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Improved Dielectric Materials for Passive Quench Protection of Superconductor Operating Near 77K

The temperature dependence of diamond thermal conductivity with the 1.1% 13C ... Thickness of diamond film will significantly suppress thermal ...

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Improved Dielectric Materials for Passive Quench Protection of Superconductor Operating Near 77K

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    1. Improved Dielectric Materials for Passive Quench Protection of Superconductor Operating Near 77K Berkeley, California 5 - 7 September 2006

    Dr. Charles E. Oberly Air Force Research Laboratory Wright-Patterson AFB, OH 45433 Ph: (937) 255-4814 Email: charles.oberly@wpafb.af.mil CHATS-06 Workshop at LBNL Presented By:

    2. Quench Experience in Superconducting Magnets

    Operating Temperature (K) ?T (K) Normal Zone Propagation Velocity Quench Modeling & Simulation Quench Detection Quench Experience Base LTS NbTc/Nb3Sn 4 - 8 < 1 High 1D/2D/3D OK Easy Huge HTS BSCCO YBCO 20 - 35+ 40 - 77+ Several Several Low Very Low 1D/2D OK? 3D Required? Difficult Very Difficult Small None * Hot Spot I T V Stabilizer Superconductor

    3. What, Me Worry?

    Gyrotron magnet successfully operated with tube Then tested way out of envelope to destruction at ~38K Quench damage occurred as might have been expected after post mortem Magnet rewound and retested successfully Be prepared! Quench protection gets much more difficult as temperature rises. Cryomagnetics will extend work to YBCO in OSD SBIR

    4. BSCCO Thermal Management / Quench Protection

    Experience is with BSCCO up to ~40K Thermal management relatively easy up to 35K Heat capacity and normal zone propagation are manageable Reasonable magnet experience Quench damage can occur when BSCCO magnets are operated out of envelope or when quench detection sensitivity is limited by excitation noise BSCCO quench protection can be managed =35K Out of design envelope quench caused by multifactors Inadequate refrigerator (37-38K) Cu resistance increases 2X BSCCO Jc decreases 1/2X Fast repeating I Ramp Very high quench detection V Gyrotron Magnet

    Coils only store 100’s of joules at 77K. 2006 Advances in YBCO Superconducting Magnets 1 0 3 2 4 2005 2006 200? Time Prototype Magnet SuperPower American Superconductor 77K 77K 50-77K ? 64K 73K Magnetic Field (Tesla) 64K

    7. Simulation of Temperature & Voltage Rise in Superconducting Tapes

    Nb3Sn Tape @ 12 K & 225 A Ul =50 cm/s Voltage Simulation Temperature Simulation Tcs=13.7 K Tc=16.5 K 25 ms 150 ms V1 V2 V3 V4 100 ms Bi-2223/Ag Tape @ 14 K & 100 A Bi-2223/Ag Tape @ 40 K & 100 A [Ul ]hts =0.23 cm/s 46 mV [Ul ]hts =1 cm/s << [Ul ]lts 10 s 60 s Tcs=44.4 K Tc=93 K Fast Response Slow Response Very Slow Response Data: Y. Iwasa Agrees well with experiment.

    8. Normal Zone Propagation Velocity vs I/Ic In YBCO - J. Schwartz, FSU

    Cu-plated Bi-2223/Ag (Ic = 119 A @77.3 K) [ORNL] 720 K No Ic-degradation 800 K Degradation (Ic = 119 A ? 50 A) 4.27 mm 0.25 mm 15-?m thick Cu [R. Duckworth (ORNL), 2004] ORNL Data

    10. YBCO Thermal Management / Quench Protection (Comments Directed at Approaching 77K)

    T(K) 600 t(sec) V(volt) 10-3 t(sec) YBCO will be very stable, but very difficult to protect against quench in energetic magnets near 77K YBCO Volcano Thermal management margin is huge (?T is several K) Heat capacity is huge (near 77K) Quench velocity and normal zone are small Quench detection is difficult / impossible Initial analysis at Yokohama Thermal Stability Conference in 1980 (Chyu & Oberly) O2 O2 O2

    430 K Cu-Plated YBCO: Slight Ic-degradation [MIT] How Hot Can a Winding be Heated up to? (cont.) Y. Iwasa (MIT)

    12. YBCO Quench Protection Methods Near 77K

    Active Protection Detect and dump Detect and actuate heater Subdivide magnet and detect locally Passive Protection Spread the quench heat fast YBCO conductor materials not modifiable Spread heat radially through thermally conductive dielectric Spread current radially through rapidly switched dielectric Hybrid Active + Passive Protection Spread the quench passively Induce higher quench voltage to detect Improved detection sensitivity / model estimator

    13. Active Quench Protection

    Detection Voltage Acoustic Emission (AE) Temperature Challenges Sensitivity Response Time Exciter / Machine Noise

    14. Passive Quench Protection

    Active quench detection may not be sensitive enough and/or too slow The real problem for YBCO conductor engineering will be quench protection near 77K in magnets Requires high thermal diffusivity to promote normal zone propagation and YBCO is crap Must promote radial heat diffusion in magnets with thermally conductive buffers and dielectrics Solutions: Thermally conductive buffers and dielectric insulation Passive elements, e.g. cryogenic zinc oxide varistor Interface control will be critical

    15. Some Potential Dielectric Materials for 3D Spreading of Heat & Current

    ZnO – CeramPhysics Diamond – Tai Yang and AFRL/PRP Sapphire (AL2O3) All have high thermal conductivity near 77K in bulk ZnO and diamond can be switched from dielectric to electrically conductive states

    16. 77K Passive Quench Protection in a Coil

    I VQ radial = cm/sec * Thermally and/or Electrically Switched Conductive Dielectric VQ azimuthal ? cm/sec Quench Initiation VQ azimuthal ? cm/sec Temperature 77K Quench Initiation Time azimuthal heat conduction 750K physical tape damage limit YBCO oxygen stoichiometry limit radial heat conduction Coil Section Quenched in ~ One Minute VQ azimuthal one turn VQ radial 1000 turns Each Turn ? 1 cm3 UNPROTECTED nickel substrate melting limit ?K 1728K PROTECTED 10kj deposited in 1cm3 will explode it - typical magnet energy is 25kj 300K maximum safe limit 150K desirable limit

    17. Idealized Concept of Passive YBCO Quench Protection

    Promote Transverse or Radial Thermal and Electric Current Spreading Typical YBCO Tape Dielectric Blocks Heat Flow YBCO has very small Thermal Conduction Typical Buffers Block Heat Flow Ag & Dielectric (Protection) YBCO (Superconductor) Buffer (Dielectric) Substrate (Metal) Ideal Multi Turn Lay-up Ag Protective Layer Only Thermally Conductive Buffer Surrounding Cu Plating Minimized Thermal Contact Resistance Maximum Dielectric Thermal Conductivity Switched (E) Electric Conductivity Fast Dielectric Switching

    18. Idealized Concept of Passive YBCO Quench Protection

    Cascading Secondary Quench Initial Quench * * * * * *

    19. Slow Quench Propagation without Passive Measures

    20. Fast Quench Propagation with Passive Thermal Transport

    21. Very Fast Quench Propagation with Passive Thermal & Current Transport

    22. Thermal Conductivity in Diamond

    Figure 4. The thermal conductivity of type II diamonds: (1): the natural type IIa crystal [50], (2): the natural type IIa crystal [48]; (3): the synthetic type IIa crystal [51]; (4): the natural type IIb crystal [49]. Figure 5. The thermal conductivity dependence on the 13C isotope content. The solid lines are the calculated data, the symbols are the experimental results (taken from the work [55]). T = 80K T = 150K T = 300K Figure 6. The temperature dependence of diamond thermal conductivity with the 1.1% 13C isotope content (corresponding to the natural content) and 0.1% content (taken form the work [55]):  - the data from the work [55]; + - the data from the work [57]; x - the data from the work [54]. Practical thermal conductivity of relatively pure synthetic diamond is 10-100X greater than Cu and 10X greater than Al2O3 near 77K Impurity Effects Copper Al2O3

    Goals of Phase I STTR Program at Ceramphysics Make an Assessment of Four Quench-Protection Strategies Diamond Films - Paper Study Cryovaristor Films - Paper Study High-K Ceramic Powders Loaded in Formvar for Draw-Tower Coating on HTS Ribbon - Prepare and Measure Bar Samples at 50 and 60 volume % Ceramic Powders in a Formvar - Demonstrate Coating on HTS Ribbon Sample - Measure Voltage Standoff High-K Ceramic Film Sputtered on HTS Ribbon - Demonstrate Sputtered Film - Measure Voltage Standoff Three-dimensional Thermal Modeling (U. of Pitt.) Assessment of Diamond Films Background Region 2 Region 1 ? ? exp(?/2T) Region 1 dominated by short phonon mean free paths Region 2 dominated by specific heat and boundary scattering of long mean free path phonons K = ?Cv?/3 Single-crystal sapphire and alumina ceramic, both Al2O3 Boundary scattering dominates thermal conductivity of ceramic alumina (30 micron grain size) and suppresses thermal conductivity by 500 times compared to single-crystal sapphire Thermal conductivity of single crystal diamond, 2.5 mm width Phonon mean free path in diamond single crystal calculated using, K = ?Cv?/3. These data are scaled to the case of a diamond film by the ratio of the film thickness to the single-crystal width (2.5 mm)

    27. Diamond Thermal Issues

    Difficult to obtain quality diamond at process temperatures <400C Phonon scatter in thin films at ~77K significantly reduces thermal conductivity Are diamond thin film thermal properties at 77K adequate?

    Thickness of diamond film will significantly suppress thermal conductivity compared to the single-crystal case Assessment of Cryovaristor Varistor switches from an insulator to a conductor in the breakdown region Pre-breakdown region is due to high-resistivity grain boundaries, and upturn region is due to conduction through ZnO grains Cryovaristor retains varistor characteristics down to 4.2 K Ceramic grain size ~ 5 microns High-K Ceramic Powders in Formvar Ceramic NN downselected for this Phase I program Discussion with California Fine Wire Co. regarding practical draw-tower considerations for coating HTS ribbon - 2 micron ceramic powder - 50 - 60 vol. % of ceramic powder in Formvar - GE 7031 varnish selected as the Formvar Status - Ceramic powders received from NexTech Mat'ls Ltd. - Loaded Formvar bars have been cast - Thermal conductivity and thermal expansion measurements will be performed Zn2GeO4 ceramic downselected for this Phase I program Sputtering target made and x-rayed Trial films sputtered on alumina substrates HTS ribbon samples from SuperPower will have films sputtered on them soon Sputtered High-k Ceramic on HTS Ribbon

    32. Thermal Contact Resistance Issues

    Issues at Cryogenic Temperature Compliant Dielectrics Kapitza Boundary Resistance

    33. Alternative Solution to Quench Protection for YBCO Magnets

    Drop Temperature to = 40K and Increase Iop/Ic (Ic will be high at = 40K) Alternative solution negates YBCO advantage by increasing refrigerator size Air Force requires small refrigerators so YBCO near 77K that is quench protected is the goal DOE and other commercial applications must choose quench protection for YBCO carefully and quench protection issues will select optimal Top and Je

    34. Summary

    LTS stability poor / quench protection good (4K) BSCCO stability good / quench protection good (=35K) YBCO stability excellent / quench protection poor to nonexistent (~77K) 3D analysis of integrated thermal and electromagnetic parameters during quench required does not yet exist for YBCO Analysis will lead to appropriate selection of strategic and tactical management of quench in energetic YBCO magnets near 77K Quench protection may be the last unresolved inhibitor for YBCO magnets at 77K

    35. Conclusions

    Rapid detection of local quench is critical for YBCO but may not be attainable at sensitivity required Passive heat spreading measures can rapidly expand the temperature “bubble” in YBCO to enable quench detection for subsequent active protection measures

    Backup

    37. Quench Modeling & Simulation Limitations

    LTS: 1D, 2D and 3D quench analyses developed “Pancake” BSCCO coils: 2D codes may adequate 3D analysis with coupled thermal and electromagnetic parameters capable of rapid transients are required for YBCO coated conductor magnets near 77K

    38. Numerical Stability/Quench Simulation Code at U. Pitt/CeramPhysics

    Complete 3-D, transient, fully coupled thermal-electric, stability/quench capability established in early 1990’s (when I was at Carnegie Mellon) Finite difference scheme with control volume formulation (Finite-Volume) - Patankar/Spalding scheme developed in early 1980’s, similar algorithm to Fluent, CFDRC … Most physics and sub-models are embedded Capable of handling complex geometry Not very user friendly at this time (actually since 1991)

    39. Publications 1991-1993

    M.K. Chyu and C.E. Oberly, "Numerical Modeling of Normal Zone Propagation and Heat Transfer in a Superconducting Composite Tape," Trans. IEEE on Magnetics, Vol. 27, 1991, pp. 2100-2103. M.K. Chyu and C.E. Oberly, "Effects of Transverse Heat Transfer on Normal Zone Propagation in Metal-Clad HTS Coil Tapes," Cryogenics, Vol. 31, 1991, pp. 680-686. M.K. Chyu and C.E. Oberly, "Influence of Operating Temperature on Stability and Quench of Oxide High-Tc Superconductors," Advances in Cryogenic Engineering, Vol. 37, Part A, 1992, pp. 307-313. M.K. Chyu and C.E. Oberly, "Influence of Operating Temperature and Contact Thermal Resistance on Normal Zone Propagation in Metal-Sheathed High-Tc Superconducting Tape," Cryogenics , Vol. 32, No. 5, 1992, pp. 519-525. M.K. Chyu, H. Ding, C.E. Oberly, "Intrinsic Stability and Analysis of Normal Zone Propagation in Superconducting Tapes," Cryogenic Engineering Conference and International Cryogenic Materials Conference, Albuquerque, July 12-16, 1993. M.K. Chyu, H. Ding, C.E. Oberly, "Importance of Three-Dimensional Modeling for HTSC Thermal Stability Analysis," Cryogenic Engineering Conference and International Cryogenic Materials Conference, Albuquerque, July 12-16, 1993. W.N. Lawless and M.K. Chyu, "Dielectric Insulations Incorporating Thermal Stabilization for Powder-In-Tube Ceramic Superconductors," accepted for presentation, Cryogenic Engineering Conference and International Cryogenic Materials Conference, Albuquerque, July 12-16, 1993.

    40. Hybrid Solutions to Quench Protection

    Maximize passive heat spread Promotes better detection Rapid detection and dumping Local ? Use of model estimator to enhance detection/protection

    42. Detection Challenges

    Source of hotspot may change over the lifetime of the machine. Voltage alone is a poor indicator due to poor signal-to-noise ratio Temperature alone is a poor indicator due to long time constants in HTS coils. Current measurement and model may predict the formation of a quenching hotspot, however, unreliable.

    45. Estimator Method Benefits / Requirements

    This method acts as a virtual sensor on a hotspot where no real sensor would be able to exist This system uses several quench detection techniques simultaneously so the strengths of each technique help cover the weaknesses of others Requires validated model and magnet experience

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