Flükiger, W.E. Heraeus Seminar, ° Lower J c values than RRP and PIT wires at intermediate &low fields Large Sn radial composition gradient Bronze Route (Summary)

Flükiger, W.E. Heraeus Seminar, The Internal Sn diffusion processing (RRP) No intermediate anneals RRP: industrial version of this technique

Flükiger, W.E. Heraeus Seminar, Ti (from NbTi) Ti additives introduced by NbTi rods. P. Lee, D. Larbalestier, J.A. Parrell, M.B. Field, Y. Zhang, S. Hong, ICMC 05, Keystone, USA Nb 3 Sn Cu Nb RRP: NbTi addition in Internal Sn wires (OST)

R. Flükiger, SSEC, Fethiye, Courtesy Parrell (OST) RRP wires for various applications

Flükiger, W.E. Heraeus Seminar, (lower cost) RRP (Internal Sn route): (Summary)

Flükiger, W.E. Heraeus Seminar, The PIT process (Powder-In-Tube process) (based on NbSn 2 pre-reaction)

Flükiger, W.E. Heraeus Seminar, Cross section of a PIT wire Godeke et al., Cryogenics,48,308 (2008) What determines J c ?

25% 40% 10% 25% Non-Cu: Ternary 64h/675°C C.M. Fischer MS thesis UW-Madison PIT measured: ~140nm, 40% A15 in non-Cu OI-ST: ~170nm, ~60% A15 (?), 24%Sn PIT, all = best bit, ~140nm, 40% A15 PIT, + = 65% A15, ~140nm, ~24%Sn PIT, + = 65% A15, ~140nm, all best bit Sn CONTENT Nb 3 Sn FRACTION AND Sn CONTENT Nb 3 Sn FRACTION Simulations on SMI-PIT (Nb,Ta) 3 Sn for J c (12T,4.2K) Godeke, LTSW (2003) Godeke, et al., J. Appl. Phys (2005) Achieved: Bruker PIT: 2600 A/mm 2 (2008) OST IT: >3000 A/mm 2 (since 2003) Hypertech IT: 3400 A/mm 2 (2008) This illustrates the potential for improving J c in PIT wires. Flükiger, W.E. Heraeus Seminar,

PIT Process (Summary)

R. Flükiger, SSEC, Fethiye, Nb 3 Sn wires in magnets: mechanical stress effects

R. Flükiger, SSEC, Fethiye, Question: What is the effect of the strong Lorentz forces at high fields in large devices? The 3D stress situation in a wire can be studied by two cases: Effect of uniaxial stress and Effect of compressive stresses Wires in magnets: mechanical stress effects

R. Flükiger, SSEC, Fethiye, The effect of applied stress on J c is mainly correlated to changes in the phonon spectrum (Markiewicz et al.). Nb 3 Sn has the highest electronic densitiy of states N(E F ) among all A15 type compounds, but this is thought to be of minor importance for understanding the effects of strain. J c is sensitive to hydrostatical pressure components, but even more to nonhydrostatical ones. * Various measuring devices: Linear strain rig (J. Ekin) Pacman (Univ. Twente) Walters spiral (Univ. Geneva) Presence of axial strain: Effects on J c

R. Flükiger, SSEC, Fethiye, T (K) Degree of the elastic tetragonal deformation of the A15 phase in the wire when cooling a, c: tetragonal lattice parameters Thermal contraction: The mechanical precompression in Nb 3 Sn wires At 4.2K, the A15 phase is not cubic anymore! 1 – c/ a Differential thermal contraction over  T = 1’000 K: The filaments are under compression. Compression is anisotropic: tetragonal distortion

R. Flükiger, SSEC, Fethiye, Neutron diffraction analysis (ILL Grenoble) R.Flükiger, W. Schauer, W. Specking, Adv. Cryo. Eng. 28(1982)361 Longitudial Compression (//) Wire Axis); Tetragonal distortion c - axis a - axis 3D Stress Distribution in a Nb 3 Sn wire

R. Flükiger, SSEC, Fethiye, Max. current: 1’000 A Wire length ≤ 0.8 m I c criterion: 0.01  V/cm Applied field: ≤ 21T Rotation of the central axis of the spiral uniaxial force on the wire Modified Walters Spiral (WASP) for J c (  ) of Wires and Tapes Univ. of Geneva design

R. Flükiger, SSEC, Fethiye, Moving part Fixed part Inverse Walters spiral (Univ. Geneva): F = 5kN, I = 1000 A, H = 21 T wir e Stainless steel anvils Voltage taps Nb 3 Sn wire Inverse Walters spiral (Univ. Geneva)

R. Flükiger, SSEC, Fethiye, Effect increas es with field Stronger precompressi on in the bronze route than in internal Sn wire : This is due to the stronger Cu- Sn matrix, which has a higher Sn content I c /I co vs. uniaxial strain e for various Nb 3 Sn wires

R. Flükiger, SSEC, Fethiye, I c does not depend on strain criterion up to 0.6%/0.7% After releasing the strain, I c depends strongly on the criterion Furukawa bronze Nb 3 Sn wire for ITER, dia. 0.8 mm; 4.2K/13T The detection of the irreversible strain limit (begin of nanocracks) depends on the I c criterion: advantage for the Walters spiral, with the longest wire length D. Uglietti, V. Abächerli, R.Flükiger,  V/cm 1  V/cm Stronger Criterion (0.01 mV/cm) leads to early detection of nanocracks

R. Flükiger, SSEC, Fethiye, Stress  (MPa) Tensile stress  a Compressive stress  t J. Ekin (1986); W. Specking, R.Flükiger, (1987)  t (I cm ) <<  a (I cm ) Nb 3 Sn wire Effect of transverse stresses: much stronger than that of uniaxial stress: Reversibility:  t : only up to 20 MPa  a : up to 150 MPa J c vs. transverse compressive stress  t

R. Flükiger, SSEC, Fethiye, Persistent Mode operation (MRI and NMR magnets)

21 Image Resonance Spectroscopy (IRM) Most important market : 10 3 Mio. $/year. (1 IRM installation for ~ 1’000 hospital beds) IRM, Presently: Magnetic field: Tesla (NbTi) IRM, Ongoing: Introduction of systems at 3T (higher resolution) R. Flükiger, SSEC, Fethiye,

High Field NMR Magnet Technology Bruker stray field shielding technology (1900 MHz; 21.3T) 900 Actively Shielded 10 m 8 m3.5 m 4.5 m 5 G (0.5 mT) NbTi/Nb 3 Sn Latest news: 1 GHz NMR magnet (22.3 T) NMR companies: Bruker Oxford Instruments JASTEC

23 The perfect superconductor Persistent mode) Industrial superconductor in a IRM magnet dI/dt ~ 0 : The initial current I o decreases very slowly in the magnet Consequence: * Very high stability of the magnetic field during the MRI analysis (20 minutes) * High precision of the medical analysis. In a NbTi magnet, a decrease of I o 0.5 I o and thus of B o 0.5 B o needs 7’500 years (!) R. Flükiger, SSEC, Fethiye, R = 0 : Current can flow several years in a superconducting loop

R. Flükiger, SSEC, Fethiye, The time decay in a superconducting loop: the Kim-Anderson law The experimental n value of the V – I transition is commonly used as a quality index for technical superconductors For homogeneous materials: electric field E can be represented as: E ~ J n (Zeldov et al.). Thus for the V – I curve, it is : V ~ I n. The n value is directly proportional to the volume pinning energy, U(B,T) : n(B,T) = U(B,T)/k B T. Definitions: F p (B,T): pinning force, determines the critical current density J c U(B,T): pinning energy, determines the thermally activated depinning a high n value is the consequence of a high pinning energy U(B,T), and corresponds to a slow decay of the persistent current. The irreversible magnetization M in a type II superconductor is ~ J when the sample is fully penetrated by the field. Kim and Anderson: M 0 : unrelaxed magnetization t 0 is a characteristi c length scale

R. Flükiger, SSEC, Fethiye, Therefore, the normalized relaxation rate S(B,T) is inversely proportional to U(B,T) : It follows that for a homogeneous superconductor: Usually, the value n is taken as a quality index of a superconducting wire. Indeed, this quantity is very easy to determine (from V – I curves). However, the relaxation rate S is a better quality index of the average pinning properties than the n value. The relaxation rate S(B,T) Magnetic relaxation measurement Example for relaxation measurement: Magnetization M(t) of a Y123 tape is measured in a vibrating sample magnetometer (VSM) at constant field B = 5T at 5K (B perpendicular to tape surface. The standard logarithmic relaxation is recovered after ~ 10 s. C. Senatore and R. Flükiger, SuST, 22,095016(2009)

R. Flükiger, SSEC, Fethiye, Relaxation rates for various superconductors C. Senatore, P. Lezza, R. Flükiger, Adv. Cryo. Eng., 52,654(2006) For a persistent mode operation in a NMR magnet, only superconductors with relaxation rates below 5%/decade can be envisaged: This is the case for the metallic superconductors NbTi, Nb 3 Sn and MgB 2. The highest magnetic field achieved so far in a NMR magnet is 23.5 T, corresponding to 1 GHz. This magnet, built by Bruker, Germany, is based on alloyed Nb 3 Sn wires.

R. Flükiger, SSEC, Fethiye, The situation in High T c superconductors is more complex: YBCO has a very low relaxation rate, even lower than Nb 3 Sn. These data suggest that YBCO would be the ideal superconductor for NMR magnets at very high fields (> 25 T). However, these are inductive measurements. In a magnet, the transport current encounters a particular problem which occurs in all superconducting oxides: the outermost layer of the HTC compound YBCO is insulating! Thus, the contact resistance is too high for persistent mode operation. For this reason, there is no NMR magnet based on YBCO operating in the persistent mode yet. Recently, a progress has been achieved, but the industrial proof is still missing. The developments are still going on. Note: * Bi-2212 and Bi-2223 are improbable for NMR, the relaxation rates being too high. * No relaxation data are presently available for Fe based Pnictides, another potential superconductor in view of high fields (see Lecture III). Persistent mode operation in HTS magnets?

R. Flükiger, SSEC, Fethiye, Examples of Nb 3 Sn magnets

R. Flükiger, SSEC, Fethiye, Fusion Power: 500 MW Plasma Current: 15 MA Plasma Volume: 837 m 3 Torus Radius: 6.2 m Plasma Radius: 2.0 m Energy Amplification: Q= ,5 T  - 12 T 12 T The ITER Machine (Tokamak) LTS conductors

R. Flükiger, SSEC, Fethiye, 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 Cable The ITER TF Model Coil Nb 3 Sn pancake

R. Flükiger, SSEC, Fethiye, LHC dipoles Magnetic fields in various magnet designs Perrmanent magnet solenoi d

R. Flükiger, SSEC, Fethiye,

R. Flükiger, SSEC, Fethiye, Low and intermediate fields: J c determined by flux pinning (grain size) At high fields, J c is determined by the value of B c2 Industrial round wires for magnets up to 23.5 T (1 GHz): Nb 3 Sn The amount of Nb 3 Sn wire in a magnet increases strongly with the produced field: at 20T, 5 times more Nb 3 Sn than for 12 T. Bronze route wires: «persistent mode» operation in NMR magnets Internal Sn (RRP) and Powder-in-Tube (PIT) wires satisfy the conditions for LHC Upgrade accelerator magnets : J c = 1’500 A/mm 2 at 4.2K/15T. Conclusions : Nb 3 Sn wires