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Progress towards High Field, Homogeneity, and Stability Magnets Using Bi-2212 Round Wire

Progress towards High Field, Homogeneity, and Stability Magnets Using Bi-2212 Round Wire.

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Progress towards High Field, Homogeneity, and Stability Magnets Using Bi-2212 Round Wire

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  1. Progress towards High Field, Homogeneity, and Stability Magnets Using Bi-2212 Round Wire Ulf P. TrociewitzD.K. Hilton, E. Bosque, P. Chen, D. Davis, F. Kametani, J. Jiang, M. Matras, E.E. Hellstrom, H. Kandel, J. Lu, G. Miller, L. English, S. Miller, P. Noyes, M.D. Bird, W. Brey, T. Cross, L. Frydman, D.C. Larbalestier 2014 Users Committee Meeting Tallahassee, FL October 10-11, 2014

  2. Overview Previous Record coils: What we want to achieve: • Ultimately: Develop high field (30+ T) NMR magnet technology using HTS • Based on success with layer-wound REBCO and Bi-2212 RW high field coils, need to build larger insert magnet with HTS and understand how to use it in high homogeneity magnet systems • Intermediate step: Investigate the possibility of reaching 24+ T aiming at ppm level homogeneity and stability Two appealing conductor choices that are currently used at the NHMFL: • REBCO tape conductor; highly anisotropic but provides high Ic as received from the manufacturer -> “32 T project” • Bi-2212 round wire; macroscopically isotropic but requires heat treatment for high Ic -> “Platypus project” LW REBCO 4.2 T (35.4 T) Bi-2212 RW 2.6 T (33.8 T)

  3. Why Focus on Bi-2212? Elimination of Porosity makes Je of Bi-2212 RW competitive J. Jiang, M. Matras Ag-alloy Bi-2212 2212: 100 bar (121 x 18 filaments) • Round wire is versatile and is not affected by field angle and less affected by shielding currents as seen in REBCO and Bi2223 tape • Before heat treatment, Bi-2212 is ~65% dense and may contain C and H2O as contaminants • Powder in wire needs to be densified and kept dense during heat treatment • De-densification issue solved by overpressure heat treatment processing, which is key to high JE in coil length Bi-2212 round wires 2212: 1 bar J. Jiang et al, SuST 24 082001 (2011) D. Larbalestier et al, Nature Materials, Mar 13(4) (2014) 375–381 20 mm twist pitch

  4. 6.58 T A Bi-2212 High Field and Homogeneity Insert Magnet (Platypus II) 0.7 km What is our concept? Platypus II is a demonstration magnet Existing 16.5 T Oxford LTS (“IMPDAHMA”) magnet as test bed Bi-2212 layer-wound central solenoid Bi-2212 layer-wound compensation coil pair Ceramic (TiO2) insulated conductor Coils epoxy impregnated HTS insert powered separately from the LTS magnet using low noise Danfysik power supply (LTS magnet to operate in persistence) Btot= 23.1 T at Iop = 400 A and BLTS = 16.5 T for 1.3 mm dia. Bi-2212 D.K. Hilton, E. Bosque 45 m

  5. How Do We Envision Platypus II? Termination point Bi-2212 compensation coil pair • Inconel600 used for HT and as bore tube • Ceramic coating on conductor and mandrel provides electric insulation • Compensation coils assembled on a G-10 sleeve slides over solenoid • Current lead extensions made with CORC (REBCO current lead made by ACT, Inc., CO) • All conductor (all twisted, one length 520 m!) received from OST VTI CORC lead extensions Lead extension support 240 mm LTS-magnet Insert magnet Bi-2212 solenoid G. Miller

  6. Coil Geometry and Homogeneity • Coil will have production tolerances besides machining tolerances • Real coil vs. model coil • Thermal expansion • Conductor (and coil?) geometry change during OPHT • Amount of Inconel600 CF LTS reference plane • Introduce spacers that can be fine tuned after coils have been made • Expected adjustment is on the order of a couple of mm Mainly affects z2, and z4 terms Mainly affects z1 terms D.K. Hilton

  7. Small bobbins: Coil Modeling and Testing Thermal Contraction (cool-down): 30 turns (38 mm), 8 layers thermal Contraction, uncompensated “10% Platypus”: 15 turns (21.5 mm), 18 layers, compensated no thermal contraction Radial Stress Component [MPa] Thermal Contraction and FL (full epoxy impregnation; 400 A in 16.4 T): Von Mises Stress [MPa] • Thermal contraction can be compensated for • Stresses more moderate than initially anticipated E. Bosque

  8. Winding Symmetry and Field Homogeneity Platypus Mock-up: Layer 2 Winding Jogs • Round wire requires orthocyclic coil winding: location of jogs affect coil geometry and symmetry and therefore field homogeneity • Need to distribute jogs evenly over coil layers • Winding tests on a full size test mandrel using Cu wire as conductor stand in are currently being carried out to verify winding procedure • Data show very good circularity in Platypus mock-up Layer 4 Daniel Davis, Steven Miller

  9. How to Process Large Coils? 3.5 m 2.65 m Stirrer to force convection and provide homogeneous O2 mix Sample space • Large furnace made by Deltech, Inc. • Hot Zone: 17 cm dia, 50 cm at up to 100 bar (1 bar pO2) • Will be key equipment for Bi-2212 coil manufacturing • Final setup about to be finished L. English, J. Jiang, E. Hellstrom

  10. Conductor Insulation is Essential Ventilation Payoff • Bi-2212 conductor requires electrical insulation that is compatible with the heat treatment process (~890oC) • Uses TiO2 in a polymer matrix applied in a dip-coating process • After heat treatment polymer burns off and a fairly homogeneous and porous (O2 permeable!) coating surface visible • Both, an in-house route and a commercial route (nGimat) are currently being pursued Traverse guide • Reel-to-reel monitoring of green and coated wire using Laser micrometer Furnaces 150 C Coating Tanks Take up “slow” 0.24 m/min “fast” 0.72 m/min Bare wire avg. J. Lu, H. Kandel • In-house Reel-to-reel system for km-length insulation, targeting 20-30 µm thick coating

  11. Summary • Previous test magnets using REBCO tape conductor caused concerns about field profile and stability in tape coils • Better understanding and significant progress in thermal processing sparked renewed interest in Bi-2212 for high field magnets • Introduced design of an HTS insert with a compensation coil pair using Bi-2212 round wire • Conductor tests and test coils are in progress to aid refinement of design and manufacturing methods

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