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Key features of the ITER-FEAT magnet system
Fusion Engineering and Design 58 – 59 (2001) 153– 157 www.elsevier.com/locate/fusengdes
Key features of the ITER-FEAT magnet system K. Okuno *, D. Bessette, M. Ferrari, M. Huguet, C. Jong, K. Kitamura, Y. Krivchenkov, N. Mitchell, H. Takigami, K. Yoshida, E. Zapretilina ITER Joint Central Team, Naka Joint Work Site, 801 -1 Mukouyama, Naka-machi, Naka-gun, Ibaraki-ken, 311 -0193, Japan
Abstract The design of the ITER magnet system is being finalized. The reference design of the winding pack of the TF coil is based on the use of circular conductors supported by radial plates. This design has been chosen for its high insulation reliability during operation. The overall TF coil structure includes pre-compression rings made of unidirectional fiber glass, which reduce the stress level in the outer intercoil structures and the coil case. The design of the central solenoid, including pre-load structure, has been developed. Two conductor jacket options are still under investigation for the CS and the final choice will be based on the results of on-going R&D. © 2001 Elsevier Science B.V. All rights reserved. Keywords: ITER-FEAT magnet system; Central solenoid; Outer intercoil structures
1. Introduction The ITER magnets are designed to meet the requirements of the plasma configuration chosen for the machine [1]. The magnet system will be the largest set of superconducting magnets ever built and it will be required to operate under demanding mechanical and electromagnetic conditions. Therefore, the design involves the selection of technically reliable and cost effective concepts among the various technologies which have been considered. The ITER magnet system consists of 18 toroidal field (TF) coils, a central solenoid (CS), * Corresponding author. Tel.: + 81-29-270-7761; fax: + 8129-270-7507. E-mail address: [email protected] (K. Okuno).
six poloidal field (PF) coils, as shown in Fig. 1, and three sets of correction coils (CCs). The TF coil winding packs are enclosed in cases which constitute the main structure of the magnet system. The inboard straight legs of the TF coils are wedged to sustain the centering forces. Friction forces at the wedge sustain part of the out-ofplane loads which result from the interaction of TF coil current with the poloidal magnetic field. The out-of-plane loads are also supported by shear keys located at the inboard curved regions of the TF coils and by four outer intercoil structures (OISs) along the outboard leg contour. A distinctive feature of the TF coils winding pack is the use of radial plates with grooves which provide support for the conductor. Each TF coil is 14 m high and 9 m wide and will operate at 12 T with a total current of 9.1 MA. The CS assembly consists of a stack of six modules. The six
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Fig. 1. ITER magnet system — isometric view. CS and correction coils are not shown.
modules are electrically independent to provide non-uniform current distributions along the vertical axis as required by plasma shaping. The CS operates at 13.5 (initial magnetization, IM) and 12.8 T (end of burn, EOB). A pre-load structure provides axial compression on the CS stack to avoid any separation between the CS modules, and supports the whole stack against net vertical forces. This paper describes some distinctive features of the ITER magnet system: the TF winding pack design, the use of pre-compression rings in the TF coil structure and the CS conductor and pre-load structure.
considering that insulation faults are the most probable cause of magnet failure and also considering the difficulties involved in the replacement of a TF coil. This concept has already been proven in the TF Model Coil Project [2]. To solve the cost issue, some R&D activities have been
2. TF coil radial plate design The reference winding pack design is based on the use of radial plates and circular conductors in a double-pancake configuration, as shown in Fig. 2. The advantages and drawbacks of this design are summarized in Table 1. This design has been chosen because of the expected high insulation reliability and the possibility to detect faults before significant damage occurs. These considerations have been given a high, overriding priority,
Fig. 2. Cross sectional view of the TF coil at nose area.
K. Okuno et al. / Fusion Engineering and Design 58–59 (2001) 153–157 Table 1 Advantages and drawbacks of the radial plate design Ad6antages A circular cross section is the optimum shape to apply insulation tapes, resulting in a robust turn insulation. The Lorentz forces acting on each conductor are transferred to the plate, without accumulation of forces on the conductor and its insulation. The conductor and ground insulations are independent and physically separated by the radial plate. A single insulation fault will not affect both insulations. A single conductor insulation fault can be detected by monitoring the resistance between conductor and radial plate. Drawbacks The radial plate manufacture is costly since it requires significant time to machine the grooves. The radial plate design implies a larger size of a winding by 30–50 mm than a conventional winding pack with square conductors. In the event of a fast discharge of the TF coils, the winding temperature rises to about 50–60 K due to eddy current in the radial plates. Recooling of the TF coils is estimated to take about 30 h.
started, including the use of an extrusion technique instead of machining to manufacture radial plates.
the inner PF coils is also limited. The most promising material for the ring is a unidirectional glass fiber epoxy composite, which will be made using a wet fiberglass (S-glass) filament winding technique. The pre-compression is applied by stretching the rings by means of radial bolts between the rings and the coil case in the upper and lower curved regions. It is expected that R&D activities will be initiated to demonstrate the manufacture of the rings and to measure material properties.
4. Central solenoid conductor jacket The main function driving the design of the central solenoid is the generation of inductive flux to ramp up and maintain the plasma current. Flux generation in the solenoid is improved by the choice of a high field and the use of the highest allowable tensile stresses in the jacket material. The requirements for the CS conductor jacket material are, therefore, primarily a high fatigue
3. Pre-compression structure The overall TF coil structure will include precompression rings at the upper and lower inboard curved regions, as shown in Fig. 3. Each ring has a mean radius of 2664 mm and its cross section has a radius of 188 mm. The rings provide a radial centripetal pre-load of 60– 70 MN per TF coil, which substantially reduces the static toroidal hoop tensile loads in the OIS [3]. More importantly, the rings reduce stresses in the coil case: the maximum principal stress at the curved inboard regions is 459 MPa and maximum cyclic stress is 194 MPa (variation in one cycle), which are within allowable limit for the reference 15 MA scenario and 30 000 tokamak cycles. To be effective, these pre-compression rings need to have high hoop (toroidal) stiffness. The space available for the rings between the CS and
155
Fig. 3. Pre-compression rings.
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K. Okuno et al. / Fusion Engineering and Design 58–59 (2001) 153–157
stainless steels, including JK2. At present, Incoloy 908 is selected as the provisional reference solution and the Ti-stainless steel (JK2) option is kept as an alternative solution. The final choice of the jacket material will be based on these R&D results.
5. CS pre-load structure
Fig. 4. CS conductors: Incoloy square jacket reinforced by a strip (left) and titanium circular jacket reinforced with U-channels (right).
resistance to stress cycling. There are two basic design options for the CS jacket, as shown in Fig. 4, both of which provide the same flux capability: The use of an extruded jacket with a square outer section made of Incoloy 908 where the structural material goes through the Nb3Sn heat treatment. A double armor option involving the use of an inner titanium circular jacket, which undergoes the Nb3Sn heat treatment, reinforced by two outer U-channels which are applied after the heat treatment. Incoloy 908 is highly sensitive to stress accelerated grain boundary oxidation (SAGBO) during the Nb3Sn heat treatment, and this requires very strict control of the heat treatment atmosphere (O2 B 0.1 ppm) [4]. On the other hand, Incoloy 908 has significant advantages. It is a precipitation hardened superalloy with (in the base metal) a very high fatigue resistance, and a thermal contraction which matches that of Nb3Sn. The use of Incoloy 908 was successfully demonstrated in the CS Model Coil [5]. For the second option, JK2 is proposed as the material of the external armor. This material has a coefficient of thermal contraction close to Nb3Sn between room temperature and 4 K. However, JK2 is not fully characterized at cryogenic temperature, especially for fatigue properties. R&D activities are underway to demonstrate the manufacture of U-channels and to establish the fatigue properties of modified
The CS consists of a stack of six electrically independent modules. The field curvature at the ends of the CS creates vertical forces on the modules. At IM and EOB, these forces are towards the center of the stack, whereas at some intermediate equilibrium configurations the end modules carry currents opposite to the central ones and are repelled. The pre-load structure is composed of the lower and upper key blocks, a set of 12 tie-plates, wedging elements and lower and upper buffer zones, as shown in Fig. 5. It applies axial pre-compression to the coil stack so that the modules remain in contact during all operating conditions. To obtain uniform compression, tie plates running axially along the CS are provided at both inner and outer diameters and connect to pressure plates at top and bottom. This structure is designed so that it can restrain the maximum vertical separating load of 75 MN acting on the end modules of the stack. The required axial tension in the structure is achieved partly by pre-tensioning at room temperature and partly by differential contraction during cooldown. This requires a jacket material of the CS conductor with a lower contraction coefficient (such as Incoloy 908 or JK2) than the tie plates, which use stainless steel.
6. Conclusions A detailed design of the ITER system, which meets the requirements of the ITER-FEAT, is being finalized. The design features include: The TF coil winding pack in the radial plate configuration which offers high reliability of the insulation system during operation.
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fiber glass– epoxy composite. Two designs are developed for the CS winding pack. The remaining issue is the final selection of the conductor jacket material. R&D activities are underway to address the cost issue of the radial plate and to allow the final selection of the CS conductor jacket material.
7. Disclaimer This paper is an account of work undertaken within the framework of the ITER EDA Agreement. The views of the authors do not necessarily reflect those of the ITER Director, the Parties to the ITER EDA Agreement, or the International Atomic Energy Agency.
References
Fig. 5. CS pre-load structure.
An improvement of the overall TF coil structure by means of a pre-compression structure using a set of rings made of a unidirectional
[1] M. Huguet, The Integrated Design of the ITER Magnets and their Auxiliary Systems, Fusion Energy 1998, 17th Conference Proceeding, vol. 3, IAEA, Yokohama, 1998, p. 957. [2] R.K. Maix et al., Completion and QA Test Results of The ITER TF Model Coil, this conference. [3] C.T.J. Jong et al., The ITER-FEAT Toroidal Field Structures, this conference. [4] N. Mitchell et al., Avoidance of Stress Accelerated Grain Boundary Oxidation (SAGBO) in Incoloy 908 Used as a Jacket Material for Nb3Sn Conductors, 15th Magnet Technology Conference, Beijing, 1997, pp. 1163 – 1166. [5] T. Kato, First Test Results for the ITER Central Solenoid Model Coil, this conference.
Key features of the ITER-FEAT magnet system K. Okuno *, D. Bessette, M. Ferrari, M. Huguet, C. Jong, K. Kitamura, Y. Krivchenkov, N. Mitchell, H. Takigami, K. Yoshida, E. Zapretilina ITER Joint Central Team, Naka Joint Work Site, 801 -1 Mukouyama, Naka-machi, Naka-gun, Ibaraki-ken, 311 -0193, Japan
Abstract The design of the ITER magnet system is being finalized. The reference design of the winding pack of the TF coil is based on the use of circular conductors supported by radial plates. This design has been chosen for its high insulation reliability during operation. The overall TF coil structure includes pre-compression rings made of unidirectional fiber glass, which reduce the stress level in the outer intercoil structures and the coil case. The design of the central solenoid, including pre-load structure, has been developed. Two conductor jacket options are still under investigation for the CS and the final choice will be based on the results of on-going R&D. © 2001 Elsevier Science B.V. All rights reserved. Keywords: ITER-FEAT magnet system; Central solenoid; Outer intercoil structures
1. Introduction The ITER magnets are designed to meet the requirements of the plasma configuration chosen for the machine [1]. The magnet system will be the largest set of superconducting magnets ever built and it will be required to operate under demanding mechanical and electromagnetic conditions. Therefore, the design involves the selection of technically reliable and cost effective concepts among the various technologies which have been considered. The ITER magnet system consists of 18 toroidal field (TF) coils, a central solenoid (CS), * Corresponding author. Tel.: + 81-29-270-7761; fax: + 8129-270-7507. E-mail address: [email protected] (K. Okuno).
six poloidal field (PF) coils, as shown in Fig. 1, and three sets of correction coils (CCs). The TF coil winding packs are enclosed in cases which constitute the main structure of the magnet system. The inboard straight legs of the TF coils are wedged to sustain the centering forces. Friction forces at the wedge sustain part of the out-ofplane loads which result from the interaction of TF coil current with the poloidal magnetic field. The out-of-plane loads are also supported by shear keys located at the inboard curved regions of the TF coils and by four outer intercoil structures (OISs) along the outboard leg contour. A distinctive feature of the TF coils winding pack is the use of radial plates with grooves which provide support for the conductor. Each TF coil is 14 m high and 9 m wide and will operate at 12 T with a total current of 9.1 MA. The CS assembly consists of a stack of six modules. The six
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 1 9 - 7
154
K. Okuno et al. / Fusion Engineering and Design 58–59 (2001) 153–157
Fig. 1. ITER magnet system — isometric view. CS and correction coils are not shown.
modules are electrically independent to provide non-uniform current distributions along the vertical axis as required by plasma shaping. The CS operates at 13.5 (initial magnetization, IM) and 12.8 T (end of burn, EOB). A pre-load structure provides axial compression on the CS stack to avoid any separation between the CS modules, and supports the whole stack against net vertical forces. This paper describes some distinctive features of the ITER magnet system: the TF winding pack design, the use of pre-compression rings in the TF coil structure and the CS conductor and pre-load structure.
considering that insulation faults are the most probable cause of magnet failure and also considering the difficulties involved in the replacement of a TF coil. This concept has already been proven in the TF Model Coil Project [2]. To solve the cost issue, some R&D activities have been
2. TF coil radial plate design The reference winding pack design is based on the use of radial plates and circular conductors in a double-pancake configuration, as shown in Fig. 2. The advantages and drawbacks of this design are summarized in Table 1. This design has been chosen because of the expected high insulation reliability and the possibility to detect faults before significant damage occurs. These considerations have been given a high, overriding priority,
Fig. 2. Cross sectional view of the TF coil at nose area.
K. Okuno et al. / Fusion Engineering and Design 58–59 (2001) 153–157 Table 1 Advantages and drawbacks of the radial plate design Ad6antages A circular cross section is the optimum shape to apply insulation tapes, resulting in a robust turn insulation. The Lorentz forces acting on each conductor are transferred to the plate, without accumulation of forces on the conductor and its insulation. The conductor and ground insulations are independent and physically separated by the radial plate. A single insulation fault will not affect both insulations. A single conductor insulation fault can be detected by monitoring the resistance between conductor and radial plate. Drawbacks The radial plate manufacture is costly since it requires significant time to machine the grooves. The radial plate design implies a larger size of a winding by 30–50 mm than a conventional winding pack with square conductors. In the event of a fast discharge of the TF coils, the winding temperature rises to about 50–60 K due to eddy current in the radial plates. Recooling of the TF coils is estimated to take about 30 h.
started, including the use of an extrusion technique instead of machining to manufacture radial plates.
the inner PF coils is also limited. The most promising material for the ring is a unidirectional glass fiber epoxy composite, which will be made using a wet fiberglass (S-glass) filament winding technique. The pre-compression is applied by stretching the rings by means of radial bolts between the rings and the coil case in the upper and lower curved regions. It is expected that R&D activities will be initiated to demonstrate the manufacture of the rings and to measure material properties.
4. Central solenoid conductor jacket The main function driving the design of the central solenoid is the generation of inductive flux to ramp up and maintain the plasma current. Flux generation in the solenoid is improved by the choice of a high field and the use of the highest allowable tensile stresses in the jacket material. The requirements for the CS conductor jacket material are, therefore, primarily a high fatigue
3. Pre-compression structure The overall TF coil structure will include precompression rings at the upper and lower inboard curved regions, as shown in Fig. 3. Each ring has a mean radius of 2664 mm and its cross section has a radius of 188 mm. The rings provide a radial centripetal pre-load of 60– 70 MN per TF coil, which substantially reduces the static toroidal hoop tensile loads in the OIS [3]. More importantly, the rings reduce stresses in the coil case: the maximum principal stress at the curved inboard regions is 459 MPa and maximum cyclic stress is 194 MPa (variation in one cycle), which are within allowable limit for the reference 15 MA scenario and 30 000 tokamak cycles. To be effective, these pre-compression rings need to have high hoop (toroidal) stiffness. The space available for the rings between the CS and
155
Fig. 3. Pre-compression rings.
156
K. Okuno et al. / Fusion Engineering and Design 58–59 (2001) 153–157
stainless steels, including JK2. At present, Incoloy 908 is selected as the provisional reference solution and the Ti-stainless steel (JK2) option is kept as an alternative solution. The final choice of the jacket material will be based on these R&D results.
5. CS pre-load structure
Fig. 4. CS conductors: Incoloy square jacket reinforced by a strip (left) and titanium circular jacket reinforced with U-channels (right).
resistance to stress cycling. There are two basic design options for the CS jacket, as shown in Fig. 4, both of which provide the same flux capability: The use of an extruded jacket with a square outer section made of Incoloy 908 where the structural material goes through the Nb3Sn heat treatment. A double armor option involving the use of an inner titanium circular jacket, which undergoes the Nb3Sn heat treatment, reinforced by two outer U-channels which are applied after the heat treatment. Incoloy 908 is highly sensitive to stress accelerated grain boundary oxidation (SAGBO) during the Nb3Sn heat treatment, and this requires very strict control of the heat treatment atmosphere (O2 B 0.1 ppm) [4]. On the other hand, Incoloy 908 has significant advantages. It is a precipitation hardened superalloy with (in the base metal) a very high fatigue resistance, and a thermal contraction which matches that of Nb3Sn. The use of Incoloy 908 was successfully demonstrated in the CS Model Coil [5]. For the second option, JK2 is proposed as the material of the external armor. This material has a coefficient of thermal contraction close to Nb3Sn between room temperature and 4 K. However, JK2 is not fully characterized at cryogenic temperature, especially for fatigue properties. R&D activities are underway to demonstrate the manufacture of U-channels and to establish the fatigue properties of modified
The CS consists of a stack of six electrically independent modules. The field curvature at the ends of the CS creates vertical forces on the modules. At IM and EOB, these forces are towards the center of the stack, whereas at some intermediate equilibrium configurations the end modules carry currents opposite to the central ones and are repelled. The pre-load structure is composed of the lower and upper key blocks, a set of 12 tie-plates, wedging elements and lower and upper buffer zones, as shown in Fig. 5. It applies axial pre-compression to the coil stack so that the modules remain in contact during all operating conditions. To obtain uniform compression, tie plates running axially along the CS are provided at both inner and outer diameters and connect to pressure plates at top and bottom. This structure is designed so that it can restrain the maximum vertical separating load of 75 MN acting on the end modules of the stack. The required axial tension in the structure is achieved partly by pre-tensioning at room temperature and partly by differential contraction during cooldown. This requires a jacket material of the CS conductor with a lower contraction coefficient (such as Incoloy 908 or JK2) than the tie plates, which use stainless steel.
6. Conclusions A detailed design of the ITER system, which meets the requirements of the ITER-FEAT, is being finalized. The design features include: The TF coil winding pack in the radial plate configuration which offers high reliability of the insulation system during operation.
K. Okuno et al. / Fusion Engineering and Design 58–59 (2001) 153–157
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fiber glass– epoxy composite. Two designs are developed for the CS winding pack. The remaining issue is the final selection of the conductor jacket material. R&D activities are underway to address the cost issue of the radial plate and to allow the final selection of the CS conductor jacket material.
7. Disclaimer This paper is an account of work undertaken within the framework of the ITER EDA Agreement. The views of the authors do not necessarily reflect those of the ITER Director, the Parties to the ITER EDA Agreement, or the International Atomic Energy Agency.
References
Fig. 5. CS pre-load structure.
An improvement of the overall TF coil structure by means of a pre-compression structure using a set of rings made of a unidirectional
[1] M. Huguet, The Integrated Design of the ITER Magnets and their Auxiliary Systems, Fusion Energy 1998, 17th Conference Proceeding, vol. 3, IAEA, Yokohama, 1998, p. 957. [2] R.K. Maix et al., Completion and QA Test Results of The ITER TF Model Coil, this conference. [3] C.T.J. Jong et al., The ITER-FEAT Toroidal Field Structures, this conference. [4] N. Mitchell et al., Avoidance of Stress Accelerated Grain Boundary Oxidation (SAGBO) in Incoloy 908 Used as a Jacket Material for Nb3Sn Conductors, 15th Magnet Technology Conference, Beijing, 1997, pp. 1163 – 1166. [5] T. Kato, First Test Results for the ITER Central Solenoid Model Coil, this conference.