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

Stress review - CERN, 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

Stress review - CERN, 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

Stress review - CERN, 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

Stress review - CERN, 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

Stress review - CERN, 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

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

Stress review - CERN, 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

Stress review - CERN,  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

Stress review - CERN, 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

Stress review - CERN, Tensile stresses 11

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, Origin of precompression in superconducting wires 12

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

Stress review - CERN, 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

Stress review - CERN, 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 : % 10% reduction of B c2 (  )/B c2m :  m : %

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

Stress review - CERN, 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

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

Ekin’s model ten Haken’s model Stress review - CERN,

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

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

Stress review - CERN, 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

Stress review - CERN, 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 =

Stress review - CERN, 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

Stress review - CERN, Transverse compressive stresses

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

tt Stress review - CERN,

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,