around 500 A/mm2. This data wasobtained in short length sample cut from around1 km length strand and heat-treated. Jc in sampleheat-treated with around 100 m length is about

Table 1Main parameters of A-SSTR2

4.5 GWFusion powerPlasma major radius 6.2 m

1.5 mPlasma minor radiusToroidal field 11 TMaximum neutron load 8 MW/m2

Plasma current 12 MALarge Q 60

T. Ando et al. / Fusion Engineering and Design 58–59 (2001) 13–16 15

Fig. 2. Winding cross-section of the TF coil.

Fig. 3. Temperature dependence of specific heat for He, Ag,Pb, Bi2223.

larger enthalpy than other metals as shown in Fig.3 and then is applied in this conductor design.The specific heat of conductor is around 340mJ/cm3/K at 20 K. Therefore, the temperaturemargin is designed with 3 K corresponding to1000 mJ/cm3. On the other hand, the design ofconductor safety in the case of quench is per-formed under the condition that the temperaturerise of the conductor due to Joule generationduring quench is less than 200 K. The quenchdetection time is 2 s and the dump time constantis 12 s. The terminal voltage of the coil is limitedto 20 kV.

3.4. AC loss design

A-SSTR2 is operated in steady state. Therefore,varying magnetic fields exposed on the conductorsis produced by plasma disruption only. It is ex-pected to be less than 0.5 T/s in 0.5 s. If all thevarying magnetic field energy is input to the con-ductor, the temperature rise is around 0.1 K.Therefore, the conductor is sufficient if the cou-pling time constant is designed with around 2 s.

3.5. Conductor configuration

The designed conductor configuration of theTF coil in the A-SSTR2 is shown in Fig. 4. Theconductor is composed of a rectangular high Tcsuperconducting cable and a rectangular coppercable. They are fixed together by Pb alloy and areinsulated with a Kapton and glass epoxy tape of 2mm thickness. The high Tc cable consists of 1200

80% of Jc in sample heat-treated with shortlength. The development of heat treatment tech-nology for long length strand is required. In thenear future, Jc in heat-treated long length strandis sufficiently expected as 1000 A/mm2 which isused in the design of the A-SSTR2.

3.3. Stability and safety design

For the stabilization of conductors of largecoils such as fusion or SMES, the heat transferbetween the conductor and coolant helium playsan important role because at 4.2 K helium haslarger enthalpy than metal. However, at 20 K theenthalpy of metal is larger than that of helium.Therefore, the stability of the conductor of A-SSTR2 TF coil is designed with enthalpy of metal,so called enthalpy stabilization. Lead (Pb) has

Table 2Characteristics of the TF coil

Number of coils 12Overall height/width 14 m/12.4 m

212Number of turns/coilCurrent per conductor 134 kATotal stored energy 181 GJ

23 TMaximum magnetic field atconductor

Centering force per coil 2400 MN20 kVTerminal voltage

Operating temperature 20 KWeight per coil 1220 tonWinding configuration In grooves in seven radial

plates

T. Ando et al. / Fusion Engineering and Design 58–59 (2001) 13–1616

Fig. 4. Conductor configuration for the TF coil.

4. Conclusion

The TF coil in Advanced Steady State Toka-mak Reactor2 (A-SSTR2) has been designed withmaximum magnetic field of 23 T using high Tcsuperconductor. As high Tc superconductor,multi-filamentary Bi2212/Ag/Ag alloy compositestrand was chosen. The operating temperature ofthe coil is 20 K. Jc of Bi2212 at 23 T and 20 K isused to be 1000 A/mm2 which is a sufficientlypossible number within several years and also isexpected as a target for its development. The 20 Koperation is expected to be reduced by around40% for the capacity of cryogenic system in com-parison with 4.5 K operation.

Acknowledgements

The authors would like to thank Drs M. Mat-suda, Y. Seki and A. Funahashi for their encour-agement on this work. The authors also wouldlike to thank Dr T. Hasagawa of Showa ElectricWire and Cable Co., Ltd. for valuable informa-tion on high Tc superconductors.

References

[1] S. Nishio et al., Conceptual design of advanced stedy-statetokomak reactor (A-SSTR2), to be presented in 18th IAEAFusion Energy Conference, Sorrent, Italy, October 2000.

[2] T. Kiyoshi, et al., Generation of 23.4 T using two Bi-2212insert coils, IEEE Trans. Applied Superconductivity 10(2000) 472–477.

[3] T. Ando et al., Design and R&D of a 60 kA HTS currentlead, presented in applied superconductivity conference2000, Virginia Beach.

[4] A. Nyilas et al., Tensile properties, fracture, and crackgrowth of nitrogen strengthened new stainless steel (Fe–25Cr–15Ni–0.35N) for cryogenic use, 3rd InternationalConference on High Nitrogen Steels ‘HNS 93’, Kiev, 1993.

[5] T. Ando et al., Consideration of high Tc superconductorapplication on magnets for Tokamak Fusion Reactors,Fusion Technology, 1998, pp. 791–794.

[6] T. Hasegawa et al., Improvement of superconducting prop-erties of Bi-2212 round wire and primary test results of largecapacity Rutherford cable, presented in Applied supercon-ductivity conference 2000, Virginia Beach.

Table 3Characteristics of the conductor for the TF coil

Pb solder impregnate cableTypeSuperconductor Bi2212Dimension 51.25×46 mm2

Enthalpy stabilizer Pb alloyBi2212:Ag:AgMgSb: 1:1.5:0.5:2.1:1.7

Cu:Pb1000 A/mm2Jc in B2212 at 20K and

23 TCooling Conduction from cooling

channel

transposed Bi2212 strands whose diameter is 1mm. The Bi2212 composite strands are composedof three phases of Bi2212, Ag and Ag alloy with avolume ratio of 1, 0.5 and 0.5. The ratio of theBi2212 phase is larger than those in presentstrands. The copper cable is composed of copperwires whose surfaces are covered with Cu–Nilayer to reduce eddy currents. The conductorstresses are designed with less than 0.2%. The sizeof the conductor is 51.2×46 mm2. The mainparameters are shown in Table 3. The conductoris cooled by a rectangular cooling channel pro-vided to be contacted at one side surface of theconductor.