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High power high temperature superconductor current leads at KIT
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High power high temperature superconductor current leads at KIT R. Heller ∗ , W.H. Fietz, A. Kienzler Karlsruhe Institute of Technology, Institute for Technical Physics, Karlsruhe, Germany
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Article history: Received 26 February 2016 Received in revised form 4 February 2017 Accepted 11 April 2017 Available online xxx Keywords: Current leads Fusion magnets High temperature superconductor
a b s t r a c t Many large fusion devices presently under construction or in operation who comprise superconducting magnets like EAST, Wendelstein 7-X (W7-X), JT-60SA and ITER use High Temperature Superconductor (HTS) current leads to reduce the cryogenic load and operational cost. The Karlsruhe Institute of Technology (KIT) designed, constructed and successfully tested a 68 kA HTS current lead demonstrator for ITER which led to the decision by ITER IO to use HTS current leads for the ITER magnet system. KIT has designed, constructed and tested the HTS current leads for W7-X, which are operational since 2015, and presently manufactures and tests also the HTS current leads for JT-60SA. All these current leads consist of a meander-flow type heat exchanger (HX) which is cooled by 50 K helium and an HTS section. In all cases the first generation HTS material BSCCO is used embedded in a low conductivity matrix of AgAu. The technology developed in the past 20 years is now mature and would be able to be used for future fusion reactors like DEMO. In the meantime industry worldwide – with the exception of one company in Japan and another one in China – does not produce BSCCO tapes anymore; the second generation HTS REBCO is now used for application because of the better field performance in particular at high temperature. As the new material can only be produced in a multilayer structure rather than as a multifilamentary tape, the technology developed for BSCCO cannot be directly transferred to REBCO. Therefore several laboratories, in particular in Europe, are presently investigating how an HTS current lead made with REBCO could look like. This development will be the prerequisite for the construction of current leads for future fusion reactors as they have to handle high currents in the range of up to 100 kA. The article discusses the status of the current lead development at KIT. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of high temperature superconductivity (HTS) in 1986 there has been a tremendous progress in R&D of HTS material, wires and applications. Especially for current lead applications HTS offers considerable economic benefits because the external field is usually well below 0.5 T allowing an operating temperature not much lower than the critical temperature of the HTS material. Superconducting magnets cooled by 4.5 K supercritical helium require a metallic connection to the power supplies located at room temperature. To limit the cooling power at 4.5 K the current leads consist of resistive HX which are actively cooled by 4.5 K helium. In general the conventional current leads are the main consumer of cooling power. Replacing their HX in the temperature range between 4.5 K and an intermediate temperature T100%HTS by HTS materials the 4.5 K heat load caused by the current leads can in
∗ Corresponding author. E-mail address: [email protected] (R. Heller).
principle be reduced by more than one order of magnitude, thanks to the low thermal conductivity and the absence of resistive losses of the HTS material. Taking into account the cooling power needed for the HX part of the HTS current lead, the overall refrigerator cooling power consumption is about a factor of 3–4 lower than that required for conventional current leads. Fig. 1 shows the basic principles of both the conventional and the HTS current lead. While HTS current leads with a current capacity of some 100 A are nowadays commonly used for powering magnet systems manufactured in industry there are other applications which deal with high current capacity in the range of 10–80 kA. Since there is no market in this area, the interest of industry for R&D is limited. The R&D for high current capacity current leads focuses mainly on two research areas namely accelerator and fusion technology. Because of the large stored magnetic energy and limitation on the electrical voltage applied during safety discharge in case of a quench of the magnet system the magnets are usually designed to have a low inductance leading to high conductor currents in the range of 10–80 kA for the desired fields. For this application, current leads for steady state or transient operation with low ramp rates
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the amount of HTS material significantly especially if a significant magnetic stray field is present. Today, most of the HTS current leads for high current capacities in operation or under construction use Bi-2223/AgAu tapes as basic component. Due to the strong and anisotropic dependence of the critical current of Bi-2223 as a function of magnetic field the HTS part has a cylindrical shape where on its outer surface the HTS tapes are located such to minimize the magnetic field perpendicular to the broad face of the tapes. Fig. 2 (left) shows the dependence of the normalized critical current as a function of the applied magnetic field parallel and perpendicular to the tape for different temperatures. Fig. 2 (right) shows the magnetic field distribution of the HTS part and a schematic of the cylindrical design of the HTS part including the stack design. Critical design parameters which influence strongly the performance are:
Fig. 1. Basic principles of (a) a conventional and (b) a HTS current lead. Pc is the cooling power at a certain temperature level and Q is the heat flow to the coil.
are needed. Two large international projects, i.e., the Large Hadron Collider LHC at CERN [1] and the International Thermonuclear Experimental Reactor ITER [2] have stimulated the superconductor community to investigate in the R&D of such HTS current leads. Already in the early 1990s and parallel to the intensive CERN activities for the Large Hadron Collider (LHC), KIT and the Swiss Plasma Center, SPC (former CRPP) Switzerland started a development program for a HTS-CL in the frame of the EU fusion technology program which ended with the construction and successful testing of the 68 kA ITER HTS current lead demonstrator [3–10]. The development and successful test of the 68 kA ITER HTS-CL demonstrator has brought ITER to the decision to use HTS-CL for the ITER magnet system which can save about 22 kW cold refrigerator power. Other fusion devices in operation like the EAST tokamak (China) and the stellarator Wendelstein 7-X, W7-X (Germany), or under construction like the satellite tokamak JT-60SA (Japan) and ITER use or intend to use HTS-CL. The Institute of Plasma Physics of the Chinese Academy of Sciences, ASIPP, has constructed LN2 cooled HTS-CL for EAST [11]. KIT designed, built and tested the HTS-CLs for W7-X [12] and is now responsible for the design, construction and testing of the HTS-CLs for JT-60SA [13].
2. State-of-the-art design of HTS current leads The general design of HTS current leads (HTS-CL) follows some basic principles which are determined by the cooling power optimization and by the electromagnetic properties of the HTS material. They consist of a HTS part covering the temperature range between 4.5 K and an intermediate temperature T100%HTS – mostly at about 60 to 65 K – and a conventional HX in the range of T100%HTS to room temperature. The HTS part is mostly cooled by heat conduction from the 4.5 K end whereas the HX is cooled with helium at approximately 50 K. The selected helium inlet temperature and the intermediate temperature T100%HTS are the result of a thermodynamic optimization taking into account the thermodynamic efficiency at 4.5 K and the Exergy efficiency at different temperatures. The result is an optimum He inlet temperature of 50 K and an optimum temperature T100%HTS of approximately 70 K [10]. Due to the broad minimum the selected temperature T100%HTS is commonly lower than 70 K, i.e., 65 K or even 60 K because this reduces
1. Contact resistances between the HTS modules and the cold end joint and between the HTS modules and the HX which determine the 4.5 K heat load as well as the 50 K He mass flow rate. 2. Efficiency of the HX which determines the 50 K He mass flow rate and the length of the HX. 3. Room temperature (RT) termination acting as the interface to the RT bus bar connection to the power supply which influences the performance of the HX. 4. Temperature margin of the HTS modules which is related to the current sharing temperature at operation conditions. It determines the performance of the current lead in case of a loss of the 50 K He flow (LOFA) and of the quench as well. 5. Paschen-tight high voltage insulation with integrated mechanical connection to the cryostat. To simplify the electrical insulation and to provide space for the instrumentation wiring routed to the RT-termination and avoiding additional HV feedthroughs at the cryostat side, a separated insulation vacuum around the current lead is advantageous. In addition, a bellow at the room temperature side of the vacuum tube is required to compensate for differential shrinkage between the stainless steel tube and the copper HX during cool down.
3. HTS current lead development at KIT 3.1. EU development program toward ITER In the middle of the 1990s an HTS current lead development program was initiated by EFDA within the frame of the EU Fusion Technology Program. Starting with a material selection phase, KIT, Germany, in collaboration with the SPC designed, fabricated and successfully tested a 10 kA HTS current lead followed by a 20 kA [14]. Afterwards a 68 kA HTS-CL demonstrator for ITER was developed and built. The design followed the design concepts presented in Section 2. The HTS module was constructed by American Superconductor Corp., AMSC. Here HTS tapes were sintered together to form stacks which were grouped together to so-called panels. A picture of the 68 kA ITER HTS-CL demonstrator and of the HTS module including instrumentation is shown in Fig. 3. The maximum steady state current of the 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 50 K He-cooling has been blocked, a current of 68 kA could be carried for more than 5.5 min which shows the stability of this current lead. During the different tests, the HTS-CL was cooled with helium at an inlet temperature of 50 K, 80 K and also with LN2 at atmospheric und sub-atmospheric pressure. Details of the development and test results can be found in [9,10,15,17–19].
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Fig. 2. Left: Typical dependence of the normalized critical current as a function of the applied magnetic field (parallel and perpendicular) for different temperatures. Center: Schematic of the cylindrical design of the HTS part.
Fig. 3. Left: 68 kA HTS current lead developed at KIT using Bi-2223/AgAu tapes, Right: HTS-module.
In a detailed study the benefits of the use of HTS current leads in ITER has been investigated. The results showed that more than 1.5 MW electrical powers can be saved if using HTS current leads [20]. The results of the study together with the good results of the 68 kA ITER HTS current lead demonstrator led to the decision by the ITER project to use HTS current leads for the ITER magnet system. 3.2. HTS current leads for W7-X The magnet system of the stellarator W7-X at the Greifswald branch of the Max-Planck-Institut für Plasmaphysik consists of 50 non-planar and 20 planar coils with a maximum conductor current of 17.6 kA. In total 14 current leads are required (nominal current Inom = 14 kA, maximum current Imax = 18.2 kA) [16]. Main requirements of the W7-X machine which dominate the design of the current leads are: 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. 2. The necessity of using low-Co stainless steel material and the limitation of the amount of silver to reduce activation by neutrons. 3. The location of the current leads very close to the magnets resulting in a rather high magnetic stray field. 4. Paschen tightness of the electrical insulation and test voltage of 13 kV of the current lead and its instrumentation has to be assured. The design of the current leads follows the design of the 68 kA ITER demonstrator current lead. It consists of a HTS part covering the temperature range between 4.5 and 60 K and a fin-type HX in the range of 60 K to room temperature. A temperature of 60 K has been chosen to limit the amount of HTS material because of the rather large stray field of 100 mT. The HTS part is cooled by heat conduction from the 4.5 K end whereas the HX is cooled with 50 K helium. The HTS material used in the current leads is Bi-2223/AgAu tape. Tapes with critical currents >110 A (at 77 K and in self-field) and sufficient mechanical strength and soldered to stacks were pro-
vided by Bruker [20,21]. Fig. 4 (left) shows the gold plated stainless steel carrier with Cu end caps where grooves are milled on the outer surface in which the HTS stacks are soldered. The design of the HX follows the design principle used by CERN for the LHC current leads [3] with some modifications. The high-voltage requirement of W7-X requested a special design of the electrical insulation system. In the KIT design, the electrical insulation is directly performed on the outer surface of the current lead vacuum vessel by a solid insulation and a G10 flange is integrated to serve as a connection part to the vacuum vessel of the cryostat. Fig. 4 (right) shows a picture of the W7-X prototype current lead after assembly and before installation into the cryostat. The acceptance tests performed so far demonstrate the operational reliability of the series CLs for W7-X. All CLs fulfilled the acceptance test requirements and the important parameters for the operation in the stellarator were identified. The heat load and He mass flow statistics provide important data for the operation of the CLs in the cooling system environment of W7-X. The measured LOFA time of the CLs of about 17 min is excellent with a sufficient time margin in case of a cooling system failure. The long-time operation showed that the performance of the CLs is highly stable and reliable. The calculated correlations between pressure drop and He inlet pressure and also between T100%HTS and THe,in could be confirmed by the experimental results. Although the acceptance test of the series CLs showed that the performance of the leads is very similar, a cold test is mandatory [22,23].
3.3. HTS current leads for JT-60SA 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 was agreed. The magnet system of JT-60SA consists of 18 toroidal field coils (25.7 kA), 4 central solenoid modules (20 kA) and 6 poloidal field coils (20 kA) [13]. Following the commitment of the German Government to the EU and described in the bilateral procurement arrangement between Japan and EU and the agreement of understanding between EU and
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Fig. 4. Left: HTS module of the W7-X current leads. Right: W7-X HTS current lead.
KIT, KIT is responsible for design, constructing and testing the current leads. In total six leads for a maximum current of 26 kA and 20 leads with a maximum current of 20 kA, mounted in vertical, upright position are required. Since the current leads for JT-60SA are located outside the biological shield, no special needs regarding material are required. The design of the HTS current leads for JT-60SA is as close as possible to the design of the current leads for W7-X. Again, 60 stacks made of 6 tapes and grouped in 12 quintuplets are used for a maximum current of 20 kA in a maximum stray field of 33 mT (CS/EF coils of JT-60SA). This design was adapted to the boundary requirements of the JT-60SA TF coils (25.7 kA) by increasing the stack number from 60 to 80 [16]. The PF current leads will supply the CS and PF coils of JT-60SA which operate in pulsed mode, a dedicated test was performed using the prototype HTS current leads of W7-X by applying triangular and trapezoidal pulses with ramp rates of 1 kA/s and 10 kA/s. AC losses were evaluated and they result in negligible losses averaged over the pulse duration in the order of mW [24]. The status of the project is that all TF current leads are manufactured, tested and delivered to QST. The PF current leads are under fabrication; 14 pairs are manufactured, 12 of them are tested and 10 are delivered to QST. 4. Current lead test facility CuLTKa For the series test of the JT-60SA current leads a dedicated test facility was built at KIT. The main goal was to perform an economic testing of the current leads. CuLTKa is integrated in the cryogenic infrastructure of the institute and directly connected to the 2 kW refrigerator. Therefore the facility was equipped with two test cryostats allowing the simultaneous cool-down and warm-up of four current leads. Beside these two test cryostats CuLTKa consists of two valve boxes and one control box with transfer lines connecting the boxes together. The signal and data acquisition systems are constructed for a HV insulation of 50 kV. The current supply is able to provide up to 50 kA, and an upgrade up to 80 kA is possible. More details can be found in [25]. 5. R&D towards an HTS current lead using REBCO material All HTS current leads under operation or under fabrication so far use BSCCO material. Industry has moved in the meantime from BSCCO to REBCO because of the better performance of this material under high magnetic fields. As a (commercial) consequence, high power HTS current leads needed for future machines e.g., for DEMO [26] will require REBCO-modules. Unfortunately the different architecture of BSCCO and REBCO tapes prohibit a direct transfer of the HTS module design: due to the layered structure of the REBCO tape a simple stacking as done for BSCCO is not possible. In addition a dedicated stabilizer material is needed to match the needs for resistance, thermal conductivity and heat capacity. KIT has started an R&D program – as a backup to the BSCCO design used in the JT-60SA current leads – to construct a 20 kA HTS current lead demonstrator using REBCO as this allows a scale-up to currents in
the 50–100 kA range required for DEMO [27]. The selection of the stabilizer material led to the decision to choose brass [28]. This choice has been already made in other projects, for example for the construction of the HTS adapter for the EDIPO text facility at SPC [29]. At present the design and fabrication of HTS stacks made of REBCO material is under investigation. KIT plans to manufacture and test a 20 kA REBCO current lead demonstrator in 2017. 6. Summary High power HTS current leads are nowadays a commonly used component in magnet systems. BSCCO material (first generation HTS) is used in all HTS current leads of large devices (LHC, EAST, W7-X, JT-60SA and ITER). KIT has designed, built and tested 68 kA HTS current lead demonstrator for ITER (maximum steady state operation current was 80 kA). It has also designed, built and tested HTS current leads for Wendelstein 7-X. KIT is at present building and testing HTS current leads for the satellite tokamak JT-60SA. For this the current lead test facility CuLTKa was built to provide a fast and economic testing. CuLTKa is principally able to be upgraded for testing components up to 80 kA. Now BSCCO material is being replaced by REBCO whose architecture is different. HTS current leads for DEMO will require application of REBCO. KIT is working on a 20 kA HTS current lead demonstrator using REBCO. As stabilizer material brass was chosen because of its low thermal conductivity and acceptable electrical resistivity. At present the preparation of the REBCO stacks is under investigation. It is planned to complete the fabrication of the REBCO current lead and to perform a detailed test in 2017. Acknowledgment The authors appreciate the effort of the main workshop of KIT and the current lead assembly crew as well as the refrigerator and testing group in ITEP. The work was performed within the European Fusion Technology Program of KIT. The work for W7X was performed in addition under contract 03/100/4500149540 between the Max-Planck-Institut für Plasmaphysik and KIT. The work related to JT-60SA is financially supported by the German Ministry of Research and Education under the grant No. 03FUS0013 and 03FUS0019 and is done in the Project JT-60SA under the Broader Approach Agreement between Europe and Japan. The views and opinions expressed herein do not reflect necessarily those of the BMBF or the European Commission. References [1] “The Large Hadron Collider; a marvel of technology”. Ed. Lyndon Evans, EPFL Press, CRC Press, 2009. [2] N. Holtkamp, An overview of the ITER project, Fusion Eng. Des. 82 (2007) 427–434. [3] R. Heller, G. Friesinger, A.M. Fuchs, T. Mito, S. Satoh, K. Takahata, M. Tasca, M. Vogel, Development of a 20 kA high temperature superconductor current lead, Cryogenics 41 (2001) 539–547. [4] R. Heller, Wissenschaftliche Berichte Kernforschungszentrum Karlsruhe (1993) KfK–KfK5172.
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High power high temperature superconductor current leads at KIT R. Heller ∗ , W.H. Fietz, A. Kienzler Karlsruhe Institute of Technology, Institute for Technical Physics, Karlsruhe, Germany
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Article history: Received 26 February 2016 Received in revised form 4 February 2017 Accepted 11 April 2017 Available online xxx Keywords: Current leads Fusion magnets High temperature superconductor
a b s t r a c t Many large fusion devices presently under construction or in operation who comprise superconducting magnets like EAST, Wendelstein 7-X (W7-X), JT-60SA and ITER use High Temperature Superconductor (HTS) current leads to reduce the cryogenic load and operational cost. The Karlsruhe Institute of Technology (KIT) designed, constructed and successfully tested a 68 kA HTS current lead demonstrator for ITER which led to the decision by ITER IO to use HTS current leads for the ITER magnet system. KIT has designed, constructed and tested the HTS current leads for W7-X, which are operational since 2015, and presently manufactures and tests also the HTS current leads for JT-60SA. All these current leads consist of a meander-flow type heat exchanger (HX) which is cooled by 50 K helium and an HTS section. In all cases the first generation HTS material BSCCO is used embedded in a low conductivity matrix of AgAu. The technology developed in the past 20 years is now mature and would be able to be used for future fusion reactors like DEMO. In the meantime industry worldwide – with the exception of one company in Japan and another one in China – does not produce BSCCO tapes anymore; the second generation HTS REBCO is now used for application because of the better field performance in particular at high temperature. As the new material can only be produced in a multilayer structure rather than as a multifilamentary tape, the technology developed for BSCCO cannot be directly transferred to REBCO. Therefore several laboratories, in particular in Europe, are presently investigating how an HTS current lead made with REBCO could look like. This development will be the prerequisite for the construction of current leads for future fusion reactors as they have to handle high currents in the range of up to 100 kA. The article discusses the status of the current lead development at KIT. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of high temperature superconductivity (HTS) in 1986 there has been a tremendous progress in R&D of HTS material, wires and applications. Especially for current lead applications HTS offers considerable economic benefits because the external field is usually well below 0.5 T allowing an operating temperature not much lower than the critical temperature of the HTS material. Superconducting magnets cooled by 4.5 K supercritical helium require a metallic connection to the power supplies located at room temperature. To limit the cooling power at 4.5 K the current leads consist of resistive HX which are actively cooled by 4.5 K helium. In general the conventional current leads are the main consumer of cooling power. Replacing their HX in the temperature range between 4.5 K and an intermediate temperature T100%HTS by HTS materials the 4.5 K heat load caused by the current leads can in
∗ Corresponding author. E-mail address: [email protected] (R. Heller).
principle be reduced by more than one order of magnitude, thanks to the low thermal conductivity and the absence of resistive losses of the HTS material. Taking into account the cooling power needed for the HX part of the HTS current lead, the overall refrigerator cooling power consumption is about a factor of 3–4 lower than that required for conventional current leads. Fig. 1 shows the basic principles of both the conventional and the HTS current lead. While HTS current leads with a current capacity of some 100 A are nowadays commonly used for powering magnet systems manufactured in industry there are other applications which deal with high current capacity in the range of 10–80 kA. Since there is no market in this area, the interest of industry for R&D is limited. The R&D for high current capacity current leads focuses mainly on two research areas namely accelerator and fusion technology. Because of the large stored magnetic energy and limitation on the electrical voltage applied during safety discharge in case of a quench of the magnet system the magnets are usually designed to have a low inductance leading to high conductor currents in the range of 10–80 kA for the desired fields. For this application, current leads for steady state or transient operation with low ramp rates
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the amount of HTS material significantly especially if a significant magnetic stray field is present. Today, most of the HTS current leads for high current capacities in operation or under construction use Bi-2223/AgAu tapes as basic component. Due to the strong and anisotropic dependence of the critical current of Bi-2223 as a function of magnetic field the HTS part has a cylindrical shape where on its outer surface the HTS tapes are located such to minimize the magnetic field perpendicular to the broad face of the tapes. Fig. 2 (left) shows the dependence of the normalized critical current as a function of the applied magnetic field parallel and perpendicular to the tape for different temperatures. Fig. 2 (right) shows the magnetic field distribution of the HTS part and a schematic of the cylindrical design of the HTS part including the stack design. Critical design parameters which influence strongly the performance are:
Fig. 1. Basic principles of (a) a conventional and (b) a HTS current lead. Pc is the cooling power at a certain temperature level and Q is the heat flow to the coil.
are needed. Two large international projects, i.e., the Large Hadron Collider LHC at CERN [1] and the International Thermonuclear Experimental Reactor ITER [2] have stimulated the superconductor community to investigate in the R&D of such HTS current leads. Already in the early 1990s and parallel to the intensive CERN activities for the Large Hadron Collider (LHC), KIT and the Swiss Plasma Center, SPC (former CRPP) Switzerland started a development program for a HTS-CL in the frame of the EU fusion technology program which ended with the construction and successful testing of the 68 kA ITER HTS current lead demonstrator [3–10]. The development and successful test of the 68 kA ITER HTS-CL demonstrator has brought ITER to the decision to use HTS-CL for the ITER magnet system which can save about 22 kW cold refrigerator power. Other fusion devices in operation like the EAST tokamak (China) and the stellarator Wendelstein 7-X, W7-X (Germany), or under construction like the satellite tokamak JT-60SA (Japan) and ITER use or intend to use HTS-CL. The Institute of Plasma Physics of the Chinese Academy of Sciences, ASIPP, has constructed LN2 cooled HTS-CL for EAST [11]. KIT designed, built and tested the HTS-CLs for W7-X [12] and is now responsible for the design, construction and testing of the HTS-CLs for JT-60SA [13].
2. State-of-the-art design of HTS current leads The general design of HTS current leads (HTS-CL) follows some basic principles which are determined by the cooling power optimization and by the electromagnetic properties of the HTS material. They consist of a HTS part covering the temperature range between 4.5 K and an intermediate temperature T100%HTS – mostly at about 60 to 65 K – and a conventional HX in the range of T100%HTS to room temperature. The HTS part is mostly cooled by heat conduction from the 4.5 K end whereas the HX is cooled with helium at approximately 50 K. The selected helium inlet temperature and the intermediate temperature T100%HTS are the result of a thermodynamic optimization taking into account the thermodynamic efficiency at 4.5 K and the Exergy efficiency at different temperatures. The result is an optimum He inlet temperature of 50 K and an optimum temperature T100%HTS of approximately 70 K [10]. Due to the broad minimum the selected temperature T100%HTS is commonly lower than 70 K, i.e., 65 K or even 60 K because this reduces
1. Contact resistances between the HTS modules and the cold end joint and between the HTS modules and the HX which determine the 4.5 K heat load as well as the 50 K He mass flow rate. 2. Efficiency of the HX which determines the 50 K He mass flow rate and the length of the HX. 3. Room temperature (RT) termination acting as the interface to the RT bus bar connection to the power supply which influences the performance of the HX. 4. Temperature margin of the HTS modules which is related to the current sharing temperature at operation conditions. It determines the performance of the current lead in case of a loss of the 50 K He flow (LOFA) and of the quench as well. 5. Paschen-tight high voltage insulation with integrated mechanical connection to the cryostat. To simplify the electrical insulation and to provide space for the instrumentation wiring routed to the RT-termination and avoiding additional HV feedthroughs at the cryostat side, a separated insulation vacuum around the current lead is advantageous. In addition, a bellow at the room temperature side of the vacuum tube is required to compensate for differential shrinkage between the stainless steel tube and the copper HX during cool down.
3. HTS current lead development at KIT 3.1. EU development program toward ITER In the middle of the 1990s an HTS current lead development program was initiated by EFDA within the frame of the EU Fusion Technology Program. Starting with a material selection phase, KIT, Germany, in collaboration with the SPC designed, fabricated and successfully tested a 10 kA HTS current lead followed by a 20 kA [14]. Afterwards a 68 kA HTS-CL demonstrator for ITER was developed and built. The design followed the design concepts presented in Section 2. The HTS module was constructed by American Superconductor Corp., AMSC. Here HTS tapes were sintered together to form stacks which were grouped together to so-called panels. A picture of the 68 kA ITER HTS-CL demonstrator and of the HTS module including instrumentation is shown in Fig. 3. The maximum steady state current of the 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 50 K He-cooling has been blocked, a current of 68 kA could be carried for more than 5.5 min which shows the stability of this current lead. During the different tests, the HTS-CL was cooled with helium at an inlet temperature of 50 K, 80 K and also with LN2 at atmospheric und sub-atmospheric pressure. Details of the development and test results can be found in [9,10,15,17–19].
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Fig. 2. Left: Typical dependence of the normalized critical current as a function of the applied magnetic field (parallel and perpendicular) for different temperatures. Center: Schematic of the cylindrical design of the HTS part.
Fig. 3. Left: 68 kA HTS current lead developed at KIT using Bi-2223/AgAu tapes, Right: HTS-module.
In a detailed study the benefits of the use of HTS current leads in ITER has been investigated. The results showed that more than 1.5 MW electrical powers can be saved if using HTS current leads [20]. The results of the study together with the good results of the 68 kA ITER HTS current lead demonstrator led to the decision by the ITER project to use HTS current leads for the ITER magnet system. 3.2. HTS current leads for W7-X The magnet system of the stellarator W7-X at the Greifswald branch of the Max-Planck-Institut für Plasmaphysik consists of 50 non-planar and 20 planar coils with a maximum conductor current of 17.6 kA. In total 14 current leads are required (nominal current Inom = 14 kA, maximum current Imax = 18.2 kA) [16]. Main requirements of the W7-X machine which dominate the design of the current leads are: 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. 2. The necessity of using low-Co stainless steel material and the limitation of the amount of silver to reduce activation by neutrons. 3. The location of the current leads very close to the magnets resulting in a rather high magnetic stray field. 4. Paschen tightness of the electrical insulation and test voltage of 13 kV of the current lead and its instrumentation has to be assured. The design of the current leads follows the design of the 68 kA ITER demonstrator current lead. It consists of a HTS part covering the temperature range between 4.5 and 60 K and a fin-type HX in the range of 60 K to room temperature. A temperature of 60 K has been chosen to limit the amount of HTS material because of the rather large stray field of 100 mT. The HTS part is cooled by heat conduction from the 4.5 K end whereas the HX is cooled with 50 K helium. The HTS material used in the current leads is Bi-2223/AgAu tape. Tapes with critical currents >110 A (at 77 K and in self-field) and sufficient mechanical strength and soldered to stacks were pro-
vided by Bruker [20,21]. Fig. 4 (left) shows the gold plated stainless steel carrier with Cu end caps where grooves are milled on the outer surface in which the HTS stacks are soldered. The design of the HX follows the design principle used by CERN for the LHC current leads [3] with some modifications. The high-voltage requirement of W7-X requested a special design of the electrical insulation system. In the KIT design, the electrical insulation is directly performed on the outer surface of the current lead vacuum vessel by a solid insulation and a G10 flange is integrated to serve as a connection part to the vacuum vessel of the cryostat. Fig. 4 (right) shows a picture of the W7-X prototype current lead after assembly and before installation into the cryostat. The acceptance tests performed so far demonstrate the operational reliability of the series CLs for W7-X. All CLs fulfilled the acceptance test requirements and the important parameters for the operation in the stellarator were identified. The heat load and He mass flow statistics provide important data for the operation of the CLs in the cooling system environment of W7-X. The measured LOFA time of the CLs of about 17 min is excellent with a sufficient time margin in case of a cooling system failure. The long-time operation showed that the performance of the CLs is highly stable and reliable. The calculated correlations between pressure drop and He inlet pressure and also between T100%HTS and THe,in could be confirmed by the experimental results. Although the acceptance test of the series CLs showed that the performance of the leads is very similar, a cold test is mandatory [22,23].
3.3. HTS current leads for JT-60SA 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 was agreed. The magnet system of JT-60SA consists of 18 toroidal field coils (25.7 kA), 4 central solenoid modules (20 kA) and 6 poloidal field coils (20 kA) [13]. Following the commitment of the German Government to the EU and described in the bilateral procurement arrangement between Japan and EU and the agreement of understanding between EU and
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Fig. 4. Left: HTS module of the W7-X current leads. Right: W7-X HTS current lead.
KIT, KIT is responsible for design, constructing and testing the current leads. In total six leads for a maximum current of 26 kA and 20 leads with a maximum current of 20 kA, mounted in vertical, upright position are required. Since the current leads for JT-60SA are located outside the biological shield, no special needs regarding material are required. The design of the HTS current leads for JT-60SA is as close as possible to the design of the current leads for W7-X. Again, 60 stacks made of 6 tapes and grouped in 12 quintuplets are used for a maximum current of 20 kA in a maximum stray field of 33 mT (CS/EF coils of JT-60SA). This design was adapted to the boundary requirements of the JT-60SA TF coils (25.7 kA) by increasing the stack number from 60 to 80 [16]. The PF current leads will supply the CS and PF coils of JT-60SA which operate in pulsed mode, a dedicated test was performed using the prototype HTS current leads of W7-X by applying triangular and trapezoidal pulses with ramp rates of 1 kA/s and 10 kA/s. AC losses were evaluated and they result in negligible losses averaged over the pulse duration in the order of mW [24]. The status of the project is that all TF current leads are manufactured, tested and delivered to QST. The PF current leads are under fabrication; 14 pairs are manufactured, 12 of them are tested and 10 are delivered to QST. 4. Current lead test facility CuLTKa For the series test of the JT-60SA current leads a dedicated test facility was built at KIT. The main goal was to perform an economic testing of the current leads. CuLTKa is integrated in the cryogenic infrastructure of the institute and directly connected to the 2 kW refrigerator. Therefore the facility was equipped with two test cryostats allowing the simultaneous cool-down and warm-up of four current leads. Beside these two test cryostats CuLTKa consists of two valve boxes and one control box with transfer lines connecting the boxes together. The signal and data acquisition systems are constructed for a HV insulation of 50 kV. The current supply is able to provide up to 50 kA, and an upgrade up to 80 kA is possible. More details can be found in [25]. 5. R&D towards an HTS current lead using REBCO material All HTS current leads under operation or under fabrication so far use BSCCO material. Industry has moved in the meantime from BSCCO to REBCO because of the better performance of this material under high magnetic fields. As a (commercial) consequence, high power HTS current leads needed for future machines e.g., for DEMO [26] will require REBCO-modules. Unfortunately the different architecture of BSCCO and REBCO tapes prohibit a direct transfer of the HTS module design: due to the layered structure of the REBCO tape a simple stacking as done for BSCCO is not possible. In addition a dedicated stabilizer material is needed to match the needs for resistance, thermal conductivity and heat capacity. KIT has started an R&D program – as a backup to the BSCCO design used in the JT-60SA current leads – to construct a 20 kA HTS current lead demonstrator using REBCO as this allows a scale-up to currents in
the 50–100 kA range required for DEMO [27]. The selection of the stabilizer material led to the decision to choose brass [28]. This choice has been already made in other projects, for example for the construction of the HTS adapter for the EDIPO text facility at SPC [29]. At present the design and fabrication of HTS stacks made of REBCO material is under investigation. KIT plans to manufacture and test a 20 kA REBCO current lead demonstrator in 2017. 6. Summary High power HTS current leads are nowadays a commonly used component in magnet systems. BSCCO material (first generation HTS) is used in all HTS current leads of large devices (LHC, EAST, W7-X, JT-60SA and ITER). KIT has designed, built and tested 68 kA HTS current lead demonstrator for ITER (maximum steady state operation current was 80 kA). It has also designed, built and tested HTS current leads for Wendelstein 7-X. KIT is at present building and testing HTS current leads for the satellite tokamak JT-60SA. For this the current lead test facility CuLTKa was built to provide a fast and economic testing. CuLTKa is principally able to be upgraded for testing components up to 80 kA. Now BSCCO material is being replaced by REBCO whose architecture is different. HTS current leads for DEMO will require application of REBCO. KIT is working on a 20 kA HTS current lead demonstrator using REBCO. As stabilizer material brass was chosen because of its low thermal conductivity and acceptable electrical resistivity. At present the preparation of the REBCO stacks is under investigation. It is planned to complete the fabrication of the REBCO current lead and to perform a detailed test in 2017. Acknowledgment The authors appreciate the effort of the main workshop of KIT and the current lead assembly crew as well as the refrigerator and testing group in ITEP. The work was performed within the European Fusion Technology Program of KIT. The work for W7X was performed in addition under contract 03/100/4500149540 between the Max-Planck-Institut für Plasmaphysik and KIT. The work related to JT-60SA is financially supported by the German Ministry of Research and Education under the grant No. 03FUS0013 and 03FUS0019 and is done in the Project JT-60SA under the Broader Approach Agreement between Europe and Japan. The views and opinions expressed herein do not reflect necessarily those of the BMBF or the European Commission. References [1] “The Large Hadron Collider; a marvel of technology”. Ed. Lyndon Evans, EPFL Press, CRC Press, 2009. [2] N. Holtkamp, An overview of the ITER project, Fusion Eng. Des. 82 (2007) 427–434. [3] R. Heller, G. Friesinger, A.M. Fuchs, T. Mito, S. Satoh, K. Takahata, M. Tasca, M. Vogel, Development of a 20 kA high temperature superconductor current lead, Cryogenics 41 (2001) 539–547. [4] R. Heller, Wissenschaftliche Berichte Kernforschungszentrum Karlsruhe (1993) KfK–KfK5172.
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