1. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 1 Better conductors for 16-20 T dipoles? David Larbalestier Applied Superconductivity Center, National High Magnetic Field Laboratory, Florida State University, Tallahassee FL USA (special thanks to Lance Cooley (FNAL), Dan Dietderich (LBNL), Arno Godeke (LBNL), Peter Lee (ASC), Mark Rikel (Nexans), Venkat Selvamanickam (TcSUH), Mike Sumption (OSU), Chiara Tarantini (ASC), and Aixia Xu (TcSUH) for input for this talk) (And Bruce Strauss for yesterday’s talk) Future Circular Collider Workshop UniMail, University of Geneva, Geneva Switzerland February 12-14, 2014 Supported by DOE-HEP, NSF, State of Florida and CERN

2. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 2 Key points – work required! Three possible conductors (5 years) New and very much improved Nb 3 Sn Further developed round wire Bi-2212 Cable-friendly REBCO coated conductors Three long shots (10 years) Round wire REBCO (2212 analog) Round wire Fe-base superconductor MgB 2 with in-grain scattering for high vortex pinning and H c2 enhancement

3. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 3 Magnet Conductors so far…. 1. Nb47Ti conductor- thousands of 8  m diameter Nb47Ti filaments in pure Cu (0.8 mm dia.), easily cabled to operate at 10-100 kA 2. Bi-2223 – the first HTS conductor – uniaxial texture developed by deformation and reaction 2  m Ag 20  m Cu 50  m Hastelloy substrate 1  m HTS ~ 30 nm LMO ~ 30 nm Homo-epi MgO ~ 10 nm IBAD MgO < 0.1 mm 3. REBCO coated conductor – extreme texture (single crystal by the mile) – for maximum GB transparency 4. Bi-2212 – high J c without macroscopic texture!

4. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 4 Isotropic, multifilament 2212 has higher conductor J c than coated conductor! 4 Requires ~100 bar 890°C processing High J c, high J e and high J w has been demonstrated in a coil already (2.4T in 31T) Much less field distortion from 2212 than from coated conductors – better for high homogeneity coils 7 times increase in long length J e by removing bubbles 2212 (25% sc) + ~1900 A/mm 2 in 2212 REBCO coated conductor (1% sc)

5. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 5 Accelerator use demands strong vortex pinning forces (F p = J c x B) Depinning from a discrete normal (N) or insulating (I) pin is better than shear along a continuous channel (e.g. GB) which must be at least a weak superconductor (S or S’) to transmit supercurrent I pins are better than N or S’ pins because the pinning energy scale is then the full condensation energy High H c2 or irreversibility field H irr tilts the pinning force curve to high field A high density of strong pins pushes to full summation of individual pinning forces (f p ) so that F p ~ n f p Meingast, Lee and DCL, J. Appl. Phys. 66, 5971

6. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 6 Nb-Ti optimizes both f p so that Fp ~ n. f p Without  -Ti precipitates, only weak GB pinning occurs  -Ti precipitates start as normal metal (N) pins but become weakly superconducting (S’) when optimized because their high density outweighs their declining pinning strength At optimum,  -Ti pin density n is several times vortex density

7. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 7 What does Nb 3 Sn need? Nb 3 Sn has sparse and weak vortex pinning by grain boundaries that allows flux sliding along the whole GB network What can be done? Strengthen pinning by increasing the superfluid density (Tarantini ASC-NHMFL) Adding point pins (Dietderich LBNL) Restricting grain growth (Sumption OSU)

8. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 8 GB pins- 30-50 times lower pin density than Nb-Ti Peter Lee’s SEM images in Tarantini et al. arXiv 1310.6729, to appear SuST 2014 620°C / 192h SEM Fractographs A15 % of non-Cu Grain size / GB density A15 layer J c Q GB =F p / S GBRRP

9. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 9 J c (16T) can be enhanced by HT reaction (RRP 54/61) – but not to 2000 A/mm 2 J c (12T) is dominated by small grain size even though HT at lower temperature leaves lots of low- Sn, low Hc2 A15 present. Higher T HT helps Hc2, even as it causes grain growth Jc(16T) first increases at medium-high temperature (680-695°C) before dropping at 750 ° C, even as the diffusion barriers break badly and leak large amounts of Sn From VSM data Strauss (FCC talk Thursday) – we want 2000 A/mm 2 at 15 T

10. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 10 Vortex pinning strength, Q GB (16T) is strongly enhanced by high HT temp. The unit pinning force exerted by GBs on vortices increases as HT increases Pinning energy scale is   T c distribution +35% +68% From VSM data Higher T reactions require better diffusion barriers (RRP 54/61)

11. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 11 The minimum to be done for high J c (16T) Raise H irr by pumping in as much Sn as possible Raises the superfluid density f(T c ) and the energy scale for f p Strengthen barriers – RRR degrades for only 1- 2% of barrier breakdown

12. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 12 Or, add insulating pins to get a full condensation energy pinning Fine grains (~50 nm with insulating (I) Al 2 O 3 pins) drives high J c and F p curve into Nb-Ti form The problem: these are thin films and so far ppts. in FM conductors have been elusive 2000 A/mm 2 at 16 T is clearly within reach

13. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 13 Nb 3 Sn Conductors with Grain size reduction and F p,max shift (a) (b) What (Aim)?: To increase J c at 15 T, 4 K in Nb 3 Sn, increase B c2, or increase flux pinning. Here we focus on pinning, by; (1)  F p, or (2) a shift of F p,max from 0.2 B irr to 0.3 to 0.5 B irr. We will use Grain size refinement. Why?: If the Nb 3 Sn grain size (in films) is refined to 15-30 nm, the peak of the F p -B curve is shifted to 0.5B irr, improving the 12 T J c by a factor of three [D. R. Dietderich and A. Godeke, Cryogenics 48, 331 (2008)] This work was funded by the US Department of Energy, Division of High Energy Physics, Grant No. DE-FG02- 95ER40900, and DE-SC0010312. How?: Grain size ↓ by HT Temp ↓ have hit the limit (further T ↓ reduces Sn %). But Rumaner [Metall. Mater. Trans. A 25, 213 (1994)] used internally oxidized Zr to reduce grain size in films. Zeitlin attempted to transfer to strands [IEEE Trans. Appl. Supercon. 15, 3393 (2005)], using internally oxidized Nb-Zr but did not see refinement. Fracture SEM images of samples reacted at 850 °C for 10 min in (a) pure Ar and (b) Ar-O 2 atmospheres. (1) We exposed Nb-Zr/Sn wires (no Cu) to Ar- Oxygen atmosphere during HT to internally oxidize Zr and refine Nb 3 Sn grains – with success! A ZrO 2 particle TEM image showing the ZrO 2 particles Average Nb 3 Sn grain size as a function of reaction temperature 45 nm Nb 3 Sn grain size Xu, Sumption, Peng, Collings Appl. Phys. Letts submitted

14. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 14 Halved grain size (45 nm) shifts F p and provides relative J c advantage (a)(b) (2) Next Step, Subelement with internal oxidation: based on a review of the Ellingham Diagram, we put SnO 2 powder between the Cu/Sn core and the Nb-1Zr tube wall: For comparison, an analog with NbO 2 was also fabricated. Grain sizes of samples with (a) NbO 2 and (b) SnO 2, reacted at 650 °C for 150 h, are 91 and 43 nm, respectively. (a) (b) The (a) F p -B, and (b) reduced F p -B curves of samples reacted at 650 °C for 150 h (note B irr normalized F p curve at right indicates peak shift, distinct from B irr shift) The F p -B curves with SnO 2 and NbO 2 peak at ~0.3B irr and ~0.2B irr, respectively. 12 T layer J c of the wire with SnO 2 is ~6.1 kA/mm 2, that for Nb 2 O strand 5.4 kA/mm 2 – both excellent, but in fact suppressed by low B irr (20.5 T), because they are binary. However, a ternary version should have a B irr of ~25 T, if so, we estimate that the 12 T layer J c should be significantly higher (perhaps ~10kA/mm 2 ). 10 μm Cu matrix Sn core Cu SnO 2 powder Nb-1Zr tube SEM image of the wire with SnO 2, ready to stack into multi-filament strands. Ellingham Diagram Conclusion: in light of the results obtained, we anticipate that this approach could lead to substantial improvement in the performance of Nb 3 Sn conductors – and is ready for ternary multifilament investigation Paper submitted to Applied Physics Letters

15. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 15 If Nb 3 Sn is plan A for a 100 TeV LHC…………. Present RRP and PIT designs are unlikely to satisfy – the lessons they teach are that higher T reactions with more homogeneous Sn can raise J c but that stronger diffusion barriers are essential – max Jc may be 1200 A/mm 2 Insulating pins and finer grains may get the required J c – layer J c of ~5000 A/mm 2 (non-Cu ~ half this) shown in thin films Fabrication of ppt-containing fine filaments has been attempted by Supergenics, SupraMagnetics and most recently Hypertech-OSU …………….a focused program will be needed to establish feasibility of a 16 T Nb 3 Sn conductor

16. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 16 Plan B: 20 T requires HTS conductors J e ≈ 600 A/mm 2 10 T 16 T 20+ T REBCO tapes developed for electric utility applications (several hundred millions) versus recent HEP-driven development (so far about $5M) for Bi-2212 Note that this is 600 A/mm2 (20T) in a conductor that is about 25% 2212, so layer Jc is ~1800 A/mm 2 DCL et al. Nature Materials accepted, arXiv 1305.1269

17. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 17 Can J c of round wire (RW) 2212 go higher? Almost certainly………. Overpressure processing removes gas bubbles but leaves high angle GBs in place However no hysteretic signature of weak links as is quite obvious in Bi-2223 Bi-2212 phase field is broad, opening up cation defect pinning Recall that Bi-2212 is the first HTS conductor like an LTS conductor twisted, multifilament, round, good normal conductor in parallel – no Diffusion Barrier needed Bi-2212 RW is an ongoing effort of US BSCCo (Bismuth Strand and Cable Collaboration at ASC-NHMFL, BNL, FNAL and LBNL with OST and Nexans (under CERN support) and in association with EUCARD2 J. Jiang “Overpressure processing as the route to high Jc in coil length Bi-2212 round wires” MT-23 July 14-20, Boston MA, USA (2013) [Ref. **] Martin et al., IEEE Trans. Appl. Supercond., (1997) Very different in Bi-2223 tape Round wire Bi-2212

18. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 18 Challenge: understand 2212 phase – complex! Mark Rikel (Nexans) in the lead (EUCARD2 and BSCCo association)

19. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 19 Cables: Large magnets are better protected when operated at high current– cables! Easy path to 2212 cables through the standard Rutherford cable Bi-2212 Rutherford cables (Arno Godeke LBNL) with mullite insulation sleeve Danko van der Laan REBCO coated conductor cable wound in many layers helically on a round form Other variants too: e.g. Roebel cable REBCO cables are harder (Coated Conductor is a single filament) – but possible (IRL, KIT, CORC, twisted stack (MIT) Cables vital for 60 T hybrid at the NHMFL, an LHC energy upgrade and a neutrino machine based on a Muon Collider at Fermilab

20. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 20 Plan C: REBCO vortex pinning engineering works – MOCVD on IBAD substrates compatible with e.g. Cable on Round Cores (CORC) Strong recent developments in Selvamanickam group at TcSUH (Aixia Xu et al. MT23 presentation) Strongly enhanced vortex pinning from 4 to 77 K in magnetic fields up to 31 T in a 15 mol% Zr- added (GdY)-Ba-Cu-O superconducting tapes - Xu, Delgado, Khatri, Liu, Selvamanickam (TcSUH) and Abraimov, Jaroszynski, Kametani and Larbalestier (ASC-NHMFL) – in final draft

21. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 21 The insulating vortex pins that one would love in Nb 3 Sn too.. BaZrO 3 and RE 2 O 3 pins give REBCO the same J c properties as Nb-Ti At 77K, not 4.2K But layer thickness is 1  m 3-5  m REBCO and thinner substrates would go far to equalize J E too Pinning force at 4.2 K now exceeds 1500 GN/m 3, 75 times Nb-Ti TEM by Kametani ASC-NHMFL

22. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 22 J E comparisons today clearly favor RW Bi-2212 – Fine filament twisted conductor is ideal for high homogeneity NMR and accelerator magnets From the cover of the MagSci report (DCL et al. arXiv 1305.1269 – to appear Nature Materials 2014) Bi-2212 conductor support by DOE–OHEP: an outcome of Bismuth Strand and Cable Collaboration (BSCCo)

23. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 23 Making common cause across many sectors is possible and desirable http://www.nap.edu/catalog.php?record_id=18355 http://www.nap.edu/catalog.php?record_id=18355 High Magnetic Field Science and Its Application in the United States: Current Status and Future Directions (Halperin Chair Met in 2012, report about to issue Report released November 2013 Note the cover image! Bi-2212 developed under OHEP support!

24. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 24 High Magnetic Field Science and Its Application in the United States: Current Status and Future Directions The recommendations (Halperin (Harvard) Chair Consider regional 32 T superconducting magnets at 3-4 locations optimized for easy user access. Establish at least 3 US 1.2 GHz NMR instruments (planned commercial) for broad access and plan for ~1.5 GHz class system development Establish high field (~30 T) facilities at neutron and photon scattering facilities Construct a 20 T MRI instrument (for R&D) A 40 T all ‐ superconducting magnet should be designed and constructed, A 60 T DC hybrid magnet that will capitalize on the success of the current 45 T hybrid magnet at the NHMFL ‐ Tallahassee should be designed and built. Very strong synergy with HEP goals (LHC energy upgrade and Muon Accelerator) for high field use – needs HTS strand AND cable development

25. David Larbalestier, Future Circular Colliders Workshop, Geneva CH, February 12-14, 2014 Slide 25 Summary 16-20 T magnets require conductor development Nb 3 Sn is probably still plan A, but: New conductor concepts needed Stability margin may be too small, so pointing to HTS…… HTS now has a round wire, multifilament, twisted, good normal metal conductor (Bi-2212) But it requires special processing Strength properties uncertain All HTS have quench protection issues Specific solutions only – need general ones Other sectors need HTS conductors too NMR, MRI, Photon, neutron, national magnet labs