1. Key issues for advancing high field superconducting magnets: quench detection & degradation limits Justin Schwartz Department of Materials Science and Engineering North Carolina State University With contributions from Wan Kan Chan, Gene Flanagan (Muons Inc.), Sasha Ishmael, Pei Li (FNAL), Federico Scurti, Tengming Shen (FNAL) and Liyang Ye, and also thanks to the team from National Instruments Funded by the U.S. Department of Energy through the SBIR/STTR program, FNAL and by ARPA-E 2014 Kyoto Workshop on HTS Magnet Technology for High Energy Physics – The 2nd Workshop on Accelerator Magnet in HTS (WAMHTS-2) 14 November 2014

2. Outline Introduction Recent progress on Rayleigh scattering based optical fiber quench detection Quench degradation limits –Experimental study of Bi2212 round wire –Computational modeling of YBCO CC Conclusions 2 “Everyone believes the experiment except the experimentalist, No one believes the theory except the theorist”. -- anon.

3. Goal: prevent degradation w/o overly reducing J E A race between the time-to-degradation & the time-to-protect Assessments in terms of a “time budget” time-to-degrade versus time-to-detect/protect 3  t lost  t DAQ  t decision  t response Must avoid false positives

4. Optical fiber based quench detection Optical fibers interrogated by Rayleigh scattering enable fully distributed temperature sensors with high spatial resolution Must also be fast … ultimately it’s the time budget Cross-correlations in wavelength domain between a reference and current scan for each sensor can be related to a change in temperature/strain The key challenge for converting the Rayleigh scatter profile into a distributed temperature/strain sensor is data volume and the associated signal-processing time Here a prototype DAQA has been built and tested by combining GPUs and FPGAs to process data from both fiber and voltage data 4

5. System description Algorithm scans fiber for segment with maximum change and the 5 neighboring around it to calculate normal zone evolution. “Focuses” on a normal zone. Normal zones on unused segments (i.e. not included in previous normal zone) also identified FPGA combines fiber data with analog voltage data  calculated evolution of the normal zone is correlated spatially with voltage data (for comparison; ultimately voltage data unnecessary) 5 Data paths, analog and fiber, combined and algorithms run on FPGA Fiber data in Fiber “hot spot regions” for decision algorithm and combination with analog data sent to board. Voltage/Thermocouple/Power supply info Rayleigh interrogator

6. Very small disturbance  recovery No signal seen in voltage data (T<T cs ) Optical fiber intrinsically more sensitive … “early notification” reduces t lost Can help manage data burden But must be wary of false positives 6 Heater pulse voltage Power supply shunt voltage Spectral shift is detected during heat pulse heat pulse

7. Increased heat pulse  recovery Fiber again detects normal zone earlier than voltage taps Both systems show recovery 7

8. Increase heat pulse  quench Fiber again shows normal zone much earlier than voltage taps and over longer length Potential for high spatial resolution is clear 8

9. Implementation/scale-up Scale-up: larger coil, longer fiber  more data/sec Migration to multiple GPUs working with FPGA(s) for increased data volume Fiber integration into coils & cables, including access to ends for junctions/terminations Cryo-optimized (enhanced) fibers  further improvements in sensitivity increase can allow decreased cycle-time or reduce front-end time-budget 9

10. Time-to-degradation: the other contestant in the race Degradation limits and underlying causes of failure is where the most significant disparity between conductors exists (LTS v Bi2212 v REBCO) Bi2212 round wire –Experimental forensics (relatively) straightforward; new results from Liyang Ye –Microstructural inhomogeneities  modeling complexity REBCO CC –Experimental forensics challenging –Highly anisotropic geometry leads to modeling complexity, but these have been overcome; emerging multiscale mechanical models 10

11. Quench-degradation in Bi2212 mostly wire-independent 11 OST 0.8 mm, 37*18 I c (s.f) = 450A OST 1.0 mm, 27*7 I c (s.f) = 160A Limited Bridging OST 1.2 mm, 85*18 I c (s.f) = 950A L. Ye, T. Shen, J. Schwartz in progress – this entire section of the talk What’s going on INSIDE the wires?

12. Bi2212 Forensics 12 11.7 mm No damage (11.0 mm) Filament damage (9.1 mm) Filament-bundle damage (4.8 mm ) No Bi2212 remains (2.4 mm) L. Ye, T. Shen, J. Schwartz in progress Internal T >> Measured surface T ? Large “internal damage gradient”

13. 9.1 mm spot: internal Bi2212 decomposition 13 11.7 mm 4.8 mm 9.3 mm left to burnt point Secondary phase region observed on the Bi2212 filament At > 862ºC, Bi2212  Liquid + AE +Cu-free CF Ag AEC Liquid L. Ye, T. Shen, J. Schwartz in progress

14. Another sample… fracture observed (80% loss of I c ) 14 V4 V5 V3V2 A B C B Fracture shows no Ag melting nor oxide phase transformations The distribution implies buckling due to compressive stress; wire wants to expand but is constrained Dynamics of these two experiments quite different due to differences in operating current

15. OP wire shows similar behavior 15 Consistent with Godeke results on I c -  of OP wire d(I/I c )/dT is higher in OP wire around 450-500 K

16. Cracks observed normal to wire axis w/reduced I c Section V4: ~40% I c degradation After light Bi-2212 etching Section V3: No I c degradation No cracks

17. Compare short wires to samples on ITER barrels of different materials Pre-strain determines temperature limit more than wire architecture or HT process 17 Increasing constraint Ti-alloy barrel G-10 barrel

18. Next level questions …. From what pre-existing defects or artifacts due the cracks nucleate or grow? What is optimum strain state in the wire after cool-down? Pre-compression increases allowable hoop-strain from Lorentz forces, but tensile strain delays quench degradation 18

19. REBCO conductor degradation Past studies showed: –Quench degradation is defect driven (Song et al., Acta Materialia) and highly localized –Delamination is also a failure mode REBCO mechanical state is complex; needs understanding of microscopic stress distributions Accumulated stresses from conductor fabrication, cable fabrication, bending, cooling, Lorentz forces, thermal stresses during a quench –Important to understand but difficult to measure in-situ the stress/strain profile of each layer within a conductor 19

20. 3D/2D electro-thermo-structural tape model Real dimensions, from  m to device scale Computationally efficient and experimentally validated Coupled electrical, magnetostatic, thermal and structural mechanics Full elastoplasticity on all layers 20 Q T TJ J c (B,θ,T) FmFm Electric (Superconductivity) Thermal Magnetostatic Structural Mechanics

21. Contact mechanics Bending radius Bending model Embed tape model into hierarchical coil model Contact mechanics model for bending Experimentally validated with coil quench studies 21 Experimental coil Multilayer tape model μm-scale 3D/2D tape model Multiscale hybrid coil model [W.K. Chan & J. Schwartz, IEEE Trans. Appl. Supercond., Vol. 22, No. 5, 2012

22. Most recent - adding a structural model 22 Fabrication Cooling Fatigue Analysis Tension @ RT, 77 K Bending @ 77 K Quench @ 77 K Experimental Validation Experimental Validation Computational Validation

23. Residual Stresses from fabrication and cooling 23 700 o C +buffer substrate 750 o C +YBCO buffer substrate 500 o C +Ag YBCO buffer substrate 25 o C +Cu Ag YBCO buffer substrate 77 K Cu Ag YBCO buffer substrate S2 S3 S4 CTE@ RT@ 77 K α C276 13.0x10 -6 10.7x10 -6 α cu 16.7x10 -6 7.8x10 -6 S2: 750 o C  500 o C S3: 500 o C  25 o C S4: 25 o C  77 K YBCO @ RT -200 MPa @ -0.08% YBCO @ 77 K -180 MPa @ -0.07% Cu @ 77 K -72 MPa @ -0.032% C276 @ RT -23 MPa @ -0.009% C276 @ 77 K +50 MPa @ 0.025% Ag @ RT -387 MPa @ 0.3% Ag @ 77 K -392 MPa @ 0.303% Stress, X-component (x10 2 MPa) State of commercial tape (Same stress profile after Cu is added) buffer @ RT No Ag Coefficient of thermal expansion

24. Validation via tensile analysis v experiment similar validation done using bending experiments 24 1.Apply axial tensile load 2.Capture key behavior: a.Yield stresses and strains of tape i.Yield strain predominately determined by Hastelloy ii.Yield stress determined by both Cu and Hastelloy b.Critical stresses and strains on YBCO layer 3.Tangent moduli (hardening slopes) are the least known data 4.Yield points in YBCO and buffer are considered as fracture points ε c = 0.0045 Experiment Simulation YBCO Stress, X-component (GPa) @ 77 K x x ε y = 0.0066 Experiment Simulation YBCO Stress, X-component (GPa) @ RT x x ε y = 0.005 ε c = 0.0065

25. Study bending + quench (500 K) in conductor 25 Tensile bending @ 77 K, R = 8.75 mmCompressive bending @ 77 K, R = 8.75 mm Dashed: T peak = 77 K Solid: T peak = 500 K Stress, X-component (x10 2 MPa) 1.Hot-spot temperature drives stresses on YBCO and buffer to be a.More tensile on tensile bending – bad! b.Less compressive on compressive bending – good! 2.For bending radius of R = 8.75 mm, stresses still < tensile limit (588 MPa) even temperature rises to 500 K – the melting temperature of solder. a.Lower stress margin under winding tension and hoop force in coil  lower maximum, safe temperature for protection

26. Study bending + quench in a coil 26 Outer turn N+2 Heater turn N 0 1.Stresses/strains become more complicated in coils 2.Stress variations depend on locations of turns and quenching temperature profile a.Inner turns near hot spot may become all compressive b.Outer turns may become all tensile c.Compressive bending not necessarily better Stress, X-component (x10 2 MPa) (Without residual stresses)

27. Conclusions Optical fiber quench detection may expand time budget by detecting incipient quenches well before current sharing Scale-up challenges are significant but solutions are evident Failure limits in Bi2212 vary weakly with wire and heat treatment, but strongly with wire initial stress state Pre-compression trade-off between maximizing “tensile strain budget” for Lorentz forces in highest field region and quench protection in low(er) field region Stress state in REBCO conductors is complex; layer-by-layer analysis may be needed (at least in highest stressed region of the magnet) 27

28. Closing thought “An overreliance on past successes is a sure blueprint for future failures.” -- Henry Petroski 28

29. Fiber integration to REBCO Coil AMSC conductor Coil ID = 96 mm Optical fiber: Single mode; 9 um core, acrylate coated fiber (1.3 m total length) Measured at: –77 K in LN 2 –Heater pulse energy range: 1- 24 J –Transport currents: 65 A to 165 A V & T direct measurements for comparison 29 Voltage taps Thermocouple Optical fiber