# Presentation: 626257

## 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:**

## Quench Experience in Superconducting Magnets

**Quench Experience in Superconducting Magnets**

**Operating Temperature (K)**

******T**
**(K)**

**Normal Zone Propagation Velocity**

**Quench Modeling & Simulation**

**Quench Detection**

**Quench Experience Base**

_**LTS**_

**NbTc/Nb****3****Sn**

**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**

## 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**

## 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 J****c**** decreases 1/2X**
**Fast repeating I Ramp**
**Very high quench detection V**

*Gyrotron Magnet*

## Coils only store 100’s of joules at 77K.

**Coils only store 100’s of joules at 77K.**

## 2006 Advances inYBCO Superconducting Magnets

**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**

## Simulation of Temperature & Voltage Rise in Superconducting Tapes

**Simulation of Temperature & Voltage Rise in Superconducting Tapes**

**Nb****3****Sn***** *****Tape @ 12 K & 225 A***** ***

***U******l *****=****50 cm/s**

_**Voltage Simulation**_

_**Temperature Simulation**_

***T******cs*****=****13.7 K**

***T******c*****=****16.5 K**

**25**** ****ms**

**150**** ****ms**

***V******1***

***V******2***

***V******3***

***V******4***

**100**** ****ms**

**Bi-2223/Ag***** *****Tape @ 14 K & 100 A***** ***

**Bi-2223/Ag***** *****Tape @ ****40 K**** & 100 A**

**[*****U******l *****]*****hts****** =*****0.23**** cm/s**

**46 mV**

**[*****U******l *****]*****hts****** =*****1**** cm/s << ****[*****U******l *****]*****lts***

**10**** ****s**

**60**** ****s**

***T******cs*****=****44.4 K**

***T******c*****=****93 K**

**Fast Response**

**Slow Response**

**Very Slow Response**

- Data:  Y. Iwasa

***Agrees well with experiment.***

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

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

## ORNL Data

- Cu-plated Bi-2223/Ag (*I****c ***= 119 A @77.3 K)  [ORNL]
**720 K**   **No *****I******c*****-degradation**
**800 K**   ***Degradation*** (*I****c ***= 119 A ****** **50 A)

**4.27 mm**

**0.25 mm**

**15-********m thick Cu**

**[R. Duckworth (ORNL), 2004]**

**ORNL Data**

## 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

## How Hot Can a Winding

**How Hot Can a Winding**
**be Heated up to? (cont.)**

- Y. Iwasa (MIT)

## 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**

## Active Quench Protection

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

## 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**

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

**ZnO – CeramPhysics**
**Diamond – Tai Yang and AFRL/PRP**
**Sapphire (AL****2****O****3****)**

**All have high thermal conductivity near 77K in bulk**

**ZnO and diamond can be switched from dielectric to electrically conductive states**

## 77K Passive Quench Protectionin a Coil

**77K Passive Quench Protection****in a Coil**

**I**

**V****Q**** radial ≤ cm/sec**

*****

**Thermally and/or Electrically Switched Conductive Dielectric**

**V****Q**** azimuthal ******** cm/sec**
**Quench Initiation**
**V****Q**** 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**

**V****Q**** azimuthal                one turn**
**V****Q**** radial                       1000 turns**

**Each Turn ******** 1 cm****3**

**UNPROTECTED**

**nickel substrate**
**melting limit**

**?K**

**1728K**

**PROTECTED**

**10kj deposited in 1cm****3**** will explode it - typical magnet energy is 25kj**

**300K**

**maximum safe limit**

**150K**

**desirable limit**

## Idealized Concept of PassiveYBCO Quench Protection

**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**

## Idealized Concept of PassiveYBCO Quench Protection

**Cascading Secondary Quench**

**Initial Quench**

*****

*****

*****

*****

*****

*****

## Slow Quench Propagationwithout Passive Measures

## Fast Quench Propagationwith Passive Thermal Transport

## Very Fast Quench Propagation with Passive Thermal & Current Transport

## Thermal Conductivity in Diamond

**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**** 13****C 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% ****13****C 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 Al****2****O****3**** near 77K**

**Impurity**
**Effects**

**Copper**

**Al****2****O****3**

## 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

- 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)

## 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

## Sputtered High-k Ceramic

-  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**

## Thermal Contact Resistance Issues

**Issues at Cryogenic Temperature**
**Compliant Dielectrics**
**Kapitza Boundary Resistance**

## Alternative Solution to Quench Protection for YBCO Magnets

**Drop Temperature to ≤ 40K**
**and**
**Increase I****op****/I****c**** (I****c**** 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 T****op**** and J****e**

## 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**

## 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

## 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**

## 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)**

## 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.**

## 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**

## Detection and Shutdown vs. the Onset of Coil Damage Timeline 

## 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.**

## Reducing Detection Timeand Improving Reliability

- _Hotspot Estimator_

**A current driven live model of the coil’s anticipated hotspot runs in real time**
**Estimators used when you can’t put a sensor on what you need to sense**

## Estimator for Simple Conductor

**Estimator utilizes current and voltage measurement for rapid response**
**Codependence on current and temp will desensitize the model to voltage noise**
**Slower temperature feedback will lock the estimator into zero steady state error**

## 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**