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Review on stress sensitivity Part I

Review on stress sensitivity Part I. R. Flükiger B. Seeber Group of Applied Physics (GAP) University of Geneva. Outline General problematics of stresses in superconductors ITER and NED requirements Uniaxial tensile stresses: J c vs. e Models for description

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Review on stress sensitivity Part I

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

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

  3. Requirement for ITER ≠ Requirement for NED Requirements for Jc and stresses: ITER: Jc(non Cu) > 1’100 A/cm2 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 Jc after > 20 years (neutrons) NED: Jc(non Cu) ≥ 1’500 A/cm2 at 15 T No cracks up to 120 MPa Impregnation, reduces problems of transverse stresses No relevant degradation of Jc after 10 years (neutrons) Stress review - CERN, 4.11.2008

  4. ø 40 mm, 1.5 mm thick steel Conduit rated current: 70 kA/11.8 T/4,6 K  1028 strands Nb3Sn + 1/3 Cu The ITER TF Model Coil Nb3Sn Conductor Stress review - CERN, 4.11.2008

  5. 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 * Jc ~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 Stress review - CERN, 4.11.2008 Courtesy A. Vostner, ITER

  6. Problem: All high field superconductors are brittle. At efracture ≤ 0.05 %: Formation of cracks Only exception: NbTi, with Tc = 10K, Bc2(0) = 14 T Question: How can one built large magnets based on superconducting wires with eirr≥ 0.6% ? Answer: Microfilamentization Reason: Relationshipbetweencontact surface and volume (or: Ratio between Interface and total filament surface) Stress review - CERN, 4.11.2008

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

  8. Internal Sn wire PIT wire Bronze Internal Sn PIT 4-5 mm 70 mm 70 mm 50 mm • Filament size D: D(bronze) << D(Internal Sn, PIT) • eirr(bronze) >eirr(Internal Sn, PIT) • > 0.8 % ≤ 0.4 % Stress review - CERN, 4.11.2008

  9. The unfulfilled dream of Nb3Sn wires: « in situ » wires, with filament sizes < 100 nm 24 h/800°C 24h/850 °C 10 mm 1 mm Main Limitation: above 560°C, the submicron size filaments are interrupted, due to the formation of Nb3Sn : “Spherodization” 10 mm 1 mm Stress review - CERN, 4.11.2008

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

  11. Tensile stresses Stress review - CERN, 4.11.2008

  12. Origin of precompression in superconducting wires Nb3Sn technical wires Cu Cu/Sn Nb • Cu and Cu/Sn in extension • Nb and Nb3Sn in compression m cool down Nb3Sn 650°C 4.2 K Stress review - CERN, 4.11.2008

  13. 1000 9 Temperature: 4.2 K 10 Magnetic Field: 8 T 100 8 10 10 Engineering Critical Current Density (Am-2) Critical Current (A) 7 10 1 23 T 6 10 0.1 5 10 -1.5 -1.0 -0.5 0.0 0.5 Why is the effect of tensile strain important? Nb3Sn Wire Applied Strain (%) Stress review - CERN, 4.11.2008

  14. Change of physical properties when applying a tensile stress In Nb3Sn, 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) Stress review - CERN, 4.11.2008

  15. Effect of tensile stress much stronger on Bc2 than on Tc Bc2(e)/Bc2m = 1 – a|eo| Bc2(e) Bc2m Tc/Tcm 10% reduction of Tc/Tcm: Dem: - 0.89% 10% reduction of Bc2(e)/Bc2m: Dem: - 0.45% Asymmetric behavior of Bc2(e) Stress review - CERN, 4.11.2008

  16. Elastic tetragonal distortion under the effect of uniaxial tensile stress Stress review - CERN, 4.11.2008

  17. Uniaxial strain behavior of Nb3Sn wires Internal Sn wires (Type I) are more strain sensitive than Bonze Route Wires. Asymmetry of Jc(e) observed for all wire types. Explanation by asymmetric distortion at both sides of em. Stress review - CERN, 4.11.2008

  18. Field and strain scaling laws for Nb3Sn Kramer’s law strain dependent critical field Ekin model ten Haken model Stress review - CERN, 4.11.2008

  19. ten Haken’s model Ekin’s model Stress review - CERN, 4.11.2008

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

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

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

  23. Steel reinforced Nb3Sn wires Stainless Steel 316LN leads to a higher precompression * higher non-hydrostatic tetragonal deformation * higher hydrostatic compression Depending on the Steel:Nb3Sn ratio, em increases from 0.25 to 0.87% For 40% steel, a decreases of Bc2 by 3 T is observed Lower Jc values are measured: for em = 0.87%, Jc/Jco = 0.12. Stress review - CERN, 4.11.2008

  24. Steel reinforced Nb3Sn wires Jc/Jco Stainless steel Cu Fe/Nb3Sn J.Ekin, W.Specking, R.Flükiger, J.Appl. Physics, 54(1983)2869 Stress review - CERN, 4.11.2008

  25. Transverse compressive stresses Stress review - CERN, 4.11.2008

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

  27. st 0 100 200 300 400 500 Stress review - CERN, 4.11.2008

  28. Conclusions • Jc(B,) have been measured at very high fields (21 T) for Nb3Sn Bronze Route, Internal Sn and PIT wires. • The results show a dominant effect of the axial components 1 D approximation: Jc(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 Jc 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, 4.11.2008

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