Key issues for advancing high field superconducting magnets: quench detection & degradation limits

Justin SchwartzDepartment of Materials Science and EngineeringNorth 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

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.

Goal: prevent degradation w/o overly reducing JE• A race between the time-to-degradation & the time-to-protect• Assessments in terms of a “time budget”

time-to-degradeversus

time-to-detect/protect

3

NZ initiation to signal

Signal detection time

Signal assessment &

decision

Protective actions

Dtlost DtDAQ Dtdecision Dtresponse

Must avoid false positives

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

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

Very small disturbance recovery

• No signal seen in voltage data (T<Tcs)

• Optical fiber intrinsically more sensitive … “early notification” reduces tlost

• Can help manage data burden

• But must be wary of false positives

60 1 2 3 4 5 6

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

Hea

ter

PS

& P

S S

hunt

Vol

tage

[V]

time [s]

0 1 2 3 4 5 60

1

2

3

4

5

x 10-4

volta

ge [V

]

HeaterPS shuntSec1Sec2Sec3Sec5Sec6Sec7Sec8Sec9SecETE

Heater pulse voltage

Power supply shunt voltage

0 1 2 3 4 5 60

5

10

15

20

25

time [s]sp

ectr

al s

hift

[GH

z]

plot 0plot 1plot 2plot 3plot 4plot 5 (hottest spot)plot 6plot 7plot 8plot 9plot 10heat pulse (not to scale)

Spectral shift is detected during heat pulse

heat pulse

Increased heat pulse recovery

• Fiber again detects normal zone earlier than voltage taps

• Both systems show recovery

7

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

10

20

30

40

50

60

70

time [s]sp

ectr

al s

hift

[GH

z]

plot 0plot 1plot 2plot 3plot 4plot 5 (hottest spot)plot 6plot 7plot 8plot 9plot 10heat pulse (not to scale)

0

0.02

0.04

Hea

ter

PS

& P

S S

hunt

Vol

tage

[V]

time [s]

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

0.05

0.1

volta

ge [V

]

Heater PSPS shuntSec1Sec2 (heater)Sec3Sec5Sec6Sec7Sec8Sec9SecETE

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

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

10

20

30

40

50

60

70

time [s]

spec

tral

shi

ft [G

Hz]

plot 0

plot 1

plot 2

plot 3

plot 4

plot 5 (hottest spot)

plot 6

plot 7

plot 8

plot 9

plot 10

heat pulse (not to scale)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

Hea

ter

PS

& P

S S

hunt

vol

tage

[V]

time [s]

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

volta

ge [V

]

HeatershuntSec1Sec2Sec3Sec5Sec6Sec7Sec8Sec9

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

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

Quench-degradation in Bi2212 mostly wire-independent

11

OST 0.8 mm, 37*18Ic(s.f) = 450A

200 300 400 500 600 7000.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

Conductor A Conductor A Conductor A

I c a

fter

qu

en

ch /

initi

al I

c

Tmax

(K)

4.2 K, Self-field

OST 1.0 mm, 27*7Ic(s.f) = 160ALimited Bridging

200 300 400 500 600 7000.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

Conductor A Conductor A Conductor A Conductor B Conductor B

I c a

fter

qu

en

ch /

initi

al I

c

Tmax

(K)

4.2 K, Self-field

OST 1.2 mm, 85*18Ic(s.f) = 950A

200 300 400 500 600 7000.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

Conductor A Conductor A Conductor A Conductor B Conductor B Conductor C

I c a

fter

qu

en

ch /

initi

al I

c

Tmax

(K)

4.2 K, Self-field

200 300 400 500 600 7000.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

Fast degradationover 500~600 K

Slow degradationin 350~500 K

No degradationbelow 300 K

Conductor A Conductor A Conductor A Conductor B Conductor B Conductor C

I c a

fter

qu

en

ch /

initi

al I

c

Tmax

(K)

4.2 K, Self-field

L. Ye, T. Shen, J. Schwartz in progress – this entire section of the talk

What’s going on INSIDE the wires?

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”

9.1 mm spot: internal Bi2212 decomposition

13

11.7 mm 4.8 mm

9.3 mm left to burnt pointSecondary 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

Another sample… fracture observed (80% loss of Ic)

14

V1 V2 V3 V4 V5 V6 V70

100

200

300

400

500

600

700

800

I c (A

)

Position

Initial Ic Final Ic

0.0

0.2

0.4

0.6

0.8

1.0

Final Ic/Initial Ic

Initi

al I

c / F

ina

l I c

Short Sample #2

V4 V5V3V2

AB

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

OP wire shows similar behavior

15

200 300 400 500 600 7000.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

1.02

1 Bar HT

100 Bar HT

I c af

ter q

uenc

h / i

nitia

l Ic

Tmax (K)

• Consistent with Godeke results on Ic-e of OP wire

• d(I/Ic)/dT is higher in OP wire around 450-500 K

100 200 300 400 500 600 700350

400

450

500

550

600

650

700

1Bar HT

100 Bar HT

I c a

fter

qu

en

ch (

A)

Tmax (K)

Cracks observed normal to wire axis w/reduced Ic

Section V4: ~40% Ic degradation After light Bi-2212 etching

Section V3: No Ic degradationNo cracks

Compare short wires to samples on ITER barrels of different materials

Pre-strain determines temperature limit more than wire architecture or HT process

17

200 300 400 500 600 7000.80

0.85

0.90

0.95

1.00

Ti-alloy Barrel #1

8cm sample

16cm sample

Ti-alloy Barrel #2

I c a

fte

r q

ue

nch

/I c b

efo

re q

ue

nch

Tmax

(K)

G-10 Barrel

4cm sample

Increasing constraintTi-alloy barrel

G-10 barrel

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

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

3D/2D electro-thermo-structural tape model• Real dimensions, from mm to device scale• Computationally efficient and experimentally validated• Coupled electrical, magnetostatic, thermal and structural mechanics• Full elastoplasticity on all layers

20

Q

TTJJc(B,θ,T)

Fm

Electric(Superconductivity)

Thermal

Magnetostatic Structural Mechanics

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 modelμm-scale 3D/2D tape model

Multiscale hybrid coil model

[W.K. Chan & J. Schwartz, IEEE Trans. Appl. Supercond., Vol. 22, No. 5, 2012

Most recent - adding a structural model

22

Fabrication Cooling

FatigueAnalysis

Tension@ RT, 77 K

Bending @ 77 K

Quench @ 77 K

ExperimentalValidation

ExperimentalValidation

ComputationalValidation

Residual Stresses from fabrication and cooling

23

700 oC +buffersubstrate

750 oC +YBCO

buffersubstrate

500 oC+Ag

YBCObuffer

substrate

25 oC+Cu Ag

YBCObuffer

substrate

77 K

CuAg

YBCObuffer

substrate

S2 S3 S4

CTE @ RT @ 77 K

αC276 13.0x10-6 10.7x10-6

αcu 16.7x10-6 7.8x10-6

S2: 750 oC 500 oC S3: 500 oC 25 oC S4: 25 oC 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%

Stre

ss, X

-com

pone

nt (x

102

MPa

)

State of commercial tape

(Same stress profile after Cu is added)

buffer @ RT buffer @ RT

No Ag

Coefficient of thermal expansion

Validation via tensile analysis v experimentsimilar validation done using bending experiments

24

1. Apply axial tensile load2. Capture key behavior:

a. Yield stresses and strains of tapei. Yield strain predominately determined by Hastelloyii. Yield stress determined by both Cu and Hastelloy

b. Critical stresses and strains on YBCO layer3. Tangent moduli (hardening slopes) are the least known data4. Yield points in YBCO and buffer are considered as fracture points

εc = 0.0045

ExperimentSimulation

YBCO

Stre

ss, X

-com

pone

nt (G

Pa)

@ 77 K

xx

εy = 0.0066

Experiment

SimulationYBCO

Stre

ss, X

-com

pone

nt (G

Pa)

@ RT

x

x

εy = 0.005 εc = 0.0065

Study bending + quench (500 K) in conductor

25Tensile bending @ 77 K, R = 8.75 mm Compressive bending @ 77 K, R = 8.75 mm

Dashed: Tpeak = 77 KSolid: Tpeak = 500 K

Stre

ss, X

-com

pone

nt (x

102

MPa

)

Stre

ss, X

-com

pone

nt (x

102

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

Study bending + quench in a coil

26

Outer turn N+2

Heater turn N

0

1. Stresses/strains become more complicated in coils2. Stress variations depend on locations of turns and quenching

temperature profilea. Inner turns near hot spot may become all compressiveb. Outer turns may become all tensilec. Compressive bending not necessarily better

Stre

ss, X

-com

pone

nt (x

102

MPa

)

(Without residual stresses)

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

Closing thought

“An overreliance on past successes is a sure blueprint for future failures.”

-- Henry Petroski

28

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 LN2 – 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