Better conductors for 16-20 T dipoles?

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


Key points – work required!

Three possible conductors (5 years)New and very much improved Nb3SnFurther developed round wire Bi-2212Cable-friendly REBCO coated conductors

Three long shots (10 years)Round wire REBCO (2212 analog)Round wire Fe-base superconductorMgB2 with in-grain scattering for high vortex pinning and Hc2 enhancement


Magnet Conductors so far….1. Nb47Ti conductor- thousands of 8 mm 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 mm Ag

20mm Cu

20mm Cu50mm Hastelloy substrate

1 mm 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 transparency4. Bi-2212 – high Jc without macroscopic texture!


Isotropic, multifilament 2212 has higher conductor Jc than coated conductor!

4

Requires ~100 bar 890°C processingHigh Jc, high Je and high Jw 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 coils7 times increase in long length Je by removing bubbles

2212 (25% sc)

+

~1900 A/mm2 in 2212

REBCO coated conductor (1% sc)


Accelerator use demands strong vortex pinning forces (Fp = Jc 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 supercurrentI pins are better than N or S’ pins because the pinning energy scale is then the full condensation energyHigh Hc2 or irreversibility field Hirr tilts the pinning force curve to high fieldA high density of strong pins pushes to full summation of individual pinning forces (fp) so that Fp ~ n fp

Equilibrium FluxoidSpacing at 5T, 4.2K

Multifilamentary Cu/Nb-TiComposite SSC Type Strandin Transverse Cross-Section

Meingast, Lee and DCL, J. Appl. Phys. 66, 5971


Nb-Ti optimizes both fp so that Fp ~ n. fp

Without a-Ti precipitates, only weak GB pinning occursa-Ti precipitates start as normal metal (N) pins but become weakly superconducting (S’) when optimized because their high density outweighs their declining pinning strengthAt optimum, a-Ti pin density n is several times vortex density

0

2

4

6

8

10

12

14

16

18

20

0 5 7 9 10 11

B (T)

F p (G

N/m

)3

ef = 5.3

= 4.4e f

= 3.4e f

= 2.5e f

= 1.1e f

precipitatesFiner and more denselypacked

864321

e f


What does Nb3Sn need?

Nb3Sn has sparse and weak vortex pinning by grain boundaries that allows flux sliding along the whole GB networkWhat can be done?

Strengthen pinning by increasing the superfluid density (Tarantini ASC-NHMFL)Adding point pins (Dietderich LBNL)Restricting grain growth (Sumption OSU)


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

SEMFractographs

A15 % of non-CuGrain size / GB density

A15 layer Jc

QGB=Fp/SGB

RRP


Jc(16T) can be enhanced by HT reaction (RRP 54/61) – but not to 2000 A/mm2

620°C

/192h

650°C

/96h

665°C

/50h

680°C

/48h

695°C

/48h

750°C

/96h

900

1000

1100

120016 T

4.2KNon

-Cu

J c @ 1

6T (A

/mm

2 )

2100

2400

2700

3000

Non

-Cu

J c @ 1

2T (A

/mm

2 )

12 T

Jc (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/mm2 at 15 T


Vortex pinning strength, QGB(16T) is strongly enhanced by high HT

temp.The unit pinning force exerted by GBs on vortices increases as HT increasesPinning energy scale is Tc distribution

+35%

+68%

620°C

/192h

650°C

/96h

665°C

/50h

680°C

/48h

695°C

/48h

750°C

/96h

3000

4000

5000

6000

16 T

4.2KA15

laye

r-Q

GB @

16T

(N/m

2 )

7000

7500

8000

8500

9000

9500

10000

A15

laye

r-Q

GB @

12T

(N/m

2 )

12 T

From VSM data

Higher T reactions require better diffusion barriers (RRP 54/61)

0.00

0.05

0.10

0.15

0.20

0.25

750°C695°C

680°C665°C

650°C620°C

2 4 6 8 10 12

8 10 120.00

0.05

0.10

0.15

0.20

0.25

0.30

192h620°C

Tc (K)

96h750°C

48h680°C

48h695°C

50h665°C

Nb3Snm0H = 16 T

96h650°C

f(Tc)

Tc ( K )

16 T


The minimum to be done for high Jc (16T)Raise Hirr by pumping in as much Sn as possible

Raises the superfluid density f(Tc) and the energy scale for fp

0 50 100 150 200 250 300 350 400

0.01

0.1

1

10

RRR

Average All Ring 1st Ring (Inner) 2nd Ring 3rd Ring (Outer) 3rd Ring - Corner 3rd Ring - Non Corner

% R

eact

-thro

ugh

(a)

0.01

0.1

1

10

100(b)

% R

eact

-thro

ugh,

% <

0.5m

m

0.6

0.8

1.0

1.2

1.4

(c)

Mea

n B

arrie

r Thi

ckne

ss (mm

)

Heat Treatment

Average All Row 1st Row (Inner) 2nd Row 3rd Row (Outer) 3rd Row - Corner 3rd Row - Non Corner

Strengthen barriers – RRR degrades for only 1-2% of barrier breakdown


Or, add insulating pins to get a full condensation energy pinning

Fine grains (~50 nm with insulating (I) Al2O3 pins) drives high Jc and Fp curve into Nb-Ti formThe problem: these are thin films and so far ppts. in FM conductors have been elusive2000 A/mm2 at 16 T is clearly within reach


Nb3Sn Conductors with Grain size reduction and Fp,max shift

(a) (b)

What (Aim)?: To increase Jc at 15 T, 4 K in Nb3Sn, increase Bc2, or increase flux pinning. Here we focus on pinning, by; (1) Fp, or (2) a shift of Fp,max from 0.2 Birr to 0.3 to 0.5 Birr. We will use Grain size refinement.

Why?: If the Nb3Sn grain size (in films) is refined to 15-30 nm, the peak of the Fp-B curve is shifted to 0.5Birr, improving the 12 T Jc 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-O2 atmospheres.

(1) We exposed Nb-Zr/Sn wires (no Cu) to Ar-Oxygen atmosphere during HT to internally oxidize Zr and refine Nb3Sn grains – with success! A ZrO2 particle

TEM image showing the ZrO2 particles

Average Nb3Sn grain size as a function of reaction temperature

45 nm Nb3Sn grain size

Xu, Sumption, Peng, Collings Appl. Phys. Letts submitted


Halved grain size (45 nm) shifts Fp and provides relative Jc advantage

(a) (b)

(2) Next Step, Subelement with internal oxidation: based on a review of the Ellingham Diagram, we put SnO2 powder between the Cu/Sn core and the Nb-1Zr tube wall:

For comparison, an analog with NbO2 was also fabricated.

Grain sizes of samples with (a) NbO2 and (b) SnO2, reacted at 650 °C for 150 h, are 91 and 43 nm, respectively.

(a) (b)

The (a) Fp-B, and (b) reduced Fp-B curves of samples reacted at 650 °C for 150 h (note Birr normalized Fp curve at right indicates peak shift, distinct from Birr shift)

The Fp-B curves with SnO2 and NbO2 peak at ~0.3Birr and ~0.2Birr, respectively.

• 12 T layer Jc of the wire with SnO2 is ~6.1 kA/mm2, that for Nb2O strand 5.4 kA/mm2 – both excellent, but in fact suppressed by low Birr (20.5 T), because they are binary.

• However, a ternary version should have a Birr of ~25 T, if so, we estimate that the 12 T layer Jc should be significantly higher (perhaps ~10kA/mm2).

10 μm

Cu matrix

Sn coreCu

SnO2 powder

Nb-1Zr tube

SEM image of the wire with SnO2, 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 Nb3Sn conductors – and is ready for ternary multifilament investigationPaper submitted to Applied Physics Letters


If Nb3Sn 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 Jc but that stronger diffusion barriers are essential – max Jc may be 1200 A/mm2

Insulating pins and finer grains may get the required Jc – layer Jc of ~5000 A/mm2 (non-Cu ~ half this) shown in thin filmsFabrication 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 Nb3Sn conductor


Plan B: 20 T requires HTS conductors

Je≈ 600 A/mm2

10 T16T

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/mm2

DCL et al. Nature Materials accepted, arXiv 1305.1269


Can Jc 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 pinningRecall 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

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16300

350

400

450

500

550

600

650

700

Field up Field down

I c(4.2

K),

A

Applied Field (T)

4.2K, H wire

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


Challenge: understand 2212 phase – complex!

Mark Rikel (Nexans) in the lead (EUCARD2 and BSCCo association)


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


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


The insulating vortex pins that one would love in Nb3Sn

too..

BaZrO3 and RE2O3 pins give REBCO the same Jc properties as Nb-Ti

At 77K, not 4.2KBut layer thickness is 1 mm

3-5 mm REBCO and thinner substrates would go far to equalize JE too

Pinning force at 4.2 K now exceeds 1500 GN/m3, 75 times Nb-Ti

TEM by Kametani ASC-NHMFL


JE 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)


Making common cause across many sectors is possible and

desirable

http://www.nap.edu/catalog.php?record_id=18355

High Magnetic Field Science and ItsApplication in the United States: Current

Status and Future Directions(Halperin Chair

Met in 2012, report about to issue

Report released November 2013Note the cover image! Bi-2212 developed under OHEP support!




High Magnetic Field Science and ItsApplication in the United States: Current

Status and Future DirectionsThe recommendations (Halperin (Harvard)

ChairConsider 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 developmentEstablish high field (~30 T) facilities at neutron and photon scattering facilitiesConstruct 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


Summary16-20 T magnets require conductor developmentNb3Sn is probably still plan A, but:

New conductor concepts neededStability 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 processingStrength properties uncertain

All HTS have quench protection issuesSpecific solutions only – need general ones

Other sectors need HTS conductors tooNMR, MRI, Photon, neutron, national magnet labs

Documents

Better conductors for 16-20 T dipoles?