- Email: [email protected]
Updating the design of the feeder components for the ITER magnet system
Fusion Engineering and Design 75–79 (2005) 241–247
Updating the design of the feeder components for the ITER magnet system K. Yoshida a,∗ , Y. Takahashi a , T. Isono b , N. Mitchell a b
a ITER Naka Joint Work Site, Ibaraki-ken, Japan Japan Atomic Energy Research Institute, 801-1 Mukaiyama, Naka-gun, Naka-machi, Ibaraki-ken 311-0193, Japan
Available online 27 July 2005
Abstract The ITER superconducting magnet system generates an average heat load of 23 kW at 4 K to the cryoplant, from nuclear and thermal radiation, conduction and electromagnetic heating, and requires current supplies 10–68 kA to 48 individual coils. The helium flow to remove this heat, consisting of supercritical helium at pressures up to 1.0 MPa and temperature between 4.3 and 4.7 K, is distributed to the coils and structures through 30 separate feeder lines. The feeders also contain the electrical supplies to the coil, helium supply pipes and the instrumentation lines, and are integrated with the current lead transitions to room temperature. The components consist of the in-cryostat feeders, the cryostat feedthroughs and the coil terminal boxes (CTBs). This paper discusses the functional requirements on the feeder system and presents the latest design concept and parameters of the feeder components. © 2005 Elsevier B.V. All rights reserved. Keywords: ITER magnet system; In-cryostat feeders; Coil terminal box
1. Introduction The ITER superconducting magnets [1] are contained in a cryostat, immediately outside of the vacuum vessel that contains the plasma and provides the main nuclear shielding for the superconducting components. The magnets and associated structures require helium and electrical power supplies and instrumentation cables, which have to be routed from the main containment building through the cryostat. The busbars ∗
Corresponding author. E-mail address: [email protected] (K. Yoshida).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.06.112
from the power supplies to the coils enter the containment building as room temperature water-cooled copper busbars, and there must therefore be a transition through a current lead to a superconducting busbar that can be connected directly to the coil.
2. Requirements 2.1. Functional requirements The room temperature transition of the busbar is placed outside of the cryostat, and the supply through
242
K. Yoshida et al. / Fusion Engineering and Design 75–79 (2005) 241–247
the cryostat wall to the coil is through a superconducting NbTi busbar in order to minimize the space used inside the cryostat. To reduce the amount of assembly work inside the cryostat, the supplies for individual coils (pairs of TF coils) are integrated into a single feeder. This is prefabricated off site with complete installation of helium supply lines, instrumentation ducts and superconducting busbars. Size restrictions during installation mean that joints are required in the feeders including joints in the superconducting busbars. Feeders have to be kept to a minimum to avoid making the complex joints. Some feeders can be preinstalled below the machine at the start of construction but most of them would block magnet or vacuum vessel assembly and have to be put in place at the end. The current leads are therefore placed distributed around the machine, at various vertical levels, corresponding to the positions of the coil current terminals, to keep the feeder length to a minimum. Once installed, the feeders have to satisfy several operational requirements. The coils move relative to the cryostat wall and building base mat, both due to cooldown and due to coil flexing under electromagnetic loads. Cooldown relative movement of the coil terminals is up to about 35 mm, both radial and vertical, and during plasma operation up to 3 or 4 mm in the radial and vertical directions plus up to 15 mm in the toroidal. In addition, the feeders themselves shrink during cooldown, typically by about 0.3%. This motion has to be accommodated within the feeder units. Earthquakes result in additional end displacements but also in dynamic loads which have to be stabilized by intermediate supports along the length of the feeders within the cryostat. From the safety viewpoint, the feeders are also the conduits that transfer the 50 GJ of stored magnetic energy into and out of the magnet system. This results in several further requirements. The superconductors and current leads must be capable of operation without external helium supplies for adequate time to detect a fault and extract the magnet energy. The feeder conduit containment must be sufficient to confine molten metal resulting from arcing caused by insulation failure either within the busbars themselves.
ible length of superconducting busbar and helium supply lines, to allow free movement of the coils. They also include the main vacuum barrier, separating the cryostat vacuum from the vacuum surrounding the current lead transitions to room temperature. Outside the cryostat, the current lead transitions to room temperature are contained in a CTB unit. This provides the vacuum containment for the leads and is the region where the helium pipes and instrumentation separate from the busbars. Also within the CTBs are the valves for the helium supply control. The nuclear radiation levels, magnetic fields and general access restrictions mean that it is undesirable to place remote control helium supply valves close to the coils and these are installed in the CTBs for each access. Each coil has one inlet and outlet supply, with manifold positioned locally on the coil. These can be accessed if required during maintenance periods. The structural feeders, supplying 18 TF coils as well as the CS pre-compression structure, contain many more supply lines. Each TF coil case has three cooling circuits, two inside and one outside, also extending to the CS structure. Individual control is required for each circuit because of differences in heat loads during cooldown and operation. These differences are created by the variability of the combinations of nuclear heating and eddy current heating, especially caused by shielding differences around the various equatorial port. The overall feeder components for the coils are shown in Fig. 1. This figure shows two toroidal field (TF) coil, a cut model of a poloidal field (PF), the central solenoid (CS) and correction coils (CC). These feeder components are installed in upper (EL + 10.56 m) and lower (EL − 11.60 m) magnet areas in the reactor building. In addition, there are three structural feeders supplying the TF coil cases and inter-coil structures and the CS pre-compression structure. One feeder supplies the inlets, placed at the lower level, and two (each taking 9 TF cases) at the upper level contain the outlets.
3. Updating design of feeder components 3.1. In-cryostat feeders
2.2. General layout requirements The feeders’ transition out of the cryostat is made through the cryostat feedthroughs. These include a flex-
A 68 kA NbTi cable-in-conduit conductor is adopted for the busbar conductor of the TF coils, CS and PF coil and a 10 kA NbTi conductor for the CC as
K. Yoshida et al. / Fusion Engineering and Design 75–79 (2005) 241–247
243
Fig. 1. ITER magnet system—feeders: 9 TF, 6 PF, 6 CS and 9 CC.
shown in Table 1. The lengths and number of joints in the busbars are summarized in Table 2. All of the joints have a common design using a circular copper sleeve and bolted clamp with a stainless steel box to contain the sleeves [2]. Various design improvements have been made since the original ITER FDR design in 2001 [2], as follows: Table 1 Typical busbar parameters (NbTi) Parameters
Large coil busbars
CC busbar
Operating current (kA) Nominal peak field (T) Operating temperature (K) Discharge time constant (s) Cable diameter (mm) Central spiral outer/inner diameter (mm) Conductor outer diameter (mm) Jacket material SC strand diameter (mm) SC strand cu: non-cu Cabling pattern SC strands SC strand weight (kg/m)
68 4 5.0 26 41 8×6
10 4 5.0 26 16.2 0
47
20.2
SS316LN 0.73 6.9 3×4×5×5×6 1800 6.1
SS316LN 0.73 6.9 3×4×5×5 300 1.0
(1) The header cylinder underneath each of the TF coils now supports the in-cryostat feeder through an electrical insulation flange to avoid eddy currents. (2) The bottom feeders for the CS were moved down to create a space for maintenance of the PF6, should it be necessary. The terminal position of PF4 was moved to provide a maintenance area. (3) The header of in-cryostat feeder for the side CC was moved from the outside of cryostat to inside. This modification removed the need for early supply and installation of this feeder before cryostat assembly. 3.2. Cryostat feedthrough All the coil components after cooldown shrink by 84 mm at 28 m radius from the machine center that is fix point of CTBs. This movement, as well as smaller vertical and toroidal movements, is taken by ‘S’ bends in the cryostat feedthroughs (Fig. 2). The feeder conductor needs two convolutions to keep the conduit stress within the allowable stress, which determines the length of the vacuum tank extending outside the main cryostat outer radius.
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K. Yoshida et al. / Fusion Engineering and Design 75–79 (2005) 241–247
Table 2 Characteristics of the feeders and specifications for the current leads TF coil
PF coil
CC
CS
Feeders No. of feeders Total length of feeder (m) No. of joints in feeder
9 58.5 54
6 49.0 24
9 283.5 54
6 72.0 24
Current leads Load factor Current (kA) He flow at Imax (g/s kA) He flow at I = 0 (g/s kA)
1.0 68 0.06 0.04
There is a vacuum barrier between the cryostat feedthrough and the CTB, both for containment reasons and to allow the cryostat vacuum to be kept with the coils cold during CTB maintenance. The thermal shield of the feedthrough is a low emissivity (0.05) single layer plate with Silver electroplating cooled by 80 K helium gas. Standard multilayer insulation is used in the thermal shield for the CTB. 3.3. Coil terminal box The coil terminal boxes (CTB) provides the housing for interconnection of the magnet systems with the Cryoplant, the Power Supplies and the Data Acquisition System and they also house the local cryogenic control components. The internal layout is shown diagrammatically in Fig. 3. Valves in the CTB control the mass flow rate of helium for each coil and current lead. These valves are also used during cooldown and warmup operations to control thermal gradients. The TF, CS and PF CTBs each contain one pair of current leads, those for the CC contain two pairs. The
0.38–0.06 0–45 0.10 0.02
0.38–0.06 0–10 0.10 0.02
0.56–0.23 0–45 0.06 0.04
current scenarios for the CS, PF and CC are pulsed, and in some plasma scenarios the coils are operating well below their maximum design values for most of the time during the dwell and burn phases. The load factors are shown in Table 2. A current lead designed to work efficiently at maximum current will generally not work at optimum performance at lower currents. The extra copper needed to reduce the resistive heating at high current produces extra heat conduction from the warm end at lower currents. A range of plasma scenarios is required and it is not possible to identify definitely the operating currents in the CS, PF and CC. Based on the inspection of the coil currents over a range of scenarios, the current leads performance and base design conditions are defined in Table 2. The leads with constant and variable cross-section along a lead are applied for TF and CS and PF and CC, respectively. ITER has only an outline current lead design used to estimate space and cryogenic requirements. Improvements can be made at the time of procurement, as long as the alternative is fully qualified and meets the base
Fig. 2. Cryostat feedthrough and CTB with valves and current leads.
K. Yoshida et al. / Fusion Engineering and Design 75–79 (2005) 241–247
245
Fig. 3. Functional diagram of a CTB.
design requirements. For example, there are many studies of high critical temperature superconducting (HTS) conductor for current leads [3,4]. However, there are still many issues before using HTS current leads in ITER magnet system. Present HTS current leads require about 20–30 K at inlet to give adequate margins. Then a special cryoplant for 20 K operation and a cryoline extending 200 m from cryoplant building are necessary specially for the HTS lead. The present ITER cryoplant consists of four identical helium refrigerators each with a capacity of 18 kW at 4.5 K giving redundancy in operation and allowing withdrawal for maintenance with minimum operational disruption. Typically, one of these is required to supply the current leads in operation. If one 4.5 K refrigerator were to be replaced by 20 K refrigerator, the cryoplant electric power would be reduced by 13% [4], but the redundancy is lost. If the supply temperature of HTS current leads could be 80 K then they can use the existing supply for the thermal shield and they become more attractive. However, the present HTS conductors cannot carry much current at 80–100 K, particularly with the leakage magnetic field in ITER at 25 m in radius of 40 mT. The present HTS leads are designed at 30 mT self field. It seems premature to propose to adopt HTS current leads
for the ITER magnet system with the present low performance at 80–100 K with 40 mT external field.
4. Protection and safety Each busbar has a double layer insulation system [5] that will allow detection of a developing busbar-toground short circuit and allow operation to be stopped before the short occurs, providing protection against further damage. The scheme consists of a double layer of insulation with a metallic shield within the double layer. The insulation will be several half-lapped layers of glass and polyimide, followed by one or two layers of metallic tape with glass in between, and finally several layers of half-lapped glass and polyimide to complete the turn insulation. An over-wrap of steel tape will be applied for mechanical protection in handling, and for grounding when installed so as to serve as an electrostatic screen. This screen serves as extra protection against a short circuit between coil terminals in addition to the steel separator plate as shown in Fig. 4. In the event of an arc developing, the arc may move into the busbars due to conductor melting since in the TF coils the main driving energy is outside the shorted coil. Calculations [6] suggest that the arc would move
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K. Yoshida et al. / Fusion Engineering and Design 75–79 (2005) 241–247
Fig. 4. Diagram of a coil feeder cross-section.
For the busbars, the rate of temperature rise during a quench must be longer than the discharge time of the coils. A discharge time of twice that of the coils is used.
interacting with in-vessel systems and requiring extra consideration during assembly. The design of feeder system has been developed with analysis and adjustments to satisfy the requirements of initial assembly and maintenance during operation. The main design options are defined and flexibility exists to adopt new technology such as HTS leads in the future if the performance can be improved. This report was prepared as an account of work undertaken within the framework of ITER transitional arrangements (ITA). These are conducted by the participants: the European Atomic Energy Community, Japan, the People’s Republic of China, the Republic of Korea, the Russian Federation, and the United States of America, under the auspices of the International Atomic Energy Agency. The views and opinions expressed herein do not necessarily reflect those of the Participants to the ITA, the IAEA or any agency thereof. Dissemination of the information in this paper is governed by the applicable terms of the former ITER EDA Agreement.
5. Conclusions
References
The feeders for the magnets are critical components that occupy valuable space within the cryostat,
[1] K. Okuno, et al., Key features of the ITER-FEAT magnet system, Fusion Eng. Des. 58/59 (2001) 153–157.
rapidly in a few seconds along the busbar. Each feeder has a robust steel outer conduit (more than 10 mm thickness) that, as well as protection against external events, provides containment of liquid metal from arcs during the transit. Both current leads and busbars are designed to operate under fault conditions. For a current lead, the most serious condition is a loss of helium flow. It is essential that even under these conditions the current leads can continue to operate for a time much longer than that required to detect the loss of flow and discharge the coils. Two conditions are adopted for a complete loss of flow (1) That the time taken for the temperature of the hot end to rise by 100 K is more than 300 s to give time for detection. (2) That the heat load at the cold end to the conductor increases by less than a factor of 3 in 600 s.
K. Yoshida et al. / Fusion Engineering and Design 75–79 (2005) 241–247 [2] ITER, Plant Description Document, 2.1 Magnets, 2001. http://www.naka.jaeri.go.jp/ITER/FDR/. [3] R. Heller, et al., Development program of a 60 kA high temperature superconducting current lead for the ITER toroidal filed coils, Fusion Eng. Des. 58/59 (2001) 105–109. [4] T. Isono, et al., Development and test results of a 60 kA HTS current lead for fusion application, J. Cryo. Soc. Jpn. 39 (2004) 122–128.
247
[5] K. Yoshida, et al., Electrical insulation design and monitoring of the ITER magnet system, in: Proceedings of the 20th Symposium on Fusion Technology, 1998, pp. 807–810. [6] ITER, Design Description Document 11 Magnet, 2. Performance Analysis, 2.4 Faults and Safety Analysis, N 11 DDD128 01-0711 R0.2, 2002.
Updating the design of the feeder components for the ITER magnet system K. Yoshida a,∗ , Y. Takahashi a , T. Isono b , N. Mitchell a b
a ITER Naka Joint Work Site, Ibaraki-ken, Japan Japan Atomic Energy Research Institute, 801-1 Mukaiyama, Naka-gun, Naka-machi, Ibaraki-ken 311-0193, Japan
Available online 27 July 2005
Abstract The ITER superconducting magnet system generates an average heat load of 23 kW at 4 K to the cryoplant, from nuclear and thermal radiation, conduction and electromagnetic heating, and requires current supplies 10–68 kA to 48 individual coils. The helium flow to remove this heat, consisting of supercritical helium at pressures up to 1.0 MPa and temperature between 4.3 and 4.7 K, is distributed to the coils and structures through 30 separate feeder lines. The feeders also contain the electrical supplies to the coil, helium supply pipes and the instrumentation lines, and are integrated with the current lead transitions to room temperature. The components consist of the in-cryostat feeders, the cryostat feedthroughs and the coil terminal boxes (CTBs). This paper discusses the functional requirements on the feeder system and presents the latest design concept and parameters of the feeder components. © 2005 Elsevier B.V. All rights reserved. Keywords: ITER magnet system; In-cryostat feeders; Coil terminal box
1. Introduction The ITER superconducting magnets [1] are contained in a cryostat, immediately outside of the vacuum vessel that contains the plasma and provides the main nuclear shielding for the superconducting components. The magnets and associated structures require helium and electrical power supplies and instrumentation cables, which have to be routed from the main containment building through the cryostat. The busbars ∗
Corresponding author. E-mail address: [email protected] (K. Yoshida).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.06.112
from the power supplies to the coils enter the containment building as room temperature water-cooled copper busbars, and there must therefore be a transition through a current lead to a superconducting busbar that can be connected directly to the coil.
2. Requirements 2.1. Functional requirements The room temperature transition of the busbar is placed outside of the cryostat, and the supply through
242
K. Yoshida et al. / Fusion Engineering and Design 75–79 (2005) 241–247
the cryostat wall to the coil is through a superconducting NbTi busbar in order to minimize the space used inside the cryostat. To reduce the amount of assembly work inside the cryostat, the supplies for individual coils (pairs of TF coils) are integrated into a single feeder. This is prefabricated off site with complete installation of helium supply lines, instrumentation ducts and superconducting busbars. Size restrictions during installation mean that joints are required in the feeders including joints in the superconducting busbars. Feeders have to be kept to a minimum to avoid making the complex joints. Some feeders can be preinstalled below the machine at the start of construction but most of them would block magnet or vacuum vessel assembly and have to be put in place at the end. The current leads are therefore placed distributed around the machine, at various vertical levels, corresponding to the positions of the coil current terminals, to keep the feeder length to a minimum. Once installed, the feeders have to satisfy several operational requirements. The coils move relative to the cryostat wall and building base mat, both due to cooldown and due to coil flexing under electromagnetic loads. Cooldown relative movement of the coil terminals is up to about 35 mm, both radial and vertical, and during plasma operation up to 3 or 4 mm in the radial and vertical directions plus up to 15 mm in the toroidal. In addition, the feeders themselves shrink during cooldown, typically by about 0.3%. This motion has to be accommodated within the feeder units. Earthquakes result in additional end displacements but also in dynamic loads which have to be stabilized by intermediate supports along the length of the feeders within the cryostat. From the safety viewpoint, the feeders are also the conduits that transfer the 50 GJ of stored magnetic energy into and out of the magnet system. This results in several further requirements. The superconductors and current leads must be capable of operation without external helium supplies for adequate time to detect a fault and extract the magnet energy. The feeder conduit containment must be sufficient to confine molten metal resulting from arcing caused by insulation failure either within the busbars themselves.
ible length of superconducting busbar and helium supply lines, to allow free movement of the coils. They also include the main vacuum barrier, separating the cryostat vacuum from the vacuum surrounding the current lead transitions to room temperature. Outside the cryostat, the current lead transitions to room temperature are contained in a CTB unit. This provides the vacuum containment for the leads and is the region where the helium pipes and instrumentation separate from the busbars. Also within the CTBs are the valves for the helium supply control. The nuclear radiation levels, magnetic fields and general access restrictions mean that it is undesirable to place remote control helium supply valves close to the coils and these are installed in the CTBs for each access. Each coil has one inlet and outlet supply, with manifold positioned locally on the coil. These can be accessed if required during maintenance periods. The structural feeders, supplying 18 TF coils as well as the CS pre-compression structure, contain many more supply lines. Each TF coil case has three cooling circuits, two inside and one outside, also extending to the CS structure. Individual control is required for each circuit because of differences in heat loads during cooldown and operation. These differences are created by the variability of the combinations of nuclear heating and eddy current heating, especially caused by shielding differences around the various equatorial port. The overall feeder components for the coils are shown in Fig. 1. This figure shows two toroidal field (TF) coil, a cut model of a poloidal field (PF), the central solenoid (CS) and correction coils (CC). These feeder components are installed in upper (EL + 10.56 m) and lower (EL − 11.60 m) magnet areas in the reactor building. In addition, there are three structural feeders supplying the TF coil cases and inter-coil structures and the CS pre-compression structure. One feeder supplies the inlets, placed at the lower level, and two (each taking 9 TF cases) at the upper level contain the outlets.
3. Updating design of feeder components 3.1. In-cryostat feeders
2.2. General layout requirements The feeders’ transition out of the cryostat is made through the cryostat feedthroughs. These include a flex-
A 68 kA NbTi cable-in-conduit conductor is adopted for the busbar conductor of the TF coils, CS and PF coil and a 10 kA NbTi conductor for the CC as
K. Yoshida et al. / Fusion Engineering and Design 75–79 (2005) 241–247
243
Fig. 1. ITER magnet system—feeders: 9 TF, 6 PF, 6 CS and 9 CC.
shown in Table 1. The lengths and number of joints in the busbars are summarized in Table 2. All of the joints have a common design using a circular copper sleeve and bolted clamp with a stainless steel box to contain the sleeves [2]. Various design improvements have been made since the original ITER FDR design in 2001 [2], as follows: Table 1 Typical busbar parameters (NbTi) Parameters
Large coil busbars
CC busbar
Operating current (kA) Nominal peak field (T) Operating temperature (K) Discharge time constant (s) Cable diameter (mm) Central spiral outer/inner diameter (mm) Conductor outer diameter (mm) Jacket material SC strand diameter (mm) SC strand cu: non-cu Cabling pattern SC strands SC strand weight (kg/m)
68 4 5.0 26 41 8×6
10 4 5.0 26 16.2 0
47
20.2
SS316LN 0.73 6.9 3×4×5×5×6 1800 6.1
SS316LN 0.73 6.9 3×4×5×5 300 1.0
(1) The header cylinder underneath each of the TF coils now supports the in-cryostat feeder through an electrical insulation flange to avoid eddy currents. (2) The bottom feeders for the CS were moved down to create a space for maintenance of the PF6, should it be necessary. The terminal position of PF4 was moved to provide a maintenance area. (3) The header of in-cryostat feeder for the side CC was moved from the outside of cryostat to inside. This modification removed the need for early supply and installation of this feeder before cryostat assembly. 3.2. Cryostat feedthrough All the coil components after cooldown shrink by 84 mm at 28 m radius from the machine center that is fix point of CTBs. This movement, as well as smaller vertical and toroidal movements, is taken by ‘S’ bends in the cryostat feedthroughs (Fig. 2). The feeder conductor needs two convolutions to keep the conduit stress within the allowable stress, which determines the length of the vacuum tank extending outside the main cryostat outer radius.
244
K. Yoshida et al. / Fusion Engineering and Design 75–79 (2005) 241–247
Table 2 Characteristics of the feeders and specifications for the current leads TF coil
PF coil
CC
CS
Feeders No. of feeders Total length of feeder (m) No. of joints in feeder
9 58.5 54
6 49.0 24
9 283.5 54
6 72.0 24
Current leads Load factor Current (kA) He flow at Imax (g/s kA) He flow at I = 0 (g/s kA)
1.0 68 0.06 0.04
There is a vacuum barrier between the cryostat feedthrough and the CTB, both for containment reasons and to allow the cryostat vacuum to be kept with the coils cold during CTB maintenance. The thermal shield of the feedthrough is a low emissivity (0.05) single layer plate with Silver electroplating cooled by 80 K helium gas. Standard multilayer insulation is used in the thermal shield for the CTB. 3.3. Coil terminal box The coil terminal boxes (CTB) provides the housing for interconnection of the magnet systems with the Cryoplant, the Power Supplies and the Data Acquisition System and they also house the local cryogenic control components. The internal layout is shown diagrammatically in Fig. 3. Valves in the CTB control the mass flow rate of helium for each coil and current lead. These valves are also used during cooldown and warmup operations to control thermal gradients. The TF, CS and PF CTBs each contain one pair of current leads, those for the CC contain two pairs. The
0.38–0.06 0–45 0.10 0.02
0.38–0.06 0–10 0.10 0.02
0.56–0.23 0–45 0.06 0.04
current scenarios for the CS, PF and CC are pulsed, and in some plasma scenarios the coils are operating well below their maximum design values for most of the time during the dwell and burn phases. The load factors are shown in Table 2. A current lead designed to work efficiently at maximum current will generally not work at optimum performance at lower currents. The extra copper needed to reduce the resistive heating at high current produces extra heat conduction from the warm end at lower currents. A range of plasma scenarios is required and it is not possible to identify definitely the operating currents in the CS, PF and CC. Based on the inspection of the coil currents over a range of scenarios, the current leads performance and base design conditions are defined in Table 2. The leads with constant and variable cross-section along a lead are applied for TF and CS and PF and CC, respectively. ITER has only an outline current lead design used to estimate space and cryogenic requirements. Improvements can be made at the time of procurement, as long as the alternative is fully qualified and meets the base
Fig. 2. Cryostat feedthrough and CTB with valves and current leads.
K. Yoshida et al. / Fusion Engineering and Design 75–79 (2005) 241–247
245
Fig. 3. Functional diagram of a CTB.
design requirements. For example, there are many studies of high critical temperature superconducting (HTS) conductor for current leads [3,4]. However, there are still many issues before using HTS current leads in ITER magnet system. Present HTS current leads require about 20–30 K at inlet to give adequate margins. Then a special cryoplant for 20 K operation and a cryoline extending 200 m from cryoplant building are necessary specially for the HTS lead. The present ITER cryoplant consists of four identical helium refrigerators each with a capacity of 18 kW at 4.5 K giving redundancy in operation and allowing withdrawal for maintenance with minimum operational disruption. Typically, one of these is required to supply the current leads in operation. If one 4.5 K refrigerator were to be replaced by 20 K refrigerator, the cryoplant electric power would be reduced by 13% [4], but the redundancy is lost. If the supply temperature of HTS current leads could be 80 K then they can use the existing supply for the thermal shield and they become more attractive. However, the present HTS conductors cannot carry much current at 80–100 K, particularly with the leakage magnetic field in ITER at 25 m in radius of 40 mT. The present HTS leads are designed at 30 mT self field. It seems premature to propose to adopt HTS current leads
for the ITER magnet system with the present low performance at 80–100 K with 40 mT external field.
4. Protection and safety Each busbar has a double layer insulation system [5] that will allow detection of a developing busbar-toground short circuit and allow operation to be stopped before the short occurs, providing protection against further damage. The scheme consists of a double layer of insulation with a metallic shield within the double layer. The insulation will be several half-lapped layers of glass and polyimide, followed by one or two layers of metallic tape with glass in between, and finally several layers of half-lapped glass and polyimide to complete the turn insulation. An over-wrap of steel tape will be applied for mechanical protection in handling, and for grounding when installed so as to serve as an electrostatic screen. This screen serves as extra protection against a short circuit between coil terminals in addition to the steel separator plate as shown in Fig. 4. In the event of an arc developing, the arc may move into the busbars due to conductor melting since in the TF coils the main driving energy is outside the shorted coil. Calculations [6] suggest that the arc would move
246
K. Yoshida et al. / Fusion Engineering and Design 75–79 (2005) 241–247
Fig. 4. Diagram of a coil feeder cross-section.
For the busbars, the rate of temperature rise during a quench must be longer than the discharge time of the coils. A discharge time of twice that of the coils is used.
interacting with in-vessel systems and requiring extra consideration during assembly. The design of feeder system has been developed with analysis and adjustments to satisfy the requirements of initial assembly and maintenance during operation. The main design options are defined and flexibility exists to adopt new technology such as HTS leads in the future if the performance can be improved. This report was prepared as an account of work undertaken within the framework of ITER transitional arrangements (ITA). These are conducted by the participants: the European Atomic Energy Community, Japan, the People’s Republic of China, the Republic of Korea, the Russian Federation, and the United States of America, under the auspices of the International Atomic Energy Agency. The views and opinions expressed herein do not necessarily reflect those of the Participants to the ITA, the IAEA or any agency thereof. Dissemination of the information in this paper is governed by the applicable terms of the former ITER EDA Agreement.
5. Conclusions
References
The feeders for the magnets are critical components that occupy valuable space within the cryostat,
[1] K. Okuno, et al., Key features of the ITER-FEAT magnet system, Fusion Eng. Des. 58/59 (2001) 153–157.
rapidly in a few seconds along the busbar. Each feeder has a robust steel outer conduit (more than 10 mm thickness) that, as well as protection against external events, provides containment of liquid metal from arcs during the transit. Both current leads and busbars are designed to operate under fault conditions. For a current lead, the most serious condition is a loss of helium flow. It is essential that even under these conditions the current leads can continue to operate for a time much longer than that required to detect the loss of flow and discharge the coils. Two conditions are adopted for a complete loss of flow (1) That the time taken for the temperature of the hot end to rise by 100 K is more than 300 s to give time for detection. (2) That the heat load at the cold end to the conductor increases by less than a factor of 3 in 600 s.
K. Yoshida et al. / Fusion Engineering and Design 75–79 (2005) 241–247 [2] ITER, Plant Description Document, 2.1 Magnets, 2001. http://www.naka.jaeri.go.jp/ITER/FDR/. [3] R. Heller, et al., Development program of a 60 kA high temperature superconducting current lead for the ITER toroidal filed coils, Fusion Eng. Des. 58/59 (2001) 105–109. [4] T. Isono, et al., Development and test results of a 60 kA HTS current lead for fusion application, J. Cryo. Soc. Jpn. 39 (2004) 122–128.
247
[5] K. Yoshida, et al., Electrical insulation design and monitoring of the ITER magnet system, in: Proceedings of the 20th Symposium on Fusion Technology, 1998, pp. 807–810. [6] ITER, Design Description Document 11 Magnet, 2. Performance Analysis, 2.4 Faults and Safety Analysis, N 11 DDD128 01-0711 R0.2, 2002.