Design study of superconducting coils for the fusion DEMO plant at JAERI

Fusion Engineering and Design 81 (2006) 1257–1261

Design study of superconducting coils for the fusion DEMO plant at JAERI T. Isono ∗ , N. Koizumi, K. Okuno, R. Kurihara, S. Nishio, K. Tobita, Demo Plant Design Team Japan Atomic Energy Research Institute, 801-1 Mukouyama, Naka-shi, Ibaraki 311-0193, Japan Received 18 February 2005; received in revised form 2 August 2005; accepted 2 August 2005 Available online 27 December 2005

Abstract A design study of the TF coil for the fusion DEMO plant at JAERI is in progress. A major issue is to estimate the maximum fields generated by the TF coils for three tokamak options and two conductor options. Three tokamak options are proposed varying the aspect ratio and the role of the CS coil. Two kinds of conductors using advanced superconducting materials are candidates for the TF coils: Nb3 Al and high temperature superconductor (HTS). In order to evaluate achievable magnetic fields, a simple method was adopted to calculate mechanical properties. The estimated maximum fields are 17–20 T by the HTS conductor and 16–17 T by the Nb3 Al conductor. There is a possibility of a 0.7 T enhancement using grading of Nb3 Al winding. © 2005 Elsevier B.V. All rights reserved. Keywords: Fusion DEMO plant; Superconducting coil; HTS; Nb3 Al

1. Introduction During the Engineering Design Activity (EDA) of ITER, technology to fabricate large superconducting coils generating a 13 T magnetic field was developed. In order to achieve an economically compet∗ Corresponding author. Tel.: +81 29 270 7548; fax: +81 29 270 7579. E-mail address: [email protected] (T. Isono).

0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.08.078

itive fusion power generation system, high field is required. The Toroidal field (TF) coils of the fusion DEMO plant at JAERI, which is the next step toward a fusion power plant, are required to generate a magnetic field of 16–20 T. To generate the fields, advanced superconducting materials, such as Nb3 Al and high temperature superconductor (HTS), are considered. The method to establish the conceptual design of the fusion DEMO plant is to integrate each design

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study of key components into a plant concept, but not to decide the principle first. In the magnet design study, estimates of the maximum fields are required according to various arrangements and winding sizes of the TF coils. From the design study of the plant, three options are proposed varying the aspect ratio and the role of the central solenoid (CS), called the CS-less, slim CS and full CS options [1]. In this study, the maximum magnetic fields generated by many kinds of the TF coils are estimated. Each of the TF coils uses HTS or Nb3 Al conductor and has a size and a position meeting one of the three tokamak options.

2. Candidate for the TF conductor 2.1. Performance of superconducting wire In order to generate a higher magnetic field of 16–20 T, superconducting wires must possess a high critical current density (Jc ) at the design field. Fig. 1 shows the Jc of typical superconducting wires and tapes at cryogenic temperature versus magnetic field. Bi2212 (Bi2 Sr2 CaCu2 Ox ) and rapid-heating, quenching and transformation (RHQT) processed Nb3 Al are candidates for the TF conductor of the fusion DEMO plant. YBCO (YBa2 Cu3 Ox ) tape has the best performance among the materials, but has not yet been fabricated in mass production. In the near future, YBCO may become one of the candidates.

Fig. 2. Cross sectional views of a proposed HTS conductor (a) and a typical Nb3 Al conductor (b).

2.2. HTS conductor design Bi-2212 wire has high critical current density of more than 1000 A/mm2 at 20 T and 4.2 K [2]. Besides, silver alloy sheathed round wires are fabricated in mass production. This is a great advantage to make a twisted cable for fusion application. Conductor designs at 20 K have been reported [3,4] and they have great advantage in stabilizing the conductor utilizing the heat capacity of metals. A 10 kA conductor was fabricated and tested [5]. However, the Jc of Bi-2212 at 20 K is very low at present. Therefore, an operating temperature of 5–8 K is chosen for the fusion DEMO plant. A sketch of the cross section of the conductor is shown in Fig. 2(a). Although HTS conductor development for fusion application is in progress by JAERI, there are many technical issues to be solved, such as accurate temperature control during heat treatment in an atmosphere of oxygen. 2.3. Nb3 Al conductor design

Fig. 1. Critical current density of typical superconducting wires.

A large Nb3 Al coil which uses jelly-roll processed Nb3 Al wires has been developed by JAERI in the ITER EDA [6]. For the fusion DEMO plant, the RHQT processed Nb3 Al wire is the candidate because Jc is about 1000 A/mm2 at 16 T and 4.2 K [7], which is much higher than that of the jelly-roll processed wire. In the RHQT process, the wire is rapidly heated up to about 2000 ◦ C. Therefore, copper addition after the rapid heating and stabilization of the conductor against thermal disturbance are major technical issues to fabricate the conductor [8]. The conductor is similar to the ITER one as shown in Fig. 2(b). For coil fabrication, major technology has been developed during the ITER EDA.

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3. Design study of the TF coil

torus coordinates. The solution of Eq. (2) is as follows:

3.1. Evaluation method

σr =(C1 · G(r) + (C2 · H(r) + C3)/r 2 + C4)/ηst (r)

In this study, magnetic fields generated by the TF coils are estimated, taking account of the critical current density of the wires, safety at quench using the hot spot method and mechanical properties. Critical current densities of Bi-2212 and RHQT Nb3 Al wires are shown in Fig. 1. Currently available conductor properties are used for this study. To guarantee coil protection, the hot spot model is used. Hot spot temperature is calculated using the following equation:


∞


RI 2 dt =

0

Tm

T0

σr +

dσr dr

C4 =

C7 + C8 2(R2TF − R2CS )

C5 = (1 + ν)R2TF R2CS (G(RTF ) − G(RCS )) C6 = (1 − ν)(R2CS H(RTF ) − R2TF H(RCS )) C7 = (1 + ν)(R2CS G(RCS ) − R2TF G(RTF ))

C dT

(1)

C8 = (1
− ν)(H(RCS ) − H(RTF )) G(r) =

where R is the resistance of the conductor. Effective components are silver and copper for the HTS conductor and copper and niobium for the Nb3 Al conductor. C is the heat capacity of metals in the conductor, such as silver, copper and superconductor for the HTS conductor, copper, niobium, stainless steel and Nb3 Al for the Nb3 Al conductor. I is the coil current, T0 the temperature at the beginning of quench and Tm is the hot spot temperature. The hot spot criteria of 300 and 250 K are used for HTS and Nb3 Al conductors, respectively. Temperature integrals of the heat capacity of copper, silver and niobium are 690, 560 and 510 MJ/m3 , respectively. Temperature integrals of the resistivity of copper and silver are 2.4 and 2.5 ␮
m K. To estimate the stress of the TF coil inboard leg according to the various cases of the radius and width of the cylinder formed by the legs, a following simple method is adopted. The basic expression of stress in a cylinder when a current is applied toward the axis of the cylinder is: 

σθ =(C1 · G(r) − (C2 · H(r) + C3)/r 2 + C4)/ηst (r) (1 + ν) (1 − ν) C1 = , C2 = 2 2 C5 + C6 C3 = 2(R2TF − R2CS )

 (r + dr) dθ = σr r dθ + 2σθ dr sin +B(r) · J(r) r dθ dr

dθ 2 (2)

H(r) =




B(r)J(r) dr r2 B(r)J(r) dr (3)

where ηst is the volume fraction of structure, RTF and RCS are the outer and inner radii of the cylinder formed by the TF coil inboard legs, respectively, and ν is the Poisson’s ratio. The allowable stress criteria are:  ((αr σr −αθ σθ )2 +(αθ σθ −θz )2 + (θz − αr σr )2 ) σp = 2 < 1.5σm and σav =



((σr −σθ )2 +(σθ − θz )2 + (θz − σr )2 ) < σm 2

(4)

where σ p and σ av are the peak and averaged stresses using the von Mises yield criterion, respectively, σ m is the allowable stress, and αr and αθ are radial and circumferential stress concentration factor, respectively. σ m = 800 MPa. αr = αθ = 2.5 is assumed for this estimation. These values αr and αθ should be evaluated by the finite element method in near future. 3.2. Study of the maximum field by HTS coils

where σ r and σ θ are the radial and the circumferential stresses, respectively, J the current density, B the magnetic field and r and θ are the radius and the angle of the

In this study, it is assumed that the number of TF coils is 12, the coil height is 14 m, the coil width is

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Fig. 3. The maximum field estimation by various figures of HTS TF coils. Inner radii of the inboard leg cylinder are 0, 0.7, 1.5 m corresponding to the CS-less, slim CS and full CS options, respectively.

7 m, the maximum terminal voltage is 20 kV and the operating current is 100 kA. The values of RCS , the inner radius of the cylinder formed by the TF coil inboard legs, are 0, 0.7 and 1.5 m corresponding to the CS-less, slim CS and full CS options, respectively. Current density and the fraction of structure are constants in this calculation. Fig. 3 shows the results of the achievable magnetic fields estimated for the HTS TF coils. RTF is the outer radius of the cylinder. Fields of 20, 17.5 and 17.5 T were calculated with the cylinder thickness (RTF − RCS ) of 2 m for CS-less, slim CS and full CS options, respectively. Component ratios are listed in Table 1.

CS less

Slim CS

0 m/2 m 19.6 T 164 515 A/mm2

100 kA 0.7 m/2.7 m 17.2 T 194 600 A/mm2

1.5 m/3.5 m 17.1 T 250 600 A/mm2

86.7% 1.8% 3.5% 3.2% 3.0% 1.8% 7.7 s

87.5% 1.6% 3.1% 3.7% 2.6% 1.5% 10.6 s

Components breakdown Structure 80.6% Superconductor 3.0% Silver 5.6% Copper 3.6% Insulator 4.5% Helium 2.7% Dump time constant 6.8 s

3.3. Study of the maximum field by Nb3 Al coils The results of the achievable magnetic field estimation by the Nb3 Al TF coils are shown as solid lines in Fig. 4. Fields of 17, 16 and 16 T were calculated with 2 m thickness (RTF − RCS ) for CS-less, slim CS and full CS options, respectively. Component ratios are listed in Table 2. The critical current density of the Nb3 Al wire decreases at a higher field, but there is a possibility to enhance the field by grading the windings, which is a common technique for a high field magnet. Dotted Table 2 Typical results of the Nb3 Al coils

Table 1 Typical results of the HTS coils

Operating current RCS /RTF Maximum field Number of turn Current density

Fig. 4. The maximum field estimation by various figures of Nb3 Al TF coils. Real and dotted lines shows the results of uniform and graded windings, respectively.

Full CS

CS less

Slim CS

Full CS

0 m/2 m 17.0 T 178 290 A/mm2

80 kA 0.7 m/2.7m 15.9 T 224 450 A/mm2

1.5 m/3.5 m 15.9 T 289 460 A/mm2

Components breakdown Structure 61.3% Superconductor 4.6% Niobium 3.7% Copper 8.4% Stainless steel 4.7% Insulator 7.7% Helium 9.6%

73.4% 2.2% 1.8% 7.2% 3.4% 5.5% 6.5%

75.3% 1.9% 1.5% 7.2% 3.0% 5.0% 6.1%

Dump time constant

8.2 s

11.4 s

Operating current RCS /RTF Maximum field Number of turn Current density

6.4 s

T. Isono et al. / Fusion Engineering and Design 81 (2006) 1257–1261 Table 3 Typical results of grading of the Nb3 Al coils Uniform

Graded Low field

RCS /RTF Maximum field (Bm) Number of turn Current density

0.7 m/2.7 m 15.9 T 224 450 A/mm2

High field

0.7 m/2.7 m 10.8 T 16.6 T 114 120 1000 A/mm2 330 A/mm2

Major components breakdown Structure 73.4% Superconductor 2.2% Copper 7.2%

78.0% 1.0% 7.2%

66.1% 3.5% 8.4%

Jacket outer diameter

42.8 mm

48.8 mm

46.3 mm

lines in Fig. 4 show the results using grading. There is a possibility to enhance the field by about 0.7 T using this technique. In the calculation, the inner cylinder whose thickness is 2/3 of total one has higher current density of more than 1000 A/mm2 , and the outer cylinder has lower current density. The component breakdowns of the uniform winding and the graded winding in a case of the slim CS option with 2 m thickness are listed in Table 3.

4. Conclusion A design study of the TF coil for the fusion DEMO plant was performed, and the maximum magnetic fields generated by the TF coils according to the three options in the tokamak design and the two options in conductor design were estimated. The maximum fields are 17–20 T using HTS conductor, and 16–17 T using Nb3 Al conductor. There is a possibility of a 0.7 T enhancement using grading of the Nb3 Al winding.

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The fraction of the structure is 61–88 % as shown in Tables 1–3, therefore accurate structural analysis is the key to improve the accuracy of the magnetic field estimation.

Acknowledgements The authors would like to thank Drs. M. Seki and S. Seki of JAERI for their helpful encouragement.

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