QUELS AIMANTS SUPRACONDUCTEURS POUR LES REACTEURS DE FUSION DU FUTUR ? J.-L. Duchateau

1. QUELS AIMANTS SUPRACONDUCTEURS POUR LES REACTEURS DE FUSION DU FUTUR ? J.-L. Duchateau Association EURATOM-CEA, CEA/DSM/DRFC CEA Cadarache (France)

2. A few questions about future tokamak reactors • - based on the experience of the model coils regarding Cable in Conduit conductors • (CICC) and superconducting materials • -and based on existing fusion reactors designs • In terms of efficiency is there an interest to operate the magnet system at 20 K instead of 5 K. • What are the requirements regarding Jnoncu for DEMO ? • Are existing Nb3Sn or Nb3Al CICC able to fulfil the requirements of DEMO ? • Do we need HTS material (Bi-2212) for DEMO ?

3. ITER TF Model Coil built and tested in Europe (FZK) (2001-2002) A first important industrial milestone on the way to tokamak reactors The plates system in which are inserted the conductors is very similar to the TF system of ITER

4. Introduction • 2. Electrical power needed for the operation of the cryogenic plant of a fusion reactor • 3. Performance of Nb3Sn in large CICC at high field • 4. Average current density in the TF systems of future tokamak reactors • 5. From ITER to DEMO which superconducting material for the TF system ? • 6. Conclusion

5. 1. Introduction : TORE SUPRA a practical experience of refrigerator associated with a fusion machine 1.8 K Winding-pack temperature during one day of operation No large temperature excursions at 1.8 K observed during long plasma pulses !

6. ITER Artist’s view

7. 2. Electrical power associated with fusion reactor cryoplant From ITER to Fusion reactor The estimation is made on the basis of the European power plant conceptual design (EPPCS) presented at SOFT 2004 Conference (model B) (ref EPPCS). Extrapolation is generally related to machine surface or volumes Ref EPPCSD. Maisonnier et al “The European power plant conceptual study” Presented at the 23 rd SOFT Conference (September 2004) Venice Italy. To be published

8. 2. From ITER to Fusion reactor Cryogenic heat load at 4.5 K Not linked to magnet system HTS current leads

9. 2. From ITER to Fusion reactor Cryogenic heat load at 80 K This cryogenic load is due to heat radiation from 300 K In ITER 700 kW only is linked to heat radiation. This fraction is extrapolated according to the volume of the machine.

10. 2. From ITER to Fusion reactor Conversion of cryogenic power to electrical power These cryogenic loads require large electrical power at the level of warm compressors. Welec = Wcryogenic/f cryoplant global efficiency : r=f   T2/(T1-T2) Carnot efficiency f 0.25 Influence of magnet temperature on cryoplant global efficiency

11. 2. From ITER to Fusion reactor Conversion of cryogenic power to electrical power Operation at 20 K instead of 4-5: 3 MW instead of 11 MWa weak benefit of 8 MW J.L Duchateau et al. “Estimation of the recycled power associated with the cryogenic refrigeration power of a fusion reactor” Nucl. Fusion 46 (2006) S94-S99

12. The Nb3Sn conductors of the two ITER model coils are a very important milestone on the way to fusion reactors CSMC 51 mm x 51 mmm 40 kA 13 T TFMC  40,7mm, 80 kA, 9.7 T

13. 3. Sensitivity of Nb3Sn to strain (from TFMC experiment) From strand to coil very large loss of performance for Nb3Sn 0 = othermal + 0BP(BI) ITER TF and TFMC similar cross sections Large Nb3Sn conductors are sensitive to stress accumulation along their characteristic thickness.

14. 3. Sensitivity of Nb3Sn to strain (from TFMC experiment) From strand to coil very large loss of performance for Nb3Sn Jnoncu (12 T, 4.2 K, -0.15 %) 800 A/mm2 Strand specification Jnoncu (12 T, 4.2 K, -0.83 %) 287 A/mm2 CICC in a TF system

15. In ITER The low value of Jconductor (12 A/mm2) is driven by the structures and not by the superconducting material 4. Average current density in Tokamak TF inner leg

16. 4. Average current density in Tokamak inner TF legCable current density in ITER TF leg Jcable in ITER  52.8 A/mm2 >> Jcond=12 A/mm2Non copper section (Jnoncu) occupies only a small part of the cable section

17. 4. Average current density in TF inner legCable current density in TF cables According to superconducting materials, Jcable decreases as a function of magnetic field from an ideal value of 74 A/mm2. Above 40 A/mm2 Jcable does not affect substantially Jcond

18. 4. Average current density in TF inner legNon copper current density for different superconducting materials According to superconducting materials, Jnoncudecreases as a function of magnetic field. Above 150 A/mm2 Jnoncu does not affect substantially Jcond

19. Average current density in TF inner legmagnetic field at target value (150 A/mm2) for Jnoncu Magnetic field at target value for Jnoncu is in the range 13.3 -13.6 T for existing industrial Nb3Sn and Nb3Al superconducting material.

20. 5. From ITER to DEMO which superconducting material ? The crucial dimensioning role of TF inner stacking from plasma center to Tokamak central axis. TF radial extension plays an important role in tokamak dimensioning. The radial extension must remain moderate.

21. 5. From ITER to DEMO which superconducting material ? Electricity generating reactor: DEMO ITER High field zone in fusion reactor magnet system constitutes a very important technological stake J.L Duchateau “Influence of the magnetic toroidal field value on the design of magnet systems for future fusion reactors”. 2006 Presented at the 15 th Int. Toki Conference.

22. 5. From ITER to DEMO which superconducting material ? For DEMO (EPPCS model C) Jnoncu=150 A/mm2appears as a target value beyond which average current density in TF inner leg is not affected substantially.

23. 5. From ITER to DEMO which superconducting material ? Bismuth strand Showa  0.81 mm Ag/Bi=2.7 At 13.2 T effective magnetic field of DEMO (EPPCS model C) existing Nb3Al and Nb3Sn CICC can already fulfil requirements.

24. 6. Conclusion For TF systems of future fusion reactors, the average current density is in the range of 10 A/mm2 and is driven by the large amount of structures necessary to resist the Lorenz forces. Thereby it is possible to accept superconducting strands with low non copper current density down to 150 A/mm2, without substantially affecting the TF radial extension of the system Due to the combined effect of differential thermal contraction and transverse forces arising in large fusion conductors, Nb3Sn strands developed in the framework of ITER program are at the limit of their capability at 13.6 T, the envisaged maximum field on the conductor However, the conductor concept can certainly be improved to mitigate the effect of the transverse magnetic field on the conductor degradation and even allow higher magnetic field for demonstration reactor if needed

25. 6. Conclusion on possible use of HTS in future fusion reactors In the present considered range of magnetic field, solutions (Bi-2212 at 4.2K, or Nb3Sn at 1.8 K ) can be envisaged but have still to prove their economical profitability. The improvement in Jnoncuwill be beneficial to the cost by allowing a decrease of the total weight of the superconducting material in future fusion reactors. Interest could grow if higher magnetic field are considered for future fusion reactors (14-16 T). Increasing the magnet temperature in operation from 5 K to 20K should provide a weak benefit < 10 MWelec