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Status of HTS current leads for WENDELSTEIN 7-X and JT-60SA
Fusion Engineering and Design 84 (2009) 776–779
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Status of HTS current leads for WENDELSTEIN 7-X and JT-60SA W.H. Fietz ∗ , R. Heller, A. Kienzler, R. Lietzow Forschungszentrum Karlsruhe, Institute for Technical Physics, P.O. Box 3640, D-76021 Karlsruhe, Germany
a r t i c l e
i n f o
Article history: Received 5 August 2008 Received in revised form 22 November 2008 Accepted 28 November 2008 Available online 20 January 2009 Keywords: Current leads High-temperature superconductor Cooling conditions WENDELSTEIN 7-X JT-60SA
a b s t r a c t The stellarator WENDELSTEIN 7-X (W7-X) and the satellite tokamak JT-60SA will use High Temperature Superconductor (HTS) current leads to reduce cryogenic load and operational cost. Forschungszentrum Karlsruhe is in charge of design, construction and test of these HTS current leads. For W7-X fourteen current leads with a nominal current of 14 kA and a maximum current of 18.2 kA are required that are oriented upside down with the room temperature end at the bottom. JT-60SA is constructed in the frame of the Broader Approach Agreement and will use 26 current leads mounted in vertical, normal position with room temperature end at the top. All current leads will consist of an HTS part using Bi-2223 conductor and a classical Cu heat exchanger. The Cu heat exchanger covers the range between 60 K and room temperature and will be cooled with 50 K He while the HTS part is conduction cooled, only. For simplicity a common basic design of the current leads for W7-X and JT-60SA is planned which requests that the current leads work independently of the orientation, and from the experimental boundaries given by W7-X and JT-60SA the current leads have to withstand a loss of He coolant for more than 10 min and allow a discharge of the magnet system although a quench of the HTS part has happened. JT-60SA gives the more critical HV requirement with a Paschen tight insulation of 21 kV. All current leads shall be designed for low thermal loads, under full current and in standby mode. The paper will give details of the actual design status of these current lead developments for W7-X and JT-60SA based on a common basic design. © 2009 Published by Elsevier B.V.
1. Introduction Current leads are a component of a sc magnet system where currently commercialized HTS material can be used because the external field is usually well below 0.5 T. The potential of HTS current leads can be explained by the ITER HTS current lead demonstrator (HTS-CL, 68 kA) which has been developed and built by the Forschungszentrum Karlsruhe in collaboration with the Centre de Recherches en Physique des Plasmas, Switzerland in the frame of an EU Fusion Technology task [1]. This HTS-CL consists of two main parts, the HTS module that uses BSCCO-2223/AgAu tapes embedded in stainless steel and the copper heat exchanger. The HTS module is cooled by heat conduction from the 4.5 K end and covers the temperature range between 4.5 and 65 K. The heat exchanger covers the temperature range from 65 K to room temperature and is actively cooled by 50 K He gas. At the 4.5 K end, a clamp contact provides the connection to the superconducting bus bar. A picture of the current lead is shown in Fig. 1. The maximum steady state current of this HTS-CL was 80 kA which exceeds the value of 68 kA needed for the ITER TF coils. In addition it was shown that even when the He-cooling has been blocked, a current of 68 kA could be
∗ Corresponding author. Tel.: +49 7247 82 4197; fax: +49 7247 82 7878. E-mail address: walter.fi[email protected] (W.H. Fietz). 0920-3796/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.fusengdes.2008.11.085
carried for more then 6 min which shows the stability of this current lead. Last but not least it could be shown that the He-refrigerator power consumption was reduced by a factor of 5 compared to conventional current leads. Details of the development and test results can be found in [1–4]. This development has brought ITER to the decision to use HTS-CL for the ITER magnet system which can save about 22 kW refrigerator power. The Chinese Domestic Agency has taken the responsibility for the design, construction and testing of all current leads for ITER. In the meantime, other fusion devices under operation or construction like EAST (China), W7-X (Germany) and JT-60SA (Japan) plan to use HTS current leads [5–7]. The Forschungszentrum Karlsruhe has taken over the responsibility for the design, construction and testing of the HTS-CL for two fusion experiments, i.e., the stellarator W7-X and the satellite tokamak JT-60SA. Critical design parameters which influence strongly the performance are: • Cold end contact resistance between the HTS elements and the copper cold end connection which determines the cold end heat load at operation current. • Warm end contact resistance between the HTS elements and the heat exchanger which determines the 50 K He mass flow rate. • Efficiency of the heat exchanger which determines the 50 K He mass flow rate and the length of the heat exchanger.
W.H. Fietz et al. / Fusion Engineering and Design 84 (2009) 776–779
777
Fig. 1. 68 kA HTS current lead developed at the Forschungszentrum Karlsruhe using Bi-2223/AgAu tape.
Fig. 2. View of W7-X from underneath showing the current lead ports.
• Temperature margin of the HTS elements which is related to the current sharing temperature at operation conditions and determines the performance of the current lead in case of Loss of Flow Accident (LOFA) and quench. 2. Current leads for WENDELSTEIN 7-X 2.1. General remarks The stellarator W7-X presently under construction at the Greifswald branch of the Max-Planck-Institute for Plasma Physics consists of 50 non-planar and 20 planar coils with a maximum conductor current of 17.6 kA. The Forschungszentrum Karlsruhe will deliver the current leads for the magnet system. In total 14 current leads are required (maximum design current Imax = 18.2 kA, nominal current Inom = 14 kA) [8]. A view of W7-X from underneath showing the current lead ports is shown in Fig. 2. The main parameters of the current leads are summarized in Table 1. 2.2. General design requirements Four main requirements of the W7-X machine dominate the design of the current leads: (1) Mounting in upside-down position, i.e., the cold end of the current lead is at the top and the warm end at the bottom side. In conventional leads this may cause cooling problems, because of Table 1 Main parameters of W7-X current leads. Parameter
Value
Number of current circuits of non-planar/planar coils Number of current leads Maximum design current Nominal current Maximum voltage to ground (test voltage) Orientation Overall length Maximum diameter Basic operation schedule
5/2 (10 coils in series each) 14 18.2 kA 14 kA ±13 kV Cold end at top 2500 mm 200 mm Steady state for 4–5 days
the large He density gradient between 4.5 K and room temperature (factor 500) can lead to free convection flow in the opposite direction to the forced convection flow. This would increase the He mass flow rate and the necessary cooling power drastically. The use of conduction-cooled HTS material between 4.5 and 60 K has the consequence that the He in the heat exchanger has to cover only the temperature range between 50 K (the inlet temperature) and room temperature. This will drastically reduce the free convection process because the density gradient between 50 K and room temperature is now only 6 instead of 500 for conventional designs using no HTS material. The penalty is the higher cost for the current leads due to the expensive HTS material. (2) The necessity of using low-Co stainless steel material and the limitation of the amount of silver: due to the design of W7-X, the current leads are located inside the biological shield, very close to the magnet system. Therefore, neutron activation has to be considered and as far as possible, Co and Ag should be avoided. However, because Bi-2223 is Ag-sheathed, silver cannot be avoided in the design of the HTS current lead. (3) The location of the current leads very close to the magnets results in a rather high magnetic stray field: due to the strong field dependence of the critical current especially at higher temperatures, a much higher amount of HTS material is needed. (4) Paschen1 tightness and relatively large high test voltage of 13 kV of the current lead and its instrumentation has to be assured. 2.3. Actual design The design of the binary current lead for W7-X follows the same principles as that of the 70 kA ITER demo current lead described above. It consists of a High Temperature Superconductor (HTS) part covering the temperature range between 4.5 and 60 K and a conventional heat exchanger in the range of 60 K to room temperature. The HTS part is cooled by heat conduction from the 4.5 K end whereas the heat exchanger is cooled with 50 K helium. Fig. 3 shows an example of the HTS module carrier made from stainless steel to reduce heat conduction. The HTS material used in the current leads is Bi-2223/AgAu tape. As AMSC has stopped the production of first-generation HTS wires in 2006, a material qualification programme was started and different suppliers have been asked to provide samples of single tapes and stacks soldered from several single tapes for electrical, thermal and mechanical testing. At the end of the qualification programme, Bi-2223/AgAu tapes with critical currents >120 A (at 77 K and in self field) became industrially available. Soldered stacks can also be made in industry [6]. The performance of heat exchangers (HEX) taking into account the natural convection is presently one branch of general investigations in fluid dynamics. Currently, it is hard to predict how large this effect would be in a current lead heat exchanger. Therefore tests with reduced-size HEX were performed that followed a spe-
1 Paschen tightness is a special property in high-voltage insulation systems. The breakthrough voltage, depends strongly on the pressure of the ambient medium. As a consequence an electrical component has to withstand the high voltage independent on the pressure of the surrounded medium, e.g. it has to operate reliably even in the case of a vacuum failure.
778
W.H. Fietz et al. / Fusion Engineering and Design 84 (2009) 776–779 Table 2 Main parameters of JT-60SA current leads. Parameter
TF
CS/EFa
Number of current circuits Number of current leads Maximum current Maximum voltage to ground (test voltage) Orientation
3 6 25.7 kA ±7 kV Vertical
4/6 8/12 20 kA ±21 kV Vertical
a
PF coils are named within the JT-60 project EF (equilibrium field) coils.
Fig. 3. HTS part mock-up of the W7-X HTS current lead developed at the Forschungszentrum Karlsruhe.
cial modified HEX design that was used by CERN for the LHC current leads [10]. The tests demonstrated that the free convection with this special design can be neglected [11]. Therefore this type of heat exchanger has been chosen as the HEX for the W7-X current leads. A picture of the 1:1 mock-up is shown in Fig. 4. The high-voltage requirement of W7-X requested a special design of the electrical insulation system. Earlier designs (e.g., for the ITER demo current lead) used a concentric electrical insulation tube around the pressure vessel of the current lead together with the use of stagnant helium as insulating gas. However, such a solution causes high thermal losses: stagnant helium around the current lead causes thermal acoustic oscillations driven by the heat flow in the room temperature region of the lead. So a different approach was chosen for the current leads currently under design and construction at FZK: the electrical insulation will be directly performed on the outer surface of the current lead pressure vessel by a solid insulation and a G10 flange will be integrated to serve as a connection part to the vacuum vessel of the facility. First tests
Fig. 4. Picture of the 1:1 mock-up of the heat exchanger for the HTS current leads of W7-X.
Fig. 5. Bird’s eye view of the tokamak JT-60SA. The picture was taken from [9].
have shown encouraging high-voltage insulation performance and satisfactory mechanical strength according to the original W7-X requirements. 3. Current leads for JT-60SA 3.1. General remarks In the frame of the Broader Approach Agreement between Japan and the EU and concomitantly to the ITER project, a satellite tokamak project called JT-60SA has been agreed. The magnet system of JT-60SA consists of 18 toroidal field coils (25.7 kA2 ), 4 central solenoid modules (20 kA) and 6 poloidal field coils (20 kA3 ) [7]. Following the commitment of the German Government to the EU, FZK shall design, construct and test the current leads. In total 6 leads for a maximum current of 26 kA and 20 leads with a maximum current of 20 kA (see Table 2), mounted in vertical, upright position are required. The plasma and basic parameters of JT-60SA are defined by Japanese and EU Satellite Tokamak Working Groups considering its mission [9]. A bird’s eye view is shown in Fig. 5. The main parameters of the current leads are summarized in Table 2.
2
Current given for JT-60SA coils follow the present status of the design.
W.H. Fietz et al. / Fusion Engineering and Design 84 (2009) 776–779
3.2. General remarks Again the current leads will be of the binary type, the HTS part covering the range between 4.5 and 60 K while the heat exchanger should cover the range between 60 K and room temperature and be cooled by 50 K He [8]. Part of the results of the qualification programme of the current leads for W7-X can also be used for the leads of JT-60SA. This is especially valid for the HTS material and the electrical insulation. Two prototype leads will be tested in CuLTKa, which is a special test facility currently under construction in FZK using JT-60SArelevant conditions. After the prototype tests 26 current leads will be built in industry with final acceptance test of all leads in CuLTKa. 4. Status of the project Presently, the design of the W7-X current leads is in its final phase. The HTS material was ordered and shall be delivered approximately end of 2008. It is planed to build two prototype current leads in the Forschungszentrum Karlsruhe and test them in a Paschen tight environment in the Current Lead Test Facility Karlsruhe (CuLTKa) which has to be newly built and commissioned. Afterwards, 14 series current leads will be built in industry whereas the final acceptance test of all leads will be done in CuLTKa. Regarding the HTS current leads for JT-60SA, the detailed design will be done after completion of the design of the current leads for W7-X. Two prototype leads will be tested in CuLTKa using JT-60SArelevant conditions. Afterwards 26 current leads will be built in industry whereas the final acceptance test of all leads will be done in CuLTKa. 5. Conclusions The Forschungszentrum Karlsruhe shall deliver current leads for two major fusion experimental devices presently under construc-
779
tion, the stellarator W7-X and the satellite tokamak JT-60SA. After the development and qualification phase ending with prototype construction and testing, the series current leads will be manufactured in industry and finally tested in a new test facility CuLTKa in Karlsruhe. References [1] R. Heller, G. Friesinger, A.M. Fuchs, R. Wesche, Development of high temperature superconductor current leads for 70 kA, IEEE Trans. Appl. Supercond. 12 (1) (2002) 1285–1288. [2] R. Heller, D. Aized, A. Akhmetov, R. Wesche, Design and fabrication of a 70 kA current lead using Ag/Au stabilized Bi-2223 tapes as a demonstrator for the ITER TF-coil system, IEEE Trans. Appl. Supercond. 14 (2) (2004) 1774–1777. [3] R. Heller, S.M. Darweschsad, G. Dittrich, W.H. Fietz, S. Fink, W. Herz, et al., Experimental results of a 70 kA high temperature superconductor current lead demonstrator for the ITER magnet system, IEEE Trans. Appl. Supercond. 15 (1) (2005) 1496–1499. [4] R. Wesche, R. Heller, P. Bruzzone, W.H. Fietz, R. Lietzow, A. Vostner, Design of high temperature superconductor current leads for ITER, Fusion Eng. Des. 82 (2007) 1385–1390. [5] S. Wu, the EAST Team, An overview of the EAST project, Fusion Eng. Des. 82 (2007) 463–471. [6] W.H. Fietz, P. Keller, B. Ringsdorf, S.I. Schlachter, M. Schwarz, et al., Electrical, mechanical and thermal characterization of Bi-2223/AgAu material for use in HTS current leads for W7-X, IEEE Trans. Appl. Supercond. 18 (2) (2008) 1443–1446. [7] A. Pizzuto, P. Bayetti, A. Cucchiaro, P. Decool, A. DellaCorte, A. di Zenobio, et al., JT-60SA toroidal field magnet system, IEEE Trans. Appl. Supercond. 18 (2) (2008) 505–508. [8] Base Document “Design, Construction and Test of the Current Leads for W7-X”, August 2, 2006, unpublished. [9] Conceptual Design Report on JT60-SA in preparation by the JA-EU satellite tokamak program under the Broader Approach (BA) Program and the JAEA’s program for national use. [10] A. Ballarino, S. Mathot, D. Milani, 13000-A HTS current leads for the LHC accelerator: from conceptual design to prototype validation, Proc. of Eucas 2003, Sorrento, Italy, LHC Project Report 696. [11] R. Lietzow, R. Heller, H. Neumann, Performance of heat exchanger models in upside-down orientation fort the use in HTS current leads for W7-X, Adv. Cryog. Eng.: Trans. Cryog. Eng. Conf. - CEC 53 (2008) 1243–1250.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Status of HTS current leads for WENDELSTEIN 7-X and JT-60SA W.H. Fietz ∗ , R. Heller, A. Kienzler, R. Lietzow Forschungszentrum Karlsruhe, Institute for Technical Physics, P.O. Box 3640, D-76021 Karlsruhe, Germany
a r t i c l e
i n f o
Article history: Received 5 August 2008 Received in revised form 22 November 2008 Accepted 28 November 2008 Available online 20 January 2009 Keywords: Current leads High-temperature superconductor Cooling conditions WENDELSTEIN 7-X JT-60SA
a b s t r a c t The stellarator WENDELSTEIN 7-X (W7-X) and the satellite tokamak JT-60SA will use High Temperature Superconductor (HTS) current leads to reduce cryogenic load and operational cost. Forschungszentrum Karlsruhe is in charge of design, construction and test of these HTS current leads. For W7-X fourteen current leads with a nominal current of 14 kA and a maximum current of 18.2 kA are required that are oriented upside down with the room temperature end at the bottom. JT-60SA is constructed in the frame of the Broader Approach Agreement and will use 26 current leads mounted in vertical, normal position with room temperature end at the top. All current leads will consist of an HTS part using Bi-2223 conductor and a classical Cu heat exchanger. The Cu heat exchanger covers the range between 60 K and room temperature and will be cooled with 50 K He while the HTS part is conduction cooled, only. For simplicity a common basic design of the current leads for W7-X and JT-60SA is planned which requests that the current leads work independently of the orientation, and from the experimental boundaries given by W7-X and JT-60SA the current leads have to withstand a loss of He coolant for more than 10 min and allow a discharge of the magnet system although a quench of the HTS part has happened. JT-60SA gives the more critical HV requirement with a Paschen tight insulation of 21 kV. All current leads shall be designed for low thermal loads, under full current and in standby mode. The paper will give details of the actual design status of these current lead developments for W7-X and JT-60SA based on a common basic design. © 2009 Published by Elsevier B.V.
1. Introduction Current leads are a component of a sc magnet system where currently commercialized HTS material can be used because the external field is usually well below 0.5 T. The potential of HTS current leads can be explained by the ITER HTS current lead demonstrator (HTS-CL, 68 kA) which has been developed and built by the Forschungszentrum Karlsruhe in collaboration with the Centre de Recherches en Physique des Plasmas, Switzerland in the frame of an EU Fusion Technology task [1]. This HTS-CL consists of two main parts, the HTS module that uses BSCCO-2223/AgAu tapes embedded in stainless steel and the copper heat exchanger. The HTS module is cooled by heat conduction from the 4.5 K end and covers the temperature range between 4.5 and 65 K. The heat exchanger covers the temperature range from 65 K to room temperature and is actively cooled by 50 K He gas. At the 4.5 K end, a clamp contact provides the connection to the superconducting bus bar. A picture of the current lead is shown in Fig. 1. The maximum steady state current of this HTS-CL was 80 kA which exceeds the value of 68 kA needed for the ITER TF coils. In addition it was shown that even when the He-cooling has been blocked, a current of 68 kA could be
∗ Corresponding author. Tel.: +49 7247 82 4197; fax: +49 7247 82 7878. E-mail address: walter.fi[email protected] (W.H. Fietz). 0920-3796/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.fusengdes.2008.11.085
carried for more then 6 min which shows the stability of this current lead. Last but not least it could be shown that the He-refrigerator power consumption was reduced by a factor of 5 compared to conventional current leads. Details of the development and test results can be found in [1–4]. This development has brought ITER to the decision to use HTS-CL for the ITER magnet system which can save about 22 kW refrigerator power. The Chinese Domestic Agency has taken the responsibility for the design, construction and testing of all current leads for ITER. In the meantime, other fusion devices under operation or construction like EAST (China), W7-X (Germany) and JT-60SA (Japan) plan to use HTS current leads [5–7]. The Forschungszentrum Karlsruhe has taken over the responsibility for the design, construction and testing of the HTS-CL for two fusion experiments, i.e., the stellarator W7-X and the satellite tokamak JT-60SA. Critical design parameters which influence strongly the performance are: • Cold end contact resistance between the HTS elements and the copper cold end connection which determines the cold end heat load at operation current. • Warm end contact resistance between the HTS elements and the heat exchanger which determines the 50 K He mass flow rate. • Efficiency of the heat exchanger which determines the 50 K He mass flow rate and the length of the heat exchanger.
W.H. Fietz et al. / Fusion Engineering and Design 84 (2009) 776–779
777
Fig. 1. 68 kA HTS current lead developed at the Forschungszentrum Karlsruhe using Bi-2223/AgAu tape.
Fig. 2. View of W7-X from underneath showing the current lead ports.
• Temperature margin of the HTS elements which is related to the current sharing temperature at operation conditions and determines the performance of the current lead in case of Loss of Flow Accident (LOFA) and quench. 2. Current leads for WENDELSTEIN 7-X 2.1. General remarks The stellarator W7-X presently under construction at the Greifswald branch of the Max-Planck-Institute for Plasma Physics consists of 50 non-planar and 20 planar coils with a maximum conductor current of 17.6 kA. The Forschungszentrum Karlsruhe will deliver the current leads for the magnet system. In total 14 current leads are required (maximum design current Imax = 18.2 kA, nominal current Inom = 14 kA) [8]. A view of W7-X from underneath showing the current lead ports is shown in Fig. 2. The main parameters of the current leads are summarized in Table 1. 2.2. General design requirements Four main requirements of the W7-X machine dominate the design of the current leads: (1) Mounting in upside-down position, i.e., the cold end of the current lead is at the top and the warm end at the bottom side. In conventional leads this may cause cooling problems, because of Table 1 Main parameters of W7-X current leads. Parameter
Value
Number of current circuits of non-planar/planar coils Number of current leads Maximum design current Nominal current Maximum voltage to ground (test voltage) Orientation Overall length Maximum diameter Basic operation schedule
5/2 (10 coils in series each) 14 18.2 kA 14 kA ±13 kV Cold end at top 2500 mm 200 mm Steady state for 4–5 days
the large He density gradient between 4.5 K and room temperature (factor 500) can lead to free convection flow in the opposite direction to the forced convection flow. This would increase the He mass flow rate and the necessary cooling power drastically. The use of conduction-cooled HTS material between 4.5 and 60 K has the consequence that the He in the heat exchanger has to cover only the temperature range between 50 K (the inlet temperature) and room temperature. This will drastically reduce the free convection process because the density gradient between 50 K and room temperature is now only 6 instead of 500 for conventional designs using no HTS material. The penalty is the higher cost for the current leads due to the expensive HTS material. (2) The necessity of using low-Co stainless steel material and the limitation of the amount of silver: due to the design of W7-X, the current leads are located inside the biological shield, very close to the magnet system. Therefore, neutron activation has to be considered and as far as possible, Co and Ag should be avoided. However, because Bi-2223 is Ag-sheathed, silver cannot be avoided in the design of the HTS current lead. (3) The location of the current leads very close to the magnets results in a rather high magnetic stray field: due to the strong field dependence of the critical current especially at higher temperatures, a much higher amount of HTS material is needed. (4) Paschen1 tightness and relatively large high test voltage of 13 kV of the current lead and its instrumentation has to be assured. 2.3. Actual design The design of the binary current lead for W7-X follows the same principles as that of the 70 kA ITER demo current lead described above. It consists of a High Temperature Superconductor (HTS) part covering the temperature range between 4.5 and 60 K and a conventional heat exchanger in the range of 60 K to room temperature. The HTS part is cooled by heat conduction from the 4.5 K end whereas the heat exchanger is cooled with 50 K helium. Fig. 3 shows an example of the HTS module carrier made from stainless steel to reduce heat conduction. The HTS material used in the current leads is Bi-2223/AgAu tape. As AMSC has stopped the production of first-generation HTS wires in 2006, a material qualification programme was started and different suppliers have been asked to provide samples of single tapes and stacks soldered from several single tapes for electrical, thermal and mechanical testing. At the end of the qualification programme, Bi-2223/AgAu tapes with critical currents >120 A (at 77 K and in self field) became industrially available. Soldered stacks can also be made in industry [6]. The performance of heat exchangers (HEX) taking into account the natural convection is presently one branch of general investigations in fluid dynamics. Currently, it is hard to predict how large this effect would be in a current lead heat exchanger. Therefore tests with reduced-size HEX were performed that followed a spe-
1 Paschen tightness is a special property in high-voltage insulation systems. The breakthrough voltage, depends strongly on the pressure of the ambient medium. As a consequence an electrical component has to withstand the high voltage independent on the pressure of the surrounded medium, e.g. it has to operate reliably even in the case of a vacuum failure.
778
W.H. Fietz et al. / Fusion Engineering and Design 84 (2009) 776–779 Table 2 Main parameters of JT-60SA current leads. Parameter
TF
CS/EFa
Number of current circuits Number of current leads Maximum current Maximum voltage to ground (test voltage) Orientation
3 6 25.7 kA ±7 kV Vertical
4/6 8/12 20 kA ±21 kV Vertical
a
PF coils are named within the JT-60 project EF (equilibrium field) coils.
Fig. 3. HTS part mock-up of the W7-X HTS current lead developed at the Forschungszentrum Karlsruhe.
cial modified HEX design that was used by CERN for the LHC current leads [10]. The tests demonstrated that the free convection with this special design can be neglected [11]. Therefore this type of heat exchanger has been chosen as the HEX for the W7-X current leads. A picture of the 1:1 mock-up is shown in Fig. 4. The high-voltage requirement of W7-X requested a special design of the electrical insulation system. Earlier designs (e.g., for the ITER demo current lead) used a concentric electrical insulation tube around the pressure vessel of the current lead together with the use of stagnant helium as insulating gas. However, such a solution causes high thermal losses: stagnant helium around the current lead causes thermal acoustic oscillations driven by the heat flow in the room temperature region of the lead. So a different approach was chosen for the current leads currently under design and construction at FZK: the electrical insulation will be directly performed on the outer surface of the current lead pressure vessel by a solid insulation and a G10 flange will be integrated to serve as a connection part to the vacuum vessel of the facility. First tests
Fig. 4. Picture of the 1:1 mock-up of the heat exchanger for the HTS current leads of W7-X.
Fig. 5. Bird’s eye view of the tokamak JT-60SA. The picture was taken from [9].
have shown encouraging high-voltage insulation performance and satisfactory mechanical strength according to the original W7-X requirements. 3. Current leads for JT-60SA 3.1. General remarks In the frame of the Broader Approach Agreement between Japan and the EU and concomitantly to the ITER project, a satellite tokamak project called JT-60SA has been agreed. The magnet system of JT-60SA consists of 18 toroidal field coils (25.7 kA2 ), 4 central solenoid modules (20 kA) and 6 poloidal field coils (20 kA3 ) [7]. Following the commitment of the German Government to the EU, FZK shall design, construct and test the current leads. In total 6 leads for a maximum current of 26 kA and 20 leads with a maximum current of 20 kA (see Table 2), mounted in vertical, upright position are required. The plasma and basic parameters of JT-60SA are defined by Japanese and EU Satellite Tokamak Working Groups considering its mission [9]. A bird’s eye view is shown in Fig. 5. The main parameters of the current leads are summarized in Table 2.
2
Current given for JT-60SA coils follow the present status of the design.
W.H. Fietz et al. / Fusion Engineering and Design 84 (2009) 776–779
3.2. General remarks Again the current leads will be of the binary type, the HTS part covering the range between 4.5 and 60 K while the heat exchanger should cover the range between 60 K and room temperature and be cooled by 50 K He [8]. Part of the results of the qualification programme of the current leads for W7-X can also be used for the leads of JT-60SA. This is especially valid for the HTS material and the electrical insulation. Two prototype leads will be tested in CuLTKa, which is a special test facility currently under construction in FZK using JT-60SArelevant conditions. After the prototype tests 26 current leads will be built in industry with final acceptance test of all leads in CuLTKa. 4. Status of the project Presently, the design of the W7-X current leads is in its final phase. The HTS material was ordered and shall be delivered approximately end of 2008. It is planed to build two prototype current leads in the Forschungszentrum Karlsruhe and test them in a Paschen tight environment in the Current Lead Test Facility Karlsruhe (CuLTKa) which has to be newly built and commissioned. Afterwards, 14 series current leads will be built in industry whereas the final acceptance test of all leads will be done in CuLTKa. Regarding the HTS current leads for JT-60SA, the detailed design will be done after completion of the design of the current leads for W7-X. Two prototype leads will be tested in CuLTKa using JT-60SArelevant conditions. Afterwards 26 current leads will be built in industry whereas the final acceptance test of all leads will be done in CuLTKa. 5. Conclusions The Forschungszentrum Karlsruhe shall deliver current leads for two major fusion experimental devices presently under construc-
779
tion, the stellarator W7-X and the satellite tokamak JT-60SA. After the development and qualification phase ending with prototype construction and testing, the series current leads will be manufactured in industry and finally tested in a new test facility CuLTKa in Karlsruhe. References [1] R. Heller, G. Friesinger, A.M. Fuchs, R. Wesche, Development of high temperature superconductor current leads for 70 kA, IEEE Trans. Appl. Supercond. 12 (1) (2002) 1285–1288. [2] R. Heller, D. Aized, A. Akhmetov, R. Wesche, Design and fabrication of a 70 kA current lead using Ag/Au stabilized Bi-2223 tapes as a demonstrator for the ITER TF-coil system, IEEE Trans. Appl. Supercond. 14 (2) (2004) 1774–1777. [3] R. Heller, S.M. Darweschsad, G. Dittrich, W.H. Fietz, S. Fink, W. Herz, et al., Experimental results of a 70 kA high temperature superconductor current lead demonstrator for the ITER magnet system, IEEE Trans. Appl. Supercond. 15 (1) (2005) 1496–1499. [4] R. Wesche, R. Heller, P. Bruzzone, W.H. Fietz, R. Lietzow, A. Vostner, Design of high temperature superconductor current leads for ITER, Fusion Eng. Des. 82 (2007) 1385–1390. [5] S. Wu, the EAST Team, An overview of the EAST project, Fusion Eng. Des. 82 (2007) 463–471. [6] W.H. Fietz, P. Keller, B. Ringsdorf, S.I. Schlachter, M. Schwarz, et al., Electrical, mechanical and thermal characterization of Bi-2223/AgAu material for use in HTS current leads for W7-X, IEEE Trans. Appl. Supercond. 18 (2) (2008) 1443–1446. [7] A. Pizzuto, P. Bayetti, A. Cucchiaro, P. Decool, A. DellaCorte, A. di Zenobio, et al., JT-60SA toroidal field magnet system, IEEE Trans. Appl. Supercond. 18 (2) (2008) 505–508. [8] Base Document “Design, Construction and Test of the Current Leads for W7-X”, August 2, 2006, unpublished. [9] Conceptual Design Report on JT60-SA in preparation by the JA-EU satellite tokamak program under the Broader Approach (BA) Program and the JAEA’s program for national use. [10] A. Ballarino, S. Mathot, D. Milani, 13000-A HTS current leads for the LHC accelerator: from conceptual design to prototype validation, Proc. of Eucas 2003, Sorrento, Italy, LHC Project Report 696. [11] R. Lietzow, R. Heller, H. Neumann, Performance of heat exchanger models in upside-down orientation fort the use in HTS current leads for W7-X, Adv. Cryog. Eng.: Trans. Cryog. Eng. Conf. - CEC 53 (2008) 1243–1250.