1. Stress review - CERN, 4.11.2008 Review on stress sensitivity Part I R. Flükiger B. Seeber Group of Applied Physics (GAP) University of Geneva 1

2. Stress review - CERN, 4.11.2008 Outline General problematics of stresses in superconductors ITER and NED requirements Uniaxial tensile stresses: J c vs.  Models for description Transverse compressive stresses: J c vs.  t What do we actually know? 2

3. Stress review - CERN, 4.11.2008 Requirement for ITER ≠ Requirement for NED Requirements for J c and stresses: ITER: J c (non Cu) > 1’100 A/cm 2 at 12 T low a.c. losses filament diameter < 20 mm No impregnation, no particular mechanical protection No cracks up to 30 MPa: twist pitch/bending Direct contact between strands: transverse stresses !!! No relevant degradation of J c after > 20 years (neutrons) NED: J c (non Cu) ≥ 1’500 A/cm 2 at 15 T No cracks up to 120 MPa Impregnation, reduces problems of transverse stresses No relevant degradation of J c after 10 years (neutrons) 3

4. Stress review - CERN, 4.11.2008 The ITER TF Model Coil ø 40 mm, 1.5 mm thick steel Conduit rated current: 70 kA/11.8 T/4,6 K  1028 strands Nb 3 Sn + 1/3 Cu Nb 3 Sn Conductor 4

5. Stress review - CERN, 4.11.2008 Internal Sn Diffusion Technique Example: Oxford Instruments, for ITER Type I) * 0.81 mm (NbTi)3Sn strand * 19 subelements *) * Single Ta barrier * Cu:non-Cu ratio 1 * J c ~1200 A/mm2 (Type I) ~1100 A/mm2 (Type II) * Non-Cu hysteresis losses: 900 kJ/m3 (Type I) 700 kJ/m3 (Type II) * Unit lengths: up to 8 km *) Agglomeration of original filaments during reaction: Characteristics of Internal Sn wires Courtesy A. Vostner, ITER 5

6. Stress review - CERN, 4.11.2008 Problem: All high field superconductors are brittle. At  fracture ≤ 0.05 %: Formation of cracks Only exception: NbTi, with T c = 10K, B c2 (0) = 14 T Question: How can one built large magnets based on superconducting wires with  irr ≥ 0.6% ? Answer: Microfilamentization Reason: Relationship between contact surface and volume (or: Ratio between Interface and total filament surface) 6

7. Stress review - CERN, 4.11.2008 Microfilaments Bronze Route wire 100 nm 2  m 7

8. Stress review - CERN, 4.11.2008 Bronze Internal Sn PIT 4-5  m 70  m 50  m Internal Sn wirePIT wire Filament size D: D(bronze) << D(Internal Sn, PIT)  irr (bronze) >  irr (Internal Sn, PIT)  > 0.8 % ≤ 0.4 % 8

9. Stress review - CERN, 4.11.2008 10  m 24 h/800°C 1  m 24h/850 °C Main Limitation: above 560°C, the submicron size filaments are interrupted, due to the formation of Nb 3 Sn : “Spherodization” 10  m 1  m The unfulfilled dream of Nb 3 Sn wires: « in situ » wires, with filament sizes < 100 nm 9

10. Stress review - CERN, 4.11.2008 Strengthening of « In Situ » wires (effect submicron filaments) Increase from 0.3 to 0.7 % Dendrite sizes after casting Final wire: Sizes < 100 nm 10 « in situ » technique given up J c too low

11. Stress review - CERN, 4.11.2008 Tensile stresses 11

12. 650°C 4.2 K Cu Cu/Sn Nb Nb 3 Sn cool down mm Cu and Cu/Sn in extension Nb and Nb 3 Sn in compression Nb 3 Sn technical wires Stress review - CERN, 4.11.2008 Origin of precompression in superconducting wires 12

13. 23 T Magnetic Field: 8 T Applied Strain (%) Nb3Sn Wire Why is the effect of tensile strain important? Stress review - CERN, 4.11.200813

14. Stress review - CERN, 4.11.2008 Change of physical properties when applying a tensile stress In Nb 3 Sn, the application of tensile stress has been recognized to change primarily the phonon spectrum rather than the electronic density of states (Markiewicz 2005, Hampshire et al., 2006) 14

15. Stress review - CERN, 4.11.2008 T c /T cm Asymmetric behavior of B c2 (  ) B c2 (  ) B c2m Effect of tensile stress much stronger on B c2 than on T c B c2 (  )/B c2m = 1 – a |  o | 15 10% reduction of T c /T cm :  m : - 0.89% 10% reduction of B c2 (  )/B c2m :  m : - 0.45%

16. Stress review - CERN, 4.11.200816 Elastic tetragonal distortion under the effect of uniaxial tensile stress

17. Stress review - CERN, 4.11.2008 Uniaxial strain behavior of Nb 3 Sn wires Internal Sn wires (Type I) are more strain sensitive than Bonze Route Wires. Asymmetry of J c (  ) observed for all wire types. Explanation by asymmetric distortion at both sides of  m. 17

18. Ekin model ten Haken model Kramer’s law Field and strain scaling laws for Nb 3 Sn strain dependent critical field Stress review - CERN, 4.11.200818

19. Ekin’s model ten Haken’s model Stress review - CERN, 4.11.200819

20. Stress review - CERN, 4.11.2008 Devices for Tensile Stress Measurements Principle: Gradual release of the precompression 20

21. Stress review - CERN, 4.11.2008 a: The Pacman strain device (University of Twente) 21

22. Stress review - CERN, 4.11.2008 b) The Walters Spiral (Univ. Geneva) Max current 1’000 A Wire length up to 1 meter Max voltage tap distance 50 cm J c criterion 0.01  V/cm Measurements: up to 21 T Strain e: applied by an axial rotation see: B. Seeber (next speaker) 22

23. Stress review - CERN, 4.11.2008 Steel reinforced Nb 3 Sn wires Stainless Steel 316LN leads to a higher precompression * higher non-hydrostatic tetragonal deformation * higher hydrostatic compression Depending on the Steel:Nb 3 Sn ratio, e m increases from 0.25 to 0.87% For 40% steel, a decreases of B c2 by 3 T is observed Lower J c values are measured: for e m = 0.87%, J c /J co = 0.12. 23

24. Stress review - CERN, 4.11.2008 Steel reinforced Nb 3 Sn wires J.Ekin, W.Specking, R.Flükiger, J.Appl. Physics, 54(1983)2869 Fe/Nb 3 Sn J c /J co Stainless steel Cu 24

25. Stress review - CERN, 4.11.200825 Transverse compressive stresses

26. Stress review - CERN, 4.11.2008 Fixed part Moving part Specifications: - F = 5KN - I = 1000 A - Field 21 T 26

27. 0 100 200 300 400 500 tt Stress review - CERN, 4.11.200827

28. Conclusions J c (B,) have been measured at very high fields (21 T) for Nb 3 Sn Bronze Route, Internal Sn and PIT wires. The results show a dominant effect of the axial components 1 D approximation: J c (B,) curves usually analysed with the Ekin and ten Haken models 3D distribution revealed by crystallography. Calculations still needed for larger filament sizes (subelements) Transverse compressive stresses: still no theoretical understanding. The very low reversibility of J c suggests that nano- and microcracks are the major responsible for the much observed effects, which are much stronger than for uniaxial tensile stresses. Stress review - CERN, 4.11.200828