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High temperature superconductors for the ITER magnet system and beyond
Fusion Engineering and Design 75–79 (2005) 105–109
High temperature superconductors for the ITER magnet system and beyond W.H. Fietz a,∗ , S. Fink a , R. Heller a , P. Komarek a , V.L. Tanna a , G. Zahn a , G. Pasztor b , R. Wesche b , E. Salpietro c , A. Vostner c a b
Forschungszentrum Karlsruhe, Institut f¨ur Technische Physik, Postfach 3640, D-76021 Karlsruhe, Germany Centre de Recherches en Physique des Plasmas, Technologie de la Fusion PSI, CH-5232 Villigen, Switzerland c EFDA-CSU, Max Planck Institute, Boltzmannstr. 2, D-85748 Garching, Germany Available online 10 August 2005
Abstract The use of high temperature superconductor (HTS) materials in future fusion machines could increase the efficiency drastically, but strong boundary conditions exist. To outline the prospects, challenges and problems, first the benefit of using HTS materials is estimated considering the saving in cryogenic power. Next, it is demonstrated that industrial available HTS materials can be used for fusion today. For this purpose, we give a short summary of results that have been obtained from an ITER conform 70 kA HTS current lead that was designed, built and tested by the Forschungszentrum Karlsruhe and the CRPP Villigen in the frame of the European Fusion Technology Programme and in cooperation with industry. This current lead consists of an HTS part that covered the temperature range from 4.5 to 70 K and a conventional part, making the connection to room temperature. Because the HTS part had no ohmic losses and poor thermal conduction, the refrigerator power necessary for cooling the current lead was reduced drastically. The saving factor could be calculated to be 5.4 at zero current and 3.7 at 68 kA. The current lead could even be operated at 80 kA and with respect to safety criteria of ITER, a complete loss of He flow was simulated showing that the HTS current lead could hold a current of 68 kA for 6 min without active cooling. These results demonstrate that today existing HTS materials can be used in ITER for current leads or bus bar systems. For fusion machines beyond ITER, the development of an HTS fusion conductor would be the key to operate the complete magnet system at higher temperatures. The option of developing fusion conductors based on Bi-2223 and YBCO are briefly discussed. For a success of such conductors, the AC loss optimisation is crucial. © 2005 Elsevier B.V. All rights reserved. Keywords: Fusion; ITER; Magnet; Superconductivity; High temperature superconductor
1. Introduction ∗
Corresponding author. Tel.: +49 7247 82 4197; fax: +49 7247 82 7878. E-mail address: [email protected] (W.H. Fietz). 0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.06.198
The construction and operation of ITER is the next step towards controlled nuclear fusion. The main goal of the ITER project is to demonstrate that power
106
W.H. Fietz et al. / Fusion Engineering and Design 75–79 (2005) 105–109
generation by nuclear fusion is feasible in an economically and environmentally acceptable way. In a fusion reactor, the hot plasma is confined by the magnetic fields generated by the superconducting toroidal field coil system and the plasma current. Thus, the superconducting coil system is a indispensable component. The main goal of the present research is the development of fusion-based power plants. For the next step after ITER, economic aspects will become increasingly important. The use of high temperature superconductors (HTS) or of MgB2 could provide economical as well as technical benefits for future fusion reactors. The main advantages of high temperature superconductors as compared to the presently used superconductors NbTi and Nb3 Sn are their much higher critical temperatures and upper critical fields. In a future fusion reactor, a considerably larger fusion power than that envisaged for ITER would be required. To reach a larger fusion power, the plasma volume and/or the toroidal magnetic field need to be enhanced. Thus, the favourable physical properties of the high temperature superconductors may be used to increase the operating temperature of the superconducting magnets or to enhance the toroidal magnetic field. An increase of the toroidal magnetic field would allow in principle a more compact design of a fusion reactor. On the other hand, the corresponding increase of forces will cause a need for much higher structural support. Furthermore, the neutron fluence increases significantly with the magnetic field leading to very high heat loads on the plasma facing components. The competing effect on the compactness of the resulting machine makes an assessment of the benefit of a field increase difficult, but details may be found in [1]. In this paper, we focus on the effect of HTS material on the operating temperature of the magnet system. First, the advantages of an enhanced operating temperature will be discussed. Next, as an example application, the results of the test of a HTS current lead which was successfully operated at 80 kA will be shown. This demonstrates that HTS materials are now beginning to reach a technological level that makes them interesting for fusion application. In a last step, the expectations and challenges of HTS materials for fusion application will be reviewed briefly.
2. Impact of the use of HTS Based on the maximum magnetic fields of 11.8 and 13 T foreseen for the toroidal field coils and the central solenoid of ITER [2], respectively, operating temperatures of 20 K seem to be feasible for the Bibased superconductors Bi2 Sr2 CaCu2 O8 (Bi-2212) and (Bi,Pb)2 Sr2 Ca2 Cu3 O10 (Bi-2223). Ag/Bi-2223 tapes are now commercially available. In the case of Bi-2212, high critical current densities have been achieved not only in tapes but also in round wires. Supposing that sufficiently high upper critical fields can be reached in dirty MgB2 , this superconductor would be also an option for 20 K operation. The use of YBa2 Cu3 O7 (YBCO) coated conductors would even allow the operation of the magnet system at a temperature of about 65 K. To calculate the benefit of a higher magnet operation temperature, the ITER outline design of January 2000 [3] is used. To remove the total heat load of 55 kW at the 4.5 K level, four 18 kW cryoplants are necessary. In addition to the removal of this thermal load, the cryoplant must supply 0.126 kg/s liquid helium used to cool the conventional current leads (66 g/s) and the torus cryopumps (fast cool down during regeneration (60 g/s)). The static heat load (approximately 17% of the thermal heat load) is expected to depend strongly on the operation temperature of the magnet system. The other contributions which are mainly the average pulsed heat load (35%) and the heat load from circulating pumps and compressors (39%) are in a first approximation independent of the operating temperature of the magnet system. Therefore, for simplicity, it can be assumed that the thermal load is independent of the operating temperature of the magnets. With this assumption, the efficiency of refrigerators have to be compared by using the needed electrical input power Pel divided by the removed thermal losses Pth as a function of Pth [4] for operating temperatures of 4.5, 20 and 65 K based on the use of 18 kW refrigerators. The values of this ratio are ≈300, ≈60 and ≈10 for operating temperatures of 4.5, 20 and 65 K, respectively. Consequently, the estimated refrigerator operational cost is reduced by factors of 5 and 30 for operating temperatures of 20 and 65 K, respectively.
W.H. Fietz et al. / Fusion Engineering and Design 75–79 (2005) 105–109
The investment cost for cryoplants is known to scale with the installed electrical input power [5]. A thermal load of 18 kW for one of the four ITER cryoplants corresponds to an electric input power of ≈5500, ≈1100 and ≈180 kW for operating temperatures of 4.5, 20 and 65 K, respectively. A rough estimate results in a reduction of the investment cost for magnet operation at 20 or 65 K instead of 4.5 K by a factor of 3 or 12, respectively. Using the present ITER design, this benefit is valid for almost all losses that arise at 4.5 K—with the exception of the necessary helium (60 g/s) for the torus cryo pumps that have to operate at 4.5 K in any circumstances. For the sake of completeness, it should be mentioned that the heat load for thermal shielding at 80 K has not been discussed here because this thermal load of 660 kW [3] has to be removed independently of the magnet operation temperature. However, when the magnet is operated at 65 K, it has to be examined if an extra thermal shielding is still necessary.
3. HTS current lead When superconducting magnets are operated at 4.5 K, a massive metallic connection is required for the transfer of current from room temperature where the power supplies are located to the low temperature region. The cooling power required by such conventional current leads is about one-third of the total dissipated cooling power of the ITER system [6]. Using HTS materials, the heat load caused by the part of the current lead that covers the temperature range from 4.5 to 65 K can be substantially reduced. Resistive losses are eliminated and only losses caused by heat conduction are present, but these losses are lower compared to conventional current leads.
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The Forschungszentrum Karlsruhe and the Centre des Recherches en Physique des Plasmas, Villigen developed, constructed and tested a 70 kA current lead using HTS for the TF coil system of ITER in the frame of a task of the European Fusion and Technology Program [7]. The current lead was designed with respect to the requirements given in the ITER magnet design document. Special attention was given to the safety requirement. The current lead has to withstand a complete loss of helium mass flow for several minutes at a nominal current of 68 kA. One further requirement was that the lead will be installed horizontally in the coil-terminal-boxes [7–9]. The developed current lead consists of a HTS part and a conventional copper part operated in the temperature ranges 4.5–65 K and 65 K to room temperature (RT), respectively. The HTS module was designed in cooperation with American Superconductor (AMSC) and built by this company using Bi-2223/AgAu superconductor. Twelve individual panels consisting of Bi2223 tapes soldered to stainless steel carriers were assembled in a circular arrangement. The HTS module is cooled by heat conduction from the 4.5 K level and the copper part is cooled with 50 K helium. This concept was also successfully applied in the former 20 kA HTS current lead program [10,11]. The optimum temperatures of both the helium and the conductor at the cold end of the heat exchanger depend on the assumed refrigerator efficiency, the engineering critical current density of the Bi-2223 tapes at the warm end operating temperature and the cross-section of the stainless steel support. The results of the optimisation studies indicate that a broad minimum of the required refrigerator input power is reached for a conductor temperature of 65 K at the warm end of the HTS and the use of 50 K helium gas to cool the heat exchanger part (65 K, RT) of the current lead. Fig. 1 shows a photograph of the 68 kA HTS current lead. At the beginning of 2004, a test loop consisting
Fig. 1. Photograph of the 68 kA HTS current lead.
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of the 68 kA HTS current lead, an 80 kA superconducting bus bar and a conventional 80 kA current lead (the last two components were remains from the ITER TFMC test) was installed in the TOSKA facility of the Forschungszentrum Karlsruhe. The experiment was performed in two test campaigns which took place between April and June 2004. The experiments cover the electrical and thermal behaviour under steady state conditions and in case of a quench. To characterise the performance of the current lead, the temperature profile, the contact resistances, the heat load at 4.5 K, the minimum required mass flow rate of 50 K helium and the critical current are evaluated. In addition, the safety margin in case of a loss of mass flow is studied. The 68 kA HTS current lead was operated stable up to 70 kA at a temperature of the warm copper end cap of the HTS module of approximately 65 K using 50 K helium for cooling of the copper heat exchanger. It was even possible to operate the lead at 80 kA at a slightly lower temperature level. The heat load at the 4.5 K level was evaluated by measuring the enthalpy change of the helium flowing in the superconducting NbTi short circuit bus bar. At nominal operation conditions, it results in about 13.5 W which is higher than expected (5.5 W). This cannot be correlated to the material data provided by the manufacturer, but must be due to additional losses originated by the facility. But nevertheless, this number is much lower than that of a conventional current lead. The soldered contact between the cold end copper bar and the copper end cap of the HTS module was about 1.9 n as expected. The contact resistance of the conical screw contact between the upper copper end cap of the HTS module and the conventional copper heat exchanger was much larger than estimated from a similar screw contact realised in the 20 kA HTS current lead constructed and tested in the year 2000. This may be correlated to a problem in the tolerance of the Cu interface between the two copper parts (details will be presented at the conference ASC 2004). For the further evaluation, the influence of the higher Cu–Cu contact resistance is considered in the calculations. At 68 kA, the necessary He mass flow rate, corrected for a correct Cu–Cu contact resistance of 10 n, is about 4.7 g/s. During the quench current measurements, the temperature at the warm end of the HTS module was
carefully increased by reducing the He mass flow rate through the heat exchanger. Measurements were performed at 50, 68 and 80 kA and for voltage levels of 50 mV and delay times up to 4 s, resulting in maximum temperatures up to 160 K caused by the quench. The current capacity of the HTS module seems to be higher than guaranteed by the manufacturer. During loss of He mass flow simulation, the He flow through the heat exchanger was stopped while the current of 68 kA was still flowing. This led to an increase of the temperature at the warm end of the HTS module. After 6 min and 18 s, the HTS module quenched at the same temperature as measured during the quench experiment. This number is more than a factor of two larger than originally specified and demonstrates the high thermal capacity and stability of the current lead. To compare the refrigerator heat load of the HTS current lead and a conventional current lead as given in [2], the load of the HTS current lead has been analysed using the experimental results for zero current as well as for 68 kA. These calculations show a reduction of the refrigerator power when a HTS current lead is used by a factor of 5.4 (zero current) and 3.7 (68 kA) or about 5 for a current operation duty cycle of 25%. All results from the tested HTS current lead demonstrate that the existing HTS materials can be used to reduce the necessary refrigerator power substantially in fusion machines. The developed technique may be expanded to a HTS bus bar system too and it would be principally possible to use such HTS components for ITER. However, coming to the magnet system itself, it is clear that a fusion conductor for magnets using HTS materials is presently not available. The possibilities and challenges will be briefly discussed in the next paragraph.
4. HTS fusion conductor HTS conductors are now available from industry for technical use. Actually two routes are favoured which are first the well developed Bi-2223 tapes and second the YBCO coated conductor line (the third option may be MgB2 , but the optimisation of this material to higher critical current and field is still a question of basic research). Bi-2223 is a candidate to build up an HTS conductor with an operation temperature ≤20 K. At such
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temperatures, a magnetic field of 13 T like it is used in fusion is not a problem for the Bi-2223-tapes and long lengths are available from industry. However, the investigation of joining methods leading to viable high amperage conductors is open and the issue of adequate cabling/bundling techniques providing high contact resistances between basic strands to limit ac losses is not solved. The concepts of classical fusion conductors using internal barriers, twisting and multistage concepts cannot be transferred to the thin tapes with a typical width in the millimetre scale. For the YBCO coated conductor, the situation is on one hand more promising but on the other hand even more difficult. For YBCO, a field of 13 T is acceptable even at 65 K. Unfortunately, high transport currents can only be achieved when within the YBCO material all grains show an almost perfect orientation within three dimensions. To solve that problem, the deposition of YBCO on biaxially oriented Ni-tapes with an intermediate buffer layer has been successfully demonstrated in the labs. At the moment, industry starts to build up fabrication lines for long lengths of such a “coated conductor”, but the usually used slow vacuum deposition techniques make the fabrication of long lengths expensive and time consuming. Therefore, alternative deposition techniques are tested (e.g. chemical deposition) and first encouraging results are obtained. With that optimisation, industrial produced coated conductor tapes will be available within the next few years. However, when these conductors are available, we are back at the same question that was asked for Bi-2223 before: how can AC losses be minimised using these tape shaped conductors? At the moment, no clear solution is available, but classical techniques may be adapted (e.g. Roebel bar conductor). To find a solution for this AC loss problem is the crucial problem when HTS materials shall be used to increase the efficiency of future fusion machines. 5. Conclusion With the developed and tested HTS current lead, it has been demonstrated that existing HTS materials can be used for current leads and bus bar systems for ITER. For fusion systems beyond ITER, an HTS fusion cable would bring a massive increase of effi-
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ciency when the complete magnet system is operated at higher temperatures. However, to achieve this, AC loss optimised cables are necessary. The development of adequate cabling and bundling techniques for an AC loss optimised design is essential for the success of an HTS fusion cable. Considering the time needed to optimise the classic fusion conductor, we should start the development of the adequate techniques now.
Acknowledgements The development and testing of the HTS current lead has been performed within the European Fusion Technology Programme. Many thanks to the TOSKA crew and the 2 kW refrigerator crew for their professional job and large effort during the test phase.
References [1] T. Ando, et al., Design of toroidal field coil for A-SSTR2 using high Tc superconductor, Fusion Eng. Design 58–59 (2001) 13–16. [2] Design Description Document DDD 11 Magnet (N11 DDD 177 04-05-12 W 0.2). [3] Technical Basis for the ITER-FEAT Outline Design, 18 January 2000 (G A0 RI 2 00-01-18 R1.0). [4] H. Frey, R.A. Haefner, in: F.X. Eder (Ed.), Tieftemperaturtechnologie, VDI Verlag, D¨usseldorf, 1981. [5] M. Wanner, B. Ziegler, Industrielle K¨alteanlagen f¨ur LHe und LN2, VDI Berichte No. 1187, Erg¨anzung, 1995. [6] Technical basis for the ITER detailed design report, cost review and safety analysis, IAEA, Vienna, 1997. [7] IER Magnet System Procurement Package 11.P5, Feeders, N11 SP 25 00-07-18 W 0.1, July 2000. [8] R. Heller, et al., Development of high temperature superconductor current leads for 70 kA, IEEE Trans. Appl. Supercond. 12 (1) (2002) 1285–1288. [9] R. Heller, et al., 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). [10] R. Heller, et al., Development of a 20 kA high-temperaturesuperconductor current lead, Cryogenics 41 (2001) 539– 547. [11] R. Heller, Design, development and constructing of high temperature superconductor current leads for 70 kA—intermediate report due to 02/02, Comparative study on HTS current leads manufactured in institutes and in industry and extrapolation of the design to 70 kA, Internal Report FE 5130.0031.0012/H, February 2003, unpublished.
High temperature superconductors for the ITER magnet system and beyond W.H. Fietz a,∗ , S. Fink a , R. Heller a , P. Komarek a , V.L. Tanna a , G. Zahn a , G. Pasztor b , R. Wesche b , E. Salpietro c , A. Vostner c a b
Forschungszentrum Karlsruhe, Institut f¨ur Technische Physik, Postfach 3640, D-76021 Karlsruhe, Germany Centre de Recherches en Physique des Plasmas, Technologie de la Fusion PSI, CH-5232 Villigen, Switzerland c EFDA-CSU, Max Planck Institute, Boltzmannstr. 2, D-85748 Garching, Germany Available online 10 August 2005
Abstract The use of high temperature superconductor (HTS) materials in future fusion machines could increase the efficiency drastically, but strong boundary conditions exist. To outline the prospects, challenges and problems, first the benefit of using HTS materials is estimated considering the saving in cryogenic power. Next, it is demonstrated that industrial available HTS materials can be used for fusion today. For this purpose, we give a short summary of results that have been obtained from an ITER conform 70 kA HTS current lead that was designed, built and tested by the Forschungszentrum Karlsruhe and the CRPP Villigen in the frame of the European Fusion Technology Programme and in cooperation with industry. This current lead consists of an HTS part that covered the temperature range from 4.5 to 70 K and a conventional part, making the connection to room temperature. Because the HTS part had no ohmic losses and poor thermal conduction, the refrigerator power necessary for cooling the current lead was reduced drastically. The saving factor could be calculated to be 5.4 at zero current and 3.7 at 68 kA. The current lead could even be operated at 80 kA and with respect to safety criteria of ITER, a complete loss of He flow was simulated showing that the HTS current lead could hold a current of 68 kA for 6 min without active cooling. These results demonstrate that today existing HTS materials can be used in ITER for current leads or bus bar systems. For fusion machines beyond ITER, the development of an HTS fusion conductor would be the key to operate the complete magnet system at higher temperatures. The option of developing fusion conductors based on Bi-2223 and YBCO are briefly discussed. For a success of such conductors, the AC loss optimisation is crucial. © 2005 Elsevier B.V. All rights reserved. Keywords: Fusion; ITER; Magnet; Superconductivity; High temperature superconductor
1. Introduction ∗
Corresponding author. Tel.: +49 7247 82 4197; fax: +49 7247 82 7878. E-mail address: [email protected] (W.H. Fietz). 0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.06.198
The construction and operation of ITER is the next step towards controlled nuclear fusion. The main goal of the ITER project is to demonstrate that power
106
W.H. Fietz et al. / Fusion Engineering and Design 75–79 (2005) 105–109
generation by nuclear fusion is feasible in an economically and environmentally acceptable way. In a fusion reactor, the hot plasma is confined by the magnetic fields generated by the superconducting toroidal field coil system and the plasma current. Thus, the superconducting coil system is a indispensable component. The main goal of the present research is the development of fusion-based power plants. For the next step after ITER, economic aspects will become increasingly important. The use of high temperature superconductors (HTS) or of MgB2 could provide economical as well as technical benefits for future fusion reactors. The main advantages of high temperature superconductors as compared to the presently used superconductors NbTi and Nb3 Sn are their much higher critical temperatures and upper critical fields. In a future fusion reactor, a considerably larger fusion power than that envisaged for ITER would be required. To reach a larger fusion power, the plasma volume and/or the toroidal magnetic field need to be enhanced. Thus, the favourable physical properties of the high temperature superconductors may be used to increase the operating temperature of the superconducting magnets or to enhance the toroidal magnetic field. An increase of the toroidal magnetic field would allow in principle a more compact design of a fusion reactor. On the other hand, the corresponding increase of forces will cause a need for much higher structural support. Furthermore, the neutron fluence increases significantly with the magnetic field leading to very high heat loads on the plasma facing components. The competing effect on the compactness of the resulting machine makes an assessment of the benefit of a field increase difficult, but details may be found in [1]. In this paper, we focus on the effect of HTS material on the operating temperature of the magnet system. First, the advantages of an enhanced operating temperature will be discussed. Next, as an example application, the results of the test of a HTS current lead which was successfully operated at 80 kA will be shown. This demonstrates that HTS materials are now beginning to reach a technological level that makes them interesting for fusion application. In a last step, the expectations and challenges of HTS materials for fusion application will be reviewed briefly.
2. Impact of the use of HTS Based on the maximum magnetic fields of 11.8 and 13 T foreseen for the toroidal field coils and the central solenoid of ITER [2], respectively, operating temperatures of 20 K seem to be feasible for the Bibased superconductors Bi2 Sr2 CaCu2 O8 (Bi-2212) and (Bi,Pb)2 Sr2 Ca2 Cu3 O10 (Bi-2223). Ag/Bi-2223 tapes are now commercially available. In the case of Bi-2212, high critical current densities have been achieved not only in tapes but also in round wires. Supposing that sufficiently high upper critical fields can be reached in dirty MgB2 , this superconductor would be also an option for 20 K operation. The use of YBa2 Cu3 O7 (YBCO) coated conductors would even allow the operation of the magnet system at a temperature of about 65 K. To calculate the benefit of a higher magnet operation temperature, the ITER outline design of January 2000 [3] is used. To remove the total heat load of 55 kW at the 4.5 K level, four 18 kW cryoplants are necessary. In addition to the removal of this thermal load, the cryoplant must supply 0.126 kg/s liquid helium used to cool the conventional current leads (66 g/s) and the torus cryopumps (fast cool down during regeneration (60 g/s)). The static heat load (approximately 17% of the thermal heat load) is expected to depend strongly on the operation temperature of the magnet system. The other contributions which are mainly the average pulsed heat load (35%) and the heat load from circulating pumps and compressors (39%) are in a first approximation independent of the operating temperature of the magnet system. Therefore, for simplicity, it can be assumed that the thermal load is independent of the operating temperature of the magnets. With this assumption, the efficiency of refrigerators have to be compared by using the needed electrical input power Pel divided by the removed thermal losses Pth as a function of Pth [4] for operating temperatures of 4.5, 20 and 65 K based on the use of 18 kW refrigerators. The values of this ratio are ≈300, ≈60 and ≈10 for operating temperatures of 4.5, 20 and 65 K, respectively. Consequently, the estimated refrigerator operational cost is reduced by factors of 5 and 30 for operating temperatures of 20 and 65 K, respectively.
W.H. Fietz et al. / Fusion Engineering and Design 75–79 (2005) 105–109
The investment cost for cryoplants is known to scale with the installed electrical input power [5]. A thermal load of 18 kW for one of the four ITER cryoplants corresponds to an electric input power of ≈5500, ≈1100 and ≈180 kW for operating temperatures of 4.5, 20 and 65 K, respectively. A rough estimate results in a reduction of the investment cost for magnet operation at 20 or 65 K instead of 4.5 K by a factor of 3 or 12, respectively. Using the present ITER design, this benefit is valid for almost all losses that arise at 4.5 K—with the exception of the necessary helium (60 g/s) for the torus cryo pumps that have to operate at 4.5 K in any circumstances. For the sake of completeness, it should be mentioned that the heat load for thermal shielding at 80 K has not been discussed here because this thermal load of 660 kW [3] has to be removed independently of the magnet operation temperature. However, when the magnet is operated at 65 K, it has to be examined if an extra thermal shielding is still necessary.
3. HTS current lead When superconducting magnets are operated at 4.5 K, a massive metallic connection is required for the transfer of current from room temperature where the power supplies are located to the low temperature region. The cooling power required by such conventional current leads is about one-third of the total dissipated cooling power of the ITER system [6]. Using HTS materials, the heat load caused by the part of the current lead that covers the temperature range from 4.5 to 65 K can be substantially reduced. Resistive losses are eliminated and only losses caused by heat conduction are present, but these losses are lower compared to conventional current leads.
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The Forschungszentrum Karlsruhe and the Centre des Recherches en Physique des Plasmas, Villigen developed, constructed and tested a 70 kA current lead using HTS for the TF coil system of ITER in the frame of a task of the European Fusion and Technology Program [7]. The current lead was designed with respect to the requirements given in the ITER magnet design document. Special attention was given to the safety requirement. The current lead has to withstand a complete loss of helium mass flow for several minutes at a nominal current of 68 kA. One further requirement was that the lead will be installed horizontally in the coil-terminal-boxes [7–9]. The developed current lead consists of a HTS part and a conventional copper part operated in the temperature ranges 4.5–65 K and 65 K to room temperature (RT), respectively. The HTS module was designed in cooperation with American Superconductor (AMSC) and built by this company using Bi-2223/AgAu superconductor. Twelve individual panels consisting of Bi2223 tapes soldered to stainless steel carriers were assembled in a circular arrangement. The HTS module is cooled by heat conduction from the 4.5 K level and the copper part is cooled with 50 K helium. This concept was also successfully applied in the former 20 kA HTS current lead program [10,11]. The optimum temperatures of both the helium and the conductor at the cold end of the heat exchanger depend on the assumed refrigerator efficiency, the engineering critical current density of the Bi-2223 tapes at the warm end operating temperature and the cross-section of the stainless steel support. The results of the optimisation studies indicate that a broad minimum of the required refrigerator input power is reached for a conductor temperature of 65 K at the warm end of the HTS and the use of 50 K helium gas to cool the heat exchanger part (65 K, RT) of the current lead. Fig. 1 shows a photograph of the 68 kA HTS current lead. At the beginning of 2004, a test loop consisting
Fig. 1. Photograph of the 68 kA HTS current lead.
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of the 68 kA HTS current lead, an 80 kA superconducting bus bar and a conventional 80 kA current lead (the last two components were remains from the ITER TFMC test) was installed in the TOSKA facility of the Forschungszentrum Karlsruhe. The experiment was performed in two test campaigns which took place between April and June 2004. The experiments cover the electrical and thermal behaviour under steady state conditions and in case of a quench. To characterise the performance of the current lead, the temperature profile, the contact resistances, the heat load at 4.5 K, the minimum required mass flow rate of 50 K helium and the critical current are evaluated. In addition, the safety margin in case of a loss of mass flow is studied. The 68 kA HTS current lead was operated stable up to 70 kA at a temperature of the warm copper end cap of the HTS module of approximately 65 K using 50 K helium for cooling of the copper heat exchanger. It was even possible to operate the lead at 80 kA at a slightly lower temperature level. The heat load at the 4.5 K level was evaluated by measuring the enthalpy change of the helium flowing in the superconducting NbTi short circuit bus bar. At nominal operation conditions, it results in about 13.5 W which is higher than expected (5.5 W). This cannot be correlated to the material data provided by the manufacturer, but must be due to additional losses originated by the facility. But nevertheless, this number is much lower than that of a conventional current lead. The soldered contact between the cold end copper bar and the copper end cap of the HTS module was about 1.9 n as expected. The contact resistance of the conical screw contact between the upper copper end cap of the HTS module and the conventional copper heat exchanger was much larger than estimated from a similar screw contact realised in the 20 kA HTS current lead constructed and tested in the year 2000. This may be correlated to a problem in the tolerance of the Cu interface between the two copper parts (details will be presented at the conference ASC 2004). For the further evaluation, the influence of the higher Cu–Cu contact resistance is considered in the calculations. At 68 kA, the necessary He mass flow rate, corrected for a correct Cu–Cu contact resistance of 10 n, is about 4.7 g/s. During the quench current measurements, the temperature at the warm end of the HTS module was
carefully increased by reducing the He mass flow rate through the heat exchanger. Measurements were performed at 50, 68 and 80 kA and for voltage levels of 50 mV and delay times up to 4 s, resulting in maximum temperatures up to 160 K caused by the quench. The current capacity of the HTS module seems to be higher than guaranteed by the manufacturer. During loss of He mass flow simulation, the He flow through the heat exchanger was stopped while the current of 68 kA was still flowing. This led to an increase of the temperature at the warm end of the HTS module. After 6 min and 18 s, the HTS module quenched at the same temperature as measured during the quench experiment. This number is more than a factor of two larger than originally specified and demonstrates the high thermal capacity and stability of the current lead. To compare the refrigerator heat load of the HTS current lead and a conventional current lead as given in [2], the load of the HTS current lead has been analysed using the experimental results for zero current as well as for 68 kA. These calculations show a reduction of the refrigerator power when a HTS current lead is used by a factor of 5.4 (zero current) and 3.7 (68 kA) or about 5 for a current operation duty cycle of 25%. All results from the tested HTS current lead demonstrate that the existing HTS materials can be used to reduce the necessary refrigerator power substantially in fusion machines. The developed technique may be expanded to a HTS bus bar system too and it would be principally possible to use such HTS components for ITER. However, coming to the magnet system itself, it is clear that a fusion conductor for magnets using HTS materials is presently not available. The possibilities and challenges will be briefly discussed in the next paragraph.
4. HTS fusion conductor HTS conductors are now available from industry for technical use. Actually two routes are favoured which are first the well developed Bi-2223 tapes and second the YBCO coated conductor line (the third option may be MgB2 , but the optimisation of this material to higher critical current and field is still a question of basic research). Bi-2223 is a candidate to build up an HTS conductor with an operation temperature ≤20 K. At such
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temperatures, a magnetic field of 13 T like it is used in fusion is not a problem for the Bi-2223-tapes and long lengths are available from industry. However, the investigation of joining methods leading to viable high amperage conductors is open and the issue of adequate cabling/bundling techniques providing high contact resistances between basic strands to limit ac losses is not solved. The concepts of classical fusion conductors using internal barriers, twisting and multistage concepts cannot be transferred to the thin tapes with a typical width in the millimetre scale. For the YBCO coated conductor, the situation is on one hand more promising but on the other hand even more difficult. For YBCO, a field of 13 T is acceptable even at 65 K. Unfortunately, high transport currents can only be achieved when within the YBCO material all grains show an almost perfect orientation within three dimensions. To solve that problem, the deposition of YBCO on biaxially oriented Ni-tapes with an intermediate buffer layer has been successfully demonstrated in the labs. At the moment, industry starts to build up fabrication lines for long lengths of such a “coated conductor”, but the usually used slow vacuum deposition techniques make the fabrication of long lengths expensive and time consuming. Therefore, alternative deposition techniques are tested (e.g. chemical deposition) and first encouraging results are obtained. With that optimisation, industrial produced coated conductor tapes will be available within the next few years. However, when these conductors are available, we are back at the same question that was asked for Bi-2223 before: how can AC losses be minimised using these tape shaped conductors? At the moment, no clear solution is available, but classical techniques may be adapted (e.g. Roebel bar conductor). To find a solution for this AC loss problem is the crucial problem when HTS materials shall be used to increase the efficiency of future fusion machines. 5. Conclusion With the developed and tested HTS current lead, it has been demonstrated that existing HTS materials can be used for current leads and bus bar systems for ITER. For fusion systems beyond ITER, an HTS fusion cable would bring a massive increase of effi-
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ciency when the complete magnet system is operated at higher temperatures. However, to achieve this, AC loss optimised cables are necessary. The development of adequate cabling and bundling techniques for an AC loss optimised design is essential for the success of an HTS fusion cable. Considering the time needed to optimise the classic fusion conductor, we should start the development of the adequate techniques now.
Acknowledgements The development and testing of the HTS current lead has been performed within the European Fusion Technology Programme. Many thanks to the TOSKA crew and the 2 kW refrigerator crew for their professional job and large effort during the test phase.
References [1] T. Ando, et al., Design of toroidal field coil for A-SSTR2 using high Tc superconductor, Fusion Eng. Design 58–59 (2001) 13–16. [2] Design Description Document DDD 11 Magnet (N11 DDD 177 04-05-12 W 0.2). [3] Technical Basis for the ITER-FEAT Outline Design, 18 January 2000 (G A0 RI 2 00-01-18 R1.0). [4] H. Frey, R.A. Haefner, in: F.X. Eder (Ed.), Tieftemperaturtechnologie, VDI Verlag, D¨usseldorf, 1981. [5] M. Wanner, B. Ziegler, Industrielle K¨alteanlagen f¨ur LHe und LN2, VDI Berichte No. 1187, Erg¨anzung, 1995. [6] Technical basis for the ITER detailed design report, cost review and safety analysis, IAEA, Vienna, 1997. [7] IER Magnet System Procurement Package 11.P5, Feeders, N11 SP 25 00-07-18 W 0.1, July 2000. [8] R. Heller, et al., Development of high temperature superconductor current leads for 70 kA, IEEE Trans. Appl. Supercond. 12 (1) (2002) 1285–1288. [9] R. Heller, et al., 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). [10] R. Heller, et al., Development of a 20 kA high-temperaturesuperconductor current lead, Cryogenics 41 (2001) 539– 547. [11] R. Heller, Design, development and constructing of high temperature superconductor current leads for 70 kA—intermediate report due to 02/02, Comparative study on HTS current leads manufactured in institutes and in industry and extrapolation of the design to 70 kA, Internal Report FE 5130.0031.0012/H, February 2003, unpublished.