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Design of the toroidal field coil for A-SSTR2 using high Tc superconductor
Fusion Engineering and Design 58 – 59 (2001) 13 – 16 www.elsevier.com/locate/fusengdes
Design of the toroidal field coil for A-SSTR2 using high Tc superconductor 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 requirements, has been designed with the thermal fusion power of 2 GW. The toroidal field (TF) coil has a maximum magnetic field of 23 T at conductor and a magnetic stored energy of 181 GJ. For the realization of this coil, large in size and with high magnetic field, the application of high Tc superconductor has been considered. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Toroidal field (TF) coil; A-SSTR2; High Tc superconductor; Tokamak reactor
1. Introduction The new tokamak reactor, Advanced Steady State Tokamak Reactor2 (A-SSTR2), has been proposed with simple concept by JAERI [1]. The outline of the reactor and the principal parameters are shown in Fig. 1 and Table 1. The coil system of A-SSTR2 consists of toroidal field (TF) coils and poroidal field (PF) coils without a central solenoid because of adoption of a non-inductive current ramp scenario. The magnetic field at
* Corresponding author. Tel.: + 81-29-270-7541; fax: + 8129-270-7579. E-mail address: [email protected] (T. Ando).
the plasma center is designed to drive be 11 T. Therefore, the maximum field at the winding of the TF coils is required to be 23 T. On the other hand, since high Tc superconductor was discovered, the development for its practical use is rapidly progressed. Recently, the generation of 23.4 T, which is a new world record as superconducting coil, has been achieved on a small coil using high Tc superconductor [2]. Furthermore, as application of high Tc superconductors to fusion reactors current lead has been already developed up to 10 kA and a current lead of 60 kA is being developed [3]. With such a background, the TF coil of A-SSTR2 has been designed using high Tc superconductor. In this paper, the conceptual design of the TF coil in A-SSTR2 is presented.
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T. Ando et al. / Fusion Engineering and Design 58–59 (2001) 13–16
2. TF coil design The TF coils are composed of 12 D-shaped coils which have a height of 15 m and a width of 12.4 m. The centering force per coil is 2400 MN and is supported with the wedge between coils and the backing cylinder installed into the center of the coil system. Each TF coil consists of six disk plates stored into the case. The conductors are installed into spiral grooves of both sides of the disk plates to support the conductors against large electromagnetic forces (Fmas =3.1 MN/m). The conductors are cooled by cooling channels co-wound with conductors. The cross-section of the TF coil winding and the parameters are shown in Fig. 2 and Table 2. In order to reduce stress in disk plates, the distance between grooves for the outer side area in the winding is larger than that for the inner side area. The maximum tresca stress is 1260 MPa in disk plate and 1192 MPa in the case, respectively. JN1 whose yield stress is 1300 MPa, is used for materials of the disk plate and the case in order to withstand these high tresca stresses [4].
3. Conductor design The conductor of the TF coil in A-SSTR2 is required to be operated in a magnetic field of 23 T with the nominal current of 134 kA. This high field requirement is difficult to achieve with low temperature superconductors. Therefore, high Tc superconductors which are expected to be still more developed in the near future, are considered to be applied to the TF coils [2].
3.1. Design base On the coil design using high Tc conductor, the operating temperature of the coil is the most important issue. As the operating temperature is higher from 4.5 to 20 K, the capacity of cryogenic system is expected to be reduced by around 40% system. So, the fabrication and operating cost of fusion reactors is cheaper. On the other hand, as the operating temperature is higher, the superconducting performance of the conductor is de-
Fig. 1. Outline of A-SSTR2.
creased. Also the mechanical properties of structural materials are decreased if operating at more than 30 K [5]. In this design, 20 K is chosen as the operating temperature.
3.2. Selection of high Tc superconductor and its performance At present, high Tc superconductor which is mostly developed for coil application, is Bi2212. Recently, fine multi-filamentary Bi2212 round shaped strands similar to low Tc superconducting strands such as NbTi, Nb3Sn are rapidly developed. The round strands have not the direction effect within transverse magnetic field on critical current density being observed in tape shaped strands and it is also easy for coil fabrication. It is useful for conductors for the TF coil which are subjected to magnetic fields with various directions. At present, Jc in Bi2212 round strand is around 2000 A/mm2 at 10 T, 4.2 K and 1 mV/cm [6]. From this value Jc at 23 T and 20 K is estimated as around 500 A/mm2. This data was obtained in short length sample cut from around 1 km length strand and heat-treated. Jc in sample heat-treated with around 100 m length is about Table 1 Main parameters of A-SSTR2 Fusion power Plasma major radius Plasma minor radius Toroidal field Maximum neutron load Plasma current Large Q
4.5 GW 6.2 m 1.5 m 11 T 8 MW/m2 12 MA 60
T. Ando et al. / Fusion Engineering and Design 58–59 (2001) 13–16
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Fig. 3. Temperature dependence of specific heat for He, Ag, Pb, Bi2223. Fig. 2. Winding cross-section of the TF coil.
80% of Jc in sample heat-treated with short length. The development of heat treatment technology for long length strand is required. In the near future, Jc in heat-treated long length strand is sufficiently expected as 1000 A/mm2 which is used in the design of the A-SSTR2.
3.3. Stability and safety design For the stabilization of conductors of large coils such as fusion or SMES, the heat transfer between the conductor and coolant helium plays an important role because at 4.2 K helium has larger enthalpy than metal. However, at 20 K the enthalpy of metal is larger than that of helium. Therefore, the stability of the conductor of ASSTR2 TF coil is designed with enthalpy of metal, so called enthalpy stabilization. Lead (Pb) has Table 2 Characteristics of the TF coil Number of coils Overall height/width Number of turns/coil Current per conductor Total stored energy Maximum magnetic field at conductor Centering force per coil Terminal voltage Operating temperature Weight per coil Winding configuration
12 14 212 134 181 23
m/12.4 m kA GJ T
2400 MN 20 kV 20 K 1220 ton In grooves in seven radial plates
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 340 mJ/cm3/K at 20 K. Therefore, the temperature margin is designed with 3 K corresponding to 1000 mJ/cm3. On the other hand, the design of conductor safety in the case of quench is performed under the condition that the temperature rise of the conductor due to Joule generation during quench is less than 200 K. The quench detection time is 2 s and the dump time constant is 12 s. The terminal voltage of the coil is limited to 20 kV.
3.4. AC loss design A-SSTR2 is operated in steady state. Therefore, varying magnetic fields exposed on the conductors is produced by plasma disruption only. It is expected to be less than 0.5 T/s in 0.5 s. If all the varying magnetic field energy is input to the conductor, the temperature rise is around 0.1 K. Therefore, the conductor is sufficient if the coupling time constant is designed with around 2 s.
3.5. Conductor configuration The designed conductor configuration of the TF coil in the A-SSTR2 is shown in Fig. 4. The conductor is composed of a rectangular high Tc superconducting cable and a rectangular copper cable. They are fixed together by Pb alloy and are insulated with a Kapton and glass epoxy tape of 2 mm thickness. The high Tc cable consists of 1200
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4. Conclusion
Fig. 4. Conductor configuration for the TF coil.
The TF coil in Advanced Steady State Tokamak Reactor2 (A-SSTR2) has been designed with maximum magnetic field of 23 T using high Tc superconductor. As high Tc superconductor, multi-filamentary Bi2212/Ag/Ag alloy composite strand was chosen. The operating temperature of the coil is 20 K. Jc of Bi2212 at 23 T and 20 K is used to be 1000 A/mm2 which is a sufficiently possible number within several years and also is expected as a target for its development. The 20 K operation is expected to be reduced by around 40% for the capacity of cryogenic system in comparison with 4.5 K operation.
Table 3 Characteristics of the conductor for the TF coil
Acknowledgements
Type Superconductor Dimension Enthalpy stabilizer Bi2212:Ag:AgMgSb: Cu:Pb Jc in B2212 at 20K and 23 T Cooling
The authors would like to thank Drs M. Matsuda, Y. Seki and A. Funahashi for their encouragement on this work. The authors also would like to thank Dr T. Hasagawa of Showa Electric Wire and Cable Co., Ltd. for valuable information on high Tc superconductors.
Pb solder impregnate cable Bi2212 51.25×46 mm2 Pb alloy 1:1.5:0.5:2.1:1.7 1000 A/mm2 Conduction from cooling channel
transposed Bi2212 strands whose diameter is 1 mm. The Bi2212 composite strands are composed of three phases of Bi2212, Ag and Ag alloy with a volume ratio of 1, 0.5 and 0.5. The ratio of the Bi2212 phase is larger than those in present strands. The copper cable is composed of copper wires whose surfaces are covered with Cu–Ni layer to reduce eddy currents. The conductor stresses are designed with less than 0.2%. The size of the conductor is 51.2×46 mm2. The main parameters are shown in Table 3. The conductor is cooled by a rectangular cooling channel provided to be contacted at one side surface of the conductor.
References [1] S. Nishio et al., Conceptual design of advanced stedy-state tokomak reactor (A-SSTR2), to be presented in 18th IAEA Fusion Energy Conference, Sorrent, Italy, October 2000. [2] T. Kiyoshi, et al., Generation of 23.4 T using two Bi-2212 insert coils, IEEE Trans. Applied Superconductivity 10 (2000) 472 – 477. [3] T. Ando et al., Design and R&D of a 60 kA HTS current lead, presented in applied superconductivity conference 2000, Virginia Beach. [4] A. Nyilas et al., Tensile properties, fracture, and crack growth of nitrogen strengthened new stainless steel (Fe – 25Cr – 15Ni – 0.35N) for cryogenic use, 3rd International Conference on High Nitrogen Steels ‘HNS 93’, Kiev, 1993. [5] T. Ando et al., Consideration of high Tc superconductor application on magnets for Tokamak Fusion Reactors, Fusion Technology, 1998, pp. 791 – 794. [6] T. Hasegawa et al., Improvement of superconducting properties of Bi-2212 round wire and primary test results of large capacity Rutherford cable, presented in Applied superconductivity conference 2000, Virginia Beach.
Design of the toroidal field coil for A-SSTR2 using high Tc superconductor 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 requirements, has been designed with the thermal fusion power of 2 GW. The toroidal field (TF) coil has a maximum magnetic field of 23 T at conductor and a magnetic stored energy of 181 GJ. For the realization of this coil, large in size and with high magnetic field, the application of high Tc superconductor has been considered. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Toroidal field (TF) coil; A-SSTR2; High Tc superconductor; Tokamak reactor
1. Introduction The new tokamak reactor, Advanced Steady State Tokamak Reactor2 (A-SSTR2), has been proposed with simple concept by JAERI [1]. The outline of the reactor and the principal parameters are shown in Fig. 1 and Table 1. The coil system of A-SSTR2 consists of toroidal field (TF) coils and poroidal field (PF) coils without a central solenoid because of adoption of a non-inductive current ramp scenario. The magnetic field at
* Corresponding author. Tel.: + 81-29-270-7541; fax: + 8129-270-7579. E-mail address: [email protected] (T. Ando).
the plasma center is designed to drive be 11 T. Therefore, the maximum field at the winding of the TF coils is required to be 23 T. On the other hand, since high Tc superconductor was discovered, the development for its practical use is rapidly progressed. Recently, the generation of 23.4 T, which is a new world record as superconducting coil, has been achieved on a small coil using high Tc superconductor [2]. Furthermore, as application of high Tc superconductors to fusion reactors current lead has been already developed up to 10 kA and a current lead of 60 kA is being developed [3]. With such a background, the TF coil of A-SSTR2 has been designed using high Tc superconductor. In this paper, the conceptual design of the TF coil in A-SSTR2 is presented.
0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 1 ) 0 0 4 6 7 - 7
14
T. Ando et al. / Fusion Engineering and Design 58–59 (2001) 13–16
2. TF coil design The TF coils are composed of 12 D-shaped coils which have a height of 15 m and a width of 12.4 m. The centering force per coil is 2400 MN and is supported with the wedge between coils and the backing cylinder installed into the center of the coil system. Each TF coil consists of six disk plates stored into the case. The conductors are installed into spiral grooves of both sides of the disk plates to support the conductors against large electromagnetic forces (Fmas =3.1 MN/m). The conductors are cooled by cooling channels co-wound with conductors. The cross-section of the TF coil winding and the parameters are shown in Fig. 2 and Table 2. In order to reduce stress in disk plates, the distance between grooves for the outer side area in the winding is larger than that for the inner side area. The maximum tresca stress is 1260 MPa in disk plate and 1192 MPa in the case, respectively. JN1 whose yield stress is 1300 MPa, is used for materials of the disk plate and the case in order to withstand these high tresca stresses [4].
3. Conductor design The conductor of the TF coil in A-SSTR2 is required to be operated in a magnetic field of 23 T with the nominal current of 134 kA. This high field requirement is difficult to achieve with low temperature superconductors. Therefore, high Tc superconductors which are expected to be still more developed in the near future, are considered to be applied to the TF coils [2].
3.1. Design base On the coil design using high Tc conductor, the operating temperature of the coil is the most important issue. As the operating temperature is higher from 4.5 to 20 K, the capacity of cryogenic system is expected to be reduced by around 40% system. So, the fabrication and operating cost of fusion reactors is cheaper. On the other hand, as the operating temperature is higher, the superconducting performance of the conductor is de-
Fig. 1. Outline of A-SSTR2.
creased. Also the mechanical properties of structural materials are decreased if operating at more than 30 K [5]. In this design, 20 K is chosen as the operating temperature.
3.2. Selection of high Tc superconductor and its performance At present, high Tc superconductor which is mostly developed for coil application, is Bi2212. Recently, fine multi-filamentary Bi2212 round shaped strands similar to low Tc superconducting strands such as NbTi, Nb3Sn are rapidly developed. The round strands have not the direction effect within transverse magnetic field on critical current density being observed in tape shaped strands and it is also easy for coil fabrication. It is useful for conductors for the TF coil which are subjected to magnetic fields with various directions. At present, Jc in Bi2212 round strand is around 2000 A/mm2 at 10 T, 4.2 K and 1 mV/cm [6]. From this value Jc at 23 T and 20 K is estimated as around 500 A/mm2. This data was obtained in short length sample cut from around 1 km length strand and heat-treated. Jc in sample heat-treated with around 100 m length is about Table 1 Main parameters of A-SSTR2 Fusion power Plasma major radius Plasma minor radius Toroidal field Maximum neutron load Plasma current Large Q
4.5 GW 6.2 m 1.5 m 11 T 8 MW/m2 12 MA 60
T. Ando et al. / Fusion Engineering and Design 58–59 (2001) 13–16
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Fig. 3. Temperature dependence of specific heat for He, Ag, Pb, Bi2223. Fig. 2. Winding cross-section of the TF coil.
80% of Jc in sample heat-treated with short length. The development of heat treatment technology for long length strand is required. In the near future, Jc in heat-treated long length strand is sufficiently expected as 1000 A/mm2 which is used in the design of the A-SSTR2.
3.3. Stability and safety design For the stabilization of conductors of large coils such as fusion or SMES, the heat transfer between the conductor and coolant helium plays an important role because at 4.2 K helium has larger enthalpy than metal. However, at 20 K the enthalpy of metal is larger than that of helium. Therefore, the stability of the conductor of ASSTR2 TF coil is designed with enthalpy of metal, so called enthalpy stabilization. Lead (Pb) has Table 2 Characteristics of the TF coil Number of coils Overall height/width Number of turns/coil Current per conductor Total stored energy Maximum magnetic field at conductor Centering force per coil Terminal voltage Operating temperature Weight per coil Winding configuration
12 14 212 134 181 23
m/12.4 m kA GJ T
2400 MN 20 kV 20 K 1220 ton In grooves in seven radial plates
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 340 mJ/cm3/K at 20 K. Therefore, the temperature margin is designed with 3 K corresponding to 1000 mJ/cm3. On the other hand, the design of conductor safety in the case of quench is performed under the condition that the temperature rise of the conductor due to Joule generation during quench is less than 200 K. The quench detection time is 2 s and the dump time constant is 12 s. The terminal voltage of the coil is limited to 20 kV.
3.4. AC loss design A-SSTR2 is operated in steady state. Therefore, varying magnetic fields exposed on the conductors is produced by plasma disruption only. It is expected to be less than 0.5 T/s in 0.5 s. If all the varying magnetic field energy is input to the conductor, the temperature rise is around 0.1 K. Therefore, the conductor is sufficient if the coupling time constant is designed with around 2 s.
3.5. Conductor configuration The designed conductor configuration of the TF coil in the A-SSTR2 is shown in Fig. 4. The conductor is composed of a rectangular high Tc superconducting cable and a rectangular copper cable. They are fixed together by Pb alloy and are insulated with a Kapton and glass epoxy tape of 2 mm thickness. The high Tc cable consists of 1200
T. Ando et al. / Fusion Engineering and Design 58–59 (2001) 13–16
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4. Conclusion
Fig. 4. Conductor configuration for the TF coil.
The TF coil in Advanced Steady State Tokamak Reactor2 (A-SSTR2) has been designed with maximum magnetic field of 23 T using high Tc superconductor. As high Tc superconductor, multi-filamentary Bi2212/Ag/Ag alloy composite strand was chosen. The operating temperature of the coil is 20 K. Jc of Bi2212 at 23 T and 20 K is used to be 1000 A/mm2 which is a sufficiently possible number within several years and also is expected as a target for its development. The 20 K operation is expected to be reduced by around 40% for the capacity of cryogenic system in comparison with 4.5 K operation.
Table 3 Characteristics of the conductor for the TF coil
Acknowledgements
Type Superconductor Dimension Enthalpy stabilizer Bi2212:Ag:AgMgSb: Cu:Pb Jc in B2212 at 20K and 23 T Cooling
The authors would like to thank Drs M. Matsuda, Y. Seki and A. Funahashi for their encouragement on this work. The authors also would like to thank Dr T. Hasagawa of Showa Electric Wire and Cable Co., Ltd. for valuable information on high Tc superconductors.
Pb solder impregnate cable Bi2212 51.25×46 mm2 Pb alloy 1:1.5:0.5:2.1:1.7 1000 A/mm2 Conduction from cooling channel
transposed Bi2212 strands whose diameter is 1 mm. The Bi2212 composite strands are composed of three phases of Bi2212, Ag and Ag alloy with a volume ratio of 1, 0.5 and 0.5. The ratio of the Bi2212 phase is larger than those in present strands. The copper cable is composed of copper wires whose surfaces are covered with Cu–Ni layer to reduce eddy currents. The conductor stresses are designed with less than 0.2%. The size of the conductor is 51.2×46 mm2. The main parameters are shown in Table 3. The conductor is cooled by a rectangular cooling channel provided to be contacted at one side surface of the conductor.
References [1] S. Nishio et al., Conceptual design of advanced stedy-state tokomak reactor (A-SSTR2), to be presented in 18th IAEA Fusion Energy Conference, Sorrent, Italy, October 2000. [2] T. Kiyoshi, et al., Generation of 23.4 T using two Bi-2212 insert coils, IEEE Trans. Applied Superconductivity 10 (2000) 472 – 477. [3] T. Ando et al., Design and R&D of a 60 kA HTS current lead, presented in applied superconductivity conference 2000, Virginia Beach. [4] A. Nyilas et al., Tensile properties, fracture, and crack growth of nitrogen strengthened new stainless steel (Fe – 25Cr – 15Ni – 0.35N) for cryogenic use, 3rd International Conference on High Nitrogen Steels ‘HNS 93’, Kiev, 1993. [5] T. Ando et al., Consideration of high Tc superconductor application on magnets for Tokamak Fusion Reactors, Fusion Technology, 1998, pp. 791 – 794. [6] T. Hasegawa et al., Improvement of superconducting properties of Bi-2212 round wire and primary test results of large capacity Rutherford cable, presented in Applied superconductivity conference 2000, Virginia Beach.